Relativity question

[Rum]

I get the consequences of relativity however I wonder if anyone can help me with the following that I have been puzzling over.

Suppose I am between two bodies each travelling away from me at 90% the speed of light. Relative to each other how fast would those bodies be travelling. Relativity, as I understand it says they can't be travelling at 180% the speed of light relative to one another. So what is their relative speed?

This isn't purely theoretical, as there are stars at the outer reaches of the universe from our perspective which are indeed travelling away from us at a significant percentage of the speed of light.

Clever people help me out please!?

[pErvinalia]

I'm not sure how it would look from your perspective. Common sense is telling me they must appear to be moving relative to each other at 180% the speed of light. I guess that doesn't break any rules, as no body is appearing to go faster than C. From the perspective of either of the bodies, the other body would appear to be going slower due to time and length dilation.

[FBM]

XC had a good answer for a very similar question not long ago: http://188.226.174.187/forum/viewtopic. ... 6#p1552152

[mistermack]

I get the consequences of relativity however I wonder if anyone can help me with the following that I have been puzzling over.

Suppose I am between two bodies each travelling away from me at 90% the speed of light. Relative to each other how fast would those bodies be travelling. Relativity, as I understand it says they can't be travelling at 180% the speed of light relative to one another. So what is their relative speed?

This isn't purely theoretical, as there are stars at the outer reaches of the universe from our perspective which are indeed travelling away from us at a significant percentage of the speed of light.

Clever people help me out please!?

I'm not clever, but what you are doing there jumping from one inertial frame of reference ( your own ) to another ( which is travelling away from you at 90% of the speed of light ) without making any adjustment for time dilation. There is a formula for making that adjustment, the Lorenz Transformation, and when you apply it to the three objects, you will find that if you are treating one of the bodies as stationary, it will observe you, and the other body, as both travelling at less than the speed of light.

The principle is that nothing can exceed the speed of light, in any inertial frame you choose.

[Tero]

This%20isn't%20purely%20theoretical,%20as%20there%20are%20stars%20at%20the%20outer%20reaches%20of%20the%20universe%20from%20our%20perspective%20which%20are%20indeed%20travelling%20away%20from%20us%20at%20a%20significant%20percentage%20of%20the%20speed%20of%20light.

But are they in fact moving "away from each other"?

It's interesting that we see stars in all directions. The universe appears to have space, dimensions. But this direction and away bit is an illusion. We think in terms of Euclidian geometry. It appears to work, on small scale.

Read one of them Asimov books for more.

Ps: I appear to have messed up the Universe with % signs.

[Tero]

Isaac Asimov's Guide to Earth and Space

Is a fun 8 dollar book. Can find used for a few bucks. It goes about earth the solars system, measuring the speed of light the first time and all kinds of stuff. You would enjoy it Rum.

Not so much relativity but things you can grasp. Like what's inside the earth.

[Xamonas Chegwé]

I get the consequences of relativity however I wonder if anyone can help me with the following that I have been puzzling over.

Suppose I am between two bodies each travelling away from me at 90% the speed of light. Relative to each other how fast would those bodies be travelling. Relativity, as I understand it says they can't be travelling at 180% the speed of light relative to one another. So what is their relative speed?

This isn't purely theoretical, as there are stars at the outer reaches of the universe from our perspective which are indeed travelling away from us at a significant percentage of the speed of light.

Clever people help me out please!?

Relative to yourself, the combined velocities of the two bodies is indeed greater than c. However, relative to either body, the other is travelling at less than c. This is down to relativistic changes in time and space affecting the measurement of velocity. Each will see the other's time slowing - to such a degree that their relative velocity never exceeds (or even reaches) c.

Basically, time passes for each body (and for you, as the observer) at different rates, depending on from which viewpoint that body is observed. None of these are "the correct" rate (or equally, they all are!)

[Rum]

Thanks XC. C has to remain constant so in effect the other variables change.

[Xamonas Chegwé]

Thanks XC. C has to remain constant so in effect the other variables change.

Pretty much.

Special relativity is derived from 2 simple propositions: 1. The laws of Physics are the same for any observer, irrespective of their motion relative to any other body. 2. The speed of light in a vacuum is constant for any observer.

All else follows from these.

[Rum]

I'm reading Einstein's biography at the moment. A rather good one too. For anyone interested:

http://www.amazon.com/Einstein-Life-Uni ... 1442348062

[cronus]

I'm reading Einstein's biography at the moment. A rather good one too. For anyone interested:

http://www.amazon.com/Einstein-Life-Uni ... 1442348062

Be satisfied with the biography Rum. You see Einstein was Einstein and he wrote at that level - heavy stuff in general. His life proves him rather mundane otherwise. He wasn't shagging every bird going like some of them do with a bit of fame or anything like that.

Try this as a intro if you really need to know the magik...

http://www.amazon.com/Introduction-Spec ... 0198539525

[pErvinalia]

Thanks XC. C has to remain constant so in effect the other variables change.

Lol. That's exactly what I said. I never get any credit around here!

[pErvinalia]

The best book on special relativity I read was by Betrand Russel. Called the "ABC of Relativity", if I remember right. I've been searching for a second hand copy for years for my collection. If you want to understand special relativity simply, then get a copy of that.

[Rum]

I'm reading Einstein's biography at the moment. A rather good one too. For anyone interested:

http://www.amazon.com/Einstein-Life-Uni ... 1442348062

Be satisfied with the biography Rum. You see Einstein was Einstein and he wrote at that level - heavy stuff in general. His life proves him rather mundane otherwise. He wasn't shagging every bird going like some of them do with a bit of fame or anything like that.

Try this as a intro if you really need to know the magik...

http://www.amazon.com/Introduction-Spec ... 0198539525

Actually it appears he was. As ever you...never mind.

[Brian Peacock]

Thanks. XC has to remain constant so, in effect, the other variables can change.

FIFY