Hi, I'm John Skieswanne, and this post is part 2 of a series on physics. In this series I will explain a few pillars of modern physics. I won't be
using any complex maths. It is my hope that this series will introduce some of you brilliant, curious-minded laymen out there to the inner circles of
So, sit back and enjoy.
the Special Theory of Relativity.
We used to think that Light could slow down. It was a natural assumption: if the Earth was to move at 30 kilometres per seconds away from, say, the
star Regulus, and if Light normally travels at 299,792.458 kilometres per seconds, then wouldn't it mean that light from Regulus should arrive slower
to us, at (299,792.458 - 30) 299,762
.458 kilometres per second?
Albert Michelson decided to put this to the test. In 1887 he devised a light speed detector - the most accurate light speed detector in his time -
which got called the "Michelson-Morley Experiment". But, to his great surprise, the speed of light remained the same - regardless of Earth's
. It goes without saying that Michelson was rather perplexed by the constancy of Light's speed.
Luckily, Albert Einstein was already working on his Special Theory of Relativity. It so happened that his theory explained (and still does)
The truth is, light speed remains the same, no matter the speed of the moving object. Since the speed of light is defined as the time interval it
takes for Light to cross a distance interval, then the new moving frame changes the definition of "distance" and "time" so in the end, light ends
up traveling the same amount of space-time intervals even when something moves.
Here's an analogy to explain Einstein's argument:
Imagine two very powerful light bulbs. One is blue and is placed on a space station some 1,200,000 kilometres from Earth. The other is red and it is
placed on Earth. Suddenly, both emit a burst of their powerful light simultaneously at the same time.
Now imagine that you are on a spaceship. I'd personally favour a popular space-ship from a TV series featuring a certain captain Kirk, but any other
spaceship will do.
Imagine you are parked just in-between the space station and the Earth. If you don't move relative to that position, you'll receive both blue and
red light pulses simultaneously:
But what would happen if you decided to move and fly right at the space station, at the speed of, say, 1/2 c
(1/2 the speed of light)?
Well, obviously, the space station's blue light pulse (the one you're going at) would reach you before
the Earth's red light pulse (from
which you're flying away). The two events won't be simultaneous anymore. Yet, this means that the two lights will come at you at different speed,
right? But Michelson proved that light speed remained the same no matter what.
Einstein had to resolve this paradox.
Einstein speculated that when an object moves, it moves space-time along it - it generates it own space-time intervals frame (its own "Minkowski
space"). Thus although the blue light does reach you first, it nevertheless traveled 3 of your moving space-time intervals. And although the red
light does reach you last, it also traveled 3 of your space-time intervals. Since one space-time interval is here equal to 299,792.458 km (for space)
and 1 second (for time), then, technically, both light rays covered 599584.916 km of warped space and 2 seconds of warped time before reaching you. In
other words, they both traveled at exactly
the speed of light (relative to you)!
On another note: Einstein also pointed out that you, in the moving ship, won't notice anything abnormal. Since you're not moving relative to your
own frame's space-time intervals, then you see your ship as if it was perfectly stationary (that is, if such a thing was possible).
If you were to fire up your thrusters on an asteroid, who would move? You away from the asteroid, or the asteroid away from you? Einstein specifies
that it makes no difference. "There are no preferred frame of reference", he would say. Everything carries its own space-time frame. If two objects
have no motion relative to each other, aka they move in the same direction at the same speed, then they share the same space-time frame. If two
objects move relative to each other, they no longer share the same space-time frame. Since Einstein's law concerns object which moves relative to
, it doesn't matter who's making the move.
movement between the two is the focus of Einstein's Special Relativity. Hence the name.
I hope you enjoyed this read; Part 3 will be coming soon, and will deal about Quantum Entanglement - a very popular aspect of modern physics.
Other part of the series:
-Part 1: the Uncertainty Principle