Slashdot Mirror
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."
http://simscience.org/membranes/advanced/essay/qua ntum_grav1.html
...has a pretty interesting explaination of quantum gravity and how it ties in with Einstien's Relativity and quantum mechanics, the two bedrocks of modern physics.
-- Your local friendly mad scientist-in-training
the fixed url.
Brief but nice overview of quantum gravity:
Quantum Gravity @ Dr. Jim Jessen
Being able to observe gravitational effects at such a small scale could be the key to unlocking the unification of our disparate scientific views of the universe.
Imagine being able to manipulate all the forces, not just electro-magnetic. Gravity producing devices operating on electric principles?
This is going to be fun!
Using the holy grail of OSes...
I wonder what effect these observations will have on superstring theory, which is supposed to combine the physics of the micro-microscopic world (quantum physics) with the physics of the gigantic universe (general relativity), two branches of study that couldn't previously be combined due to huge inconsistancies in the math.
Superstring theory was supposed to have some profound effect on the theory of gravity, last I remember, but then, I haven't read up on it in a year or so, and there have probably been big developments since.
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.
In Capitalist America, bank robs you!
So if I understand quantum gravity correctly, it is possible for a neutron to stand still for a while, and to suddenly start falling at 1.7 cm/sec? So the way Wile E. Coyote is falling off a cliff isn't completely *wrong*, it is in fact a kid's first introduction to quantum phenomena.
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.)
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.
However, they really don't tell us anything about gravitons, any more than the Schroedinger equation for an atomic electron tells us about photons. All it really says is that "energy is lost somehow". It doesn't let us derive the detailed properties of whatever it's being lost to, not even their masslessness. To do that, you have to actually quantize the gravitational field, and get gravitons. It's analogous to going from QM to QED. Your thought experiment is purely QM, not on the level of quantum field theory. So it can't tell us about gravitons.
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).
Quantum falling was first used to measure the charge of the electron, where charged balls fell in gravity against a field. No-one knew at the time that it was the electron that was doing it.
The other amusing thing is the diversity of units, none of which are "SI": cm/s, electron volts, rather than m/s, J.
OS/2 - because choice is a terrible thing to waste.
To reiterate previous posts, this is just standard quantum mechanics with gravity thrown in. Not quantum gravity! Something quantum gravity- related would involve observing gravitons or something sensational like that.
But there have been older experiments which involve quantum mechanics and gravity. For example, Colella + Overhauser + Werner wrote "Observation of Gravitationally Induced Quantum Interference," Phys. Rev. Lett. 34, 1472 (1975). For any budding physicist, you can check chapter 2 of Sakurai.
For non-physicists, the experiment involves the idea of Feynman path integrals, which is a beautiful, but normal quantum mechanics, idea. Roughly, it says that a quantum wave of particles (let's say, neutrons!) traveling through some potential (let's say, a gravity potential!) will acquire a phase. Now, to pick up this phase, we can combine it with another wave of particles which DIDN'T go through the same path and see if there's interference effects. The result was "yes it does." Thus establishing the applicability of quantum mechanics to regular old gravitational wells.
Now, in this recent Nesvizhevsky et al. paper in Nature, the results are exciting because the authors picked up bound states in a gravitational well, just as one would pick up bound states in a nucleon well (gives us atoms and orbitals and that stuff.)
I'm not a particle physicist, so I got this question. My question is what happens you a neutron makes a transition from one bound state to another? In the atom, you can spontaneously emit a photon and cause a transition, which sometimes comes out in the visible regime so you can see color. Like when you burn cobalt and it turns blue (well, I don't know whether it's really blue or not.) So if a neutron in the Nesvizhevsky experiment made a transition from one height to a lower height, it's gotta be emitting gravitons, right? Or should I wait till the development of Quantum Gravity for an answer?
I see you have accepted questions from the public at large and done a fine job in answering them so here's mine....
This isn't GR, but it's at least associated with Da Man himself. Say you cool a cloud of radioactive atoms into a Bose-Einstein condensate and hold the condensate together for a period longer than the half life (or more accurately, a period long enough to where there is an overwhelmingly high probability that one radioactive decay would sponaneously occur). What happens? If the wave functions of the atoms all merge into a single wave function (admittedly a QM situation, not a GR one) then when the BEC is warmed, how is it "decided" which atom underwent decay? Maybe you could float this around your physics dept and see what the concensus is....
I only have a BS in Physics, marriage, kids, divorce and a job got in the way of my PhD, but I have the requisite curiosity in abundance, since these are such amazing times in which we live....
First, here's a link to the original article in Nature, where you can download the paper in PDF format.
Secondly, the electrical analogy is an excellent one. Basically, quantum theory started in 1900 with Planck postulating that atoms radiate energy (light, heat) only in discrete quantities. He used this as a "mathematical trick" to derive the spectrum of black-body radiation. (However, he didn't believe his "trick" was true in any literal sense until much later, about 1913). Then in 1905 Einstein postulated the existence of photons, and used them to explain the photoelectrical effect. I'll briefly explain what that is:
When you shine light on a metal plate, it can free electrons from the metal, which can then fly a short distance to a second plate and produce an electric current. What happens is that the electrons in the metal absorb some light and use this energy to break free from the metal (they need a certain threshold energy for this). Any additional energy they have left is then invested in their movement. According to the wave theory of light, the brighter the light you shine on the plate, the more energy the electrons absorb, and the more of them should be able to break free. But, that's not what happens. If you shine a very bright red light at the plate, you don't get any electrons, but a faint blue light, even if it contains much less energy in total, will liberate plenty of electrons. Einstein's explanation was that the photons of red light, having a longer wavelength, each contain less energy. If the light is very bright then you might have LOTS of photons, but each photon only has a relatively low energy. Now, typically, the probability that a given electron is hit by a photon is quite small. This means that those (lucky few) electrons that do aborb a photon will generally only absorb one, not more. If this is a red light photon, then this energy is simply not enough to break free of the metal, so there's no photo effect. But if you shine blue light at the plate, then each photon carries enough energy to liberate an electron, which is why you expect the effect to work with blue light. If you make the light brighter, then there are more photons, hence more electrons are released. But they each still have the same amount of energy. Incidentally, this is what Einstein got his Nobel prize for, not relativity.
Now for the analogy. What has been done in the Grenoble experiment is to confirm the analogue of Planck's result. So we now know (as we had guessed for a long time) that gravitational energy, at least in bound states, comes in discrete quantities. This does not yet imply the existence of gravitons, which would be analogous to photons. So the next experiment we would need is a gravitational version of the photo effect:
Imagine a system in which neutrons are bound in some state and need a little tug to be freed (I have no idea how to bind a neutron in a state such that such a weak tug could pull it free - remember that all other forces are SO much stronger than gravity). Then maybe we could see them pulled free by gravity, and notice the strange effect, that if we increase the gravitational field (by moving a large object near to it - with the experiment done in zero gee) we can pull free MORE neutrons, but each liberated neutron still starts off with the same energy (i.e. speed).
Anybody have any ideas for such a setup? Maybe we should study neutrons orbiting a small lead ball in a zero gee?
"...Look on my works, ye mighty, and despair!"