Special Relativity (1 Viewer)

[Petyo]

Hi does anyone have any ideas about how Einstein came to the conclusion that the speed of light was constant during his postulation of the Principle of Relativity? I can't see why he went from the point that the speed of light must be the same for passenger on the train and the observer ouside to stating that the speed of light is always constant.
I know people have proofs for it later on; but what's written in my textbooks is not thoroughly clear about those postulations.
It seems like they never provide sufficient amount of information or I'm just too dull to understand the words. T_T

 

[darkchild69]

The constancy of the speed of light stems from Lorentz' interpretation of Maxwells equations, of which Einstein was a 'fan' more or less (Ether is immobile)

If the Ether is immobile, then as a direct consequence of this the velocity of light relative to the ether is constant, irrespective of the motion of the observer.

Basically, Lorentz came up with the idea of constancy of the speed of light relative to the ether and Einstein applied this idea of constancy relative to and 'inertial frame of reference' as Einstein had kind of disbanded the idea of the Ether by this stage

According to relativity, the laws of physics are the same in each and every inertial frame of reference. So thus, if the speed of light is constant in one frame, it must be constant in all inertial frames, remembering that there is no specific 'rest' frame -- no way to proritise which frame is at rest -- it is relative to the observer.

 

[Petyo]

Come back to his thought experiment about a train traveling at the speed of light, I do not see why the situation that the observer on the embankment perceive light on the train at as twice as its usual speed is impossible, if the principle of relativity is to hold true in all inertial frame of reference: the passenger on the train can see his reflection.

 

[Pwnage101]

um, i'm not sure what you are asking, but i'll try to explain some stuff and then ask specific questions and id be happy to help.

In one of einstein's famous thought experiments, he proposed that if there was a passanger on a train travelling at c, would he see his reflection in a mirror held in front of him?

there are 2 possible answers, one of which is correct:

-yes. In order for this to occur, light from his face would have to travel at some speed > 0 relative to the person on the train (who is travelling at c relative to someone on a train platform). So, to the person on the platform, the light would seem to be travelling at a speed > c.

-no, because since he is travelling at c, and light travels at c, the light from his face ccant reach the mirror and back again because he is travlling as fast as light.

Now, to put it simply, he conlcuded the answer was Yes. The reason is because of the 'Principle of Relativity'. This concept is often misunderstood as something einstein postulated, but he didn't - this principle dates back to galileo/Newton and even further back, and simply states all inertial motion is relative and cannot be detected withut reference to an external point. Thus, in an inertial frame of reference, you cannot perform any mechanical experiment or observation that would reveal to you whether you were moving with uniform velocity or standing still. [The principle also states that the laws of physics are the same in all inertial frames of reference]

If the answer was no, then the person in the train (einstein used himself as the person he imagined in the train, so Einstein) would know that he was travelling at c, and not standing still, which violates the prionciple of relativity, which einstein believed Must hold at all times (his Special Theory of Relativity contained 2 postulates, and a statement - this statement was that the principle of relativity always holds).

So, using this logic, the answer must be yes - now he concluded if this is indeed the answer, the light must be travelling at a speed greater than c, relative to the person on the platform - but this was impossible.

So, he concluded c is constant, regardless of the motionof teh observer, but this has serious implications (time dilation, relativistic amss increase, length contraction, relativity fo simultaneity)

any questions, i'd be happy to answer

 

[Petyo]

My problem is that I can't see why the situation that "light travel at a speed greater than c relative to the person on the platform" is wrong. Did I miss something?

 

[Petyo]

i know my question in the previous posts has been unclear, because I, myself feel unsure about where I've been lost. I read the whole thing over and over again and I think i can now recite every single word from the textbook but still the concept just puzzles me.
It's like, in the case that we accept that "principle of Relativity" always holds true, and that the answer to Einstein's question in his thought experiment is "yes", so now the speed of light relative to the person on the train is c, and as the train is also moving, the speed of light relative to the person outside the train (c+v) with v being the speed of the train. I can't see why the speed of light must always be constant regardless of motion of tHe observer.

 

[cutemouse]

Any frame of reference is good as another.

So say you were standing on a platform with a mirror, and a train goes past you at c. That means your velocity relative to the train is c. Will you still see your image in the mirror? Obviously you can.

Therefore, if you are in the train, then your velocity relative to the platform is c. Therefore you will be able to see your image in the the mirror.

Also, if you couldn't see your image, then this violates the principle of relativity.

As for 'c' being constant in a vacuum, it just is. The proof of it is out of the scope of the HSC Physics course. IIRC, it can be proven using trigonometry (not 100% sure though).

 

[spartan31234]

Did u know relativity existed before Einstein!!!

This was called Classical relativity, Galileo (i think..) conducted experiments and made up the definitions of inertial frames and accelerated frames of reference, and he postulated that u cannot measure the speed of a inertial frame of reference form within .ect…………


Einstein Thought
Say a train is travelling at the speed of light ( or very close too it) people on the train would see their reflections after some lagg – this would be a violation of classical relativity as they would know they are travelling close to the light, by conducting experiments from within the frame.

Thus Einsten said classical relativity must be held so he postulated that light is measured the same in all frames of reference in order to comply with the classical theory of relativity

 

[spartan31234]

I got the ansewer yes!

 

[shady145]

Did u know relativity existed before Einstein!!!

This was called Classical relativity, Galileo (i think..) conducted experiments and made up the definitions of inertial frames and accelerated frames of reference, and he postulated that u cannot measure the speed of a inertial frame of reference form within .ect…………

umm u mean galileo said u cannot tell if you are moving at a constant velocity or at rest. then he stated you cannot perform any mechanical experiment wholly within the frame of reference to determine if you are moving in costant velocity or at rest.
AND he didnt come up with accelerated frames of reference, Newton took his work furthur and did accelerated frames. called newtonian relativity.

it has nothing to do with measuring the speed of an inertial frame of reference.

Now for petyo
if einsteins train was going at c, and he could not see his reflection in the mirror, then he has violated galileos relativity, as you could perform a mechanical experiment wholly within the frame to tell if you were moving at constant velocity.
so he assumed that c was constant, which reulted in time and distance to be relative.
the jacarnada text book explains this fairly good i thought.

 

[Petyo]

Well, if he didn't want the Principle of Relativity to be violated, he could have assumed that both the passenger sitting on the train and the observer on the embankment perceive light as having the same constant speed c [so that even if we set up experiments to measure the speed of light on a constantly moving object, we would not be able to conclude whether we're moving constantly or standing still as now the speed of light is always equal to 3x10^8, regardless of our motion]; but then the light on the train would be moving at (c+v) [ v is the speed of the train]relative to the person standing at the station i think...
@ Shady145: I have that textbook as well, I know that its explanation is the best of all textbooks but still ...=.=!!

 

[Petyo]

but wait, is it true that in the laws of electromagnetism described by James Clerk Maxwell in 1860's the speed of light emerges from the equation as a constant and independent of the velocity of the observer? Is it also possible that this is the reason why the speed of light can't be more than c relative to the observer on the platform?

 

[Pwnage101]

Read through this (skip the uni-level stuff, but a lot of it is HSC level, and is a great read):

http://www.physics.mq.edu.au/~jcresser/Phys378/LectureNotes/SpecialRelativityNotes.pdf

 

[darkchild69]

but wait, is it true that in the laws of electromagnetism described by James Clerk Maxwell in 1860's the speed of light emerges from the equation as a constant and independent of the velocity of the observer? Is it also possible that this is the reason why the speed of light can't be more than c relative to the observer on the platform?

Perhaps you should read my first post?

Lorentz' interpretation of Maxwells equations was that if the Ether was immobile, then because of this, the speed of light relative to the Ether is constant and does not depend on the motion of the observer

Eg. A person moving at half the speed of light, will still judge the speed of light to be the same as a stationary person

Einstein just applied this theory of constancy of the speed of light to 'inertial frames of reference' rather than the 'Ether' --

IT WAS ALL LORENTZ' IDEA

 

[Petyo]

Read through this (skip the uni-level stuff, but a lot of it is HSC level, and is a great read):

http://www.physics.mq.edu.au/~jcresser/Phys378/LectureNotes/SpecialRelativityNotes.pdf

Thanks pwnage for your help so far. I've had a quick look at the note you suggested; its explanation is just excellent and I'll read in details when I could find a more quiet place. The thing is at first I do understand the concept. It's the way the thought experiment written in my textbook confuses me
I guess I'll leave this issue for a while...

 

[Petyo]

Perhaps you should read my first post?

Lorentz' interpretation of Maxwells equations was that if the Ether was immobile, then because of this, the speed of light relative to the Ether is constant and does not depend on the motion of the observer

Eg. A person moving at half the speed of light, will still judge the speed of light to be the same as a stationary person

Einstein just applied this theory of constancy of the speed of light to 'inertial frames of reference' rather than the 'Ether' --

IT WAS ALL LORENTZ' IDEA

yea you're right. It was what i found in some other sources as well, but it is not mentioned in the textbook as the reason why the speed of light is assumed "constant" .

 

[Petyo]

Well maybe I should stop thinking about it for a while

 

[Pwnage101]

Thanks pwnage for your help so far. I've had a quick look at the note you suggested; its explanation is just excellent and I'll read in details when I could find a more quiet place. The thing is at first I do understand the concept. It's the way the thought experiment written in my textbook confuses me
I guess I'll leave this issue for a while...

LOL no probs, well i've heard a funny quote, and from my experience in HSC physics it's quite true:

"An erection is like the [Special] Theory of Relativity: the more you think about it, the harder it gets"

 

[Petyo]

LOL. I don't know what I should compare it to...
The funny thing is I used to understand it, like when we came to this topic in class I also did heaps of research and I think I did find out something which made me got the concept. Then 3 weeks after the holidays had started, yea I posted this thread. It's irritating really, it's in my head all the time and I can't not just drop it. It makes me feel like a loser...

 

[Pwnage101]

yeh, that happened to me as well - the good news is the more you think about it, and struggle with it, the more you'll understand it