Hello everyone! This week I’m going to try and answer one of the questions I’ve received regarding my piece on the Block Universe. And that is, why does observing a quantum system change it. It is a very good question, and it’s an extremely tricky one to answer as it’s one of those aspects of the teeny tiny stuff that took me a while to wrap my head around.

So what is the answer? Why does observing a quantum system change it? And the answer is…

Well, actually, nobody really knows why.

It is, in fact, one of the deepest questions in physics. And nearly a century after quantum mechanics was developed, nobody knows with certainty why measurement produces definite outcomes. The mathematics predicts experimental results with extraordinary accuracy, but what the mathematics means is still debated.

To take a closer look, imagine you’re in a casino where every roulette wheel is somehow landing on every number at once. Not one result. All of them. Red, black, odd, even, 17, 32, 8, 0. Every possibility exists simultaneously in a cloud of uncertainty. You walk over, look at the wheel, and suddenly there’s only one answer staring back at you. Red 17. One outcome. One reality. The mystery at the heart of quantum physics is that nature seems to work a little like this, and nobody is entirely sure why.

For centuries, scientists imagined the Universe as a giant clockwork machine. Objects had definite properties whether anyone looked at them or not. A football occupied a particular position in the air or on the field, a cannonball travelled along a specific trajectory, and a cat was either asleep or awake.

Then quantum mechanics came along, yet again, and politely informed us that, at the smallest scales, reality appears to be playing by very different rules indeed.

According to quantum theory, particles such as electrons are described by something called a wave function. Rather than telling us exactly where a particle is, the wave function describes all the possible places it could be found. Before measurement, an electron can exist in what physicists call a superposition, which is a state containing multiple possibilities at the same time.

Imagine flipping a coin spinning in the air. While it’s rotating, you can’t honestly call it heads or tails. In a loose sense, it represents both possibilities. The quantum world takes that idea far beyond anything we’re used to. Before measurement, particles don’t merely hide their properties. According to the mathematics, the multiple possibilities genuinely matter.

The famous double-slit experiment, which I mentioned in my Five Things… article, reveals this bizarre behaviour in dramatic fashion. Fire electrons through two narrow slits and they create an interference pattern, exactly what you would expect from waves passing through both openings simultaneously. Somehow, each electron behaves as though it explores multiple possible paths at once. Yet the moment scientists place detectors at the slits to discover which route the electron takes, the interference pattern disappears. Suddenly the electrons behave like ordinary particles. It’s as though nature says, “If you’re not asking, I’ll keep my options open. If you are asking though, I’ll commit to an answer.”

This puzzle sits at the centre of what physicists call the Measurement Problem.

The equations of quantum mechanics describe the wave function evolving smoothly and predictably through time. Left undisturbed, the mathematics allows all these possibilities to continue existing in superposition. Yet whenever a measurement is performed, we observe only one result. Not many. Not a mixture. Just one. The electron appears here. The detector clicks. The cat is alive or dead. The equations predict probabilities perfectly, yet they don’t clearly explain why one specific reality emerges from many possibilities.

Popular science often makes this sound even stranger by claiming that consciousness creates reality. As catchy a headline as this is, most modern physicists don’t believe the human mind is the crucial ingredient. In quantum mechanics, “observation” doesn’t mean a conscious person staring at something. It means a physical interaction that extracts information. A detector can do it. A photon can do it. Even the environment can do it. This is the tricky bit, folks, and it’s the one that I wrestled with when first studying this stuff. Once you can get your head past a person looking at it as the observer, it really does make perfect sense, trust me!

That’s because obtaining information isn’t passive. To discover where an electron is, something must interact with it. Light must bounce off it, a detector must absorb it, or some other physical process must occur. At the quantum scale you cannot simply peek without participating. The act of learning something about the system inevitably becomes part of the system’s story. With me? Of course you are.

For decades this left physicists wondering why the weird quantum world of that pesky teeny tiny stuff doesn’t spill over into everyday life. If electrons can occupy multiple states at once, why can’t cats? Why don’t we see cars parked in several places simultaneously?

Which leads to something you will all be very familiar with, even if some of you struggle to understand it, and no discussion of the measurement problem would be complete without the most famous feline in science, Schrödinger’s Cat.

Are you ready? I’m going to attempt an explanation that hopefully makes sense. In 1935, physicist Erwin Schrödinger invented a deliberately ridiculous thought experiment to show how strange quantum mechanics becomes if you apply it to everyday objects. Imagine a cat sealed inside a box with a tiny amount of radioactive material, a detector, and a vial of poison. If the radioactive atom decays, the detector triggers and releases the poison. If the atom doesn’t decay, the cat lives.

According to quantum mechanics, before anyone opens the box, the atom exists in a superposition of both decayed and undecayed states. With me so far? Good. OK, if you follow the mathematics all the way through, the cat should also end up in a superposition of being both alive and dead at the same time. Schrödinger wasn’t suggesting that cats really exist as zombie-like half-alive creatures. It was quite the opposite. He had designed the experiment to highlight the apparent absurdity of extending quantum superpositions into the everyday world. Yet when the box is opened, nobody ever finds a cat that’s simultaneously alive and dead. They find one or the other. The actual question is: at what point did reality make up its mind? Was it when the atom decayed? When the detector clicked? Was it when the box was opened? Or did both outcomes somehow continue to exist? That deceptively simple question lies right at the heart of the measurement problem and continues to fuel debate among physicists nearly a century later.

This brings us to one of the most important advances in modern quantum theory, which is decoherence.

I could write a whole article on this, but in another attempt to keep it simple, this isn’t my analogy, but it is the most beautifully simplistic one I could find. Imagine dropping a single drop of blue food colouring into a glass of clear water. At first, the drop is obvious and distinct. But within moments the water molecules begin colliding with it. The blue colour spreads throughout the glass until the original blob is impossible to identify. The information about where it started hasn’t vanished. It’s just been dispersed among trillions of molecules.

And something remarkably similar appears to happen in the quantum world. A quantum system begins with delicate patterns of possibility that allow superpositions and interference to exist. But the system never remains perfectly isolated. Photons strike it. Air molecules collide with it. Heat radiates through it. The environment is constantly interacting with everything around it, and as this happens, information about the quantum state leaks into the surroundings and becomes distributed among enormous numbers of particles. The special quantum coherence that made the system behave in a wave-like manner effectively gets diluted into the environment.

The result is that quantum weirdness fades astonishingly quickly for large objects. Your coffee cup isn’t in two places at once because it’s interacting with countless particles every second. The environment is continually monitoring it, as it were. Quantum coherence becomes spread out so rapidly that classical reality emerges almost instantly.

For a while, many physicists hoped decoherence would completely solve the Measurement Problem, and although it solved a huge part of it, it didn’t solve all of it.

The food-colouring analogy helps explain why. Imagine the colouring spreading throughout the water. You’ve explained why the original neat drop is no longer visible. But you haven’t explained why you should focus on one particular blue molecule rather than another.

Likewise, decoherence explains why quantum superpositions stop being observable and why the world looks classical. What it doesn’t fully explain is why a specific outcome appears. It tells us why the roulette wheel no longer displays all possibilities simultaneously. It doesn’t completely explain why you experience red 17 instead of black 32.

This is why competing interpretations still exist. The Copenhagen interpretation accepts collapse as a fundamental part of nature. Many-Worlds suggests every outcome occurs and reality branches into multiple universes. Objective collapse theories propose that collapse is a real physical process waiting to be discovered. Bohmian mechanics argues that particles always possess definite properties guided by hidden quantum processes. All reproduce the experimental data, however, none of them have achieved any universal agreement.

And that’s what makes the measurement problem so extraordinary. Quantum mechanics may be the most successful scientific theory ever devised. It powers modern electronics, lasers, GPS corrections, MRI scanners and emerging quantum computers. Yet beneath its incredible success lies an unanswered question that goes right to the heart of reality itself.

When you look closely enough at nature, possibility seems to come before certainty, but how certainty finally emerges from possibility remains one of the deepest mysteries in all of science.

Phew. I made it! I hope that makes sense, as I stated at the beginning, it’s a very tricky question to answer, and there are so many different ways an attempt at answering it can be made. A bit like the subject matter itself, I guess!