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John Huth | USLHC | United States

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The great vacuum in the sky

This is the zone rockets traverse in Thomas Pynchon’s novel Gravity’s Rainbow. I got e-mail from a reader who didn’t understand the concept of the vacuum. The writer didn’t think it possible, and is in good company. Neither Plato, nor Aristotle, nor even Descartes believed that a pure vacuum could exist.

A ‘vacuum’ in the most common sense is simply the absence of matter in some volume. Early experiments by physicists Torricelli and Boyle with vacuum pumps demonstrated that at least a partial vacuum was possible and could be created on earth. A standard measure of the purity of a vacuum is often expressed in the unit of pressure called a “Torr”, after Torricelli. The pressure at the surface of the earth is 760 Torr. The creation of vacuums of increasingly rarefaction has been possible with more and more powerful pumps. First, there is a mechanical pump, much like a piston engine in a car, which can achieve a pressure of about 10E-5 Torr. Then, there is a turbomolecular pump that uses a high-speed turbine to rid a chamber of gas. Beyond this, there are ion pumps, which trap atoms in a chamber by bombarding them with ionized atoms. At very low temperatures, physicists can take advantage of cryopumping where molecules can be made to stick to cold surfaces.

Why are vacuums important to the LHC? As you might be aware, we have to cool the magnets to a degree or so above absolute zero. In order to do this, we effectively have to create a giant thermos bottle to help keep the magnets cold. This uses a vacuum as the first stage of insulation from the outside world, which prevents the transmission of heat across the barrier of the vacuum.

The beam pipes of the LHC must have a very clean vacuum in order to keep protons circulating in the accelerator tubes without colliding with errant gas molecules. To do this, the pipes the protons travel through are typically maintained to a vacuum of 10E-9 Torr. At the interaction points, where the collisions take place in the middle of the detectors, extra care has to be taken to reduce the number of gas molecules even further, so more cryopumping is used to get the vacuum down to a level of 10E-11 Torr.

To give you some idea of what 10E-11 Torr is like, it’s akin to the pressure in interplanetary space. Present estimates of the vacuum of space far between galaxies is more than 1000 times lower than that, with 6 hydrogen atoms per cubic meter.

In a sense, these are all ‘partial vacuums’ – meaning that there are still atoms floating around. But, if we were able to make a perfect vacuum pump, would this mean that there’s absolutely nothing but space in such a creation?

The answer is ‘no’ and somewhat bizarre. In quantum field theory, there is a concept of ‘virtual’ particles, which are always being created and destroyed in empty space. For example, an electron and an anti-electron (called a positron) can be created momentarily in free space and can then fall back together again. If we introduced a free charge to this perfect vacuum, these electron-positron pairs would polarize and tend to screen the charge of the particle.

Beyond these virtual pairs of particles, there is something even stranger, that we sometimes associate with the Higgs boson, called a ‘vacuum expectation value’. This is to say, in a perfect vacuum we expect that there is some non-zero amount of the Higgs field floating around. Now, one may be quick to dismiss this as just some figment of a theorist’s imagination that has no consequence. Measurements of the rate of expansion of the universe, however, indicate a strange ‘dark energy’ that permeates free space and is forcing the universe to accelerate its expansion. This dark energy appears to be an energy that will inhabit space devoid of any matter whatsoever and is akin to the ‘vacuum expectation value’ in many ways. No one knows why this dark energy exists, but it is permitted by Einstein’s equations describing the large-scale structure of the universe. We just didn’t expect to see it, and it seems to lurk everywhere.

So, perhaps the ancient philosophers were right: there may not be a pure vacuum in nature after all.

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