Just a quick note before we start: I got a bit carried away with this one. What was meant to be one article on quantum entanglement turned into something closer to a small dissertation, so I’ve split it into two parts rather than dumping the whole lot on you in one sitting which will also give me an extra week do the research and get Part Two much more readable.

This week, Part One: entanglement itself, what Einstein got wrong (and sort of right), and why it doesn’t break the universe. Next Friday, Part Two: what entanglement actually builds, and how deep the rabbit hole goes. Right, on with the show…

Hello, my lovely readers, how about today we take a friendly deep dive into the universe’s strangest connection: Quantum Entanglement.

At some point in your life, you were probably told the universe works like a well-behaved machine. Things move, collide, push, and pull. Causes lead neatly to effects. Objects exist whether or not you’re looking at them.

Then quantum physics came along and politely, but firmly, said:

“Er… Excuse me? I like your thinking, but that’s not quite right.”

And at the heart of that disruption is something both beautiful and unsettling, something the physics boffins call quantum entanglement. The term was first coined by Erwin Schrödinger, you know, the guy with the cat in the box you’re always hearing about?

Einstein famously mocked it as “spooky action at a distance.” He thought it was a flaw, but it actually turned out to be a feature.

Let me try and unpack what that really means, without getting lost in too much maths, (as, I’ve mentioned before, maths isn’t one of my strong points), but still going deeper than the usual surface explanations.

So, what is entanglement?

Imagine two particles created together, say, two photons born from the same event. From that moment on, their properties become linked. They are not just similar, and not just predictable. They are shared, which means if you measure something about one such as its orientation or “spin”, you instantly know the corresponding property of the other.

Even, and this is the truly mind blowing bit, if it’s on the other side of the galaxy.

Well, that doesn’t sound so weird, right? At first glance, it feels normal, I’ll go with the classic analogy, say you have two gloves, one left and one right and you put them in separate boxes. You send one to Norwich and the other one to Waitangi (I chose these two places as I live in Norwich and I googled what is the antipodal point of Norwich City UK; here’s a link for no reason: https://tinyurl.com/4ap8w4d3). When you open one you immediately know what the other one is. No mystery, right? Except this analogy quietly assumes something critical and that is the gloves already had definite identities before you looked.

Unfortunately, quantum particles don’t behave that way.

The twist is that reality isn’t really set until you look. In quantum mechanics, particles can exist in a superposition which is a blend of possibilities. Before measurement, an electron doesn’t have a definite “spin” like up or down, it simply exists in a cloud of probabilities. Are you still with me? I’m going to carry on regardless even if you’re not as you’ll get it at the end, I promise. If I can understand it, absolutely anyone can.

So, when two particles are entangled, it’s not that they secretly agreed in advance that is what they were going to do, it’s more like they didn’t fully decide until one of them was observed and then they both snapped into agreement instantly!

Einstein didn’t like this at all and that’s the part he just couldn’t accept, and was quite vocal about it for many years. He believed in two key ideas: locality, where nothing can influence something else faster than light, and realism: where physical properties exist whether we observe them or not.

Einstein’s big problem with entanglement was that it seems to violate both. So, in 1935, Einstein and colleagues proposed the EPR paradox (I’ll attempt to actually explain this a bit later).

“Quantum mechanics must be incomplete. Something deeper must be going on.”

He suspected hidden variables, unknown factors quietly determining outcomes behind the scenes.

This debate simmered on and on for decades, it sounded philosophical and almost untestable until in 1964, physicist John Bell made a breakthrough.

He showed that if Einstein was right, that is, if hidden variables governed everything, then there should be strict limits on how correlated entangled particles can be. Right?

These limits are called Bell inequalities.

Basically, (without attempting any heavy maths), Bell’s logic boiled down to this:

If particles carry pre-existing answers, and no influence travels faster than light, then their measurement correlations must stay within certain bounds.

Right? Get it? No? Me neither at first and it’ll take yet another blog post to explain it fully, so just go with it for now and I’ll have to tell you all about Bell Inequalities in a different post.

Anyway, back to where we were: quantum mechanics predicts something different. It predicts stronger correlations than classical physics allows.

Physicists tested Bell’s predictions with real experiments, over and over, by taking entangled particles, separating them, and measuring them in different ways.

The result showed that quantum mechanics wins. Every time. Without fail. The observed correlations broke Bell’s Inequality, they couldn’t be explained with hidden variables, and they couldn’t be explained by local interactions either. Furthermore, recent experiments have ruled out all the usual loopholes.

This means we are forced to accept something different and more radical; the universe isn’t both local and real in the classical sense, which means something about our everyday intuition is wrong.

Hang on just a minute, didn’t you say in your last article that you couldn’t have faster than light messaging? If one particle instantly affects another, why can’t we use this to send messages faster than light?

And the answer to that question is quite a simple one, you can’t control the outcome of your measurement. When you measure a particle, you can’t choose “spin up” versus “spin down”; you can only observe whatever happens. Which means you get a random result. While the correlation is instant, there’s no way to encode and transmit information. Nature gives you Einstein’s spooky connection but keeps communications firmly limited by relativity.

And there we have it, the end of Part One, a short one this week as all the extra fun stuff will be packed in to Part Two, which judging by my draft, will be a tad longer. Thanks again for reading, and stay tuned folks, if I pull my finger out and crack on I’ll have Part Two ready for this time next week.