Friday, April 21, 2017

No, physicists have not created “negative mass”

Thanks to BBC, I will now for several years get emails from know-it-alls who think physicists are idiots not to realize the expansion of the universe is caused by negative mass. Because that negative mass, you must know, has actually been created in the lab:

The Independent declares this turns physics “completely upside down”

And if you think that was crappy science journalism, The Telegraph goes so far to insists it’s got something to do with black holes

Not that they offer so much as a hint of an explanation what black holes have to do with anything.

These disastrous news items purport to summarize a paper that recently got published in Physics Review Letters, one of the top journals in the field:
    Negative mass hydrodynamics in a Spin-Orbit--Coupled Bose-Einstein Condensate
    M. A. Khamehchi, Khalid Hossain, M. E. Mossman, Yongping Zhang, Th. Busch, Michael McNeil Forbes, P. Engels
    Phys. Rev. Lett. 118, 155301 (2017)
    arXiv:1612.04055 [cond-mat.quant-gas]

This paper reports the results of an experiment in which the physicists created a condensate that behaves as if it has a negative effective mass.

The little word “effective” does not appear in the paper’s title – and not in the screaming headlines – but it is important. Physicists use the preamble “effective” to indicate something that is not fundamental but emergent, and the exact definition of such a term is often a matter of convention.

The “effective radius” of a galaxy, for example, is not its radius. The “effective nuclear charge” is not the charge of the nucleus. And the “effective negative mass” – you guessed it – is not a negative mass.

The effective mass is merely a handy mathematical quantity to describe the condensate’s behavior.

The condensate in question here is a supercooled cloud of about ten thousand Rubidium atoms. To derive its effective mass, you look at the dispersion relation – ie the relation between energy and momentum – of the condensate’s constituents, and take the second derivative of the energy with respect to the momentum. That thing you call the inverse effective mass. And yes, it can take on negative values.
If you plot the energy against the momentum, you can read off the regions of negative mass from the curvature of the resulting curve. It’s clear to see in Fig 1 of the paper, see below. I added the red arrow to point to the region where the effective mass is negative.
Fig 1 from arXiv:1612.04055 [cond-mat.quant-gas]

As to why that thing is called effective mass, I had to consult a friend, David Abergel, who works with cold atom gases. His best explanation is that it’s a “historical artefact.” And it’s not deep: It’s called an effective mass because in the usual non-relativistic limit E=p2/m, so if you take two derivatives of E with respect to p, you get the inverse mass. Then, if you do the same for any other relation between E and p you call the result an inverse effective mass.

It's a nomenclature that makes sense in context, but it probably doesn’t sound very headline-worthy:
“Physicists created what’s by historical accident still called an effective negative mass.”
In any case, if you use this definition, you can rewrite the equations of motion of the fluid. They then resemble the usual hydrodynamic equations with a term that contains the inverse effective mass multiplied by a force.

What this “negative mass” hence means is that if you release the condensate from a trapping potential that holds it in place, it will first start to run apart. And then no longer run apart. That pretty much sums up the paper.

The remaining force which the fluid acts against, it must be emphasized, is then not even an external force. It’s a force that comes from the quantum pressure of the fluid itself.

So here’s another accurate headline:
“Physicists observe fluid not running apart.”
This is by no means to say that the result is uninteresting! Indeed, it’s pretty cool that this fluid self-limits its expansion thanks to long-range correlations which come from quantum effects. I’ll even admit that thinking of the behavior as if the fluid had a negative effective mass may be a useful interpretation. But that still doesn’t mean physicists have actually created negative mass.

And it has nothing to do with black holes, dark energy, wormholes, and the like. Trust me, physics is still upside up.

Wednesday, April 19, 2017

Catching Light – New Video!

I have many shortcomings, like leaving people uncertain whether they’re supposed to laugh or not. But you can’t blame me for lack of vision. I see a future in which science has become a cultural good, like sports, music, and movies. We’re not quite there yet, but thanks to the Foundational Questions Institute (FQXi) we’re a step closer today.

This is the first music video in a series of three, sponsored by FQXi, for which I’ve teamed up with Timo Alho and Apostolos Vasileiadis. And, believe it or not, all three music videos are about physics!

You’ve met Apostolos before on this blog. He’s the one who, incredibly enough, used his spare time as an undergraduate to make a short film about gauge symmetry. I know him from my stay in Stockholm, where he completed a masters degree in physics. Apostolos then, however, decided that research wasn’t for him. He has since founded a company – Third Panda  – and works as freelance videographer.

Timo Alho is one of the serendipitous encounters I’ve made on this blog. After he left some comments on my songs (mostly to point out they’re crappy) it turned out not only is he a theoretical physicist too, but we were both attending the same conference a few weeks later. Besides working on what passes as string theory these days, Timo also plays the keyboard in two bands and knows more than I about literally everything to do with songwriting and audio processing and, yes, about string theory too.

Then I got a mini-grant from FQXi that allowed me to coax the two young men into putting up with me, and five months later I stood in the hail, in a sleeveless white dress, on a beach in Crete, trying to impersonate electromagnetic radiation.

This first music video is about Einstein’s famous thought experiment in which he imagined trying to catch light. It takes on the question how much can be learned by introspection. You see me in the role of light (I am part of the master plan), standing in for nature more generally, and Timo as the theorist trying to understand nature’s working while barely taking notice of it (I can hear her talk to me at night).

The two other videos will follow early May and mid of May, so stay tuned for more!

Update April 21: 

Since several people asked, here are the lyrics. The YouTube video has captions - to see them, click on the CC icon in the bottom bar.

I am part of the master plan
Every woman, every man
I have seen them come and go
Go with the flow

I have seen that we all are one
I know all and every one
I was here when the sun was born
Ages ago

In my mind
I have tried
Catching light
Catching light

In my mind
I have left the world behind

Every time I close my eyes
All of nature's open wide
I can hear her
Talk to me at night

In my mind I have been trying
Catching light outside of time
I collect it in a box
Collect it in a box

Every time I close my eyes
All of nature's open wide
I can hear her
Talk to me at night

[Repeat Chorus]

[Interlude, Einstein recording]
The scientific method itself
would not have led anywhere,
it would not even have been formed
Without a passionate striving for a clear understanding.
Perfection of means
and confusion of goals
seem in my opinion
to characterize our age.

[Repeat Chorus]

Monday, April 17, 2017

Book review: “A Big Bang in a Little Room” by Zeeya Merali

A Big Bang in a Little Room: The Quest to Create New Universes
Zeeya Merali
Basic Books (February 14, 2017)

When I heard that Zeeya Merali had written a book, I expected something like a Worst Of New Scientist compilation. But A Big Bang in A Little Room turned out to be both interesting and enjoyable, if maybe not for the reason the author intended.

If you follow the popular science news on physics foundations, you almost certainly have come across Zeeya’s writing before. She was the one to break news about the surfer dude’s theory of everything and brought black hole echoes to Nature News. She also does much of the outreach work for the Foundational Questions Institute (FQXi).

Judged by the comments I get when sharing Zeeya’s articles, for some of my colleagues she embodies the decline of science journalism to bottomless speculation. Personally, I think what’s decaying to speculation is my colleagues’ research, and if so then Nature’s readership deserves to know about this. But, yes, Zeeya is frequently to be found on the wild side of physics. So, a book about creating universes in the lab seems in line.

To get it out of the way, the idea that we might grow a baby universe has, to date, no scientific basis. It’s an interesting speculation but the papers that have been written about it are little more than math-enriched fiction. To create a universe, we’d first have to understand how our universe began, and we don’t. The theories necessary for this – inflation and quantum gravity – are not anywhere close to being settled. Nobody has a clue how to create a universe, and for what I am concerned that’s really all there is to say about it.

But baby universes are a great excuse to feed real science to the reader, and if that’s the sugar-coating to get medicine down, I approve. And indeed, Zeeya’s book is quite nutritious: From entanglement to general relativity, structure formation, and inflation, to loop quantum cosmology and string theory, it’s all part of her story.

The narrative of A Big Bang in A Little Room starts with the question whether there might be a message encoded in the cosmic microwave background, and then moves on to bubble- and baby-universes, the multiverse, mini-black holes at the LHC, and eventually – my pet peeve! – the hypothesis that we might be living in a computer simulation.

Thankfully, on the latter issue Zeeya spoke to Seth Lloyd who – like me – doesn’t buy Bostrom’s estimate that we likely live in a computer simulation:
“Arguments such as Bostrom’s that hinge on the assumption that in the future physically evolved cosmoses will be outnumbered by a plethora of simulated universes, making it vastly more likely that we are artificial intelligences rather than biological beings, also fail to take into account the immense resources needed to create even basic simulations, says Lloyd.”
So, I’ve found nothing to complain even about the simulation argument!

Zeeya has a PhD in physics, cosmology more specifically, so she has all the necessary background to understand the topics she writes about. Her explanations are both elegant and, for all I can tell, almost entirely correct. I’d have some quibbles on one or the other point, eg her explanation of entanglement doesn’t make clear what’s the difference between classical and quantum correlations, but then it doesn’t matter for the rest of the book. Zeeya is also careful to state that neither inflation nor string theory are established theories, and the book is both well-referenced and has useful endnotes for the reader who wants more details.

Overall, however, Zeeya doesn’t offer the reader much guidance, but rather presents one thought-provoking idea after the other – like that there are infinitely many copies of each of us in the multiverse, making every possible decision – and then hurries on.

Furthermore, between the chapters there are various loose ends that she never ties together. For example, if the creator of our universe could write a message into the cosmic microwave background, then why do we need inflation to solve the horizon problem? How do baby universes fit together with string theory, or AdS/CFT more specifically, and why was the idea mostly abandoned? It’s funny also that Lee Smolin’s cosmological natural selection – an idea according to which we should live in a universe that amply procreates and is hence hugely supportive of the whole universe-creation issue  – is mentioned merely as an aside, and when it comes to loop quantum gravity, both Smolin and Rovelli are bypassed as Ashtekhar’s “collaborators,” (which I’m sure the two gentlemen will just love to hear).

For what I am concerned, the most interesting aspect of Zeeya’s book is that she spoke to various scientists about their creation beliefs: Anthony Zee, Stephen Hsu, Abhay Ashtekar, Joe Polchinski, Alan Guth, Eduardo Guendelman, Alexander Vilenkin, Don Page, Greg Landsberg, and Seth Lloyd are familiar names that appear on the pages. (The majority of these people are FQXi members.)

What we believe to be true is a topic physicists rarely talk about, and I think this is unfortunate. We all believe in something – most scientists, for example believe in an external reality – but fessing up to the limits of our rationality isn’t something we like to get caught with. For this reason I find Zeeya’s book very valuable.

About the value of discussing baby universes I’m not so sure. As Zeeya notes towards the end of her book, of the physicists she spoke to, besides Don Page no one seems to have thought about the ethics of creating new universes. Let me offer a simple explanation for this: It’s that besides Page no one believes the idea has scientific merit.

In summary: It’s a great book if you don’t take the idea of universe-creation too seriously. I liked the book as much as you can possibly like a book whose topic you think is nonsense.

[Disclaimer: Free review copy.]

Wednesday, April 12, 2017

Why doesn’t anti-matter anti-gravitate?

Flying pig.
Why aren’t there any particles that fall up in the gravitational field of Earth? It would be so handy – If I had to move the couch, rather than waiting for the husband to flex his muscles, I’d just tie an anti-gravitating weight to it and the couch would float to the other side of the room.

Newton’s law of gravity and Coulomb’s law for the electric force between two charges have the same mathematical form, so how come we have both positive and negative electric charges but not both negative and positive gravitational masses?

The quick answer to the question is, well, we’ve never seen anything fall up. But if there was anti-gravitating matter, it would be repelled by our planet. So maybe it’s not so surprising we don’t see any of it here. Might there be anti-gravitating matter elsewhere?

It’s a difficult question, more difficult than even most physicists appreciate. The difference between gravity and the electromagnetic interaction – which gives rise to Coulomb’s law – is the type of messenger field. Interactions between particles are mediated by fields. For electromagnetism the mediator is a vector-field. For gravity it’s a more complicated field, a 2nd rank tensor-field, which describes space-time itself.

In case an interaction is quantized, the interaction’s field is accompanied by a particle: For electromagnetism that’s the photon, for gravity it’s the (hypothetical) graviton. The particles share the properties of the field, but for the question of whether or not there’s anti-gravity the quantization of the field doesn’t play a role.

The major difference between the two cases comes down to a sign. For a vector-field, as in the case of electromagnetism, like charges repel and unlike charges attract. For a 2nd rank tensor field, in contrast, like charges attract and unlike charges repel. This already tells us that an anti-gravitating particle would not be repelled by everything. It would be repelled by normally gravitating mass – which we may agree to call “positive” – but be attracted by gravitational masses of its own kind – which we may call “negative.”

The question then becomes: Where are the particles of negative gravitational mass?

To better understand the theoretical backdrop, we must distinguish between inertial mass and gravitational mass. The inertial mass is what gives rise to an object’s inertia, ie its resistance to acceleration, and is always positive valued. The gravitational mass, on the other hand, is what creates the gravitational field of the object. In usual general relativity, the two masses are identical by assumption: This is Einstein’s equivalence principle in a nutshell. In more detail, we’d not only talk about the equivalence for masses, but for all types of energies, collected in what is known as the stress-energy-tensor. Again, the details get mathematical very fast, but aren’t so relevant to understand the general structure.

All the particles we presently know of are collected in the standard model of particle physics, which is in agreement with data to very high precision. The standard model also includes all anti-particles, which are identical to their partner-particles except for having opposite electric charge. Is it possible that the anti-particles also anti-gravitate?

Theory clearly answer this question with “No.” From the standard model, we can derive how anti-matter gravitates – it gravitates exactly the same way as normal matter. And observational evidence supports this conclusion as follows.

We don’t normally see anti-particles around us because they annihilate when they come in contact with normal matter, leaving behind merely a flash of light. Why there isn’t the same amount of matter and anti-matter in the universe nobody really knows – it’s a big mystery that goes under the name “baryon asymmetry” – but evidence shows the universe is dominated by matter. If we see anti-particles – in cosmic rays or in particle colliders – it’s usually as single particles, which are both too light and too short-lived to reliably measure their gravitational mass.

That, however, doesn’t mean we don’t know how anti-matter behaves under the influence of gravity. Both matter and anti-matter particles hold together the quarks that make up neutrons and protons. Indeed, the anti-particles’ energy makes a pretty large contribution to the total mass of neutrons and protons, and hence to the total mass of pretty much everything around us. This means if anti-matter had a negative gravitational mass, the equivalence principle would be badly violated. It isn’t, and so we already know anti-matter doesn’t anti-gravitate.

Those with little faith in theoretical arguments might want to argue that maybe it’s possible to find a way to make anti-matter anti-gravitate only sometimes. I am not aware of any theorem which strictly proves this to be impossible, but neither is there – to my best knowledge – any example of a consistent theory in which this has been shown to work.

And if that still wasn’t enough to convince you, the ALPHA experiment at CERN has not only created neutral anti-hydrogen, made of an anti-proton and a positron (an anti-electron), but has taken great strides towards measuring exactly how anti-hydrogen behaves in Earth’s gravitation field. Guess what? So far there is no evidence that anti-hydrogen falls upwards – though the present measurement precision only rules out that the anti-hydrogen’s gravitational mass is not larger than (minus!) 65 times its inertial mass.

[Correction added April 19: There is not one but three approved experiments at CERN to measure the free fall of anti hydrogen: AEGIS, ALPHA-g and GBAR.]

So, at least theoretical physicists are pretty sure that none of the particles we know anti-gravitates. But could there be other particles, which we haven’t yet discovered, that anti-gravitate?

In principle, yes, but there is no observational evidence for this. In contrast to what is often said, dark energy does not anti-gravitate. The distinctive property of dark energy is that the ratio of energy-density over pressure is negative. For anti-gravitating matter, however, both energy-density and pressure change sign, so the ratio stays positive. This means anti-gravitating matter, if it exists, behaves just the same way as normal matter does, except that the two types of matter repel each other. It also doesn’t give rise to anything like dark matter, because negative gravitational mass would have the exact opposite effect as needed to explain dark matter.

To be fair, I also don’t know of any experiment that explicitly looks for signatures of anti-gravitational matter, like for example concave gravitational lensing. So, strictly speaking, it hasn’t been ruled out, but it’s a hypothesis that also hasn’t attracted much professional interest. Many theoretical physicists who I have talked to believe that negative gravitational masses would induce vacuum-decay because particle pairs could be produced out of nothing. This argument, however, doesn’t take into account that the inertial masses remain positive which prohibits pair production. (On a more technical note, it is a little appreciated fact that the canonical stress-energy tensor isn’t the same as the gravitational stress-energy tensor.)

Even so, let us suppose that the theoretically possible anti-gravitating matter is somewhere out there. What would it be good for? Not for much, it turns out. The stuff would interact with our normal matter even more weakly than neutrinos. This means even if we’d manage to find some of it in our vicinity – which is implausible already – we wouldn’t be able to catch it and use it for anything. It would simply pass right through us.

The anti-gravitating weight that I’d want to tie to the couch, therefore, will unfortunately remain fiction.

[This post previously appeared on Starts With A Bang.]

Friday, April 07, 2017

Book review reviewed: “The Particle Zoo” by Gavin Hesketh

The Particle Zoo: The Search for the Fundamental Nature of Reality
By Gavin Hesketh
Paperback Edition
Quercus (15 Jun. 2017)

A few weeks ago, I reviewed Gavin Heskeths book The Particle Zoo. I found his introduction to quantum field theory very well done. Considering that he can’t rely on equations, Hesketh gets across a lot of details (notably, what Feynman diagrams do and don’t depict).

However, I was quite unhappy with various inaccuracies in the book, particularly concerning the search for physics beyond the standard model.

But then something amazing happened! Hesketh sent me an email a few days ago, saying he read my review and revised the manuscript for the paperback edition to address the criticism. While the changes between the two editions will not be large, it usually doesn’t take more than a sentence or two to add some context or a word of caution. And so, I’m happy to endorse the paperback edition of The Particle Zoo which (according to amazon) will appear on June 15th.

Thursday, April 06, 2017

Dear Dr. B: Why do physicists worry so much about the black hole information paradox?

    “Dear Dr. B,

    Why do physicists worry so much about the black hole information paradox, since it looks like there are several, more mundane processes that are also not reversible? One obvious example is the increase of the entropy in an isolated system and another one is performing a measurement according to quantum mechanics.

    Regards, Petteri”

Dear Petteri,

This is a very good question. Confusion orbits the information paradox like accretion disks orbit supermassive black holes. A few weeks ago, I figured even my husband doesn’t really know what the problem is, and he doesn’t only have a PhD in physics, he has also endured me rambling about the topic for more than 15 years!

So, I’m happy to elaborate on why theorists worry so much about black hole information. There are two aspects to this worry: one scientific and one sociological. Let me start with the scientific aspect. I’ll comment on the sociology below.

In classical general relativity, black holes aren’t much trouble. Yes, they contain a singularity where curvature becomes infinitely large – and that’s deemed unphysical – but the singularity is hidden behind the horizon and does no harm.

As Stephen Hawking pointed out, however, if you take into account that the universe – even vacuum – is filled with quantum fields of matter, you can calculate that black holes emit particles, now called “Hawking radiation.” This combination of unquantized gravity with quantum fields of matter is known as “semi-classical” gravity, and it should be a good approximation as long as quantum effects of gravity can be neglected, which means as long as you’re not close by the singularity.

Illustration of black hole with jet and accretion disk.
Image credits: NASA.

Hawking radiation consists of pairs of entangled particles. Of each pair, one particle falls into the black hole while the other one escapes. This leads to a net loss of mass of the black hole, ie the black hole shrinks. It loses mass until entirely evaporated and all that’s left are the particles of the Hawking radiation which escaped.

Problem is, the surviving particles don’t contain any information about what formed the black hole. And not only that, information of the particles’ partners that went into the black hole is also lost. If you investigate the end-products of black hole evaporation, you therefore can’t tell what the initial state was; the only quantities you can extract are the total mass, charge, and angular momentum- the three “hairs” of black holes (plus one qubit). Black hole evaporation is therefore irreversible.

Irreversible processes however don’t exist in quantum field theory. In technical jargon, black holes can turn pure states into mixed states, something that shouldn’t ever happen. Black hole evaporation thus gives rise to an internal contradiction, or “inconsistency”: You combine quantum field theory with general relativity, but the result isn’t compatible with quantum field theory.

To address your questions: Entropy increase usually does not imply a fundamental irreversibility, but merely a practical one. Entropy increases because the probability to observe the reverse process is small. But fundamentally, any process is reversible: Unbreaking eggs, unmixing dough, unburning books – mathematically, all of this can be described just fine. We merely never see this happening because such processes would require exquisitely finetuned initial conditions. A large entropy increase makes a process irreversible in practice, but not irreversible in principle.

That is true for all processes except black hole evaporation. No amount of finetuning will bring back the information that was lost in a black hole. It’s the only known case of a fundamental irreversibility. We know it’s wrong, but we don’t know exactly what’s wrong. That’s why we worry about it.

The irreversibility in quantum mechanics, which you are referring to, comes from the measurement process, but black hole evaporation is irreversible already before a measurement was made. You could argue then, why should it bother us if everything we can possibly observe requires a measurement anyway? Indeed, that’s an argument which can and has been made. But in and by itself it doesn’t remove the inconsistency. You still have to demonstrate just how to reconcile the two mathematical frameworks.

This problem has attracted so much attention because the mathematics is so clear-cut and the implications are so deep. Hawking evaporation relies on the quantum properties of matter fields, but it does not take into account the quantum properties of space and time. It is hence widely believed that quantizing space-time is necessary to remove the inconsistency. Figuring out just what it would take to prevent information loss would teach us something about the still unknown theory of quantum gravity. Black hole information loss, therefore, is a lovely logical puzzle with large potential pay-off – that’s what makes it so addictive.

Now some words on the sociology. It will not have escaped your attention that the problem isn’t exactly new. Indeed, its origin predates my birth. Thousands of papers have been written about it during my lifetime, and hundreds of solutions have been proposed, but theorists just can’t agree on one. The reason is that they don’t have to: For the black holes which we observe (eg at the center of our galaxy), the temperature of the Hawking radiation is so tiny there’s no chance of measuring any of the emitted particles. And so, black hole evaporation is the perfect playground for mathematical speculation.

[Lots of Papers. Img: 123RF]
There is an obvious solution to the black hole information loss problem which was pointed out already in early days. The reason that black holes destroy information is that whatever falls through the horizon ends up in the singularity where it is ultimately destroyed. The singularity, however, is believed to be a mathematical artifact that should no longer be present in a theory of quantum gravity. Remove the singularity and you remove the problem.

Indeed, Hawking’s calculation breaks down when the black hole has lost almost all of its mass and has become so small that quantum gravity is important. This would mean the information would just come out in the very late, quantum gravitational, phase and no contradiction ever occurs.

This obvious solution, however, is also inconvenient because it means that nothing can be calculated if one doesn’t know what happens nearby the singularity and in strong curvature regimes which would require quantum gravity. It is, therefore, not a fruitful idea. Not many papers can be written about it and not many have been written about it. It’s much more fruitful to assume that something else must go wrong with Hawking’s calculation.

Sadly, if you dig into the literature and try to find out on which grounds the idea that information comes out in the strong curvature phase was discarded, you’ll find it’s mostly sociology and not scientific reasoning.

If the information is kept by the black hole until late, this means that small black holes must be able to keep many different combinations of information inside. There are a few papers which have claimed that these black holes then must emit their information slowly, which means small black holes would behave like a technically infinite number of particles. In this case, so the claim, they should be produced in infinite amounts even in weak background fields (say, nearby Earth), which is clearly incompatible with observation.

Unfortunately, these arguments are based on an unwarranted assumption, namely that the interior of small black holes has a small volume. In GR, however, there isn’t any obvious relation between surface area and volume because space can be curved. The assumption that such small black holes, for which quantum gravity is strong, can be effectively described as particles is equally shaky. (For details and references, please see this paper I wrote with Lee some years ago.)

What happened, to make a long story short, is that Lenny Susskind wrote a dismissive paper about the idea that information is kept in black holes until late. This dismissal gave everybody else the opportunity to claim that the obvious solution doesn’t work and to henceforth produce endless amounts of papers on other speculations.

Excuse the cynicism, but that’s my take on the situation. I’ll even admit having contributed to the paper pile because that’s how academia works. I too have to make a living somehow.

So that’s the other reason why physicists worry so much about the black hole information loss problem: Because it’s speculation unconstrained by data, it’s easy to write papers about it, and there are so many people working on it that citations aren’t hard to come by either.

Thanks for an interesting question, and sorry for the overly honest answer.

Friday, March 31, 2017

Book rant: “Universal” by Brian Cox and Jeff Forshaw

Universal: A Guide to the Cosmos
Brian Cox and Jeff Forshaw
Da Capo Press (March 28, 2017)
(UK Edition, Allen Lane (22 Sept. 2016))

I was meant to love this book.

In “Universal” Cox and Forshaw take on astrophysics and cosmology, but rather than using the well-trodden historic path, they offer do-it-yourself instructions.

The first chapters of the book start with every-day observations and simple calculations, by help of which the reader can estimate eg the radius of Earth and its mass, or – if you let a backyard telescope with a 300mm lens and equatorial mount count as every-day items – the distance to other planets in the solar system.

Then, the authors move on to distances beyond the solar system. With that, self-made observations understandably fade out, but are replaced with publicly available data. Cox and Forshaw continue to explain the “cosmic distance ladder,” variable stars, supernovae, redshift, solar emission spectra, Hubble’s law, the Herzsprung-Russell diagram.

Set apart from the main text, the book has “boxes” (actually pages printed white on black) with details of the example calculations and the science behind them. The first half of the book reads quickly and fluidly and reminds me in style of school textbooks: They make an effort to illuminate the logic of scientific reasoning, with some historical asides, and concrete numbers. Along the way, Cox and Forshaw emphasize that the great power of science lies in the consistency of its explanations, and they highlight the necessity of taking into account uncertainty both in the data and in the theories.

The only thing I found wanting in the first half of the book is that they use the speed of light without explaining why it’s constant or where to get it from, even though that too could have been done with every-day items. But then maybe that’s explained in their first book (which I haven’t read).

For me, the fascinating aspect of astrophysics and cosmology is that it connects the physics of the very small scales with that of the very large scales, and allows us to extrapolate both into the distant past and future of our universe. Even though I’m familiar with the research, it still amazes me just how much information about the universe we have been able to extract from the data in the last two decades.

So, yes, I was meant to love this book. I would have been an easy catch.

Then the book continues to explain the dark matter hypothesis as a settled fact, without so much as mentioning any shortcomings of LambdaCDM, and not a single word on modified gravity. The Bullet Cluster is, once again, used as a shut-up argument – a gross misrepresentation of the actual situation, which I previously complained about here.

Inflation gets the same treatment: It’s presented as if it’s a generally accepted model, with no discussion given to the problem of under-determination, or whether inflation actually solves problems that need a solution (or solves the problems period).

To round things off, the authors close the final chapter with some words on eternal inflation and bubble universes, making a vague reference to string theory (because that’s also got something to do with multiverses you see), and then they suggest this might mean we live in a computer simulation:

“Today, the cosmologists responsible for those simulations are hampered by insufficient computing power, which means that they can only produce a small number of simulations, each with different values for a few key parameters, like the amount of dark matter and the nature of the primordial perturbations delivered at the end of inflation. But imagine that there are super-cosmologists who know the String Theory that describes the inflationary Multiverse. Imagine that they run a simulation in their mighty computers – would the simulated creatures living within one of the simulated bubble universes be able to tell that they were in a simulation of cosmic proportions?”
Wow. After all the talk about how important it is to keep track of uncertainty in scientific reasoning, this idea is thrown at the reader with little more than a sentence which mentions that, btw, “evidence for inflation” is “not yet absolutely compelling” and there is “no firm evidence for the validity of String Theory or the Multiverse.” But, hey, maybe we live in a computer simulation, how cool is that?

Worse than demonstrating slippery logic, their careless portrayal of speculative hypotheses as almost settled is dumb. Most of the readers who buy the book will have heard of modified gravity as dark matter’s competitor, and will know the controversies around inflation, string theory, and the multiverse: It’s been all over the popular science news for several years. That Cox and Forshaw don’t give space to discussing the pros and cons in a manner that at least pretends to be objective will merely convince the scientifically-minded reader that the authors can’t be trusted.

The last time I thought of Brian Cox – before receiving the review copy of this book – it was because a colleague confided to me that his wife thinks Brian is sexy. I managed to maneuver around the obviously implied question, but I’ll answer this one straight: The book is distinctly unsexy. It’s not worthy of a scientist.

I might have been meant to love the book, but I ended up disappointed about what science communication has become.

[Disclaimer: Free review copy.]

Monday, March 27, 2017

Book review: “Anomaly!” by Tommaso Dorigo

Anomaly! Collider Physics and the Quest for New Phenomena at Fermilab
Tommaso Dorigo
World Scientific Publishing Europe Ltd (November 17, 2016)

Tommaso Dorigo is a familiar name in the blogosphere. Over at “A Quantum’s Diary’s Survivor”, he reliably comments on everything going on in particle physics. Located in Venice, Tommaso is a member of the CMS collaboration at CERN and was part of the CDF collaboration at Tevatron – a US particle collider that ceased operation in 2011.

Anomaly! Is Tommaso’s first book and it chronicles his time in the CDF collaboration from the late 1980s until 2000. This covers the measurement of the mass of the Z-boson, the discovery of the top-quark and the – eventually unsuccessful – search for supersymmetric particles. In his book, Tommaso weaves together the scientific background about particle physics with brief stories of the people involved and their – often conflict-laden – discussions.

The first chapters of the book contain a brief summary of the standard model and quantum field theory and can be skipped by those familiar with these topics. The book is mostly self-contained in that Tommaso provides all the knowledge necessary to understand what’s going on (with a few omissions that I believe don’t matter much). But the pace is swift. I sincerely doubt a reader without background in particle physics will be able to get through the book without re-reading some passages many times.

It is worth emphasizing that Tommaso is an experimentalist. I think I hadn’t previously realized how much the popular science literature in particle physics has, so-far, been dominated by theorists. This makes Anomaly! a unique resource. Here, the reader can learn how particle physics is really done! From the various detectors and their designs, to parton distribution functions, to triggers and Monte Carlo simulations, Tommaso doesn’t shy away from going into all the details. At the same time, his anecdotes showcase how a large collaboration like CDF – with more than 500 members – work.

That having been said, the book is also somewhat odd in that it simply ends without summary, or conclusion, or outlook. Given that the events Tommaso writes about date back 30 years, I’d have been interested to hear whether something has changed since. Is the software development now better managed? Is there still so much competition between collaborations? Is the relation to the media still as fraught? I got the impression an editor pulled the manuscript out under Tommaso’s still typing fingers because no end was in sight 😉

Besides this, I have little to complain about. Tommaso’s writing style is clear and clean, and also in terms of structure – mostly chronological – nothing seems amiss. My major criticism is that the book doesn’t have any references, meaning the reader is stuck there without any guide for how to proceed in case he or she wants to find out more.

So should you, or should you not buy the book? If you’re considering to become a particle physicist, I strongly recommend you read this book to find out if you fit the bill. And if you’re a science writer who regularly reports on particle physics, I also recommend you read this book to get an idea of what’s really going on. All the rest of you I have to warn that while the book is packed with information, it’s for the lovers. It’s about how the author tracked down a factor of 1.25^2 to explain why his data analysis came up with 588 rather than 497 Z \to b\bar b decays. And you’re expected to understand why that’s exciting.

On a personal note, the book brought back a lot of memories. All the talk of Herwig and Pythia, of Bjorken-x, rapidity and pseudorapidity, missing transverse energy, the CTEQ tables, hadronization, lost log-files, missed back-ups, and various fudge-factors reminded me of my PhD thesis – and of all the reasons I decided that particle physics isn’t for me.

[Disclaimer: Free review copy.]

Wednesday, March 22, 2017

Academia is fucked-up. So why isn’t anyone doing something about it?

A week or so ago, a list of perverse incentives in academia made rounds. It offers examples like “rewarding an increased number of citations” that – instead of encouraging work of high quality and impact – results in inflated citation lists, an academic tit-for-tat which has become standard practice. Likewise, rewarding a high number of publications doesn’t produce more good science, but merely finer slices of the same science.

Perverse incentives in academia.
Source: Edwards and Roy (2017). Via.

It’s not like perverse incentives in academia is news. I wrote about this problem ten years ago, referring to it as the confusion of primary goals (good science) with secondary criteria (like, for example, the number of publications). I later learned that Steven Pinker made the same distinction for evolutionary goals, referring to it as ‘proximate’ vs ‘ultimate’ causes.

The difference can be illustrated in a simple diagram (see below). A primary goal is a local optimum in some fitness landscape – it’s where you want to go. A secondary criterion is the first approximation for the direction towards the local optimum. But once you’re on the way, higher-order corrections must be taken into account, otherwise the secondary criterion will miss the goal – often badly.

The number of publications, to come back to this example, is a good first-order approximation. Publications demonstrate that a scientist is alive and working, is able to think up and finish research projects, and – provided the paper are published in peer reviewed journals – that their research meets the quality standard of the field.

To second approximation, however, increasing the number of publications does not necessarily also lead to more good science. Two short papers don’t fit as much research as do two long ones. Thus, to second approximation we could take into account the length of papers. Then again, the length of a paper is only meaningful if it’s published in a journal that has a policy of cutting superfluous content. Hence, you have to further refine the measure. And so on.

This type of refinement isn’t specific to science. You can see in many other areas of our lives that, as time passes, the means to reach desired goals must be more carefully defined to make sure they still lead where we want to go.

Take sports as example. As new technologies arise, the Olympic committee has added many additional criteria on what shoes or clothes athletes are admitted to wear, which drugs make for an unfair advantage, and they’ve had to rethink what distinguishes a man from a woman.

Or tax laws. The Bible left it at “When the crop comes in, give a fifth of it to Pharaoh.” Today we have books full of ifs and thens and whatnots so incomprehensible I suspect it’s no coincidence suicide rates peak during tax season.

It’s debatable of course whether current tax laws indeed serve a desirable goal, but I don’t want to stray into politics. Relevant here is only the trend: Collective human behavior is difficult to organize, and it’s normal that secondary criteria to reach primary goals must be refined as time passes.

The need to quantify academic success is a recent development. It’s a consequence of changes in our societies, of globalization, increased mobility and connectivity, and is driven by the increased total number of people in academic research.

Academia has reached a size where accountability is both important and increasingly difficult. Unless you work in a tiny subfield, you almost certainly don’t know everyone in your community and can’t read every single publication. At the same time, people are more mobile than ever, and applying for positions has never been easier.

This means academics need ways to judge colleagues and their work quickly and accurately. It’s not optional – it’s necessary. Our society changes, and academia has to change with it. It’s either adapt or die.

But what has been academics’ reaction to this challenge?

The most prevalent reaction I witness is nostalgia: The wish to return to the good old times. Back then, you know, when everyone on the committee had the time to actually read all the application documents and was familiar with all the applicants’ work anyway. Back then when nobody asked us to explain the impact of our work and when we didn’t have to come up with 5-year plans. Back then, when they recommended that pregnant women smoke.

Well, there’s no going back in time, and I’m glad the past has passed. I therefore have little patience for such romantic talk: It’s not going to happen, period. Good measures for scientific success are necessary – there’s no way around it.

Another common reaction is the claim that quality isn’t measurable – more romantic nonsense. Everything is measurable, at least in principle. In practice, many things are difficult to measure. That’s exactly why measures have to be improved constantly.

Then, inevitably, someone will bring up Goodhart’s Law: “When a measure becomes a target, it ceases to be a good measure.” But that is clearly wrong. Sorry, Goodhard. If you want to indeed optimize the measure, you get exactly what you asked for. The problem is that often the measure wasn’t what you wanted to begin with.

With use of the terminology introduced above, Goodhard’s Law can be reformulated as: “When people optimize a secondary criterion, they will eventually reach a point where further optimization diverts from the main goal.” But our reaction to this should be to improve the measure, not throw the towel and complain “It’s not possible.”

This stubborn denial of reality, however, has an unfortunate consequence: Academia has gotten stuck with the simple-but-bad secondary criteria that are currently in use: number of publications, the infamous h-index, the journal impact factor, renown co-authors, positions held at prestigious places, and so on. 

We all know they’re bad measures. But we use them anyway because we simply don’t have anything better. If your director/dean/head/board is asked to demonstrate how great your place is, they’ll fall back on the familiar number of publications, and as a bonus point out who has recently published in Nature. I’ve seen it happen. I just had to fill in a form for the institute’s board in which I was asked for my h-index and my paper count.

Last week, someone asked me if I’d changed my mind in the ten years since I wrote about this problem first. Needless to say, I still think bad measures are bad for science. But I think that I was very, very naïve to believe just drawing attention to the problem would make any difference. Did I really think that scientists would see the risk to their discipline and do something about it? Apparently that’s exactly what I did believe.

Of course nothing like this happened. And it’s not just because I’m a nobody who nobody’s listening to. Similar concerns like mine have been raised with increasing frequency by more widely known people in more popular outlets, like Nature and Wired. But nothing’s changed.

The biggest obstacle to progress is that academics don’t want to admit the problem is of their own making. Instead, they blame others: policy makers, university administrators, funding agencies. But these merely use measures that academics themselves are using.

The result has been lots of talk and little action. But what we really need is a practical solution. And of course I have one on offer: An open-source software that allows every researcher to customize their own measure for what they think is “good science” based on the available data. That would include the number of publications and their citations. But there is much more information in the data which currently isn’t used.

You might want to know whether someone’s research connects areas that are only loosely connected. Or how many single-authored papers they have. You might want to know how well their keyword-cloud overlaps with that of your institute. You might want to develop a measure for how “deep” and “broad” someone’s research is – two terms that are often used in recommendation letters but that are extremely vague.

Such individualized measures wouldn’t only automatically update as people revise criteria, but they would also counteract the streamlining of global research and encourage local variety.

Why isn’t this happening? Well, besides me there’s no one to do it. And I have given up trying to get funding for interdisciplinary research. The inevitable response I get is that I’m not qualified. Of course it’s correct – I’m not qualified to code and design a user-interface. But I’m totally qualified to hire some people and kick their asses. Trust me, I have experience kicking ass. Price tag to save academia: An estimated 2 million Euro for 5 years.

What else has changed in the last ten years? I’ve found out that it’s possible to get paid for writing. My freelance work has been going well. The main obstacle I’ve faced is lack of time, not lack of opportunity. And so, when I look at academia now, I do it with one leg outside. What I see is that academia needs me more than I need academia.

The current incentives are extremely inefficient and waste a lot of money. But nothing is going to change until we admit that solving the problem is our own responsibility.

Maybe, when I write about this again, ten years from now, I’ll not refer to academics as “us” but as “they.”

Wednesday, March 15, 2017

No, we probably don’t live in a computer simulation

According to Nick Bostrom of the Future of Humanity Institute, it is likely that we live in a computer simulation. And one of our biggest existential risks is that the superintelligence running our simulation shuts it down.

The simulation hypothesis, as it’s called, enjoys a certain popularity among people who like to think of themselves as intellectual, believing it speaks for their mental flexibility. Unfortunately it primarily speaks for their lacking knowledge of physics.

Among physicists, the simulation hypothesis is not popular and that’s for a good reason – we know that it is difficult to find consistent explanations for our observations. After all, finding consistent explanations is what we get paid to do.

Proclaiming that “the programmer did it” doesn’t only not explain anything - it teleports us back to the age of mythology. The simulation hypothesis annoys me because it intrudes on the terrain of physicists. It’s a bold claim about the laws of nature that however doesn’t pay any attention to what we know about the laws of nature.

First, to get it out of the way, there’s a trivial way in which the simulation hypothesis is correct: You could just interpret the presently accepted theories to mean that our universe computes the laws of nature. Then it’s tautologically true that we live in a computer simulation. It’s also a meaningless statement.

A stricter way to speak of the computational universe is to make more precise what is meant by ‘computing.’ You could say, for example, that the universe is made of bits and an algorithm encodes an ordered time-series which is executed on these bits. Good - but already we’re deep in the realm of physics.

If you try to build the universe from classical bits, you won’t get quantum effects, so forget about this – it doesn’t work. This might be somebody’s universe, maybe, but not ours. You either have to overthrow quantum mechanics (good luck), or you have to use qubits. [Note added for clarity: You might be able to get quantum mechanics from a classical, nonlocal approach, but nobody knows how to get quantum field theory from that.]

Even from qubits, however, nobody’s been able to recover the presently accepted fundamental theories – general relativity and the standard model of particle physics. The best attempt to date is that by Xiao-Gang Wen and collaborators, but they are still far away from getting back general relativity. It’s not easy.

Indeed, there are good reasons to believe it’s not possible. The idea that our universe is discretized clashes with observations because it runs into conflict with special relativity. The effects of violating the symmetries of special relativity aren’t necessarily small and have been looked for – and nothing’s been found.

For the purpose of this present post, the details don’t actually matter all that much. What’s more important is that these difficulties of getting the physics right are rarely even mentioned when it comes to the simulation hypothesis. Instead there’s some fog about how the programmer could prevent simulated brains from ever noticing contradictions, for example contradictions between discretization and special relativity.

But how does the programmer notice a simulated mind is about to notice contradictions and how does he or she manage to quickly fix the problem? If the programmer could predict in advance what the brain will investigate next, it would be pointless to run the simulation to begin with. So how does he or she know what are the consistent data to feed the artificial brain with when it decides to probe a specific hypothesis? Where does the data come from? The programmer could presumably get consistent data from their own environment, but then the brain wouldn’t live in a simulation.

It’s not that I believe it’s impossible to simulate a conscious mind with human-built ‘artificial’ networks – I don’t see why this should not be possible. I think, however, it is much harder than many future-optimists would like us to believe. Whatever the artificial brains will be made of, they won’t be any easier to copy and reproduce than human brains. They’ll be one-of-a-kind. They’ll be individuals.

It therefore seems implausible to me that we will soon be outnumbered by artificial intelligences with cognitive skills exceeding ours. More likely, we will see a future in which rich nations can afford raising one or two artificial consciousnesses and then consult them on questions of importance.

So, yes, I think artificial consciousness is on the horizon. I also think it’s possible to convince a mind with cognitive abilities comparable to that of humans that their environment is not what they believe it is. Easy enough to put the artificial brain in a metaphoric vat: If you don’t give it any input, it would never be any wiser. But that’s not the environment I experience and, if you read this, it’s not the environment you experience either. We have a lot of observations. And it’s not easy to consistently compute all the data we have.

Besides, if the reason you build an artificial intelligences is consultation, making them believe reality is not what it seems is about the last thing you’d want.

Hence, the first major problem with the simulation hypothesis is to consistently create all the data which we observe by any means other than the standard model and general relativity – because these are, for all we know, not compatible with the universe-as-a-computer.

Maybe you want to argue it is only you alone who is being simulated, and I am merely another part of the simulation. I’m quite sympathetic to this reincarnation of solipsism, for sometimes my best attempt of explaining the world is that it’s all an artifact of my subconscious nightmares. But the one-brain-only idea doesn’t work if you want to claim that it is likely we live in a computer simulation.

To claim it is likely we are simulated, the number of simulated conscious minds must vastly outnumber those of non-simulated minds. This means the programmer will have to create a lot of brains. Now, they could separately simulate all these brains and try to fake an environment with other brains for each, but that would be nonsensical. The computationally more efficient way to convince one brain that the other brains are “real” is to combine them in one simulation.

Then, however, you get simulated societies that, like ours, will set out to understand the laws that govern their environment to better use it. They will, in other words, do science. And now the programmer has a problem, because it must keep close track of exactly what all these artificial brains are trying to probe.

The programmer could of course just simulate the whole universe (or multiverse?) but that again doesn’t work for the simulation argument. Problem is, in this case it would have to be possible to encode a whole universe in part of another universe, and parts of the simulation would attempt to run their own simulation, and so on. This has the effect of attempting to reproduce the laws on shorter and shorter distance scales. That, too, isn’t compatible with what we know about the laws of nature. Sorry.

Stephen Wolfram (from Wolfram research) recently told John Horgan that:
    “[Maybe] down at the Planck scale we’d find a whole civilization that’s setting things up so our universe works the way it does.”

I cried a few tears over this.

The idea that the universe is self-similar and repeats on small scales – so that elementary particles are built of universes which again contain atoms and so on – seems to hold a great appeal for many. It’s another one of these nice ideas that work badly. Nobody’s ever been able to write down a consistent theory that achieves this – consistent both internally and with our observations. The best attempt I know of are limit cycles in theory space but to my knowledge that too doesn’t really work.

Again, however, the details don’t matter all that much – just take my word for it: It’s not easy to find a consistent theory for universes within atoms. What matters is the stunning display of ignorance – for not to mention arrogance –, demonstrated by the belief that for physics at the Planck scale anything goes. Hey, maybe there’s civilizations down there. Let’s make a TED talk about it next. For someone who, like me, actually works on Planck scale physics, this is pretty painful.

To be fair, in the interview, Wolfram also explains that he doesn’t believe in the simulation hypothesis, in the sense that there’s no programmer and no superior intelligence laughing at our attempts to pin down evidence for their existence. I get the impression he just likes the idea that the universe is a computer. (Note added: As a commenter points out, he likes the idea that the universe can be described as a computer.)

In summary, it isn’t easy to develop theories that explain the universe as we see it. Our presently best theories are the standard model and general relativity, and whatever other explanation you have for our observations must first be able to reproduce these theories’ achievements. “The programmer did it” isn’t science. It’s not even pseudoscience. It’s just words.

All this talk about how we might be living in a computer simulation pisses me off not because I’m afraid people will actually believe it. No, I think most people are much smarter than many self-declared intellectuals like to admit. Most readers will instead correctly conclude that today’s intelligencia is full of shit. And I can’t even blame them for it.

Saturday, March 11, 2017

Is Verlinde’s Emergent Gravity compatible with General Relativity?

Dark matter filaments, Millenium Simulation
Image: Volker Springel
A few months ago, Erik Verlinde published an update of his 2010 idea that gravity might originate in the entropy of so-far undetected microscopic constituents of space-time. Gravity, then, would not be fundamental but emergent.

With the new formalism, he derived an equation for a modified gravitational law that, on galactic scales, results in an effect similar to dark matter.

Verlinde’s emergent gravity builds on the idea that gravity can be reformulated as a thermodynamic theory, that is as if it was caused by the dynamics of a large number of small entities whose exact identity is unknown and also unnecessary to describe their bulk behavior.

If one wants to get back usual general relativity from the thermodynamic approach, one uses an entropy that scales with the surface area of a volume. Verlinde postulates there is another contribution to the entropy which scales with the volume itself. It’s this additional entropy that causes the deviations from general relativity.

However, in the vicinity of matter the volume-scaling entropy decreases until it’s entirely gone. Then, one is left with only the area-scaling part and gets normal general relativity. That’s why on scales where the average density is high – high compared to galaxies or galaxy clusters – the equation which Verlinde derives doesn’t apply. This would be the case, for example, near stars.

The idea quickly attracted attention in the astrophysics community, where a number of papers have since appeared which confront said equation with data. Not all of these papers are correct. Two of them seemed to have missed entirely that the equation which they are using doesn’t apply on solar-system scales. Of the remaining papers, three are fairly neutral in the conclusions, while one – by Lelli et al – is critical. The authors find that Verlinde’s equation – which assumes spherical symmetry – is a worse fit to the data than particle dark matter.

There has not, however, so far been much response from theoretical physicists. I’m not sure why that is. I spoke with science writer Anil Ananthaswamy some weeks ago and he told me he didn’t have an easy time finding a theorist willing to do as much as comment on Verlinde’s paper. In a recent Nautilus article, Anil speculates on why that might be:
“A handful of theorists that I contacted declined to comment, saying they hadn’t read the paper; in physics, this silent treatment can sometimes be a polite way to reject an idea, although, in fairness, Verlinde’s paper is not an easy read even for physicists.”
Verlinde’s paper is indeed not an easy read. I spent some time trying to make sense of it and originally didn’t get very far. The whole framework that he uses – dealing with an elastic medium and a strain-tensor and all that – isn’t only unfamiliar but also doesn’t fit together with general relativity.

The basic tenet of general relativity is coordinate invariance, and it’s absolutely not clear how it’s respected in Verlinde’s framework. So, I tried to see whether there is a way to make Verlinde’s approach generally covariant. The answer is yes, it’s possible. And it actually works better than I expected. I’ve written up my findings in a paper which just appeared on the arxiv:

It took some trying around, but I finally managed to guess a covariant Lagrangian that would produce the equations in Verlinde’s paper when one makes the same approximations. Without these approximations, the equations are fully compatible with general relativity. They are however – as so often in general relativity – hideously difficult to solve.

Making some simplifying assumptions allows one to at least find an approximate solution. It turns out however, that even if one makes the same approximations as in Verlinde’s paper, the equation one obtains is not exactly the same that he has – it has an additional integration constant.

My first impulse was to set that constant to zero, but upon closer inspection that didn’t make sense: The constant has to be determined by a boundary condition that ensures the gravitational field of a galaxy (or galaxy cluster) asymptotes to Friedmann-Robertson-Walker space filled with normal matter and a cosmological constant. Unfortunately, I haven’t been able to find the solution that one should get in the asymptotic limit, hence wasn’t able to fix the integration constant.

This means, importantly, that the data fits which assume the additional constant is zero do not actually constrain Verlinde’s model.

With the Lagrangian approach that I have tried, the interpretation of Verlinde’s model is very different – I dare to say far less outlandish. There’s an additional vector-field which permeates space-time and which interacts with normal matter. It’s a strange vector field both because it’s not – as the other vector-fields we know of – a gauge-boson, and has a different kinetic energy term. In addition, the kinetic term also appears in a way one doesn’t commonly have in particle physics but instead in condensed matter physics.

Interestingly, if you look at what this field would do if there was no other matter, it would behave exactly like a cosmological constant.

This, however, isn’t to say I’m sold on the idea. What I am missing is, most importantly, some clue that would tell me the additional field actually behaves like matter on cosmological scales, or at least sufficiently similar to reproduce other observables, like eg baryon acoustic oscillation. This should be possible to find out with the equations in my paper – if one manages to actually solve them.

Finding solutions to Einstein’s field equations is a specialized discipline and I’m not familiar with all the relevant techniques. I will admit that my primary method of solving the equations – to the big frustration of my reviewers – is to guess solutions. It works until it doesn’t. In the case of Friedmann-Robertson-Walker with two coupled fluids, one of which is the new vector field, it hasn’t worked. At least not so far. But the equations are in the paper and maybe someone else will be able to find a solution.

In summary, Verlinde’s emergent gravity has withstood the first-line bullshit test. Yes, it’s compatible with general relativity.

Thursday, March 02, 2017

Yes, a violation of energy conservation can explain the cosmological constant

Chad Orzel recently pointed me towards an article in Physics World according to which “Dark energy emerges when energy conservation is violated.” Quoted in the Physics World article are George Ellis, who enthusiastically notes that the idea is “no more fanciful than many other ideas being explored in theoretical physics at present,” and Lee Smolin, according to whom it’s “speculative, but in the best way.” Chad clearly found this somewhat too polite to be convincing and asked me for some open words:

I had seen the headline flashing by earlier but ignored it because – forgive me – it’s obvious energy non-conservation can mimic a cosmological constant.

Reason is that usually, in General Relativity, the expansion of space-time is described by two equations, known as the Friedmann-equations. They relate the velocity and acceleration of the universe’s normalized distance measures – called the ‘scale factor’ – with the average energy density and pressure of matter and radiation in the universe. If you put in energy-density and pressure, you can calculate how the universe expands. That, basically, is what cosmologists do for a living.

The two Friedmann-equations, however, are not independent of each other because General Relativity presumes that the various forms of energy-densities are locally conserved. That means if you take only the first Friedmann-equation and use energy-conservation, you get the second Friedmann-equation, which contains the cosmological constant. If you turn this statement around it means that if you throw out energy conservation, you can produce an accelerated expansion.

It’s an idea I’ve toyed with years ago, but it’s not a particularly appealing solution to the cosmological constant problem. The issue is you can’t just selectively throw out some equations from a theory because you don’t like them. You have to make everything work in a mathematically consistent way. In particular, it doesn’t make sense to throw out local energy-conservation if you used this assumption to derive the theory to begin with.

Upon closer inspection, the Physics World piece summarizes the paper:
which got published in PRL a few weeks ago, but has been on the arxiv for almost a year. Indeed, when I looked at it, I recalled I had read the paper and found it very interesting. I didn’t write about it here because the point they make is quite technical. But since Chad asked, here we go.

Modifying General Relativity is chronically hard because the derivation of the theory is so straight-forward that much violence is needed to avoid Einstein’s Field Equations. It took Einstein a decade to get the equations right, but if you know your differential geometry it’s a three-liner really. This isn’t to belittle Einstein’s achievement – the mathematical apparatus wasn’t then fully developed and he was guessing its way around underived theorems – but merely to emphasize that General Relativity is easy to get but hard to amend.

One of the few known ways to consistently amend General Relativity is ‘unimodular gravity,’ which works as follows.

In General Relativity the central dynamical quantity is the metric tensor (or just “metric”) which you need to measure the ratio of distances relative to each other. From the metric tensor and its first and second derivative you can calculate the curvature of space-time.

General Relativity can be derived from an optimization principle by asking: “From all the possible metrics, which is the one that minimizes curvature given certain sources of energy?” This leads you to Einstein’s Field Equations. In unimodular gravity in contrast, you don’t look at all possible metrics but only those with a fixed metric determinant, which means you don’t allow a rescaling of volumes. (A very readable introduction to unimodular gravity by George Ellis can be found here.)

Unimodular gravity does not result in Einstein’s Field Equations, but only in a reduced version thereof because the variation of the metric is limited. The result is that in unimodular gravity, energy is not automatically locally conserved. Because of the limited variation of the metric that is allowed in unimodular gravity, the theory has fewer symmetries. And, as Emmy Noether taught us, symmetries give rise to conservation laws. Therefore, unimodular gravity has fewer conservation laws.

I must emphasize that this is not the ‘usual’ non-conservation of total energy one already has in General Relativity, but a new violation of local energy-densities does that not occur in General Relativity.

If, however, you then add energy-conservation to unimodular gravity, you get back Einstein’s field equations, though this re-derivation comes with a twist: The cosmological constant now appears as an integration constant. For some people this solves a problem, but personally I don’t see what difference it makes just where the constant comes from – its value is unexplained either way. Therefore, I’ve never found unimodular gravity particularly interesting, thinking, if you get back General Relativity you could as well have used General Relativity to begin with.

But in the new paper the authors correctly point out that you don’t necessarily have to add energy conservation to the equations you get in unimodular gravity. And if you don’t, you don’t get back general relativity, but a modification of general relativity in which energy conservation is violated – in a mathematically consistent way.

Now, the authors don’t look at all allowed violations of energy-conservation in their paper and I think smartly so, because most of them will probably result in a complete mess, by which I mean be crudely in conflict with observation. They instead look at a particularly simple type of energy conservation and show that this effectively mimics a cosmological constant.

They then argue that on the average such a type of energy-violation might arise from certain quantum gravitational effects, which is not entirely implausible. If space-time isn’t fundamental, but is an emergent description that arises from an underlying discrete structure, it isn’t a priori obvious what happens to conservation laws.

The framework proposed in the new paper, therefore, could be useful to quantify the observable effects that arise from this. To demonstrate this, the authors look at the example of 1) diffusion from causal sets and 2) spontaneous collapse models in quantum mechanics. In both cases, they show, one can use the general description derived in the paper to find constraints on the parameters in this model. I find this very useful because it is a simple, new way to test approaches to quantum gravity using cosmological data.

Of course this leaves many open questions. Most importantly, while the authors offer some general arguments for why such violations of energy conservation would be too small to be noticeable in any other way than from the accelerated expansion of the universe, they have no actual proof for this. In addition, they have only looked at this modification from the side of General Relativity, but I would like to also know what happens to Quantum Field Theory when waving good-bye to energy conservation. We want to make sure this doesn’t ruin the standard model’s fit of any high-precision data. Also, their predictions crucially depend on their assumption about when energy violation begins, which strikes me as quite arbitrary and lacking a physical motivation.

In summary, I think it’s a so-far very theoretical but also interesting idea. I don’t even find it all that speculative. It is also clear, however, that it will require much more work to convince anybody this doesn’t lead to conflicts with observation.

Thursday, February 23, 2017

Book Review: “The Particle Zoo” by Gavin Hesketh

The Particle Zoo: The Search for the Fundamental Nature of Reality
By Gavin Hesketh
Quercus (1 Sept. 2016)

The first word in Gavin Hesketh’s book The Particle Zoo is “Beauty.” I read the word, closed the book, and didn’t reopen it for several months. Having just myself finished writing a book about the role of beauty in theoretical physics, it was the absolutely last thing I wanted to hear about.

I finally gave Hesketh’s book a second chance and took it along on a recent flight. Turned out once I passed the somewhat nauseating sales pitch in the beginning, the content considerably improved.

Hesketh provides a readable and accessible no-nonsense introduction to the standard model and quantum field theory. He explains everything as well as possible without using equations.

The author is an experimentalist and part of the LHC’s ATLAS collaboration. The Particle Zoo also has a few paragraphs about how it is to work in such large collaborations. Personally, I found this the most interesting part of the book. Hesketh also does a great job to describe how the various types of particle detectors work.

Had the book ended here, it would have been a well-done job. But Hesketh goes on to elaborate on physics beyond the standard model. And there he’s clearly out of his water.

Problems start when he begins laying out the shortcomings of the standard model, leaving the reader with the impression that it’s non-renormalizable. I suspect (or hope) he wasn’t referring to non-renormalizability but maybe Landau poles or the non-convergence of the perturbative expansion, but the explanation is murky.

Murky is bad, but wrong is worse. And wrong follows. Fore example, to generate excitement for new physics, Hesketh writes:
“Some theories suggest that antimatter responds to gravity in a different way: matter and antimatter may repel each other… [W]hile this is a strange idea, so far it is one that we cannot rule out.”
I do not know of any consistent theory that suggests antimatter responds differently to gravity than matter, and I say that as one of the three theorists on the planet who have worked on antigravity. I have no idea what Hesketh is referring to in this paragraph.

It does not help that “The Particle Zoo” does not have any references. I understand that a popular science book isn’t a review article, but I would expect that a scientist at least quotes sources for historical facts and quotations, which isn’t the case.

He then confuses a “Theory of Everything” with quantum gravity, and about supersymmetry (SuSy) he writes:
“[I]f SuSy is possible and it makes everything much neater, it really should exist. Otherwise it seems that nature has apparently gone out of its way to avoid it, making the equations uglier at the same time, and we would have to explain why that is.”
Which is a statement that should be embarrassing for any scientist to make.

Hesketh’s attitude to supersymmetry is however somewhat schizophrenic because he later writes that:
“[T]his is really why SuSy has lived for so long: whenever an experiment finds no signs of the super-particles, it is possible merely to adjust some of these free parameters so that these super-particles must be just a little bit heavier, just a little bit further out of reach. By never being specific, it is never wrong.”
Only to then reassure the reader
“SuSy may end up as another beautiful theory destroyed by an ugly fact, and we should find out in the next years.”
I am left to wonder which fact he thinks will destroy a theory that he just told us is never wrong.

Up to this point I might have blamed the inaccuracies on an editor, but then Hesketh goes on to explain the (ADD model of) large extra dimensions and claims that it solves the hierarchy problem. This isn’t so – the model reformulates one hierarchy (the weakness of gravity) as another hierarchy (extra dimensions much larger than the Planck length) and hence doesn’t solve the problem. I am not sure whether he is being intentionally misleading or really didn’t understand this, but either way, it’s wrong.

Hesketh furthermore states that if there were such large extra dimensions the LHC might produce microscopic black holes – but he doesn’t mention with a single word that not the faintest evidence for this has been found.

When it comes to dark matter, he waves away the possibility that the observations are due to a modification of gravity with the magic word “Bullet Cluster” – a distortion of facts about which I have previously complained. I am afraid he actually might not know any better since this myth has been so widely spread, but if he doesn’t care to look at the subject he shouldn’t write a book about it. To round things up, Hesketh misspells “Noether” as “Nöther,” though I am willing to believe that this egg was laid by someone else.

In summary, the first two thirds of the book about the standard model, quantum field theory, and particle detectors are recommendable. But when it comes to new physics the author doesn’t know what he’s talking about.

Update April 7th 2017: Most of these bummers have been fixed in the paperback edition.

Sunday, February 19, 2017

Fake news wasn’t hard to predict – But what’s next?

In 2008, I wrote a blogpost which began with a dark vision – a presidential election led astray by fake news.

I’m not much of a prophet, but it wasn’t hard to predict. Journalism, for too long, attempted the impossible: Make people pay for news they don’t want to hear.

It worked, because news providers, by and large, shared an ethical code. Journalists aspired to tell the truth; their passion was unearthing and publicizing facts – especially those that nobody wanted to hear. And as long as the professional community held the power, they controlled access to the press – the device – and kept up the quality.

But the internet made it infinitely easy to produce and distribute news, both correct and incorrect. Fat headlines suddenly became what economists call an “intangible good.” No longer does it rely on a physical resource or a process of manufacture. News now can be created, copied, and shared by anyone, anywhere, with almost zero investment.

By the early 00s, anybody could set up a webpage and produce headlines. From thereon, quality went down. News makes the most profit if it’s cheap and widely shared. Consequently, more and more outlets offer the news people want to read –that’s how the law of supply and demand is supposed to work after all.

What we have seen so far, however, is only the beginning. Here’s what’s up next:
  • 1. Fake News Get Organized

    An army of shadow journalists specializes in fake news, pitching it to alternative news outlets. These outlets will mix real and fake news. It becomes increasingly hard to tell one from the other.

  • 2. Fake News Becomes Visual

    “Picture or it didn’t happen,” will soon be a thing of the past. Today, it’s still difficult to forge photos and videos. But software becomes better, and cheaper, and easier to obtain, and soon it will take experts to tell real from fake.

  • 3. Fake News Get Cozy

    Anger isn’t sustainable. In the long run, most people want good news – they want to be reassured everything’s fine. The war in Syria is over. The earthquake risk in California is low. The economy is up. The chocolate ratio has been raised again.

  • 4. Cooperations Throw the Towel

    Facebook and Google and Yahoo conclude out it’s too costly to assess the truth value of information passed on by their platforms, and decide it’s not their task. They’re right.
  • 5. Fake News Has Real-World Consequences

    We’ll see denial of facts leading to deaths of thousands of people. I mean lack of earthquake warning systems because the risk was believed fear-mongering. I mean riots over terrorist attacks that never happened. I mean collapsed buildings and toxic infant formula because who cares about science. We’ll get there.

The problem that fake news poses for democratic societies attracted academic interest already a decade ago. Triggered by the sudden dominance of Google as search engine, it entered the literature under the name “Googlearchy.”

Democracy relies on informed decision making. If the electorate doesn’t know what’s real, democratic societies can’t identify good ways to carry out the people’s will. You’d think that couldn’t be in anybody’s interest, but it is – if you can make money from misinformation.

Back then, the main worry focused on search engines as primary information providers. Someone with more prophetic skills might have predicted that social networks would come to play the central role for news distribution, but the root of the problem is the same: Algorithms are designed to deliver news which users like. That optimizes profit, but degrades the quality of news.

Economists of the Chicago School would tell you that this can’t be. People’s behavior reveals what they really want, and any regulation of the free market merely makes the fulfillment of their wants less efficient. If people read fake news, that’s what they want – the math proves it!

But no proof is better than its assumptions, and one central assumption for this conclusion is that people can’t have mutually inconsistent desires. We’re supposed to have factored in long-term consequences of today’s actions, properly future-discounted and risk-assessed. In other words, we’re supposed to know what’s good for us and our children and grand-grand-children and make rational decisions to work towards that goal.

In reality, however, we often want what’s logically impossible. Problem is, a free market, left unattended, caters predominantly to our short-term wants.

On the risk of appearing to be inconsistent, economists are right when they speak of revealed preferences as the tangible conclusion of our internal dialogues. It’s just that economists, being economists, like to forget that people have a second way of revealing preferences – they vote.

We use democratic decision making to ensure the long-term consequences of our actions are consistent with the short-term ones, like putting a price on carbon. One of the major flaws of current economic theory is that it treats the two systems, economic and political, as separate, when really they’re two sides of the same coin. But free markets don’t work without a way to punish forgery, lies, and empty promises.

This is especially important for intangible goods – those which can be reproduced with near-zero effort. Intangible goods, like information, need enforced copyright, or else quality becomes economically unsustainable. Hence, it will take regulation, subsidies, or both to prevent us from tumbling down into the valley of alternative facts.

In the last months I’ve seen a lot of finger-pointing at scientists for not communicating enough or not communicating correctly, as if we were the ones to blame for fake news. But this isn’t our fault. It’s the media which has a problem – and it’s a problem scientists solved long ago.

The main reason why fake news is hard to identify, and why it remains profitable to reproduce what other outlets have already covered, is that journalists – in contrast to scientists – are utterly intransparent about their doings.

As a blogger, I see this happening constantly. I know that many, if not most, science writers closely follow science blogs. And the professional writers frequently report on topics previously covered by bloggers – without doing as much as naming their sources, not to mention referencing them.

This isn’t merely a personal paranoia. I know this because in several instances science writers actually told me that my blogpost about this-or-that has been so very useful. Some even asked me to share links to their articles they wrote based on it. Let that sink in for a moment – they make money from my expertise, don’t give me credits, and think that this is entirely appropriate behavior. And you wonder why fake news is economically profitable?

For a scientist, that’s mindboggling. Our currency is citations. Proper credits is pretty much all we want. Keep the money, but say my name.

I understand that journalists have to protect some sources, so don’t misunderstand me. I don’t mean they have to spill beans about their exclusive secrets. What I mean is simply that a supposed news outlet that merely echoes what’s been reported elsewhere should be required to refer to the earlier article.

Of course this would imply that the vast majority of existing news sites were revealed as copy-cats and lose readers. And of course it isn’t going to happen because nobody’s going to enforce it. If I saw even a remote chance of this happening, I wouldn’t have made the above predictions, would I?

What’s even more perplexing for a scientist, however, is that news outlets, to the extent that they do fact-checks, don’t tell customers that they fact-check, or what they fact-check, or how they fact-check.

Do you know, for example, which science magazines fact-check their articles? Some do, some don’t. I know for a few because I’ve been border-crossing between scientists and writers for a while. But largely it’s insider knowledge – I think it should be front-page information. Listen, Editor-in-Chief: If you fact-check, tell us.

It isn’t going to stop fake news, but I think a more open journalistic practice and publicly stated adherence to voluntary guidelines could greatly alleviate it. It probably makes you want to puke, but academics are good at a few things and high community standards are one of them. And that is what journalisms need right now.

I know, this isn’t exactly the cozy, shallow, good news that shares well. But it will be a great pleasure when, in ten years, I can say: I told you so.