# Thread: The Speed of Light is the same to all observers?

1. I know the theory has been substantiated pretty well, but intuitively it seems weird to imagine that all observers would perceive light to be moving at exactly the same speed regardless of direction and temporal perspective.

It seems like if you were moving at 1/2 of C in one direction, and a beam of light passed you going the opposite direction that you'd perceive it as going faster than C relative to you.

Is that perhaps not what the theory suggests? I'm just curious how it all comes together, because there could be some things I'm not factoring into it.

What if you're traveling at 1/2 of C and two beams of light pass you, one from in front of you going the opposite direction, and the other from behind you going the same direction as you? Would you perceive both beams as traveling at the same speed?

2.

3. i think ive heard somewhere that all light seems to move the same relative to you because time is dilated more as you approach c

dont take my word on it though

4. It’s all in the maths.

Let’s say two objects are travelling at velocities v and w relative to an observer O. Then according to special relativity, the relative velocity between the two objects is

v<sub>rel</sub> = (v+w)*∕*[1+(vw*⁄*c<sup>2</sup>)]

In the first case, when the light beam is travelling in the opposite direction to you, w = −c. Substitute that into the equation for v<sub>rel</sub>. You’ll get v<sub>rel</sub> = −c. (The minus sign indicates that motion is in the opposite direction to yours.)

In the second case, the light beam is moving in the same direction as you, so w = c. This yields v<sub>rel</sub> = c.

Either case, you won’t get any speed greater than c for the light beam.

5. Originally Posted by kojax
I know the theory has been substantiated pretty well, but intuitively it seems weird to imagine that all observers would perceive light to be moving at exactly the same speed regardless of direction and temporal perspective.

It seems like if you were moving at 1/2 of C in one direction, and a beam of light passed you going the opposite direction that you'd perceive it as going faster than C relative to you.

Is that perhaps not what the theory suggests? I'm just curious how it all comes together, because there could be some things I'm not factoring into it.

What if you're traveling at 1/2 of C and two beams of light pass you, one from in front of you going the opposite direction, and the other from behind you going the same direction as you? Would you perceive both beams as traveling at the same speed?
My own opinion is that light speed is radiated at 'c' from the radiating source only.
It is the electric fields that transmit the light and so you would have to add or subtract the spatial velocity of the radiating source to get the 'c' as relative to space.

So if the observers are looking at this light source from different directions, you have to either add or subtract the local space velocity to the light speed to get the real velocity relative to the various positions of the different observers.
A radial motion of the object would require adding or subtracting but when the motion is transverse, there would be no change in 'c' relative to the observer.

Cosmo

6. JaneBennet, so when w=-c, then v=0, and when w=c, then v=c? Otherwise, if v=c in both cases, then when w=-c, the result is zero divided by zero.

7. Yes, the formula breaks down for objects travelling at light speed in opposite directions. :? It does work in all other cases though – even for objects moving at light speed in the same direction.

8. The idea is that the faster you move, the slower time moves for you through "time dilation". This would only work if time were a dimension (though it seems to only work one way, as in slowing down). If moving fast merely slows down things on a quantum scale, even nuclear decay would take longer. It is assumed that EMR would not slow down but remain at a constant speed. However, a Bose-Einstein Condensate can slow down light as can dense materials (as in neutrinos coming from the sun's centre) so in a "slower medium", light may even slow down.

9. Wait, I think I have the answer.

Let one of the objects have velocity v and set the velocity of the other object at −c (i.e. it’s moving at light speed in the opposite direction). Then

This is equal to −c so long as v < c. However, as v approaches light speed, notice that

So the relative speed between two objects moving at light speed in opposite directions is still not greater than c. We can’t insert the values directly into the relativistic relative-velocity formula – but we can do some mathematical juggling instead.

10. This still leaves me wondering something, though. If I were traveling at 1/2 C, with the one beam of light coming from in front of me, and one from behind me, would I perceive them to be approaching me at the same rate?

If they started out at equal distances from me, would I perceive both to be hitting me at the same time?

11. Use the formula. When w = ±c, the v cancels out of the equation. It doesn’t matter whether v is ½c, or any other value.

12. Originally Posted by kojax
This still leaves me wondering something, though. If I were traveling at 1/2 C, with the one beam of light coming from in front of me, and one from behind me, would I perceive them to be approaching me at the same rate?

If they started out at equal distances from me, would I perceive both to be hitting me at the same time?
Yes to both.

13. Originally Posted by Cyberia
The idea is that the faster you move, the slower time moves for you through "time dilation". This would only work if time were a dimension (though it seems to only work one way, as in slowing down). If moving fast merely slows down things on a quantum scale, even nuclear decay would take longer. It is assumed that EMR would not slow down but remain at a constant speed. However, a Bose-Einstein Condensate can slow down light as can dense materials (as in neutrinos coming from the sun's centre) so in a "slower medium", light may even slow down.
The slowing down thing is only relative though, if you were used to travelling faster, it would seem that time gets 'faster' when you slow down.

I found this on wikipedia and was thought i would just quote it in relation to your post

'In 2003, Mikhail Lukin, with scientists at Harvard University and the Lebedev Institute in Moscow, succeeded in completely halting light by directing it into a Bose–Einstein condensate of the element rubidium, the atoms of which, in Lukin's words, behaved "like tiny mirrors" due to an interference pattern in two "control" beams.'

14. i dont know

15. Describing things like this through math is lazy.

The people who made these equations did not say, "Hey, lets use my equation that I will discover to find itself out!" No, they actually described the process through words , then made the equations for quick manipulation of the laws of nature to find things out much faster. Ideally we would not use equations ever, but people have week minds (including myself) and must in an effort to preserve efficiency.

There is an explanation for this that does not deal with math; maybe someone would care to share it? or maybe, no one really knows why.

16. Originally Posted by kojax
I know the theory has been substantiated pretty well, but intuitively it seems weird to imagine that all observers would perceive light to be moving at exactly the same speed regardless of direction and temporal perspective.

It seems like if you were moving at 1/2 of C in one direction, and a beam of light passed you going the opposite direction that you'd perceive it as going faster than C relative to you.

Is that perhaps not what the theory suggests? I'm just curious how it all comes together, because there could be some things I'm not factoring into it.

What if you're traveling at 1/2 of C and two beams of light pass you, one from in front of you going the opposite direction, and the other from behind you going the same direction as you? Would you perceive both beams as traveling at the same speed?

If you were God perceiving the system, as some scientists think they are..........you would, if travelling at 1/2C with C beams approaching you, you would perceive a "wormhole" relevant enough to your perception that justifies this distortion. The wormhole you would perceive would beckon you in. Basically you would create a "c" mandate, like you would by your abrupt actions create a hole in space-time.

Think about it, then get back to me.

Obviously we currently officially do not have the technology to achieve this, but that does not stop us from hypothesising.

Why should technology, proof, stop us from hypothesising........seriously, why?

17. Originally Posted by Stuart Thomson
Originally Posted by Cyberia
The idea is that the faster you move, the slower time moves for you through "time dilation". This would only work if time were a dimension (though it seems to only work one way, as in slowing down). If moving fast merely slows down things on a quantum scale, even nuclear decay would take longer. It is assumed that EMR would not slow down but remain at a constant speed. However, a Bose-Einstein Condensate can slow down light as can dense materials (as in neutrinos coming from the sun's centre) so in a "slower medium", light may even slow down.
The slowing down thing is only relative though, if you were used to travelling faster, it would seem that time gets 'faster' when you slow down.
And that's the funny thing. If you had always been moving at 1/2 C in one direction (say "North"), and you slowed down, you'd think you were speeding up in the "South" direction. Wouldn't you?

I found this on wikipedia and was thought i would just quote it in relation to your post

'In 2003, Mikhail Lukin, with scientists at Harvard University and the Lebedev Institute in Moscow, succeeded in completely halting light by directing it into a Bose–Einstein condensate of the element rubidium, the atoms of which, in Lukin's words, behaved "like tiny mirrors" due to an interference pattern in two "control" beams.'
That is a very interesting point. Even though the condensate is moving at the rate that the Earth is moving (because it's on Earth), it has so little internal motion that it seems, for some purposes, to be experiencing frozen time, like it might if it were traveling at the speed of light or something.

I wonder how that fits in with relativity.

18. Originally Posted by kojax
I know the theory has been substantiated pretty well, but intuitively it seems weird to imagine that all observers would perceive light to be moving at exactly the same speed regardless of direction and temporal perspective.
a stationary observer would see light moving at a diffenert speed than that of a moving observer that is what special theory of relativity states.

19. Originally Posted by unoscooter
a stationary observer would see light moving at a different speed than that of a moving observer that is what special theory of relativity states.
NO! It is the opposite. The basic law is that the speed of light is independent of any reference frame. "Relativity" only means that phenomena like time and space can only be described relative to an observer. There is no universal timescale or space fabric.

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