A friend of mine is a fan of No Ordinary Family and was asking about how the speed of light works with the effect of time. Since this is a fun topic I thought I'd write something over here and point him at it rather than just doing it in an email or a Facebook Page.

Here's the thing with the speed of light: Light ~always~ travels at the speed of light or "c". It does not matter where you are or how fast you are going, light appears to travel at "c" regardless of your frame of reference.

Let's keep the math easy. Let's pretend that the speed of light is 1000 mph. It's not. It's a LOT more than that. But it's a lot easier to talk about if it's a number we can relate to.

So imagine yourself standing at the starting line of a straight bit of race track. You turn on a flash light. How fast does light shoot away from you? At the speed of light or 1000 mph. That makes sense, right? You can imagine the little photons of light shooting out of the flashlight in a beam traveling at that high speed.

Now, suppose you have a friend A that takes off running at a gentle 10 mph. From where you are standing he is moving away from you at 10 mph, and he sees you moving away from himself at 10 mph. Again, that makes sense right?

A second friend B takes off running after your first A. This friend B runs at 15 mph. You see her running away from you at 15 mph. Your first friend A sees B coming towards him at 5 mph (10-15 = 5). So far this should all work. Let's recap:

You stationary see friend A running away at 10 mph and B at 15 mph.

Friend A running at 10 mph sees you moving away at 10 mph, and B approaching at 5 mph.

Friend B running at 15 sees you moving away at 15mph and A getting closer at 5 mph.

All is right in the world.

Except for light.

Let's say you turn on your flash light. You see the light photons shoot out at the speed of light (which we are calling 1000 mph). That's easy.

Friend A who is running at 10 mph should see the light approaching at 990 mph, right? That would be 1000 - 10 = 990 mph. But, and this is where Light gets weird, he doesn't. He sees the light coming at him ~At The Speed Of Light~ or 1000 mph.

Friend B who is running at 15 mph should see the light coming at her at 985 mph from the same logic. At the very least, you would think that because she is running faster, the light would be coming at a slower rate. That's logical.

However, according to Einstein, the speed of light is CONSTANT in ALL frames of reference. That means that you, friend A and friend B all see the photons of light traveling at the speed of light or 1000 mph. Not close to 1000, not around 1000, but exactly 1000 mph.

How does this work? Again, according to Einstein, time slows down based on how fast you are traveling. Likewise distances appear shorter (as long as they're in line with your motion). So because Friend A and Friend B are in motion, time for them runs a fraction slower than it does for you who is not in motion. Because time is a ~tiny~ bit shorter, the light beams appear to be traveling at the same speed.

Now, because the speed of light is ~Really~ fast, we don't perceive these time shifts in our day to day lives. Humans cannot get up to the required speeds to really notice that seconds or minutes are lost at high speed travel. Unless they have superpowers, as happens on some TV shows or in comic books.

A final word: Time never travels backwards and nothing besides light ever actually ~reaches~ the speed of light. It can come very close, but never actually be that speed.

There are some really interesting reads based on this. If you find this interesting let me recommend you look up things such as this experiment, or the classic Twin Paradox. Or simply pester your physics teacher.

Thank you so much for posting this! Very thought provoking!

ReplyDeleteLindsay