Question:
Time paradox and the speed of light?
anonymous
2008-12-20 17:22:09 UTC
If time is relative to the observer, doesn't that suggest the speed of light is not finite? and in 'theory' we could go faster than the speed of light, relative to an outside observer?
Eight answers:
Larry454
2008-12-20 17:39:25 UTC
Hi Al -



Time is relative, but the speed of light is not. One of the tenets that got Einstein thinking in the first place was the observation - not the theory, but the real data - that the speed of light is constant, regardless of the motion of the source or the observer (within inertial reference frames). What this means is that if you travel away from the sun at 99% of the speed of light, the light from the sun will still pass you at the speed of light - relative to you. And if a galaxy is receding from us at 99% of the speed of light, the light from that galaxy will still pass us at exactly the speed of light - relative to us (in fact, this is nearly the case with some recent observations of very distant galaxies).



This is not terribly intuitive, but it is absolutely accurate. No matter how fast you move away from (or toward) the sun, the light from the sun will still pass you at the speed of light. You cannot catch those photons. In fact, in a way, you are no closer to the speed of light when you are moving that fast than when you are standing "still" on the earth's surface.



The net effect of this is that, since the speed of light does not change with moving reference frames, other things like time, length, and mass must change as viewed from one frame to another. A simple equation like distance = rate x time changes dramatically when the rate is fixed.



As far as we know today, this is the way the universe works.



ADDED: OK, think of it this way. You are standing on an asteroid in empty space. A star ship passes by , and that ship is built entirely of transparent material, with mirrored floors. As it passes at near light speed, the pilot shines a flashlight straight down toward the floor. In his frame of reference, the light from the beam travels straight down and bounces straight back up, like this "|". But as you see him pass, the beam, in your frame of reference, travels down and then back up at an angle, because of the relative speed of his frame compared to yours. To you, the photons travel in a path that looks like a "V". The distance that he measures for the light path is shorter than the distance that you measuer for the same light path. You are both right.



The strange part occurs when you measure the speed. He sees the beam traveling at the speed of light relative to him. You also see the beam traveling at the speed of light relative to you. That doesn't seem like it could be right in our every day world, but it is the way that light works. Now, since the distance the light travels changes, but the speed remains the same, you can then see that the equation "distance = rate x time" is affected in a way that is not normal in our Newtonian experience. That is, if the distance changes and the rate remains constant, then the time must change. And that is indeed what occurs. The time to see the event of the light shining to the floor and bouncing back up again is different in your frame of reference than it is for the pilot. Neither is wrong. They are just different.



If you are saying that the speed of light cannot be constant, I'm afraid you will have to take that up with all the folks who have been measuring it for the last century or so. All light travels at exactly the same speed, whether we are moving toward or away from the source. Light from distant galaxies that are receding at 90% of the speed of light still arrives here at the speed of light relative to us.
Tahir Akbar
2014-04-29 00:36:16 UTC
As per Einstein's theory if an object attains speed of light its mass becomes infinite which is not possible. So attaining speed of light is impossible.
William W
2008-12-20 17:35:05 UTC
Relative to an outside observer, yes. Imagine 2 spaceships travelling at 0.6c (more than half the speed of light) away from each other. A third party observer would observe the combined separation of the two spaceships to be faster than the speed of light.



The speed of light is finite, and is very close to 300,000 km/s. However, if you are travelling at speed, from your perspective the speed of light will always be travelling the speed of light faster than you.



'in 'theory' we could go faster than the speed of light, relative to an outside observer?' Unfortunately this wouldn't be faster than the speed of light. As far as relativity tells us, c is the universal speed limit.
anonymous
2008-12-20 17:42:57 UTC
Actually the speed of light is finite. Whether you are traveling at the speed of light and some is approaching you also at the speed of light you are both observing each other as going the speed of light, not twice the speed of light. Hard to explain but this will cause time to slow down.
science_guy
2008-12-20 17:28:54 UTC
That is the idea yes - but thats what theory of relativity says - heres an easy example to understand:



if there is a train traveling at 60 kmph and in one of the carrages you throw a ball in the same direction at 10kmph then from an observer outside the train the ball is moving at 70kmph. However, if you fire a laser inside the train in the same direction, you'd expect the speed of the the laser to appear as speed of light + 60 from an observer, but its doesn't it stays as the speed of light and distorts time.
mattsmasher0000
2008-12-20 17:36:00 UTC
Firstly, time paradoxes are confusing! I like them, but they're still confusing. Secondly, the speed of light is the speed of light, regardless of relativity. Keep in mind that as you reach the speed of light, time slows down, relative to you. Nothing that we know of can even go the speed of light, except for light itself, so how would we test it anyway?

Hope this helps

o7
Chris S
2008-12-20 21:03:52 UTC
nope, the formulation of special relativity makes it so no matter how fast you are moving to a relative observer it will always be measured less than the speed of light. wikipedia has a good article on it, it is under Velocity-addition formula.
quet
2008-12-20 21:50:39 UTC
Yes ,Einstein's theory proves that u can go faster than speed of light and slower than it but not at the speed of light


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