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 Light at the end of a long tunnel

Background:  This is another one from you the public

Sent: Thursday, November 07, 2002 9:30 PM
Subject: My Stupid Question

Hey everyone,
     Here is my stupid question, a question which has intrigued me since
Iwas 5.  Get a really long straight tube, about 11,160,000 miles (speed
oflight approx 186,000 miles per second times 60 seconds) long. Seal one
end. Coat the interior with some sort of miracle mirror material offering
100% reflectivity.  Fire a high power visible laser into it for 30 seconds.
The beam should now be approximately half the length of the tube.  Seal the
tube.   Please answer this:  How long will the light keep bouncing in the
tube?  If I open the tube will the light be visible for 30 seconds or will the
light have filled out the tube and take a full 60 seconds to empty?  Can
this idea be useful?  I was a strange child, and probably shouldn't have
read so many National Geographic magazines. 



Well this one has us stumped...we asked Aladdin but he just said *woof* so it looks like we will have to call a college campus on this one and the next =) Well we got some user input on this one and we can't argue because no one wanted to talk to us :( But until then here is his input:

Info:  Let me make sure I understand the question first:

At one end (spot A) we shine a laser for 30 seconds into a tube that is so long it takes light a minute to go through. Is the person asking what you see at the other end (spot B)? If that's the case, you just have to remember that light is traveling at a finite speed, so once the laser is turned off, the person at the other end will not see *anything*, since the first photons (think of them as light particles) are only halfway down the tube. So 30 seconds after the laser shut off, the person at B will see the first part of the laser; see it lit for 30 seconds, then nothing again.

You simply can only see light if it:
a) Is traveling towards you
b) Has had time enough to reach you

Oh, and I'm assuming a straight tube with the laser being shined in the middle -- no need for the mirrored coating. If the light is bouncing off mirrors, that means it was pointed at an angle with respect to the tube. So replace all "30"s above with "30*cos (a)" where "a" is the angle the laser was at, because the light is now traveling at longer distances since some distance is "wasted" going back and forth in the tube.

What's funny is that this question doesn't touch on the fascinating thing about light -- that its speed is constant, even in different frames of reference (think different speeds). Check this out:

The above problem could have been done with anything... sound waves for example. You blow a whistle, and someone will hear it later on at the other end.

Scenario 1:
Now let's say we've got a noisemaker in the middle of the tube, and a person at spot A and a person at spot B. Will they both hear the sound at the same time? Of course... the sound is going in opposite directions, but it's still going the same distance.

Scenario 2:
Now, let's say the tube, and the people, are all on a train going really fast (where the person in the back is at spot A, so think of the train going left to right, with A on the left). Again, the sound goes off in the middle of the tube... will they hear it at the same time? Nope.
The person a spot A "catches up" to where the noisemaker made its noise, and so the sound didn't travel as far. While the sound had to travel it's usual distance, plus some extra, for person B since by the time the sound got to where person B *was*, the train moved and person B was now up ahead a little ways.

Okay, that was easy stuff. Now for light... instead of a noisemaker we have two laser pointers pointed in opposite directions.

Scenario 1 is the same. No one moving, the light travels the same distance from the middle of the tube to each observer. They see it at the same time.

Scenario 2 is different. The light still travels less distance to the person at spot A, and more distance to the person at spot B, but if there was some guy sitting still off the train track watching the beam of light, he would report that they see them at the *same time*. It's freaky. But light always travels at the same speed to anyone who's measuring it. I could go into, but I'm at work, and this email is long enough...

Can't argue will not argue and probably don't understand it, is what we are saying here at S.Q.A. although we are interested in these types of questions, so don't be afraid to ask...Still trying to get a hold of someone that will take us seriously.


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