Quantum entanglement is a counterintuitive phenomenon in quantum physics in which two particles become deeply intertwined, so that a change in one particle immediately affects the other – even if they are billions of light-years apart. This effect occurs regardless of distance, meaning that an action performed on one particle will be immediately reflected in the other.
In 1964, physicist John Bell introduced the idea that these instantaneous changes could actually be real and measurable even at great distances, formulating what is now known as Bell's theorem.
This theory challenges the established laws of physics, in particular the principle that information cannot travel faster than the speed of lightweight – a principle confirmed decades earlier by Albert Einstein. Einstein called quantum entanglement “spooky action at a distance.”
What is quantum entanglement?
Quantum entanglement is based on a fundamental concept of quantum mechanics called superposition, which means that particles can exist in multiple states at once until they are measured or observed.
Think of a coin toss: after the coin is tossed, but before we look at it, we know that it will come up heads or tails, but the exact result is not known. In quantum terms, this “unknown” extends further – it's as if the coin doesn't decide on heads until you observe it.
Superposition is indispensable to understanding entanglement because it shows that quantum particles have no fixed properties until they are observed.
Entanglement goes a step further by combining two particles into a special superposition that connects them in space. In our coin example, imagine that two coins, one in New York and one on Mars, are entangled.
Flip a coin in New York and watch it, and a coin on Mars immediately “knows” the result, even if it hasn't been tossed. This entangled state between two locations is characteristic of quantum mechanics and contradicts our everyday experience.
Examples of quantum entanglement
A typical example of entanglement are pairs of photons (lightweight particles) emitted simultaneously from one source. When photons are entangled, their polarizations – the direction in which they oscillate – are related.
If you measure the polarization of one photon as horizontal, the other photon will also be horizontal during the measurement, regardless of the distance between them. Simply put, imagine these photons as two dice that, when rolled, always land on matching numbers, no matter how far apart they are.
Polarization is a property related to the oscillation of the electric field of a lightweight wave as it moves through space. It can oscillate in different directions – vertically, horizontally or anywhere in between.
So, when the entangled photons are separated and the polarization of one of them is measured, the polarization of the other will match, as if the result of the first measurement was immediately “known”.
Is quantum entanglement faster than the speed of lightweight?
At first glance, it seems that entanglement allows information to be transmitted faster than lightweight. If we measure one entangled photon on Earth and a pair of it on Pluto, it should take about six hours for information to flow between them at the speed of lightweight. However, the measurement of the second photon will still match the measurement of the first, regardless of distance and delay, suggesting an instantaneous connection.
However, this does not mean that information really travels faster than lightweight. Scientists be careful this is not about “transferring” information, but rather about the natural correlation that arises when particles become entangled.
Think of it as two predetermined cards, one red and one black, shuffled and dealt to two people on opposite sides of the galaxy. When one person checks their card, they know the suit of the other person's card without transmitting any information. Similarly, when we measure entangled particles, we get the impression that they “knew” they fit, but nothing traveled to convey it.
It shows that particles can be combined in ways that do not fit classical models of cause and effect or distance. This has led to numerous experiments aimed at understanding whether space and time are fundamental or emergent properties.
Better understood principles behind entanglement could revolutionize theories of the universe and create a modern framework that combines quantum mechanics with general relativity.
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Have scientists proven quantum entanglement?
For over 50 years, scientists have been trying to experimentally test Bell's theorem. In 2015, physicists performed one of the most thorough tests yet, finding mighty evidence that particles in an entangled state actually influence each other, supporting Bell's theory.
2022 test reinforced this further by showing that any realistic model of the Universe that includes “hidden variables” (unknown factors that can explain these phenomena) must account for the “spooky” influence between entangled particles.
Ongoing research confirms that quantum entanglement is real and measurable, even though it still contradicts our classical understanding of space, time and causality.
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