Rainbow Gravity

In the past week, I received two emails from some folks concerned that a speculative physics proposal called "rainbow gravity" eliminates the Big Bang and hence the beginning of the universe.  They are worried that this undermines Christianity.  Presumably if two different people living in different countries took the trouble to email me about it, there are hundreds of people out there, equally worried about it, who didn't bother to email me.

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Now I already wrote a big long series about whether the universe began and I don't want to repeat everything again.  But let me say a bit more about the virtue of faith as it relates to scientific inquiry.  As a scientist I think it is a real shame if, when Christians learn about Way Cool New Science, their first instinct is to doubt their faith and wonder if the new stuff undermines what they believe about God.

I think it's a lot healthier to be able to learn new and amazing things about the world—the world that God created—without worrying that every new discovery will undermine your religious beliefs and make it so you have to be an atheist.  I would like to propose that it is a virtue for a Christian be able to learn new things about the world, and to keep an open mind towards new discoveries without continually engaging in the torture of nagging doubt and worry, so that scientific discovery starts seeming like a hostile force.  That is not the confidence which comes from faith.  A happily married wife shouldn't spend all her time worrying (without good cause) that her husband is cheating on her whenever he goes off on a business meeting.

Even if it were established that there was time before the Big Bang, that would not establish that God did not create the universe.  Perhaps the beginning of the universe was in fact a long time before what we think of as the Big Bang.  Or perhaps we need to be flexible about what we mean by creation, and say that God created a universe which goes back infinitely in time.

Don't get me wrong; I am an evidentialist.  I think people should only believe in things for which there is enough evidence, and that merely being consistent with the scientific data is not (by itself) strong evidence.  But I also think that there's more than one type of evidence, since we have the historical records of Christ and the inner testimony of God's Spirit to help us.  Your personal relationship with Christ obviously does not consist primarily of speculation about what happened (or didn't happen) before the Big Bang.  If Christianity is important enough to worry about, that's because it's relevant to your personal life, not just to scientific questions.

If you are solidly rooted in Christ then you don't have to be ''tossed back and forth by the waves, and blown here and there by every wind of teaching" (Eph. 4:14).  That in turn gives one the ability to explore new knowledge with a feeling of freedom and security (and paradoxically this probably puts you in a better position to know whether Christianity is true or not, then constantly worrying about it all the time would).

But is rainbow gravity in fact Way Cool New Science?  Let's explore and see.

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My correspondents were concerned by an article by Sarah Knapton which appeared in two slightly different forms in the National Post and Telegraph.  These in turn appear to mostly be cannibalized versions of this Phys.org article, which in turn describes the article "Absence of Black Holes at LHC due to Gravity's Rainbow" by Ahmed Farag Ali, Mir Faizal, and Mohammed M. Khalil.

However, there is nothing about the Big Bang in these last two links, so Knapton must have done some some additional investigation.  It is true that Faizel also wrote another article suggesting that rainbow gravity might avoid the Big Bang.  (But more likely she got this information from a Scientific American blog article based on arXiv:1308.4343).

Knapton's article belongs to a long and venerable tradition of journalists taking the marginal, speculative ideas and making it sound like they are taken seriously by the scientific community.  She states that

Scientists at Cern in Switzerland believe the particle accelerator, which will be restarted this week, might find miniature black holes at a certain energy level.

This could prove the controversial theory of “rainbow gravity” which suggests that the universe stretches back in time infinitely with no singular point where it started, and so no Big Bang. The theory was postulated to reconcile Einstein’s theory of general relativity, which controls very large objects, and quantum mechanics, which affects the tiniest building blocks of the universe. It takes its name from a suggestion that gravity’s effect on the cosmos is felt differently by varying wavelengths of light.

This makes it sound like Ali, Faizal, and Khalil are located at CERN (which they are not), that many researchers at CERN take the idea of rainbow gravity seriously, and that there is some sort of epic "controversy"-battle taking place between those who believe it and those who don't.  But the reality on the ground is that most particle physicists and quantum gravity researchers probably haven't even had this idea show up on their radar screen.  (Now the multiverse?  That rises to the level of being controversial.)

Sentences like:

Scientists believe they could find the first proof of alternative realities that exist outside ou[r] own universe.

make it sound as if the scientific community takes something seriously, when actually it just means that somebody (with a science job) wrote a article (with equations) proposing it—and that they have at least one coauthor, since the noun "scientists" is plural!

The Scientific American article is a bit better.  It correctly states that

The idea is not a complete theory for describing quantum effects on gravity, and is not widely accepted.

but then immediately thereafter we have the inevitable "at least 2 scientists are willing to indulge in speculation" construction:

Nevertheless, physicists have now applied the concept to the question of how the universe began, and found that if rainbow gravity is correct, spacetime may have a drastically different origin story than the widely accepted picture of the big bang.

Unfortunately, these types of inflated articles make it difficult for non-scientists to tell which new ideas in science are actually taken seriously or not.  How can non-scientists tell whether something is legit?  The fact that the article made it through the peer-review process of a top journal?  (Hold on for a moment while I stop laughing.)  While peer-review tends to filter out the worst crackpots, quite a few lemons still manage to get through.  Conversely, good articles are frequently rejected, although this is mitigated by the tactic of simply submitting to enough journals that one of them accepts it—but this tactic is also open to authors of bad papers!

What makes good science is observational support, elegance, precise models, and so on.  Unfortunatly non-scientists usually have to take the word of the scientific community about the extent to which any given proposal meets these tests.

(Honestly, given the awfulness of pop-science venues in this respect, I would say if you aren't a scientist, it's probably best not to take any new scientific idea you read in the news all that seriously, at least not until you find out that a broad cross-section of the relevant experts believe in it.  It's a better use of your time to learn about the Way Cool Old Science which has already been established!)

The main trouble with "Absence of Black Holes" is that it's a combination of two different speculative ideas (each with individual problems), and together they become even worse.  I don't want to call this paper crackpot exactly, but let's just say that it has a very, very small probability of being correct.  The main ingredients are:

1) Rainbow gravity (which seems to have originated from something called "Doubly Special Relativity") is an idea based on very speculative quantum gravity models suggesting Special Relativity should be modified for particles with energy very close to the Planck scale, $10^{16}$ TeV, when quantum gravity effects become important.  The idea is that the spacetime seen by different particles should depend on how energetic those particles are.

Unfortunately nobody knows how to make this model into a mathematically consistent field theory (like every other successful fundamental theory to date).  So they just make crude approximations, like proposing that the geometry felt by a particle at a given spacetime position depends on its energy, by means of a function $f(E)$ (which they just make up rather than actually deriving it honestly from any deeper theory).

But simultaneously measuring (a) energy-momentum and (b) spacetime position conflicts with the Heisenberg Uncertainty Principle, so this is hard to reconcile with quantum mechanics even though it was inspired by quantum gravity.  It seems that these theories would have to be fundamentally nonlocal.

2) Large Extra Dimensions.  The idea here is that there are additional dimensions, besides the usual 4 spacetime dimensions we see.  One has to explain why we can't see these dimensions normally; in this particular approach one says that ordinary matter fields are stuck on a 4 dimensional membrane and that only gravity can propagate in the extra dimensions.

This has the effect of strengthening gravity at short distances, and could conceivably even lower the Planck scale to smaller values, perhaps even to a few TeV.  Of course it was no conicidence that people were most interested in models in which the Planck scale was moved to energies accessible to the LHC, thus making people excited by the possibility of seeing things like quantum black holes experimentally!  (Nobody gets a Nobel prize for saying that we will never observe quantum gravity effects because the energy scales are much too high.)  This was also an extremely speculative idea, and what's more, after turning on the LHC we haven't yet seen any black holes or anything like that.

The sane conclusion to draw, of course, is that probably there are no large extra dimensions (or if there are, the Planck scale is still considerably above what we can see).  These authors instead propose that if rainbow gravity is also true, the minimum size of black holes might be bigger, explaining why we haven't seen them yet.

I was going to write a more detailed critique, but I find that Sabine Hossenfelder has already done most of the work for me.  She writes that:

In rainbow gravity the metric is energy-dependent which it normally is not. This energy-dependence is a non-standard modification that is not confirmed by any evidence. It is neither a theory nor a model, it is just an idea that, despite more than a decade of work, never developed into a proper model. Rainbow gravity has not been shown to be compatible with the standard model. There is no known quantization of this approach and one cannot describe interactions in this framework at all. Moreover, it is known to lead to non-localities with are ruled out already. For what I am concerned, no papers should get published on the topic until these issues have been resolved.

Rainbow gravity enjoys some popularity because it leads to Planck scale effects that can affect the propagation of particles, which could potentially be observable. Alas, no such effects have been found. No such effects have been found if the Planck scale is the normal one! The absolutely last thing you want to do at this point is argue that rainbow gravity should be combined with large extra dimensions, because then its effects would get stronger and probably be ruled out already. At the very least you would have to revisit all existing constraints on modified dispersion relations and reaction thresholds and so on. This isn't even mentioned in the paper.

That isn't all there is to say though. In their paper, the authors also unashamedly claim that such a modification has been predicted by Loop Quantum Gravity, and that it is a natural incorporation of effects found in string theory. Both of these statements are manifestly wrong. Modifications like this have been motivated by, but never been derived from Loop Quantum Gravity. And String Theory gives rise to some kind of minimal length, yes, but certainly not to rainbow gravity; in fact, the expression of the minimal length relation in string theory is known to be incompatible with the one the authors use. The claims that this model they use has some kind of derivation or even a semi-plausible motivation from other theories is just marketing. If I had been a referee of this paper, I would have requested that all these wrong claims be scraped.

I also briefly looked at the other article by Faizel about avoiding the Big Bang, and it seemed just as ad hoc as any of the other papers on this subject, and I wasn't convinced it makes sense.  I don't think we should expect to find any pots of gold at the end of this particular rainbow.