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banquo's_bumble_puppy
2015-Jun-13, 08:07 AM
Is it possible to make a spacecraft "stand still" in space? If so, would the natural expansion of space in effect cause the craft to move? I suspect that this would take a lot of energy and would probably be highly inefficient, or unworkable.

Noclevername
2015-Jun-13, 10:13 AM
Since all motion is relative, no, not the way you're describing it.

Expansion of space does make objects "move" relative to distant galaxies, that's why they are red-shifted. Nearer galaxies are gravitationally bound and thus orbit each other in complex paths, and stars rotate around the galactic centers. A hypothetical non-gravitationally bound non-rotating object would still see the universe moving away from it, and not "move".

banquo's_bumble_puppy
2015-Jun-13, 10:29 AM
Since all motion is relative, no, not the way you're describing it.

Expansion of space does make objects "move" relative to distant galaxies, that's why they are red-shifted. Nearer galaxies are gravitationally bound and thus orbit each other in complex paths, and stars rotate around the galactic centers. A hypothetical non-gravitationally bound non-rotating object would still see the universe moving away from it, and not "move".

Thanks! You always come through with an answer.

Cougar
2015-Jun-13, 06:10 PM
Is it possible to make a spacecraft "stand still" in space?
...all motion is relative...

This is the main point. "Stand still" relative to what? There is no "absolute space" that Newton postulated. However, one may choose any convenient frame of reference and describe all other motions relative to that, understanding that there is no one choice that is preferred over another.

cjameshuff
2015-Jun-13, 07:15 PM
This is the main point. "Stand still" relative to what? There is no "absolute space" that Newton postulated. However, one may choose any convenient frame of reference and describe all other motions relative to that, understanding that there is no one choice that is preferred over another.

Note that there is a frame (the "comoving frame") where the doppler shift of the cosmic background is equal in all directions, but this is basically the frame an observer would have if they hadn't accelerated since the universe became transparent...not truly "absolute" in any way.

The laws of physics behave the same in every inertial frame. Locally, if you are not accelerating, you are at rest. Objects in inertial frames only have velocities in relation to objects in other frames. Put another way, velocity is a relationship between frames, not a property of objects.

grapes
2015-Jun-13, 11:52 PM
Is it possible to make a spacecraft "stand still" in space? If so, would the natural expansion of space in effect cause the craft to move? I suspect that this would take a lot of energy and would probably be highly inefficient, or unworkable.
Along with the other answers, your suspicion about inefficiency is well-founded.

The Hubble constant is about 68 (km/s)/Mpc. That 68 km/s makes it look rather large, but remember Mpc stands for Megaparsec, a very large unit, about 3.3 million lightyears!

So, if something were half way to the nearest star from us, about two light years (the farther away, the greater the effect), its velocity as a result of the assumed expansion would be 2 ly x 68 km/s / 3.3 million ly, or about .15 km per hour

Probably could achieve that just by huffing and puffing. :)

glappkaeft
2015-Jun-15, 09:12 PM
This is the main point. "Stand still" relative to what? There is no "absolute space" that Newton postulated. However, one may choose any convenient frame of reference and describe all other motions relative to that, understanding that there is no one choice that is preferred over another.

Indeed the notion of relative moment is vital even to Newtonian mechanics and was first published 10 years before Newton was born by that Italian guy.

Noclevername
2015-Jun-15, 10:09 PM
Indeed the notion of relative moment is vital even to Newtonian mechanics and was first published 10 years before Newton was born by that Italian guy.

There have been a lot of Italian guys. Any particular one?

glappkaeft
2015-Jun-15, 11:10 PM
There have been a lot of Italian guys. Any particular one?

He's quite famous. ;)

The Italian Guy (https://en.wikipedia.org/wiki/Galileo_Galilei)
His Book (https://en.wikipedia.org/wiki/Dialogue_Concerning_the_Two_Chief_World_Systems)
His Idea (https://en.wikipedia.org/wiki/Galilean_invariance)

grapes
2015-Jun-16, 02:20 AM
Galilean relativity

Jens
2015-Jun-16, 05:11 AM
He's quite famous. ;)


You mean the guy who's in the Queen song?

Cougar
2015-Jun-16, 12:59 PM
Put another way, velocity is a relationship between frames, not a property of objects.

That's a good way to put it.

publiusr
2015-Jun-19, 10:29 PM
I'd like to see a statite directly above one of the Sun's poles.

You could see all the solar system at once, hovering there.