Question about the speed of light barrier

[Juked07]

When we say we cannot exceed the speed of light, what reference point are we using to measure speed? I am not an established physicist of any sort, but it seems to me that speed is only meaningful given a reference point...

Is there an accepted point that is considered absolutely at rest?

If so, if you were part of a solar system which is already moving away from this reference point at say, 0.8c, would you then be limited to travelling in that same direction at a speed of only 0.2c?

If you could, however, travel at a full c from your solar system, which is already travelling at 0.8c, then you would be moving at 1.8c away from the absolute reference point.

This issue confuses me.

Furthermore, if you are moving through a vacuum at any speed X relative to a reference point, how is this different in a physical sense from being at rest with a different reference point? Why should it be impossible or more difficult to accelerate in one case or the other?

Hopefully in the drivel above my shortcomings in understanding the issue has become apparent and those with fuller understandings can correct me =)

 

[KIAman]

Relativistic speed.

If not, you could spin around and the moon would be going "faster" than light in relation to your pov.

 

[PolymerTim]

I would love to understand this better as well. I've read a bit about it and understand what the basic effects are, but not so much why.

To answer your first question, there is no position of absolute rest. Einstein's Special Theory of Relativity was specifically intended to make this point. As to the implications of that in relation to the speed of light, I am still pretty confused. If I understand correctly, whether you are stationary relative to another object of reference or moving at a significant fraction of the speed of light relative to that same object of reference, you will still measure the speed of light to be the same. I would love to hear an explanation of why if anyone can manage it.

One famous null experiment that specifically addresses the relativistic motion of the planet in the solar system in the galaxy etc happened right here at my university if you're interested.
http://en.wikipedia.org/wiki/M...lson-Morley_experiment
At the time, they believed there was a medium that light propagated through called the luminiferous aether. The idea was that if the earth is moving so fast relative to an absolute zero velocity point (in this case their proposed aether) then you should be able to detect a change in the speed of light depending on the direction it was travelling. The hard part was measuring small changes in the speed of light in 1887. This experiment, and improved future versions eventually proved that there is no measurable change in the speed of light with direction. Similarly, emission theory sought to show that the speed of light was dependent on the emitter, but is inconsistent with observations.

 

[silverpig]

Originally posted by: PolymerTim
I would love to understand this better as well. I've read a bit about it and understand what the basic effects are, but not so much why.

To answer your first question, there is no position of absolute rest. Einstein's Special Theory of Relativity was specifically intended to make this point. As to the implications of that in relation to the speed of light, I am still pretty confused. If I understand correctly, whether you are stationary relative to another object of reference or moving at a significant fraction of the speed of light relative to that same object of reference, you will still measure the speed of light to be the same. I would love to hear an explanation of why if anyone can manage it.

One famous null experiment that specifically addresses the relativistic motion of the planet in the solar system in the galaxy etc happened right here at my university if you're interested.
http://en.wikipedia.org/wiki/M...lson-Morley_experiment
At the time, they believed there was a medium that light propagated through called the luminiferous aether. The idea was that if the earth is moving so fast relative to an absolute zero velocity point (in this case their proposed aether) then you should be able to detect a change in the speed of light depending on the direction it was travelling. The hard part was measuring small changes in the speed of light in 1887. This experiment, and improved future versions eventually proved that there is no measurable change in the speed of light with direction. Similarly, emission theory sought to show that the speed of light was dependent on the emitter, but is inconsistent with observations.

Actually, that's not quite true. If you look into the sky (with a radio telescope) you'll see a cosmic microwave background in all directions. This background is very uniform, with only very very small anisotropies in temperature. The motion of the earth through space causes this uniform radiation to have a dipole (ie, it's hotter in the direction we're headed in, and colder in the direction we're coming from). You can subtract a dipole term from the CMB to get a frame of reference that is at rest with respect to the average of the rest of the universe

Of course this is just another valid reference frame to do physics in, and is no different than any other, except for the fact that you can say you're at rest with respect to the universe

What Einstein's theory of relativity says is that there is no special frame of reference from which physics looks different from any other.

As to the OP's question, I'll pull a Fermat and say that while the answer is quite beautiful, I cannot fit it all in this one post...

Check wikipedia though. I'm sure someone with a lot of time on their hands has written a good article.

 

[PowerEngineer]

The crux of special relativity is that the speed of light is constant for observers in any and all (unaccelerating) frames of reference (regardless of their velocities with respect to one another).

"At rest" with respect to what? There is no frame of reference that can claim to be uniquely "at rest"; every frame of reference can make that claim with equal validity.

Observers in two different frames of reference that are moving with respect to each other will still measure the speed of light they see from a common source as...well...the speed of light. Their speed relative to one another does not change the speed of light they measure.

This idea of a maximum speed does not fit comfortably with our Newtonian sense of how the universe works, and Albert saw that it meant our notions of distance and time had to be adjusted to accommodate it.

The two observers in different frames of reference measure distances and experience time differently. The differences are tied to their velocity relative to one another, and mesh together in a way that makes their measured speeds of light the same.

I agree with you that this is confusing, largely because it seems contrary to our day-to-day experience in which these "relativistic" effects are imperceptible. If we lived in a universe where the speed of light was maybe 1000 mph, then maybe it'd seem obvious and natural.

 

[PolymerTim]

Originally posted by: silverpig
Actually, that's not quite true. If you look into the sky (with a radio telescope) you'll see a cosmic microwave background in all directions. This background is very uniform, with only very very small anisotropies in temperature. The motion of the earth through space causes this uniform radiation to have a dipole (ie, it's hotter in the direction we're headed in, and colder in the direction we're coming from). You can subtract a dipole term from the CMB to get a frame of reference that is at rest with respect to the average of the rest of the universe

That's an excellent piece of info I wasn't aware of. Thanks for sharing.

Originally posted by: PowerEngineer
This idea of a maximum speed does not fit comfortably with our Newtonian sense of how the universe works, and Albert saw that it meant our notions of distance and time had to be adjusted to accommodate it.

The two observers in different frames of reference measure distances and experience time differently. The differences are tied to their velocity relative to one another, and mesh together in a way that makes their measured speeds of light the same.

I agree with you that this is confusing, largely because it seems contrary to our day-to-day experience in which these "relativistic" effects are imperceptible. If we lived in a universe where the speed of light was maybe 1000 mph, then maybe it'd seem obvious and natural.

I had a feeling things like time dilation and length contraction somehow made up the difference. I think you have to see how all these pieces fit together to see the consistency with our observations. I also agree that some facts will go against our "common sense" since they explain phenomenon that are, for the most part, outside the realm of our daily perception.

 

[hellokeith]

Attempting to find an absolute point of reference

Arminius originally suggested using the quasar BR 1202-0725 as a point
of reference. Doing so would give a somewhat surprising result:

Taking this quasar as a reference point, the earth would be moving at
282,000 kilometers per second!

This enormous speed comes from the fact that our universe is
expanding. As the universe expands, the galaxies move away from one
another. And the further an object is, the faster it is receeding from
us.

As Quasar BR 1202-0725 is one of the most distant objects known, it
moves away from us at the incredible speed of 282,000 kilometers per
second (94% the speed of light). And if we take the quasar as a point
of reference, the earth will be the one moving at this mind-boggling
speed.

Kinda off topic, but I'm intrigued with, when dealing on scales of this magnitude distance/size (i.e. over multiple galaxies), at what distance is gravity truly effectual?

 

[silverpig]

Originally posted by: hellokeith
Attempting to find an absolute point of reference

Arminius originally suggested using the quasar BR 1202-0725 as a point
of reference. Doing so would give a somewhat surprising result:

Taking this quasar as a reference point, the earth would be moving at
282,000 kilometers per second!

This enormous speed comes from the fact that our universe is
expanding. As the universe expands, the galaxies move away from one
another. And the further an object is, the faster it is receeding from
us.

As Quasar BR 1202-0725 is one of the most distant objects known, it
moves away from us at the incredible speed of 282,000 kilometers per
second (94% the speed of light). And if we take the quasar as a point
of reference, the earth will be the one moving at this mind-boggling
speed.

Kinda off topic, but I'm intrigued with, when dealing on scales of this magnitude distance/size (i.e. over multiple galaxies), at what distance is gravity truly effectual?

Eh, that's kind of misleading because space itself is expanding. It's hard to really say what is moving and what isn't because you can imagine both objects being "at rest" in a local reference frame and the space in between them just getting larger... if you want to call them moving or not is up to you, but it's not easy to say.