The Shalimar Treaty

[Slick]

That thump you just heard was Einstein rolling over in his grave.

[Spoo]

AngleWyrm_2 said:
The Doppler effect applies to both sound (pitch changes) and light (red shift). It does not change the speed, only the frequency. The speed of sound changes with the rigidness of the medium through which it travels, and the speed of light changes with the refraction index of the medium through which it travels.

I'm not talking about Doppler shift (which is different than red shift, by the way). Sound travels at roughly 340m/s at sea level. If I'm moving at 100m/s away from the sound source, as far as I'm concerned the sound is moving at 240m/s. This is not the case with light, however. Light (or information, if you prefer) travels at 3x10^8m/s. If I'm moving away from the light source at 1x10^8m/s, then as far as I'm concerned, the light is still moving at 3x10^8m/s.

These two examples are not orthinagonal. In the sound example, the traveller is accepted as travelling at the speed of sound, but in the light example the traveller is accepted as travelling less than the speed of light. It could be just as valid to switch the roles, and say running towards the target at light speed means you never see the gun go off, but running at some speed arbitrarily close to the speed of sound you still hear the gun go off.

Again the difference is that's possible to travel faster than sound, while it is impossible to travel faster than light (information).

EDIT: Let me add that it would take infinite energy to accelerate a massive object (like a spaceship/person/electron) to light speed (3x10^8m/s).

[AngleWyrm]

These guys seem to think red shift and Doppler shift describe the same phenomenon:
http://www.answers.com/topic/redshift
http://zebu.uoregon.edu/~soper/Light/doppler.html
http://hyperphysics.phy-astr.gsu.edu...ro/redshf.html

If light is being emitted from a source at 3x10^8m/s and I travel away from that source at 1x10^8m/s, then the light will approach me at 3x10^8m/s - 1x10^8m/s = 2x10^8m/s.

[Spoo]

AngleWyrm_2 said:
These guys seem to think red shift and Doppler shift describe the same phenomenon:
http://www.answers.com/topic/redshift
http://zebu.uoregon.edu/~soper/Light/doppler.html
http://hyperphysics.phy-astr.gsu.edu...ro/redshf.html

Using "red shift" and "Doppler effect" interchangably is a common mistake. While it is true that a Doppler effect is measurable for stars in our galaxy and for nearby galaxies (the shift can be "red" or "blue"), in most cases, when an astronomer refers to "red shift" they mean the stretching of photons by the expansion of the universe. To quote your first source,
"...all distant galaxies show a red shift proportional to their distance from the earth as a result of the general expansion of the universe (see Hubble's law)..."
In other words, the Doppler effect is an example of red shift, but not all red shifts are caused by the Doppler effect.

But this is getting off topic.

If light is being emitted from a source at 3x10^8m/s and I travel away from that source at 1x10^8m/s, then the light will approach me at 3x10^8m/s - 1x10^8m/s = 2x10^8m/s.

Wrong. "The speed of light is the same for all observers, no matter what their relative speeds."

[AngleWyrm]

If light is being emitted from a source at 3x10^8m/s and I travel away from that source at 1x10^8m/s, then do you believe that it will still approach me at 3x10^8m/s? What about the guy I pass along the way; does it approach him at 3x10^8m/s also?

It seems to me that the light will reach the guy I pass before it reaches me.

[Phoenix-D]

Spoo said:

That's why I specified that I was refering to the speed that photons travel at. Light appears to move slowly through certain materials, but the individual photons still move at ~3x10^8m/s. In these cases what's happening is that photons are constantly being absorbed and reemitted, which makes the light appear to be moving slowly.

True, but somewhat irrelevent..Take the extreme example: a system set up so sound travels unimpreded, but light must go through a large barrier than slows it down to below the speed of sound.

You'll now hear the blast, THEN see the shot. And this still doesn't violate cause and effect.

Teleportation is science fiction.

Thank you for missing the point. And 700 years ago a weapon that could fire projectiles at greater than the speed of sound was the same thing.

The point is it doesn't matter what the observer sees first because that doesn't change the order of events.

Wrong. "The speed of light is the same for all observers, no matter what their relative speeds."

Which leads to some incredibly -weird- physics.

Two people are accelerating away from me, one at twice the speed of the other. I fire a laser at both. Both, if they could measure it, would see that the light is approaching at C..despite the fact that light's speed is 'constant' and one is moving faster than the other.

[douglas]

AngleWyrm_2 said:
If light is being emitted from a source at 3x10^8m/s and I travel away from that source at 1x10^8m/s, then do you believe that it will still approach me at 3x10^8m/s? What about the guy I pass along the way; does it approach him at 3x10^8m/s also?

It seems to me that the light will reach the guy I pass before it reaches me.

The stationary observer will indeed measure that the light's velocity relative to you is only 2x10^8m/s. For you, however, time will pass more slowly. When you measure the speed of light, you might come up with the same distance covered as the stationary guy measured, but your figure for the time will be lower. As a result, your measurement of light speed relative to yourself will come out exactly the same as the stationary observer's measurement of light speed relative to himself. Actually, your measurement of the distance will be somewhat off as well, but the net effect is that no matter what your speed, any attempt to measure the speed of light in a vacuum relative to yourself will always give the same result.

[AngleWyrm]

So I'm zooming away from an pulsar, and pass a stationary witness, who observes me travelling at 1/3c and the light from the pulsar flying along at c, closing the gap between itself an myself at a rate of 2/3c (twice my speed).

On the one hand, the pulses would reach me with longer gaps between (from the stationary witness point of view), because I am racing away from them (doppler effect). Does this time compression thing say the amount of time between pulses would be the same to me as it was when I was stationary, because time has slowed down for me?

This seems to say that I cannot perceive a change in pulsar timing, no matter what my speed.

[douglas]

I don't know the exact equations for it, but you actually would observe a difference in the pulsar's frequency depending on your speed. Because you are moving away from it, each pulse would have a greater distance to travel to reach you, and so the observed period would increase by the time required for the light to cover the extra distance since the last pulse reached you. This increase in period is exactly linear with respect to your speed relative to the pulsar. The time dilation due to high speed movement is not linear, so it will generally not exactly cancel the doppler effect. There might be one particular speed where you would observe the same pulsar frequency as the stationary observer, but in most circumstances it would be different.

[AngleWyrm]

If I use the doppler effect of pulsar timings to measure my speed, then these two statements seem to indirectly conflict:

"As a result [of lower time figure], your measurement of light speed relative to yourself will come out exactly the same as the stationary observer's measurement of light speed relative to himself."

"The time dilation due to high speed movement is not linear, so it will generally not exactly cancel the doppler effect."

It would help if we had the formula used for this.
EDIT: Found a derivation of time dilation which is easy to read.