Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Titanscape on 14/02/2012 16:47:53
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How does it work, traveling at light speed means you reach your destination via curved space, getting home before you left?
Does the sun's light reach the earth before it leaves the sun, or is it eight minutes old?
Can we see the billion year old sunlight?
Have you recommended reading?
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How does it work, traveling at light speed means you reach your destination via curved space, getting home before you left?
Nope - causality is not violated. Only massless particles travel at light speed and the concept of change and time for a massless particle is more philosophy than science at present; but there is no thought of anything happening before the cause. You might want to explain more why you think there might be - and we can nail down the problem
Does the sun's light reach the earth before it leaves the sun, or is it eight minutes old?
It has been travelling from leaving the outside of the sun to the earth for about 8 minutes - it definitely does not reach us before it has left.
Can we see the billion year old sunlight?
(ignoring the fact that the "light" is initially created deep within the star and takes a long time to work its way out through multiple plasma interactions) Billion year old starlight yes - sunlight from Sol only if we were a Glyr away - that's a billion light years
Have you recommended reading?
On what area in particular - and how much of a mental workout do you want? I would always recommend Hyperphysics (http://hyperphysics.phy-astr.gsu.edu/hbase/hph.html) as a great resource. It is well maintained, at a single level of difficulty, and has a nice academic stamp of approval. Wikipedia (http://en.wikipedia.org/wiki/Astronomy) has some great science pages - especially the introductions; but it does suffer from a very uneven editing style and a range of complexity from infantile to post-postdoc (often in a few paragraphs)
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I'm sure I read or heard that the light takes circa 100,000 years before it erupts from the sun...bumping and grinding inside before figuring the way out !
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As you say, I think it does take a very long time.
The quasi-philosophical question is, "Once it escapes, how long does it take to get here?"
There does seem to be a bit of a paradox. A clock that travels at the speed of light would not advance (no ticks or tocks) but a clock cannot travel at the speed of light because it has mass, and light has no mass, so light cannot have a clock.
Personally (hate mail to follow) I think this is a bit of a "cop out". If you try to extrapolate what happens to time and mass when you get to the speed of light, the math can't really answer the question because it simply blows up! (Really nasty hate mail to follow.)
But fear not! There is a big clue (unbelievably nasty hate mail to follow). EVERYTHING that has mass in this Universe ages. Given enough time, it all decays. In other words, it is susceptible to the effect of time. But light has no mass, so it could care less about time.
The evidence is that light does not decay in the slightest, even when it has traveled from the other side of our known Universe to get here. My contention is that it could only possibly do that if, for it, there was no such thing as time.
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Better minds than mine would be able to explain strange ideas that might help. I once ran headlong into a discussion (http://www.scienceforums.net/topic/63263-why-is-the-speed-of-light/page__view__findpost__p__654925) on this matter that included the lines:
Define what you mean by "timeless".
If it is a statement like: the affine parameter describing a null geodesic cannot by an affine reparametrisation be understood as the time measured by a standard clock moving along the null geodesic, then yes, photons are timeless.
The problem is that the space-time interval for null geodesics, that is the paths of light is always zero. This messes things up.
No - I don't understand it either. The upshot of the discussion between two heavyweight mathematical physicists was I doubt this is a useful concept, but is as close to "photons being timeless" as I think we can get.
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I think the upshot of that discussion (in slightly more detail) is that there is no way to stretch our current theories to extend our notion of time intervals to a photon's reference frame (or even to define 'a photon's reference frame'), since current theories don't cover that case at all.
Introducing a special case that covers photons might be useful if we could test it by placing a clock on a photon, but we can't do that (light always moves at the speed of light compared to a clock, so it couldn't ride along on a photon). Introducing a special case without being able to test it is useless as science.
So we're left with no consistent theory describing time intervals for photons and no way to test any new theories we invent, so it's a bit of a scientific dead end.
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Well, it's about 'time', and that 'arrow' that create our 'reality'. In it we have 'propagation/speed', as defined by a clock and a ruler. Without a 'clock' there could be no propagation to be measured, and to take it one step further, without a measurable 'clock' you can't have a arrow.
Turn it around, can you imagine any circumstance in where you have a geometrical three dimensional room as ours, excluding the arrow and so all clocks, in where you could find anything 'moving/propagating'?