[MUSIC]
In February 2016, the final major prediction put forth by Einstein’s theory of general
relativity was confirmed, more than 100 years after he initially proposed it, proving yet
again: The greatest physicist, of ALL time, is JOHN CE--
No. Just no. It’s Albert Einstein
[OPEN]
Let’s face it, in the world of physics, Einstein is like Beyonce, Kanye, and Taylor
Swift all rolled into one, and a touch of Lady Gaga in the hair. He's famous, but he's got the
skills to back it up. By age 26, Einstein had already completely
changed physics, but who would be satisfied with that?
He still wanted to integrate gravity into his theory of relativity.
Einstein’s idol Isaac Newton had claimed gravity was mediated by an attractive force
between two bodies, and that an object that feels no force will either remain motionless
or move in one direction at a constant speed.
But this way of looking at things really bugged Einstein, so he did what bored patent clerks
do and daydreamed his way into the history books…
Einstein imagined himself falling from a great height in a sealed container.
Everything inside would be weightless, floating around him,
but there’s no way he’d be able to distinguish this from floating in deep space, far from a massive object like Earth.
Now, suppose that sealed container is accelerating through space at 9.8 meters per second squared,
There’s no experiment we can do to distinguish this from the feeling of standing on Earth’s
surface. If we drop an apple, we can’t tell if it is accelerating towards the ground,
or if the ground is accelerating toward the apple. This means acceleration from gravity
and acceleration from any other force are indistinguishable.
Or to put it another way, gravity isn’t a force at all, but a result of our surroundings accelerating *relative* to us.
Einstein’s theory of general relativity joined these two ideas into one. Rather than
gravity being a special force between two bodies, massive things warp spacetime, like
dimples in a fabric, and falling objects are simply moving in straight lines around these
curves.
Of course, a beautiful theory’s just a beautiful theory if it can’t make observable predictions.
But for the past 100 years, physicists have been putting Einstein to the test
The first test was the gravitational effects
of massive objects at close distances. People had known for a long time that the long axis
of Mercury’s elliptical orbit rotates around the sun over time, called precession. But
new measurements of this rotation made in the late 19th century were off by 43 arcseconds
per century from what Newton’s physics predicted. This is just a few thousandths of a degree,
but it’s still something. When Einstein applied his spacetime curvature, the numbers
lined up.
Einstein’s next prediction was that massive
objects should bend passing light, and scientists were able to test this just a few years later
in the form of a solar eclipse. If Einstein’s relativity was correct, then stars visible
near the edge of the eclipsed sun should appear in different positions from when they were
viewed away from the sun. Newtonian gravity also predicts that light can be bent by a
gravitational field, but it’s based on some bad assumptions, and gives a number just half
the size of Einstein’s prediction. Astronomer Arthur Eddington sent teams to
Brazil and west Africa to observe the event, and their data confirmed Einstein’s model
over Newton’s. This resulted in what might be the greatest scientific newspaper headline
of all time, and made Einstein a global celebrity.
We’ve talked about the effects of Einstein’s
*special* relativity on time and distance before, but general relativity has its own
effects on how clocks tick. Let’s say two observers each have a photon
clock. Instead of ticking seconds, these clocks tick when a photon bounces between two mirrors.
Without any other factors, each observer should see the other’s clock “ticking” at the
same rate as their own, but if we accelerate one clock upward, it ticks more slowly because
the top mirror is moving away from the rising photon.
Now remember, Einstein’s equivalence principle says we can’t distinguish an accelerating
frame from a gravitational field, so clocks tick more slowly, time passes slower in stronger
gravitational fields. We don’t have to go near something like
a black hole to see this at work. Clocks aboard our GPS satellites, far away from Earth, have
to correct for this effect when beaming time information to our devices.
A clock on Mount Everest, if it had been ticking there for the entire history of Earth, would
be 39 hours ahead of a clock at sea level. A clock on your head would even tick ever
so slightly faster than a clock at your feet.
Perhaps the wildest prediction of general relativity was that massive objects could
create waves in spacetime itself. We’re talking huge things –spinning pairs of neutron
stars or colliding black holes. As these waves traveled across the universe, they’d pass
right through Earth, squishing and pulling us like Jell-o.
But these ripples are tiny and had remained
undetectable… until now. Last year, the LIGO
Observatory detected a passing G wave using tiny fluctuations in laser light beams, and
in February 2016, scientists confirmed these spacetime ripples had been directly observed
for the first time ever. The waves originated 1.3 billion years ago,
far outside our own galaxy, from the collision of two black holes, and were detected here
a century after Einstein made his prediction. If you want to dig deeper into how these gravitational
waves are formed, and how LIGO detected them, check out these two videos from our friends
at PBS Space Time, they are really great.
Gravitational waves let us see a totally new spectrum of physics beyond electromagnetic
radiation, letting us study the most massive objects in the universe through completely
new eyes.
With this new discovery, and the final confirmation of general relativity’s predictions, Einstein
cements his place as THE spacetime lord.
Stay Curious.