alternate Train carriage-embankment thought experiment.

[Liam.lah]

First post here, Hi all.

 

I had thought about this some years ago, but visiting here, this seemed like the place to posit it.

 

Many of you will be familiar, in relativity with the thought experiment of the light bulb in the moving train carriage. The light bulb is in the centre of the carriage, and when turned on, depending on the observer, the light may reach the back of the carriage before the first, or both at the same time, whether you are looking in from an embankment, or sitting inside the carriage respectively.

 

Say we were to take this further...

 

Inside the carriage, on either side are small light sensitive switches, both of which are connected to a board with two LEDs on it, if the light sensor at the back of the carriage is hit first, the red light will come on, if it hits the front light sensor, or both at the same time, the green LED will flash on.

 

If we work through this again, the man sitting on the embankment will observe the light hitting the sensor at the back first, and should therefore expect to see a red LED turn on. The man inside the carriage should see both light sensor activated simultaneously and should expect to see the green LED turn on.

 

 

This doesn't seem possible at all in the real world. Have I missed some important part of this thought experiment?

[swansont]

The measurement is being made in the rest frame of the carriage; that's physically where the circuitry is. All this will do is confirm that the events are simultaneous in that frame.

[insane_alien]

you should look up 'relativity of simultaneity'

[Zarnaxus]

Light travels at the same speed no matter how fast the source of light is moving. The sensors are moving the same speed in relation to eachother and the person outside observing. I cant find any reason for the light to not reach the sensors at the same time. I beleive the green LED would turn on.

 

 

[md65536]

Okay I wrote up a perfectly wonderful response along the lines of everyone else and then spotted a source of confusion.

 

Yes, an exterior observer would "observe the light reaching the back of the traincar first" as that end moved forward to meet the light signal that is moving backward.

 

The situation you describe requires that the LED switch device is causally connected to the "light hitting the sensors" events. This means that the LED switch event must happen long enough after the sensor event, that information from the 2 sensors can travel to the switch. The sensor can't know instantly when a sensor was triggered; this information reaches it at best at the speed of light.

 

An example setup might have the LED switch in the middle of the train, connected by wires of the same length to each sensor.

 

The hobo (assumed) on the embankment with his perfect observation device "observes" the light hitting the rear of the train quickly, but then observes the signal from the sensor taking longer to reach the switch (as both are moving forward). Meanwhile he sees the light take longer to reach the front sensor (as the front sensor is moving away while the light travels), but then observes the signal from the sensor taking less time to reach the switch.

 

Regardless, the sensor/switch setup is "causally connected", and no matter how you calculate it, I guarantee that no observer is going to see the order of any 2 causally connected events to be reversed from what another observer sees.

Edited December 11, 2010 by md65536