"The vacuum is not empty. It may sound like magic to laypeople but the problem has preoccupied physicists since the birth of quantum mechanics. The apparent void bubbles incessantly and produces fluctuations of light even at absolute zero temperature. In a sense, these virtual photons are just waiting to be used. They can carry forces and change the properties of matter. The force of the vacuum, for instance, is known to produce the Casimir effect."
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What is the Casimir effect?
To understand the Casimir Effect, one first has to understand something about a vacuum in space as it is viewed in quantum field theory.
Far from being empty, modern physics assumes that a vacuum is full of fluctuating electromagnetic waves that can never be completely eliminated, like an ocean with waves that are always present and can never be stopped.
These waves come in all possible wavelengths, and their presence implies that empty space contains a certain amount of energy—an energy that we can’t tap, but that is always there.
Now, if mirrors are placed facing each other in a vacuum, some of the waves will fit between them, bouncing back and forth, while others will not.
As the two mirrors move closer to each other, the longer waves will no longer fit—the result being that the total amount of energy in the vacuum between the plates will be a bit less than the amount elsewhere in the vacuum.
Thus, the mirrors will attract each other, just as two objects held together by a stretched spring will move together as the energy stored in the spring decreases.
This effect, that two mirrors in a vacuum will be attracted to each other, is the Casimir Effect.
It was first predicted in 1948 by Dutch physicist Hendrick Casimir. Steve K. Lamoreaux, now at Los Alamos National Laboratory, initially measured the tiny force in 1996.
It is generally true that the amount of energy in a piece of vacuum can be altered by material around it, and the term “Casimir Effect” is also used in this broader context.
If the mirrors move rapidly, some of the vacuum waves can become real waves. Julian Schwinger and many others have suggested that this “dynamical Casimir effect” may be responsible for the mysterious phenomenon known as sonoluminescence.
One of the most interesting aspects of vacuum energy (with or without mirrors) is that, calculated in quantum field theory, it is infinite! To some, this finding implies that the vacuum of space could be an enormous source of energy—called “zero point energy.”
But the finding also raises a physical problem: there’s nothing to stop arbitrarily small waves from fitting between two mirrors, and there is an infinite number of these wavelengths.
The mathematical solution is to temporarily do the calculation for a finite number of waves for two different separations of the mirrors, find the associated difference in vacuum energies and then argue that the difference remains finite as one allows the number of wavelengths to go to infinity.
Although this trick works, and gives answers in agreement with experiment, the problem of an infinite vacuum energy is a serious one.
Einstein’s theory of gravitation implies that this energy must produce an infinite gravitational curvature of spacetime—something we most definitely do not observe.
The resolution of this problem is still an open research question.
https://www.scientificamerican.com/article/what-is-the-casimir-effec/
Ultra-cold mirrors could reveal gravity’s quantum side
By Adam Becker
An experiment not much bigger than a tabletop, using ultra-cold metal plates, could serve up a cosmic feast. It could give us a glimpse of quantum gravity and so lead to a “theory of everything“: one that unites the laws of quantum mechanics, governing the very small, and those of general relativity, concerning the monstrously huge.
Such theories are difficult to test in the lab because they probe such extreme scales. But quantum effects have a way of showing up unexpectedly. In a strange quantum phenomenon known as the Casimir effect, two sheets of metal held very close together in a vacuum will attract each other.
The effect occurs because, even in empty space, there is an electromagnetic field that fluctuates slightly all the time. Placing two metal sheets very close to one another limits the fluctuations between them, because the sheets reflect electromagnetic waves. But elsewhere the fluctuations are unrestricted, and this pushes the plates together.
James Quach at the University of Tokyo suggests that we might be able to observe the equivalent effect for gravity. That would, in turn, be direct evidence of the quantum nature of gravity: the Casimir effect depends on vacuum fluctuations, which are only predicted by quantum physics.
But in order to detect it, you would need something that reflects gravitational waves – the ripples in space-time predicted by general relativity. Earlier research suggested that superconductors (for example, metals cooled to close to absolute zero such that they lose all electrical resistance) might act as mirrors in this way.
“The quantum properties of superconductors may reflect gravitational waves. If this is correct, then the gravitational Casimir effect for superconductors should be large,” says Quach. “The experiment I propose is feasible with current technology.”
It’s still unclear if superconductors actually reflect gravitational waves, however. “The exciting part of this paper has to do with a speculative idea about gravitational waves and superconductors,” says Dimitra Karabali at Lehman College in New York. “But if it’s right, it’s wonderful.”
Journal reference: Physical Review Letters, DOI: 10.1103/PhysRevLett.114.081104
source:
https://www.newscientist.com/article/dn27060-ultra-cold-mirrors-could-reveal-gravitys-quantum-side/
