Gravitational Waves Detected: An Experimental Foundation for Quantum Gravity

Jernal wrote: »

This blog post explains the significance of the discovery rather brilliantly.

Not sure the thread title is correct. The observaiton of GW waves is still an indirect measurement. So can we really say we've detected them?

Labarbapostiza wrote: »

Yeah, those are the pictures. Though I had only seen the prediction/simulation picture before.




It's not trivial. But it makes a great difference when you know the ranges of what you're looking for. There's also a huge advantage in the scale of the universe. The result is fantastic in that it's a proof of the technique, that I imagine could be of great use looking at other phenomena. Even very slight biases in measured polarity could reveal a lot.


The technique seems to have more potential for measuring cosmic gravity events, than building a long armed thing on earth and hoping it works.




Well, the current CMB picture, gives indications from its' slight blobbiness, that the universe is a blown up quantum fluctuation. The BICEP2 pictures show the polarisation around the known mass. If there were quantum fluctuations in the primordial gravity, I imagine, that would result in a blobby pattern different to the visible, but probably much weaker.



There is the theory of the big annihilation. Right after the big bang, a huge annihilation of matter and anti-matter, just leaving behind a small excess. I don't know if that can be seen. Or if it happened. But...just twiddling off the top of my head, if it did happen, the resulting gravity wave would have been like a very large person doing a cosmic scale belly flop.

I'm not sure what cosmic gravity phenomena you can mean. Would be interested to hear more, because if you mean gravitational waves from black hole binaries etc, then you are incorrect to think of them this way.

The 'blobbiness' you mention, I presume are the acoustic peaks due to the photon-baryon fluid around 400,000 years after the big bang. These are not the quantum fluctuations (though they are sourced by them), but the distribution will tell us about the spectrum of quantum fluctuations. The acoustic peak structure from this time *is* present in the polarisation pattern. However the peak structure reflects the horizon size at this last scattering surface at 400,000 years. Since gravitational waves are not effected by such pressure terms, they quickly decay within the horizon at the time they last scattered off electrons (creating the polarisation pattern that has been detected). Therefore you should only expect to see one peak or maybe two - as appear to have been seen in the BICEP2 data set.

The baryogenesis thing you are talking about could occur at any scale about the electroweak scale (which is at around 10 orders of magnitude below the inflationary scale). Not sure why you think this would produce a bigger gravitational wave signal than the creation of the universe! In addition there is no reason to believe that this would produce a strong effect on the metric - or cause a large production of gravitons - either of which is necessary to give the gravitational wave signal you're talking about.


I should add that I've looked at this in a bit of detail so if anyone is interested in a bit of a lowdown on what the results are, and the possible implications for cosmology I could try and write something (though perhaps the blogs will already have covered what I could say anyway).

Labarbapostiza wrote: »

No. That's not what I meant, but now you say that, the acoustic peaks would be explained by bog standard thermodynamics. I've heard the blobs described as pre-inflationary quantum fluctuations, and that they were "blown up" in the inflation. But, thermodynamics after 400,000 might explain them too.

Thermodynamics 400,000 years after the big bang on it's own is insufficient to explain the homogeneity of the temperature of the universe. The CMB radiation left over from the big bang - i.e. the relic photons from the big bang that we can observe today - reveal that the temperature is uniform to 1 part in 100,000. To explain where the 400,000 years comes from; that is the time (roughly) when electrons no longer had enough energy to scatter the photons. [Since the universe is expanding, this means it is getting cooler. Cooler means less energetic.] So we cannot see beyond 400,000 years with the CMB as earlier times are opaque, in the same sense as due to dust particles in water you cannot see the light on the helmet of a deep sea diver after a certain distance.

Anyway to get back to the point, we need cosmology to explain this uniformity - and some other phenomena. So there are two main approaches: (1) that there was an infinite amount of time, to allow for thermal equilibrium to be established; or (2) that there was a period of extremely rapid expansion of a single tiny region. This region will become very uniform due to such extreme stretching.

For case (1), there's an issue because the Hubble expansion rate that we see today along with its evolution inferred from general relativity tells us that the universe is around 14 billion years old. The CMB observations also tell us that the universe must have been in an extremely dense state in the early universe. So the way around it is to suppose that the universe has been expanding and contracting slowly (not quite to a singularity) for an infinite amount of time. This theory is known as the ekpyrotic/cyclic universe theory.


There are many different ways to create scenario (2), i.e. inflation, but they all really rely on needing an outward pressure term to exist so that pressure works with gravity in the early universe rather than against it. The simplest models predict that due to the extreme stretching of space-time, that (an observable amount of) ripples will be created. These ripples are gravitational waves. They will affect the photons on the last scattering surface, where they last scattered off electrons - and are the photons we see on our CMB maps of the sky. The ripples will mean that certain directions of scattering will be preferred, and these preferred directions mean that the light from the CMB gets polarised. There's a subtlety to this point, by which this polarisation pattern may be mimicked on smaller scales by galaxies "lensing" the light from the last scattering surface as it comes towards us, but that occurs (mainly) on different scales to that polarisation seen due to gravitational waves.

Since the ekpyrotic scenario does not cause any rapid change in the fabric of space-time there will be no such ripple.
Therefore, seeing gravitational waves rule out a whole class of models.

Labarbapostiza wrote: »

I meant pre-inflationary quantum fluctuations in gravity, then blown up by expansion. I don't know if they're there, or if it's possible. If inflation was a period of negative gravity, and if the quantum fluctuations in gravity had existed pre-inflation, the might either vanish just by the scale of inflation, or they may have been there at the large scale. And I'm not thinking about black holes, but that there may be large scale gravity anomalies out there. They could be incredibly difficult to detect. But that they might emerge, by studying star light polarity.

Or maybe they're not there.

Not sure again what you mean here. Perhaps my post above has explained that inflation does blow up the quantum fluctuations of the early universe. That's the whole point of the paradigm.

Labarbapostiza wrote: »

Go ahead, I would be interested. I wasn't following it until the big media thing.

There is something else I'm curious about. Is it expected that gravitational waves emitted photons as they dissipate? And I don't mean by encountering electrons.

No, gravitational waves interact via gravitational effects. To emit photons you would have to couple them to electromagnetism, i.e. come up with a different theory. Of course the photons will be affected by the change in space-time they encounter. But no the gravitons themselves will not emit photons, at least not at the energy scales we are observing. Perhaps at an energy scale a few orders of magnitude higher - at the Planck scale where the gravitational force is anticipated to be of the same strength as the electromagnetic force, and where a quantum gravity theory is definitely required, this may be true.

Labarbapostiza wrote: »

No, I was think in terms like the creation of Hawking radiation. That the photons are due to vacuum fluctuations. But the gravity gradient allows the virtual photons to become real.

You're a bit confused here. The vacuum fluctuations that produced inflation are the same as vacuum fluctuations that are in Hawking radiation - the subtlety being that in the latter case the vacuum fluctuations creation electron and positron pairs at the boundary (horizon) of a black hole. Normally these particle and anti-particles annihilate almost immediately but due to being at the boundary of the black hole, there's a small probability that one of the pair may escape before the annihilation can occur.
Quite what you mean by the gravity gradient allowing the virtual photons to become real I don't know.

Labarbapostiza wrote: »

But, the primary problem of gravitons is how they affect space-time, or what that relationship is. Gravitational lensing has been observed. The basis of General Relativity is gravity altering the geometry of space-time. It's a taller order than the photon has. The graviton has to do a lot; it has to couple all the particles we can observe, and it has to do some magic on space-time.

This is rehashing what I said but in a confused way. GR says that due to the presence of matter, gravity will be altered. This does *not* mean that gravity is coupled to matter in the dynamical way you are talking about. And also the force of gravity is generally much much weaker than the electromagnetic force, - aside from energy scales that are probably higher than those present even at inflation.

Labarbapostiza wrote: »

Or maybe it just doesn't unify at that scale at all. If gravity/gravitons act on space-time, then on the Planck scale they cannot, or the fluctuations would quickly knot space up. Even if it doesn't knot, there should be an observable effect.

Taking Newton and proposing a Graviton, isn't too difficult. It's GR where you hit major snags. Even if GR is incomplete, you have to explain the observations.

It is impossible to say that "the fluctuations would quickly knot space up" at the Planck scale since we do not know that theory of quantum gravity rules the universe on such scales. Again not sure of you're last point, - of course there are snags with GR at the quantum gravity scale, i.e. the Planck scale. The problems with Newton's theory are much more profound and present themselves at observable energy scales.

Labarbapostiza wrote: »

Yes, but there are problems with both ideas. Why aren't we seeing "little bangs" happening all around us. I believe the net mass in a cubic metre of vacuum due to quantum fluctuations is that of a proton. I don't know how the circle is squared on all this.

We don't see the fluctuations because they are on scales far too small for us to observe. The time between creation and annihilation of the particle-antiparticle pair is too small for us to observe.

Labarbapostiza wrote: »

There's an assumption Hawking's makes that I'm not sure of the grounds for. That the anti-particle falls into the hole, and the ""particle escapes. And that's his theory for black hole evaporation. I don't know why he expects the anti-particle to fall in.

Yeah that's more an issue to do with the fact that we are describing the phenomenon using particles and not fields, i.e. using QFT. A somewhat simple, but approximate, way to think of it is that given that the fluctuations come from the vacuum configuration, then you should not lower the energy via the creation of negative energy particles, on either side of the black hole horizon. Inside the horizon the direction of time flips, so negative energy for us, is positive energy inside the black hole, i.e. what we count as the anti-particle has positive energy with respect to an observer inside the black hole. So the energy of the vacuum is not being lowered on either side of the horizon by Hawking radiation. So we see a positive energy particle being emitted from the black hole, and therefore witness the decay of the black hole.

Labarbapostiza wrote: »

Quite what you mean by the gravity gradient allowing the virtual photons to
become real I don't know.

Well, it's a similar idea to Hawking radiation or Unruh radiation. I'm assuming it doesn't happen as it's not observed. But the idea is, a particle pair are created in the vacuum, on a gravity gradient. The particles have different potential energy, let's say they're an electron positron pair. If there's a net difference in the momentum when they annihilate, I imagine the excess is released as a photon. We don't see these. I imagine it could be because the probability of another electron positron pair with the same but opposite excess momentum is the same, so two photons of equal energy are created in different annihilations and annihilate each other.

I cannot see how this could be possible unless the force of gravity was much much higher. Again this is a speculation on what is the theory of quantum gravity.

Labarbapostiza wrote: »

Well, gravity appears to be coupled to space time. The GPS satellites require both a GR and SR correction. If the graviton was a force carrier, the relativity effects might be caused by acceleration due to the graviton. Explaining gravity as a spacial contraction works better in a way; if you dropped an apple from the top of a tall building, it's velocity is not increasing, space is decreasing, in the frame at the bottom of the building its' velocity has increased relative to the contraction of space; energy is conserved.

With the graviton, unless there's a much better explanation, the apple accelerates as it falls, because the graviton density is increasing. Then you're getting momentum but where is it coming from, and how to you explain the need for GR corrections on the GPS satellites.

Not sure why you are explaining the concept of GR to me, but just to rephrase, the coupling between matter and gravity given by the equations of GR is not a QFT equation. It does not say that gravitons will emit photons when they decay as there is not a direct coupling, i.e. a term that looks like the Riemann tensor multiplied by the F_{mu nu} term of electromagnetism in this equation.

Labarbapostiza wrote: »

At the large scale gravity appears very strong, it also appears to be linear. Is there any evidence to suggest it isn't at short scales or higher energy?

What do you mean strong at large scales. If you mean galactic scales, then no gravity is still weak. It's much much weaker than the other 3 forces, strong, weak and electromagnetic except in very very early universe- even at inflationary scales it's still a few orders of magnitude weaker than the others, which appear to have the same magnitude (actually maybe the same GUT force) during the early inflationary epoch.

Labarbapostiza wrote: »

No, but it's possible to say space is not knotted by fluctuations, for the simple reason we wouldn't be here to witness it, or anything else, if it were there. The idea of quantum space-time foam, with loops and wormholes, and mini black holes, was popular. If there are space-time quantum fluctuations then they can only be limited to compression/decompression of space time, with no loops or wormholes. The looping would lead to Klein Bottle like geometries. A mini black hole, might annihilate with a mini white hole, but I'm not sure what would happen with knots. Or maybe there's a complex sequence of annihilations were all knots are cancelled.

Don't understand this logic. You are saying on the one hand it could have been knotted in the early universe, and now you're invoking an anthropic argument to say it can't have been. Again this is speculation on what would be a quantum gravity theory. Presumably if knots existed in the early universe, via some complicated manifold, then you can only invoke the anthropic argument to say that the knots must have unfurled before the inflationary epoch.

PS I write off these posts without re-reading them and just realised that a couple of the phrases like "You're a bit confused here", and "Don't understand this logic" may come across as a bit rude. Certainly don't mean it in this way. I find this forum great for ironing out my own misconceptions so I hope you will find flaws in my arguments and set me on the right path!

Labarbapostiza wrote: »

Yes, the fluctuations are small, but not (at least in theory) unobservable. Looking at the Wiki page for quantum foam, it looks like John Wheeler was the first to propose it in the mid 50s.

The experiment to observe quantum space-time fluctuations is relatively simple and straightforward. Observe a very distant stellar event; like a super nova. If the fluctuations are there, then due to the law of large numbers, some photons will have a longer path and some a shorter path, to the detector. ......What to expect; there should be a ramp of early photons.

This sounds like it would be an incredibly small effect. The energy scales of supernovae, though large by terrestrial standards (and galactic standards!) are still a good bit below those at which quantum gravity effects should have observable consequences. Sure they would have an effect very near the supernovae, but to appreciably affect the path distance surely they would have to cause excess photon production very far from the supernova. Also how would you distinguish this annihilation event from the possibility of dark matter annihilation occurring somewhere along the path towards that supernova.

Labarbapostiza wrote: »

So far no one has seen this, though there's a claim on the Wiki page that MAGIC observed something, but no one else has. (just thinking about it, maybe they should be looking for spectral broadening - I'm not thinking too deeply about it, but maybe the effect appears in spectral broadening and not in arrival time of the photons).

Again I think the MAGIC result has been generally looked on as a possible hint of dark matter annihilation.

Labarbapostiza wrote: »

Or maybe again, there would be a theoretical reason that yes, the space time fluctuations happen but all photons from a distant stellar event take the same path of shortest time, and there is no discernible effect. (This would have to mean that the result of the fluctuations is a single path of shortest time.)

There is a theoretical reason that photons (from anywhere) take the shortest path, this is the geodesic equation of general relativity, but as that's true of all photons that means they are loyal tracers of distance which is what you want!

Labarbapostiza wrote: »

I'd like to respond to your other points, but not at the minute, as some of them literally give me a headache just trying to think about them. You'll see what I mean when I respond.

No problem. Will try and help. However, most of your points hint at wanting a quantum gravity explanation, which tends to be the more exotic type of solution to the issue. I do work in cosmology but some of these theories are a bit far fetched so I may not know a great deal about them.