Last month, billionaire Yuri Milner and astrophysicist Stephen Hawking announced Breakthrough Starshot: an incredibly ambitious plan to send the first human-made spacecraft to another star system in our galaxy. A giant laser array could launch a microchip-sized apparatus to another star at 20% light speed. But it is unclear how this small device would be able to communicate with us through the vast interstellar space. How about quantum entanglement? Can it be applied to such a connection?
This idea certainly deserves attention.
Imagine two coins, each of which can come up heads or tails. You have one coin, I have another, and we are extremely far from each other. We toss our coins into the air, catch them and slap them on the table. Before looking at a piece that has landed, we expect a 50/50 probability to come up tails, and of course heads as well. In an ordinary, untangled universe, your results and mine will be independent of each other. If you come up tails, my coin has a 50% chance of falling heads or tails. But under certain conditions, these results can be confusing: if you run this experiment and get tails, you will know that my coin has a 100% chance of showing heads before I even tell you. You will know about it instantly, even if we are separated by light years and not a single second has passed.
In quantum physics, we usually entangle not coins, but individual particles, such as electrons and photons, where, for example, each photon can have a spin of +1 or -1. If you measure the spin of one photon, you instantly recognize the spin of another, even if it is half a universe away from us. Until you measure the spin of one photon, they both exist in an indeterminate state; but as soon as one has been measured, you instantly know about it. On Earth, we conducted such an experiment, separating two entangled photons by many kilometers and measuring their spins over a nanosecond. It turned out that if we measure the spin of one and it turns out to be +1, we find out that the spin of the other -1 is 10,000 times faster than the speed of light could allow us.
And here's the question: could we use this property - quantum entanglement - to communicate with a distant star system? Answer: yes, if we consider taking a measurement at a remote location as a form of communication. But when you say connect, you usually want to know something about the place you are connecting with. You can, for example, hold an entangled particle in an indeterminate state, send it aboard a spacecraft to a nearby star, and tell it to look for signs of rocky planets within that star's habitable zone. Seeing one, he makes a measurement, which leads to the fact that your particle will be in state +1, and if not, then the measurement will show that your particle is in state -1.
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So, you suppose, a particle on Earth should be in state -1 when you measure it, which indicates that the spacecraft has found a planet in the habitable zone, or in state +1, which indicates that the spacecraft has a planet not found. If you know that a measurement has been taken, you can take your own measurement and instantly know about the state of another particle, even if it is many light years away.
Wave pattern for electrons passing through a double slit. If you measure which slit the electron passes through, you will destroy the pattern of quantum interference.
The plan is fine. But there is a problem: entanglement only works if you ask the particle: what state are you in? If you place an entangled particle in a certain state, you break the entanglement, and the measurement on Earth will be completely independent of the measurement of a distant star. If you simply measured a distant particle (and found out: +1 or -1), then your measurement on Earth will also be -1 or +1 (respectively) and will give you information about a particle located light years away from you. If you immerse a particle in the +1 or -1 state, then regardless of the result, your particle on Earth will have a 50% probability of +1 or -1 and will not say anything about the particle for many light years.
This is one of the most misunderstood things in quantum physics: entanglement can be used to get information about a component of a system when you know its full state and measure another component (s), but not to create and transfer information from one part of an entangled system to another. … Therefore, there is no opportunity for communication faster than light.
Quantum entanglement is an amazing property that we can use for tons of different things, like a perfect encryption system for information. But communication is faster than light? To understand why this is not possible, we need to understand a key property of quantum physics: that forcibly plunging at least part of an entangled system into one state prevents you from gaining information about this plunge through measuring the rest of the system. As Niels Bohr once pointed out, "if quantum mechanics has not yet deeply shocked you, you have not yet understood it."
The universe plays dice with us all the time, much to Einstein's chagrin. Even our best attempts to cheat in this game are brought out by nature.