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Physics: A Perfect Vacuum, Not - by John Prytz

One of the big scientific oops often depicted in Hollywood (and other country’s equivalents) sci-fi space operas is an external view of spaceships, etc. in deep space chug-chug-chugging along with all appropriate sound effects, and/or blasting away with laser cannons or photon torpedos, ditto with appropriate sound effects. Of course, if you really were an external viewer, what would you actually hear? - Absolutely nothing. It would be, or should be, like viewing a silent film from the pre-talkies era. But, for the sake of dramatics, Hollywood (etc.) ignores the physics of it all. ‘Artistic licence’ is what it’s called I believe. 


However, in the physics classroom, where artistic licence isn’t allowed, I’m sure we can all recall from our student days a demonstration of the ringing alarm clock inside the bell jar. As the air was pumped out from the bell jar, the ringing got softer and softer and softer until you heard nothing at all, even though the alarm clock was still jangling away. Of course the science teacher or physics professor told you there was now a total vacuum inside the bell jar and thus no sound could travel from point A – the alarm clock inside the vacuum inside the bell jar, to point B – your ears which were outside the bell jar.


Of course it should be obvious to blind Freddy that there was no such thing as a total or perfect vacuum inside the bell jar. Firstly, no pump is good enough or efficient enough or strong enough to remove each and every last molecule of air from the bell jar. [Of course there is no such thing as a molecule of air, rather atoms and molecules of the various substances that together make up what we call air, but for the sake of simplicity let’s make believe there are molecules of air.] Even discounting those rarefied air molecules left within the bell jar, molecules few and far enough in-between as to prevent the sound of the alarm clock from reaching your ears, the inside of the bell jar still wasn’t a total vacuum.


Why? Well assuming the bell jar was made of glass; light was pouring through the bell jar, and light is a something. The inside of the bell jar was full of visible light. Okay, let’s make the bell jar out of solid lead. That blocks out the light – right? Wrong. Visible light is but a small part of the entire electromagnetic spectrum. Radio waves (a form of non-visible ‘light’) will probably pass through the leaden bell jar, or gamma rays. Even if you succeed in blocking out all of the wavelengths and frequencies of the electromagnetic spectrum, there are still cosmic rays. If you make the leaden bell jar thick enough you might block out all of the cosmic rays, but that still leaves neutrinos, and in order to block all of those, you’d need a bell jar that had a lead thickness of hundreds of light years. Neutrinos can pass through leaden walls as easily as Casper the Friendly Ghost – even easier!


Okay, so you’ve got a perfect pump to remove all the air molecules and sufficient shielding to prevent even neutrinos from entering and passing through. That’s that; now you have your perfect vacuum – right? Wrong again.


You still have gravity not only surrounding but inside the otherwise perfectly shielded bell jar. Even if there were no other matter in the entire Universe, the bell jar itself is made of matter and has its own gravity which resides inside the jar as well as extending to infinity outside of it. Since nothing we know of can block the force of gravity, well that alone puts the kibosh on the perfect vacuum. Gravity is really a real thing, and if you doubt it, I invite you to try to leap tall buildings at a single bound!


Now a perfect vacuum would have to have a temperature of absolute zero, given the total absence of any material substance within. Temperature of course is just a measurement of the average molecular or atomic motion of molecules, atoms, even their fundamental particles. No stuff means a temperature of absolute zero. However the Universe isn’t lacking in stuff which moves around from place to place


For quite another reason however, absolute zero, zero degrees on the Kelvin scale, (or minus 273.15 degrees Celsius or minus 459.67 degrees Fahrenheit) is only theoretical and can never be obtained. That’s because the concept of absolute zero violates the Heisenberg Uncertainty Principle which states that it is impossible, no matter how good your measuring instruments are, to simultaneously know the position and momentum of anything, because the very act of measuring something must alter both parameters. That’s because something must bounce off what you are trying to measure and hence interact with your measuring device, but that bounce alters the position and momentum of that in which you are interested. At a theoretical absolute zero, nothing moves and thus you would know both the position and momentum of that or any object, even say an electron, with absolution precision. The momentum is zero and thus your object is in, and stays in, a fixed location – and besides, there’s nothing to bounce off it anyway, because that something (usually a photon) must also have zero energy and is thus at rest.


And thus we have the concept of the vacuum energy, or quantum fluctuations or the quantum jitters. That is to say, at the extreme micro level, there is always activity and thus motion and thus temperature above the theoretical minimum of absolute zero - thus, no perfect vacuum is possible. Even the best vacuum will have the quantum jitters. Oh, and by the way, the quantum jitters or the vacuum energy has been experimentally verified.


So, if you were somehow expecting that there was such a thing as a perfect vacuum, you’re out of luck, not that that I’m sure will cause anyone any loss of shuteye. However, that doesn’t alter the fact that in space, no one can hear you scream! You don’t need a perfect vacuum for that to be true.

Science librarian; retired.

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