This episode is supported by 23andMe. This is a box. And now it’s empty. Well, not really. We can’t see it, but it’s still full of
dust particles, water vapor, air molecules, and on a smaller scale, photons, electrons
and other elementary particles. But if we could somehow remove every last
bit of matter, could we make that space really, truly EMPTY? Is space just an absence of things… or is
space a thing itself? [INTRO] Ancient Greeks gave us the word “aether,”
which has been used for centuries to describe space as a thing. Whether space is a “something” or a “nothing”
sounds like one of those questions your philosophy major friend likes to ask late at night. But the very nature of reality hinges on this
question. It all has to do with relativity. No, not his relativity. Old-school relativity. Way before Einstein, physicists knew velocity
was relative. To say an object is moving at a certain speed
in a certain direction only has meaning relative to other objects. If two astronauts pass each other in deep
space, each can claim that they were at rest and the other was moving. And they’re both equally right. But accelerated motion is different. Changes in speed or direction, daily experience
tells us these seem to have value even without other objects as reference points. Isaac Newton illustrated this with a thought
experiment: You have a bucket of water at the end of a
rope. As you twist the rope, the bucket starts to
spin. Eventually the water is pushed against the
sides. You’re probably like “so what?” but think about it. The bucket and water are moving together. If all motion were relative, for the water
to be pushed against the bucket, it should be moving relative to some other thing. That’s fine, we have plenty of options:
“the rope!” or “the air!” or “the earth!” But now let’s picture this experiment in
the vacuum of space. No Earth, no sun… nothing. And instead of water, it’s you strapped
to the inside of the bucket. As it starts to spin, you feel pressed against
the side. But why? Relative to the bucket, you seem to be still. What are you accelerating relative to? And if the answer is NOTHING, then what’s
making you feeel thiiiiis waaaaaay? Newton’s solution to this riddle was that
SPACE MUST BE A THING. An invisible but very real structure that
gives all motion in the cosmos a master point of reference. He called this absolute space. Nobody could agree what it was made of, but
for the next couple hundred years, scientists accepted that space was something--until another
famous physicist offered a new, crazy idea. NOPE, NOT QUITE THERE
YET. In the mid-1800s, Austrian physicist Ernst
Mach decided he didn’t want to ride up Isaac’s bucket. He pointed out a flaw with Newton’s reasoning:
our universe is NOT actually empty. We share the cosmos with billions of stars
and other things, and even though they’re really far away, they can still act as points
of reference. According to Mach, in a truly empty universe,
with just you in it, acceleration really would have no meaning, there really would be no
way to tell spinning from not spinning. In a universe with half as much matter, you’d
feel half as much. It isn’t space that gives meaning to acceleration,
it’s all the stuff in space. SPACE IS NOT A THING. Around the same time, another brilliant physicist--NO, NOT YET! YOU’LL KNOW WHEN IT’S TIME!--James Clerk
Maxwell was discovering some important things about light: It travels in a wave-like manner
at exactly one speed. Now, sound waves and ocean waves both need
a medium to travel through, so scientists figured light traveled through a medium too,
one that filled space itself. They called it the luminiferous aether, and
once again space was A THING. But there was a problem with this idea: Two
boats will measure an ocean wave’s speed differently depending on their own velocities
through the water. But light waves, unlike anything else in the
universe, ALWAYS have the same speed no matter who’s measuring. An object moving toward a light source at
a million meters per second, an object moving away from a light source at a million meters
per second and an object sitting still will all measure the light approaching them at
the exact same speed. HOW CAN THIS BE TRUE? GOSH, IF ONLY THERE WERE SOME SORT OF FAMOUS
GENIUS WHO COULD EXPLAIN THIS PARADOX. ONE WITH, LIKE, CRAZY HAIR WHOSE FACE IS ON
A LOT OF DORM ROOM POSTERS… Oh, forget it, it was Albert Einstein. NOPE TOO LATE! YOU MISSED YOUR CHANCE! Einstein said “forget the aether.” He said that light, different from any other
kind of wave, could travel through empty space. But without a medium, what is light’s speed
relative to? This is where Einstein blew our minds. He explained light’s bizarre behavior by
reasoning that space and time are not fixed. Distances and durations depend on the perspective
of the observer. But if space is not absolute, as Newton argued,
it can’t be used as an ultimate point of reference for motion, so MAYBE SPACE ISN’T
REALLY A THING? Not so fast. According to Einstein, even though space isn’t
absolute, and time isn’t absolute, what we get when we put them together, SPACETIME
is absolute. The faster an object moves through space,
the slower it moves through time. An object not moving through space is moving
at top speed through time. An object moving as fast as possible, at the
speed of light, no longer moves through time. None of these objects will agree about distances
and durations, but there is one thing they will all agree on: That an accelerating object
will always have a curved trajectory through this spacetime. This is how spacetime provides a reference
point for acceleration all by itself. Space may not be a thing, but SPACETIME IS
A THING. You’ll feel that bucket spinning even if
there’s no other matter in the universe. Sorry, Mach. I know this might make your head spin faster
than Newton’s bucket, it may even sound unbelievable, but Einstein’s equations have
survived decades of experimentation, and we’ve witnessed the malleability of space and time
in actual experiments, like this. The idea that we’re all swimming through
an omnipresent invisible something, a master framework of space and time, may sound a bit
like Einstein invented a new “aether,” but that’s just like, your perspective man. Stay curious. Thanks to 23andme for sponsoring this episode. The name ‘23andMe’ comes from the fact
that human DNA is organized into 23 pairs of chromosomes. 23andMe is a personal genetic analysis company
created to help people understand their DNA. You can see which regions around the world
your ancestors come from, learn how DNA impacts your health, your facial features, hair, even
your sense of smell, and how you sleep. You just have to provide a DNA sample by spitting
in a tube. My DNA, even the DNA in my mitochondria, has
let me trace my family tree and ancestry on a molecular level. By comparing my DNA sequences with sequences
from other people around the world, and seeing what we have in common, I learned that I have
a much better chance of being related to Isaac Newton, who was British, than Albert Einstein. I’ll take it. You can learn more about your personal DNA
story and support our show by going to 23andMe dot com/OKAY. If you give it a try, let us know!
