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."

Comments

Impact

[SkulkCU]

There's no mention in the article of the impact or importance of this observation.

Anyone? Anyone?

Strings & gravity

[rice_burners_suck]

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.

Re:Very misleading, not "proof of quantum gravity"

[Anonymous Coward]

What you're describing is basically the kind of thing the experiment being discussed was testing. The difference is that their experiment was on a small enough scale that the field they were examining was essentially uniform, instead of noticeably central, and their test mass was small and quantum, not macroscopic as you are suggesting. (That means that the energy levels of the macroscopic body will be much more finely divided than for something like a single neutron.) But yes, your arguments are correct.

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.

They used an old trick

[os2fan]

The article says they used the downwards drift of slowly moving neutrons. The idea was that they slowed neutrons down enough that they could see it fall under gravity. Being relatively small, the neutrons should interact with individual gravitons. If this were the case, the quantum nature would be visiable and measurable. They did not measure the energy directly [as you can't: Energy is a product of two measurables].

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.

Re:They used an old trick

[barawn]

Actually, the standard set is:

MKS: physics
English: engineering
CGS: astrophysics

A physicist will say 9.8 m/s^2, but an astrophysicist will say 2x10^33 grams. There's then the adage of "if you see things in a schematic like 10 cm x 50 cm, it's never been built".

Physics does typically use MKS - it's astrophysics that's on the CGS trip. There are reasons for it (What's the magnetic field of the earth? Bout a gauss - tesla are WAY too huge to be convenient except for certain wacky guys).

As for the reason for electrostatic units (e.s.u.), that's simple: they're not a holdover for anything, it's just that when you're not talking about things that are measured in a lab (like volts, amps) you might as well use convenient math, and for e.s.u., the Coloumb force is just qq/r^2. For astrophysics, that's the easiest way to do it.

There will always be a split in the physics community, though, between theorists and experimentalists. Keeping track of constants is a pain in theory - the math is hard enough already without shoving constants in front of everything, and the constants really confuse a lot of the underlying structure, as well. Therefore, theorists will always use natural units, with everything set to 1, basically (Heaviside-Lorentz units for electric charge: set epsilon-naught to 1, mu-naught to 1, so the speed of light is 1). Unfortunately, those numbers are ridiculously inconvenient for actually DOING any physics, since we don't live on a subatomic scale, so real physicists (er... experimentalists :) ) will always use MKS.

Re:Not 'Quantum gravity'

[os2fan]

The evidence for photons lies in the photoelectric effect. If you shine light at different wave-lengths onto a material, than it will not issue current until the wave-length becomes shorter than a certian length: ie it has enough energy to knock the electron out of its orbit. This is indirect proof of the photon.

Quantum Gravity and Dark Energy

[gnovos]

I have a feeling the the Quantum Gravity people need to team up with the Dark Energy people, because I suspect they are tackling the same issue. Case in point: Dark Energy is thought to have a "negative pressure" (i.e. the less dense, the more pressure), which is similar to the way "gravitons" work (as the more of them that strike an object, that is to say, the greater density, the less the pressure keeping two objects apart). Also, somehow, mass never seems to run out of gravitons. Stars eventually run out of photons, but gravitons never stop. What happens to all these hojillion gravitons? They can't ALL be absorbed by matter, can they? If they had even a nutrino's nutrino worth of mass, they could easily make up all the dark matter in the universe. Some food for thought...

One other thing, I wonder if there is a such thing as a gravatic black hole. Something so powerfully repulsive that gravitons cannot escape...

Re:Quantum Gravity and Dark Energy

[Anonymous Coward]

Virtual particles are not measurable! There is no particle detector that will ever detect a virtual particle. There is no apparatus that will ever produce a single one in a controllable (let alone measurable) way. All possible virtual histories simultaneously contribute to whatever the real process is. (That's the basis for the many-worlds interpretation of quantum mechanics.)

Other gravity + QM experiments done before.

[wilgamesh]

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?

Re:Very misleading, not "proof of quantum gravity"

[cybrpnk]

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....

Re:Not 'Quantum gravity'

[Anonymous Coward]

If gravity was quantised, then it would have a particle like nature. These particles would have mass, therefore would have a gravitational effect, ie. would emit more gravity particles. As it was quantised there would be a lower limit of particles emit. Therefore when summing we would get infinite amouts of gravity in all directions.....hummmm.....

Analogue of the photoelectrical effect?

[nairolF]

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?