[Another] question on relativity

roosh

A combination of reading that Morbert has joined the mods and having 2 weeks holidays started me thinking about relativity again and a question popped into my mind that I was hoping to get some help with. It is more of a question this time, rather than a "relativity is wrong" kind of post. I've attached a diagram to try to clarify the question.

Hopefully the below makes sense.


Diagram
In the diagram there are two lasers - represented by the red lines. The idea is that, when the path of the lasers is unbroken the LEDs (or some other system) emit a light signal which travels to the receiver mid-way between the two LEDs. The lasers are at rest relative to an observer on the platform.

In the diagram also, you can see a metre stick in between the two lasers (at rest relative to them); when the metre stick is at rest relative to, and between the two lasers, the path of each laser is unbroken, and a signal is emitted from both LEDs.


Observer on the platform
Now, if we imagine a metre stick moving relative to the observer on the platform and the lasers, from right to left, as we look at it; the metre stick will, according to the observer on the platform, be contracted and, as it moves from left to right, will break the path of the laser labelled 'R' first - shutting off the LED; it will move to a position between the lasers such that both LEDs are switched on and two signals are emitted; then it will break the path of the laser labelled 'L' - shutting off the LED.

This sequence of events would lead to the reciever in the middle recording a pattern of: LR - LR - L - L - L - LR - R - R - R

Where LR indicates that a signal was received from both sides at the same time and either L or R indicates from which LED the signal was received.


Relatively moving observer
From the perspective of an observer moving along with the metre stick, say in a train where the relative velocity is 0, the distance between the lasers is shorter and the metre stick is uncontracted, such that the metre stick will never arrive at a position between the two lasers in a way that will give rise to both LEDs being switched on; that is, it will not give rise to the LR signal that is sandwiched between the Ls and Rs above.


Causality
I presume some information could be coded into each signal such that the pattern could be used to set some sequence of events in motion, such as launch a rocket or just make one of those guns fire, where the little flag that says "bang" comes out; or not set them in motion, as the case may be. Could the difference in pattern violate causality?

Double-slit
Could the set-up be linked to a double-slit experiment, such that the pattern represents the order in which particles are fired at the screen?



I presume there is a solution to the above, that I am overlooking; I've tried to visualise it but bcos I'm not familiar with relativity to the point that it is intuitive, I'm having trouble seeing the solution. I've tried to imagine the LED labelled 'R' switching on before L and the signal traveling slower in one direction than the other, but I haven't been able to see the solution doing that.

Apologies if this is going over old ground; any thoughts would be greatly welcomed.



Diagram
In the diagram I have only shown the metre stick at rest relative to the lasers, in between them; I'm hoping that is sufficient to visualise the rest, where the metre stick moves left, from a position to the right of laser 'R', cuts through 'R' and then cuts through 'L' and continues moving left.

Morbert

According to the observer on the platform, the time it takes a signal to travel from R to the receiver is the same as the time it takes a signal to travel from L to the receiver. However, according to to the second observer, the time it takes a signal to travel from R to the receiver is much shorter than the time it takes a signal to travel from L to the receiver (This is because the receiver is heading into the signal from R, and away from the signal from L).

Therefore, even though the second observer says there is a time when both beams are broken together, the receiver will receive the information from the right LED much faster than the left LED, and the sequence of events will be

LR - LR * L - L - L * LR * R - R - R

The first star indicates when the "broken" signal from R reaches the receiver. The second star indicates when the "no longer broken" signal from R reaches the receiver, and the third star indicates when the "broken" signal from L reaches the receiver.

roosh

Morbert wrote: »

According to the observer on the platform, the time it takes a signal to travel from R to the receiver is the same as the time it takes a signal to travel from L to the receiver. However, according to to the second observer, the time it takes a signal to travel from R to the receiver is much shorter than the time it takes a signal to travel from L to the receiver (This is because the receiver is heading into the signal from R, and away from the signal from L).

Therefore, even though the second observer says there is a time when both beams are broken together, the receiver will receive the information from the right LED much faster than the left LED, and the sequence of events will be

LR - LR * L - L - L * LR * R - R - R

The first star indicates when the "broken" signal from R reaches the receiver. The second star indicates when the "no longer broken" signal from R reaches the receiver, and the third star indicates when the "broken" signal from L reaches the receiver.

cheers. that's what I was thinking the answer would be, but I was just having trouble trying to visualise it.