THE SOUTHWORTH PLANETARIUM
207-780-4249   www.usm.maine.edu/planet
70 Falmouth Street   Portland, Maine 04103
43.6667° N                   70.2667° W 
Altitude:  10 feet below sea level
Founded January 1970
Julian Date: 2458877.16
2019-2020:  XCIV
       “Experience is a hard teacher. She gives the test first and the lessons afterwards."
Anonymous

My apologies!
I inadvertently sent the first part of the article without having finished it earlier.   You see, when a DA is in progress, I'll e-mail it to myself because I generally don't find e-mails from me to be that annoying, generally.  

THE DAILY ASTRONOMER
Wednesday, January 29, 2020
Traveling Light

It is not uncommon for the material in one DA to elicit questions that spawn into another DA:  akin to a black hole in one Universe given rise to another Universe: to use the most appallingly self-important analogy possible.  The DA written to the subscriber who wanted to escape to a nearby nebula prompted yesterday's DA and today's, as well, as another subscriber was curious about the notion of light speed.

"Why would somebody on a vessel not experience any time when traveling at light speed?  How can we possibly know that? Why don't science fiction movies take this into account."
                              -Helen  (no, not THAT Helen.)

Greetings, not that Helen,

Time dilation.
The tendency of time to dilate when a given vessel moves.  Such an effect is negligible unless the craft travels at an appreciable fraction of light speed.    This strange phenomenon is part of Albert Einstein's Theory of Special Relativity, published in 1905.        It derives from one deceptively simple statement:

The speed of light is constant in all inertial reference frames.

To explain how this simple statement could affect time, let's put ourselves in a spaceship.  As we needn't worry about such pesky matters as acceleration times and other real world nonsense, we can set the speed to any value we desire.    Moreover, we're going to play tag with a light beam. 

First, we simply hover in space and let the light beam move across our viewfield.  We instruct the on-board science geek to measure the beam's velocity.  Unsurprisingly, she measured the speed of light to be, well, speed of light, or 'c'.     You'll see why we have denoted light speed as "c" soon."

Next, we accelerate from a dead stop to half the speed of light as we try -in vain, of course- to catch up to the beam.  This time, the science geek measures the speed of light again and discovers that it still measures c, the speed of light.     Flummoxed, we accelerate to 85% light speed (the geek calls it 0.85c) and she measures the light beam again. Again, the beam is going at light speed and not at 15% light speed as we would expect.   Now that we're really all mad (well, the geek seems content), we accelerate to 99% light speed and measure the beam again. Of course, the beam's speed remains stubbornly the same:  c, or the speed of light.

In ordinary circumstances, our speed affects the measurement of any other moving object.   A person in a car moving at 100 mph toward another car also moving at 100 mph measures the oncoming car's speed as being 200 mph!    
Light is different. No matter how one travels relative to the light beam, the beam's speed remains constant.      That is the startling consequence of the speed of light being constant in all inertial reference frames.   For this reason scientists use the letter "c" to specify light speed.   The ramification is startling:   time dilates on a moving vessel.  The faster the vessel travels, the greater the time dilation becomes.  IF a vessel could move at light speed, time aboard the vessel stops.  

The problem is that mass increases, as well.  As the vessel's mass increases, it becomes exceedingly more difficult to accelerate it to higher velocities.  

Why don't sci-fi movies take that into account.
Because most sci-fi movie producers consider science to be a real inconvenience that is best ignored.  After all, starship battles would be truly boring if you couldn't see the beams.


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