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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In my layman's-level grasp of things (someone more qualified can always clean up my mess later on):
In the quantum scheme of things, a field is a region of space in which some property is altered. In the case of gravity, it's the geometric curvature. In the cases of electricty and magnetism, it's the permittivity and permeability, respectively.
When the object creating the fields is moved, it exchanges virtual particles--"vector bosons" of the force--with the surrounding space so the fields can change. (This gets around spooky "action at a distance" ideas of Newtonian/classical physics.) Virtual particles differ from their real cousins in that they're free. They don't really have to be accounted for in the mass/energy balance sheet of things because their energy is small and they don't exist for long. They're basically quantum hiccups.
Electric and magnetic fields are aspects of the force of electromagnetism; thus the vector boson is the photon. The presumed vector boson of the gravitational force is the graviton. Why it is presumed to propagate at photon speed, I'm not sure, but it probably has to do with relativity theory.
Perhaps its because the Intelligent Designer won't give him a bigger allowance. (It's a slow day.)
What are we talking about here and have you any links?
Yes, but you forgot it has proportionately more mass to accelerate, so it falls at the same rate.
If you do a Google search on "quantized redshift" you will be amazed.
ROFL!!
Oh, I see what you're driving at! OK. Deep breath. Let's see whether I can make this clear.
Imagine that you and I are standing at the end of a long hallway. I'm throwing superballs at you, and you're catching them. Whenever you catch one, you count it.
Our game is fraught with problems, however. One problem is that I throw like a girl. Sometimes I throw things straight up, and sometimes I throw things straight ahead. Another problem is that the ceiling is very sticky. Whenever a superball hits the ceiling, it gets stuck and it never makes it to you. Fortunately, you can move the ceiling up and down.
If you could see the superballs bouncing, you'd notice that they always seem to bounce to certain specific heights. But you can't see the superballs. All you know is how many I throw, how many you catch, and how high the ceiling is.
If the ceiling is too low, none of the balls make it through. That's because no matter how I through the balls, they always bounce to at least a certain height. You move the ceiling a bit higher, and still see nothing. A bit higher, still nothing, and so on, until you put the ceiling high enough to let the least-bouncy balls through.
Suddenly, you are counting a significant number of balls. "OK," you think, "since I believe the balls can have any old energy above the minimum, I expect that there will be some more balls bouncing only a little bit higher than the minimum. I'll raise the ceiling a little bit, and I'll catch a few more of the balls." So you raise the ceiling a bit, but you still see balls coming down the hallway at the same rate. So you raise it a bit more; still you get the same result. So you raise it more and more, and you realize that the rate at which balls make it down the hall has plateaued as a function of ceiling height.
Then you raise it a bit more. Suddenly, the rate of the superballs makes another big jump! This is because you're suddenly admitting the next energy level: the ceiling has been raised higher than the fixed height to which balls of this energy can bounce.
In the case of the experiment (which uses neutrons instead of superballs, a neutron absorber instead of a sticky ceiling, and a neutron detector instead of a fancy West-coast lawyer), they clearly resolve the first jump, the plateau, and the second jump. There is weak evidence for a second and a third plateau, but the finite resolution of the detector washes them out. But I'd say that they have two discrete energy levels firmly in hand.
"through" = throw
Yes. The bouncing neutrons can't have zero energy; there's a minimum energy they can have. You see the population of neutrons having that energy with the first jump. The subsequent plateau tells you that there are no other neutrons with a slightly higher energy.
(It's tempting to think of the plateau as representing the first energy level, but that's wrong; remember, it's a plateau in counting rate, not in energy. The first jump represents the first energy level.)
And if so, what is the second level they've found?
The sudden increase of neutrons after the first plateau represents the sudden acceptance of a second population of neutrons having a higher energy. If these neutrons had not a single energy, but a distribution of energies, you'd see the population slowly ramping up with absorber height after the first plateau, but that isn't what's seen. The counting rate jumps up sharply after the first plateau.
And then finally, when Stockholm calls you, will you get me a ticket to the ceremony?
Stockholm keeps calling me, but I won't accept the call as long as they keep reversing the charges!
That's probably Ingrid. She's 23, single, very lonely, and she gets excited discussing cosmology. I gave her your number.
I suppose it's not, now that I reread it. It's not the clearest piece of science journalism.
I read the paper on www.nature.com (courtesy of Penn's sitewide license). The first plateau is clearly defined on the data plot (which tells you that the jumps up to it and from it are sharp). The second plateau is more of a shoulder.
How thoughtful of you to recall our discussion from many months ago.
You should also recall that I stipulated up front in that discussion that the quantum world was discrete.
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