# A perfect vacuum

## Main Question or Discussion Point

I have no qualifications in science but find it fascinating in general. Could anyone answer the following for me?

Imagine an immensely strong pipe closed at one end with an airtight piston pressed tightly against the closed end.

(a) Is it possible to pull the piston any distance along the pipe using a powerful engine?

(b) If so, is there a limit as to how far can it be pulled?

(c) Assuming a perfect vacuum is created, would moving the piston farther be stretching space?

(d) Is it possible to stretch space under any conditions?

Thanks

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This is not such a special problem. Even if the piston and the bottom of the cylinder were a single piece (or welded together) you can break them apart. This is done routinely by the machines used to test tensile strength.
If you worry just about the pressure of air on the other side of the piston, you only need a force equal to whatever air pressure force is on the piston.
How far does not matter, the force is the same.

There is nothing about "space stretching" in this.

Bandersnatch

Imagine you take the same pipe+piston, only filled with air at normal atmospheric pressure, and submerge it 10 metres deep in water.
Since at that depth the pressure is equal to about twice the atmospheric pressure, the difference in pressures between outside and inside of the pipe would be 1 atm. I.e., the same difference as in the scenario you described, with a pipe with perfect vacuum inside and air outside.
That piston would be just as hard to pull 10 metres deep in water as it would be with perfect vacuum on the surface, and the pipe just as likely to collapse.

If you replaced the air inside with vacuum, and take the setup underwater, it'd require just twice the force from the previous case to move.

CWatters
Homework Helper
Gold Member
(a) Is it possible to pull the piston any distance along the pipe using a powerful engine?
What force do you think has to be overcome? Typically it's only the air pressing on the outside of the piston (say about 15lbs per square inch).

(b) If so, is there a limit as to how far can it be pulled?
No. Except perhaps the size of the universe?

(c) Assuming a perfect vacuum is created, would moving the piston farther be stretching space?
No.

(d) Is it possible to stretch space under any conditions?
I don't think that's easy to answer. Really strange things happen near a black hole or when travelling close to the speed of light.