Our top 5 mind-breaking facts about physics

After writing this we're just rocking in the corner, dazed

The universe is a weird place. It might seem normal in our daily lives but we don’t have a very good view of the universe. We’re medium-sized and it’s difficult to think about things that are much bigger or smaller than us. Try imagining events on the scale of galaxies and distances across billions of light-years. It makes us feel tiny. However, we’re giants if you consider the opposite perspective; it’s equally difficult to imagine events on the tiny scale of particles.

At our size, physics works a certain way. Every time you drop your toothbrush, it always falls down and never floats up the ceiling. Time keeps marching on and in the right direction. We know how the world works and expect the rest of the universe to work the same way, but physics looks very different at other scales. Here are our favourite mind-breaking facts about physics. Warning: this article is going to get weirder and weirder as it goes, so brace yourself.

5. The speed of light isn’t always the same

The speed of light in a vacuum is constant: 300,000 km per second. We’re not denying that. But a lot of the universe isn’t a vacuum. For example, light travels at 225,000 km per second when it travels through water. OK, that’s absurdly fast but it’s also a big difference.

An interesting phenomena caused by the slowing of light through a medium is Cherenkov radiation. Have you ever seen the eerie blue glow of underwater nuclear reactors? That’s Cherenkov radiation and it’s because the reactor is causing some particles to travel at extremely high speeds. Sometimes these particles travel through a substance that’s particularly good at slowing down light. The blue glow occurs when the particles travel through the same substance faster than the light. So the next time someone tells you nothing can travel faster than the speed of light, you can remind them that sometimes light travels slower than other things.

Image © Argonne National Laboratory

Scientists have been creating materials to slow down light as much as possible and Lene Vestergaard Hau made history in 1998 by slowing light down to 40 mph. Yeah that’s right, you could drive faster than this light. If that isn’t quite weird enough for you, you’ll be happy to know that she recently pushed these methods further to slow down light to a complete stop and then restart it again.

4. Almost all of the universe is unaccounted for

Physicists and astronomers use sophisticated methods to measure how much mass there is in the universe but there are two different answers. When you try to measure the total mass in the universe based on all available evidence, you get a very big number. When you do the same for everything we can observe – that’s all the mass in the galaxies, stars, and planets – you get a much smaller number. Literally everything we can see out there only accounts for about 2% of the mass in the universe. How can we be so wrong? Where is all the missing mass in space?

Image © Giphy

The best explanation for now is that most of the universe contains something we can’t see: dark matter and dark energy. Something has to be out there because we can see its affect on observable objects via gravity. Some galaxies are pushed and pulled around the universe in ways that can’t be explained by the gravity from the mass we can actually see. Has anyone seen dark matter? No. Do we know what it actually is? Not really. But we can measure its effect and there has to be something out there to account for all the missing mass.

3. Two particles on opposite sides of the universe can affect each other instantly

You might want to sit down for this one. At the smallest scales, physics operates in a way that seems alien and unintuitive to us. The laws of physics are the same at those scales, of course, but we’ve evolved at our own scale so we’re used to objects behaving in certain way. You have to be quite open-minded to take in some of the discoveries from quantum physics.

It’s important to understand that these particles have physical properties that aren’t set until we actually observe them. For example, particles have a property we call “spin” and when we observe a particle it could spin in one direction or another (it’s not really spinning but that’s not the point). The weirdness of quantum physics means that until we actually observe the particle, its spin is clockwise and anti-clockwise, all at once. It’s only when we observe it that it has a specific spin value. I know, right?

Image © Giphy

Quantum particles, perhaps photons of light, can be linked together in a way that makes them parts of a single entity. We call this entanglement and when I say it’s really weird I’m not sensationalising. This is about as weird as it gets. This is weird on stilts. Once two particles have become entangled, you can separate them in space but they act like they’re still together. Their spin cancels each other out, so if one particle has a clockwise spin, the other would be anti-clockwise. But until we observe it, they’re both spinning in all possible ways.

Putting all this together, it means that if you observe one of the particles and it has a set spin value, the other particle will instantly have a different value (e.g. the opposite) just because you observed the first. What’s mind-blowing about this is that no signal is sent between the particles; it’s more like they’re still physically connected. You can theoretically put one particle on Earth and the other in another galaxy and they would change value instantly when one is observed. This is weird when you think about nothing travelling faster than light, but to the particles nothing is travelling. It’s like they’re still together in spacetime despite being separated by huge distances and this effect is real and has been tested on Earth over miles. Future experiments on the International Space Station will test it even further.

2. The double slit experiment

We all know what particles are: tiny things, little blobs of matter. We all know what waves are: waves of energy like microwaves and other forms of radiation. So what’s light? It’s both. Yeah. This isn’t even the weird part.

One of the most bizarre things you’ll likely ever hear about is the double-slit experiment used to demonstrate the wave-particle duality of light. Picture a solid screen with two slits in it facing a wall. Scientists can fire a single beam of light through the slits to see where it hits the wall. If light is a wave, the slits should cause the light to become diffracted and make a specific diffraction pattern on the wall. This happens because the slits are close together so the light passing through one slit interferes with the light passing through another. If light is a particle, it should just shoot right through one of the slits and hit the wall at a single point like you fired a bullet through. This experiment should settle it once and for all!

If you set up this experiment right now you will see a diffraction pattern. Light must be a wave! What’s absolutely absurd is that if you actually observe a single photon passing through a slit, you see it goes through only one. Obviously a particle couldn’t go through to slits. You couldn’t kick a football through both goals at once. To simplify, imagine you had a camera that captures the moment light goes through the slit much like the photograph at the end of a horse race to determine the winner. When we actually witness the photon travel through one slit as a particle, it influences the pattern of light on the wall so that it is no longer diffracted.

Let than sink in. Usually light acts as a wave so it ends up going through both slits and makes a diffraction pattern on the wall. But if we merely watch the photon going through, we see it goes through only one slit and there’s no diffraction pattern on the wall. This is the Observer’s Effect. The very act of noticing that the photon couldn’t possibly travel through both slits causes the light to hit the wall like it’s a particle. But if we didn’t look closely, the light would act as a wave.

Here’s Jim Al-Khalili explaining it visually:

1. The future can affect the past

The double-slit experiment is weird enough but let’s make it weirder. Light can travel through the slits as a wave or as particles, but not both at the same time. In 1999, a group of scientists proposed a variation known as a delayed choice quantum eraser. In extremely complicated experiments, scientists have shown they can influence the way light behaves (diffraction pattern or not) using a method that takes place slightly after the photon has been observed going through the slit. In other words, we can make a last-moment decision just before the light hits the wall, and it will determine whether the light was a wave or particle when it passed through the slit in the first place.

Image © Giphy

This messes with our intuitive understanding of cause and effect. In this experiment scientists seem to be changing the effect, which in turn changes the cause. It sounds bonkers but only because we’re so used to the way physics works in our daily lives; this kind of thing is seemingly commonplace in the quantum world. The most bizarre aspect of this is that it also messes with the way we imagine the flow of time. It’s only a thought experiment, but theoretically we could build this exact experiment on an interstellar scale. Imagine the distance between the screen and the wall being millions of light-years. Because light travels at a constant speed in a vacuum, it would take millions of years for the light from the slit to reach the wall. If we repeated the delayed quantum choice eraser experiment at this scale, the last-minute decision would cause a photon to decide to be a wave or a particle millions of years ago. Imagine it: a decision a scientist on Earth made today could influence how another part of the universe behaved before there even was an Earth.

Physics is weird.

Main image © Flickr/Aaron Landry