Some Relativity (was Re: Faster Than Light Travel - Reply)

At 05:19 PM 9/14/97 -0700, Donald wrote:
>
>The SCI.ASTRO FAQ page is COOL!! 8-D  What these astronomy/physics
texts
>don't explain to me...If light has no mass (so it can travel at the
>speed of light), how can anyone feel it? 

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
>(from the light piling up inside the bulb...the car's speed plus that
of
>light, puts it over the speed limit).

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,
>it looks like a partical (I think it is...)  and when you look at it
>another, it looks like a wave (an ocean wave is made up of water
>molocules).  You can feel the sun shining on you.

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.

Allan Goodall:	agoodall@sympatico.ca 
"You'll want to hear about my new obsession.
 I'm riding high upon a deep depression. 
 I'm only happy when it rains."    - Garbage