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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."
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
It would be a Bad Thing (tm) if quantum mechanics were inconsistant on a macroscopic scale *phew*.
-Leo
The experiment is basically the same one that discovered the electron (with a few details changed).
In essence, if you select a mass small enough, you may be able to observe its interaction with individual quanta. What these people did was to slow the neutron down so much that they could see it fall under gravity. Their idea was that in stead of falling in a parabola, you should be able to see the polygon sides as the individual quanta hit, or downwards speeds quantised at different multiples of a base. It is this second element that they observed.
Since in the past, this yielded the experimental evidence for the electron, here it yields what could be experimental data for the graviton.
OS/2 - because choice is a terrible thing to waste.
This effectively demonstrates the existence of gravitons. A graviton is simply a quantum of gravitational force, and showing that particles exist in discrete gravitational quantum states demonstrates that gravitation is quantized (duh). The next step is to reconcile quantized gravitation with general relativity, which ain't gonna be fun.
Bugrit! Millenium hand and shrimp!
To use the electrical analogy, the existence of discrete atomic spectra did not prove the existence of photons. It only proved that atoms can only emit and absorb electromagnetic energy in discrete amounts. To actually prove the existence of photons, in quantum electrodynamics, required more subtle work. I think the first airtight experimental proof was in spontaneous emission of photons, actually.
Indirect evidence... you can explain it, however, by assuming that matter can only transition in particular energy levels, without assuming that radiation itself is always quantized. IIRC, spontaneous emission is the only phenomenon that provides airtight direct evidence of the mandatory quantization of the electric field. See Milonni's The Quantum Vacuum for details.
So I'm guessing the picture in your head is something like a big sun, and there's the earth going around the sun in circles. And then over time, you observe that the earth's orbit moves closer to the sun. Is there a loss of angular momentum here, or does the earth somehow speed up to compensate for it? Well, we can check it-
.x. v) for angular momentum. And noting that GM/r^2 = v^2/r, we can arrive at an expression for the angular momentum as a function of distance to the sun.
By some quick calculations, assuming circular orbits, you write L = m (r
L(r) = m (GMr)^(1/2)
So you realize that as the earth moves closer in, it's orbital angular momentum is dropping as sqrt(r). Is this a loss in angular momentum? Sure, but it has to go somewhere.
I'm no planetary physicist, but I'll point to the example of the moon Io orbiting Jupiter. There is also a loss of orbital angular momentum, and that gets transferred into tidal forces that stretch and compress Io, which is thought to be the source of Io's volcanic activity. So we should have to account for the change in angular momentum by looking at the internal degrees of freedom in the massive objects, like axial rotation of Sun and Earth objects. In other words, the internal rotation of the massive objects will get bumped up if orbits start to decay... That is, if we only have Newtonian physics.
By perhaps you're thinking of something more exotic. Let's say that instead of massive objects, these are totally point-like objects, so that you can't transfer angular momentum to them. And then you also observe that orbits decay! So what would cause these orbits to decay? Well, I've forgotten most of my general relativity now, but I think accelerating masses generate gravitons, so angular momentum can be carried off by gravitons too! This, I think, is a very small effect that'll usually get washed out by the other mundane business I mention above.
The GR gravitational wave decay was thought to be observed in some binary neutron star system. Sorry, I don't have a reference for it.
However, gravitons do not work the way you describe; what you're talking about is the old classical LeSage model of "particulate pressure" for gravity, which doesn't work. (Try a Google search for sci.physics* discussions.) The properties of gravitons do not intrinsically have anything to do with the cosmological constant; you can have graviton-based theories with and without one.
You're also getting confused between real and virtual particles. Massive bodies always gravitate, because they radiate virtual gravitons. Charged bodies have an electric field, because they radiate virtual photons. These particles don't contribute any real energy, and are not even measurable, so you don't have to worry about "running out" of them. Radiation of real particles leads to energy loss. Stars radiate real photons, and can run out of energy; orbiting bodies can radiate gravitational waves (and hence real gravitons), and can inspiral into each other.
Real gravitons cannot escape a black hole any more than real photons can; virtual gravitons and photons can escape, and are responsible respectively for the hole's external gravitational field and electric fields. See the FAQ.
No astrophysicist will say that 2*10^33 grams is equivalent to 9.8m/(s^2). That is totally wrong as grams (actually some form of kg) is a unit of mass not weight. Only weight includes acceleration as part of its magnitude.
What good is a used up world, and how could it be worth having? --Sting
Absolutely none. String theory contains a theory of quantum gravity. But as pointed out correctly by the Anonymous Coward above, this discovery is not a discovery of "quantum gravity", as the term is usually used. They have discovered quantization of neutron orbits in the classical gravitational potential, analagous to the quantization of electron orbits around a proton. (You know, the s,p,d,f energy levels from chemistry?) At low energies (and these are VERY low energies), our classical picture of gravity is extremely accurate, and there's not a graviton in sight. Experimental proof that the graviton exists would be proof of quantum gravity.
Discovering quantum gravity is much, much harder. The energy scale at which quantum gravity becomes important is 10^19 GeV (note 1 GeV/c^2=mass of proton). The accelerators we're building now are 2000 GeV. We won't get to 10^19 in our lifetimes, if ever. There has been a flurry of papers recently saying that we might see quantum gravity at current or near-term accelerators, but they do this by invoking extra dimensions. In other words, curled up extra space-time dimensions that are as big as 100um, and only gravity propegates in the extra dimensions. This has the effect of lowering the energy scale at which gravity becomes important, so that we might be able to see it.
But if that 10^19 figure is really correct, we ain't gonna see quantum gravity anytime soon. Unfortunately...
--Bob
1^2=1; (-1)^2=1; 1^2=(-1)^2; 1=-1; 1=0.
I just read half the article, and no, Viadd is right, os2fan is wrong. They don't measure any trajectory. They measure the vertical distribution of the bouncing neutrons and observe that it (the distribution) has oscillations. This is "just" another confirmation that neutrons can behave as waves. I don't see how it can teach us anything about quantum gravity.
I should add that I also agree with Viadd that it is an impressive experimental feat anyway.
Svein.
The "quantum gravity" that Mssrs. Hawking, Thorne, etc. are looking for (and is likely to revolutionize both science and technology in many fundamental ways) is not this. What they're looking for is large-scale manifestation of real quantum gravity particles... in the form of gravity waves.
What this experiment measured was the small-scale effect of VIRTUAL quantum gravity particles. The particles themselves were still not detectable.
Why all the hub-bub? Because now that virtual quantum gravity particles are being characterized, it might be easier to build dectectors for real particles.
Or to find out *some* data about real particles from this data. I doubt we'll see a full characterization, however.
I am disrespectful to dirt! Can you see that I am serious?!
The Higgs Boson is part of the Standard Model, which is a Quantum Field Theory. (Note the "Quantum" in "Quantum Field Theory") It is not a classical theory. Perhaps you meant "Standard Model" rather than "Classical Theory". The word "Classical" to a physicist means "non-quantum".
As Schrodinger might say, there's more than one way to skin a cat, and there are is more than one way to give mass to the W and Z bosons (which is why we want the higgs). It has been proposed, for instance, that there are several higgs', separated in energy by about 10GeV. This could be responsible for the experimental evidince seen at LEP2 just before it was shut down. One thing is clear, however. Whether or not we find the higgs boson, the LHC (next accelerator at CERN) must find new phenomena. Some of our calculations simply don't make sense as you increase the energy. The higgs is a good, simple solution to part of the problem, but that doesn't mean it's necessarily correct. We theorists like to pretend we know what we'll see (SUSY/Extra Dimensions/Higgs) but I like to keep my mind open and hope that we'll be surprised by what we see at the LHC.
--Bob
1^2=1; (-1)^2=1; 1^2=(-1)^2; 1=-1; 1=0.
The experiment treats the Earth's gravity well as just another semiclassical potential well. You could get the same effect with protons and a very, very weak electric field (for example).
Not to belittle the experiment -- it's groundbreaking and interesting., and I (for one) can't wait to see a semiclassical quantum verification of the equivalence principle.
It's just not "quantum gravity" in the sense one might naturally think.
You have to imagine that in the quantum regime these things are waves. What happens when you confine a light wave to a box? The boundary conditions make the light turn into a standing wave; the lowest energy one of these is essentially an unmoving half-wave of light. In a similar way, in the quantum states in question the lowest energy has no vertical velocity expectation value; the next has a 1.7 cm/sec one, etc. A quantum jump from one to the other would lead to that Wile E. Coyote behavior, so familiar in the quantum world and so foreign to the classical one we seem to inhabit.
Going back to the boundard conditions issue, this is how the experiment works. There is an absorption plate which essential determines the width of the channel. Classically a few neutrons will get through even the narrowest of channels, but quantum mechanically it has to be wider than the wavelength of the relevant particle. The curious thing about this experiment is that the channel is much wider (15 microns) than the neutron wavelength (0.01 micron) and visible light (0.6 micron) but the visible light gets through while the neutron does not! A straighforward explaination is to include the gravitational interaction quantum mechanically; then you get a neutron-graviton quasiparticle with a much longer wavelength that cannot fit through the slit. However, as the mass of light is darn small it couples very weakly and goes through essentially unchanged. The neutron, on the other hand, is sufficiently massive to cause a "strong" coupling and thus doesn't get through.