Can you determine absolute motion?

[rede96]

So what about this thought experiment?

I am in a ship at rest wrt to earth. In my ship I have a pendulum that can move in any direction. As I accelerate away I see my pendulum swing opposite to the direction I am moving.

At some point I switch my engine off and my pendulum centres again.

I am now in an inertial frame of reference and at rest wrt to myself.

I now decide to use my lateral thrusters to move at right angles to my current direction (wrt to earth). However, as I am in an inertial reference frame what I would expect to see is my pendulum swing in the exact opposite direction to my thrust. So if I put my left booster rocket on to 'turn' right, I would expect to see the pendulum move left.

However, I would suspect that as I have momentum, the path my ship will take would not be an immediate change in direction at right angles to my forward motion; I would move off at some angle to my intial direction.

So I wouldn't see my pendulum move to 270 degrees (assuming 0 degrees is forward) I would see it move to somewhere around 200 degrees say.

So I can deduce from my pendulum movement that I was not at rest prior to engaging my lateral rocket. If I was at rest, I would have seen my pendulum swing to 270 degrees.

Therefore when I accelerate I can always tell what momentum I had before I accelerated.

 

[jeppetrost]

So what about this thought experiment?

I am in a ship at rest wrt to earth. In my ship I have a pendulum that can move in any direction. As I accelerate away I see my pendulum swing opposite to the direction I am moving.

At some point I switch my engine off and my pendulum centres again.

I am now in an inertial frame of reference and at rest wrt to myself.

I now decide to use my lateral thrusters to move at right angles to my current direction (wrt to earth). However, as I am in an inertial reference frame what I would expect to see is my pendulum swing in the exact opposite direction to my thrust. So if I put my left booster rocket on to 'turn' right, I would expect to see the pendulum move left.

However, I would suspect that as I have momentum, the path my ship will take would not be an immediate change in direction at right angles to my forward motion; I would take a curved path until I was traveling at right angles to the initial direction.

So I wouldn't see my pendulum move to 270 degrees (assuming 0 degrees is forward) I would see it move to somewhere around 200 degrees say.

So I can deduce from my pendulum movement that I was not at rest prior to engaging my lateral rocket. If I was at rest, I would have seen my pendulum swing to 270 degrees.

Therefore when I accelerate I can always tell what momentum I had before I accelerated.

The short answer is: this is wrong. Even in Newtonian mechanics this doesn't hold.

 

[rede96]

The short answer is: this is wrong. Even in Newtonian mechanics this doesn't hold.

OK, can you explain why please? (If it was the curved path bit, I changed that.)

 

[jeppetrost]

OK, can you explain why please?

Well, you can boost to the frame, which is moving and everything would look as if you were standing still - even if you were going at some high velocity. Point is, relative to you, the pendulum would go left if you accelerate right.

 

[HallsofIvy]

So what about this thought experiment?

I am in a ship at rest wrt to earth. In my ship I have a pendulum that can move in any direction. As I accelerate away I see my pendulum swing opposite to the direction I am moving.

At some point I switch my engine off and my pendulum centres again.

I am now in an inertial frame of reference and at rest wrt to myself.

I now decide to use my lateral thrusters to move at right angles to my current direction (wrt to earth). However, as I am in an inertial reference frame what I would expect to see is my pendulum swing in the exact opposite direction to my thrust. So if I put my left booster rocket on to 'turn' right, I would expect to see the pendulum move left.

However, I would suspect that as I have momentum, the path my ship will take would not be an immediate change in direction at right angles to my forward motion; I would take a curved path until I was traveling at right angles to the initial direction.

So I wouldn't see my pendulum move to 270 degrees (assuming 0 degrees is forward) I would see it move to somewhere around 200 degrees say.

This is wrong. The pendulum, before the second acceleration, is at rest with respect to your ship. If you apply a force at right angles to the motion of the ship (at 270 degrees) the pendulum will move in that direction. Of course, an observer with respect to whom the ship is moving- that is an observer who was at rest with respect to the ship before the first acceleration- would see that the ship and pendulum were not making a 270 degree move but at an angle. But that is internal to the ship.

So I can deduce from my pendulum movement that I was not at rest prior to engaging my lateral rocket. If I was at rest, I would have seen my pendulum swing to 270 degrees.

Therefore when I accelerate I can always tell what momentum I had before I accelerated.

No, you cannot. The pendulum, as well as any object inside or attached to the ship, share in the ship's motion and does not have any difference in motion relative to the ship that can be observed.

That does not require "relativity"- Gallileo knew that. The problem of relativity appeared when Maxwell's equations for electro-magnetic fields seemed to show that the magnetic field inside a moving object depended upon the speed of the object so that some kind of electro-magnetic experiment could determine "speed" in some absolute sense. That was what the Michaelson-Moreley experiment was intended to determine.

 

[rede96]

This is wrong. The pendulum, before the second acceleration, is at rest with respect to your ship. If you apply a force at right angles to the motion of the ship (at 270 degrees) the pendulum will move in that direction. Of course, an observer with respect to whom the ship is moving- that is an observer who was at rest with respect to the ship before the first acceleration- would see that the ship and pendulum were not making a 270 degree move but at an angle. But that is internal to the ship.

OK, I may be being a bit simple here so please bear with me!

If I jet off in any particular direction and then apply some force to move at right angles to that direction, surely I can't 'instantly' do this? Otherwise I would always be traveling around in straight lines? (wrt to me!)

EDIT: OK, I think I see where I was confusing myself. I'm not accelerating anymore, so pendulum moves in the direction of force. *Doh!* Well it's been a long week.

So does that mean that in my frame of reference, I am always traveling in straight lines?

 

[Dale]

So does that mean that in my frame of reference, I am always traveling in straight lines?

In your frame of reference you are not moving at all, by definition.

 

[Dale]

Also I am still trying to find out how (and what) experiments were done that prove that Light approaches any reference point at C instead of C-V.

see the FAQ:

http://www.edu-observatory.org/physics-faq/Relativity/SR/experiments.html#moving-source_tests

 

[rede96]

In your frame of reference you are not moving at all, by definition.

Whilst I don’t understand relativity in any great depth I do get the gist of it. I accept that in the context of relativity it is perfectly fine to say that there is no absolute motion.

However if I am in a spaceship and I turn on my thrusters I know through various laws of physics that I can say with 100% certainty that the result of my thrusters will lead to acceleration and thus motion.

So I think it is a perfectly valid statement in the real world to say that I am in motion without it having any effect on the principles of relativity.

I obviously can’t say that I am traveling at any given speed as that is relative to other frames of reference.

Moreover, I would also agree that it is impossible for anyone to say that “I am at rest.” I know of some laws of physics that will predict motion as an outcome but I don’t know of any laws of physics that state that the outcome is that you will be ‘at rest’.

So I would argue that as there is no absolute rest frame, then by default all frames are moving.

But it is only possible to quantify the movement wrt other frames of reference.

However, suppose there was some way to map all movement / acceleration from the point of the big bang and to calculate the entire relative effects caused. I would then be able to have an absolute frame of reference wrt to origin of the big bang both in terms of motion and time.

 

[Dale]

It has nothing to do with relativity. When you say "your frame" you mean "a frame where you are at rest". Therefore, by definition, you do not move at all in your frame since that is how your frame is defined.

Obviously, if you turn on thrusters then your frame is a non-inertial frame and there will be fictitious forces in it. If you turn them on and off then the fictitious forces will be time varying, etc.

 

[rede96]

It has nothing to do with relativity. When you say "your frame" you mean "a frame where you are at rest". Therefore, by definition, you do not move at all in your frame since that is how your frame is defined.

a frame where I am at rest wrt to what? I can't say I am at rest.

If I am at rest wrt to the Earth and then turn my thrusters on for a short burst and head off in some direction, relativity says that it is just as valid to say that the Earth is moving and I am not. I am not disputing this.

What I am saying is that common sense would prevail, it is obvious that I am the one moving away from the Earth and that the whole of the universe is not moving wrt to me.

This doesn’t invalidate either statement (I.e. the Earth is moving wrt to me or I am moving wrt to earth.) It just simply puts the movement in context.

In fact I would argue that it is impossible for the whole visible universe to suddenly start to move wrt to me and that I am at rest. That would suggest that the universe was finite.

 

[Dale]

a frame where I am at rest wrt to what? I can't say I am at rest.

At rest wrt yourself. In your reference frame all velocities and positions are measured wrt you. And yes, you can always say that you are at rest wrt yourself.

 

[1MileCrash]

What I am saying is that common sense would prevail, it is obvious that I am the one moving away from the Earth and that the whole of the universe is not moving wrt to me.

Nope, that is not common sense. There is no "physical reality" to this, it's relative.

In other words, what if I defined a reference frame in which the Earth was moving at a constant speed equal to the speed of your ship relative to the Earth but in the exact opposite direction? That implies that relative to that reference, starting your ship made you stop, and the Earth kept moving away from you, and this point of view and reference is just as valid as any other!

 

[rede96]

Nope, that is not common sense. There is no "physical reality" to this, it's relative.

In other words, what if I defined a reference frame in which the Earth was moving at a constant speed equal to the speed of your ship relative to the Earth but in the exact opposite direction? That implies that relative to that reference, starting your ship made you stop, and the Earth kept moving away from you, and this point of view and reference is just as valid as any other!

This is getting silly. Of course there is a physical reality, although I suspect not everyone lives in it. lol

IF the Earth had moved away from me what would I see? I would see it leaving the moon behind for starters.

But when I look out my window, I still see the moon in the same orbit. So the moon must be moving with the Earth too. But then I see the Earth in the same orbit relative to all the other planets too. As would any observer.

So that must mean that the galaxy is moving too, but I see the solor system in the same relative distance from other objects in the galaxy, and the galaxy the same relative distance from other galaxies and so on.

Therefore, either I moved away from the Earth or the whole universe moved wrt to me, which is impossible.

Also, when I turn around and go back to Earth I find that I am younger than my twin brother. Why? Because I am the one that went through the acceleration, just as the twin paradox predicts.

So I can say with absolute certainty that I moved away from the earth.

All this does is put the movement in context. It does not dismiss SR or GR in anyway.

 

[Dale]

IF the Earth had moved away from me what would I see? I would see it leaving the moon behind for starters.

But when I look out my window, I still see the moon in the same orbit. So the moon must be moving with the Earth too. But then I see the Earth in the same orbit relative to all the other planets too. As would any observer.

So that must mean that the galaxy is moving too, but I see the solor system in the same relative distance from other objects in the galaxy, and the galaxy the same relative distance from other galaxies and so on.

Therefore, either I moved away from the Earth or the whole universe moved wrt to me, which is impossible.

There is nothing wrong with using a reference frame where the whole universe is moving.

Also, when I turn around and go back to Earth I find that I am younger than my twin brother. Why? Because I am the one that went through the acceleration, just as the twin paradox predicts.

Yes, proper acceleration is absolute, even in a non-inertial reference frame.

So I can say with absolute certainty that I moved away from the earth.

No, it is not true in any frame-independent sense.

 

[1MileCrash]

This is getting silly. Of course there is a physical reality, although I suspect not everyone lives in it. lol

No.

IF the Earth had moved away from me what would I see? I would see it leaving the moon behind for starters.

No. That changes nothing. You're confusing yourself here.

Who's to say the moon and Earth weren't drifting in a constant speed in any direction I want? No one, because it's relative. And who's to say that when you moved away from the Earth wrt the earth, you ceased motion while it continued (along with the moon, stars, whathaveyou) in a constant speed and direction?

The universe is a big, big place. The stars and the moon that you can see are literally just about nothing, they are relatively moving just as everything else is and they are not absolute reference frames.

But when I look out my window, I still see the moon in the same orbit. So the moon must be moving with the Earth too. But then I see the Earth in the same orbit relative to all the other planets too. As would any observer.

See above, so?

You are moving relative to the moon, along with the earth, and they are moving relative to you. Neither is a reality because you could never define any of them as being "still" to begin with!

Pretend there is an absolute reference frame that we've located, just to put this into your noodle. Relative to it, our galaxy is moving in a given direction (X) at .12 C.

You blast off in your ship going .12 C in direction Q (opposite of direction X). So, who's moving? According to our pretend absolute reference frame, when you turned on your thrusters, you stopped moving while the galaxy kept moving at .12 C in direction X.

Therefore, either I moved away from the Earth or the whole universe moved wrt to me, which is impossible.

No. The statement you're looking for is "I moved away from the earth, AND the observable bodies not the whole universe moved away from me. Our net change in speed with regards to each other is absolute, but whether I slowed down, sped up, or changed direction with regards to the Earth or the Earth with regards to me is relative. Which is perfectly possible."

Also, when I turn around and go back to Earth I find that I am younger than my twin brother. Why? Because I am the one that went through the acceleration, just as the twin paradox predicts.

See above. You absolutely underwent acceleration and the Earth did not, therefore you are younger. That means nothing regarding who was "moving."

According to any reference frame, you accelerated, that's why you're the younger twin. But did you stop, speed up, or change direction? That's relative! Only the net change in speed of your ship is absolute. Therefore no, the twin paradox in no way suggests that the spaceship twin is "absolutely moving" only that it absolutely changed it's speed.

If I could paint a mental picture, ignoring the "speeds not exactly adding" fact for simplicity:

From the Earth's reference, you blast off at .1 C in a -> direction, then after some time you turn around and travel back towards Earth at a <- direction at the same speed.

Reference Frame A, which is whatever the hell I want, initially observed Earth as traveling .1 C in -> direction. Therefore when you got in your ship and left, you traveled .2 C at direction ->, then when you "turned around," according to A, you ceased motion while the Earth "caught up with you" by traveling it's constant .1 C. You absolutely accelerated.

You absolutely changed speed by .2 C, and A observed that change of speed as you stopping motion.

Reference Frame B, which is also whatever the hell I want, initially observed Earth as traveling .1 C in <- direction. WRT B when you launched your space ship, you stopped! You completely ceased motion. You are traveling .0 C, while the Earth continues <- @ .1 C. Now, it's time for you to "turn around", which B observes as you starting to move again, this time in a <- direction, at .2 C.

So according to reference frame B, the Earth was in constant motion, you stopped motion when you left the earth, and then you accelerated back towards the Earth at a speed of .2 C.

You absolutely changed speed by .2 C, and when you turned around, B observed you starting to move, relatively.

Now, reference frame C, which is also whatever the hell I want (this is physics, I can do that.) initially observes Earth traveling .7 C in direction ->. It sees you speed away from Earth at .8 C, and perceives your "turning around" as a mere "slowing down" to .6 C while the Earth catches up with you.

You still absolutely changed your speed by .2 C when "turning around." But here, you just slowed down, relatively.

None of these reference frames are preferred, because none of them can be concluded as being of "absolute rest" wrt space itself, therefore, all observations are equally correct.

So I can say with absolute certainty that I moved away from the earth.

Nope. You can only say with absolutely certainty that you experienced a net change in speed.

It does not dismiss SR or GR in anyway.

Yes it does.

 

[rede96]

Firstly, thanks for the replies.

I think the penny has dropped, but I am still struggling with one thing.

If I am at rest wrt to the Earth and then I accelerate for a short time then stop accelerating, I would notice that the Earth and I are moving apart. So two things could have happened.

a) I slowed down and the Earth carried on its merry way

or

b) I speed up and moved away from the earth.


In both those cases it was I that accelerated. So I can say that I broke the symmetry not the Earth (As it did not accelerate.)

But I can't say that I moved away or the Earth moved away.

However, if someone on Earth sent a beam of light in my direction, couldn’t I tell using the Doppler effect if it was the Earth receding or if I was moving away?

 

[DaveC426913]

In both those cases it was I that accelerated. So I can say that I broke the symmetry not the Earth (As it did not accelerate.)

Yes, if your craft accelerates you can conclude that it is you moving away from the Earth rather than the Earth moving away from you.

However, if someone on Earth sent a beam of light in my direction, couldn’t I tell using the Doppler effect if it was the Earth receding or if I was moving away?

Both scenarios would result in the same Doppler shift, so no.

 

[rede96]

Yes, if your craft accelerates you can conclude that it is you moving away from the Earth rather than the Earth moving away from you.



Both scenarios would result in the same Doppler shift, so no.

Thanks.

 

[DaveC426913]

Thanks.

HOWEVER, once you stop accelerating, you revert to an inertial frame of reference, from which you will determine that you are stationary, like any other FoR.

(Note that, even though you know you accelerated away from Earth, you have no way of claiming that the Earth was stationary while you were on it, so it is not stationary either. All you can say is that you and the Earth are equally valid inertial FoR).

 

[Dale]

If I am at rest wrt to the Earth and then I accelerate for a short time then stop accelerating, I would notice that the Earth and I are moving apart. So two things could have happened.

a) I slowed down and the Earth carried on its merry way

or

b) I speed up and moved away from the earth.


In both those cases it was I that accelerated. So I can say that I broke the symmetry not the Earth (As it did not accelerate.)

But I can't say that I moved away or the Earth moved away.

Yes, exactly.

However, if someone on Earth sent a beam of light in my direction, couldn’t I tell using the Doppler effect if it was the Earth receding or if I was moving away?

No, the Doppler effect is fully relativistic.

 

[harrylin]

[..] I am still trying to find out how (and what) experiments were done that prove that Light approaches any reference point at C instead of C-V. [..]

Hi Physicist1231,
Was your first post of this thread sufficiently answered?

Assuming that relativity is correct, "The Omnipotent Point of view" cannot be disproved by experiments - as JesseM also mentioned. Perhaps because it has no practical use (how could we use a view that we can't determine!), it's less well known from the peer reviewed literature.

Relativity jargon is positivistic: only operational definitions are used, based on phenomena (appearances). Now, "Light approaches any reference point at C" is poorly stated, and therefore true or false depending on your references. The approaching speed of light is "relative" to the used reference system: it is set by definition equal to the receding speed in special relativity. You can read the definition here, in section 1:
http://www.fourmilab.ch/etexts/einstein/specrel/www/

As a result, the (apparent) "closing" or "approaching" speed has been made c wrt a reference point that is at rest in the reference system that you use, by appropriately regulating the clocks (see also Einstein 1907, Jahrbuch Radioelectr. Electr.4, 414).
It seems that you correctly understood that it is not c relative to a point that is "moving" in your reference system - indeed, that would be paradoxical, as the vector subtraction (c-v) with c=constant and v=/=0 cannot equal c.

The impossibility to determine absolute motion (in the original, Newtonian sense) is directly related to the impossibility to determine the absolute one-way closing speed of light, which in turn is directly related to relativity of simultaneity.

Harald

 

[xxxx0xxxx]

I was thinking that if I was to use lateral thrusters, the amount of thrust I would need to turn the ship 90 degrees would depend on the speed I was travelling. The faster I was going the more thrust I need.

Something like it takes more force to change the direction of moving object then a static one, as a moving object’s mass increases. (E= mc2) the faster it goes. So if it took more force to change direction then I must have gathered more than my rest mass and thus must be ‘moving’

The force required to change direction is applied in your rest frame, and is independent of relative motion. For you, your mass is always your rest mass. For others not at rest with you, you have additional energy/mass in the form of momentum.

Your thrusters do not change your net momentum, since they only produce angular acceleration (you do lose rest mass however).

 

[chingel]

But can you determine absolute rotation? The further away you go on a merry-go-around the harder it tries to throw you out. If the universe is spinning around a center, can we absolutely determine by how much?

 

[Dale]

But can you determine absolute rotation?

Yes. Rotation is a non-inertial motion.

 

[xxxx0xxxx]

But can you determine absolute rotation? The further away you go on a merry-go-around the harder it tries to throw you out. If the universe is spinning around a center, can we absolutely determine by how much?

Well, from the point of view of the moon, Earth rotates around it, and I would say the universe rotates around us every 24 hours :)

If you are within some sort of rotating structure, you will experience centripetal force if you appear to be stationary with the structure, and you will observe coriolis motion of objects moving relative to the structure; otherwise the interior structure will be moving with respect to you in a circle ... and yes these would be measurable absolute effects.