Einstein's theory of special relativity teaches us that time is relative.
But what does it mean for time to be relative? How can we live in a universe
where nobody can agree on when events start, when they end and how long they
last? The infamous "twin paradox" showcases what living in a truly
relativistic world is like.
Put simply, special relativity tells us that moving clocks run slowly. This
is a phenomenon called time dilation, and it's a cold, hard fact of the
universe. Einstein had to introduce the concept to rescue something he
thought was much more fundamental: the universality of physical laws. If we
want the laws of physics — which also means the experience of physics — to
be the same regardless of your reference frame, perspectives have to shift
for observers at different speeds.
Special relativity says that a clock attached to a moving object will tick
at a slower rate than one standing still. And what goes for moving clocks
goes for everything else; physics, chemistry and biology all operate at a
slower pace in moving frames of reference.
It's very difficult to measure this effect at human scales, because it only
shows up to a significant degree as one approaches the speed of light.
Still, it's measurable. In 1971, physicists Joseph Hafele and Richard
Keating put atomic clocks onboard jet aircraft and sent them flying around
the world. When the clocks returned, they were out of sync with a
ground-based clock by exactly the amount predicted by special relativity.
To explore the full implications of this idea, let's start with some
hypothetical twins, Alice and Bob. As long as they remain near each other,
their clocks will stay synchronized and they will age at the same rate. But
if Bob were to get in a rocket ship and cruise around the galaxy at near
light speed, things would be a bit different.
For Bob, maybe only a few months or a couple of years passed on his journey.
But depending on how fast he went, it could have been decades, or even
centuries, for Alice.
The actual paradox
As head-scratching as this tale is, it's not a paradox. This is exactly what
the physics of special relativity demands: Different observers in the
universe will have different calculations of the flow of time depending on
their velocity.
So here's the actual paradox. Nothing feels different for Bob when he's
traveling. It's not like he moves in slow motion or anything. Everything
just feels totally normal — so normal, in fact, that as long as he's not
accelerating, he can't even tell he's moving at all.
This is the crucial thought experiment that helped Einstein develop special
relativity. If you're moving at the exact same speed (no accelerations or
decelerations), it's impossible to tell if you're the one who's moving, or
if you're staying still and the rest of the universe is moving.
Have you ever been in a car and you see the car next to you reverse, and for
a moment, you think you're moving forward? Your mind is relying on outside
cues to help tell you if you're in motion.
So, from Bob's perspective, he's staying perfectly still, and it's Alice
(and the whole Earth) that's racing away from him. Yes, it sounds
improbable, but from the point of view of physics, Bob can't tell the
difference. In Bob's calculations, his clock is running normally, and it's
Alice's that should be running slowly. So, when they meet again, she should
be younger than him, because not as much time has passed for her.
But Alice says the exact same thing about Bob, because according to her,
she's the one staying still and he's the one moving.
Who's correct, Alice or Bob? This is the real paradox.
A certain point of view
We can solve the twin paradox by looking at the whole picture. It's true
that as long as Bob is flying along at a constant speed, he and Alice will
fundamentally disagree about the flow of time, and both perspectives will be
valid. But for them to compare notes, Bob has to return to Earth. In other
words, he needs to slow down, stop, turn his ship around, reaccelerate and
come back home.
This act of turning around has a major impact on Bob's reckoning of time. He
will be doing something that shifts him away from his special frame of
reference. And he'll be able to tell he's doing it not by looking at the
outside universe but by making observations while inside his rocket — for
example, the vibrations of the engines and the push of his body against the
floor during deceleration.
Because of this asymmetry, Bob's perspective is no longer as valid as
Alice's, and he can no longer claim that he is standing still while the
universe is moving around him. When he finally makes it back to Earth, he
will know that he was the traveling twin and that he was the one moving at
close to the speed of light.
The mathematics of special relativity tells us how much Alice and Bob will
age in their own frames of reference, and that same math demonstrates that
they will end up agreeing on the numbers in the end.