Because the speed of light is always at the fastest possible
speed the universe allows this has a weird byproduct: light’s speed is not
relative to the observer. It is always
the same. Here’s a little more detail
about what that means and following, a little about why that’s weird. Imagine someone on a train car going 100 km/h,
you’re standing still watching the train car go by. Now imagine they throw a ball at 50 km/h. From their perspective the ball is going 50
km/h. But from your perspective the
train and the ball add their speeds together and the ball is going 150 km/h. For most moving objects in the universe you
have to measure their speed from a particular frame of reference. That is, are we measuring the ball’s speed
from your frame or from the person who is throwing the ball? We will get different answers depending on
which frame we decide to measure from.
But because light always goes at the maximum speed allowed by the
universe, this does not happen for light.
Let’s go back to the train car only this time the person on board has a
flashlight. They’re traveling along
again at 100 km/h. They turn on the
flashlight. From their perspective the
photons exiting the flashlight are going c (the speed of light, about 300,000
km/s). From your perspective the photons are traveling? Also c, exactly the same speed. Not c + 100 km/h.
Here’s the weird thing…at least weird from our day to day
experience: the fact that light travels
at the universal speed limit and its speed doesn’t change based on any frames
of reference leads to time being experienced differently when travelling at
different speeds. Let’s go back to the
train car. This time, imagine there are
mirrors inside the car and a beam of light bouncing between them. The light beam is bouncing straight back and
forth and takes some very small amount of time to make one bounce. This is what it might look like, the back
bars being the mirrors and the blue bar being the light beam:
Now, let’s set the train car moving. Now the light beam must bounce at a diagonal
and travel a slightly longer distance in order to “catch up” with the moving
car. Here’s what this looks like, again,
the black bars are the mirrors, spaced out to show the movement of the train
car:
Just like with the ball thrower, with most objects the train
will just impart its speed on the object and we will see it go faster to keep
up with the moving train across the slightly longer diagonals. But here’s where things get weird. Remember that light cannot go any faster and
that it always goes at the speed c regardless of frame of reference (so its
speed is the same if you are observing from outside the train or on the
car). If the speed can’t increase it
seems we may be stuck with a problem: the light has to travel a longer distance
in the same amount of time but it simply can’t because the speed of light is
conserved no matter what. Here’s the
punchline: instead of adjusting the speed of light, the universe adjusts how
much time passes for each frame of reference.
From outside the train time on the train actually slows down so the
light has enough time to get back and forth from mirror to mirror. From inside the train time appears to move at
a normal rate. But from outside, time is
moving slower.
This is the phenomenon known as time dilation. We don’t really start to see significant
effects until we get pretty close to the speed of light but this happens every
time you move with respect to someone/something else. Every time you get in a car, train, plane,
skateboard, bicycle, heck even walking, time slows down for you just a little
bit. So be wise, friends. Choose something productive to do with all
your extra nanoseconds.
Some references:
Some references:
Cox, Brian and Jeff Forshaw.
Why Does E=MC2?
Boston: Da Capo Press, 2009.
