Posted on 01/17/2002 4:06:29 PM PST by Ernest_at_the_Beach
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Gravity's quantum leaps detected |
| 19:00 16 January 02 |
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Gravity's subtle influence in the quantum world has been directly observed for the first time. On tiny scales, nature makes particles behave according to curiously rigid rules. For instance, negatively charged electrons trapped around a positive nucleus under the pull of the electromagnetic force cannot have any energy they want -they have to fall into a set of distinct energy levels. In the same way, the pull of gravity should make particles fall into discrete energy levels. But because gravity is extremely weak on small scales, the effect has been impossible to spot. "To be able to measure it, you need to suppress interference from all the other fields," says Valery Nesvizhevsky of the Laue-Langevin Institute in Grenoble, France. Now Nesvizhevsky and his colleagues have achieved the feat using a beam of neutrons. Neutrons were ideal because they're neutral, so they don't feel the electromagnetic force and can ignore its quantum rules. Experts say it is a convincing result from an extremely tricky experiment. "The difficulty of this measurement should not be underestimated," says Thomas Bowles of Los Alamos National Laboratory in New Mexico. "In the quantum realm, the gravitational force is so weak that it is difficult to observe quantum effects."
Nesvizhevsky's team took a beam of ultracold neutrons with tiny energies, moving from left to right at less than eight metres per second. Under the force of gravity, the neutrons fell down onto a reflecting mirror and bounced off it before arriving at a detector. The team could limit the energies of the neutrons arriving at the detector by placing an absorbing material at different heights above the mirror. The material mopped up all the neutrons that bounced too high. Forgetting quantum mechanics, you would expect neutrons with any energy to arrive at the detector. But no neutrons appeared unless the neutron-mop was at least 15 micrometres above the mirror. This means the neutrons have to have a certain, minimum energy (equal to 1.41 x 10-12 electronvolts) in the Earth's gravitational field.
There were also hints that neutron transmission took little leaps at different, higher energies, corresponding to higher quantum levels. However, the team has still to confirm this. Nesvizhevsky says the technology is exciting because it could test some other key ideas in physics - for instance, whether or not the neutron carries some minuscule amount of electric charge. "If it's there, it's very, very small," says Nesvizhevsky. It could also put on trial the equivalence principle, a famous concept of Einstein's. It says that all particles, regardless of their mass or composition, should fall with the same acceleration in a uniform gravitational field. Journal reference: Nature (vol 415, p 297) |
| 19:00 16 January 02 |
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nature makes particles behave according to curiously rigid rules.
Huh ... doesn't sound at all like that Gaia chick.
This study also depends on the gravitational field of the Earth for it's results, and should be replicated in greater and lesser gravitational fields for confirmation. Is that right?
The team could limit the energies of the neutrons arriving at the detector by placing an absorbing material at different heights above the mirror. The material mopped up all the neutrons that bounced too high.
At last, we know why the toast always falls buttered-side down. The butter mops up all the neutrons.
All your gravity are belong to us.
As I said, the primary value of this is the technique and not the result. They do claim in the article to have evidence for higher energy levels; presumably that will be the subject of their next publication.
This study also depends on the gravitational field of the Earth for it's results, and should be replicated in greater and lesser gravitational fields for confirmation. Is that right?
I don't know whether their apparatus is sensitive enough to distinguish variations in the gravitational field at different places on the Earth. It would be expensive to do it anywhere else right now. Personally, I think it's enough to show that the energies are discrete, and to show that the energy levels agree or disagree with what theory predicts.
I agree: if for no other reason, because the heavier object ATTRACTS the other body more, which will have a small but real velocity component up towards the falling object!
A repeat in a way of the Thompson experiment where the charge on an electron was measured.
What is the speed of electrical attraction?
I agree that this experiment doesn't directly address other energy levels (although it handles the edge condition). The effect will be less at higher energy levels. As an analogy, the two-slit experiment (the hypothetical, intro quantum theory experiment that shows that electrons appear to be in multiple places at the same time) is done with particles, where the effect can be noted, not baseballs.
Physical theories can be verified under different conditions, but not proven in the mathematical sense. Remember Mr. Spock's: "We may be in a region of space where our physical laws do not apply."
BTW, to show I'm not all "math" and "guy", I saw "The Music Man" recently. You came to mind when the mother blames her daughter's singleness on "her Irish imagination, her Iowa stubborness and her library full-of-books."
But I forget why.
The reason you can't tell that the energy of, say, a baseball is quantized is because the energy levels are so close together at that energy scale. In order to see the quantization, you need to look at extremely light objects that move slowly.
If the spacings were much, much farther apart, you might notice that while you can throw a baseball upwards at, say, 10 mph, which allows it to reach a certain height, you can't throw it upwards at 12 mph. Put a little more arm into it, and you can throw it at 14 mph, but not a little bit faster or slower than that. Put still more arm into it, and you can throw it at, say, 17.5 mph.
But yes, I suppose in the real world, you wouldn't be able to demonstrate energy quantization in the case of a baseball, since you'd never get the resolution you'd need. But lighter, slower objects do apparently behave in the counterintuitive way I described.
laconas: I don't think anybody really knows.
It's the speed of light. Virtual photons fly around at their accustomed speed, creating the electrical field in the space around them. The speed of gravity is thought to be the same, mediated by a so-far undetected particle called the graviton.
I know what a virtual image is. I have been an optical engineer in a prior incarnation. But I do not know what a virtual photon might be.
For the speed of gravity determination they are using massive but highly sensitive devices. If something heavy moves, like a supernova, they might be able to get a measurement. I think they are waiting for a suitable event so they can get some numbers. Until then they are detecting neutrinos and doing Aetvos experiments with artificial satellites. Maybe we will be the generation that gets a clue.
Your opinion was once universally held; but experimentation has shown that heavy and light objects fall at the same speed. Each falling object is not only attracted to the earth in proportion to its mass, but it [the falling object] also has inertia that resists the motion, so the falling object's mass cancels itself out. All that's left is the earth and the gravitational constant, which are the same for all falling objects.
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