Can you determine absolute motion?

[rede96]

Interesting topic!

What about this scenario using gravity or acceleration to prove absolute motion. By the way, I don’t know the math on this but it still seems a good thought experiment so here goes:

Let’s say I am in a space ship, with no thrusters or power on, so all I know is that I could be traveling at some speed. So I can say that I am in an inertial reference frame.

I know that if I change direction but not speed I will still be accelerating. I also know that I will be able to measure the rate I accelerate by using something like a pendulum or some other device that would move when I accelerate.

I know the mass of my ship. So I can calculate that if I was traveling at speed ‘x’how much thrust (power) I would need to turn the ship through 90 degrees in a particular direction. I would also be able to calculate the effects of gravity (acceleration) on my pendulum.

So I execute the manoeuvre and measure the effect on my pendulum. If the pendulum swings the calculated amount I know what speed I was traveling at prior to the manoeuvre.

If the pendulum doesn’t swing the calculated amount I could then work out what speed I was moving at prior to the manoeuvre.

Therefore I can show that I was in motion (or not) without making reference to another frame.

 

[JesseM]

Let’s say I am in a space ship, with no thrusters or power on, so all I know is that I could be traveling at some speed. So I can say that I am in an inertial reference frame.

I know that there if I change direction but not speed I will still be accelerating. I also know that I will be able to measure the rate I accelerate by using something like a pendulum or some other device that would move when I accelerate.

I know the mass of my ship. So I can calculate that if I was traveling at speed ‘x’how much thrust (power) I would need to turn the ship through 90 degrees in a particular direction. I would also be able to calculate the effects of gravity (acceleration) on my pendulum.

So I execute the manoeuvre and measure the effect on my pendulum. If the pendulum swings the calculated amount I know what speed I was traveling at prior to the manoeuvre.

Why do you think it would depend in any way on your initial speed? Of course in relativity there is no objective truth about your initial speed, but different frames who define your coordinate speed differently can apply the same laws to your ship and pendulum to predict how it will behave, and all will predict the same thing for the angle the pendulum moves (not necessarily the angle in their frame, but if the pendulum string is initially lined up with the 0-degree mark on a circular protractor on board your ship, all frames predict the same thing about what mark it's lined up with at the end of the maneuver).

 

[SeventhSigma]

I don't think we can ever prove absolute space. Everything sitting still and everything moving at constant velocity V would look the same. We can only define motion in the relative sense.

 

[xxxx0xxxx]

Nay sirs,

He who accelerates has the slow clock,

the spacefarer must launch and accelerate to leave, then decelerate, turn around and accelerate to return, then decelerate to land.

The people who stay behind do not experience these effects which are general (not special) relativistic, the difference in clock times are due to general relativity.

 

[JesseM]

Nay sirs,

He who accelerates has the slow clock,

the spacefarer must launch and accelerate to leave, then decelerate, turn around and accelerate to return, then decelerate to land.

The people who stay behind do not experience these effects which are general (not special) relativistic, the difference in clock times are due to general relativity.

You're correct that if one accelerates and the other doesn't, then the one that accelerated is always the one to have aged less. However, as long as spacetime is flat (no curvature due to mass/energy) there is no need for general relativity, you can calculate the behavior of an accelerating object (including the time that elapses between two points on its worldline) just fine using an SR inertial frame.

 

[rede96]

Why do you think it would depend in any way on your initial speed?

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’

 

[DaveC426913]

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’

No. In your reference frame your mass does not increase. You are - as far as any (local) experiment can detect (including accelerating your craft in any direction) - as good as stationary.

 

[rede96]

No. In your reference frame your mass does not increase. You are - as far as any (local) experiment can detect (including accelerating your craft in any direction) - as good as stationary.

As I understood it the reason I can’t travel at c is that the energy I would need would reach infinity as my mass would keep increasing proportionately.

So I assume that mass does increase but would revert back to its rest mass once I stop accelerating. Is that correct?

 

[Mentz114]

As I understood it the reason I can’t travel at c is that the energy I would need would reach infinity as my mass would keep increasing proportionately.

This is how it would look to an observer you left behind when you started accelerating. In your frame ( spaceship ?) you would notice nothing untoward, your motors will push you along just as they did when you started. If you keep firing the engines long enough, a horizon will spring up between you and the left-behind observer.

So I assume that mass does increase but would revert back to its rest mass once I stop accelerating. Is that correct?

There's no actual mass increase, but when you stop accelerating the horizon will disappear.

Have a look at this article

http://gregegan.customer.netspace.net.au/SCIENCE/Rindler/RindlerHorizon.html

 

[DaveC426913]

In your frame ( spaceship ?) you would notice nothing untoward, your motors will push you along just as they did when you started.

Agreed. This is key.

Under your constant thrust, your pendulum would continue to stay as tilted a year from now as it does today. Again, no local experiment would indicate your velocity. (Though you could look out the window and see the galaxy passing by at .9999c.)

 

[HallsofIvy]

As I understood it the reason I can’t travel at c is that the energy I would need would reach infinity as my mass would keep increasing proportionately.

So I assume that mass does increase but would revert back to its rest mass once I stop accelerating. Is that correct?

No, you are not correct. You are missing the word "relative"- which you should constantly be using in a discussion of relativity. You cannot travel at c relative to another frame of reference because your mass relative to that frame of refererence would keep increasing. If you stop accelerating, your mass, relative to that frame of reference, would remain whatever the Lorentz transformation law for mass gives for your current speed. Your mass would "revert back to its rest mass", relative to that frame of reference only if you slowed down so you were stationary relative to that frame of reference. At any given time, you are at rest relative to yourself and so your mass, relative to yourself, will always be your rest mass.

(Any many here would argue that talking about increasing mass is out of date- your "mass" remains your rest mass while your momentum, relative to another frame of referrence increases non-linearly with speed.)

 

[Physicist1231]

Why don't you start with the results of the experiments? We have ample evidence of time dilation; Einsteinain SR and GR explain it very well. The Newtonian model does not.

Why are you trying to fix what ain't broke? What is the impetous driving your desire to find another answer when we have an answer?

Actually I am looking at a lot of experiments that have been done and the assumptions made. Some of them I do not understand why other assumptions were not made that still coincided with what was previously known.

For instance the Michelson-Morley experiment to see if aether winds existed. It was a pretty good test that showed that these "winds" that would affect a photon was affected by said winds. The verdict was that these "winds" did not exist. Why not say they just don't affect light?

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.

The emmitter theory was doomed for failure from the start because it had a bad definition as a reference point. Light is cannot be "thrown" like a ball or a bullet from a moving vehicle. Rather it is proven that light (like sound) is emitted and radiates from its original location (regardless of the new location of its moving light maker). This is why we see things like redshift and blueshift and time dilation.

(i tried to paste a picture of my understanding of this but had to make it an attachment)

Granted this is not drawn to scale but the point shows that the the distance between waves in the direction of travel is shorter so a person observing this would see the incident appear to take less time than the event actually took where as a person behind the light bulb path would see a much longer time elaps between light waves.


One other experiment I have an issue with is the Twin Paradox (started a new thread for this question). Please let me know what you guys have

 

[Physicist1231]

No, you are not correct. You are missing the word "relative"- which you should constantly be using in a discussion of relativity. You cannot travel at c relative to another frame of reference because your mass relative to that frame of refererence would keep increasing. If you stop accelerating, your mass, relative to that frame of reference, would remain whatever the Lorentz transformation law for mass gives for your current speed. Your mass would "revert back to its rest mass", relative to that frame of reference only if you slowed down so you were stationary relative to that frame of reference. At any given time, you are at rest relative to yourself and so your mass, relative to yourself, will always be your rest mass.

(Any many here would argue that talking about increasing mass is out of date- your "mass" remains your rest mass while your momentum, relative to another frame of referrence increases non-linearly with speed.)

Then now I am confused about E=mc^2... I thought that was the underlying foundation for the argument about mass needed to exceed/reach C. (and is counter acted with the guestimated size of a photon that travels at C yet any observable mass (from our reference point) is extremely minute if any)

 

[rede96]

No, you are not correct. You are missing the word "relative"- which you should constantly be using in a discussion of relativity. You cannot travel at c relative to another frame of reference because your mass relative to that frame of refererence would keep increasing. If you stop accelerating, your mass, relative to that frame of reference, would remain whatever the Lorentz transformation law for mass gives for your current speed. Your mass would "revert back to its rest mass", relative to that frame of reference only if you slowed down so you were stationary relative to that frame of reference. At any given time, you are at rest relative to yourself and so your mass, relative to yourself, will always be your rest mass.

(Any many here would argue that talking about increasing mass is out of date- your "mass" remains your rest mass while your momentum, relative to another frame of referrence increases non-linearly with speed.)

OK, Thanks.

So if I understand it, that means that even though I could start to accelerate up to 0.7c for example, stop and accelerate again at the same rate, my speed wrt to another frame of reference would keep increasing each time (as would my relative mass) but it would never reach c, and I would have to add velocities relatively.

 

[DaveC426913]

OK, Thanks.

So if I understand it, that means that even though I could start to accelerate up to 0.7c for example, stop and accelerate again at the same rate, my speed wrt to another frame of reference would keep increasing each time (as would my relative mass) but it would never reach c, and I would have to add velocities relatively.

Yes. After two successive (internal) accelerations of .7c, your velocity externally will be .94c.

 

[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.