[MikeS (OP)]

If the Universe is expanding at a significant fraction of the speed of light then we are moving at a significant fraction of the speed of light. If so then what we call rest mass is actually relativistic mass and is subject to change depending upon speed?  The question is if there is anywhere in the Universe that was absolutely stationary would mass have weight?  Or for that matter would it have mass?

[Soul Surfer]

No this is definitely not correct because if it was the masses of things could be different in different  places and times.  This is clearly not what is observed.  The mass of a particle or an object is just a measure of the amount of energy that has been trapped and located by the particle or object.

[MikeS (OP)]

No this is definitely not correct because if it was the masses of things could be different in different  places and times.  This is clearly not what is observed.  The mass of a particle or an object is just a measure of the amount of energy that has been trapped and located by the particle or object.

I agree but if that mass is travelling at relatavistic velocity then is should have an extra relativistic mass.  If if does not then we have to ask why not?

Can we observe it though?
As all local reference frames are travelling effectively parallel to each other and at the same relativistic speed.  Likewise, (local) clocks are all dilating at the same rate.

[yor_on]

If you by mass mean the weight perceived, then that only change in a acceleration. there you can find yourself heavier. We have uniform motion and accelerated motion existing. Then we have what is called invariant proper mass, aka the mass of a planet for example. That mass will always be the same in a uniform motion, no matter if you could accelerate Earth relative the Sun, as soon as it stops accelerating your mass will be the same as before the acceleration.

[MikeS (OP)]

If you by mass mean the weight perceived, then that only change in a acceleration. there you can find yourself heavier. We have uniform motion and accelerated motion existing. Then we have what is called invariant proper mass, aka the mass of a planet for example. That mass will always be the same in a uniform motion, no matter if you could accelerate Earth relative the Sun, as soon as it stops accelerating your mass will be the same as before the acceleration.

That makes sense but what happens when you remove the accelerating force from the object.  Does it continue in a straight line at a constant velocity.  If it does then where has the relatavistic mass gone?  Or does it loose it by slowing down and if so what causes it to slow?

[MikeS (OP)]

If you by mass mean the weight perceived, then that only change in a acceleration. there you can find yourself heavier. We have uniform motion and accelerated motion existing. Then we have what is called invariant proper mass, aka the mass of a planet for example. That mass will always be the same in a uniform motion, no matter if you could accelerate Earth relative the Sun, as soon as it stops accelerating your mass will be the same as before the acceleration.

Having read your answer again, no I did not mean perceived weight.  I meant relativistic mass.

[yor_on]

Let's discuss it in form of energy instead. When you accelerate you expend energy, but when you move uniformly you do not. So what exactly gives the added energy in a 'free fall' aka uniform motion/geodesic, as you meet the floor?

When it comes to relativistic mass you only gain that in a acceleration, not in a uniform motion as far as I can see. If it was otherwise you would contain a untold number of different 'relativistic mass' on any uniformly moving object of matter, as our Earth. That because all uniform motion are indistinguishable from eachother. It doesn't matter what you measure against, or if you like, it do matter and with each measurement you can define a new 'speed' to our Earth, depending on what you define it against, and so a new 'relativistic mass' if now uniform motion was related to a relativistic mass. Because with a system of two uniformly moving objects in space you are free to define all 'motion' to either one, naming the other as being 'still' according to relativity.

In fact I think you would be able to define both as being 'still', instead assuming a expansion of the 'space' in between too? I'm not entirely sure on that one, but I have trouble seeing how you would differ between a expansion and uniform motion there. "Einstein was willing to generalize the equivalence principle, and to conclude that the classical idea of a distinguished class of frames of reference has no physical basis. Any frame that we might regard as inertial might be, for all we can tell by experiment, in free fall. By the same token, any frame that is uniformly accelerating is indistinguishable from one that is at rest in a uniform gravitational field."

From Space and Time: Inertial Frames.

[lightarrow]

If the Universe is expanding at a significant fraction of the speed of light then we are moving at a significant fraction of the speed of light.

Wrong. Every time you say "A is moving" you are making a mistake. You have to say: "A is moving with respect to B".
So you see that "movement" is not an intrinsic property of a body, because it always depends on other bodies.

Mass, instead, *is* an itrinsic property of a body.

[MikeS (OP)]

If the Universe is expanding at a significant fraction of the speed of light then we are moving at a significant fraction of the speed of light.

Wrong. Every time you say "A is moving" you are making a mistake. You have to say: "A is moving with respect to B".
So you see that "movement" is not an intrinsic property of a body, because it always depends on other bodies.

Mass, instead, *is* an itrinsic property of a body.

Ok so lets call all distant points A and our local reference frame B.  Isn't this basicaly the same as "If the Universe is expanding at a significant fraction of the speed of light then we are moving at a significant fraction of the speed of light."

Invariant mass is an intrinsic property of a body but I am talking about relativistic mass which isn't.

[MikeS (OP)]

Let's discuss it in form of energy instead. When you accelerate you expend energy, but when you move uniformly you do not. So what exactly gives the added energy in a 'free fall' aka uniform motion/geodesic, as you meet the floor?

When it comes to relativistic mass you only gain that in a acceleration, not in a uniform motion as far as I can see. If it was otherwise you would contain a untold number of different 'relativistic mass' on any uniformly moving object of matter, as our Earth. That because all uniform motion are indistinguishable from eachother. It doesn't matter what you measure against, or if you like, it do matter and with each measurement you can define a new 'speed' to our Earth, depending on what you define it against, and so a new 'relativistic mass' if now uniform motion was related to a relativistic mass. Because with a system of two uniformly moving objects in space you are free to define all 'motion' to either one, naming the other as being 'still' according to relativity.

In fact I think you would be able to define both as being 'still', instead assuming a expansion of the 'space' in between too? I'm not entirely sure on that one, but I have trouble seeing how you would differ between a expansion and uniform motion there. "Einstein was willing to generalize the equivalence principle, and to conclude that the classical idea of a distinguished class of frames of reference has no physical basis. Any frame that we might regard as inertial might be, for all we can tell by experiment, in free fall. By the same token, any frame that is uniformly accelerating is indistinguishable from one that is at rest in a uniform gravitational field."

From Space and Time: Inertial Frames.

In a free fall, gravitational potential energy is converted into kinetic energy. 

You gain relativistic mass by accelerating but you keep it in uniform motion.  Relativistic mass increases with velocity.

[Pmb]

You gain relativistic mass by accelerating but you keep it in uniform motion.  Relativistic mass increases with velocity.

Let us not forget that if we first measure (from an inertial frame in a flat spacetime) the mass of the particle mass, we will get the value of the proper mass. But if we change our frame of reference to one which is moving relative to the first one then when we measure its mass now we will get a higher value.

Saying that a frame of light is moving is meaningless. It's wrong to say that we are moving through the universe. However we can speak of our velocity relative to the matter in the universe. Its like a balloon being filled with air with two ants on the surface on the balloon. IF the ant moves relative to the surface of the balloon then we can give a definite value for the ants motion. If both the ants are not moving with respect to the surface they will still determine that they are moving apart. In fact if the surface had a bunch of ants on it then each ant will determine that each and every ant is moving away from the still ant. Same thing in cosmology.

[lightarrow]

Ok so lets call all distant points A and our local reference frame B.  Isn't this basicaly the same as "If the Universe is expanding at a significant fraction of the speed of light then we are moving at a significant fraction of the speed of light."

If you move with respect to A, you will have kinetic energy with respect to A; you move with a different velocity with respect to C, you have a different kinetic energy with respect to C...

Invariant mass is an intrinsic property of a body but I am talking about relativistic mass which isn't.

No because you were talking of rest mass:
"If so then what we call rest mass is actually relativistic mass and is subject to change depending upon speed?"

If you talk of rest mass you mean that the frame of reference you are...referring to is the one of the object itself, so...the object doesn't move by definition, so you can't say the rest energy comes from the object movement  []

[yor_on]

There is two ways to look at it. if you imagine something moving uniformly at the surface of our earth, then accelerate it, and then let it move uniformly after I agree that it will have a larger momentum relative that earth. We also can prove that by referring to its history, aka it accelerating before.

And when it comes to all objects moving in SpaceTime we have the 'Big Bang'. But that was a inflation more than a normal 'explosion'. But we might assume that it contained a acceleration. After that we have relative motion, relative as there is no frame of reference defining what 'no motion' would be in here.

So using Earth as a example I would agree, but when using the theory of relativity I'm not as sure. Because that uses 'local experiments' to define motion, and in those a uniform constant acceleration becomes inseparable from 'gravity', and a uniform motion becomes inseparable from having no 'motion' at all.

Although the momentum will change with a higher 'uniform speed', relative any predefined standard, as some other planet, your weight won't. Only in a acceleration will it do that. So if you mean momentum I will agree, but when it comes to relativistic mass I'm not as sure. If we had a universal definition of something 'not moving' to define it from it would be much simpler.

[yor_on]

If we discussed it in terms of inertia I think I would agree too. Expressed by its refusal to 'stop' at a collision in this case. But in all uniform motions the only gravity you will experience is of the 'invariant' kind, the matter you are surrounded by, sort of.

So 'energy' 'inertia' and 'momentum' will all change with different uniform motion, the motion defined relative some 'inertial frame' of your choice. Which makes me wonder about if gravity (invariant mass) and inertia is one and the same btw

As in a uniform motion you can be said to represent different inertia, depending on your relative motion. But your local gravity should be one and the same in all uniform motions, defined by what invariant mass there is existing?