First, what is quantum physics? Put simply, it is the study of the very small. In a microscope, we can see things that are too small to see with our own eyes, like the cells that make up all living things. This is still far too big to be quantum. Zooming waaaaay in, we find organelles, chromosomes, DNA strands, proteins, and finally the basic building blocks of all matter: atoms. Now we have arrived at the level of quantum physics.
“Quantum” means the smallest possible amount. It does not mean consciousness, alternate histories, or time travel, so put those thoughts out of your mind. Quantum physics is the study of things that cannot be divided. For instance, an electron orbiting an atomic nucleus cannot have less than a certain amount of energy, called its ground state. Above that is the first excited state. To gain more energy, an electron must receive enough to make the difference between the ground state and the first excited state. After that, comes the second excited state, and so on. Think of it like climbing a ladder. If you want to get to the next rung, you have to lift your hand all the way up to it. If your hand isn’t high enough, it won’t be able to grab onto anything, and will fall back to where it was before.
Electrons have a little-known property called spin. They don’t actually rotate, because that doesn’t make sense, but they have a magnetic field as if they were rotating. If you measure an electron’s spin, you will either find it to be aligned with your measurement, spin up, or opposite your measurement, spin down. The details get complicated, but the important thing is that there are only two possible spins that an electron can have. Also, it is impossible for an electron to be without spin.
A quantum state is the way that a particle or system of particles is at any given time. An electron’s state includes its energy level, which atom it is a part of, or lack thereof, and its spin. Electrons are a type of particle called fermions, which means that two of them cannot have their total state be exactly the same at the same time. Looking at an atom again, there can only be two electrons in the ground state of energy, since there are only two possible spins. A third electron would have to go in the first excited state, and would not be able to get to the ground state.
There is one more important fact about quantum physics: a particle can be in two or more states at once. An electron can be 50% spin up and 50% spin down. This is called quantum superposition. To be clear, it is not halfway between spin up and spin down; it is both at the same time. If you measure something in quantum superposition, it becomes 100% one of its options, and we say it collapses. This means that if you measure an electron in a 50% spin up, 50% spin down state, it will collapse to be either 100% spin up or 100% spin down.
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| Think of it like these two sine waves. There cannot be two fermions with the same wave, but they can be antisymmetric like this. |
Note: this does not mean consciousness creates reality. What exactly counts as a measurement is still debated, but there is no evidence that it has anything to do with consciousness. There is no counter-evidence either, but the burden of proof is on the person who wants to make the claim that there is a connection.
So if you have two electrons in an atom’s ground state, you would think that one has spin up and the other has spin down. But in reality, one is 50% spin up and 50% spin down, and the other is 50% spin down and 50% spin up. You might ask, how can this be? We just talked about how it is impossible for two electrons to be in the same total state. Well, unlike in everyday life, in quantum physics, order matters. Half-down-half-up is different from half-up-half-down—different in exactly the right was so that the two electrons occupy different total states.
If you take an electron away from its atom, its spin will still be in a superposition of up and down. This means that if you measure it, it will have a 50% chance of collapsing to spin up, and a 50% chance of collapsing to spin down. The same is true for the other electron, 50% for spin up, 50% for spin down. But here’s where the magic happens: When the two electrons are in the atom, they must be opposite from each other. If you take them out of the atom, they must still be opposite from each other. So if you measure one and it collapses to spin up, you immediately know the other one is going to be spin down. The thing is, it does not matter how far apart the electrons are. You could separate them by light years, and measuring one will collapse the spin state for both of them at once. This is quantum entanglement. It can happen for all kinds of particle combinations, and for many properties besides spin. A lot of neat stuff regarding it has been observed in labs, and there are even theoretical technologies based on it like quantum computers.
To reiterate, when two particles are entangled, measuring the entangled property of one will immediately determine what you will get when you measure the entangled property of the other, even if two people do the measuring at faraway places at the same time. Although it seems like instantaneous action faster than the speed of light, there is unfortunately no way to use entangled particles to communicate, so it does not break the speed of light barrier. After all, when you measure your particle, you have no idea if the one on the other end has been measured yet. Too bad for humanity’s galactic civilization 10 million years from now. But even if it doesn’t solve communication light-lag, quantum entanglement has plenty of other uses, and it’s quite a marvelous mystery of the universe.
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