I don’t mean they will both “see” the same events “at the same instantaneous moment”. They see the flashes at different moments, but the whole process of the flashes and the movement of the light and the observers’ individual “seeing” of the flashes at different instantaneous moments takes place in both frames “at the same time”, ie during the course of the thought experiment. When one sees a flash, something is going on in both frames at the same time, whether they both “see” the flashes at the same moment or not.

Einstein says in Chapter 9 that the moving train observer encounters the two flashes of light at c + v and c – v. That’s why that observer sees the B flash first. If the moving observer saw both flashes arrive at him at the same time, then he’d be seeing the fronts of both beams travel toward him at c. But if that happened, the embankment observer would see the A flash first at the speed of c + v.

The “relativity of simultaneity” was noticed by way back in history. Soldiers on the battlefield in Newton’s time knew that they heard cannon sound later than the actual firing of the cannon and delayed by more time the further they were away from the cannon. Renaissance scientists knew that simultaneity of “seeing” light and “hearing” sound was just relative and depended on the position and state of motion of the observer. Doppler wrote papers about this in the 1840s. In 1676 Romer knew that when he saw the moons of Jupiter, he was seeing them on a delayed basis.

I'm not sure that follows from general relativity though, does it?

Anyone remember Bell's rope?

That's not right, Eroica. If we assume normal deceleration and re-acceleration, B will see A's clock running fast during the deceleration and re-acceleration. If we assume "instantaneous" changing of direction without changing speed, B will see A's clock "jump ahead" at the switching point.

At no point, in either case, will A see B's clock running fast.

Note: The word "see" in this post does not refer to the actual detection of light waves by eyes or other detector. It refers to the perception of how and when events occurred.

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But it is accepted that in the atmosphere, the light and sound generated at the same distance simultaneously will not reach the observer at the same time.

However, under SR, it is required that two light beams generated at the same distance simultaneously will reach the observer at the same time.

If they don't, either they weren't at the same distance or they weren't simultaneous.

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Doh.

The Chinese discovered that thousands of years ago, by observing exploding skyrockets and firecrackers at a distance.

That is true for observers that are stationary relative to the two emitters. But the moving train observer is not stationary relative to the two emitters. In the Chapter 9 example, the flashes were at the same distance and occurred simultaneously when the train observer was at Point M’ along the track. But since it takes time for light to travel, and since the train observer was moving during that time, he moved closer to the oncoming beam from B, whilst he was “riding on ahead” of the beam coming from A. So he saw flash B first. There is nothing mysterious about this.

Doesn't matter. The speed of light is a constant regardless of the velocity of the emitters, and in SR the emitters are in motion relative to the stationary train observer.

Speaking of the constancy of c, Einstein mentions de Sitter in Chapter 7, The Apparent Incompatibility of the Law of Propagation of Light with the Principle of Relativity. I'd really like to hear how your "local c-regulator" theory gets around those binary stars.

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I think we're arguing about two completely different meanings of the phrase running fast. I mean that if B looked back at A with his telescope, he would think that A's clock was ticking more slowly than his. But when he turned for home, A's clock would suddenly seem to be ticking faster than his.

Imagine the clocks are transmitting pulses every second. If A's clock is, say, 30 seconds behind B's, how can it jump instantaneously to being 30 seconds ahead? What happened to the 59 other pulses in between?

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In the event of an instantaneous direction change, B would still receive all those pulses, and in the proper order. When B tries to "undo" the Doppler effect, though, figuring out where and when A was when it transmitted those pulses, things would look weird. It would almost seem (I think) kind of like A was in two different places transmitting two different pulses at the same time.

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SeanF

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The train is not in SR, it’s on the earth. SR is a science fiction story. It should have won the Nobel Prize for Literature.

If the “the emitters are in motion relative to the stationary train observer”, then you’ve got to make a choice. If you want the B flash to travel at c relative to the train, then its speed of light has to slow down relative to the source. In Einstein’s Chapter 9 it’s traveling at c + v relative to the train observer and c relative to the source. So you want to slow it down to c at the train observer? Why would you want to do that? Einstein said, “Now in reality.....” the train observer and the light are moving together at c + v. Why would you want to change that?

Ok, then you’ve got to make it depart the moving source at less than c. It can’t both leave the source at c and encounter the train observer at c while the source is moving. If it leaves the source at c, then you’ve got to add the velocity of the source, v, to c, and that would cause the light to encounter the train observer at c + v. But if you want the light to encounter the train observer at c, then it would have to leave the moving source at c – v, relative to the source, with v being the velocity of the source. And then you would have the velocity of the light at the surface of the earth traveling less than c.

If you want to consider the distance between the two as being great, so that the Vtot light speed transforms between two different comoving spaces, then it could leave the source at c, it could gradually change its Vtot while in route by slowing down relative to the train observer, then it could reach the train observer at c. Much like what we observe in the light from the distant galaxies. But by the time it reaches the train observer, it will be traveling at c – v relative to the source.

Man, this SR stuff has really messed up your way of thinking about how waves, light, and clocks work.


“By means of similar considerations based on observations of double stars, the Dutch astronomer De Sitter was also able to show that the velocity of propagation of light cannot depend on the velocity of motion of the body emitting the light.”

The velocity at the earth, when you view the light, when the photons finally reach us, is c relative to you and the earth. The motion of the “double stars” doesn’t change that. The Vtot change takes place in space, probably closer to the stars than to the earth, so that’s why we see red and blue shifts of revolving binaries, even though we encounter the light at c.

I don't understand why you guys keep arguing with Sam5 about the implications of SR. His last post there clearly illustrates that he doesn't understand what SR says, nor does he understand what observations show. Until he understands that, you can't have a conversation about its implications - you can't understand the twins paradox without first understanding SR.

Sam5, I'm trying to decide if I should even attempt to explain it to you. You seem reluctant to learn. But I'll give it one little try: I'll start with the basic error in your post about C and see if we can go from there.

The speed of light is always measured by everyone everywhere - on the train, on earth, on a satellite in space - to be C. Exactly, precisely C.

Yes, that's a postulate of SR, but set relativity aside for now: this is DATA and OBSERVATIONS. It is real and its the way light works and is indisputable even without SR to explain it.

I've explained that "Now in reality" phrase to you twice already. Ignoring explanations and continuing to bring up the same wrong concepts after they've been explained multiple times is the kind of thing that's gotten people banned from this BB before.
Read about de Sitter and the binary stars before you attempt to dismiss it, JW. It's got nothing to do with Doppler, and it's got nothing to do with the speed of the light when it reaches us.

Start with this page (it is, after all, the first response from Google on a search for "de Sitter" and "binary stars") and get back to me on the de Sitter effect once you understand what it is.

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Just making sure that we understand it. Or trying to...

So, uhh, this stuff about scientists measuring the speed of light passing the sun as slowing down is wrong? And so light can easily go out of "black holes" at the speed of c? Light is measured to go through glass at c and water at c?

After going back and reading this thread and the relativity thread, I'm not sure I understand any of it now.... :wink:

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Although you would have us believe you have a very good understanding of SR (you must if you claim it's wrong), you have either misunderstood some basic physics and GR analogies or you are being purposely obtuse with these questions. Light doesn't slow down passing the sun, its path is bent by the sun's gravity. Yes, light is moving at c trying to get out of a black hole. Depending on how you want to look at it, light's frequency is infintely redshifted or it's path is curved so much it stays within the horizon. Either way, it's not visible. And it's the time it takes photons to be absorbed and reemitted from the atoms in the water or glass that slows it down, but it's still moving at c between those interactions.

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And so, uhh, you believe in the “tired light” theory? The light just gets tired of traveling at “c” but getting no where while trying to get out of the black hole, and it just “gives up”?

My, my you are being obtuse. Ok, light's frequency is infintely redshifted by gravity. Now, please explain how a gravitational redshift is the same as tired light, if that's what you believe.

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