Debunking quantum myths: Entanglement allows faster-than-light communication

Entanglement allows faster than speed of light communication.

Entanglement is one of the most misinterpreted concepts in quantum physics. It has been described as a mystical way of connecting particles that allows for instant communications across the space. This has led to the believe that entangled particles can send data instantaneously, breaking the universal law that nothing can travel faster than light.

Let’s do a thought experiment to understand where the idea of faster-than-light communications comes from. Let’s create two entangled particles, that is, their properties are bounded somehow. We separate them a great distance (one stays here on Earth and we send the other particle on a spaceship to the edge of the galaxy). Then we measure a quantum property (like the spin) of the particle we kept, so now we can know information about the quantum state of the other particle instantaneously (faster than the speed of light!). The catch is that we are not actually sending any information. By “observing” (measuring) the properties of our particle we are only constraining the “possible properties” (probable outcomes) of the other particle.

Let’s try with a classical example, imagine we have two lightbulbs instead of two particles. One can flash red and the other blue. We keep one in a box and send the other far away. When we open the box and plug the light, we see that is flashing red, that means that the one we sent over must be blue. Does this imply instant communication? Now it’s obvious the trick we were playing before. We already had information about the “state” of the lightbulbs, that is, the lights had “collapsed” before even measuring (opening the box). By knowing that one light was red and the other was blue we intervened in the initial “state preparation”, we interfered in the creation of the entanglement. Applying this to qubits, for example, we can measure one and immediately know the state of the other, it only implies that we knew how they were entangled all along, just as we knew how the two light bulbs related.

It is true that in quantum physics, qubits exist in a superposition of 0 and 1 value, and only after measuring the qubit, it collapses into 0 or 1. They remain in a superposition (we don’t know which lightbulb we have) until we observe them. So yes, it can seem that the qubits are communicating instantly. However, if someone on the other side of the universe measures their particle, they won’t have any way of knowing that we had already observed ours, therefore the only information they have gained is the state of their particle but if they haven’t interfered in the preparation of the entanglement, they won’t know how to interpret that information. To know this information, we would have to use a classical channel (we can have a video-call with the astronaut) that is limited by the speed of light. So now it’s clear that there isn’t faster-than-light communication, we had intervened in the preparation of the states so we just knew the entanglement beforehand.