# The Naked Scientists Forum

### Author Topic: How fast is "stopped"???  (Read 2559 times)

#### Teakhat

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##### How fast is "stopped"???
« on: 05/04/2015 12:35:15 »
What vector and speed would be needed to "stop" dead in space? Once there, how would physics be perceived?

#### Ethos_

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##### Re: How fast is "stopped"???
« Reply #1 on: 05/04/2015 15:03:25 »
What vector and speed would be needed to "stop" dead in space? Once there, how would physics be perceived?
All motion in space is frame dependent, this means one can only know their relative velocity compared to another object within their area of observation. It is impossible to calculate velocity relative to empty space. It is however possible to calculate acceleration and deceleration by measuring inertial forces upon the body in question. But determining a position as "dead in space" as motionless relative to space itself is impossible.

#### PmbPhy

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##### Re: How fast is "stopped"???
« Reply #2 on: 05/04/2015 15:14:54 »
What vector and speed would be needed to "stop" dead in space? Once there, how would physics be perceived?
What Ethos said is quite correct. It's actually a law from the special theory of relativity known as the Principle of Relativity defined here:
http://en.wikipedia.org/wiki/Principle_of_relativity

#### Ethos_

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##### Re: How fast is "stopped"???
« Reply #3 on: 05/04/2015 15:26:57 »
What vector and speed would be needed to "stop" dead in space? Once there, how would physics be perceived?
What Ethos said is quite correct. It's actually a law from the special theory of relativity known as the Principle of Relativity defined here:
http://en.wikipedia.org/wiki/Principle_of_relativity
Thanks Pete for providing that link, good source material is always very helpful and Wiki is usually an excellent source.

#### jeffreyH

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##### Re: How fast is "stopped"???
« Reply #4 on: 05/04/2015 17:42:11 »
To overcome this you could suggest, erroneously, that as all galaxies are receding away from earth, that earth is stationary. However, if you move to a point far away from earth then the same rate of expansion will be observed so that this new position will now appear as stationary. Since space itself is expanding there is no fixed stationary reference point. If you consider the universe as infinite then that excludes any fixed stationary point at all as you would need to divide infinity in half to find it, which is not possible as infinity is not a number.

#### alancalverd

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##### Re: How fast is "stopped"???
« Reply #5 on: 05/04/2015 18:09:37 »
There being no origin, and all motion being relative, you can consider yourself stationary and everything else to be in motion around you. The starting point of relativity (hence its name) is that physics is the same from the point of view of every observer, so we have to derive laws of physics that are invariant between moving observers.

#### jeffreyH

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##### Re: How fast is "stopped"???
« Reply #6 on: 05/04/2015 18:23:41 »
There being no origin, and all motion being relative, you can consider yourself stationary and everything else to be in motion around you. The starting point of relativity (hence its name) is that physics is the same from the point of view of every observer, so we have to derive laws of physics that are invariant between moving observers.

Exactly and hence frames of reference.

#### Colin2B

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##### Re: How fast is "stopped"???
« Reply #7 on: 08/04/2015 13:56:21 »
............ Once there, how would physics be perceived?
I assume you have read and understood the above replies.

If you are in a spaceship and are not accelerating, even though you may be moving, you will find that the laws of physics are the same as in any other spaceship which is not accelerating. The fact that the other spaceship may appear to be passing you will not change things. In general the laws of physics are the same as we are familiar with, newtons laws, thermodynamics, electromagnetism etc.
If you are accelerating that is a wholly different can of worms - assuming you have some worms with you

#### Teakhat

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##### Re: How fast is "stopped"???
« Reply #8 on: 14/04/2015 11:32:56 »
So...nobody can answer my question, then.......

#### Ethos_

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##### Re: How fast is "stopped"???
« Reply #9 on: 14/04/2015 13:26:57 »
So...nobody can answer my question, then.......
Not true, your question was answered. One can not ever consider themselves stopped relative to the essence of space itself.

In the first place, the question; "How fast is stopped?" is a contradiction in terms. To assume any object has any velocity while being simultaneously at rest is illogical.

Furthermore, the word "fast" denoting velocity, has no association with the word "stopped" and therefore could not have any reasonable proportionality to it when used in the same sentence.

« Last Edit: 14/04/2015 15:31:04 by Ethos_ »

#### jeffreyH

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##### Re: How fast is "stopped"???
« Reply #10 on: 16/04/2015 00:15:10 »
So...nobody can answer my question, then.......

How are you going to determine that you have actually stopped? You need a fixed reference point that is also stationary. Since space is expanding you can't use that. The dimensions of space are changing dynamically. You are always in motion relative to other objects. Also, why would you consider that the laws of physics would be any different? You first need to clarify why you think this may be true.

#### PmbPhy

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##### Re: How fast is "stopped"???
« Reply #11 on: 16/04/2015 22:04:59 »
Quote from: Teakhat
So...nobody can answer my question, then.......
No. That's not what we've said. We have answered your question. It's just that the answer is not what you might have imagined.

Quote from: Teakhat
What vector and speed would be needed to "stop" dead in space?
This is an odd question in itself. Suppose you asked us What vector and speed would be needed to "stop" dead on the highway?  Stopping is not a question of speed but a question of acceleration. Had you asked what acceleration would be required to stop your car if it was moving at 90 mph within a time period of 30 seconds. Then we'd be able to answer that.

#### The Naked Scientists Forum

##### Re: How fast is "stopped"???
« Reply #11 on: 16/04/2015 22:04:59 »