Warning - I might be remembering this wrong. And Albert Einstein may have been wrong. I don’t know.
If you’re in a spaceship you can’t tell if you’re moving smoothly or standing still without looking outside. It’s the principle of relativity. Easy.
What would happen if you flew at the speed of light and looked in a mirror? Is light leaving your face faster than the speed of light to get to the mirror? But nothing can travel faster than the speed of light. Maybe when you hit the speed of light, your reflection disappears from the mirror? Well, you could tell from inside your spaceship that you were travelling at the speed of light just by looking in a mirror which breaks the principle of relativity.
Imagine we have a box containing a light bulb which gives out regular pulses of light which go to a mirror, get reflected and bounce back to a counter which goes click-click. The path of the light from light bulb, to mirror, to counter is kind of like throwing a ping pong ball up in the air from your left hand, and catching it in your right hand. Except the path is straight.
You take this box on a train. Stop thinking about ping pong balls. It's light. When the train’s moving, it works exactly like it did before you took it on the train (otherwise if it changed you could use this to tell you were moving without looking outside which breaks the principle of relativity). When you are on the moving train next to the box, you see the path the light takes as:
/\
/_\
/_\
Someone on the embankment watching the train go past sees the path the light takes as:
/\
/_____\
/_____\
The light beam leaves to bulb, and by the time it reaches the mirror, the train had moved forward a bit. For the observer sat on the embankment, the distance the light has travelled is longer, but the speed of light is constant so the only thing that can have happened is time has changed. When you are on the moving train, time has slowed down. Yep. Moving clocks run slower than stationary clocks.
How about this… a light bulb is placed in the middle of a train carriage. When light from the bulb hits either the front or back door it opens. To someone travelling on the train, when the light is turned on, the front and back doors open at the same time.
An observer on the embankment watching the train pass sees the back door open before the front door as the back door moves forward to meet the light pulse. Two events happening in two different locations that occur simultaneously to one observer, may occur at different times to another observer.
2 comments:
I read a good book on this kind of thing some time ago. Made sense at the time but can't remember exactly how it went.
Try thinking of two trains passing each other. Although the trains are the same length, to the passengers the other train will appear shorter.
Now think what happens when they pass, the front of your train will pass the back of the other before their front reaches your back. The same (but opposite) will be true for the passengers on the other train. But to somebody watching at the station both fronts and backs pass at the same time.
So the faster things move the shorter they get. It's called 'Length contraction', look it up on wikipedia.
or take a look at:
http://www.phys.unsw.edu.au/einsteinlight/jw/module4_time_dilation.htm
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