Trying to Understand Light in Motion: A Frustrating Puzzle

[PhysicsLaura]

Hi, I feel really out of my depth and this is just to satisfy a personal frustration. I can see that the light from both flashes should reach a passenger on board the train at the same time and also that it should hit from the front first, but I just can't seem to make it work in my head. Are there equations I could try and work through to show what point the light has physically reached at each point in actual time? Each line of reasoning makes sense but I am still struggling to figure it out as a whole.

Sorry if it's too simplistic.

 

[Muphrid]

We have three objects. Sensor A starts at the origin, Sensor B at 2d from the origin in the x-direction, and Light Source C at d from the origin in the same direction.

All three object have some four-velocity u that is a linear combination of e_t (the timelike direction) and e_x. Specifically, u = \gamma (e_t + \beta e_x) with, let's say, e_t \cdot e_t = -1.

When the light source C creates a pulse, photons go along n_\pm = e_t \pm e_x. Now, from here, we can calculate the spacetime points where these null lines intersect the worldlines of our sensors.

For sensor A, only n_- intersects its trajectory. Let \alpha be some scalar, and...

x_A = \tau_A \gamma (e_t + \beta e_x) = \alpha_A (e_t - e_x) + d e_x

Applying equalities on each component gives us

\alpha_A = \tau_A \gamma, \quad \tau_A = \frac{d}{\gamma (1 + \beta)}

Doing the same thing for sensor B gives

\alpha_B = \tau_B \gamma, \quad \tau_B = \frac{d}{\gamma (1 - \beta)}

To measure the coordinate time elapses between events, all me must do is calculate take the vector from the source C to each of A and B and dot it with e^t = -e_t to extract the time component. The result is that, from our perspective, the beam reaches A faster (A is at the back of the car). This is the correct result when we impose the condition that A, B, and C all be synchronized with respect to our frame, which is exactly what we've done here. We ensured that their positions all had no component in our e_t direction at some initial time.

When instead we synchronize with respect to the train, then we must enforce the notion that each of the "initial" position vectors of A, B, and C will have no component in the direction of the train's four-velocity. In that framework, we'll get the result that both sensors detect the pulse at the same time. You can try that if you want; the only difference is in setting up the initial position vectors or, if you feel confident in the math, you can set it up as a stationary problem and Lorentz boost it at the end.

 

[PhysicsLaura]

Nope, guess I'm no where near your level I lost it here.

Specifically, u = \gamma (e_t + \beta e_x) with, let's say, e_t \cdot e_t = -1.

It would be great if someone could explain this to me, and I would LOVE to have an image of this graph as a visual aid! It's partly that I don't know what gamma and beta are representing here!

Laura

 

[Muphrid]

\beta denotes some speed relative to the speed of light. So if you're traveling at half of lightspeed, \beta = 0.5. \gamma = 1/\sqrt{1-\beta^2} is a normalization factor to ensure that u \cdot u = -1. This is in c=1 units, so what that really means is that the four-velocity's magnitude is always the speed of light.

See this:

Flat spacetime is a Minkowski or hyperbolic space. Where a unit vector in a Euclidean plane would be confined to the unit circle, unit vectors in spacetime are confined to a unit hyperbola instead. The asymptotes of the hyperbola are what light rays follow from the origin. This is why no real particle can ever travel like a light ray--no matter how it goes along the hyperbola, it will never have the same four-velocity as a light ray.

 

[jartsa]

Hi, I feel really out of my depth and this is just to satisfy a personal frustration. I can see that the light from both flashes should reach a passenger on board the train at the same time and also that it should hit from the front first, but I just can't seem to make it work in my head. Are there equations I could try and work through to show what point the light has physically reached at each point in actual time? Each line of reasoning makes sense but I am still struggling to figure it out as a whole.

Sorry if it's too simplistic.

Here is a very simple way:

Here is a table of positions of train cabin front wall, relative to the ground, measured at times 0,1,2,3,4,5,6,7,8,9,10:

Table1: 50 51 52 53 54 55 56 57 58 59 60

Here is a table of positions of light, relative to the ground, measured at times 0,1,2,3,4,5,6,7,8,9,10:

Table2: 40 42 44 46 48 50 52 54 56 58 60

This is a table of the difference between the light position table and the cabin wall position table:

Table3: 10 9 8 7 6 5 4 3 2 1 0


At time 10 the difference becomes 0, light hits the cabin front wall, and this happens at position 60


(we have here a train that moves at half the speed of light)

 

[ghwellsjr]

Hi, I feel really out of my depth and this is just to satisfy a personal frustration. I can see that the light from both flashes should reach a passenger on board the train at the same time and also that it should hit from the front first, but I just can't seem to make it work in my head. Are there equations I could try and work through to show what point the light has physically reached at each point in actual time? Each line of reasoning makes sense but I am still struggling to figure it out as a whole.

Sorry if it's too simplistic.

I hope you realize that you are talking about two different scenarios here because the two flashes of light cannot both hit the passenger at the same time and at different times.

If we set up the scenario such that the two flashes are emitted at the same time in the passenger's rest frame, then they will hit the passenger at the same time. That's how we define "emitted at the same time". Another way of saying this is that define "emitted at the same time" when the two flashes arrive at the passenger at the same time.

If, on the other hand, we set up the scenario so that the two flashes are emitted at the same time in the ground's rest frame, then they will hit the passenger at different times and that means that they were emitted at different times in the passenger's rest frame. Again, this is how we define what "at the same time" means for remote events.

Just remember, they are two different scenarios and both cannot happen in a single scenario.

 

[solarflare]

i said that the light from both flashes hits the passenger of the train at the same time.

Accordng to relativity light travels at the same speed ( the speed of light ) relative to EVERYTHING. Therefore it must travel at the speed of light relative to the train in all directions. so the light must hit her at the same time or light does not travel at the speed of light relative to everything.

im assuming that the train behind the passenger would go through length contraction to make it work.

 

[TheEtherWind]

Imagine a light cone diagram, place two points on the x-axis at ct=0. These events are space-like. Now consider a moving frame's coordinates. They're slightly angled in towards the light line. Now you can see that in that frame one event happens at negative ct and another at ct=0.

 

[Doc Al]

i got a warning for giving misinformation on this subject so i will take that as an admission that the person thinks einstein is wrong.

No, it was you who were wrong, not Einstein.

i said that the light from both flashes hits the passenger of the train at the same time.

Well, that's not true. In that scenario (that you commented on), lightning strikes the ends of the train simultaneously according to an observer on the platform.

Accordng to relativity light travels at the same speed ( the speed of light ) relative to EVERYTHING. Therefore it must travel at the speed of light relative to the train in all directions.

Right!

so the light must hit her at the same time or light does not travel at the speed of light relative to everything.

Wrong!

im assuming that the train behind the passenger would go through length contraction to make it work.

 

[solarflare]

in the video it states the observer is actually the exact distance and that it is a fact that the strikes happen at the same time.

 

[Doc Al]

in the video it states the observer is actually the exact distance and that it is a fact that the strikes happen at the same time.

(1) Which observer? The observer on the platform is exactly at the center of the train when he sees the lightning strikes.
(2) The strikes happen at the same time for the platform observer, not for the observer on the train.

 

[solarflare]

anyway the observer on the platform has nothing to do with what the video says happens. it says that the light from the rear passes her at a different time even tho they strike at the same time.

what if there was no observer on the platform - and there were two lightning strikes that did actually hit at the same time - what would the passenger see?

 

[Jimmy]

The train is moving toward the light from one flash and away from the flash at the rear. The light from the flash at the rear has to travel further to reach the passenger.

 

[Doc Al]

anyway the observer on the platform has nothing to do with what the video says happens.

Watch it again.

it says that the light from the rear passes her at a different time even tho they strike at the same time.

Even though they strike at the same time according to the platform observer.

what if there was no observer on the platform - and there were two lightning strikes that did actually hit at the same time - what would the passenger see?

At the same time according to what frame of reference?

 

[solarflare]

The train is moving toward the light from one flash and away from the flash at the rear. The light from the flash at the rear has to travel further to reach the passenger.

Relativity states that light travels at the speed of light RELATIVE to everything. so it would still take the same time no matter how fast the train was moving. that is the whole reason we have time dilation and length contraction.

 

[Doc Al]

Relativity states that light travels at the speed of light RELATIVE to everything.

Right. Every observer will measure the speed of light with respect to themselves to be the same speed c.

so it would still take the same time no matter how fast the train was moving.

For train observers it does take the same amount of time for the light to travel from each end of the train to the middle. And since the light does not arrive at the middle of the train at the same time, the train observer concludes that the lightning strikes did not happen at the same time.

Of course, for platform observers the light from the front of the train has a shorter distance to travel so it takes less time to reach the middle. (Since the middle of the train is moving towards the oncoming light.) Similarly, it takes longer for the light from the rear of the train to reach the middle, according to platform observers.

 

[Muphrid]

This is fairly easy to demonstrate with math, solarflare.

Let some person S be standing stationary at x=0 on a train platform watching a train go by. Another person, T, is riding this train and, at time t=0, is also at x=0. There are two light sources F and B at the front and back of our train, respectively; F is at x=+d and B at x=-d at time t=0. Let the four-velocity of T (and also of the sources F and B) be e_t' = u = \gamma (e_t + \beta e_x).

Case 1: Person S perceives two light pulses simultaneously from sources F and B.

Person T will then measure the time at which F and B emitted these pulses relative to his own four-velocity. As with all vectors, you can measure the component of a vector along another with a dot product. The vectors involved are s_F = d e_x, the position of the front source, s_B = -d e_x, the position of the back source, and {e^t}' = \gamma (e^t - \beta e^x).

t'_F = s_F \cdot {e^t}' = -d \gamma \beta, \quad t'_B = s_B \cdot {e^t}' = +d \gamma \beta

In the limit that \beta \to 0, the person on the train perceives the flashes simultaneously also, but he clearly does not for any nonzero x velocity.


Case 2: Person T on the train perceives two flashes simultaneously.

Change \beta to -\beta, and you can now work the problem in reverse. No additional logic is required because both frames are inertial; we need only say that the train platform is moving backwards relative to the inertial train.

 

[solarflare]

Right. Every observer will measure the speed of light with respect to themselves to be the same speed c.

For train observers it does take the same amount of time for the light to travel from each end of the train to the middle. And since the light does not arrive at the middle of the train at the same time, the train observer concludes that the lightning strikes did not happen at the same time.

Of course, for platform observers the light from the front of the train has a shorter distance to travel so it takes less time to reach the middle. (Since the middle of the train is moving towards the oncoming light.) Similarly, it takes longer for the light from the rear of the train to reach the middle, according to platform observers.

how can it take the same amount of time - but at the same time not arrive in the middle at the same time?


how can an observer see light traveling parrallel to them. an observer can only see light coming towards them. vision is based on light hitting the eye. what the video is saying is that the platform observer is equidistant from both strikes so the light moves towards him and reaches him at the same time. he is not seeing the light moving towards the passenger because that light is not moving towards him. light moves at the speed of light relative to everything - not just the passenger - therefore the light must get there from both directions at the same time.

 

[Doc Al]

how can an observer see light traveling parrallel to them. an observer can only see light coming towards them. vision is based on light hitting the eye. what the video is saying is that the platform observer is equidistant from both strikes so the light moves towards him and reaches him at the same time. he is not seeing the light moving towards the passenger because that light is not moving towards him. light moves at the speed of light relative to everything - not just the passenger - therefore the light must get there from both directions at the same time.

Since you seem to accept that light travels at speed c with respect to any observer, how about viewing things from the platform--since that's the frame where we are told that the lightning strikes at the same time. If the train wasn't moving, then the platform observer (and everyone else) would agree that the light from each strike would hit the middle of the train at the same time. But the train is moving. So, if you accept that the speed of light is constant, you must agree that the middle of the train moves toward the light from the front of the train and away from the light from the rear of the train. So the light flashes reach the middle of the train at different times. (Everyone agrees with this.)

Tell me if you agree with that or not. (Note that all observations are from the platform frame, so no need for any relativistic effects...yet.) If you don't agree, point out where you are stuck.

 

[Muphrid]

The light rays are invariant in every frame, but the distances between objects and whether two rays are emitted at the same time can and will change. That is the very nature of Lorentz transformations. You're trying to use the invariance of these light rays to say nothing changes, which is silly.

 

[Doc Al]

how can it take the same amount of time - but at the same time not arrive in the middle at the same time?

Example: You and I are exactly 1 mile from the same point and we travel exactly at the same speed. Do we necessarily arrive at that point at the same time? Of course not: I started off at 1pm and you started off at 1:15pm. We only arrive at the same time if we left at the same time.

 

[ghwellsjr]

Solarflare, what video are you talking about?

 

[Doc Al]

Solarflare, what video are you talking about?

https://www.youtube.com/watch?v=wteiuxyqtoM

 

[solarflare]

we are told that the lightning strikes the train at the same time - and the observer on the platform sees the light strike at the same time because he is equidistant from each strike. and then we are told that the passenger will see them strike at different tmes.

 

[solarflare]

let me ask the same question but in definite terms.

the two bolts of lightning do actually strike at the same time in the trains frame of reference

the observer on the platform is exactly the same distance from each strike

does the woman see the two strikes at the same time?

 

[solarflare]

now imagine that the train had two wires, one from each end, to conduct the electricity from the strikes to the centre to turn on a light. a red light from the front strike and a green light from the rear strike.

would the lights come on together or would they not?

 

[Doc Al]

we are told that the lightning strikes the train at the same time - and the observer on the platform sees the light strike at the same time because he is equidistant from each strike.

Right. (Note that we are told that the lightning strikes the train ends at the same time in the platform frame.)

and then we are told that the passenger will see them strike at different tmes.

We can deduce that. Given the above, we must conclude that the light flashes reach the train passenger at different times.

 

[Doc Al]

let me ask the same question but in definite terms.

the two bolts of lightning do actually strike at the same time in the trains frame of reference

the observer on the platform is exactly the same distance from each strike

does the woman see the two strikes at the same time?

Sure. The lightning struck the ends at the same time in her frame and since they traveled the same distance they reach her at the same time. No mystery there.

 

[solarflare]

Right. (Note that we are told that the lightning strikes the train ends at the same time in the platform frame.)

We can deduce that. Given the above, we must conclude that the light flashes reach the train passenger at different times.

we must only conclude that if we assume that the speed of light is not the same relative to everything.

if we conclude that the light will arrive at her at different times - then we are saying that we can add and subtract the trains speed from that of the speed of light. but relativity says that we cannot because light travels at the speed of light relative to the trains motion

 

[Doc Al]

now imagine that the train had two wires, one from each end, to conduct the electricity from the strikes to the centre to turn on a light. a red light from the front strike and a green light from the rear strike.

would the lights come on together or would they not?

What scenario are you talking about? If the lightning struck the ends simultaneously in the train frame, then the signals would reach the middle at the same time and the lights would turn on together. (I see no particular advantage to using electrical signals instead of light flashes. Realize that the signals would take time to travel. Light flashes are easier to analyze, since their behavior is simple to describe.)