A COMPLETELY DIFFERENT SET OF FUNDAMENTAL PARTICLES,

OR A REALITY WHERE YOU CAME UP WITH THAT HILARIOUS COMEBACK

JUST IN TIME,

YOU HAVE TO ADMIT THAT THE IDEA OF PARALLEL UNIVERSES

IS PRETTY EXCITING.

AND WITH MODERN ADVANCES IN COSMOLOGY AND PARTICLE PHYSICS,

WE’RE REFINING OUR IDEAS OF WHAT, EXACTLY, OUR OWN UNIVERSE IS MADE OF,

HOW IT GOT STARTED, AND WHETHER THERE MIGHT JUST BE ANOTHER ONE,

TWO, OR INFINITELY MANY MORE OUT THERE....

OR RIGHT IN HERE.

SO, HOW CLOSE ARE WE TO FINDING A PARALLEL UNIVERSE?

WHEN WE CONSIDER WHAT MIGHT LIE BEYOND OUR

OBSERVABLE UNIVERSE, A FEW PRETTY COOL THEORIES EMERGE.

MANY ARE NOT DIRECTLY TESTABLE JUST YET; AFTER ALL, IF WE COULD OBSERVE IT,

IT WOULD BECOME PART OF OUR OBSERVABLE UNIVERSE, RIGHT?

BUT AS IT TURNS OUT, WE HAVE PLENTY OF PLACES TO LOOK

IN THE GAME OF HIDE-AND-SEEK FOR OTHER UNIVERSES.

SO LET’S START WITH THE MOST BASIC, VANILLA THEORY, AND WORK OUR WAY UP

TO THE MORE “GHOST-PEPPER-ICE-CREAM-WITH-RAINBOW-SPRINKLES”-TYPE IDEAS.

THERE’S A TYPE I MULTIVERSE, A QUANTUM MULTIVERSE,

A MIRROR UNIVERSE, BUBBLE UNIVERSES, AND BABY UNIVERSES INSIDE BLACK HOLES.

THE FIRST PROPOSAL IS WHAT PHYSICIST MAX TEGMARK COINED ‘THE TYPE 1 MULTIVERSE,’

OR WHAT I LIKE TO CALL,  “MORE OF THE SAME.”

- One version is just to say that we can observe some finite region in the universe, but there's

probably more out there.

So that sort of multiverse almost certainly exists.

It just says that the universe is much bigger than the universe that we can see,

but that's not that much of a surprise.

OKAY, BUT IF OUR OWN UNIVERSE IS DEFINED BY WHAT WE CAN OBSERVE, WHAT IF IT’S THOSE

OBSERVATIONS THEMSELVES THAT SPAWN MORE UNIVERSES?

WHAT IF, EVERY TIME YOU CHOSE AN ICE CREAM FLAVOR, FOR EXAMPLE, THIRTY OTHER UNIVERSES

BRANCHED OFF WHERE ALL OTHER POSSIBLE OUTCOMES CAME TO FRUITION?

THIS IS THE NEXT POSSIBILITY, WHAT YOU MIGHT CALL THE “QUANTUM MULTIVERSE”...

WHERE THINGS GET A LITTLE MORE INTERESTING.

The multiverse where, whenever something in the quantum world happens, the universe kind

of splits into two copies is the so-called many-worlds version of quantum mechanics.

In quantum mechanics you can have version A of something, and version B and usually

that's something like an electron, but it doesn't weird us out that much that there's

two versions of an electron.

There are experiments that are now making two superposed, that is, kind of added-up

versions of complex molecules.

There are people who are even trying the simplest viruses and as technology gets better, we

might find that there's a limit to how big of an object we can make two versions of...

or there might not be.

At some point, we might have two versions of a person.

BUT DON’T GET TOO EXCITED TO MEET YOUR NEW EVIL TWIN, OR SPOCK WITH A GOATEE,

ANYTIME SOON.

QUANTUM PHENOMENA ARE FAMOUSLY TOUGH TO UNDERSTAND AND MANIPULATE,

BECAUSE BY DEFINITION, OBSERVING THEM

COLLAPSES THEIR COEXISTING REALITIES INTO JUST ONE: OUR OWN.

BUT WHAT IF WE COULD OBSERVE QUANTUM EFFECTS ON OTHER PARTICLES?

COULD THERE BE CLUES ABOUT ANOTHER UNIVERSE LURKING RIGHT UNDER OUR NOSE?

AND THAT BRINGS US TO THE THIRD IDEA: MIRROR MATTER.

- Mirror matter was originally hypothesized because of a discomfort with the fact that

the weak force, one of the four fundamental forces in the universe, actually prefers to

couple to left-handed particles.

We realized that in order for this to be a valid theory, it probably shouldn't interact

very strongly with our own universe.

Mirror matter was actually one of the first and oldest possible candidates for dark matter.

THAT’S WHAT LEAH AND HER COLLEAGUES AT THE OAK RIDGE NATIONAL LABORATORY

ARE BUSY HUNTING FOR,

USING ONE PARTICULARLY PERTINENT PARTICLE:

THE NEUTRON.

What's really interesting about the neutron is because it is electrically neutral, it

can undergo a process called oscillation.

This is a quantum mechanical phenomenon in which a particle can just move back and forth

from one state to another.

So if there were some mirror partner to the neutron, some dark neutral twin, then the

neutron, in principle, could undergo an oscillation into that particle.

And if so, that's something we can detect.

- Beyond this wall is a large liquid mercury target.

And through a process called spallation, it shakes free a bunch of neutrons from this

very heavy mercury nucleus.

Once the neutrons are cooled down, they travel down these beam guides

into the sample area of the instrument.

We use a wall made of a really thick boron carbide to absorb any neutrons

that try to pass through.

If a neutron can oscillate into a mirror neutron, they'll pass through the wall.

And then on the other side of this wall,

those mirror neutrons then have a chance to oscillate back into regular neutrons.

If we're able to confirm that what we see

really acts like a mirror neutron, that would be very strong evidence that something like

mirror matter, this mirror universe must exist.

WHILE LEAH AND HER TEAM CONTINUE TO SEARCH FOR THAT ELUSIVE MIRROR UNIVERSE,

THIS THEORY IS STILL UP IN THE AIR.

BUT SHE CAN TELL YOU ONE THING FOR SURE:

I definitely don't have a pet Demogorgon. [laughs]

WHILE MIRROR MATTER WOULD BE ONE ALTERNATE REALITY RIGHT HERE IN THIS ROOM,

WHAT IF THE ANSWER TO THE MULTIVERSE LAY WAY OUT

IN WHAT’S KNOWN AS THE COSMIC MICROWAVE BACKGROUND?

THERE, AT THE EDGE OF EVERYTHING, LIE CLUES ABOUT THE BEGINNING OF OUR OWN COSMOS.

AND IT MAY NOT BE AS STRAIGHTFORWARD AS IT SEEMS.

- You can think of the Big Bang as that kind of fireball that was the hot plasma that was

expanding, but you can also think of the Big Bang as the beginning of everything, the beginning

of time and space and so on.

Those two things it turns out are not exactly the same.

Could there be a whole era of cosmic history

in between the beginning of the universe as a whole

and that fireball that led to our observable universe?

THIS IS KNOWN AS “INFLATION,” AND, IN THE VERSION WHERE IT GOES ON FOREVER,

“ETERNAL INFLATION.”

INFLATION IS THE IDEA THAT SOMETHING AS SMALL AS A KILOGRAM OF MATERIAL CAN BE BLOWN UP

TO BECOME THE ENTIRE OBSERVABLE UNIVERSE, AND MORE.

This inflation process is, is, it turns out, very, very, very effective.

It doesn't just create the observable universe or even a million times bigger,

but it generally keeps going.

Creates huge amounts of space and time,

including, possibly, regions of space-time with very

different properties than the one that we inhabit.

There are these other big regions we might call universes, but they might have very different

properties, different laws of physics, different constants, that might have beings in them

and so on, that might even have a different number of dimensions,

all brought about by a single process.

AND THERE ARE A FEW IDEAS ABOUT WHAT, EXACTLY, THAT PROCESS ENTAILS.

ONE FLAVOR SUGGESTS THAT THE MULTIVERSE MIGHT LOOK SOMETHING

LIKE A ROOT BEER FLOAT – WHERE OUR UNIVERSE IS JUST ONE

IN A SERIES OF BUBBLES.

- It's a little bit too far away. [laughs]

Now imagine that there's some sphere that

smashes into this, right?

It'll bruise, it'll dent one side of this

and that's exactly what a collision with another bubble would do;

it would create a sort of disc on the sky that was a little bit different than

every other direction.

My collaborators and I have actually done this

taken the microwave background data,

looked for these signatures, looked for patches on the sky.

We found some,

that's the exciting thing.

The unexcited thing is that the things that we found

are totally explainable as just random variations in the regular microwave background.

WITH MORE AND BETTER DATA ON THE EDGE OF THE UNIVERSE,

WE COULD STUDY FEATURES LIKE TEMPERATURE

AND LIGHT POLARIZATION OF THE CMB IN MORE DETAIL,

TO DO MORE SENSITIVE TESTS FOR THIS KIND OF IMPACT.

AND IN THE REALM OF INFLATION EXISTS ANOTHER OUT-OF-THIS-WORLD THEORY – AND THIS ONE

MIGHT JUST BE THE CHERRY ON TOP.

A fairly natural process, just in the dynamics of inflation, before our sort of hot fireball

Big Bang could have created a whole bunch of baby universes.

They would appear to us as black holes, but inside each black hole would be a sort of

wormhole that connects to a baby universe.

SO, IF ALL WE NEED IS TO DO IS FIGURE OUT HOW TO JUMP INTO A LUCKY BLACK HOLE,

FIND A BRUISE IN THE FARTHEST STARS, CONFIRM SOME NEUTRONS ARE WHERE THEY

SHOULDN’T BE, OR BUILD A QUANTUM COPY MACHINE TO PIN DOWN QUANTUM DYNAMICS FOR GOOD, HOW

CLOSE ARE WE TO FINDING A PARALLEL UNIVERSE?

Very close.

It's right here.

For the quantum multiverse, it's here all the time.

We just have to believe it.

For a mirror sector, it's right here, we just have to detect it.

For a baby universe inside a black hole, we have to go and find it.

- Over the next five years, we're going to be exploring a lot more possible mechanisms

by which the neutron can oscillate into mirror neutrons.

And as even better, brighter sources of neutrons appear around the world, on the ten year timescale

we think that we can really probe a lot of interesting space

where parallel universes might be hiding.

- If we're looking for a bubble that's running into our bubble universe, those really could

appear in the data at any time, with lots of luck, but that's something that even in five

or ten years, more data will certainly exist and that's data

that we can check for that effect.

So somewhere between right now and thousands of years,

I think we'll definitely

maybe find one.

- To discover something like that would revolutionize physics.

I don't even know where we'd go next.

But it would be very, very exciting.

There's a parallel universe somewhere

where you're the smartest person in the world,

up to date on all the latest scientific discoveries.

You can jump closer to that universe right now

by hitting the subscribe button and the bell down below.

Thanks for watching and I'll see you next time!