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Emergent Gravity and the Dark Universe

arxiv.org

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Re: Emergent Gravity and the Dark Universe

#31

Post by zamalek »

jcoffland wrote: post #18
cshreve wrote: post #15

So I too love the idea that we don't need dark matter to explain all the gravitational effects we observe in the universe. However, is it just me or does the falsification of this seem a bit sketchy: if we find a dark matter particle...then my theory is false. That's a bit like saying...oh yeah if we find a different way to unify general relativity and quantum mechanics...then String Theory is false. It seems like th…

I think you will find all the models are false to a degree. A model is only an approximation of the real thing. Quantum mechanics and General relatively cannot both be right. The obvious answer is that they are both wrong.

A thought experiment of mine.

Assume that all possible hypothesis or theories (explanations in general) form part of an "explanation space." Some explanations are good (theories), some have no evidence (hypothesis) and some are false (we are riding a giant turtle). Given the number of false explanations, the overall space is infinite. Hypothesis spaces would be islands within the false explanation space, theories islands within hypothesis and the truth as a single point within one of those theory spaces.

The question is, does that mean that the theory space is infinite? If so, that would mean that it would be impossible to know that we've found the truth (as a single point within this infinite space) - we'd only ever be able to conclude that we've found a closer approximation. Scientists may never run out of work.

Re: Emergent Gravity and the Dark Universe

#32

Post by empath75 »

ars wrote: post #24
mrfusion wrote: post #12

Like others have mentioned I'm not sure what happens now to the gravity waves we've detected.

I'm not convinced we have in fact detected gravity waves. From our point of view merging black holes would not move due to time dilation, but the waves detected are modeled after black holes without time dilation. Although it's possible we detected a wave from a different source, not a black hole.

While we can never see them merging completely, we can see them asymptotically approach a merger, which does in fact take about 1/5th of a second to get most of the way there.

Re: Emergent Gravity and the Dark Universe

#33

Post by maverick_iceman »

slowmovintarget wrote: post #25

In the summary, it says that these theories are "best understood in Anti de Sitter space...". I thought current observations ran against a spacetime that had a negative curvature. Are they just saying the math is easier in that geometry, but that it still applies to flat (or ever-so-slightly-positively curved) spacetime? Or are they arguing in favor of a "saddle-shaped" universe?

Spacetime has a positive cosmological constant, so it's closer to de-Sitter than Anti de-Sitter. It doesn't really have to do with curvature exactly, which is given by the Riemann tensor (so it doesn't make sense to say it's positive/negative).

Re: Emergent Gravity and the Dark Universe

#34

Post by joadha »

ars wrote: post #23
TeMPOraL wrote: post #19

Earlier quoted context omitted.

> the existence of gravitational waves constitute a proof of the existence of gravitons, due to wave/particle duality I always thought this is the case. But IANAP either. Could anyone with actual knowledge chime in?

No, a gravitational wave has mass (carries mass is more accurate), but a graviton does not. They are not two aspects of the same thing.

Can you elaborate on what you mean by "a gravitational wave has mass (carries mass is more accurate)"? That doesn't fit my understanding of gravitational waves.

Re: Emergent Gravity and the Dark Universe

#35

Post by maverick_iceman »

hammock wrote: post #6

Gravitons always struck me as a goofy half-baked concoction (even though we've supposedly observed them now), and so did dark matter. Emergent gravity theories are very interesting.

Gravitons are simply quanta of gravitational waves. They arise in the same way as photons in electromagnetism. There's nothing goofy about them.

Re: Emergent Gravity and the Dark Universe

#36

Post by maverick_iceman »

ars wrote: post #23
TeMPOraL wrote: post #19

Earlier quoted context omitted.

> the existence of gravitational waves constitute a proof of the existence of gravitons, due to wave/particle duality I always thought this is the case. But IANAP either. Could anyone with actual knowledge chime in?

No, a gravitational wave has mass (carries mass is more accurate), but a graviton does not. They are not two aspects of the same thing.

Gravitational wave doesn't carry mass. It travels at the speed of light, so it has to be massless. In fact, in 4D gravitational waves can arise even without any mass/energy source.

Re: Emergent Gravity and the Dark Universe

#39

Post by theptip »

hammock wrote: post #6

Gravitons always struck me as a goofy half-baked concoction (even though we've supposedly observed them now), and so did dark matter. Emergent gravity theories are very interesting.

Goofy in what way? All other known forces are carried by particles, so why not gravity?

Re: Emergent Gravity and the Dark Universe

#40

Post by raattgift »

hammock wrote: post #6

Gravitons always struck me as a goofy half-baked concoction (even though we've supposedly observed them now), and so did dark matter. Emergent gravity theories are very interesting.

Gravitons appear in several different families of theory.

We have not observed any of them; the interaction cross-section is so small (because gravity is so weak) that we have no hope of observing a single graviton, or failing to observe an expected one, any time soon.

Gravitational waves have spin 2 symmetry and are massless. A straightforward quantization of a gravitational wave leads to gravitons as a spin 2 massless gauge boson. Conversely, in almost any gauge theory that admits a massless spin 2 particle, that particle mediates a force almost exactly like gravitation, and is amenable to analysis under the Paramaterized Post-Newtonian Formalism.

Indeed, when gravity is weak, such theories essentially exactly reproduce General Relativity. It's only in strong gravity that non-negligible differences appear, and in this case strong gravity is when the quantum uncertainty in curvature distorts distances in ways that can't be worked around e.g by a change of basis or system of coordinates. Alternatively, gravity is strong when you have multiple loops of gravitons in a Feynman diagram.

So in the type of quantization above, the problem is that the momentum carried by an individual graviton is much much much much smaller than that carried by an individual low frequency photon. For example, a near-infrared photon has a momentum-energy on the order of about 1 eV/c^2, a 100 MHz photon has an energy on the order of 10^-7 eV/c^2, and a graviton will be less than 10^-22 eV/c^2 (LIGO collab., https://arxiv.org/abs/1602.03837, about six paragraphs before section VIII).

In other words, even the extremely weak signals LIGO's detected represent waves comprising huuuuuuge numbers of gravitons. But each graviton is so weak (or equilvalent long-wavelengthed) that we have no hope of detecting it individually. Think of how much fun you would have trying to isolate with your fingers an individual molecule of water in a surfers-beware ocean wave -- that'd be easier than isolating a graviton from a presently-detectable GW.

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