Problem with Relativity/Simultaneity (1 Viewer)

[andysoul]

length contraction comes into this in absolutely no way.
the observer is standing in the middle of the train, and therefore each end will, according to the observer on the ground, shrink by exactly the same amount, as predicted by the Lorentz Transformations. therefore the distance from each end is shorter, but still equal.

the problem i had with the explanation on:

http://www.bartleby.com/173/9.html

is that it uses the premise that the observer on the train will actually move towards the forward lightning strike's beam of light at faster than 'c', and they will move away from the rearward lightbeam so relative to them it it travelling slower than 'c'. All we have been taught and indeed one of the basic principles of relativity it that light moves at 'c' no matter who does the measuring.

That explanation conforms to the stationary observer's point of view, true, but it is then analysed by the train's observer using the fact that light travels at 'c', relative to him/her. the speed of light observed from the moving frame of reference, relative to the ground cannot be used to explain the observations of the person on the ground.
It in fact uses to observation of the outside observer to 'put words in the mouth' of the observer on the train by saying that the forward strike occured before the rearward one (i have no problem with that statement, merely the methods used to obtain it)

you must see that the observer on the train can ONLY have the light move at 'c' relative to him or her, the observation of the outside observer is not applicable to the person who is moving.

 

[zeropoint]

ok
based upon your reasoning, the opposite is indeed also possible, that the person on the train (who is in an inertial frame of reference) viewed both flashes as simultaneous. all you need to do is reverse positions. because both flashes had the same distance to travel (end of train to the middle), at the same speed, 'c', from either end of the train. therefore the observer in the train will only start their stopwatch and never stop it? you see? and they will have, as you said, observed the person on the ground recieve two seperate flashes.

both yours, and my arguments are both perfectly logical, and therein lies the paradox, no matter which way you look at it.

i think, however, that both of them, upon meeting would have still running stopwatches, as both viewed the lightning as simultaneous. the theory of relativity throws up seemingly illogical paradox's in many of it's applications. However, as has been stated in many places, both observers are correct in their observations, even though they are contradictory, due to the nature of light.

i think this is one of the few cases where it is in fact CORRECT to agree to disagree, as both are correct.

We're not discussing the situation whereby the flashes of lightning are simultaneous onboard the train. However, if you desire to arbitrarily shift the simultaneity from one inertial frame to another, that's your prerogative, and we can deal with that situation equally well, as it is symmetric with respect to all inertial observers. To illustrate, I'll restate the argument in the new configuration whereby the lightning flashes are simultaneous for the train observer. If the lightning flashes are simultaneous with respect to the train, then they are not simultaneous with respect to the embankment. If your hypothesis is true, the embankment observer will watch the train observer start and cease his stopwatch, whereas the train observer will leave the stopwatch running. So you see, my argument holds true in any configuration. However, your hypothesis results in paradox in _every_ possible configuration, a worthwhile reason to discredit it.

 

[Shuter]

is that it uses the premise that the observer on the train will actually move towards the forward lightning strike's beam of light at faster than 'c', and they will move away from the rearward lightbeam so relative to them it it travelling slower than 'c'. All we have been taught and indeed one of the basic principles of relativity it that light moves at 'c' no matter who does the measuring.

No what I perceived it to be saying was that it will take different times for each strike to reach the observer on the train, hence this explains why the light 'appears' to travel at different speeds. It is actually being received at different times and hence there is different distances between the person and where the lightning strike occur (on the assumption light travels at c constantly).

 

[zeropoint]

length contraction comes into this in absolutely no way.
the observer is standing in the middle of the train, and therefore each end will, according to the observer on the ground, shrink by exactly the same amount, as predicted by the Lorentz Transformations. therefore the distance from each end is shorter, but still equal.

the problem i had with the explanation on:

http://www.bartleby.com/173/9.html

is that it uses the premise that the observer on the train will actually move towards the forward lightning strike's beam of light at faster than 'c', and they will move away from the rearward lightbeam so relative to them it it travelling slower than 'c'. All we have been taught and indeed one of the basic principles of relativity it that light moves at 'c' no matter who does the measuring.

When I use the term c + v, I am not referring to the velocity of any particular object. I am saying that the gap between the front wavefront and the centre of the train is diminishing at a rate of c + v with respect to embankment. Nowhere in my deliberations has anything exceeded the speed of light.

That explanation conforms to the stationary observer's point of view, true, but it is then analysed by the train's observer using the fact that light travels at 'c', relative to him/her. the speed of light observed from the moving frame of reference, relative to the ground cannot be used to explain the observations of the person on the ground.

Bwah? Could you rephrase that please?

It in fact uses to observation of the outside observer to 'put words in the mouth' of the observer on the train by saying that the forward strike occured before the rearward one (i have no problem with that statement, merely the methods used to obtain it)

Light propagates isotropically in every inertial frame of reference, even you agree on that right?

you must see that the observer on the train can ONLY have the light move at 'c' relative to him or her, the observation of the outside observer is not applicable to the person who is moving.

Regardless of whether this is true or not, I have analysed the situation from both frames and it is entirely consistent.

 

[Jase]

Well.. i've lost track. But im with the whole "time/space" independence thing.

Here have a look at this nice lightcone thought experiment:

Relativity of Simultaneity

Conclusion: The inevitable conclusion is that A must measure space and time along axes which are skewed relative to B. Events which happen at the same time according to Ahappen at different times according to B; and vice versa. A's hypersurfaces of simultaneity are not the same as B's.

 

[andysoul]

jase

that site is great, made perfect sense to me. explains things very well and logically.

now zeropoint, i shall explain the problem i have with this experiment and the accompanying explanation as clearly as i can. and yes, i agree that light travels at 'c' no matter what your viewpoint.

http://www.bartleby.com/173/9.html

http://www.cord.edu/dept/physics/credo/etrain_credo.html

these sites set out the experiment very well, and clearly.
the graphic on the second site is clearly contradictory to me, as it shows the person on the train moving towards the light beam, which must be travelling at 'c', therefore their relative velocities are > c, and this is impossible, therein lies my problem with your explanation, that the person on the train sees the front light beam first. they cannot, simply because the light cannot travel faster (or slower in the case of the rear strike) than 'c' relative to the observer on the train.
therefore the light must travel at 'c' relative to either/both observers, it merely depends upon your viewpoint. this means that BOTH observers see the light beams as simultaneous, because they are both equidistant from either end of the carriage.

if you could perform this simple thought experiment for me. forget all of the above discussion, and simply imagine that you are the one on the train. light beams strike both end of the carriage at the same time, yet you know nothing of it yet, because the light hasn't reached you. because you are in an inertial frame of reference you KNOW for a fact that light will travel at 'c' in any direction. you're also at the middle of the carriage. now, as you are equidistant from either end of the carriage, and the light travels at 'c' from either end, you will see both light beams as simultaneous.

now, from outside the train, same situation. when the train passes, lightning strikes both ends of it at the same moment, as you are equidistant from each of the strikes, and light moves at 'c', you see both of the strikes as simultaneous.

now i REALISE that this question is based upon the statement that both the lighning strikes are simultaneous (from whose point of view you may ask?) but that is the question, straight out of the catholic schools trial paper.

now, here comes the bit you may disagree with, or find illogical.
from the observer outside the train: you have seen both lightning strikes as simultaneous. looking at the train you see the other observer looking at you. because he has moved towards the front flash in the instant it took the light to reach you, you see the forward facing part of him become illuminated before the rearward sideof him. therefore you surmise that he MUST have seen the front lightning strike first.

from the observer on the train's point of view: you have seen both flashes as simultaneous (stated above), to you it looks like the observer outside is moving toward the rear of the train, and in the instant it took the light to reach you, he has moved toward the rear of the train. therefore he is now closer to the rear lightning flash, and you see the rearward facing part of him illuminated before the forward facing part of him. therefore you assume that he MUST have seen the rearward lightning strike first.

both these observations, although contradictory, are correct. both observers have reported exactly what they saw.

i see no paradox here, its just the way that light travels

 

[zeropoint]

jase
both these observations, although contradictory, are correct.

It is pretty clear that we are not going to agree on this for a while. So this will be my last attempt at convincing you before my biology exam on monday.

Although our relative perception of reality is contradictory, you don't seem to realise that the real world still exists! The world of special relativity is a fusion of three spatial dimensions and one time dimension into a single 4-dimensional Minkowski space. Points in Minkowski space are referred to as events. The relativity of simultaneity can be thought of as hyperbolic rotations in spacetime, so every observer has their own perspective of the same events from a different angle in spacetime. If an event exists in Minkowski space (such as the simultaneous reception of two light rays) then all observers must agree on the existence of this event. However, events that are separated in spacetime (such as two distinct lightning flashes) will be perceived differently by observers occupying different planes of simultaneity in spacetime.

 

[jang]

zeropoint, that q (which appears in context) is in the CSSA 2004 paper. it is 5 marks to explain. n i ur answer that they both see the 2 lightnings simultaneously is wrong.
the text book is right, due to the guy in the train is moving towards the lightning at relativistic speeds hence the lightning at the back of the train will take longer to reach his eyes.
the guy outside... what, he's just bumming there and the 2 lighting strikes, so he sees them as simultaneous events coz they occured in his own inertial frame of reference.
that guy that was talkin bout light activated doors, yes indeed he was talkin bout jacaranda text book's explanation of relativity of simultaneity...
anyways gL.
and jase r u han from nomanhizzle?

 

[Shuter]

Yes, the main concept here is.

If I'm standing in the middle of a cricket pitch, and someone takes a flash photograph of me. For the purposes of this, lets say the cricket pitch it extremely large and it takes 5 seconds to reach me (if I'm standing still), therefore it would take 2.5 seconds to reach halfway. ok?

Now, say at the same point in time as when the flash photograph was taken, I was initially standing still, but then run really fast towards the direction of the flash, at say 0.9c. Now, I would meet this beam of light then just a little before I reach the halfway point between me and the flash.

Therefore it takes less time to reach me and travels a shorter distance.

The same principle applys to the train, with say someone took a flash photograph from the other side of the field at the same time. I would have put extra distance in between me and the flash, in the time it takes to reach me, so therefore it would take longer to reach me.

 

[Jase]

yeh, Jangers i am . ahaha

 

[andysoul]

as they say shuter, everything is relative.

if you're running down the cricket pitch at .9c, as in your thought experiment, and you were to see the flash when you're almost halfway down the pitch. the light would have to have been approaching you (relative to you) at 1.9c, which is of course impossible. this is the only way that the light could meet you just before halfway.

now, you would actually make it 9/10 of the way down the pitch before you saw the light flash. because it HAS to travel, relative to you, and according to you, at a velocity of 'c'. therefore, because you're already travelling at .9 'c', it approaches you (from your point of view) at 'c', using a little subtraction (1 - .9 = .1) we see that it is only travelling at .1 c relative to the cricket pitch. (there's no way to actually measure this, because someone who is stationary would still measure the light travelling at 'c' relative to him/her).

the only way to see the flash at the middle is to move at .5c, then it'll approach you at 'c' (relative to you), and it'll meet you halfway.

this is similar to the train experiment in that the person on the train will have the light approach him/her at 'c', not > or < 'c', which would have to occur otherwise.

 

[mojako]

as they say shuter, everything is relative.

if you're running down the cricket pitch at .9c, as in your thought experiment, and you were to see the flash when you're almost halfway down the pitch. the light would have to have been approaching you (relative to you) at 1.9c, which is of course impossible. this is the only way that the light could meet you just before halfway.

now, you would actually make it 9/10 of the way down the pitch before you saw the light flash. because it HAS to travel, relative to you, and according to you, at a velocity of 'c'. therefore, because you're already travelling at .9 'c', it approaches you (from your point of view) at 'c', using a little subtraction (1 - .9 = .1) we see that it is only travelling at .1 c relative to the cricket pitch. (there's no way to actually measure this, because someone who is stationary would still measure the light travelling at 'c' relative to him/her).

the only way to see the flash at the middle is to move at .5c, then it'll approach you at 'c' (relative to you), and it'll meet you halfway.

this is similar to the train experiment in that the person on the train will have the light approach him/her at 'c', not > or < 'c', which would have to occur otherwise.

and the explanation to this strange phenomenon would be to say that time is dilated OR length is contracted depending on how you look at it.

 

[Shuter]

as they say shuter, everything is relative.

if you're running down the cricket pitch at .9c, as in your thought experiment, and you were to see the flash when you're almost halfway down the pitch. the light would have to have been approaching you (relative to you) at 1.9c, which is of course impossible. this is the only way that the light could meet you just before halfway.

now, you would actually make it 9/10 of the way down the pitch before you saw the light flash. because it HAS to travel, relative to you, and according to you, at a velocity of 'c'. therefore, because you're already travelling at .9 'c', it approaches you (from your point of view) at 'c', using a little subtraction (1 - .9 = .1) we see that it is only travelling at .1 c relative to the cricket pitch. (there's no way to actually measure this, because someone who is stationary would still measure the light travelling at 'c' relative to him/her).

the only way to see the flash at the middle is to move at .5c, then it'll approach you at 'c' (relative to you), and it'll meet you halfway.

this is similar to the train experiment in that the person on the train will have the light approach him/her at 'c', not > or < 'c', which would have to occur otherwise.

Argh, my head hurts.

But what I am saying is that, if you move closer to the object the light is coming from, is that there will be a shorter distance and hence the light will arrive sooner (in a shorter time).

It's like saying these stars we look at are 100 million light years away. Therefore the light is travelling for 100 million years to reach us (we are seeing things from 100 million years ago).

If you are to board on a spacecraft and fly towards these planets (at whatever speed you want), you should see the light get younger as you move closer towards the star. e.g. say at halfway on the journey you would then be seeing light from 50 million years ago, agree?

This is the same principle I'm applying to the cricket pitch, if you move closer, the light will have LESS distance to travel to reach you, therefore will take LESS time, therefore is still actually travelling at the speed of light the whole time.

 

[Rorix]

ITT people ignore the fact that the observer inside the train moves

 

[jumb]

You guys need to stop thinking so much. Seriously.

 

[Brad]

The lightening is in the same frame of reference of the guy standing still isnt it???


The guy in the train is in a different frame of reference......... thats why something or other... i dunno if this has already been said.... i couldnt be botehred reading it all.

 

[Jase]

I agree. You guys are thinking too much. Think less. Less.. lesser!.. moer lesser.. thast betaaaer.

 

[scorpi0n]

argh what conflicting arguments. ok here's mine:
Let Observer A be the Observer inside the train
Let Observer B be the Observer on the platform
The train is moving to the right at velocity v
Lightning strikes whenthe train is in the centre of the platform

velocity -->
%[=====A=====]%
_______ B_________

tada theres my diagram. % = lightning, [==] is the train, ___ the platform. Now:

Observer B would suggest the two lightning strikes have occured "simultaneously", as the distance from each strike to B is equal, and as t = d/s where s and d are equal, time would be equal for each of these events to "reach" B.
However observer A would suggest the strike on the right occured first. This is as he/she is moving towards the strike, and hence the distance the light must travel would be shorter.
Similarly, the strike on the left would have further to travel to reach Observer A, and as c is constant, would take longer.
Thus their respective statements suggest simultaenity is relative depending on the frame of reference.

There. now feel free to point out any mistakes in that, but it makes perfect sense in my head.

 

[Rorix]

i agree 100% with scorpi0n

 

[Shuter]

Yes, that's exactly what i've been trying to say, some other people seem to argue "but but veloicty of light always has to be c relative to everything!". That may be true, but the distance is getting closer as the person moves hence the lightning will arrive faster, yet still be moving at the same speed.