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Moving through space

Skeptic believer

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A simple science question that occured to me

Would it be possible to travel through empty space by continually changing your shape?

E.g. Would an amoeba have the ability to freely move within a weightless environment?
 
Skeptic believer said:
A simple science question that occured to me

Would it be possible to travel through empty space by continually changing your shape?

E.g. Would an amoeba have the ability to freely move within a weightless environment?
Click to expand...

No. Not due to weightlessness, which isn't relevant, but due to not having anything to push or squirm or slither against.

You can see this because of the conservation of momentum. Actually, it's even easier than that. This has to be true for everything, but let's take a rocket. You have the rocket and all its fuel and everything. It has a center of mass. Then the rockets go on, and it moves. If you take everything into account, the rocket and the stuff that comes out the back, the center of mass hasn't changed.

ETA: A snake can move by slithering in grass, but it can only go forward if the grass goes backward. The grass bone is connected to the Earth bone. So, for even a snake to move in one direction, the Earth has to move in the opposite direction. Not much, because snakes aren't very big, but some.
 
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I suppose if the organism has a lot of time on its hands, changing shape in order to make thermal radiation pressure uneven would work in a vacuum in a very, very limited way. But yes, momentum conservation is a harsh mistress.
 
epepke said:
ETA: A snake can move by slithering in grass, but it can only go forward if the grass goes backward. The grass bone is connected to the Earth bone. So, for even a snake to move in one direction, the Earth has to move in the opposite direction. Not much, because snakes aren't very big, but some.
Click to expand...

But an amoeba doesn't need anything to push against to change its shape, which is why I used it as an example


epepke said:
You can see this because of the conservation of momentum.
Click to expand...

Assume that. for the purposes of the experiment. you define yourself as stationary within space - Thus no momentum (in your frame of reference).

If you are sitting on a long seat in space, which has the ability to elongate or contract, either left or right -

Without moving through space, you elongate the right side, and then move to your right - Then you contract the left side.
Thus, you will in effect have moved slightly towards the right, but your momentum hasn't changed, as you remain stationary (in the unaltered frame of reference).

You are simply changing your position by using the changing shape of the seat as a sort of step ladder - but your momentum doesn't change, so that doesn't come into it

In this way, you will be able to slowly cross space

or will you (??)
 
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Skeptic believer said:
But an amoeba doesn't need anything to push against to change its shape, which is why I used it as an example
Click to expand...

An amoeba lives in water, which it can push against.

Without moving through space, you elongate the right side, and then move to you right - Then you comtact the left side.
Click to expand...

No. The center of mass does not move. When you elongate the right side, it pushes the rest to the left, and the center of mass does not move.
 
Skeptic believer said:
Assume that. for the purposes of the experiment. you define yourself as stationary within space - Thus no momentum (in your frame of reference).

If you are sitting on a long seat in space, which has the ability to elongate or contract, either left or right -

Without moving through space, you elongate the right side, and then move to you right - Then you comtact the left side.
Click to expand...

Newton's third law of motion. Trying to "move to your right" will move the seat to your left. Your ultimate position will be the same.
 
Actually, it IS possible to "swim" in a gravitational field without pushing against anything, just by changing shape. Essentially what you're doing is radiating gravitational waves. But it's horrifically inefficient, and the effects would be far too small at non-relativistic speeds to make any noticeable difference.
 
There was an American scientist article on this a while back. It is possible to do, but as Ziggurat points out, it's nearly pointless.
 
Oho! 'Swimming' in electromagnetic radiation is bad enough already. But theoretically a good idea; didn't think of that.
 
Skeptic believer said:
A simple science question that occured to me

Would it be possible to travel through empty space by continually changing your shape?

E.g. Would an amoeba have the ability to freely move within a weightless environment?
Click to expand...

Yes, by jet power, if ejecting a part could be considered changing shape.
 
In line with what others have pointed out -- if you can split yourself, you can move part of yourself anywhere by having the other part go the other way. This is exactly how a rocket moves.
Similarly, if you can stretch yourself out, you can get as far as you can stretch. But again, your center of mass stays in the same place, so any stretching should ideally involve stretching in both directions.
 
Farting could work.

I wondered something similar in trying to figure out how 'scootching' works.
You can propel a canoe in still water without a paddle, by lurching forward, and slowly coming back. It doesn't seem like it should work, but it does.
 
Vorpal said:
I suppose if the organism has a lot of time on its hands, changing shape in order to make thermal radiation pressure uneven would work in a vacuum in a very, very limited way. But yes, momentum conservation is a harsh mistress.
Click to expand...
.
A solar sail.
 
quarky said:
Farting could work.

I wondered something similar in trying to figure out how 'scootching' works.
You can propel a canoe in still water without a paddle, by lurching forward, and slowly coming back. It doesn't seem like it should work, but it does.
Click to expand...
.
Farting loses mass.
Expanding to a sail lets the solar wind push you.
 
quarky said:
I wondered something similar in trying to figure out how 'scootching' works.
You can propel a canoe in still water without a paddle, by lurching forward, and slowly coming back. It doesn't seem like it should work, but it does.
Click to expand...

Resistance doesn't scale evenly with speed.
 
Ziggurat said:
Actually, it IS possible to "swim" in a gravitational field without pushing against anything, just by changing shape. Essentially what you're doing is radiating gravitational waves. But it's horrifically inefficient, and the effects would be far too small at non-relativistic speeds to make any noticeable difference.
Click to expand...

Since you are radiating gravitational waves, are you not also radiating energy and losing mass in the process? I would say a loss of mass does not equate to a strict "change of shape."

By the same token, a body could also radiate EM waves by changing its internal charge distribution in an appropriate way, and move without "pushing against anything." Again, energy is being radiated away and a loss of mass would result. This would be much more efficient than radiating gravitational waves as the EM force is much stronger.
 
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