from a physics perspective.

Unlike when you play a video backwards in time (where it’s pretty obvious that something

weird is going on), when compared with a normal video, motion in a mirrored video still looks

totally physically normal - just mirrored.

In fact, without outside context, there’s no way to tell which was the original and

which was mirrored!

Which is why horizontally flopped shots are used in movies all the time.

In fact, as far as we know, everything in the universe governed by electromagnetism

and gravity and the strong nuclear force behaves this way - if you set up two experiments that

are mirror images of one another, they’ll produce results that look like mirror images

of each other.

Which presents a problem if we ever need to communicate with aliens from afar: if all

physics is mirror-symmetric, that would mean left handed and right handed are relative

- from a physics perspective they’re interchangeable, just like up and down and forward and back

- so if we were simply talking to aliens and didn’t have any shared reference objects,

we’d we have no way of explaining what we mean by left-handed and right-handed using

physics.

. This Left/Right ambiguity is called “the Ozma Problem.”

.

And the distinction between left and right IS important, because earth-based life mostly

relies on sugars with right-handed symmetry and amino acids with left-handed symmetry.

This isn’t a physics constraint - it just as easily could have been the other way around

- but the point is, the molecules in our food and our bodies DO have a specific orientation,

so not knowing left from right could impair intergalactic culinary relations.

However, there is a solution: the weak nuclear force doesn’t always play nicely when mirrored

. For example, when uranium nuclei beta decay they emit (mostly?) electrons spinning like

left-handed corkscrews, but if you perform the mirror image of the experiment using a

mirror-image uranium nucleus, the nucleus still emits electrons spinning like left-handed

corkscrews (rather than right-handed, as they would in a mirror) . It turns out that in

our universe, the mirror-image of a physical process doesn’t always result in the mirror

image of the outcome - uranium always decays more into left-handed electrons, no matter

how you look at it.

So we’d tell the aliens “you know how electrons spin when uranium decays?

That direction is what we call ‘left’”.

Which would solve the Ozma problem - except, there’s a problem with this solution.

Because what if the distant aliens were made entirely of antimatter?

I mean, in principle they could be and we wouldn’t know.

Antimatter interacting with itself behaves exactly like matter interacting with itself:

antihydrogen has the same atomic spectrum as hydrogen, and antimatter-you looks and

behaves exactly like matter-you (until it interacts with matter).

And here’s the problem: while the matter version of a uranium nucleus decays into left-handed

electrons whether it's in a mirror or not, the anti-matter version always decays into…

right-handed anti-electrons whether it's in a mirror or not.

So if we told the aliens “look at the beta decay of the nucleus with atomic weight 239

- that’s always the orientation we call left-handed” we’d be wrong: for aliens

made of antimatter, it would in fact be what we call right handed.

And you definitely don’t want to shake either hand of an alien made of antimatter.

So how can you figure out, from afar, if a distant alien is made of antimatter?

This is the Ozma problem, level 2.

Essentially, antimatter is another kind of mirror we can hold up to the universe, which

combined with the possibility of regular mirroring means we can’t use beta decay to define

left vs right.

But luckily, there’s a next level solution, again thanks to the weak nuclear force.

Enter the Kaon, a fast-decaying subatomic particle.

Whether they’re mirrored or not, around 20.3% of the time Kaons decay into right-handed

anti-electrons , while around 20.1% of the time - slightly less often - they decay into

left-handed electrons.

And the key is this : if you instead take antimatter Kaons, whether mirrored or not,

they still decay slightly less often into left-handed electrons, rather than right-handed

anti-electrons as you might have expected from the way antimatter-uranium decays.

That is, normal kaons - whether mirrored or not - and anti-kaons - whether mirrored or

not - both decay less often into left-handed electrons.

And this is how distant aliens could figure out if they’re made of matter or antimatter

and whether or not they’re using the same concept of “left” as us: simply build

a particle accelerator and look at the decays of neutral kaon particles; the electron-like

thing that they decay into slightly less frequently is made of what we call matter, and it’ll

be moving in what we call a left-handed way.

The universe doesn’t distinguish between left and right or antimatter for electromagnetism,

for gravity, and for the strong nuclear force.

But for some reason, the weak force allows us to tell the difference.

This video was made with the generous support of the Heising Simons foundation, which also

supports research into the violation of antimatter-mirror symmetry in our universe.

Processes that violate antimatter-mirror symmetry (which is called “CP Violation”) are necessary

to explain why there’s so much more matter in universe than antimatter, and while the

weak nuclear force does violate that symmetry, it doesn’t come anywhere close to accounting

for the observed imbalance between matter and antimatter in the universe.

So physicists around the world (among them researchers supported by the Heising Simons

foundation) are searching for other possible processes that might break antimatter-mirror

symmetry to help explain how the universe ended up made mostly of matter, and thus,

why it was possible for us to exist.

Thanks Heising Simons!