Saturday, January 24, 2009

Relativity for Kids

there's no link.
A friend sent me this email regarding her son:

Hi there. I need help with a question that Max just asked.

If someone is traveling at the speed of light, could they see themselves in a mirror?
This was my reply. I'll point her here if you'd like to chime in with supplements.
Well, that's very similar to the question that Einstein asked -- what happens if you run as fast as a beam of light -- and to answer it, you have to change the way you think about the world. Fundamentally.

The speed of light isn't a speed you can go. It's just the end of what 'fast' is. A fuel tank has a gauge, it reads 'empty' to 'full.' For speed, 'Empty' is analogous to not moving. 'Full' is the speed of light. It doesn't make sense to talk about speeds past that any more than it would make sense to add gas to a full tank.

So, OK. The speed of light is the end of acceleration. You can't go the speed of light, because if you could, you could just juice it a little more and go faster, which wouldn't make any sense. You can only approach the speed of light. Acceleration gets harder and harder the closer you get.

Now, what does speed mean? If I leave my house, and trot along at 3 MPH, I'm moving at 3 MPH relative to my house. But, what about when I can't see my house? How would I know how fast I was moving? There's a bunch of other stuff in my neighborhood that moves at the same speed as my house, and I can choose one of those things to measure myself against. But, I have to choose something. And, as I measure it, I'm really measuring how fast that thing is moving relative to me. I really never move, from my perspective, right? We're all the center of our worlds, just like it feels.

Those are two points.
(1) I don't ever actually move. Things move relative to me.
(2) Those things never go as fast as the speed of light.

I forget how old Max is. Ah... you got married in, what, 1993? So, he was born in 1994? He's 14? Einstein was 16, you know. Anyway, I'm not going to muddy the waters with quantum dynamics. I'm not super sure I resolve them off the top of my head, anyway -- I think there's some outstanding paradox; I believe there was some news in recent years, but I didn't follow.

Clearly, there are some implications for time, here, and resolving that has some implication for linear distance. But, I'll let Max work those out for himself.


nephos said...


Myself, I always found it unsatisfying that the speed of light was this fundamental limit. It's as if Einstein started off with "if you assume some really non-intuitive ideas is correct, the it follows that..."

Of course he got lucky and perhaps that's how a lot of physics works, but still...

mark said...

1) "'Empty' is analogous to not moving." ..... HEY!!!!!

2) Well said, I wish Hawkings could have been that succinct in his writings.... unless... you're wrong, in which case I retract.

3) We've got a real problem if no one really moves and everything moves relative to everything else. We're all dynamic, and I can't see how that that little theory fits into all these non-static bodies moving about relative to everything else. Hey, Stephen...!

4) If Chalmers is experiencing the Grand Ennui at his office and he has ten minutes to type 1906 characters while Max is 14 and aging further at a rate of 1 year every 3.155815E+07 seconds, is the speed of his creeping arthritic bent relative to his mounting anxiety of paying off his honeymoon debt incurred while hiking Guatemala's ATM cave?

5) It takes a LONG time for your page to load!

Anonymous said...

that's outright wrong. there are particles that go faster than the speed of light. the rule is nothing can go AT the speed of light, you can still go faster and, obviously, slower.

if you're traveling faster than the speed of light, you would see the house for a brief moment and then it would disappear. an analogy will clarify, it's like that part in big daddy where sandler puts a tree branch in front of the rollerblader, the rollerblader is on its path and the tree branch happens to be there. the two objects meet and hilarity ensues, likewise, the light particle still has the ability to hit your eyes regardless of the speed you're going at. if his teacher complains, he/she is a fucking idiot teaching freshman science, clearly their scope is limited.

Anonymous said...

Hilarity does ensue. Thanks for stopping by the blog.

Anonymous said...

I think that you explained it very simply and well I'm in middle school and this made sense for me. Thanks!

Anonymous said...

Thanks! That's nice to hear.

Venkatesan said...

The speed of light comes from maxwells equations. It is made up
from the electric pertivity and magnetic permeability which is independant of any coordinate. So
it was a puzzle that light always
travelled at a speed independant of the coordinate system, which contradicts the galilean relativity. But the basic postulate that gets rid of absolute velocity must hold good in the new theory that Einstein was contemplating. This can happen ONLY if we are ready to give up the traditional notion of space time.

Anonymous said...

Permittivity, which for some reason is not in my spell checker. I don't know that starting from Maxwell's equations is the most intuitive way to go, but Go Scotland! Thanks for coming by the blog.