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Quantum Gravity Observed
Lawrence_Bird writes "AIP News is reporting the first observations of quantum gravitational states by researchers in Grenoble using a beam of ultra cold neutrons. This is an incredible observational achievement when you consider the energies involved - order of 1 pico electron volt (10^ -12eV). The full paper is in the 17 Jan Nature."
This really puts the nail in the coffin of General Relativity. We now know for sure that there will have to be an overhaul of that side of the physics.
It's great that they detected somthing so weak like gravity at a quantum level. This may finally help us understand what is it's like in a black hole.
If you don't understand any of my sayings, come to me in private and I shall take you in my German mouth.
the first thing i did when i saw that headline was make a quick mental check that it wasn't the first of april
if the results stand up, this could very well be the first major steps in what would easily be one of the greatest scientific achievements of the 21st century: the completion of einstein's dream of a grand unified theory
quantum gravity (when fully understood) will be the last step at showing the four fundamental forces of nature are in fact driven by a unified underlying principal, that on some level, they are the same.
various other people have posted good links for explanations.
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Instead, the neutron is in a quantum state in a potential well. The fact that the potential well is due to gravity, rather than electrical or some other force, has nothing to do with the quantum nature of gravity itself.
Quantum gravity would be if the gravitational force itself were quantized, rather than the neutron state.
That doesn't mean that it isn't a great achievement in a difficult experimental field, which can be used to test fundamental physics including theories of gravity. It merely means that the /. headline is misleading.
This is not what a quantum gravity researcher would call "a test of quantum gravity", insofar as it does not demonstrate that the gravitational field is quantized. What this is, is a test of the effects on quantum matter of a classical gravitational field. In other words, as the Nature article says, it shows that gravity "can have a quantum effect" on other particles. But it does not show that gravity itself is quantized.
If you have a classical potential well, such as that due to a Maxwellian electric field, or a Newtonian or general relativistic gravitational field, a matter particle in that potential well will exhibit quantization of energy, momentum, etc. As the article says, this happens when the well is confining (when you don't have enough energy to escape the well).
An example is the energy levels of an electron in the electric field of an atomic nucleus, the standard orbitals you get when you solve the ordinary Schroedinger equation. Note that this assumes a potential due to an ordinary, classical electric field.
There are atomic effects due to the quantized electromagnetic field, like the quantum electrodynamics (QED) corrections to the Lamb shift coming from vacuum pair production. They crop up when you assume that the electromagnetic field is made up of individual quanta (photons). These effects are much smaller than the dominant, lowest-order classical effect.
So, what these researchers have done is demonstrated that a classical gravitational potential well can lead to quantized observables for matter, like the electronic orbitals of an atom. This is interesting by itself, because the gravitational field is so weak that the Earth's gravitational potential well is relatively much more "shallow" than the electric potential well of an atomic nucleus, as far as the strengths of the forces are concerned.
However, they have not shown that the gravitational field is itself quantized, any more than the quantized orbitals of electrons demonstrate the electromagnetic field is quantized. So they have not provided evidence for quantum gravity (i.e., a quantized gravitational field), any more than Bohr's law for atomic energy transitions provides evidence for QED.
True tests of quantum gravity are much harder than even this difficult experiment. To read about some proposals, try this paper on Planck scale phenomenology by Amelino-Camelia. (You can also see some of his other papers.)
Forgive me for being an amateur, but all they are saying here is that some scientists got some neutrons to display observable QM behavior in response to gravity. Quantum gravity as theorized requires a particle to bear the force (gravitons). If they had discovered gravitons interacting with the neutrons this would have been an epoch-making discovery. What we have here is a "stunning observational achievment" but to say we're all just going to pack up GR and move on to the next level is a bit premature.
speaking as a nuclear structure physicist, this is a major accomplishment. Some may say, "what new news is there? We already knew gravity was quantized." Until now, the actual quantization has never been measured. However, the resolution still is not good enough. The resolution will still need to improve quite a bit. However, once we have highly resolved measurements of the quantization, we will have the eigenvalue matrix elements. Once the matrix elements are obtained, we can confirm its symmetry group classification. As of now, the problem with unified field theory is primarily gravity. Just as EM fields have a virtual photon with spin 1 as a field particle, gravity has a "gravitron" with spin 2 as field particle. All the fields other than gravity are now believed to be subgroups of the SO(5) symmtery group. The symmetry group of gravity is not currently believed to be a subgroup of SO(5). To be honest, I've no predictions on what the future result will be. Gravity behaves so weirdly in comparison to everything else that it's a bit of a pain in the ass with respect to modeling. Don't fear though, the data will pave the way. People tend to put faith in wild theory that has no relationship to reality. Experiemental research and the constraints of its data is the only way to truly proceed. Without it, anyway can invent wild theories that sound nice but it has no use without the constraints of data.
Just to clarify, what's being talked about here is not what physicists usually refer to as 'quantum gravity'. Quantum gravitational effects are relevant at *extremely* large energies, much larger than the energy scales that characterize the processes that we associate with typical particle physics phenomena. It is very unlikely that we will learn much of anything about quantum gravity by looking at such low energy processes as the ones described in this story. There are some scenarios that bring down the scale that characterize quantum gravity to something on the order of TeV, but those are speculative.
Furthermore, learning about quantum gravity *does not* mean that we toss General Relativity. Regardless of what kind of physics goes on at the Planck scale, GR is absurdly accurate over a tremendous range of energies, much more so than we have any right to expect. For instance, even if we develop a consistent theory of Quantum Gravity you'd never use it to explain how the orbit of Mercury differs from the predictions of Newtonian
celestial mechanics, GR does this with as much precision as we'll ever be able to measure.
The results of the experiment in this story, while they may have to do with quantum mechanics in an external gravitational potential, are not the result of quantum gravity effects.
The /. title is wrong. The experiment had merely observed the quantization of neutron momentum in the external gravitation field. The gravitation in that model (the external field approximation) is a purely classical (non-quantum) potential, i.e. it afects the quantum particle (neutron) but it is not affected by the particle. To detect quantum gravity one would need an experiment that detects quantization of that field (e.g. particle-like aspect of the gravitational field, the same way that photons are manifestation of the quantization of the EM field).
draw the vectors and you'll answer that question yourself.
From the slapdowns by the informed set here, I get the feeling that this is showing quantization in the motion of the neutron, which proves about zip about the forces acting on it. I'm not even sure about whether it's the velocity or the acceleration that's quantised, but either way it's only a very tenuous suggestion (at best) that the gravitons acting on it might be quantized.
What strikes me is the comparison with computer models. I used to work on physics engines for games along with a maths geek who was most disgruntled at the dreadful granularity that we had to work with (double precision floats, how primitive!). He was horrified to discover that such engines often use a dt timestep to do things like (v += a * dt), and to be fair, at 30fps, this requires a little fudging to keep orbits circular or whatever.
So articles like this give me a fuzzy flow, because they intimate that reality is granular. More than a double precision float, or a 33ms timestep, sure, but only by degree. If my poor head is getting this right, the universe seems pixellated at a very fine level, so all us games developers need to do to model it accurately is to get our frame rates way up and our dt's way down. There's a goal to aim for. ;-)
If you were blocking sigs, you wouldn't have to read this.