# Physics Newbie Here

1. Dec 7, 2003

### TARSIER

I am a sophmore in H.S. and would like to learn the basics of physics and physicists. What are some good physics colleges? How much do physicists make? Do you have to teach in a university as a physics career? Where can I learn more about basic physics? My main question about physics are: What is space-time? I read a little of Stephen Hawkings A Brief History of Time and I could not understand exactly what space-time was. Also, how many dimensions of the universe are there, how do they interact or correlate, and what are their purposes? Does light have mass? If so, is it the lightest particle? Is it a particle? Does this correspond with the Wave Particle Duality? If not, how does light bend? Another question of mine is quantum computing. What is all this about? What is the purpose?

Also, is there a relationship between physics and philosophy? I really enjoy philosophy.

2. Dec 7, 2003

Staff Emeritus
Welcome to the MKaku and Physics Forums, tarsier!

Your career questions would be better answered down on the physics board, where they have real physicists who can give you the scoop.

I'll try to clue you in on spacetime.

When Einstein discovered his special theory of relativity in 1905, he saw that people who were in motion relative to each other would see each others lengths shortened and each others clocks slowed. This is because the speed of light would be the same for both of them. Their speeds would NOT add with the light speed in the way you would think, but each of them would measure the SAME speed of light.

Einstein wrote down linear equations for the length of one in the coordinates of the other and for the time. Each equation had two terms, one for the observed length and the other for the observed time. So each of the length and time of one observer was a function of the length and time of the other. You couln't separate length from time, as Newton had assumed, because they always varied together.

A mathematician named Minkowski read Einstein's paper and declared that space and time had lost their identity and had beccome merged. He called this merged state Spacetime.

Now what is it like. It has four dimensions. three of them are our familiar three, and the other one is time. Geometrically it's a little odd because the Pythagoras theorem has negative signs in it, and the time coordinate is multiplied by c, the speed of light. So hypotentuse equals c-squared times t-squared MINUS x-squared minus y-squared minus z-squared..

I hope this is enough to help you with Elegant Universe which is a wonderful book. If you have any questions about this just post them. I check several times a day.

3. Dec 19, 2003

### TARSIER

Im still not sure i get it

Is it that two people who are parrellel, moving at the same speed share the same space time? And then a jet who is also moving parrallel to them shares a different space time? Is time slowed for the jet? So, is space time the relationship between space and time? Is this a variation equation?

4. Dec 19, 2003

### FZ+

Ignoring gravitional effects, then yes.

Depends on your frame of reference.

Spacetime is the union of space and time. Spacetime incorporates space and time as facets of the same thing.

5. Dec 19, 2003

Staff Emeritus
Re: Im still not sure i get it

All three share the same spacetime. what the two who are moving parallel at the same speed share is called an inertial frame or just a frame. The requirement for to things to be in the same inertial frame is that they don't see any motion relative between them. The inertial requirement is that they aren't accelerated.

The Jet is presumably moving faster than the first two, and so it would be in a second inertial frame, provided that it wasn't accelerating.

Now here is something that gives people problems, but its's tru in relativity. Both the people in the first frame AND the jet in the second frame are entitled to regard themselves at rest. Each is entitled to say "I am at rest and the other one is moving". Of course we can guess the jet is moving but the physics doesn't know that. What Einstein said - and Galileo before him - was that physics works the same in the first frame or the second frame just as it would if it were at rest in Newton's universe.

So the question "Is time slowed in the jet?" should read, "do the two folks in the first frame see the time of the jet slowed". Yes they do. But the jet pilot measures his time as OK, and he sees the time in the first frame as slowed. And length in the direction of apparent motion is shortened to. This is the coordinated change in length and time I menrtioned in my earlier post.

The equation betwen the two frames is two linear equations

time_1 = A*length_2 + B*time_2
lenth_1 = -B*length_2 + A*time_2

Where A and B are certain numbers that depend on the speed between the two frames.