- “It’s crazy — don’t waste my time.”
- “It’s possible, but it’s not worth doing.”
- “I always said it was a good idea.”

Maybe I’m getting old and bold rather than wise and nice, but when it comes to quantum gravity phenomenology, craziness seems to thrive particularly well. My mother asked me the other day what I tell a journalist who wants a comment on somebody else’s work which I think is nonsense. I told her I normally say “It’s very implausible.” No, I’m not nice enough to bite my tongue if somebody asks for an opinion. And so, let me tell you that most of what gets published under the name of quantum gravity phenomenology is, well, very implausible.

But quantum gravity phenomenology is just an extreme example of a general tension that you find in theoretical physics. Consider you’d rank all unconfirmed theories on two scales, one the spectrum from exciting to boring, the other the spectrum from very implausible to likely correct. Then put a dot for each theory in a plane with these two scales as axes. You’d see that the two measures are strongly correlated: The nonsense is exciting, and the truth is boring, and most of what scientists work on falls on a diagonal from exiting nonsense to boring truths.If you’d break this down by research area you’d also find that the more boring the truth, the more people work on nonsense. Wouldn’t you too? And that’s why there is so much exciting nonsense in quantum gravity phenomenology - because the truth is boring indeed.

Conservative wisdom says that quantum gravitational effects are tiny unless space-time curvature is very strong, which only happens in the early universe and inside black holes. This expectation comes from treating quantum gravity as an effective field theory, and quantizing it perturbatively, ie when the fluctuations of space-time are small. The so quantized theory does not make sense as a fundamental theory of gravity because it breaks down at high energies, but it should be fine for calculation in weak gravitational fields.

Most of the exciting ideas in quantum gravity phenomenology assume that this effective limit does not hold for one reason or the other. The most conservative way to be non-conservative is to allow the violation of certain symmetries that are leftover from a fundamental theory of quantum gravity which does not ultimately respect them. Violations of Lorentz-invariance, CPT invariance, space-time homogeneity, or unitarity are such cases that can be accommodated within the effective field theory framework, and that have received much attention as possible signatures of quantum gravity.

Other more exotic proposals implicitly assume that the effective limit does not apply for unexplained reasons. It is known that effective field theories can fail under certain circumstances, but I can’t see how any of these cases play a role in the weak-field limit of gravity. Then again, strong curvature is one of the reasons of failure, and we do not understand what the curvature of space-time is microscopically. So sometimes, when I feel generous, I promote “implausible” to “far-fetched”.

John Donoghue is one of the few heroically pushing through calculations in the true-but-boring corner of quantum gravity phenomenology. In a recent paper, he and his coauthors calculated the quantum contributions to the bending of light in general relativity from 1-loop effects in perturbatively quantized gravity. From their result they define a semi-classical gravitational potential and derive the quantum corrections to Einstein’s classical test of General Relativity by light deflection.

They find a correction term that is suppressed by a factor ℏ G/b

^{2}relative to the classical result, where b is the impact parameter and G is Newton’s constant. This is the typical result you’d expect from dimensional reasons. It’s a loop correction, it must have an extra G in it, it must have an inverse power of the impact parameter so it gets smaller with distance, thus G/b

^{2}is a first guess. Of course you don’t get tenure for guessing, and the actual calculation is quite nasty, see paper for details.

In the paper the authors write “we conclude that the quantum effect is even tinier than the current precision in the measurement of light deflection”, which is an understatement if I have ever seen one. If you are generous and put in a black hole of mass M and a photon that just about manages to avoid being swallowed, the quantum effect is smaller by a factor (m

_{p}/M)

^{2}than the classical term, where m

^{p}is the Planck mass. For a solar mass black hole this is about 70 orders of magnitude suppression. (Though on such a close approach the approximation with a small deflection doesn’t make sense any more.) If you have a Planck-mass black hole, the correction term is of order one – again that’s what you’d expect.

Yes, that is a very plausible result indeed. I would be happy to tell this any journalist, but unfortunately news items seem to be almost exclusively picked from the ever increasing selection of exciting nonsense.

I will admit that it is hard to communicate the relevance of rather technical calculations that don’t lead to stunning results, but please bear with me while I try. The reason this work is so important is that we have to face the bitter truth to find out whether that’s really all that there is or whether we indeed have reason to expect the truth isn’t as bitter as it said on the wrapping. You have to deal with a theory and its nasty details to figure out where it defies your expectations and where your guesses go wrong. And so, we will have to deal with effective quantum gravity to understand its limits. I always said it was a good idea. Even better that somebody else did the calculation so I can continue thinking about the exciting nonsense.

Bonus: True love.

## 32 comments:

If we imagine we can rotate this excitement and boring graph 90 degrees Feynman like what might this tell us of interpreting the main axis of the diagonal?

How is it that muons can be a catalyst to induce fusion? Is this not the deeper question? I mean is there a level beneath the quark and combinational description that is analogous to at least one level of such a catalyst between neutron and proton structures and asymmetry? Kaons? Boring truths give us better controls as well as point to new ideas.

1) This is not the solution we desire.

2) Absent precedent, it is wrong.

3) It would have been proposed earlier if important.

4) It contradicts accepted theory.

Every atom in an α-quartz single crystal forms a 3-fold helix, either all right-handed 3(1) or all left-handed 3(2). Pairs of single crystals in space groups

P3(1)21 versusP3(2)21 violate the Equivalence Principle. Parity violations, symmetry breakings, chiral anomalies, baryogenesis, Chern-Simons repair of Einstein-Hilbert action are sourced; dark matter is Milgrom acceleration, string theory violently contracts. Test spacetime geometry with geometry. A testable good idea is believable after testing."

quantizing it perturbatively" New symmetries cannot appear within perturbation treatments. Trace violate spacetime isotropy absent an external preferred direction.Hi Sabine. A very nice post this one. However, I suppose it would be nicer if you were a bit more specific on what you mean by 'exciting' and 'boring'. I have a sense of course what you are trying to mean here nevertheless for the sake of completeness, i think it would be nice to spell out what you think the word ought to mean in this context for the sake of completeness.

I think the quantum correction is bigger; why you compare with the classical Newtonian term only?

Uncle AI

Extraordinary symmetries can be contained within pertubative spaces appearing directly measurable or not.

Some truths or lies are absolutely boring even uncertainty after a century of excitement in the mystery.

Does a sphere represent minimum quantization, a point, a volume, a line or string? Boredom in theory evokes a search for new exciting theory so I have heard. Is a photon bigger than what it collapses into space wise?

The cited paper I find a little boring for it litterally just restates number theory .

On the other hand the mechanism that sober engineers implied in your chiral concerns is most exciting - are you excited, are you now bored?

While we measure a theory as sober we should not forget the leading role of imagination and scientific progress by our leading theoretical physicists.

And Giotis, what are you asking Bee? If there is some deeper reason behind it that would be a plausably exciting theory indeed.

I showed my theory to the following

1. Ali Shamsdine(NCG), he said but physics is complicated

2. Polchinski, he said it looks simple , let your local university check it out.

3. A guy with his own theories, IT is not scientific.

4.Another guy like above(all well known), It can't be right even if it is correct!

5. Another, nice needs a lot more work(i.e. I doubt if it will work)

6. Bee , no comment.

“Theories have four stages of acceptance: i. This is worthless; ii. This is interesting, but perverse; iii. This is true, but quite unimportant; iv. I always said so.” J. B. S. Haldane (1963)

The paper is too complex for me and I do not understand the math inside. I understand only the simple one and the equations I've written here ( https://www.dropbox.com/s/ejkj84bsmr7xmna/EN_Singularity.pdf?dl=0 ). However as the ratio between r_n and r_s is enormous at quantum level, and due to the fact that matter can't implode into a singularity, the effect of a bending space-time becomes irrelevant as the time belong from (1-r_s / r) dt^2 while space from (1-r_s / r)^-1 ds^2.

The most fun thing for me is to have a completed model of the quantum gravity. The sad one is that even it is not difficult at all my poor understanding of math cause to me major difficulties to write it down... but I'm not scared from it and I will try to do it in the future! :-D

You should watch one of Nima Arkani-Hamed's talks, like the recent one at Perimeter. Then you would understand that exotic nonsense can be extremely boring.

Uncle Al

1. Two branes of opposite handedness either intersect in all points or none.

2. In a sequence alternating chiralities may cancel between any successive sub loops of 4 objects.

3. There are 16 possibilities of 1,2,3, or 4 objects preferred or not, considered isotropic or not.

4. But two are thought superimposed as a null mirror of 1 null 4 object.

5. That leaves 15 observable states of say methane excited by microwaves.

Does this make the theory more believable? Does it settle the distance as some limit or not of curvature toward excluded singularity?

G:

No, I will not. If you think I was serious with this, you misunderstood. I sometimes make a joke along these lines in my seminars and there have been a few people who, like you, take this seriously, so let me give you a serious answer, because this is is very closely related to Richard Dawid's idea of post-empirical assessment.

You can either assess a theory by its compatibility with existing data, or you can make a judgement of belief, which is either subjective (if a single person) or some sort of vote (if you aggregate opinions). However, the believes are either partly based on empirical facts that are not properly taken into account in hard data, or they should not be taken into account because science isn't about believes. There is no such thing as post-empirical science.

For the same reason there is no way of defining "exciting" or "plausible" that is not subjective and it's thus not scientifically relevant. The only way to define it in a useful way is to rely on data and empirical facts, and this should be taken into account already.

If this was too many words, let me summarize this briefly: It's a joke. Best,

B.

Giotis:

Well, what do you think it is? Tell me your estimate then. Best,

B.

Rastus: I don't think I want to waste my time on this...

believes ---> beliefs

Phillip: Well, at least this time I was thinking as I was typing "is it believes or is it beliefs?". And then I was thinking one life, two lives... Maybe next time I'll actually get it right.

Sabine

Is it not great news for quantum gravity when

a) The Baryonic-Tully Fisher relation is derived from QG

b)The galaxy rotation curves are derived from QG

c)The high velocities of population II Halo stars and ancient lenticular galaxies is explained by QG

d) DE and DM are also explained including calculating the precise value of the cosmological constant from QG

Well peers who include prominent astronomers agree.

And the bitterest truth of all from which all excesses originate?

The energy and information of the universe is finite. What you give to one is taken from another. The pill no one wants to swallow.

Eric, you can take and you can give. You can find and you can lose. Entropy and meaning are not necessarily conserved

Why must an ordinary truth be "bitter"? That a finite net can only hold 153 fish does not mean there are no larger numbers.

An ordinary truth can be quite exciting just like the extraordinary mystery the universe makes some sense and persists in the breath it does.

But can science ever resolve this long philosophic paradox between minimalism and the radical empiricists? True love lost is by definition tragic and like cosmology is the source of poetry. What sort of gods would create a world as a practical joke only to load the dice and walk away?

Nominalism not minamalism although we raise issues of terminology a lot these days.

Otto, when I said the bitterest truth I was being ironic. Conservation of energy and information to me is the most beautiful aspect of the universe. But to the perverse, and there are many, it is a limiting factor. People not being able to see things, or not feeling the need to see things from another's perspective comes from the belief that acquiring or claiming things that is not rightfully claimable for oneself will not hurt someone else.

Belief in non-conservation of energy and information in the universe is why wars start. It leads to the idea that unfairness does not have consequences.

@Eric: General Relativity contains no conservation laws.

http://math.ucr.edu/home/baez/physics/Relativity/GR/energy_gr.html

Hello Sabine,

What I’m saying is that as I understand it when you say that

“correction term that is suppressed by a factor ℏ G/b2 relative to the classical result”

you compare with the first classical Newtonian term in equation 20.

But there is also a *classical* correction (the second term in the same equation) due to the non linearities of GR of the order G^2xM^2/b^2.

So your statement “relative to the classical result” is ambiguous.

Hi Giotis,

I see, sorry for being imprecise. Yes, I was comparing to the first term. If you take the second term and multiply it with another \hbar G/b^2, you just get a correction which is even smaller, so it doesn't matter much. Best,

B.

I think the important issue is that we need at least two term corrections for horizons. Then we can ask if any further ghosts of departed quantities matter physically at all. It is too soon to dare more not just a speculative guess.

Jeff Shallit wrote an interesting essay on the history behind quotes similar to Arthur Clarke's about the "phases of truth" (although he didn't mention Clarke's contribution). One of the most common such quotes is usually attributed to Arthur Schopenhauer (often on web sites of questionable repute): "All truth passes through three stages. First, it is ridiculed. Second, it is violently opposed. Third, it is accepted as being self-evident." Presumably this quote appeals to people whose beliefs are ridiculed and opposed, because it seems to suggest that universal acceptance must be right around the corner. However, even aside from the question of whether one should want to associate oneself with Schopenhauer, it appears that Schopenhauer never actually wrote those words. No scholars have ever been able to find it in his writings. He did, however, write something vaguely related in the 1818 preface to "The World as Will and Representation", where he said "To truth only a brief celebration of victory is allowed between the two long periods during which it is condemned as paradoxical or disparaged as trivial". It would be interesting to know if and how the common mis-quotation somehow evolved from this actual statement, or if it's just a coincidence, and the common mis-quotation is a pure fabrication.

Bee - May I dub this then the Hossenfelder index, or perhaps the Bee index? That is,

Excitement/Plausability

I'd suggest that the Hossenfelder index likely cannot be renormalized but is still quite useful over typical ranges.

Alice,

All twins like bonuses.

Grown-ups call our exciting nonsense silly.

This can not be. True love is not silly. No matter what expressions you invent to titillate your love.

What is infinite?

Let me count the ways*

*Prelude to:

How do I love thee?

Elizabeth Barrett Browning

speaks of the infinite.

If only she had included the physics...

Bob

I loved your bonus. It reminds me of an article describing Kaluza-Klein theory as having a haunting beauty. It's good to see someone actually grinding through the hard stuff and pick out a gem or two. Sometimes just doing the math is important, especially with all the speculation and assumptions out there.

Post a Comment