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The image above represents two mirrors with a photon bouncing back and forth between them.

In this example we will pretend that the speed of this photon represents the speed of light, and the two mirrors are one foot apart. It takes one nanosecond for the light to go between the mirrors.

A nanosecond is one billionth of a second.

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The slider bar to the right controls the velocity of the mirrors across the screen.

If you haven't already, try moving the slider bar up and down and observe what happens.

 f'w W#(

The number to the lower right represents the velocity of the mirrors as a proportion to the speed of light.

I like to use the units "feet/nanosecond" because the speed of light is just under 1 foot per nanosecond. I find it much easier to work with this unit than the more traditional "186,000 miles per second" or "300,000 km per second."

 (w #(

The time here, is the amount of time it takes for one photon of the light wave to go from one mirror to the other.

 )w +#(

You have probably noticed that when you move the mirrors faster, they become shorter. This is an effect called length contraction.

 *w L0#(

You will notice that whenever you change the velocity,

the light wave fades out and is soon replaced by a new one.

The speed of light is not constant with respect to objects under acceleration.

You can use acceleration here to make the mirrors dodge out of the way of an oncoming photon.

 8+w W#(

Put the slider so that it is at around .99 times the speed of light.

Notice that it takes a very long time for the photon to fly back and forth between the two mirrors.

This is because the light has to travel a much greater distance.

 ,w L0#(

Imagine that you are watching me travel with the mirrors, and I am measuring the time it takes for the photon to go back and forth.

From my perspective, the mirrors are not moving. Therefore, I still time the photon going between the mirrors in one nanosecond

 b-w L0#(

You could watch me take my measurements, and agree that my watch said it took one nanosecond for the light to reach the other mirror.

However, while one nanosecond passed for me, you would see a larger amount of time pass by, as indicated where the arrow is pointing.

I would appear to be moving in slow motion.

 .w W#(

Go ahead and bring the speed up to the MAX.

When you reach the speed of light the photon must use all of its velocity to moves the right. It cannot reach the other mirror.

In fact, a photon couldn't even exit the mirror, because there is no passage of time.

And finally, you would not be able to see anything from the side, because it is all flatter than a pancake.

 l/w W#(

You may be thinking, now, if I am moving in slow-motion, then it must appear, from my perspective that you are moving in fast-motion.

In fact, this is not the case at all. You would appear to be moving in slow motion to me as well.

I will explain this seeming paradox in another lesson.

 W0w +#(

You may also be wondering, if I am moving at the speed of light, and I go to Alpha-Centauri and back without experiencing the passage of time, shouldn't I feel I have traveled faster than the speed of light?

 V1w W#(

The solution is simple.

Like time, distance is relative.

When I travel at the speed of light, time does not pass, and distance shrinks to nothing. To an object traveling at the speed of light, the earth is adjacent to Alpha Centauri, and would take no time to pass between them.

 2w AP#(

The speed of light is most definitely not relative.

It is constant, no matter how fast you're going.

When we discovered this, it forced us to completely re-think everything that we once thought was relative.

 3w W#(

Some of the eternal constants like time and space have had to be re-worked.

We now have to think in terms of

"How long does time take?"

and

"How big is distance?"

and we find that these, once concrete concepts, are now malleable, and... relative.

 R4w zW#(

Now, imagine if instead of one photon bouncing back and forth, we had three photons bouncing back and forth.

While the mirrors are stopped, these photons appear to all bounce at the same time.

However, when you accelerate the mirrors you will see a very interesting effect.

 5w fAP#(

This effect is what I like to call de-synchronization.

Events that occur in different places can appear to be simultaneous to one observer, but not to another.

This is a very real effect

 6w W#(

For some reason, people like to get into heated debates

about whether light travels as a wave or a particle.

I've always had trouble understanding this debate since

every wave I've ever heard of was made of particles.

But as we add more photons to the line, it starts to look

more like a wave, and less like a bunch of particles

 7w qAP#(

What we have developing here with our crowded string

of photons is a wave FRONT.

In general though, a light wave has a spherical wave front originating from a source like a star or a light bulb, or a radio antenna or a warm body.

 8w #(

Let's play with some spherical, or at least circular wave fronts next.

 9w R 3(

The Next Lesson: Spherical Wave Fronts.

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