This SciShow video is supported by LMNT.
You can head to DrinkLMNT.com/SciShow for a free sample pack with any order.
The universe is filled with invisible light.
That might sound surprising, but it’s because we only learned
light didn’t have to be visible to the human eye a couple centuries ago.
Radio waves, infrared radiation, x-rays?
They’re all part of one big electromagnetic spectrum,
with a thin sliver of visible light sitting roughly in the middle of it.
The particles that carry all these types of light,
invisible or otherwise, are known as photons.
But a lot more recently,
we learned that the universe is also filled with invisible matter.
We call it dark matter.
And some scientists have proposed that this dark sector of reality
may have a way of making itself visible,
and it involves particles that might sound as oxymoronic as invisible light.
They call them dark photons.
[♪ INTRO]
Back in the 20th century, astronomers started noticing something wonky going on
with the universe’s gravity.
For example, galaxies and whole galaxy clusters tend to move like they’re
way more massive than they seem when we look through our fancy telescopes.
And occasionally, astronomers find regions of seemingly empty space
with an inexplicable gravitational pull.
You can try to add up all the conventional matter in the universe,
including the super dim stuff like black holes and rogue planets,
but there’s just not enough of it to produce the effects we observe.
So like it or not…and many people don’t…
about five-sixths of the mass in our universe seems to come from something
we can’t see and don’t understand.
A.K.A. dark matter.
And these days, one of the most common ways that scientists
poke at the dark matter problem is to try adding new particles
to the Standard Model of particle physics,
which describes how reality works on a subatomic level.
But doing so has two big problems.
First, while we know for a fact that the Standard Model is incomplete,
it might just be the best-tested idea humanity has ever had.
If you squish the Standard Model enough to insert one or more kinds
of dark matter particles, you run the risk of
contradicting trillions of experiments that support the current version.
And year after year, data from satellites and particle accelerator experiments
are narrowing the properties that dark matter is allowed to have,
and shrinking the amount of squish the standard model can handle.
The second problem is that dark matter interacts
with just one of the universe’s four fundamental forces: Gravity.
That’s why it’s invisible.
Photons are the representatives of the electromagnetic force.
But unfortunately, the Standard Model doesn’t explain how gravity works.
That’s General Relativity’s job,
and those two theories tend not to play well together.
But that hasn’t stopped theoretical particle physicists.
Because when they say that dark matter only interacts gravitationally,
they don’t really mean it.
Instead, most assume that every once in a while, a dark matter particle
will smack into a regular particle and interact via the weak nuclear force,
that fundamental force that governs stuff like radioactive decay.
The Standard Model’s all over explaining that one.
But despite scientists having several massive detectors that should be able to find
evidence of this weak interaction, nothing’s come up, yet.
Big ol’ goose egg.
So some researchers have gone back to the drawing board,
proposing that dark matter, through those hypothetical dark photons,
can interact with the electromagnetic force.
Now, just like dark matter isn’t really “dark” in color,
dark photons aren’t, like, the propagators of dark.
As awesome as that would be.
But they are a sort of twisted version of conventional light.
For example, regular photons don’t have mass.
Well, technically they have energy…
and Einstein famously showed us that E=MC^2.
So, that energy could do things that real mass does.
But as far as we’re concerned, they’re massless.
And in contrast, the more popular model of dark photons
gives them a little bit of mass.
If you’ve got mass, you’ve got gravity,
so these massive dark photons could act like your run of the mill dark matter.
They’d exert a gravitational tug on normal matter,
but otherwise pretty much do their own thing.
Of course, that means that detecting dark photons
poses the same potential problem as any other dark matter particle:
If they only interact through gravity, the only way to detect them is through gravity.
And gravity between subatomic particles is too weak to measure.
But here’s where dark photons change things up:
Just like Darth Vader can harness his love for his son,
throw the Emperor down a shaft, and die as Anakin Skywalker,
a massive dark photon should be able to transform into a massless photon.
And just like Anakin Skywalker can slice and dice a bunch of younglings,
a photon should be able to turn to the dark side.
So hypothetically, if this dark light really does exist,
we should be able to indirectly detect it based on its relationship with regular light.
And of course, we’ve got experiments aiming to do just that.
One way to detect dark photons is to see if regular photons ever disappear.
And at least some physicists aim to do that by using an experiment called…
and I am not making this up…a “light-shining-through-wall experiment”.
The idea is to bounce a bunch of light around inside
of a hollow, inescapable cavity.
Basically, a mirror box.
And if the amount of light inside the box ever decreases,
you can conclude that some of the photons
transformed into something that can tunnel through the box’s walls.
And that “something” includes dark photons, which don’t give a wombat’s
rear end about mirrors, walls, or the humans trying to discover them.
Meanwhile, the NA64 experiment at CERN
is going more of the particle accelerator route.
It creates a bunch of light by smashing some super speedy electrons
into a detector, and compares how much energy goes into
and comes out of the experiment.
If photons really are turning into dark photons,
the proverbial After photo will be missing some energy.
Of course, the team will have to make sure there aren’t some other,
non dark matter particles capable of running away with it
before they can shout “Eureka!”.
But not all dark photon experiments are looking for something that’s missing.
Others want to watch what happens
when dark photons smash into things themselves.
According to some researchers, dark photons may collectively carry enough
momentum to jiggle a nanoscale drumhead, slightly nudge the mirrors
inside gravitational wave-detectors,
or even make the antennas on large radio telescopes hum ever so lightly.
Like all research at the edges of physics,
we’re very much in the middle of a story being written.
But if you ever see an announcement that physicists have added dark photons
to the Standard Model, it’s not because they’re going through their goth phase.
This SciShow video is supported by LMNT: a sugar free electrolyte drink mix.
LMNT is designed to help reduce muscle cramps, fatigue, sleeplessness,
and headaches by replacing the sodium lost through sweat during exercise.
It contains 200 mg potassium, 1000 mg sodium, and 60 mg magnesium.
These mixes come in different flavors, like their winter chocolate medley!
That includes chocolate chai, chocolate mint, and chocolate raspberry,
but without the sugar that usually comes with all of that chocolate.
Since LMNT supports SciShow and you do too,
you get a free sample pack of eight single serving packets with any order.
You can take that opportunity to try all eight flavors or share with a friend!
And it’s a no risk purchase because LMNT offers no-questions-asked refunds.
To get yours, just head to DrinkLMNT.com/SciShow
or click the link in the description below.
Thank you and thanks to LMNT for supporting SciShow!
[♪ OUTRO]