What if You Were Completely Still in Space? Exploring the Effects of Motion

[Nathan991]

The Earth is spinning and orbiting the Sun, and that is orbiting the center of the Milky Way, and that too moving in some way...

Say I were to hop in a spaceship and travel in such a way that it counters all of this motion, leaving me and the spaceship completely motionless,
would anything change? i.e. Does the speed at which we move through space affect our experience?

 

[Ibix]

The idea of "counter all of this motion" does not make sense, I'm afraid. There is no way to define a "fixed point in space" - you can only define motion relative to something else.

Basically, then, it doesn't matter what you do you have the same experience of the laws of physics. This is the point of the "principle of relativity" that underlies the theory of relativity - physics is the same for all observers who are not accelerating.

 

[etotheipi]

leaving me and the spaceship completely motionless

motionless with respect to what?

 

[Nathan991]

you can only define motion relative to something else.

motionless with respect to what?

Okay I get that, but is there then no posibility of a fixed point in space outside of what we can observe?

 

[Ibix]

Okay I get that, but is there then no posibility of a fixed point in space outside of what we can observe?

How would you even define a "fixed point in space"? There are no scratches in space, no markers you could use to recognise one point when you saw it again. You can only say you are stationary with respect to something you can see and touch, or bounce a radar pulse off or whatever.

 

[Will Learn]

Hi @Nathan991
The answers already given are good. There's no universal reference frame, so you should give up on trying to define anything as "motionless" unless you specify with respect to what.

What could you compare your motion against?
One of the more sensible things you could choose is the Cosmic Microwave background radiation (CMBR). You could try to adjust your motion so that the CMBR looked the same in every direction. This is a special class of motions that is (arguably) most like being motionless in space. It is described as being "co-moving".

would anything change?

The whole universe would be as symmetric as you can get it to be. Everything should be much the same in every direction. That sounds important but on a practical level, nothing would be noticeably different.

 

[Ibix]

That sounds important but on a practical level, nothing would be noticeably different.

Indeed. It's a larger scale version of being at rest on the Earth's surface. It's an interesting state of motion because it's in some sense the same as the average state of motion of most of the matter around you, but it's not special in any fundamental physics sense.

 

[synch]

Hmmmm. I am bit puzzled now. If a mass has no velocity...it has no momentum...so the de Broglie wavelength is infinite ie not bounded by the size of the universe...so the mass doesn't (edited - mainly)exist in the universe !
If the velocity is defined as relative to me...I can see several masses at the moment, and they are certainly not relatively moving to any great extent, and thay are existing just fine. I guess there is thermal movement, but that is possibly a different kettle of fish.

 

[phinds]

Hmmmm. I am bit puzzled now. If a mass has no velocity...it has no momentum

That is correct. If something is stationary relative to you it has no momentum relative to you. That same object DOES have momentum relative to me, however, if I am in motion relative to you. Momentum is frame dependent. That's been known for several hundred years. You're late to the party.

 

[cmb]

The Earth is spinning and orbiting the Sun, and that is orbiting the center of the Milky Way, and that too moving in some way...

Say I were to hop in a spaceship and travel in such a way that it counters all of this motion, leaving me and the spaceship completely motionless...

Well, if you were stationary with respect to the centre of our galaxy then I think you'd be moving at around 80km/s relative to everything else around you in the solar system, so it certainly wouldn't 'look' like you are stationary from where you are sitting.

If you were to be stationary wrt the solar system you're in, as you'd not be in a stable orbit, you'd get pulled in towards the Sun (and/or nearest planet/moon) and as you crash and burn up you can then reflect on why your quest for motionless ended up causing you to crash into something.

I mean, the only real world comparison is that being motionless in the middle of the highway is not actually a very safe place to be. Best to keep up with the motion of the solar system/rotation of the earth/etc. would be my advice. Keep up with the prevailing astronomical-body traffic speed.

[italic is edited text, per posts below]

 

[PeterDonis]

the de Broglie wavelength

This is the relativity forum, not the quantum physics forum. So the de Broglie wavelength is not an applicable concept for this thread.

 

[PeterDonis]

As you'd not be in a stable orbit, you'd get pulled in towards the Sun

No, you wouldn't; you'd be moving much faster than escape velocity from the Sun (assuming you were near the Earth), so you would most likely escape the solar system altogether. Only for a very narrow range of trajectories would you hit the Sun.

 

[cmb]

No, you wouldn't; you'd be moving much faster than escape velocity from the Sun (assuming you were near the Earth), so you would most likely escape the solar system altogether. Only for a very narrow range of trajectories would you hit the Sun.

Sorry, yes as this, I mis-edited a second sentence saying if one was 'stationary' in the solar system.

(Started off as falling towards centre of galaxy but then I thought it was pointless mentioning that, as everyone on board would die of old age, millions of years earlier than getting there).

 

[crastinus]

Maybe OP is also trying to get at a question like this:

Is everything always moving relative to something else?

I would think the answer is “yes”, if nothing else just based on the evidence of the history of the cosmos. There was a massive expansion to start it off, so it makes sense that everything would be in relative motion. I think it’s also true just from relativity theory, since there is no absolute motion and thus no absolute rest either.

 

[phinds]

I think it’s also true just from relativity theory, since there is no absolute motion and thus no absolute rest either.

That is why it is true. It has nothing to do with everything starting off with an "explosion" (which is itself nonsense, but that's another story). That has been known at least since the time of Galileo.

 

[Ibix]

Is everything always moving relative to something else?

The literal answer to this is no, since if I throw a ball upwards it returns - somewhere in there it must be stationary with respect to me even if only instantaneously. But it's certainly the case that you can only define motion with respect to something else, if that's what you mean. There is no absolute state of motion or rest in a relativistic universe.

 

[crastinus]

But even in the case of the ball being momentarily stationary with respect to me, it is still moving with respect to lots of other things, e.g. the sun, etc., right?

As to the expansion of the universe from the beginning to now, of course, it’s not just expansion in the ordinary sense, since space time itself expands. But it’s not as if everything else is stationary while space time expands. Large scale structures in the universe are moving relative to each other. So, it seems that one can appeal to the facts about cosmology in order to see another reason for why everything is moving relative to something else.

 

[Ibix]

But even in the case of the ball being momentarily stationary with respect to me, it is still moving with respect to lots of other things, e.g. the sun, etc., right?

Yes. I guess it depends how you read 'something else' in what I quoted.

space time itself expands

No - to the extent anything expands, it's space that expands not spacetime.

So, it seems that one can appeal to the facts about cosmology in order to see another reason for why everything is moving relative to something else.

Seems to me to be unnecessary. The idea that there's no detectable state of rest leads directly to either special relativity or Newtonian physics. We eliminate Newtonian physics by experiment, and we've never detected any violation of special relativity within its domain of applicability.

 

[synch]

Hmm. Can, then, a mass be stationary relative to the speed of light ?

 

[Ibix]

Hmm. Can, then, a mass be stationary relative to the speed of light ?

That doesn't make sense. Can a mass be stationary with respect to 30mph? Or 75mph? Or any other speed? What would that even mean?

 

[russ_watters]

Hmm. Can, then, a mass be stationary relative to the speed of light ?

Perhaps a different framing: light (and other things that travel at C) is always traveling at the speed of light with respect to local objects.

Note that when you describe something's speed with respect to something else, you're declaring the "something else" to be stationary.

 

[Grasshopper]

Didn’t someone write up a fantastic article on the principle of relativity? I could have sworn there was one here somewhere, and if I recall it went into some detail on Galileo’s ship thought experiment.

 

[synch]

Perhaps a different framing: light (and other things that travel at C) is always traveling at the speed of light with respect to local objects.

Note that when you describe something's speed with respect to something else, you're declaring the "something else" to be stationary.

Yes it was badly phrased. There is a point there though, my understanding (now that I have thought about it more !) was that speed is a scalar whereas velocity is a vector ie velocity has to have a located point of reference somehow whereas speed can simply be a numeric and can be numerically compared to 0 even if it cannot be be measured or determined absolutely.
So I guess there is a boundary condition, it may seem idiotic but is not, that an entity's speed/velocity wrt itself is defined as 0. So it can never have momentum wrt itself ? How can it then have wavelength ?
(edit with aoplogies) OK so that is for a different forum.

 

[russ_watters]

Yes it was badly phrased. There is a point there though, my understanding (now that I have thought about it more !) was that speed is a scalar whereas velocity is a vector ie velocity has to have a located point of reference somehow whereas speed can simply be a numeric and can be numerically compared to 0 even if it cannot be be measured or determined absolutely.

Speed is just the magnitude of velocity. It's what you use when you don't care about direction or describing it would be problematic/pointless - it doesn't mean there isn't a direction that you could use if desired. And this "even if it cannot be measured or determined absolutely" bit -- you just have to discard it. There is no such thing as absolute speed/velocity. It's not a matter of difficulty in measuring it. It just plain isn't a thing.

So I guess there is a boundary condition, it may seem idiotic but is not, that an entity's speed/velocity wrt itself is defined as 0. So it can never have momentum wrt itself ? How can it then have wavelength ?
(edit with aoplogies) OK so that is for a different forum.

How can what have wavelength? An object like a person? That's not a thing either (classically).

 

[Ibix]

There is a point there though, my understanding (now that I have thought about it more !) was that speed is a scalar whereas velocity is a vector ie velocity has to have a located point of reference somehow whereas speed can simply be a numeric and can be numerically compared to 0 even if it cannot be be measured or determined absolutely.

No. Speed is just the magnitude of velocity and if you can't define velocity without a reference then you can't define speed without a reference.

You may be being confused by sources that comment that Lorentz scalars are frame independent. Speed isn't a Lorentz scalar. Speed relative to a specified reference is, but once again you need that reference.

 

[synch]

Yes that is sensible. [ I think I lost the plot there somewhere :( ]