# How Did Einstein Discover Near-Light Velocity's Affect on Other Things? **URL:** \ **Category:** Factual Questions\ **Created:** [October 15, 2003, 6:07am UTC](https://boards.straightdope.com/t/how-did-einstein-discover-near-light-velocitys-affect-on-other-things/207517 "2003-10-15T06:07:00Z")\ **Posts on this page:** 6\ **Page:** 2
**Author:** ![MC\_Master\_of\_Ceremonies](https://avatars.discourse-cdn.com/v4/letter/m/bb73d2/32.png) [@MC\_Master\_of\_Ceremonies](https://boards.straightdope.com/u/MC_Master_of_Ceremonies)\ **Post date:** [October 15, 2003, 8:58pm UTC](https://boards.straightdope.com/t/how-did-einstein-discover-near-light-velocitys-affect-on-other-things/207517/21 "2003-10-15T20:58:20Z")
> [@](#): > > \*Originally posted by Whack-a-Mole \* > \*\*It probably helps some people but my dim mind is balking. > > Why is the front of the train not causally related to the back of the train? They are attached to each other and moving at the same speed. Slow the front down and you slow the back down…it is essentially a single object. > > Further, I have a problem with defining who sees what and when. Lets take observation/reaction out of the equation (if this is ok to do). Say we time this experiemtn very well and set a computer at either end of the barn to shut the doors at a predetermined time. We will be sure to have the Relativity Express Train in the barn at that time. Our 200m long train is passing through the barn and the doors slam shut (I guess for this experiment we can ignore the time it takes to close the doors and just assume they do so really fast). Is the train in the building or not? \*\* You can view the relatsically train as a sinlge rigid object as long as it’s not accelrating, the problem as phrased doesn’t involved accelrtation which makes it simpler as accelration breaks the symmetry of the situation. The back and the front aren’;t causally connected as an action on the front won’t instaneously effect the back. Yes you can assume the doors close instaneously as it doesn’t matter in this apparent paradox. You can imagine that the framer sets up a computer exactly at the midway point in the barn which sends two light pulse in either direction to trigger the doors when the train is fully in the barn (or just before in order to take into account the time the pulse takes to travel to the door), so from the farmer’s point of view they both shut at exactly the same time. Now consider the train travelling at a relativistic speed and how it views this light pulses. So imagine now, at the time the front of the train reaches the front door, the point on the train that was level with the computer when the pulses were emitted. According to relativity from it’s point of view both pulses must of travelled at c, but it’s moved some distance since the pulses were emitted, so from it’s point of view (and indeed the whole train as it is in the same refernce frame) the front door will close before the second as it now has less distance to travel. From the point of view of a staionery observer the train will fit in the barn, from the point of view of the train it won’t. ---
**Author:** ![MC\_Master\_of\_Ceremonies](https://avatars.discourse-cdn.com/v4/letter/m/bb73d2/32.png) [@MC\_Master\_of\_Ceremonies](https://boards.straightdope.com/u/MC_Master_of_Ceremonies)\ **Post date:** [October 15, 2003, 9:02pm UTC](https://boards.straightdope.com/t/how-did-einstein-discover-near-light-velocitys-affect-on-other-things/207517/22 "2003-10-15T21:02:19Z")
I should of made this clearer in the above explanation: From the point of view of the train it is sitting still and it’s the barn that is travelling at a relatvistic speed. ---
**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\ **Post date:** [October 15, 2003, 10:00pm UTC](https://boards.straightdope.com/t/how-did-einstein-discover-near-light-velocitys-affect-on-other-things/207517/23 "2003-10-15T22:00:29Z")
Thanks **MC**. I’m not trying to be thick about all of this but… The premise seems to be relying on a signal sent trhough the barn and thus the people on the train see things differently than the computers/people in the barn do. Does it change anything if you go back to my original idea that the whole thing is timed? That is, you have two computers, one at each end of the barn, controlling the door there. Each computer has hyper-accurate clocks and we tell them to shut the doors at time X. We now send our train hurtling down the tracks at near light speed and time it such that the train will be ‘inside’ the barn at the precise moment (time X) the computers tell the doors to shut. Does this make any difference to ‘catching’ our train inside the barn now? I assume caclulations to see this sort of timing would be possible (even if the actual details of setting up the experiment would be difficult). [sub]If this is too much of a hijack at this point let me know and I’ll restart this in another thread.[/sub] ---
**Author:** ![MC\_Master\_of\_Ceremonies](https://avatars.discourse-cdn.com/v4/letter/m/bb73d2/32.png) [@MC\_Master\_of\_Ceremonies](https://boards.straightdope.com/u/MC_Master_of_Ceremonies)\ **Post date:** [October 15, 2003, 10:19pm UTC](https://boards.straightdope.com/t/how-did-einstein-discover-near-light-velocitys-affect-on-other-things/207517/24 "2003-10-15T22:19:05Z")
Yes the same explanation still works when you have two clocks ‘staionery’ 100% accurate clocks as in order to agree with the time in the farmers reference frame then they must make the doors close at the same time as the two pulses of light would of in the above example. ---
**Author:** ![MC\_Master\_of\_Ceremonies](https://avatars.discourse-cdn.com/v4/letter/m/bb73d2/32.png) [@MC\_Master\_of\_Ceremonies](https://boards.straightdope.com/u/MC_Master_of_Ceremonies)\ **Post date:** [October 15, 2003, 10:21pm UTC](https://boards.straightdope.com/t/how-did-einstein-discover-near-light-velocitys-affect-on-other-things/207517/25 "2003-10-15T22:21:03Z")
\*“when you have two ‘stationery’, 100% accurate clocks” ---
**Author:** ![The\_Hamster\_King](https://avatars.discourse-cdn.com/v4/letter/t/8edcca/32.png) [@The\_Hamster\_King](https://boards.straightdope.com/u/The_Hamster_King)\ **Post date:** [October 15, 2003, 11:43pm UTC](https://boards.straightdope.com/t/how-did-einstein-discover-near-light-velocitys-affect-on-other-things/207517/26 "2003-10-15T23:43:13Z")
Here’s another way to think about it, Whack. Imagine trying to put a plastic tube in a cardboard box where the box is slightly shorter than the tube. You can’t put it in straight, but if you rotate it at an angle it will fit. Now imagine the barn as a four-dimensional box where the fourth dimension is time. And the train is the tube. When you accelerate the train, its frame of reference rotates with respect to the barn. The full length is still there, but some if it is in the time dimension instead of in XYZ. Now the tube fits in the box. To a stationary observer the train looks like it got shorter. To someone on the train it looks like the only part of the train is in the box at a time. Both are accurate 3-D descriptions of the 4-D reality. Does that help? [Previous page](https://boards.straightdope.com/t/how-did-einstein-discover-near-light-velocitys-affect-on-other-things/207517.md?page=1)