Swimming in space, it is a possible propulsion?

[Trikenstein]

TL;DR Summary

In the void of space, is it possible to use own limbs to generate kinetic energy as propulsion?

Let's imagine a space tourism program where there is a fun part, the tourist can wander freely in space with a jetpack.

Let's assume I stray randomly in space, floating leisurely in zero gravity. Then I detach my jetpack, just the time to make a selfie. While fiddling with the phone to setup the camera, I forgot to hold the jetpack which then drifts slowly, roughly 50 meters away from me.

Alone in space and far from the group. The jetpack is the only way to get back to the space station. Is it possible that I swim in space to catch up with the jetpack? The swim stroke I think of is a kind of half-breast stroke. The legs stay immobile, only extend the arms quickly in front (towards the jetpack I suppose). Then slowly bring back the hands under the chin, and repeat again the quick arm extension. Swimming my way to the jetpack.

It is possible to create propulsion by self-generating momentum with own limbs? In case it is possible, what would be the most efficient way to "swim" in space?

 

[phinds]

It is possible to create propulsion by self-generating momentum with own limbs?

No, not even remotely. You need to study the basics. Start with Newton's Laws.

 

[Ibix]

It is possible to create propulsion by self-generating momentum with own limbs?

No. You'd have to violate the conservation of momentum to do this.

Similar schemes are possible on Earth, even in vacuum, but they all utilise friction forces between you and the ground. And similar schemes are available in air in zero g, but they utilise momentum transfer to the air just like swimming in water. Neither strategy is available in vacuum in zero g, though.

Throw your phone hard and directly away from the jetpack, or radio for assistance.

 

[A.T.]

Throw your phone hard and directly away from the jetpack

Or release some of your pressurized oxygen in the right direction.

 

[Vanadium 50]

Or release some of your pressurized oxygen in the right direction.

Is it wrong of me to think of the campfire scene in Blazing Saddles?

I agree with the others. The key phrase is 'self generated momentum", No good. Momentum is conserved.

 

[Rive]

Swimming in space, it is a possible propulsion?

Given that 'space' is not perfect vacuum there may be some loopholes for generating some negligible thrust, but for the described scenario with human 'equipment' the answer is a solid screaming 'NO'.

 

[kuruman]

I forgot to hold the jetpack which then drifts slowly, roughly 50 meters away from me.

Literally a bad move. I agree with what others have said and have this to add. Both you and the jetpack will be drifting away from the point in space where you lost contact. Furthermore, the jetpack will not stop at 50 m but keep on going because there is nothing to stop it.

Removing your jetpack in free space to take a selfie reminds me of this

 

[Ibix]

Removing your jetpack in free space to take a selfie reminds me of this

I guess you might want to be able to remove an expended jetpack, but it's definitely something that should need several intentional actions to perform, including accepting "you'll probably die if you do this" warnings.

 

[Trikenstein]

Some thought experiments to clarify about the self-generated momentum.

Experiment1: Attach a rock with a rope. Tie the rope around your waist. Stand on the ground and throw the rock hard in front of you. What would happen to the position of body+rock relative to the ground?

Experiment2: You lay flat on the bed, looking to the ceiling. You want to sit up (ie. legs on bed, body perpendicular to the bed). Some could try to muscle their abs and could sit up naturally. However if you bend your legs and kick hard to extend the legs, then the sit up is much easier. Because the body kind of benefit from the momentum, moving in the same direction than the kicking motion.

In swimming, two swimmers identical in every aspect. About to finish at the same time. However, at the finish line, the one who throws the arm faster in front will touch the wall a fraction of a second faster. What is the Newton law principle that could explain this?

 

[Trikenstein]

I guess you might want to be able to remove an expended jetpack, but it's definitely something that should need several intentional actions to perform, including accepting "you'll probably die if you do this" warnings.

Please be lenient. The silliness of the story is just to set the stage for the "swimming in space" question. ie. is there a way to self-propulse in space in case of emergency.

 

[Ibix]

Experiment1: Attach a rock with a rope. Tie the rope around your waist. Stand on the ground and throw the rock hard in front of you. What would happen to the position of body+rock relative to the ground?

The rock goes one way; the body goes the other; the center of mass does not move.

Experiment2: You lay flat on the bed, looking to the ceiling. You want to sit up (ie. legs on bed, body perpendicular to the bed). Some could try to muscle their abs and could sit up naturally. However if you bend your legs and kick hard to extend the legs, then the sit up is much easier. Because the body kind of benefit from the momentum, moving in the same direction than the kicking motion.

This is one of the "utilising friction" strategies I mentioned in #3 (it also involves a reaction force from the bed - again not something available in zero g).

In swimming, two swimmers identical in every aspect. About to finish at the same time. However, at the finish line, the one who throws the arm faster in front will touch the wall a fraction of a second faster. What is the Newton law principle that could explain this?

They are reaching in front of themselves; they are slightly slowed by that but that's all. The relevant point about swimming is that a swimming motion pushes the water backwards. There's no water to push in space.

is there a way to self-propulse in space in case of emergency.

No, as literally everyone has said already.

 

[Vanadium 50]

The silliness of the story is just to set the stage

This is a bad start. If you can ask a question without silliness, please do so. It is respectful.

Some thought experiments to clarify about the self-generated momentum.

"Thought experiment" is also kind of off-putting. And, as pointed out, there is no suchg thing as self-generated momentum. If you are uncertain about this, and can ask questions about it without introducing what you called "silliness", please do so.

 

[kuruman]

is there a way to self-propulse in space in case of emergency.

The answer is no, not without something to push against by expelling it. That's how the jetpack works. It expels gases in one direction and you move in the opposite direction. In essence you push against the gases and the gases push against you. In posts #3 and #4 suggestions for expelling something are offered.

 

[jbriggs444]

In swimming, two swimmers identical in every aspect. About to finish at the same time. However, at the finish line, the one who throws the arm faster in front will touch the wall a fraction of a second faster.

The one who throws the arm faster has more time to finish his final stroke. Otherwise, there is no advantage to be gained.

In fact, given quadratic drag, extending the arm too rapidly would result in more net push away from the wall than a slower extension.

 

[berkeman]

The silliness of the story is just to set the stage for the "swimming in space" question

Back at cha'!

 

[A.T.]

the one who throws the arm faster in front will touch the wall a fraction of a second faster.

The analogy in space would be throwing the arm so fast that you can still grab the escaping jet pack. Nothing to do with propulsion.

 

[Trikenstein]

This is a bad start. If you can ask a question without silliness, please do so. It is respectful.

"Thought experiment" is also kind of off-putting. And, as pointed out, there is no suchg thing as self-generated momentum. If you are uncertain about this, and can ask questions about it without introducing what you called "silliness", please do so.

Sir, I am trying to understand something. I ask the question the way I see it. I don't target anyone and I try to better visualize the concept with some examples to give a context. I am sorry if the way I formulate the questions could offend someone. Although I sincerely think that my questions are polite.

Now that you know exactly the context of my question. After the fact, you find the question not elegant because of its silly portion. If I started the question drily as "it is possible to move forward by swimming in space" ? Then someone or maybe even you would wonder "why do someone would want to do such a thing in space?" It's hard to please everybody. Is it OK to forgive some clumsiness for this time?

I accept any opinion, even when someone answering off-topic with the sawing of own branch. I understand the funny implication of the answer and didn't feel offended at all.

 

[Haborix]

I guess you could flail around a lot to heat up your body and hope that the your suit will asymmetrically radiate electromagnetic waves preferentially in a direction opposite the object of your desire. Although, you'd certainly run out of oxygen before that got you anywhere...

 

[DaveC426913]

> It is possible to create propulsion by self-generating momentum with own limbs?

Technically yes, but you can only do it a maximum of four times and you'll need

• a hacksaw,
• a self-sealing suit
• a lot of pain meds.

 

[jbriggs444]

Reactionless drives are on the list of forbidden topics at PF. This would appear to include attempts using body gyrations.

 

[Trikenstein]

Gentlemen, I do understand (at least I hope) Newton laws and the principle of conservation of momentum. And of course, I know the solutions you recommended: like throwing a mass to the opposite direction, expelling gases. Those are somehow simple to "visualize", It is possible to calculate the resulting speed. Just need to know the mass and speed of the object which is ejected (leaving the body) in the opposite direction.

I am not trying to promote the violation of the laws of physics. It's just somehow, my intuition deceived me when I imagined that swimming in space scenario. Here is what bothered me:

The movement which is self-generated is asymmetric:

• Phase1: Throwing the arms fast forward, let's say V1
• Phase2: Recovering the arms slowly V2.

Let's assume V1 = 5*V2. As the kinetic energy is v squared. And the center of gravity does change between Phase1 and 2. Would that results in some kind of imbalance?

Is it correct to say whether Phase1 or 2, whatever the arms do, the body recoils in a proportion so that there is no net displacement?

The question is about swimming straight here but could be as well swinging one arm to try to spin the body. It's just that it's unintuitive to be convinced that nothing will happen.

 

[Vanadium 50]

On the one hand, you have a bunch of messages showing how conservation of momentum prevents what you describe.

On the other, you say it disagrees with your intuition.

I'd say that this tells you that your intuition is wrong. I know you said this hurts your feelings, but we don't have many alternatives. Wanting something to be correct is not the same as it actually being correct.

 

[Ibix]

And the center of gravity does change between Phase1 and 2

No it does not. It cannot without violating the conservation of momentum.

However you slice it, if you move a mass ##m## forward a distance ##l## (the arms) then a mass ##M## must move a distance ##L## (the body) in the opposite direction so that ##ml-ML=0##. Then when you move the arms back through ##l## the body will move forward ##L## and you are back where you started. The speed with which you do this isn't relevant except to the time taken.

The reason speed matters on Earth is that you can use a large force for a short time that overcomes static friction between you and the floor, so you move. Then you use a small force for a long time that does not overcome static friction and you do not move. Thus you can transfer momentum to the Earth and move yourself. But there is nothing to push against in space - no friction. So you move as much with the quick motion as the slow, and return to where you started.

 

[DaveC426913]

Is it correct to say whether Phase1 or 2, whatever the arms do, the body recoils in a proportion so that there is no net displacement?

Correct. No net displacement.

The question is about swimming straight here but could be as well swinging one arm to try to spin the body. It's just that it's unintuitive to be convinced that nothing will happen.

Yes, you can set yourself spinning. You centre of mass will remain stationary.

 

[Ibix]

The question is about swimming straight here but could be as well swinging one arm to try to spin the body.

Rotation is different. You can keep swinging your arm in a circle forever and your body will counterrotate forever. You could use this to reorient yourself.

You can only move yourself as long as something is moving in the opposite direction. Because you can swing your arm forever you can rotate forever. But you cannot keep extending your arms forever unless you detach them as @DaveC426913 suggests.

 

[berkeman]

The movement which is self-generated is asymmetric:

That is at the core of many Reactionless Drive mechanisms, which do not work. That is why they are on the list of forbidden topics here at PF (you can do a Google search of that topic to get more info). Per the PF rules, this thread is now closed.

EMDrive and other reactionless drives
See https://www.physicsforums.com/threads/nasas-em-drive.884753/

 

[PeterDonis]

I'm adding this post to provide some more information that might be useful.

the center of gravity does change between Phase1 and 2.

Here is where you go wrong. Your center of gravity does not change (with one tiny caveat that I'll describe further below). When you fling your arms out one way, the rest of you moves the other way. And when you draw your arms back in, the rest of you moves back in too. Your center of gravity never changes. That is what conservation of momentum requires. (You did raise the possibility of the rest of the body recoiling, but I'm not sure you fully realized that doing that means the center of gravity does not change at all.)

That said, you will find the term "swimming in space" used in the literature to describe a counterintuitive phenomenon in curved spacetime, as for example in these papers:

https://arxiv.org/pdf/2211.04654.pdf

https://web.mit.edu/wisdom/www/swimming.pdf

As these papers will tell you, in a curved spacetime it actually is possible in principle to displace yourself by making carefully chosen asymmetric movements. However, first, understanding how this works and why it still satisfies conservation of momentum is a matter for a separate "A" level thread (since this phenomenon requires advanced GR analysis, as you will see in the papers). And second, as the papers note, this phenomenon, with the amount of spacetime curvature available in the vicinity of the Earth, is about 24 orders of magnitude too small to help the OP move 50 meters to recover the jetpack. So it's entirely negligible, which is why the many good responses in this thread have quite correctly ignored it.

This thread will remain closed.