Relativity of simultaneity help

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Relativity of simultaneity help

Postby massownage » Thu Feb 11, 2010 3:13 pm

My physics class has been learning this recently and the teacher has posed a thought experiment relating to the above topic, problem is I'm terribly confused here's the jist of it:

" There exists a train and a special tunnel, at rest the train is longer than the tunnel. Anyway the train attains relativistic speeds upon entring the tunnel, an observer sees the train just fit into the tunnel but the train driver sees his train has not fully entered. At the moment the observer sees the train just fit he flicks a switch causing the ends of the tunnel to close, what does the observer and train driver see?"

I believe that the train driver will see his 'back of the train' crumpled then will smash into front door since the light from the back has a longer distance to travel to his eyes. Whilst for the observer the train crashes into the front door only.....

So what i have left 2 crashes for train driver and one crash for observer.......

Could anyone help me resolve this?

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Re: Relativity of simultaneity help

Postby joe » Wed Feb 17, 2010 2:24 am

This paradox is resolved by the relativity of simultaneity. What are simultaneous events (doors closing) in one frame are not necessarily simultaneous in another.

1. Someone at rest with respect to the doors see them close simultaneously.

2. To the train driver, however, one door closes just in front of the train (and he crashes through it) and, some short time later, the second door closes behind the train.

This situation (a standard puzzle in relativity, called the 'train in tunnel' or 'pole in barn') is analysed in detail in


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