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

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