In science, a perfect vacuum is an ideal vacuum that contains no particles and has a pressure of zero (in any pressure units). A perfect vacuum is a theoretical concept that cannot be achieved in the real world. But, it’s possible to come close, both in nature and in the lab.
How a Vacuum Works
In order to understand why a perfect vacuum isn’t possible, it’s helpful to understand how a vacuum works. By definition, a vacuum is a volume that contains little or no matter. Any region with fewer particles than air at atmospheric pressure is a vacuum. Familiar examples of (imperfect) vacuums include vacuum cleaner suction, the interior of an incandescent bulb, and the atmosphere of the Moon.
One way of forming a vacuum is using suction. Suction pulls particles out of a region. For example, the motor on a vacuum cleaner powers a fan that sucks up air and small objects. If you attach a vacuum cleaner to a rigid container, like a plastic bottle, you empty some of its air. But, you don’t form a perfect (or even especially good) vacuum.
The other way of forming a vacuum is by expanding the volume of a fixed amount of matter. For example, if you cap the end of an “empty” syringe and pull back on the plunger you increase the volume for the fixed amount of air. Expanding the volume infinitely produces a perfect vacuum.
Why a Perfect Vacuum Is Impossible
Forming a perfect vacuum is impossible because no device removes every single atom or molecules from a space, we can’t expand a volume infinitely, and we can’t prevent all outside particles from getting inside a container.
Researchers achieve near-perfect vacuums using multiple vacuum pumps. But, there are other considerations, too. As pressure falls, the walls of the container experience outgassing. Outgassing is when water, air, or other molecules trapped on the surface evaporate or sublimate. Using a desiccant or baking the container helps. Also, lining the walls of a container with a special coating that attracts and traps stray molecules (a “getter”) improves the vacuum.
Even if scientists somehow remove every single atom from a chamber, it’s impossible to shield the interior from external radiation. Muons from cosmic rays, neutrinos from the Big Bang and the Sun, and photons from cosmic background radiation pass through containers into the otherwise empty space. It’s possible to shield a container from muons and photons, but neutrinos still enter any man-made vacuum.
Even perfect shielding doesn’t result in a perfect vacuum. This is because, according to quantum mechanics and the Heisenberg uncertainty principle, there is still a connection between the apparent emptiness inside a container and the matter outside the container. In other words, there is always a vacuum fluctuation in any region of space.
How Close to a Perfect Vacuum Can You Get?
In nature, the closest you can get to a perfect vacuum is intergalactic space. There is still residual radiation and the odd atom, ion, and subatomic particle. Vacuum fluctuation still occurs. But, there are around 10-6 particles per cubic meter of space. Another way to look at it is that if you examine a random cubic meter of intergalactic space, chances are good it wouldn’t contain any matter.
The best vacuum in a laboratory setting has a pressure around 13 picoPascals (13 x 10-12 Pa). A cryogenic vacuum system achieves a near-perfect vacuum with a pressure around 6.7 femtoPascals (6.7 x 10-15 Pa). In comparison, atmospheric pressure is around 100 kPa or 100,000 Pa.
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