Stuck on thought experiment about light

Question

Say we have a very long fluid pipe with the width of a few astronomical units, and that this pipe is perfectly resistant to sustain the stress of a perfectly incompressible fluid going through it without collapsing.

Assume also that there are no external fields acting on this pipe, nor the fluid; the only force is a device which will apply a pressure source pushing this fluid in one end of the pipe. The pipe is filled with this fluid.

If I look at the other end of the pipe, is the fluid going to come out as soon as the device is activated on the other end? It sounds like it shouldn't be possible because it would be transmitting information faster than a pulse of light travelling across the length of the pipe, but I can't see what is absurd about it.

What's wrong with it?

Basically a duplicate of Is it possible for information to be transmitted faster than light by using a rigid pole? — John Rennie, Mar 07 '23 at 06:41
@JonCuster I would say there are no solids at that scale for another. — m4r35n357, Mar 07 '23 at 09:34

score 14 · Accepted Answer · answered Mar 06 '23 at 21:37

14

As @JonCuster already pointed out in the comments, you are assuming an incompressible fluid. Under unrealistic assumptions you of course get unrealistic results. In reality, you will get a simple pressure wave traveling through your pipe at the speed of sound.

answered Mar 06 '23 at 21:37

rfl

6,455

As I commented above, you cannot have the pipe either. – m4r35n357 Mar 07 '23 at 09:34
true, though I would say the scale of the setup in the OP is a red herring and doesn't matter at all – rfl Mar 07 '23 at 12:54

Stuck on thought experiment about light

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