I happened onto an article on the New York Times abut Erik Verlinde’s take on gravity as an Entropic force. The article was written by Dennis Overbye who most of the time does a good job of covering high energy physics. Erik’s work dates from earlier this year and can be found here. To tell the truth, I don’t understand what he’s trying to say in that paper and to me it feels like it’s almost certainly wrong.
However, I don’t want to discuss that paper. What I want to discuss is the following provocative quote
“We’ve known for a long time gravity doesn’t exist,” Dr. Verlinde said, “It’s time to yell it.”
I don’t believe this is taken out of context, so we should take it at face value. The statement is obviously wrong, so it sounds like ultra-post-modern pap and makes all physicists working on the subject of quantum gravity look like crazy mad men. I’m sure this sells newspapers, but that is not the point.
When asked for a sound byte can’t people at least say something that is correct and not just provocative?
The proper way to write that statement is that “Gravity is not really a fundamental force “, which is more correct and does not deny gravity its proper place as something that has been observed in nature, however it is less catchy. If we apply the same criteria as used in the above construction, all of the following statements are also correct:
- Hydrodynamics does not exist (it only happens for collections of atoms, but not for individual ones)
- Space and time do not exist (often used when talking about quantum gravity being emergent from somewhere else)
- All emergent phenomena do not exist (they are not fundamental after all).
- I do not exist (I’m an emergent phenomenon).
Reminds me of discussions I have read before at Backreaction, here and see also here in the Discover magazine about time not existing.
You should also read the following from Asymptotia: But is it real? and also a discussion on What is fundamental, Anyway?
The article is pretty bad.
Having said that, one could make the case that the phrase
“Gravity is not really a fundamental force “ is almost as wrong.
The bottomline is that english vulgarizations of physics concepts always suffer from lost in translation effects and are pretty subjective in the sense that it really is a question of how wrong do you accept to be (rather a tradeoff between accuracy and prose length).
I do agree with the punchline though, and have always felt that its a disservice to laymen to throw catch phrases around as it ultimately confuses more than anything else.
What do we expect when even top tier physicists in their writings equivocate everything, and can’t even settle on fixed definitions… Of course average people will end up completely mystified and think we are crazy.
— Feynman, The Character of Physical Law
— Gell-Mann, The Quark and the Jaguar
Either nothing exists or everything exists – is this one of the conversations that eventually devolves into “that depends on what the definition of ‘is’ is?” Even if we are just experiencing holographic hallucinations projected up from the true quantum dynamic foam of reality, does that make it not exist, as existence itself is merely a human concept and for that reason may be considered to not exist?
Gravity may not be one of the fundamental forces but I’ll be damned if you don’t feel it the same way I do, and therefor it exists. Solids don’t exist but I experience them. For all practical purposes in the average human’s experience, that’s all that matters. Quantum physicists generally ignore the matter of the conscious observation that is needed to make quantum physics work, because there’s considered to practical purpose to exploring that nebulous area.
Gravity must exist even if our description of it is wrong, because it happens. We can measure it. Something makes me not shoot off this non-solid ball of non-orbiting particlewaves non-spinning through non-space. As to what may cause that and whether or not that cause exists almost sounds philosophical rather than physicistical.
Coming up with a new name for an effect or a new explanation for how an effect works doesn’t invalidate that effect. After light became photons it didn’t cease being light, after it became electromagnetic energy it didn’t cease being light and affecting us. We just gained a better understanding of how to use it.
And then there’s the dreaded question of what happens when we truly do find the fundamental force(s) of what we currently experience as nature. What do those forces ride on?
However, I’ll agree the article was pretty bad. The NYT these days seems to exist to inflame commentary – the more comments they get the more ads they’ve served, and thanks to Fox News we now know it’s been judicially declared legal that a news company doesn’t have to refrain from lying.
I do know one thing though – if you can’t mathematically prove you theory it doesn’t exist. 😀
But you think, therefore you are! In contrast to gravity, presumably? 😉
Thanks for the link,
B.
Already within the context of GR you can say that Gravity (as a force) does not exist. Within GR when you follow the geodesic no force is exerted to you. But we know that this is just a geometric description. There is a *real* gravitational force mediated by gravitons.
In String theory a graviton is a state of the closed string upon quantization. If you take the sigma model action on a background with a certain metric you see that a small perturbation of that metric is equivalent with the emission of gravitons. You can even consider the whole background metric and see it as a coherent state of gravitons just like light is a coherent state of photons.
If you require Weyl anomaly cancelation in the low energy approximation you take the Einstein field equations and the Einstein Hilbert action. Our 4D Einstein Hilbert action descends from that action.
These things are essential for string theory and you can find them in any text book. I don’t see how you can accept this picture on one hand (being a string theorist) and at the same time to make such claims. Something is wrong with that picture. You can have various believes at high energies beyond the string scale but in our 4D low energy world the general picture should be clear for a string theorist like Verlinde.
Hi Giotis:
All I can say in few words is read this:
http://xkcd.com/123/
That’s a funny and geeky Bond cartoon. 😉 Needless to say, I completely share Giotis’ interpretation, including his question about the consistency of a string theorist’s mind. It’s just not consistent.
And I think that Erik actually doesn’t believe that gravity doesn’t exist – and if someone told him to jump out of the window to prove his belief, he wouldn’t jump.
I guess that David B. also agrees with Giotis but is slightly afraid to say so explicitly. I also hope it’s legal to post a link to the explanation why gravity can’t be entropic in Verlinde’s sense
http://motls.blogspot.com/2010/01/erik-verlinde-why-gravity-cant-be.html
because the phenomena would 1) be irreversible, as any phenomenon based on the increase of entropy, and much like the defunct LeSage gravity of the 17th century which is actually a special example of Verlinde’s gravity, and 2) gravity wouldn’t correctly affect interference patterns that have been tested with neutron interferometry – even the tidal forces have been seen on the interference pattern!
Gravity that follows the equivalence principle – phenomena in a freely falling frame are indistinguishable from those in the absence of gravitational fields – has to hold for all phenomena, including individual events with particles. That much is known empirically. So gravity can’t be just an effect of having a large number of degrees of freedom.
There can’t be any large, black-hole-entropy-sized entropy of ordinary objects just because of their gravitational potential. Only hot/warm matter and/or event horizons can carry substantial entropy. Of course, black holes themselves are dual to non-gravitational systems, and the whole gravity may be derived from AdS dual pictures etc. But it’s still true, even in the dual pictures, that two simple but heavy mutually attracting objects don’t carry any macroscopic entropy just because of their relative position or gravitational potential.
I think it’s insane if physicists – and not even string theorists – can’t agree on an obvious statement that Verlinde’s picture is just wrong. What’s wrong with Clifford Johnson, Andy Strominger, and others? On the other hand, Raphael assures us that none at Berkeley believes it, not even the early enthusiasts, but he wasn’t given any space to explain why the picture is wrong so it remains unclear whether they actually know the reasons why it’s wrong.
REALITY does not exist. Now, an empirical proof:
Chemical & Engineering News 88(28) 3 (2010) 12 July, “From the Editor.” The ACS 2009 Salary Survey is published.
“…unemployment among chemists had reached 3.9%, the highest rate of unemployment among chemists in at least the past 20 years.”
What spacetime continuum does the American Chemical Society inhabit? Folks who find this Official Truth personally offensive may drop the Editor a line of two of professional dissenting opinion,
edit.cen@acs.org
If reality does not exist, measurements of its observables cannot be any more realistic.
As we have repeatedly discussed, Luboš, nobody knows if opposite shoes violate the Equivalence Principle. Inverse geometric parity atomic mass distributions are chemistry not physics. Physics illusion of knowledge denies any fundamental meaning of emergent phenomena like chirality. Nevertheless, the proper challenge of spacetime geometry is test mass geometry,
Somebody should look. The worst it can do is succeed.
arxiv:1007.0587, arxiv:1006.4106, arxiv:0912.4852
The EP is political, Luboš, for no new experiment may be performed without precedent, lest it fail. Guaranteed failure carries zero risk, and so is fundable.
Hi David, For my taste the best semantics is that gravity is an emergent phenomena, in that it is a long distance approximation to something more detailed (rather than being a complete story). I think pretty much everyone agrees that gravity is emergent in that sense, but the story involving thermodynamics is much more fishy (hard to come up with a version of such claims that is not obviously wrong). So I think it is important to emphasize the difference between the two: the emergence of approximate structures in some limit is more general than, and should not be confused with, the coarse-graining associated with the transition from statistical mechanics to thermodynamics.
An excellent and concise way to summarize the difference between the two levels of “emergence” claims – one of them right, one of them wrong.
Hi Moshe:
I agree completely. Can you make it into a one-liner?
Why, are you running out of storage space on wordpress?
Also, complete agreement is too easy, I am not so sure I completely agree with myself. After all, the hints from black holes etc. have to do specifically with thermodynamics.
Hi Moshe:
The reason for the one liner is that that is what one needs as a ‘sound byte’ for talking to the public, or at least outside our field. Something for people to take home and tell their friends on twitter. Its about talking to the press.
I also don’t doubt that some thermodynamics is part of the problem of understanding gravity, in particular black holes, but its definitely true that gravity is not the same thing as just thermodynamics with no extra dynamics controlling it.
Yeah, I got that, but I will leave that to more talented people. I am still not sure about Jacobson paper, which highlights the tension between the thermodynamic description of Rindler space, and the fact that it is empty space. Anyhow, this is probably not a blog discussion. Next time, over beer or something…
Dear Moshe,
if you don’t agree with yourself, it’s just called being a scientist – or suffering from schizophrenia, depending on the preferred terminology. 🙂
The (large) entropy in the case of the black holes always resides at the horizons. No horizons means no (large) entropy. The rest is derivable – the temperature is linked to the surface gravity at the horizons, and so on.
Jacobson’s map of thermodynamics and Einstein’s equations is really about the reinterpretation of the local light cones at each point as event horizons. But when you look at the whole spacetime etc., it’s clear that there can’t be any large entropy (comparable to BH entropy) coming from the gravitational potential – e.g. between two cold neutron stars.
The Rindler space carries all the entropy at the (D-2)-dimensional t=r=0 wedge, and it may be interpreted as counting the possible states outside the Rindler (right) wedge, in the future triangle. Averaging over all (or generic) microstates of the Rindler space is equivalent to making the future triangle outside the Rindler geometry empty, I think.
All the best
Luboš
I think its quite clear that gravity is the result of the convolution of multiple distributions, forcing the emergence of objects which appear “normal” (or rather the product of several characteristic functions).
http://en.wikipedia.org/wiki/Central_limit_theorem#Alternative_statements_of_the_theorem
On another note:
I thought this article,
http://motls.blogspot.com/2010/03/defending-statistical-methods.html
Could use a little help from these articles:
http://en.wikipedia.org/wiki/Signal-to-noise_ratio#Alternative_definition
http://www.opticsinfobase.org/abstract.cfm?&id=49771
Great recommendations for resources, Just Learning. Too bad that I am not writing the text now because I would surely follow your advice. (And I won’t be forcing everyone to read a similar text again.)
By the way, Erik Verlinde was just on the radio, The Takeaway,
http://www.thetakeaway.org/2010/jul/14/argument-against-gravity/
The audio is 7 minutes long. Gravity is not like magnetism, it’s great to throw away assumptions and be like Einstein, gravity comes from information.
The host actually proposed something that does follow from Verlinde’s picture – and is silly at the same moment. 😉 When a man dies, his entropy goes up, so he can no longer resist gravity, so he falls down.
Verlinde confirms that this is a good description of the idea. Gravity comes from being partially dead and disordered. 🙂 Verlinde also says that “eventually”, this zombie theory of gravity may be tested by colliders and especially cosmological observations.
Hilarious.
Giotis said: “Already within the context of GR you can say that Gravity (as a force) does not exist. Within GR when you follow the geodesic no force is exerted to you. But we know that this is just a geometric description. There is a *real* gravitational force mediated by gravitons.”
And it is precisely the failure to reconcile these points of view that is responsible for the failure of all quantum gravity theories to date. How do we get geometry out of gravitons? One approach, of course, is to deny that geometry matters. And that leads you straight to this crazy “entropic everything” rubbish.
[…] the subject came from dberenstein over on Shores of the Dirac Sea posted four days ago and entitled Gravity does not exist? which is also certainly worth a look! However, I don’t want to discuss that paper. What I […]
There’s a simple thought experiment that I think anyone who believes “gravity is an entropic force” should know how to address, but I haven’t seen it discussed. Maybe a blog comment is a suitable forum for posing the question publicly.
There’s one case where we all more or less understand and agree on what it means for gravity to be emergent: AdS/CFT. So let’s consider taking classical AdS and putting a massive object in it at some position z = z0 (in coordinates ds^2 = (R/z)^2 (dz^2+dx^2)). I don’t think it matters much whether you think of this as a pointlike object or a string, but let’s say it’s a fundamental string since we know these exist in well-defined AdS theories.
Now, this object will fall “down” in AdS (toward larger z), because of gravity. What does this mean in the CFT picture that gravity emerged from? Our object at position z0 corresponded to some field configuration with characteristic size z0 on the boundary theory. As the object in the bulk falls, this corresponds to the field configuration on the boundary expanding, as localized excitations in any CFT will tend to do. We injected energy in a small space and it will want to expand outward and occupy a larger volume.
If there’s a reasonable way to think of bulk gravity as being an entropic force, there should be some coarse-graining in the boundary theory that lets us think of this expanding field configuration as increasing in entropy. We should probably also ask that this coarse-graining doesn’t do violence to the symmetries of our theory, so e.g. we don’t want to divide space up into boxes of fixed size, which would break the dilatation symmetry of the CFT.
Is there any candidate for such a coarse-graining? I can’t think of one, but I haven’t tried very hard, and maybe I’m just lacking imagination.
There are other situations in AdS/CFT where we obviously can think of a falling object as increasing in entropy, e.g. if we look at an AdS-Schwarzschild background, where on the boundary we’re starting out with some localized excitation in a finite-temperature background which over time thermalizes. But I don’t see how entropy plays a role in the simplest case of a falling object in AdS.
(Another fact that might be relevant is that at small enough acceleration, uniformly accelerating observers in AdS don’t see Unruh radiation. Again, I haven’t given any of this much thought, but I think anyone who advocates entropic gravity should have a consistent story that makes sense of this basic example.)
without having thought about it in any depth, expanding does sound like diffusion…. which follows from conservation laws and positivity of entropy production………
There’s a case where I agree that there is something like diffusion which produces entropy: at finite N, the falling string will fluctuate, and explore a bigger configuration space (this is dual to something like a parton shower). But for strictly classical gravity, it’s not clear to me that there’s a way to interpret the dual of the falling string as increasing in entropy. (I’m not saying I’m sure there isn’t. I just think it would be a useful example for someone to flesh out, if they want to examine the concept of “entropic gravity” in a setting where we all agree on what it means for gravity to emerge.)
you don’t need the internal degrees of freedom of a string. any energy density if localised will spread out. this is entropically driven and will occur in classical gravity (as seen e.g. via the fluid gravity correspondence). it follows from hydrodynamics which itself follows from (in this minimalist case) conservation of energy and `fick’s law’ which is a statement about positivity of entropy production (as explained somewhere in landau and lifshitz and probably elsewhere).
I am not a physicist, but I really like this subject and I would really like to hear your opinion about this crazy idea I have. I do think gravity does not exist, but for an entire different reason. Not all objects fall down, some fall “up”. Case in point: a helium balloon.
So the ideas is like this: Gravity is just the manifestation of objects arranging in space base on their density. Same principle applies at the bottom of the ocean. Then you might ask: why hydrogen does not fly off the earth. And the answer is: space is not a vaccum. Space time matrix is of a much lower density than the hidrogen atoms at the upper atmosphere, but it fills up all interplanitary space.
I know Archimedes and Newton might be rolling in their graves, but to me, this is just logic.
Thanks
Luis
hi david — i don’t have a problem with the quote (that being what the discussion is about, not the broader correctness or not of the entropic picture). i think you are being overly pedantic 🙂
it is sufficiently obvious to everyone, scientists and non-scientists alike, that gravity does exist in the sense that you want to — that people fall etc. — that using a more provocative phrasing is entirely in order. the point of the quote is obviously to challenge people to think about what it means for gravity to exist. which indeed is what your post does to some extent.
actually your comparison to `postmodern pap’ is perhaps revealing here… 99% of `postmodernists’ don’t deny that things which are obviously true are true. they are trying to say something more subtle about the nature of this truth, which i at least find interesting 🙂
I’m sure David will have his own response, but as someone who has let his mind wonder around some of these ideas, if taken at face value, Verlinde’s proposal is wrong. If not taken at face value, and merely taken as a statement to challenge intellectualism, then its utility is at the asympotitic limit.
The driving principle for physics for the last 70 years has been that forces are described by certain observable symmetries that occur in nature. The observation of supersymmetry will be the first step in confirming that gravity is a fundamental force, albeit fundamentally different from the other forces.
The closest logical interpretation that I can make of Verlinde’s conjecture, is a statement that the particles most people think of are ones that are objects. People want to believe that their world is built up from a substrate of these “objective” particles.
Those particles are the ones that don’t exist. What does exist are certain symmetries that cause energy to diffract into certain quantifiable patterns (energy produced by the power spike called the big bang…or alternatively, energy remaining after the downward transition of the background vacuum energy). Each pattern having some sort of characteristic properties determined by the particular symmetry group responsible.
So in that sort of semantic sense, one can argue that particles don’t exist, including gravitons. However, in Verlinde’s case, his statement about gravitons not existing is tantamount to an indictment of supersymmetry and the approach physics has taken since the 70’s. (Although, it would be a valid point that the motivation to study supersymmetry has been to resolve issues beyond those resolved by the standard model and not necessarily quantum gravity, but discovery of supersymmetry would open the gateway to unification).
I think this is sympotamatic of a generation of physicists that have lost their way and are trying to protect their legacies by closing doors to future generations of real thinkers (although, it is clearly a subconscious effort). The issue being debated is not whether we have a new path, but whether we should throw in the towl.
Hi Sean:
I am overly pedantic sometimes :). I think in this case it is warranted. This is because the vast majority of the people out there who are reading the article don’t know what gravity is or how to interpret that sentence within a scientific discussion, so the point is completely lost on them and it becomes a joke.
By the way, I’ m not against postmodernism per se. Just against some stereotypical downgrades of their ideas that are used colloquially into ‘feel good’ pap with no content.
If gravitation is coupled to entropy, one would discover weight anomalies in trivial cases: crystalline vs. amorphous quartz (near absolute zero), macroscopic objects cooled to their quantum ground states, Bose-Einstein condensate lattices, laser beams coherent in time and space; strongly magnetized neutron starts/pulsars/magnetars, presumably superconductors…
…amorphous metal alloys. Is LiquidMetal/Vitreloy gravitationally different from a pure metal single crystal? Entropy arises from disorder and mixing.
http://en.wikipedia.org/wiki/Liquidmetal
First, I think Erik Verlinde should jump off a short building just enough to survive. On the way down he should yell “there is no gravity” I am sure after the fall he would change his mind.
Einstein’s gravity is good enough and well proven, only thing missing is to extend GR to account for things still not complete.
I have taken such a step and published some at my website.
cosmicdarkmatter.
Erik Verlinde may not believe in gravity, but gravity believes in Erik Verlinde.