Quantum entanglement is a counterintuitive phenomenon in quantum physics in which two particles become deeply connected, such 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 is immediately reflected in the other.
In 1964, physicist John Bell introduced the idea that these instantaneous changes could indeed be real and measurable, even at extreme distances, formulating what is now known as Bell’s theorem.
This theory challenged the established laws of physics, especially the principle that information cannot travel faster than the speed of light – a principle confirmed decades earlier by Albert Einstein. Einstein called quantum entanglement “spooky action at a distance.”
What is quantum entanglement?
Quantum entanglement depends on a fundamental concept in quantum mechanics called superposition, which means that particles can exist in multiple states at once until they are measured or observed.
Think of tossing a coin: after tossing the coin, but before looking at it, we know it will come up heads or tails, but the exact outcome is unknown. In quantum terms, this ‘unknown’ extends even further: it is as if the coin only turns heads or tails when you actually observe it.
Superposition is essential to understanding entanglement because it shows how quantum particles do not have solid properties until they are observed.
Entanglement takes this concept one step further by connecting two particles in a special superposition that connects them across space. In our coin example, imagine two coins, one in New York and the other on Mars, are entangled.
Flip the coin in New York and observe it, and the coin on Mars would immediately “know” the result, even if it is not flipped. This entangled state between two locations is unique to quantum mechanics and defies our everyday experience.
Examples of quantum entanglement
A common example of entanglement is the use of pairs of photons (particles of light) emitted simultaneously from a single source. When photons are entangled, their polarizations (the direction in which they oscillate) are linked.
If you measure the polarization of one photon to be horizontal, the other photon, when measured, will also be horizontal, regardless of the distance between them. Simply put, imagine that these photons, like two dice, always land on the same numbers when rolled, no matter how far apart they are.
Polarization is a property related to the oscillation of the electric field of the light wave as it moves through space. It can oscillate in different directions: vertical, horizontal or somewhere in between.
So when entangled photons are separated and the polarization of one is measured, the polarization of the other will match, as if you immediately ‘know’ the first measurement result.
Is quantum entanglement faster than the speed of light?
At first glance, entanglement seems to allow information to be transferred faster than the speed of light. If we measured one entangled photon on Earth and its pair on Pluto, it would take about six hours for information to travel between the photons at the speed of light. However, the measurement of the second photon will still match the first regardless of distance or delay, indicating an instantaneous connection.
However, this does not mean that information actually travels faster than light. Scientists to believe it is not a matter of “conveying” information, but rather of a natural correlation that occurs when the particles become entangled.
Think of it as two prearranged cards, one red and one black, shuffled and dealt to two people on opposite sides of the galaxy. When one person checks his card, he knows the color of the other’s card without transmitting any information. Likewise, when we measure entangled particles, it is as if they “knew” they fit together, but nothing traveled to convey this.
It shows that particles can be connected 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 properties or emergent properties.
If the principles behind entanglement are better understood, it could revolutionize theories of the universe and lead to a new framework that unites quantum mechanics with general relativity.
Have scientists proven quantum entanglement?
For more than 50 years, scientists have been trying to test Bell’s theorem experimentally. In 2015, physicists performed one of the most accurate tests to date, finding strong evidence that particles in an entangled state do indeed influence each other, supporting Bell’s theory.
A 2022 study further reinforced this and showed that any realistic model of the universe that includes ‘hidden variables’ (unknown factors that could explain these phenomena) must take into account the ‘ghostly’ influence between entangled particles.
This ongoing research confirms that quantum entanglement is real and measurable, even though it still defies our classical understanding of space, time and causality.
