The Red Green Experiment
Nov. 14th, 2004 06:59 pm![[personal profile]](https://www.dreamwidth.org/img/silk/identity/user.png){kind=small}
tcepsaOkay, taking a break from the saga of the weekend to share a physics thought experiment involving relativity that's been on my mind for awhile. This is a variation on what I think of as the cart experiment, where one person stands on a cart moving at relativistic speeds and the other person stands still on the ground. The cart has two light detectors, one at the front and one at the back, and they are hooked up to a device that will turn green if both detectors detect a flash simultaneously, and red if they detect flashes sequentially (one after the other). There are two other detectors, the same distance apart, on either side of the person standing on the ground. Normally, the observers are two famous physicists, such as Einstein and Heisenberg, but I'm going to use you and me instead because Heisenberg is a pain to type out and besides, you've always wanted to be part of a big physics experiment, haven't you?
So, in the first scenario, I'll stand on the ground and let you ride on the cart. The cart comes zipping along at relitivistic speeds, and the instant that you, on the center of the cart, pass me, standing on the ground, there's a flash of light between us that begins spreading out in a sphere. This is where things start to get good.
From my point of view:
Since my detectors are both the same distance from the point at which the flash of light started, the light will hit them at the same time and so my device will indicate green. Since you continue to move after you pass me, the light will hit the back detector on your cart before it hits the front one, and so your device will indicate red. All perfectly normal and explainable from the standpoint of classical physics.
From your point of view:
One of the things that the theory of relativity states is that light has a constant speed relative to the observer. In the instant that the light flash starts, your detectors are lined up with mine and both the same distance from the point that the flash starts at. As you move on, you observe the light traveling at the speed of light relative to you, so it will hit both of your detectors at the same time; your device will display green. Likewise, since you continue to move after the light flashes, it will hit one of my detectors before the other one, turning my device red. Not normal at all from the standpoint of classical physics, but easy enough to understand with that extra twist of general relativity.
This is basically where the experiment, as I am familiar with it from school, ends. What I'm curious about actually starts right about here, and it is absolutely critical that you pay close attention that we are talking about this from your point of view, with your device being green and mine being red:
Now that the experiment is done, you pick up your device--which appears green to you--and come back to see me. When you show it to me, what happens?
Do I freak out? I was supposed to see your device turn red, and here you are showing me that it is green, and furthermore, that my own device is red, when it should have been green! From your point of view, to me, the laws of physics have just been badly violated.
Or, if we flip it around to use my point of view, do I see you staggering back towards me, a red device in your hand and bewilderment on your face at the fact that the equations you did before the experiment turned out to be completely wrong?
We both, from our own perspective, see the experiment carried out successfully and as expected. Who, then, are these poor doppelgangers for whom the results are precisely opposite what they ought to be--to whom are we actually showing the results of the experiment?
So, in the first scenario, I'll stand on the ground and let you ride on the cart. The cart comes zipping along at relitivistic speeds, and the instant that you, on the center of the cart, pass me, standing on the ground, there's a flash of light between us that begins spreading out in a sphere. This is where things start to get good.
From my point of view:
Since my detectors are both the same distance from the point at which the flash of light started, the light will hit them at the same time and so my device will indicate green. Since you continue to move after you pass me, the light will hit the back detector on your cart before it hits the front one, and so your device will indicate red. All perfectly normal and explainable from the standpoint of classical physics.
From your point of view:
One of the things that the theory of relativity states is that light has a constant speed relative to the observer. In the instant that the light flash starts, your detectors are lined up with mine and both the same distance from the point that the flash starts at. As you move on, you observe the light traveling at the speed of light relative to you, so it will hit both of your detectors at the same time; your device will display green. Likewise, since you continue to move after the light flashes, it will hit one of my detectors before the other one, turning my device red. Not normal at all from the standpoint of classical physics, but easy enough to understand with that extra twist of general relativity.
This is basically where the experiment, as I am familiar with it from school, ends. What I'm curious about actually starts right about here, and it is absolutely critical that you pay close attention that we are talking about this from your point of view, with your device being green and mine being red:
Now that the experiment is done, you pick up your device--which appears green to you--and come back to see me. When you show it to me, what happens?
Do I freak out? I was supposed to see your device turn red, and here you are showing me that it is green, and furthermore, that my own device is red, when it should have been green! From your point of view, to me, the laws of physics have just been badly violated.
Or, if we flip it around to use my point of view, do I see you staggering back towards me, a red device in your hand and bewilderment on your face at the fact that the equations you did before the experiment turned out to be completely wrong?
We both, from our own perspective, see the experiment carried out successfully and as expected. Who, then, are these poor doppelgangers for whom the results are precisely opposite what they ought to be--to whom are we actually showing the results of the experiment?
