From: Allan Goodall <agoodall@a...>
Date: Sun, 14 Sep 1997 18:54:30 -0400
Subject: Some Relativity (was Re: Faster Than Light Travel - Reply)
> At 05:19 PM 9/14/97 -0700, Donald wrote: Light has mass. What light doesn't have is a REST MASS. Now bear with me, it's been a decade since I studied physics, but my understanding is that nothing with a rest mass (that is, a mass when it isn't moving) can hit the speed of light. As you increase velocity your mass increases. It's imperceptible until you get to a reasonably large fraction of light speed. Of course, the more massive you are the more energy you need to keep on accelerating you, thus to accelerate an object with a rest mass to light speed you need infinite energy (at least, that's what I remember). Photons, on the other hand, have no rest mass. They travel at light speed, and thus have a mass at that speed. Slow a photon down to zero speed and it goes poof. > Automobile headlights don't explode when you are driving down the road Beep! Wrong answer, but thanks for playing. Okay, that does it. Wait a minute while I dig out my Modern Physics text. Okay, I was right about the rest mass (whew, the old noodle is still good for something). Okay, your car's speed is NOT added to the speed of light, thus pushing the speed of light from your car's lights over light speed. This is the old "if I'm flying at the speed of light and I turn on my lights, do I see anything?" paradox. One of Einstein's postulates is that the speed of light has the same speed (known as c, or approximately 300,000 km/sec) in all inertial frames of reference. Due to the principles of relativity, there is no difference--from a physics point of view--between you travelling 100 km per hour down the road as you fly past the scenery and you sitting still while the scenery flies past you at 100 km per hour. So, you are travelling near the speed of light and you turn on your lights. Big deal. It's like you weren't moving at all and the rest of the universe was flying past you in the opposite direction when you turned on your lights. Your lights shine out in front of you, travelling at light speed (c). Here's another question. I'm in a NAC ship travelling at.75c (three quarters light speed). You are in an NSL ship travelling at.75c in the exact opposite direction. If I fired a laser at you, would it hit you? You might answer "No, because we are both travelling away from each other at a combined velocity of 1.5c. Your laser can only fire at the speed of light (c) so it would never touch me." Nope, wrong answer laser breath! Relativity. I can just pretend I'm not moving at all and you are flying past me at high speed. In that case, you'd be travelling at .75c + .75c = 1.5c which means you are breaking the universal speed limit. However, things travelling near the speed of light have special effects. As you travel near the speed of light, lengths contract and time dilates (i.e. lengths and distances get shorter, and time gets longer; this happens at all speeds but is only noticable near light speed). The answer is (if I've done the math correctly) that my NAC ship sees your NSU ship travelling away from me at.96c, which is under the speed of light so my laser hits you (this is using Lorentz's velocity transformation, a pretty simple equation that I can post if you want it). > When you look at light one way, Ah, yes, good old wave particle duality. To put it simply, for the longest time everyone thought light was a particle. It moved like little balls of energy. Then someone shone a light through a screen with two slits in it. At the other side the light formed a pattern of dark and light bands. These are like the crests and troughs of a wave of water. Ah ha! Light was a wave! And the particle theory of light died in an instant. There were some problems, though. If it was a wave, in what was it waving? This resulted in the idea of an odourless, colourless (and eventually massless) substance known as the ether, which no one could detect. There were other problems, too, but for the most part the wave theory stood. Then along came Albert Einstein and his Photoelectric Effect (the fact that light on certain substances emits electricity), showing once again that light was a particle. So, someone got the bright idea of using electrons instead of photons and doing the double slit experiment again. Only this time they would put an amp meter around the slits and see how many particles went through. They turned on the experiment and saw the nice wave patterns. They then completed the circuit for the amp meter. They could measure the number of electrons going through each slit but the wave pattern went away! You can't detect light as a wave and a particle simultaneously! This, to me, is the spookiest thing in physics. The experiment was recently conducted with a very serious set up using a laser with the beam split so that the same light could be measured for particles and waves (I can't remember the exact set up). The same thing happened: if they looked for a wave they couldn't measure the particles and if they measured the particles the wave went away. It's as though someone is saying, "Uh, uh, uh. Not so fast, monkey boy. This is a secret and you just don't know the password." Thus endeth the physics lesson for today. No homework tonight. Class dismissed.