Re: Would light stay inside a flash light at light speed.

Einstein's theory of special relativity is based on two postulates:

First: the laws of physics are the same for all inertial observers
Second: the speed of light (c=300 million meters per second) is the same for all inertial obserevers, independent of their velocity.

"Inertial observer" simply means someone who is not accelerating, i.e., an observer who is in "free fall".

These two postulates and a bit of imagination can take you a long way. Imagine that I have that flashlight and I am running past you at one-half of the speed of light (0.5c). I turn on the flashlight, and according to postulate number two, I see the light leave at the speed c (300 million meters per second). Now you might think that you, standing still, will see me moving at 0.5c and the light moving at c, so the total speed of the light would be 1.5c, right? Wrong.

Postulate number two demands that you see the light from my flashlight as travelling at c. How can this be? The anwer lies in understanding Einstein's great achievement- the unification of space and time. You will perceive that my light travels at the speed c. However, you will also observe that my perception of time and distance is "messed up".

We could decide to do an experiment to see how fast light travels. I can put my hand in front of the flashlight and count how long it takes for the light to hit my hand and bounce back. If the distance from my hand to the flashlight is d, then I'll say that it took an amount of time t=2d/c for the light to travel to my hand and back. But you will say "no", that you perceived that the distance from my hand to the flashlight is d', and that it took a time of t'=2d'/c for the light to travel to your hand and back. Your distance d' is shorter than my distance d, and your time t' is shorter than my time t. You think that I am thinner and slower than I think I am!

What is happening is that as I run faster, I experience a dilation of time and space. From your point of view, the faster I go, the slower I become. I, on the other hand, perceive everything happening normally. But as I increase my speed, the time dilation keeps increasing. From your point of view, the faster I go, the slower my time goes, so that it would take me an infinite time to reach the speed of light- I'll never make it to that speed!

This is a property of all things that have mass- they can never go to the speed of light, no matter how hard they try, not even your flashlight.

Everyone, including me travelling with the flashlight, and you standing still, will perceive that the light leaves the flashlight at c. This may sound strange, but that's how relativity works. It only seems odd because you and I are products of a sub-relativistic world, where things move at a snail's pace when compared to the speed of light. The fastest thing we and other animals experience is usually the speed of sound. The speed of sound is only about 300 meters per second, a million times slower than the speed of light! It is only in the modern era, when we can imagine flight to other stars, and we can communicate by radio waves (also travelling at the speed of light) that relativistic velocities have become important to mankind. But the evolution of our brains is much slower than the growth of our technology- so we can be baffled by the strange world, not only of relativity, but also of other modern concepts such as quantum mechanics.

For an excellent reference on relativity, with many well-thought examples, I recommend the book "Spacetime Physics" by Edwin Taylor and John Wheeler (New York: W. H. Freeman 1992).