1. Mar 2, 2013

### John Cox

I'm not a physics person (did not take it in high school), but I was wondering why the speed of light is the constant (in relativity) and not the half life of an electron or positron.
The name of a good introductory book to physics would be good too.

2. Mar 2, 2013

### Simon Bridge

Welcome to PF;
The speed that photons travel through a vacuum is also the invarient speed in special relativity because photons happen to be the first zero mass particle for which this was verified. All massless particles are measured by all observers to travel at this speed regardless of the relative velocities of the observers.

A half-life is a measure of time, not speed.
The half-life of an electron or a positron is "infinity" anyway. These are stable particles.
But we could ask why not define the time for some other particular process to be invarient (say, the period of oscilations of caesium)?
The answer is because we have yet to find any process which takes the same time for different observers regardless of the relative speed of the observers.
There are no absolute clocks.

It is tricky to know how to advise you on a book - since you didn't study physics in High School then you should probably start with a primer in physics for secondary students. It'll be dry but get you through the basics fast.

You are not going to be able to follow relativity, your interest(?), well unless you have the basic classical theory, and the math, down. Anything aimed at the layman or more pop-sciencey will just be doing you a disservice.

That said - my fave quick-and-dirty intro to relativity is:
http://www.physicsguy.com/ftl/html/FTL_intro.html
... try to follow chapter 1 - it wil tell you what else you need to know.
You should also read II and III and treat IV (FTL) as a bit of fun on the side.

Last edited: Mar 2, 2013
3. Mar 2, 2013

### mrspeedybob

Einstein's original paper on special reletivity is an excelent read. His writing style is easy to read, not dry. He describes "thought experiments" that make his conclusions obvious. The math involved isn't too bad. I was able to understand it when I first read his paper in the 5'th grade.

4. Mar 2, 2013

### John Cox

I'm aware that a half life is not movement speed. It just seemed that something like an electron's lifespan was more set in stone so to speak than the speed of light (Weird to say that, but whatever).

I also thought that after a certain point of travel a photon would accumulate the energy of an electron (P=MV^2), and at a certain speed the electron would have a spent lifespan equal to that of a photon, and that these velocities and distances would coincide.

For the book, there's a great book called freakonomics, I'm not sure if there's something similar for physics.

Edit:
Thanks I'll look that up

Last edited: Mar 2, 2013