speed of light confusion

[goomadeer]

Howdy folks I'm new here. Just another silly question regarding this stuff!

 

I think when I stumble over layman's descriptions of the Theory of Relativity its telling me that light travels past objects at the same speed no matter how the source of the light and the destination are moving in relation to each other which seems insane to me. Am I interpreting this right? And if so.. what would happen in the below scenario?

 

A light is switched on in a vacuum with two moving objects a light second away at that exact time. Object 1 is moving away from the source at 1/3 of the speed of light, and Object 2 is moving away at 2/3 of the speed of light. Surely this theory says that the light will reach both at the same time still.. is this right? And if it is right how can this be so when Object 2 is further away than Object 1 - and therefore the light has moved further or faster to reach both objects at the same time?

 

Any help appreciated!

 

Paul

Edited August 10, 2010 by goomadeer

[timo]

Descriptions of the Theory of Relativity its tell me that light travels past objects at the same speed no matter how the source of the light and the destination are moving in relation to each other which seems insane to me.[...]

A light is switched on in a vacuum with two moving objects a light second away at that exact time. Object 1 is moving away from the source at 1/3 of the speed of light, and Object 2 is moving away at 2/3 of the speed of light. Surely this theory says that the light will reach both at the same time still.. is this right?

Relativity says that the light pulse travels at c, regardless of the motion of object 1 and object 2 (that's only expected: when I throw a ball towards you, why should the ball suddenly change its motion just because you start running?). It also tells you that the light pulse travels at c regardless of the motion of the source (that's the unexpected part which seems illogical to you because you're used to think that velocities did add up). It definitely does not say, that a light pulse takes the same time from A to B as it takes from A to C, regardless of A, B and C. In other words: since the two objects are not the same distance from the light source, a direct consequence of the light traveling at c is that is will not arrive at the same time.

[goomadeer]

It definitely does not say, that a light pulse takes the same time from A to B as it takes from A to C, regardless of A, B and C. In other words: since the two objects are not the same distance from the light source, a direct consequence of the light traveling at c is that is will not arrive at the same time.

 

 

But B & C were the same distance from A when the light started.. and light is going at both B & C at the same speed relative to them - if it takes longer to get to C because C is moving away faster doesn't that disprove the theory? Sorry I'm obviously missing something here

[pioneer]

Light is not part of relative reference but is an absolute reference. If C is the same in all references, there must also be an absolute zero reference, since all the references would need a zero state that is not relative, so they can measure the same value of C. I theorized once that the generation and creation of photons, via matter, must occur at the zero reference or else all references would not be able to measure C, regardless of their reference.

[Sisyphus]

But B & C were the same distance from A when the light started.. and light is going at both B & C at the same speed relative to them - if it takes longer to get to C because C is moving away faster doesn't that disprove the theory? Sorry I'm obviously missing something here

 

Sure. If they're the same distance away when the light is emitted, it will take the same amount of time to reach you, always.

[swansont]

But B & C were the same distance from A when the light started.. and light is going at both B & C at the same speed relative to them - if it takes longer to get to C because C is moving away faster doesn't that disprove the theory? Sorry I'm obviously missing something here

 

 

They are not in the same reference frame as you. Light moving relative to you (and your frame) moves at c. Light moving relative to something else does not move at c, relative to those objects, as measured in your frame.

 

——————

 

 

 

Light is not part of relative reference but is an absolute reference. If C is the same in all references, there must also be an absolute zero reference, since all the references would need a zero state that is not relative, so they can measure the same value of C. I theorized once that the generation and creation of photons, via matter, must occur at the zero reference or else all references would not be able to measure C, regardless of their reference.

 

!

Moderator Note

If it's theorized by you and not part of standard physics, it belongs in speculations, not here

Edited August 10, 2010 by swansont
add clarification

[timo]

The time it takes the ball I throw to you to reach you depends on

1) it's speed and

2) the distance from the point at which I stood when I threw the ball and the point where you stand when you catch the ball.

Where you stood when I threw the ball or where I stood when you caught it does not matter.

 

Perhaps it helps your understanding to know that relativity is completely irrelevant for the point I am trying to make. My point is almost trivial. The reason why you are confused -and I only understood that after reading Swansont's comment which looked very strange at first- is that you are making additional assumptions that you do not mention and which are wrong.

 

MAJOR EDIT/REWRITE:

 

Goomadeed, ignore the above, I overread the bold part in your following statement

But B & C were the same distance from A when the light started.. and light is going at both B & C at the same speed relative to them.

There is two answers:

1) Light moves at c absolutely. That does not rule out that it moves at some other distance relative to some other moving object. That is the easy answer. And also the one above by Swansont.

 

2) Now the tricky part:

Considering the light source as static and the objects as moving is somewhat arbitrary. Couldn't you just watch the scene from a different perspective where object 1 is static and the rest moves? Then, the light takes t1 = d1/c to reach object 1 where d1 is the distance of object 1 to the source. Or an angle where object 2 is static and the rest moves. You'd have t2=d2/c, then (the speed of light, c, is the same in both cases by definition). And since in your construction, in the perspective where the source is static, the distance of the source to the objects at the point of emission was equal, you'd expect d1=d2 and hence t1=t2. So obviously, a constant c is not possible by that reasoning. That was your reasoning, right?

Problem is: the assumption that d1=d2 seems so obvious that you'd never question it (except if you already know Relativity, of course). But it's wrong. To allow for constant c you must allow that in different perspectives distances are measured differently. It gets worse: in relativity you allow that different perspectives measure time intervals differently (as a matter of fact, t1=t2 would indeed not mean that the light arrives at the two objects simultaneously). It gets even worse: relativity allows for distances and time intervals to convert into another. What might be 1 second for A could be 1.1 second and 100 kilometers from the perspective of B (except that the numbers probably don't match). This mixing is not completely arbitrary but well defined, its rules are the Lorentz transformations. You'll understand that I won't start explaining Relativity now (your example would be a very bad starting point anyways because it approaches it from the direction where it looks ridiculous) so just a few more or less random comments:

* If you stay within my point 1) you get the correct result and you didn't need Relativity for it.

* Unequal distances for seemingly the same ... "distance" ... seem completely ridiculous. There's two things to do about it (in my opinion): 1) accept that it's unintuitive but experimentally proven and live with it. 2) Learn Relativity by the math, not by examples and so-called "thought experiments". Physical Relativity is not intuitive so you can as well go with math which not intuitive but rigorous rather than so-called examples which are neither. I doubt that there's any other way to learn relativity than to learn the math, then develop an intuition for the math, then try applying to actual problems.

Edited August 10, 2010 by timo

[Fuzzwood]

It's very simple. Will the ball accelerate to a new speed when you start moving in order to reach you in the same time as if you would have stood still? Same goes for EM waves

[HiggsBoson]

Object 1 will see the light first. Although the speed of light will appear the same to both of them, object 2 is further from the source therefore the light takes more time to get there.

[Zarnaxus]

I believe that since time around thos objects moving at 1/3c and 2/3c slows down, and time with the 2/3c slows down more than the 1/3c. The time it takes the light to reach the objects will be the same to the perspectives of those objects. From an outside perspective the light seems to take a longer amount of time to reach the faster object; we will have forgotten that time is relatively slower where that object is.

 

A stationary object in relation to the light is going to experience time regularly. It might take a second to reach it.

If light takes the same amount of time to catch up to a moving object, say an object moving at 1/2c, the only option, is that time is actually moving 1/2 slower, and it is still taking one second for the light to reach it from the perspective of that object.

 

It all comes down to just manipulating time to make sure that c is a constant.

[lemur]

Has any (small) vehicle actually been used to test whether it can accelerate faster than the speed of light? I'm sure many observations have been made of particles launched with high amount of force, but I'm talking about a vehicle that can move through interstellar space at high speed, stabilize, and then accelerate again and again, etc. until it nears C without impinging electric fields, etc. to contend with.

Edited December 4, 2010 by lemur

[between3and26characterslon]

Has any (small) vehicle actually been used to test whether it can accelerate faster than the speed of light? I'm sure many observations have been made of particles launched with high amount of force, but I'm talking about a vehicle that can move through interstellar space at high speed, stabilize, and then accelerate again and again, etc. until it nears C without impinging electric fields, etc. to contend with.

 

Does light accelerate? If you apply a 1N force to a battle ship for one second you won't notice any acceleration at all, if you apply a 1N force to a H atom for one second it will accelerate incredibly fast because it has less mass. Light, which has no mass, can only travel at the speed of light, never slower and never faster so it doesn't accelerate.

 

Going back to the OP the light will take 1 and 1/3 seconds to reach the ball travelling at 1/3c and it will take 1 and 2/3 seconds to reach the ball travelling at 2/3c.

[swansont]

I believe that since time around thos objects moving at 1/3c and 2/3c slows down, and time with the 2/3c slows down more than the 1/3c. The time it takes the light to reach the objects will be the same to the perspectives of those objects. From an outside perspective the light seems to take a longer amount of time to reach the faster object; we will have forgotten that time is relatively slower where that object is.

 

A stationary object in relation to the light is going to experience time regularly. It might take a second to reach it.

If light takes the same amount of time to catch up to a moving object, say an object moving at 1/2c, the only option, is that time is actually moving 1/2 slower, and it is still taking one second for the light to reach it from the perspective of that object.

 

It all comes down to just manipulating time to make sure that c is a constant.

 

The amount that time slows down is not linear. Dilation of something moving at 2/3 c is not twice that of something moving at 1/3 c.

[PhysicsNut]

Has any (small) vehicle actually been used to test whether it can accelerate faster than the speed of light? I'm sure many observations have been made of particles launched with high amount of force, but I'm talking about a vehicle that can move through interstellar space at high speed, stabilize, and then accelerate again and again, etc. until it nears C without impinging electric fields, etc. to contend with.

 

As someone already said light doesn't accelerate, but if you are asking whether a vehicle can accelerate to speeds faster than the speed of light, then by simply accelerating no, because photons don't have any mass and so nothing can travel faster than them, but theoretically a vehicle can travel faster than the speed of light if it bends time in front of it and behind it.

[Zarnaxus]

The amount that time slows down is not linear. Dilation of something moving at 2/3 c is not twice that of something moving at 1/3 c.

 

 

Hmm.. I actually didn't know that... Thanks for that info, but my point will still stand: You will always have to continually manipulate time to make sure that c is a constant.

 

So, is there an equation that can accurately measure the amount of dilation for an object moving at any speed?

[swansont]

Hmm.. I actually didn't know that... Thanks for that info, but my point will still stand: You will always have to continually manipulate time to make sure that c is a constant.

 

So, is there an equation that can accurately measure the amount of dilation for an object moving at any speed?

 

The amount of time dilation and the amount of length contraction are inverses of each other.

 

http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/tdil.html