The dancer astronaut

[michel123456]

Reminding the Mach's principle that stated that:

You are standing in a field looking at the stars. Your arms are resting freely at your side, and you see that the distant stars are not moving. Now start spinning. The stars are whirling around you and your arms are pulled away from your body. Why should your arms be pulled away when the stars are whirling? Why should they be dangling freely when the stars don't move?

I have enhanced in bold the part I'd like to discuss.

Imagine you are an astronaut lost in deep space

And you want to spin, like a dancer executing a pirouette

Without a floor, without a grip anywhere, can you execute a pirouette? Ad if Yes (like an acrobatic diver), then will your arms be pulled away by the laws of physics?

(last picture from https://prezi.com/c_0irhrx1vpo/pirouette-the-aspects-of-physics/

 

Edited March 23, 2018 by michel123456

[Strange]

17 minutes ago, michel123456 said:

I have enhanced in bold the part I'd like to discuss.

OK. How do you start spinning?

Quote

Why should your arms be pulled away when the stars are whirling? 

https://en.wikipedia.org/wiki/Centrifugal_force

17 minutes ago, michel123456 said:

Without a floor, without a grip anywhere, can you execute a pirouette?

No. You need to push against something.

p.s. Mach's principle seems so vague as to be useless. I suppose there may be some way of relating the total mass distribution in the universe to a local frame of reference, but I have no idea what it is or if it is useful.

Edited March 23, 2018 by Strange

[koti]

3 minutes ago, Strange said:

OK. How do you start spinning?

You could take your helmet off and use it as leverage to start spinning.

[Raider5678]

1 hour ago, koti said:

You could take your helmet off and use it as leverage to start spinning.

Or RCS.

[koti]

38 minutes ago, Raider5678 said:

Or RCS.

RCS is fuel based propulsion used in spcecraft, the one in space suits is compressed gas based propulsion:

https://en.m.wikipedia.org/wiki/Astronaut_propulsion_unit

[John Cuthber]

One way to start spinning would be to punch a hole in your space suit so the air rushes out.

Les lethal options would also ne possible.

 

Imagine doing a similar experiment, but in a big black box floating in space.

You start spinning and you notice that your arms get flung out to the sides because you are spinning, but because of the box, you can't see the stars.
OK, stop the spinning .

Now imagine that there are "stars " painted on the box  in luminous paint and that, while you are not spinning, someone sets the box rotating round you- your arms don't suddenly get flung out by centrifugal force because you are not spinning.

 

You might recognise that the stars have nothing to do with it.

The forces on your arms are a function of your rotation.

 

[MigL]

Never mind the box, John.

What if you spin the universe around the astronaut ?
Do his arms still fly out to the sides as they would if he was spinning ?
( this question has always fascinated me )

[dimreepr]

1 hour ago, MigL said:

Never mind the box, John.

What if you spin the universe around the astronaut ?
Do his arms still fly out to the sides as they would if he was spinning ?
( this question has always fascinated me )

Good question, what happens if he tucks his arms in?

[koti]

2 hours ago, MigL said:

Never mind the box, John.

What if you spin the universe around the astronaut ?
Do his arms still fly out to the sides as they would if he was spinning ?
( this question has always fascinated me )

That is interesting. The universe would have to have a frame of reference to which it would be spinning in relation to. If somehow we would establish that frame, I guess we could treat the astronauts rotation in realtion to the universe the same way as to any other object he’d be inside. But then again, the spacetime rotation is something I never thought about, first thing which comes to my mind is a reference frame but how can you have one.

[Bender]

I vote for passing a screwdriver (or anything else which doesn't result in death)  behind your back. That way you can easily stop spinning after the pirouette.

We just had a thread about absolute rotation. It ended with the assumption that it is possible for spacetime to rotate (as is the case around a spinning black hole) , and as a result it could theoretically be possible to see the stars spinning without your arms moving out.

As for the answer to the question what would happen if the universe was spun around you, I think the answer depends on whether all of spacetime is rotating, or only the stuff in it.

Then again,I don't think either makes any sense. The latter very quickly encounters speed of light issues. The former is meaningless as nothing would change. 

Edited March 24, 2018 by Bender

[michel123456]

14 hours ago, John Cuthber said:

One way to start spinning would be to punch a hole in your space suit so the air rushes out.

Les lethal options would also ne possible.

 

Imagine doing a similar experiment, but in a big black box floating in space.

You start spinning and you notice that your arms get flung out to the sides because you are spinning, but because of the box, you can't see the stars.
OK, stop the spinning .

Now imagine that there are "stars " painted on the box  in luminous paint and that, while you are not spinning, someone sets the box rotating round you- your arms don't suddenly get flung out by centrifugal force because you are not spinning.

 

You might recognise that the stars have nothing to do with it.

The forces on your arms are a function of your rotation.

 

I think that is the point.

If you try to spin yourself without any mechanical device, you may use your arms and the entire upper part of your body to begin spinning. As would do an ice skater. In this case your arms will use their own weight (momentum) for spinning the rest of the body. And as a skater, once spinning, you would pull by your own force your arms close to the rotational axis of your body (in order to spin faster)

All this has nothing to do with the rest of the Universe IMHO. The stars around you play no role.

The rotation is a relative motion, from a state at time stamp zero to another state at time stamp 1, then at time stamp 2, etc.

The rotational transformation from one time stamp to another is an acceleration (because there is a change in direction). And the stars around play no role in this. IMHO.

 

Edited March 24, 2018 by michel123456

[Bender]

3 hours ago, michel123456 said:

If you try to spin yourself without any mechanical device, you may use your arms and the entire upper part of your body to begin spinning

That does not work in space. The ice skater has the ice to push against to start the rotation.

[koti]

10 hours ago, Bender said:

I vote for passing a screwdriver (or anything else which doesn't result in death)  behind your back. That way you can easily stop spinning after the pirouette.

I wonder if its possible to join hands behind your back in a space suit. Assuming it is, its a great idea...a few hundred gram tool could actually do the job. 

[Strange]

40 minutes ago, Bender said:

That does not work in space. 

Or maybe it does: http://web.mit.edu/wisdom/www/swimming.pdf

[J.C.MacSwell]

6 minutes ago, Strange said:

Or maybe it does: http://web.mit.edu/wisdom/www/swimming.pdf

You can reorientate yourself in space, but without pushing against something external or jettisoning something internal you can't change your angular momentum. So if you start with none in an isolated system that system will never have any.

[koti]

9 minutes ago, Strange said:

Or maybe it does: http://web.mit.edu/wisdom/www/swimming.pdf

Strange...Im picturing you performing fast body movements in zero gravity. On a serious note, it sounds interesting, I will save that article and read later. 

[John Cuthber]

I think we need to clarify that, while you are passing the screwdriver round yourself, you will rotate (a bit), but when you stop  passing it, you will stop moving.

You may be facing in a different direction at this point.

[koti]

5 minutes ago, John Cuthber said:

I think we need to clarify that, while you are passing the screwdriver round yourself, you will rotate (a bit), but when you stop  passing it, you will stop moving.

You may be facing in a different direction at this point.

Unless you do a few passes and throw the screwdriver away. Then you will keep on rotating. 

[swansont]

6 hours ago, michel123456 said:

The rotation is a relative motion, from a state at time stamp zero to another state at time stamp 1, then at time stamp 2, etc.

No it's not. If one of us is spinning, we can do physics to tell which one of us it is. 

[michel123456]

6 hours ago, Strange said:

Or maybe it does: http://web.mit.edu/wisdom/www/swimming.pdf

From my understanding of the paper, the conditions for it to work are rather extreme. Bender is right, in practice it doesn't work.

Quoting the pdf

 

Quote

 

Swimming on a frictionless plane cannot be

accomplished by cyclic deformation of shape,

but it is possible to swim on a frictionless

sphere. The plane has no intrinsic or extrinsic

curvature. The sphere has both. Therefore....

 

 

Quote

 

Swimming in curved spacetime.

A qua-

si-rigid body can swim on a manifold with

intrinsic curvature through cyclic deforma-

tions of shape. General relativity portrays

spacetime as a curved four-dimensional man-

ifold. Is it possible to swim in spacetime

through cyclic deformations?

In relativity, forces of constraint move with

finite velocity, so if one part of the system re-

ceives an impulse then there will be a delay in the

response of other parts of the system. Naturally

occurring bodies are not described by pure posi-

tional constraints. However, there is no obstacle

to engineering a quasi-rigid body that does main-

tain positional constraints, as long as the schedule

of deformations of the body is known sufficiently

in advance. In this case, the internal stresses that

are required to maintain the positional constraints

can be precomputed and prespecified, and then

executed simultaneously. The engineered quasi-

rigid body is choreographed for a particular

frame, which defines simultaneity. Ballet is not

Lorentz invariant...

 

"Ballet is not Lorentz invariant." Nice quote.

 

 

 

 

[MigL]

It cannot work.
Angular momentum is a conserved quantity.
If the system has none it will continue to have none.
the only way to change the situation is by external ( to the system ) effects, or by altering the system ( ejecting mass/energy ).

[Area54]

22 hours ago, dimreepr said:

Good question, what happens if he tucks his arms in?

The expansion of the universe stops and it all collapses towards a Big Crunch.