# Is there a limit to how strong a vacuum can be?

Apologies for using an absolute term (vacuum) in a relative way, but the question revolves around such a contradiction.

If we could suck matter out of a region with the full power allowed by the laws of physics, would there be much or any difference between such a vacuum and the vacuum of a weaker experiment, or empty space?

• yes there is a limit. there is a finite amount of matter you can take away from a given volume Commented Feb 26, 2018 at 13:49
• But is the approach to this limit asymptotic in difficulty and/or effect? Commented Feb 26, 2018 at 23:45
• Actually there’s no such thing as a vacuum there is only low pressure Commented Aug 14, 2023 at 18:27

Vacuum has no "strength". The only reason why a vacuum vessel on Earth must be strong is to resist the crushing pressure of Earth's atmosphere.

Also, High vacuum systems do not "suck" matter out of the vacuum vessel. When the system is first started up, the mechanical action of the fore pump continually creates a region of lower pressure, and the higher pressure of the air inside the vacuum vessel pushes air toward the pump.

Once a high vacuum is achieved, that means that free molecules in the chamber collide with the chamber walls more often than they collide with each other. At that point, a Turbomolecular pump or a diffusion pump comes in to play. It's job is to trap any molecules that happen by chance to bounce its way and, to keep them from returning to the chamber; but there is no way that the pumping system can "suck" those molecules toward the exit.

• My question was about vacuums, not vacuum vessels. My seemingly erroneous definition of a 'strong vacuum' correlating with what you call a 'high vacuum'. You have confirmed that there can be vacuums made with differing amounts of 'success' or "height", but you did not answer whether there is a limit to the degree of success possible theoretically, i.e., how little matter is left confined to space and how the strength/time required to remove it might grow. Commented Feb 26, 2018 at 23:52
• @user7778287, A perfect vacuum would be a volume of space that is completely devoid of free molecules. You can't get any more vacuuous than that. Commented Feb 27, 2018 at 14:34
• @user7778287, It doesn't take "strength" to get the last few molecules out of a vacuum system: It takes patience. You can't just reach in, grab them, and pull them out. They're in there, randomly bouncing around at approximately the speed of sound, and you have to wait until they happen to bounce in to the exit hole, and then you have to have some mechanism (e.g. the turbomolecular pump that I mentioned earlier) that decreases the liklihood that they will simply bounce back into the chamber. You also have to contend with new molecules getting in (e.g, by leaks or by outgassing.) Commented Feb 27, 2018 at 14:39
• @user7778287, another trick is to include a getter in the vacuum system: That is, a substance that free molecules tend to stick to if they happen to hit it. Commented Feb 27, 2018 at 14:41

I assume that you're discussing the mechanical pressure differential produced between a vacuum and air at ambient atmospheric pressure.

The answer is, "not much". Ambient atmospheric pressure is roughly 103.1 kPa = 14.7 PSI = 760 torr (mm of mercury).

An inexpensive, simple, mechanical rotary vane vacuum pump can evacuate a chamber to a residual pressure of about 20 millitorr = 2.6 Pa = 0.00039 PSI.

That is... not very much pressure.

So a perfect vacuum, in which the chamber contains no free-flying molecules, is only a fraction of a percent "stronger" than one that's been evacuated with a cheap pump used by refrigerator repairmen.

If you look at the way CRT tubes were made, you get a clear view of the problem of extracting all air molecules. The tube is positioned face up, electron gun down. At first, the pump is working hard to fight again all the weight of the air molecules. The pressure at see level is about 15 psi. Then, when the CRT is almost empty, the remaining molecules are falling out by gravity. Who knew that it would not be possible to build CRT in any factories located in orbit (so, feeling weightless by definition). The maximum strength of vacuum at see level is about -15 psi. A related phenomenon that could help you better understand the limit of vacuum is: what is the loudest sinusoidal wave possible at see level? The answer turns out to be about 194 db. This is the limit where the cone of a speaker would push the air at twice the speed of sound and would retract so fast that air would not have time to catchup. So, the wave would propagate as a growing sphere, like any compression wave and would be composed of high pressure point at about 30 psi and low pressure point at 0 psi, local vacuum.

• Welcome to Physics SE! I am not sure if this completely answers the question, as it does not discuss the "relativity" of different vacuums. Commented Feb 16, 2019 at 0:54
• This isn't quite correct. By the time you reach the final pumpdown of a vacuum instrument like a CRT, much of the pumping is being done by gettering where the molecules stick to the walls. Gravity isn't as relevant. Commented Sep 5, 2019 at 3:00

I used to vacuum test welds. At best with an air compressor and a special pass-through pipe fitting to pull pressure, I could usually get about $$\mathrm{-9psi}$$. The perfect vacuum at sea level on our planet is $$\mathrm{14.7 psi}$$. That’s the best vacuum you’re going to get.

• I have downvoted this answer. While you may be expressing things that are true in the context of a trade-specific way of communicating information, in physics there are no negative pressures and 14.7 PSI is regular air pressure, not a vacuum.
– g s
Commented Jul 22, 2023 at 0:41