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How Much Does 'Nothing' Weigh?

MrRING

Android Futureman
Joined
Aug 7, 2002
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Interesting proposal to do just that:
https://www.scientificamerican.com/article/how-much-does-nothing-weigh/
Geologically, Sardinia is one of the quietest places in Europe. The island, along with its neighbor Corsica, is located on a particularly secure block of Earth's crust that is among the most stable areas of the Mediterranean, with very few earthquakes in its entire recorded history and only one (offshore) event that ever reached the relatively mild category of magnitude 5. Physicists chose this geologically uneventful place because the Archimedes experiment requires extreme isolation from the outside environment. It involves a high-precision experimental setup designed to investigate the worst theoretical prediction in the history of physics—the amount of energy in the empty space that fills the universe.

Researchers can calculate the energy of the vacuum in two ways. From a cosmological perspective, they can use Albert Einstein's equations of general relativity to calculate how much energy is needed to explain the fact that the universe is expanding at an accelerated rate. They can also work from the bottom up, using quantum field theory to predict the value based on the masses of all the “virtual particles” that can briefly arise and then disappear in “empty” space (more on this later). These two methods produce numbers that differ by more than 120 orders of magnitude (1 followed by 120 zeros). It's an embarrassingly absurd discrepancy that has important implications for our understanding of the expansion of the universe—and even its ultimate fate. To figure out where the error lies, scientists are hauling a two-meter-tall cylindrical vacuum chamber and other equipment down into an old Sardinian mine where they will attempt to create their own vacuum and weigh the nothing inside.

What's in Empty Space?

A vacuum is not completely empty. This is because of an idea in quantum physics called Heisenberg's uncertainty principle. The principle states that you can't determine the position and the velocity of a particle at the same time with any precision—the more precisely you know one value, the less precisely you can know the other. This principle also applies to other measurements, such as those involving energy and time. Its consequences are considerable. It means that nature can “borrow” energy for extremely short amounts of time. These changes in energy, known as vacuum fluctuations, often take the form of virtual particles, which can appear out of nowhere and disappear again immediately.

Vacuum fluctuations have to respect some rules. A single electrical charge, for example, cannot suddenly appear where there was none (this would violate the law of charge conservation). This means that only electrically neutral particles such as photons can pop out of the vacuum by themselves. Electrically charged particles have to emerge paired with their antiparticle matches. An electron, for instance, can appear along with a positron, which is positively charged; the two charges cancel each other out to preserve the total charge of zero. The result is that the vacuum is continuously filled with a stream of short-lived particles buzzing around.

Even if we can't capture these virtual particles in detectors, their presence is measurable. One example is the Casimir effect, predicted by Dutch physicist Hendrik Casimir in 1948. According to his calculations, two opposing metal plates should attract each other in a vacuum, even without taking into account the slight gravitational pull they exert on each other. The reason? Virtual particles. The presence of the plates imposes certain limits on which virtual particles can emerge from the vacuum. For example, photons (particles of light) with certain energies can't appear between the plates. That's because the metal plates act like mirrors that reflect the photons back and forth. Photons with certain wavelengths will end up with wave troughs overlapping wave crests, effectively canceling themselves out. Other wavelengths will be amplified if two wave peaks overlap. The result is that certain energies are preferred, and others are suppressed as if those photons were never there. This means that only virtual particles with certain energy values can exist between the plates. Outside them, however, any virtual particles can emerge.

The result is that there are fewer possibilities—and therefore fewer virtual particles—between the plates than around them. The comparative abundance of particles on the outside exerts pressure on the plates, pressing them together. This effect, strange as it may sound, is measurable. Physicist Steven Lamoreaux confirmed the phenomenon experimentally at the University of Washington in 1997, almost 50 years after Casimir's prediction. Now Calloni and his colleagues hope to use the Casimir effect to measure the energy of the void.

This energy has important consequences for the universe as a whole. General relativity tells us that energy (for example, in the form of mass) curves spacetime. That means virtual particles, which change the energy of the vacuum for a short time, have an effect on the shape and the development of our universe. When this connection first became clear, cosmologists hoped it would solve a major puzzle in their field: the value of the cosmological constant, another way of describing the energy in empty space.
 
The ex was a science teacher and would show pupils how to weight air. You basically weighed balloons before and after inflating them, and the difference was the air.
I pointed out that if the balloons were being personally blown up by humans they'd contain moisture well as air. Didn't go down well.
 
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