Why You Can’t Use Entanglement to Communicate Faster than Light (Without Math)
I often get asked to explain pop-sci concepts like how you can travel back in time. But one of the more recent ones has been to clarify how you could (or couldn’t) communicate faster than light using entangled states, so here’s a no-math explanation.
The Scenario
Quantum entanglement is generally understood as this: Two particles have some property in which a change in one instantly causes a change in the other. So, in theory, you could change something about one entangled particle on one side of the universe and then use the change in the other particle to instantly communicate to the other side of the universe . Thus, bypassing the universal speed limit.
This would be a big deal if it were true, and there’d be a ton of applications. For example, we could put a particle with an entangled property of spin into a spaceship and send it off looking for life in a distant galaxy. If it finds life, we tell a machine to change the spin of the entangled particle. From the safety of the earth, we can check on the other entangled particle to see if the spin changed. But, things aren’t that simple in quantum mechanics (of course).
A Quick Primer on Quantum Measurements
Before going on, it’s crucial to have an idea of how weird stuff happens on the quantum level. Let’s start off with an analogy. Imagine we have a quantum green particle. Since the color green is a combination, or superposition, of the colors yellow and blue, the quantum green particle is half yellow particle and half blue particle.
To create entanglement, we take our green particle and split it. When we split the particle, it separates into two entangled constituent particles. The two particles are still in a superposition of blue/yellow. When we measure the color of one of the entangled particles, we get a 50/50 chance of measuring blue/yellow.
However, when we measure the color of the other entangled particle afterwards, it will always be the opposite color of the other particle. So if we measure one particle to be blue, the other particle will always be yellow, and vice versa. Entangled particles are in a superposition of states until you measure them. When you measure one, the superposition collapses into a random available state, and the other entangled particle collapses into the complementary state, always.
The Explanation
Now, back to the scenario. Let’s split the green particle into the two entangled particles and (without measuring the color), and put them into separate boxes. We then place each box into different space ships and fly them to opposite sides of the universe. Later, when one person opens the box and sees they have a yellow particle, they know with 100% certainty that the other spaceship has a blue particle.
This is where the dilemma happens. Yes, measuring the color of one entangled particle instantly collapsed the superposition of the other across the universe. However, the other spaceship wouldn’t be able to tell the superposition collapsed until they opened their box. By then, how would they know they didn’t collapse the state first? You’d have to send information, and that is still governed by the speed of light. The collapse of a superposition, no matter the distance, happens instantly, but it does not carry any information.
Nothing we do to entangled particles gets instantly transmitted across the universe except the collapse of the states due to measurement, as described above. If we perform a measurement then force a change in the state of one particle, the other entangled particle does not instantly change state in response to the forced change of the other because both superpositions already collapsed into a state. Or, in terms of our color-particle scenario: If I have an entangled blue particle, and I change something about the color after already measuring the color, the other entangled yellow particle does not change in response.
Conclusion
What often gets confused with entanglement is what actually gets instantly changed across the universe. It is true, we can harness entanglement to perform faster-than-light actions, but that action cannot be used to transmit information. However, we can still harness entanglement’s other amazing uses like quantum computing..
Originally published at https://www.groundstatecuriosity.com on December 5, 2019.
