# Speed of light

I dont understand how the speed of light can be constant to all observers. For example if one observer travels alongside a beam of light and another observer travels towards the same oncoming beam of light then that one beam of light would have to be travelling at two different speeds simultaneously and would seem to take two different times to get from A to B. Can someone explain why I'm wrong?

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#### Dr.Brain

The second postulate of Special Relativity states that:
"The speed of light in vacuum remains same for all inertial reference frame"
What you just typed in your post was equally thought by the scientific community before einstein . There are many specific reasons for the second postulate . This also follows from the logic as said by man 'cocktailparty-philosophers'.

In first case when the person travels along the beam with the same speed as that of light , he is infact no more a person who can see , but now his whole mass will change into a form energy. In second case when a person travels towards the light , lets say with speed 0.5c , then the velocity of light as seen by him is given by:

$\frac { 0.5c +c }{ 1+ \frac{(0.5c)(c)}{c^2} }$

which is equal to c.

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#### rbj

I dont understand how the speed of light can be constant to all observers. For example if one observer travels alongside a beam of light and another observer travels towards the same oncoming beam of light then that one beam of light would have to be travelling at two different speeds simultaneously and would seem to take two different times to get from A to B. Can someone explain why I'm wrong?
you might be assuming that both observers view the other's clock (which is used to measure speed of light) as ticking at the same rate as their own clock. that assumption is not correct.

r b-j

#### nykill

the speed of any thing is nothing but the distance covered by the body in the particular time and its always the same no matter from which point of reference u see. its only varies when consider a point which is to be crossed by the body and u observe from two different dimentions, in that case u find two different times.

#### Kazza_765

if one observer travels alongside a beam of light and another observer travels towards the same oncoming beam of light then that one beam of light would have to be travelling at two different speeds simultaneously and would seem to take two different times to get from A to B. Can someone explain why I'm wrong?
Try reading up on the special theory of relativity. It is surprisingly easy to understand if you let go of a lot of prejudices and can understand pythagoras' theorum, and a remarkably beautiful theory in my opinion. I'm pretty sure (though its been a little while since I studied relativity) that your absolutely correct - it would seem to take two different times to get from A to B. In fact, from a reference frame travelling at the speed of light the time taken to get from A to B would be zero no matter how far apart they are (and you can probably start to see why you cant travel faster than light).

The theory of relativity doesn't prove that two observers will always measure the speed of light at 3*10^9 m/s, this has been shown experimentally, relativity explains it though.

#### brahmaphysics

THis was the same question occurred to me when i first glimpsed special relativity. Remember why you do see it only in the context of speed of light and not on the context of distance travelled by light? I'd suggest you a very simple suggestion. When you are going toward the incoming light then the light will have to cover a smaller distance. if you are going away from it than the light have to cover a larger distance. so at a constant speed light will cover the same distance at two different amount of times which clearly depends upon your frame of reference. i suggest u read the book on special relativity by resnick.

#### Integral

Staff Emeritus
Gold Member
You will never find the answer to "Why is the speed of light constant" in a book on special relativity. Recall that Einstein made the constancy of the speed of light a POSTULATE, he makes no effort to prove that it is indeed constant. He only explores the EFFECTS of a constant speed of light. If you wish to see the source of his postulate you must study Maxwell's equations. Even there you will not find WHY it is constant, only that it is.

Physics cannot answer the question WHY, it observes, measures and makes predictions based on those observations and measurements but can not tell us why the physical universe is the way it is.

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