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Emergent Gravity and the Dark Universe

arxiv.org

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Re: Emergent Gravity and the Dark Universe

#61

Post by codethief »

empath75 wrote: post #30
ars wrote: post #23

Earlier quoted context omitted.

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.

In a quantum field theory, a wave implies a force carrying particle, does it not?

To be precise, particles (with precise momentum) are certain modes or excitations of a quantum field which is often governed by something like a wave equation. However, not all quantum fields (and their associated particles) are force carriers. These are usually just the gauge bosons.

In any case, though, noone has managed to quantize gravity yet (i.e. describe gravity as a quantum field theory with gravitons as force carriers), so we've detected just gravitational waves but no gravitons.

Re: Emergent Gravity and the Dark Universe

#63

Post by tpeo »

wcoenen wrote: post #52
tpeo wrote: post #29

Earlier quoted context omitted.

Frankly, I'd be surprised if anything ever suggested that Leibniz's colorful metaphysics was correct, even or incorrect. Leibnizian monads are unextended and unable to act on one another (from §1 and §7 of the Monadology), which is to say that they're non-physical. On the other hand, as they're somehow supposed each to have a complete picture to the world (§56), they imply some form of non-locality. Which is to say t…

> But Leibniz's idea was more like that there is an arbitrarily large number of computers running the same simulation, while some of them also run artificial intelligences which just watch the simulation unfold. I think this was just Leibniz awkward attempt at explaining modal realism. The idea of many possible worlds is much more mainstream now in physics, so I wouldn't call it "completely nuts".

I wasn't previously acquainted with the term "modal realism" (been avoiding contemporary philosophy for a while now), but there doesn't seem to be an immediate connection in this case. There aren't actually many worlds being simulated here, as each computer simulates the same world and they all get the same results. Leibniz does introduce the idea of many possible worlds (§53), but as a set of designs for this whole thing (computers plus the stuff running inside them) from which God draws one particular design to be the actual world.

He might be closer to modal realists in other regards, though. I don't know.

Re: Emergent Gravity and the Dark Universe

#64

Post by snissn »

ars wrote: post #48
maverick_iceman wrote: post #36

Earlier quoted context omitted.

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.

See here: https://en.wikipedia.org/wiki/Sticky_bead_argument I suppose I should have said energy, not mass, but unlike a photon which can disappear into nothing, leaving just the energy, a gravitational wave can't ever vanish entirely, so it's more like mass in that regard. And don't forget that a wave can travel at the speed of light even if the wave is carried by massy particles that can not.

> And don't forget that a wave can travel at the speed of light even if the wave is carried by massy particles that can not.

I would love an example of that!

Re: Emergent Gravity and the Dark Universe

#65

Post by npgatech »

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…

> "So I too love the idea that we don't need dark matter to explain all the gravitational effects we observe in the universe."

It is an attempt to explain. Just like an attempt to explain (prediction) of Higgs Boson. It was only true once we observed it via experimentation.

Falsification makes sense if the theory has been experimentally validated. You can't falsify something that has not been proven.

There is no one claiming String theory or the theory discussed here is true - no experimental evidence has been produced.

Re: Emergent Gravity and the Dark Universe

#66

Post by M_Grey »

SomeStupidPoint wrote: post #59
M_Grey wrote: post #51

Earlier quoted context omitted.

Not even remotely the case, although I'd be fascinated to see where you got this notion from.

I mean, one could argue that perhaps EM is really a special kind of gravitational ripple (eg, geons might work out in quantum gravity to model electrons and photons, and then the question arises that if all EM is gravity induced and modeled, is EM not part of gravity?), but I don't know that I've heard it the other way. It's also certainly not the mainstream view, as classical geons are extremely unstable.

I'm not sure how EM and Gravity could be unified as a single force, especially since the best theory of gravity that's out there describes gravity as curvature in spacetime, not a force.

Re: Emergent Gravity and the Dark Universe

#67

Post by MrQuincle »

There seems to be considerable disagreement in the field. Why is it news (I really would like to know!)?

+ Kobakhidze explaining neutron experiments: https://arxiv.org/abs/1108.4161

+ http://physics.stackexchange.com/questions/4289/is-gravity-a...

+ Lubos Motl: http://motls.blogspot.nl/2010/01/erik-verlinde-why-gravity-c... and http://motls.blogspot.nl/2010/01/erik-verlinde-comments-abou...

It's nice to see that also experts in these highly specified fields quarrel about what it means to be "entropic" and "irreversible", etc.

Re: Emergent Gravity and the Dark Universe

#68

Post by ars »

snissn wrote: post #64
ars wrote: post #48

Earlier quoted context omitted.

See here: https://en.wikipedia.org/wiki/Sticky_bead_argument I suppose I should have said energy, not mass, but unlike a photon which can disappear into nothing, leaving just the energy, a gravitational wave can't ever vanish entirely, so it's more like mass in that regard. And don't forget that a wave can travel at the speed of light even if the wave is carried by massy particles that can not.

> And don't forget that a wave can travel at the speed of light even if the wave is carried by massy particles that can not. I would love an example of that!

Electricity. The electrons barely move, but the wave (the voltage in this case) travels at almost the speed of light.

Re: Emergent Gravity and the Dark Universe

#69

Post by taliesinb »

If you're wondering how this isn't some weird form of MOND, which is basically dead as an explanation, here's an important paragraph from the paper:

> We like to emphasize that we have not derived the theory of modified Newtonian dynamics as proposed by Milgrom. In our description there is no modification of the law of inertia, nor is our result (7.43) to be interpreted as a modified gravitational field equation. It is derived from an estimate of an effect induced by the displacement of the free energy of the underlying microscopic state of de Sitter space due to matter. This elastic response is then reformulated as an estimate of the gravitational self-energy due to the apparent dark matter in the form of the integral relation (7.40). Hence, although we derived the same relation as modified Newtonian dynamics, the physics is very different. For this reason we referred to the relation (7.43) as a fitting formula, since it is important to make a clear separation between an empirical relation and a proposed law of nature. There is little dispute about the observed scaling relation (7.43), but the disagreement in the scientific community has mainly been about whether it represents a new law of physics. In our description it does not.

Re: Emergent Gravity and the Dark Universe

#70

Post by auntienomen »

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 basically unavoidable. They may not be fundamental particles, but they'll always show up in the long-wavelength approximation of any theory of quantum gravity that extends general relativity.
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