If you throw a ball straight into the air, you can easily predict its trajectory and catch it. This is one of everyday phenomena of physics which you have experienced time and again. But, what if you bring this physics you live through every day down to the size of atoms? Can you predict the motion of an electron around the nucleus of a hydrogen atom?

Of course not. That’s because the physics that controls the behavior of particles at such scales is markedly different from the laws of physics that govern things you see around you all the time. Phenomena in the macroscopic world we are familiar with function according to the laws of classical mechanics, while phenomena of the systems on the atomic level are governed by the laws of quantum mechanics.

Schrödinger’s Thought Experiment

Bizarreness is the hallmark of the quantum realm. An illustration of this characteristic feature is demonstrated by a famous thought experiment called the Schrödinger’s cat.

The experiment involves putting a live cat in a box, along with a radioactive atom that has a 50 percent chance of decaying. A Geiger counter is also placed inside the box so when it detects atomic decay, it discharges and through a relay causes a hammer to fall and shatter a flask containing cyanide, a poisonous gas, which will kill the cat. However, there is no way of knowing whether the atom has decayed unless someone opens the box and thus, there is no way of knowing if the cat is alive or dead.

Even before we made the observation, we could say that the cat would be in objectively indefinite states – neither alive nor dead but instead in an infinite mixture of both possibilities, with a 50-50 chance for each. In terms of quantum physics, we can say the cat is in a superposition state.

Particles that dwell at quantum scales undergo the same sort of thing. Take an electron orbiting around a hydrogen atom for example. It isn’t exactly orbiting at all; it sort of happens to exist everywhere in space, all at the same time and the only way we can determine its location at that exact moment is when we measure its position.

If you think about it, it’s very much alike how we couldn’t figure the condition of the cat until someone opened the box. So now where does this take us to? A spooky yet fascinating phenomenon known as quantum entanglement.

What is Quantum Entanglement?

Quantum entanglement is one of the quirkiest facets of quantum physics where two particles are inextricably tied-up to one another regardless of their distance in time and space. These two entangled particles can exchange information between them as if they have a clandestine network of communication and they do so instantaneously – apparently violating one of the core concepts of Einstein’s Theory of Special Relativity that nothing travels faster than light.

To have a better grasp of this phenomenon, let’s make another assumption where we have, not just one, but two cats in two separate boxes. And if we run the Schrödinger’s cat experiment with these two cats, the outcome can be one of four possibilities where either both cats are alive, or both dead, or where one is alive, or the other dead, or vice versa. Again, both cats are in a superposition state, however the difference is that in this system of two cats each outcome has 25 percent chance rather than 50 percent as in the original experiment envisioned by Erwin Schrödinger in 1935.

But here’s when things get interesting: Quantum mechanics says that erasing the outcomes where both cats are alive and dead from the superposition of states is a possibility. To put it simply, there can be two system where the outcome will always be one cat alive and the other dead. So the cats in essence are in entangled states.

Quantum entanglement explained on the basis of Schrodinger's thought experiment
Schrödinger’s thought experiment teaches us a lot about quantum mechanics. [Image: Gabriela Barreto Lemos via University of Vienna]
Now let’s say you have two cats in two boxes, and they are in this entangled state. These two boxes will be mysteriously linked to one another no matter what their distance is. So even when you push one box to the opposite side of the universe, the outcome will always be the same. That is, when one cat comes out alive, you automatically know the condition of the other cat even though which cat comes out alive or ends up dead is cloaked by overwhelming uncertainty before we measure the outcome. But, how is this even possible? How can two cats that are light years apart from each other be entangled in this way? They are too far to even maintain contact with each other, but seem to carry through all in an instant.

Well, our minds are too frail to make sense of what’s happening there. But this recondite inexplicable cosmic connection is the magic of quantum entanglement. Even Albert Einstein was startled by this strange quantum property of entanglement and he called it “spooky action at a distance.”

Real World Lab Experiment

Quantum entanglement is not just another conjectural hooey as many people would have thought. In fact, physicists have confirmed it in real world experiments. Two particles entangled in a superposition state maintained a sort of inexplicable relationship with each other, that is when one particles spins clockwise, the other spins the other way even when there’s not a whit of possibility for information to pass from one particle to the other dictating which way to spin.

Entanglement is the heart of quantum information science. If we could somehow exploit this property and apply in our macroscopic world, it would indubitably open up a whole new dimension of applications which we don’t yet understand such as quantum cryptography, quantum computing, quantum communication or even quantum teleportation.