Relativity - the idea that everything is relative, right?
Relative to your perspective, your upbringing, your age, your place and orientation in space
and time?
Except that plenty of things - in fact, perhaps most things - aren't relative.
For example, George Washington was the first president of the United States, World War
I happened before the movie Star Wars was made, this picture shows three apples, and
so on.
It is true that certain things ARE relative to one's perspective - is the apple on your
left, or my right? - is 50°F hot or cold? - is a car fast or slow?
big or small?
- and that's precisely what makes these concepts less interesting to scientists.
In physics (and in most science), anything that changes if you change perspective can't
be a fundamental property of the universe - only things that are absolute are considered
'physical' or 'real'.
And for a long time, physicists thought that distances in space and intervals of time were
absolute, fundamental properties in the universe.
The special theory of relativity first described by Albert Einstein was merely a statement
of the realization that we were wrong: distances in space and time are actually relative - they
change depending on how fast you're moving.
But more importantly, Einstein also described several quantities related to space and time
which *are* absolute: the distance between two events in spacetime, the energy-momentum
of an object, and of course, the speed of light.
Similarly, the general theory of relativity was essentially the recognition that in fact
neither the acceleration nor the gravitational force experienced by an object are absolute
quantities.
Accelerations can transform into gravitational fields (and vice versa), depending on your
perspective and the path you take through spacetime.
The more fundamental absolute quantity is the *curvature* of spacetime, which you can
think of as a kind of "underlying" or "absolute" gravity.
Special and General relativity, are, at their core, not about what's relative - they're
about what's real irrespective of perspective.
If everything were relative, then there could be no science, no laws, no justice - just
opinion.
Science exists because it turns out there *are* absolutes in the universe - truths which
are the same regardless of your perspective.
You might even say that science is simply about finding the truths that will still be
true if you remove
the scientist.
So... goodbye!