IIRC, the main reason we don’t have a quantum theory of gravity is because it is so weak compared to other Forces by orders of magnitude. We would need access to an ultra-intense gravity source, like a quantum sungularity.
Einstein showed that gravity and accelleration are equivalent.
What if we were to take a sensor packageand apply some serious muscle-power to the acceleration vehicle…could we detect quantum gravitational effects in high-accelleration applications?
The main reason we don’t have a quantum theory of gravity is because we’ve never seen any evidence that gravity IS quantized. No measurement has ever shown a deviation from general relativity. The idea that gravity is quantized is just speculation - an analogy from electromagnetism. But in that case, experiment showed the way; classical theory could not explain such things as the black-body spectrum and the emission lines of hydrogen, but quantum theory (once it came along) did so beautifully. Another reason physicists look for a quantum gravity is that general relativity predicts singularities, regions of infinite density or discontinuities in space-time, and that just seems wrong. The idea is that at high gravity, quantization would appear and the singularities wouldn’t happen. But again, there is no experimental evidence for it. So anyway, yes, you’re right; if quantum gravity effects show up it would be in high gravity/acceleration conditions, but probably much greater than anything humans would ever be able to produce. Evidence would more likely come from observing subtle properties of the cosmic background radiation.
Well, that may or may not be true. In order to explain the observed rate of expansion of the universe, we have to add dark matter and dark energy to the equations, even though “dark” means we can’t directly observe either one. It’s possible that general relativity may need corrections on a universal scale that would eliminate the need for dark energy much as general relativity itself eliminated the need for a planet “Vulcan” to explain the precession of Mercury’s orbit.
Not to mention that no measurement has shown a deviation from quantum mechanics either, yet that theory is incompatible with general relativity. We need some kind of new theory that can explain both.
Well, the problem is that we know that quantum mechanics and relativity are at some point incompatible; they can’t both be completely true.
So there is a possibility that in fact gravity and acceleration aren’t equivalent in all situations, and if they differ it’s going to be in exactly those situations that show quantum gravity effects.
Bottom line, we can’t say anything for sure about quantum gravity by looking at acceleration.
Wouldnt it be slightly more accurate to say if we looked hard at acceleration and found quantum acceleration that would not neccessarily mean gravity was quantized?
Just a minor (well intended) nitpick I know.
You can make an order-of-magnitude, ballpark (read: probably wrong) estimate of what kind of acceleration would be necessary to observe quantum gravity effects. Most physicists assume that quantum gravity effects will take over when you reach the Planck scale for whatever phenomenon you’re looking at; for acceleration, the “Planck acceleration” works out to be somewhere around 10[sup]51[/sup] m/s[sup]2[/sup]. Roughly speaking, the gravitational field observed by an observer accelerating at this rate would be so strong that we would expect classical general relativity to break down.
It’d still be interesting to do this experiment at (relatively) lower accelerations, though, if only to observe the Unruh effect, a predicted effect very similar to Hawking radiation. Getting a better handle on how quantum fields other than gravity behave in curved (but classical) spacetime might well be an important step to getting a full-blown theory of quantum gravity. Unfortunately, you’d still need ridiculous accelerations to observe Unruh radiation of any temperature above a few attokelvin; to observe radiation with a temperature of 300 K, you’d need to be accelerating at around 10[sup]22[/sup] m/s[sup]2[/sup]. It’s more accessible than the Planck acceleration, I suppose, but good luck finding any particle detectors that can survive that number of g’s.
For any dopers who needed to look something up to explain this (dopers… like me 
), the issue here is that Gravity is not known to be quantized. That is, we have ways to measure gravity’s effects but no evidence that it can be seperated into any particle or charge. In science ficiton or physics speculation, such particles or charges are called gravitons sometimes. And graviton is a very cool name.
Let’s say we did have a source of extreme gravity: a mini-black hole maybe the mass of the moon, orbiting out in the Kupier belt. So we send probes to fly past it, probe it’s event horizon with radio and laser beams, look for Hawking radiation, etc. Would this provide any clues to a quantum interpretation of gravity, or would it more or less just confirm general relativity?
Or refute GR. After all, what’s the point of experimenting if there is no hope of finding something new?
A black hole that size and easily accessible would emit significant Hawking Radiation, a near certain theoretical process that evaporates black holes, which would be of great interest. It would provide avenues for many GR investigations, but, using it to investigate quantum gravity would probably take real cleverness.
Now, what would give us some serious insight into quantum gravity, assuming they exist, would be miniature black holes of the sort some think the LHC might produce. Frankly, it’s probably wishful thinking, but if it works, great (especially considering that that’s not why the LHC is being built in the first place).
Incidentally, we can say confidently that there must be some way to reconcile GR and quantum mechanics, since scenarios can arise in the Universe where both would be relevant. Extremely small black holes (much smaller than moon-sized; think more like the mass of a bacterium) would be such a scenario, for instance. And a “normal” sized black hole would, according to current theories, eventually evaporate down to such a size. Either they do so, in which case quantum gravity is whatever describes that Planck black hole, or they don’t evaporate all the way to that point, in which case quantum gravity is what’s preventing them from reaching that point.
And the best prospect currently for investigating quantum gravity, incidentally, is some as-yet unexplained noise from the GEO gravitational-wave detector in Germany: It appears to just about match the noise that would be expected from the holographic versions of the string model. As an educated WAG, I’d say that there’s about a 90% chance that it’ll turn out to be something mundane and boring instead, but still, it would be a big enough jackpot that a 10% chance of winning it is something to be excited about.
I’d give Octoberfest at least a 1 percent in there somewhere 
All that mass of liderhosen and beer steins concentrated in the general area cant be go for the measureing process…
If you thought that was a joke, then you obviously don’t know just what crazy sources of noise really can throw off a gravitational wave detector. At one of the LIGO instruments, they literally have to account for the gravitational fields of tumbleweeds blowing past the detector. Probably the only thing that saves them from Oktoberfest noise is the fact that it happens on a yearly schedule, and they already throw out everything with a period of one year, anyway.
I remembered your past reference to tumbleweeds and other stuff, so this was a great opportunity. How often do get to warn/comment about something so absurd that might actually be a real problem?
And I wish that hologram universe story would die…in some ways it creeps me out.