I was just reading an amusing article by Jeremy Clarkson in which, in relation to rising sea levels, he suggests:
“Space is only 75 miles from the surface of the Earth, so why not make a giant hosepipe, dip one end in the sea and take the other end out into the void, where, of course, there is a vacuum. That means the water will be sucked up the pipe without the need for any energy-absorbing pumps.”
I’m wondering about the physics involved. Why exactly would this not work? I know I should be able to fathom this form myself but it’s late…
Jeremy Clarkson is amusing and Top Gear is a great programme, but he doesn’t really have the first clue when it comes to anything other than cars.
I think gravity stops the idea working.
The hosepipe would just be a long tube with atmosphere in it, the atmosphere would get thinner as the hospipe got higher (as our atmosphere does), and there would be a vacuum in the pipe when it was actually out of the atmosphere, but the same gravity that prevents our atmosphere from be “sucked” away by the vacuum of space would prevent any water from being sucked up the hosepipe.
Even if you had a hard vacuum in the pipe, and seawater contained no dissolved gases and had no propensity to boil at low pressure, water would rise only about 30’. The vacuum does not suck water up; air pressure pushes the water up. By the time your column of water reaches about 30’, it’s exerting the same downward pressure as all the many miles of atmosphere. Hence it can’t be raised higher no matter how good the vacuum.
If Clarkson had ever read and inwardly digested the Children’s Encyclopaedia as a boy, he’d know you can’t lift water more than the amount stated above with a suction pump; if you want it higher, you have to push it from underneath. Torricelli knew all this three or four hundred years ago.
It won’t work for many of the same reasons that the ocean and atmosphere doesn’t just spontaneously launch itself into space. There’s nothing particularly special about a pipe - the whole of the atmosphere is exposed to space at the top - and it doesn’t all get sucked away.
Or to put it another way, try this thought experiment:
-We build a wall, perpendicular to the earth’s surface - all around the equator - it’s made of scrith, so we can ignore the structural issues - this wall is taller than the atmosphere and goes all the way down to bedrock, so it divides the ocean and atmosphere in half.
Do we expect half the atmosphere to be sucked into space just because it’s been divided in half?
Of course not. Now, we tear down the wall and rebuild it on the tropic of cancer - now it divides everything, but not into equal halves any more.
We keep on tearing down and rebuilding the wall on lines of latitude further and further north until eventually, it’s very conspicuously a tube, rising out of the North Pole.
The water and air in the tube doesn’t get sucked out by the vacuum of space for exactly the same reasons as half the atmosphere didn’t get sucked away when the wall went around the equator.
Psst… I don’t think Jeremy Clarkson seriously thinks this would work. He’s not as dumb as he likes to let people think he is. He writes for comic effect, and cultivates a philistine know-nothing persona to further that effect. (And I think he does it rather well.)
A key point to remember which helps with thinking about this sort of physics problem is to remember that vacuums do not suck. They just don’t push. Forget the vacuum in space. Think instead about what is going to push. Is there anything to push the water 75 miles straight up? Hell. No.
Actually, now that I think about it, saying that vacuums do not suck is dead wrong: of course sucking is by definition applying a vacuum. What I mean is, you can’t apply a force to something by sucking (ie applying a vacuum to it).