Clarifying Confusion: Understanding Einstein's Theory of Relativity

[Alkatran]

I've just read part of Einstein's book on relativity (for a physics project in high school). I just need clarification on a few points:

The theory was figured out because it was found that light speed is the same relative to you, and your friend, and... (everything).
Things get shorter when they go faster, they move slower through time when they go faster, etc. Right?
Does the change in speed/distance cancel out the whole "light is different for me than it is for you" thing? (I don't think it does, I'm not sure if it should?)
It's impossible to reach lightspeed because the energy required goes up. (Lightspeed relative to what? I thought it was relative to you? How can you know if you're approching light speed if light is still going c faster than you??) :grumpy:

The general language in the book is "ALMOST" over my head. I think I'll need to read it twice..

 

[cookiemonster]

Yes, you probably want to re-read the book. You'll understand more the second time around. It's hard to read science just once and take everything away from it.

Your questions are some of the most major effects of SR. They are doubtlessly explained in the book.

cookiemonster

 

[turin]

The theory was figured out because it was found that light speed is the same relative to you, and your friend, and... (everything).

The theory arose based on a postulate, not because it was found that ... Later, the postulate would suffer various tests and always prove true, but originally, it was simply postulated. Note that there is a subtle distinction that should be addressed. Perhaps this (very basic version of the) history will help clarify:

- Issac Newton made a huge step to organize physical phenomena into a concise set of mathematical relations.
- Electromagnetism began to culminate in the late-mid 19th century with Maxwell's unification. The (classical) culmination basically concluded with Hertz demonstration of E&M waves at the end of the 19th century.
- Either Newton's mechanics or electromagnetism needed to be adjusted because they behaved fundamentally differently. There was a lot of work to force E&M to play nicely with Newton. Einstein was playing for the other team and officialized the modification of classical mechanics (i.e. Newton's laws) into what we now call special relativity.

Does the change in speed/distance cancel out the whole "light is different for me than it is for you" thing?

Sort of. But, I recommend to think of the length contraction and time dilation as a consequence of the constancy of c rather than what enables the constancy of c. This is, of course, a very difficult thing to do at first, after a life of having Euclid's postulates crammed down your throat in a misleading physical context (i.e. visual high school geometry).

Lightspeed relative to what? I thought it was relative to you? How can you know if you're approching light speed if light is still going c faster than you??

EXCELLENT! This is exactly what should have occurred to you. This is basically a major facet of the other postulate of special relativity that says the laws of physics do not depend on the relative velocity of the observer to any other observer, in so many words. Of course, there are several problems with the way I have just stated it, so make sure that, if you want to memorize it, you find a very rigorous source.

 

[VantagePoint72]

Lightspeed relative to what? I thought it was relative to you? How can you know if you're approching light speed if light is still going c faster than you??

OK, there is what turin said but also: yes, no matter how much you increase your speed, the beam of light in front of you will always race away at c, however relative to a non-light object, you can still say your approaching c. c is just a number. While that pesky beam of light is traveling at c, you can still measure yourself traveling at say 0.5c relative to your friend who's watching you pathetically try and catch it. As for light speed relative to what...it's the theory of relativity, pick something! If you're traveling at 0.99c relative to one object, it will take a tremendous amount of energy to increase your velocity relative to it. If someone is traveling with you at 0.98c, it will be comparatively easy for you to increase your velocity relative to THAT observer (remember in Einstein's Universe 1+1 no longer equals 2.) Approaching the speed of light doesn't mean catching up with a light beam, it just means getting closer to that all important 300,000 km/s relative to whatever reference point you pick. The light beam will still be traveling away from you at the same speed. Always remember, c is just number.