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Post by abacus9900 on Oct 10, 2010 10:05:41 GMT 1
We have been told many times that two observers, one moving at a large fraction of the speed of light in a spaceship and the other one on earth (say), will each see the other's clock as moving much more slowly than their own clock and that this is somehow tied up with the constancy of the speed of light, but why? How does light decide how quickly or slowly clocks, either mechanical or biological, run?
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Post by olmy on Oct 10, 2010 10:24:20 GMT 1
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Post by abacus9900 on Oct 10, 2010 10:33:33 GMT 1
Thanks, Olmy, I have looked at that but it is very hard going. I was hoping for a more intuitive explanation.
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Post by olmy on Oct 10, 2010 10:41:11 GMT 1
Thanks, Olmy, I have looked at that but it is very hard going. I was hoping for a more intuitive explanation. OK, well, you don't have to follow the maths to see why it's happening. If you set up a simple light clock that measures time by bouncing a light pulse between two mirrors... upload.wikimedia.org/wikipedia/en/5/5b/Time-dilation-001.svgYou can see that the light just has to travel twice the distance between the mirrors for one 'tick'. If you look at this from the point of view of someone rushing past at high speed, though, the light has to travel further.... upload.wikimedia.org/wikipedia/en/a/a5/Time-dilation-002.svgAs the speed of light is constant, relative to both observers, the time between ticks must be different.
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Post by abacus9900 on Oct 10, 2010 11:30:05 GMT 1
Ok, I get that bit, but does that mean then that all physical processes slow down at great speeds due to the way light mediates forces between matter? In other words, does light itself act as the great material 'clock' of all matter? And why does the clock of the other observer appear to slow down in both cases?
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Post by olmy on Oct 10, 2010 11:47:26 GMT 1
Ok, I get that bit, but does that mean then that all physical processes slow down at great speeds due to the way light mediates forces between matter? In other words, does light itself act as the great material 'clock' of all matter? And why does the clock of the other observer appear to slow down in both cases? Second bit first. The situation is symmetrical because the first diagram relates to what I would see if I were stationary relative to the light clock and the second to what I would see if I were travelling relative to the clock. We have not made any assumption about which observer is moving, in fact one of the points is that there is no way to do so. On your first point, the argument does not depend on how one measures time - obviously we could compare light clock to light clock but you could actually use any timing mechanism to time the light pulses in the light clocks. The fact that the situations are symmetrical means that we cannot be looking for some sort of process that affects 'moving' clocks.
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Post by abacus9900 on Oct 10, 2010 13:34:08 GMT 1
Ok, I think I get that it is the *relative* motion of observers that is the crucial idea, regardless whether one is actually moving or not, but why is it so important that the speed of light must always be constant?
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Post by eamonnshute on Oct 10, 2010 13:58:26 GMT 1
why is it so important that the speed of light must always be constant? It is an observed fact (in vacuo). Time dilation is a consequence of this fact.
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Post by olmy on Oct 10, 2010 14:23:26 GMT 1
...why is it so important that the speed of light must always be constant? Not sure how one would interpret 'important' here but it's really just a matter of whether it is or not and, as eamonn says, it i s observed to be. At the time, I think, it was based on the Michelson and Morley experiment and Maxwell's theory of electromagnetism. Of course, since then, there have been many more experimental tests of special relativity.... math.ucr.edu/home/baez/physics/Relativity/SR/experiments.html
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Post by abacus9900 on Oct 10, 2010 14:54:41 GMT 1
Well, I appreciate that time dilation is counter-intuitive but do we really know what causal link there is between time slowing down (the more the difference in relative velocities of two different observers) and the speed of light? I know the speed of light is always going to be constant but it seems to be to be a rather circular argument to say that light beams take longer to traverse distances the faster you are moving (relative to some other observer) because it always matches its speed to the on-board observer. I seem to be missing some underlying key idea here. Or is that just the way it is as observed and you can't really use logic or intuition to work it out? And why do two frames of references appear to one another to slow down all physical processes? I'm more confused than ever.
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Post by abacus9900 on Oct 10, 2010 14:56:39 GMT 1
...why is it so important that the speed of light must always be constant? Not sure how one would interpret 'important' here but it's really just a matter of whether it is or not and, as eamonn says, it i s observed to be. At the time, I think, it was based on the Michelson and Morley experiment and Maxwell's theory of electromagnetism. Of course, since then, there have been many more experimental tests of special relativity.... math.ucr.edu/home/baez/physics/Relativity/SR/experiments.htmlOlmy, so would it be true to say that we don't really know why time dilation occurs?
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Post by olmy on Oct 11, 2010 11:08:34 GMT 1
I must admit that I'm not sure where your confusion lies, abacus.
You say that "[light] always matches its speed to the on-board observer", which isn't really the case. What happens is that every observer sees light travel at the same speed relative to them.
This is odd but leads directly to the conclusion that time and space are relative. That is why time dilation occurs.
I'm not sure what other sort of 'why' answer you might expect. Why is the universe the way it is and not different?..... no idea.
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Post by speakertoanimals on Oct 11, 2010 17:10:10 GMT 1
THere isn't a causal link other than light (like everything else) moves in the same space and time.
What we mean by space and by time depends on your point of view, so that different observers have different opinions about lengths and times. Just happens that the way distances and times are related rests on a cosmic speed limit, and that light just happens to travel at that speed limit. But it isn't light that's funny, its just the way space and time knit together.
So, if I'm moving, part of my space becomes part of your time, and vice versa. Hence time dilation, length contraction and so on. But you're getting it wrong if you're looking for a causal link, as if it is all some sort of great illusion. It just seems to be that that is the way that time and space ARE, and that a fast-moving object (like light) is the easiest way to detect that.
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Post by abacus9900 on Oct 11, 2010 19:09:43 GMT 1
Well, what I meant was that light always travels at the same speed regardless of the frame of reference of the observer. That's correct, isn't it? It is very odd though, and I wonder if the answer will ever be known. It is as if, to maintain the light speed limit a trade-off between space and time takes place where time must slow down in order for two inertial frames of reference to see light as the same speed. Why isn't it much simpler where an absolute time rules everywhere in the universe? Things would be much simpler and Newton would have been correct!
Olmy, is it true, at least in theory, that if you could exceed the speed of light you could travel back in time, or is this just an 'old wives tale?'
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Post by abacus9900 on Oct 11, 2010 19:18:20 GMT 1
Well, obviously and it can only mean one thing: that time and space and light are all unified in some way we do not yet understand.
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