First, he doesn’t need to shine his torch at anything. We see galaxies moving away from us at 98% of the speed of light just fine from their own light. They look entirely normal. Why? Because time moves normally from them just as it does for us. It doesn’t matter which reference frame we pick. We’re moving away from them at the same rate. Doesn’t matter, because there is no absolute frame of reference with which to compare both of them simultaneously. It’s that impossibility of simultaneous reference that is, again, Einstein’s insight.
But we cannot synchronize our clocks with them because they have never been in the same frame.
The astronaut who has been on earth cannot synchronize his clock with the earth clock while he is an accelerating frame of reference. He does have to wait until both clocks are in the same frame of reference. The earth has not been accelerating; only the astronaut has. They have been through two different space-time routes. General theory, not special theory. Once the astronaut gets back, it is then and only then possible to see the effects on his clock of his having taken a longer space-time route to get back to the same place, thus affecting his time.
So the theory doesn’t tell you “what you will see from any particular frame of reference.” It tells you what your clocks will read once they have been brought back to the same frame of reference. That’s a totally different thing.
And if you want to be believed when you say “I’m not at all rejecting the theory” you’ve got to stop saying things like “Count me in as an anti revolutionary.”
You’ve again misunderstood what I’ve said. The torch was there merely to show that within his frame of reference the astronaut is stationary and everything relative to him behaves normally, including light.
And let’s do away with acceleration entirely and assume (for arguments sake - ignoring the practicalities of such operation) that the spaceship does not accelerate all of its journey, that, say, the star system is 10000 light years away and it accelerates/decelerates for only 1% of it’s total journey time - insignificant to the journey as a whole. The rest of the time it coasts at approximately 0.98c relative to the earth.
I guess my central problem with all the explanations given are that respondents repeatedly state that the spaceship is moving whilst the earth observer is ‘stationary’ indicating that somehow earth has a magical position on the universe as a unique stationary point which runs against the whole basic premise of relativity.
The problem with your Europan analogy is that we aren’t very different from them in the sense that we can only detect a small band of all possibilities using our evolved senses. We can only see light around the 400-700 nm range and we can’t hear very high pitched or very low pitched sounds. We have no idea what it would be like to be an ant, who lives in a world of complex chemical signals from hundreds of invididuals in her immediate vicinity. Or a mole, who uses hyper sensitive tactile sensations to explore his world…
However, we are a technological race and can build machines that can detect such phenomena and translate it into a language we can understand. Presumably an intelligent, social, and technological echolocating race would as well. There’s no reason they wouldn’t know as much about EM radiation as we do.
“Anyhoo such considerations are irrelevant as relativity (wisely) does away such fixed points of reference and correctly predicts what, from your own frame of reference, will be observed (as I have said before, this I do not dispute at all).”
You need to understand the difference between speed and acceleration. The earth is moving through space, but it is not accelerating though space. Moving in a direction opposite from earth is not deaccelleration (ignoring for the moment that there is only acceleration). Again, think of the elevator. Suppose you were blindfolded and told you were either in an elevator or standing outside an elevator. You can tell the difference because in the elevator you feel the acceleration (you momentarily feel heavier, then normal, then lighter).
Maybe Dave doesn’t realize there’s a difference between speed and acceleration? Earth, relative to the sun, is traveling quite fast, but it’s not accelerating. That’s why we don’t feel like we’re moving. If earth started speeding up (accelerating), we’d all be pushed back in our chairs so to speak, and would notice we are accelerating. This is general relativity. Acceleration is exactly like being in a gravitational field. Earth’s gravity, makes it feel as if the earth is pushing us against the ground at 1G of acceleration. So, accelerated movement, and being in a gravitational field is the same thing to the universe. Correct me if I’m wrong, but if you could somehow stand at the threshold of the event horizon of a black hole, it would have he same time-like effect of traveling at the speed of light.
Your astronaut is accelerating, and the people of earth are not. THERE is where the time dilation thing happens.
The Europans - they were an abstract example. Yes I know they would probably have discovered EM radiation were they real - i was postulating an artificially created race who could perceive the world only through sound to illustrate my point.
… in the same way physicists use the idea of 2 dimensional species (also extremely unlikely to exist) to explain such issues as the curvature of space and extra dimensions in space time.
It’s assumptions like these that make me roll my eyes. How stupid do you think scientists are? Everything imaginable is taken into consideration, especially the idea that we might not be able to detect something directly. It’s almost what science is all about. Reverse engineering the universe, so to speak. This is also what makes science hard… especially when the results of such don’t meet intuitive expectations (and then explain this to the laymen). Sound familiar?
Ah … so what you are saying is that velocity is unimportant (I assume you mean velocity rather than speed)? So … if two objects have been travelling toward each other at a constant velocity having once been at rest relative to each other and accelerated to .98c would notice no relativistic effects?
i.e. each would have accelerated approximately 0.49c (I can’t be bothered to do the maths) relative to an observer at either of the starting points?
Each would have accelerated the same and so any time dilation due to accelleration would be the same…
An observer on either one of these objects wouldn’t see time slowed down on the other relative to their frame of reference?
That’s very much different to my understanding…
As for the earth accelerating, were the spaceship moving at a constant velocity, the earth would actually be the one changing velocity all the time - relative to the spaceship at least. And if I am not mistaken a change in velocity amounts to what is commonly termed acceleration. (Please note i want only to consider relativity due to velocity here - let’s concentrate on that for now I have no real problem with the effects of gravitation).
Say that there was a prearranged agreement that NASA control on earth would send out a message when the astronaut was halfway to Lalande 21185. So 5 years after launch they send out a message at light speed.
The message needs 4.11 years to reach the halfway point. But the astronaut is almost to Lalande by then, so he doesn’t see the message.
He reaches Lalande and starts to return home. He and the message finally meet, about nine-tenths of the way to Lalande. (Remember the astronaut is moving slower than the light beam. You can use calculus to figure this out exactly, but there’s no need for that level of precision.) The message has been traveling for about 7.5 years.
The astronaut, by his internal time, has been traveling for 4.5 years plus the slow part of his return journey for another one-tenth of the distance.* Say, another 3 years. So about 7.5 years total have elapsed for him.
So he reads the message and compares times. The elapsed time on both messages is about the same. But the date stamp on the message is five years after he started. He’s lived 7.5 years. The message has lived 7.5 years. The people on Earth have lived for 12.5 years.
Now that they are in the same frame of reference he can see what has been happening on earth. He and the message feel the same amount of aging because both have lived their lives at relativistic speeds. They haven’t felt a thing different. Time has just passed normally for them. Same for the people on earth who haven’t lived at relativistic speeds in their frame of reference. Their time passes normally too. It just isn’t the same amount of time. But they can never directly check this until the clocks are in the same frame of reference.
Frame of reference and the absolute speed of light. They make all the difference in the universe.
*He is accelerating at 1 gravity. It takes time to ramp up to that speed, so most of the relativistic effects happen in the last part of the accelerated voyage.
Again you’re missing my point. Relativity accurately predicts the behaviour of light and allows for corrections in the received signal I don’t refute this.
Huh? That comment was in response to the idea that scientists don’t take anything but their own evolved senses into consideration. Which is utter nonsense. The thought experiment of the Europans I was not knocking in any way (no matter how flawed it actually is). You were making a direct and insulting criticism of science in general, and I was merely pointing out how invalid and silly it was.
Thought experiments I regard when they are accurate and actually expose a truth. Einstein had a lot of these… look into them.