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Wave Particle Duality for Feckin Eejits

Physics gets a lot of media attention for its pretty pictures of space and rumours of people in Switzerland recreating the Big Bang and blowing us all up. Most of us read the headlines or know the lingo like “E=mc2” but presume it’s too complicated for us to understand. Beneath the jargon some of these concepts aren’t as difficult to understand as they seem.

One exception is quantum mechanics, which the scientists don’t even understand themselves, and that’s what makes it so cool and interesting. I’m going to try to explain some quantum mechanics, famous for being unexplainable… so bear with me. It’s called wave-particle duality, maybe you haven’t heard of it, but it’s the basis of the theory of quantum mechanics. The idea is that light is both a wave and a particle at the same time.

So, what even is a wave? We all know waves on the sea, and that’s basically what all waves are. Waves can travel through different mediums, water being one of those. When sound waves travel through the air we can hear them, but if there was no air at all (like in a vacuum) we’d hear nothing. 

All waves have a frequency and a wavelength. Sticking to water waves because they’re the easiest to visualise, the frequency is how often the waves hit the shore. The wavelength is the distance between the top of one wave and the next. 

For light this is important because a different wavelength or frequency is what gives light its colour. Red light has a long wavelength and low frequency and purple/blue light has a short wavelength and high frequency. Yellow and green are in between, hence the order of colours in a rainbow!

And a particle? Just a piece of matter. A physical thing.

The point is that one behaves in a particular way and the other behaves entirely differently. For example, if you throw a ball (a particle) at a wall with a hole in it, it will either go through and continue on or hit the wall and bounce back.

If you imagine a water wave travelling towards the wall, on the other hand, it will spread out on its way and when it hits the wall some will splash back. The water that travels through will start spreading out again from that point. If there’s more than one hole in the wall it will start a new wave at both points and as they spread out they’ll hit each other and create a new pattern.

Light does this pattern thing like a wave. For many years, physicists were in agreement that light was a wave until in the 1900s it was discovered that sometimes it behaves like a physical particle. It could hit electrons out of an atom like it was a ball on a pool table. One little bunch of light fired at the atom equals one electron fired out of the atom. Light behaves as a wave in some situations and a particle in others, and this is what we call wave-particle duality. This confuses everyone in physics, so some of us just decide to call it a warticle.

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Later, however, we discovered that anything can behave this way! Even you! Well, if you’re about as small as an atom, because quantum mechanics really only makes a difference at the teeny tiny scale. That is why we didn’t notice its effects for so long and this is such a relatively recent discovery. 

An electron for a long time was the smallest particle we knew about, approximately 5 million billion billion times lighter than a grain of sand. Why am I talking about electrons? Because they too can behave like waves! This is super confusing, because unlike with light we can actually see that they are particles under special microscopes. 

We know they can do this because someone did the hole in the wall experiment with electrons. They fired an electron beam at two slits and looked at the pattern left on a screen at the other side. The pattern left on the screen was the type you would get from a wave rather than a particle.

The pattern meant that one electron was going through both holes at the same time, like a wave and hitting into itself at the other side. No, it doesn’t make sense and we have no idea how it does this, so they tried to take a closer look. A detector was put at the slits to try watch the electrons as they approached and see what in the world was going on, but the pattern disappeared on the far screen! 

The electrons just went back to behaving as particles going through either one slit or the other, as if they knew we were watching. The detector was removed, and the wave pattern returned. I know, we asked every question under the sun when it came up in our first lecture on this stuff, but they’ve tried everything we could think of. It seems it’s just that weird and wonderful. For now, it’s anyone’s guess how it works or why but that’s one of the final questions physics has left to answer.

 

Georgia Stynes – Science Writer

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