Monthly Archives: July 2026

The Black Hole Glow: A Journey into Hawking Radiation

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Howdy my loyal readers. I’d like to start my saying thank you for the messages I’ve received asking if there is anything else they should know about black holes, and as they are a common theme through most of my articles, I’m going to indulge you with this one, on how Stephen Hawking discovered that the darkest objects in the universe may slowly evaporate away

By any reasonable definition, black holes should be simple. They pull things in. Nothing gets out. End of story. Yet in 1974, Stephen Hawking discovered something astonishing, and that was, black holes are not completely black. Given enough time, they glow, lose mass, shrink, and may eventually disappear altogether. That prediction is now known as Hawking radiation and remains one of the most profound (you’ve heard my ear worm already) insights in the history of physics.

This was a problem nobody expected, although I’m pretty sure Stephen Hawking had an inkling. You see, Black holes emerge naturally from Einstein’s theory of General Relativity. According to the classical picture, they are regions of space where gravity becomes so powerful that nothing, not even light, can escape once it crosses the event horizon. I know, I know, this is old news to you guys, I’ve stated it several times in previous articles.

For decades, physicists assumed black holes could only grow. Matter falls in. Energy falls in. Nothing comes out. Yeah? Then along came Stephen Hawking, and he combined Einstein’s gravity with quantum mechanics and arrived at an answer nobody could have anticipated. Black holes should emit a faint thermal glow. And this implication was extraordinary. Simply put, it meant that Black Holes could die.

As humoungously massive as it is, the Universe is never truly empty, and to understand Hawking radiation, we need to start with a strange feature of quantum mechanics, and that is what appears to be empty space isn’t actually empty.

According to quantum field theory, the vacuum of space is filled with fluctuating quantum fields. That is, teeny tiny disturbances constantly occur throughout space, causing particle-like excitations to briefly appear and disappear. Something that physicists call vacuum fluctuations.

OK. This bizarre activity actually happens everywhere in the universe, and this includes some of the most extreme environments surrounding a black hole.

And this takes us to the famous story that everyone learns. And that is, the most common explanation of Hawking radiation goes something like this:

Near the event horizon, a particle and antiparticle appear as a pair. Now then, normally they would immediately annihilate each other, but if the pair forms close enough to the event horizon, gravity can separate them before they reunite. Which means, one particle escapes into space, while the other one falls back into the black hole. To a distant observer, the escaping particle looks like radiation emitted by the black hole itself.

And this is because, the particle that falls inward, effectively carries negative energy relative to faraway observers, thus reducing the black hole’s mass, so over immense stretches of time the black hole slowly evaporates.

It is an absolutely, beautiful explanation.

But, and there’s always a but, you know this by know, surely guys, there’s one problem.

This isn’t really how Hawking derived the result.

To look at the deeper truth, among physicists, this particle-pair story is considered a useful mental picture, but is not the actual mathematics (maths again, I know. Regular readers already know my issue with maths). Anyway, Hawking’s original calculation used quantum field theory in curved spacetime. And that is, the essential idea is subtle. Different observers can disagree about what counts as empty space. A vacuum for one observer may appear to contain particles to another observer. But, near a black hole’s event horizon, spacetime is distorted so severely that these differing perspectives become physically important

When Hawking compared the state of quantum fields before and after a black hole forms, he discovered that an observer far away would detect radiation emerging from the black hole. So, in other words, Hawking radiation is less about particles popping out of nowhere, and more about the deep relationship between quantum fields, spacetime, and observation itself.

One of Hawking’s greatest discoveries was that black holes possess temperature, and the temperature of a black hole is inversely proportional to its mass, so the heavier the black hole, the colder it becomes, and this leads to a surprising consequence.

A supermassive black hole containing millions or billions of solar masses is incredibly cold, and a tiny black hole is extraordinarily hot, and this relationship means, small black holes radiate far more efficiently than large ones.

Because Hawking radiation carries energy away, the black hole loses mass, and this results in the slow death of a black hole.

As it shrinks, something curious happens. The black hole gets smaller as its temperature rises, then radiation increases which causes the mass to decrease even faster, and therefore the process accelerates.

If the theory is correct, then the final phase could end up with an enormous burst of energy, however, what exactly happens during those final moments remains unknown.

If you got the time to watch one, don’t expect to watch one evaporate as the evaporation process is incredibly slow. A black hole with roughly the mass of our Sun would survive for approximately 10⁶⁷ years, and this is one hell of a lot of zeros, and an incredibly long time. Trillions upon trillions upon trillions of times longer than the current age of the universe. Told you it was a long time.

And what’s more, supermassive black holes live even longer! Fortunately for the universe, black hole death is not an urgent problem, and nothing to worry about. Not for a verrrrrry longtime, at least!

Before Hawking’s discovery, physicist Jacob Bekenstein, argued that black holes should possess entropy, a measure of information content (remember my last article? You did read it, didn’t you?), and Hawking’s calculations confirmed this. In fact, black holes possess staggering amounts of entropy. Even more surprisingly, that entropy scales with the surface area of the event horizon rather than the black hole’s volume!

This observation helped inspire one of the deepest concepts in modern theoretical physics, and that is the Holographic Principle (you know this, I mentioned it way back in my original Black Hole article). This principle suggests that information contained within a region of space may be fully encoded on its boundary surface. It may sound like science fiction, but it has become one of the central ideas in attempts to understand quantum gravity

There’s a problem though, and that is the Information Paradox, and that is the question that changed theoretical physics forever.

Let us suppose a black hole swallows, my laptop I’m writing this article on, books, planets, stars, people, and entire civilizations and the specific details seem to vanish behind the event horizon.

If the black hole later evaporates and only thermal Hawking radiation remains, where did the information go? And that, is what has become known as the Black Hole Information Paradox.

The paradox appears to pit two fundamental pillars of physics against one another. Again, you know this, I’ve mentioned it before. It pits General Relativity against Quantum Mechanics. And this is because quantum theory normally forbids information destruction. What’s surprising is, Hawking’s original result seemed to imply exactly that.  And that is why many physicists consider this one of the deepest unresolved problems in science.

Modern research increasingly views Hawking radiation as an entanglement phenomenon. The radiation escaping from a black hole appears linked to degrees of freedom hidden behind the event horizon, and these quantum correlations may play a crucial role in preserving information.

This current line of thinking has driven decades of research involving Quantum information theory, string theory, holography (my old, now dead, dad was a big fan of holography), and quantum gravity, as well as many other areas.

Moving on, there is another remarkable discovery that Hawking radiation appears closely related to the Unruh Effect.

The what, I hear you say? Another topic for later? Sorry, yes (picture me doing an emoji grimace in apology as I type this), the Unruh Effect predicts that an accelerating observer can perceive empty space as containing thermal radiation. And that is what one observer sees as a vacuum, another observer sees as heat. Many physicists suspect both effects arise from the same underlying physics linking horizons, acceleration, and quantum fields.

Which brings us to the puzzles of tine scales. And that is, there is another challenge lurking inside Hawking’s theory.

When researchers trace Hawking radiation backward in time, some calculations appear to involve wavelengths smaller than the Planck scale (you know what that is), where known physics may stop working completely, and this issue is called the Trans-Planckian Problem. Which suggests we may need a deeper theory of nature to fully understand what is happening near the event horizon of a black hole.

Ah, who was that at the back, another question do I hear? Have we ever seen Hawking Radiation?

Surprisingly, the answer is…

No. We haven’t.

Despite being one of the most famous predictions in science, Hawking radiation has never been directly observed from a real astronomical black hole, and that is because the effect is simply too faint. Remember how long it would take to observe it? That is how faint it is. Even paint dries at the speed of light in comparison. (It doesn’t, bit that sounded cool, didn’t it!).

However, researchers have built laboratory systems that mimic the behaviour of event horizons using optical fibre, fluid systems, and Bose-Einstein condensates, and these analogue systems have produced Hawking-like radiation and, yes there’s another and, in recent experiments, even evidence of the associated back-reaction effects predicted by theory! Cool huh?

While these are not actual black holes, they do provide valuable support for the underlying physics, which leaves us with the final mystery.

And that is perhaps that the most important thing about Hawking radiation is not the radiation itself. There is a deeper lesson, that black holes sit at the intersection of humanity’s three greatest theories, General Relativity. Quantum Field Theory, and Thermodynamics. Unfortunately, those theories do not fit together perfectly, but Hawking radiation exposes the cracks.

Hawking radiation tells us that spacetime has temperature, that gravity may be linked to information, and that entropy and geometry are somehow connected. Oh, and that our deepest theories are still incomplete.

More than fifty years after Hawking’s discovery, physicists are still trying to understand what the phenomenon is really telling us.

The black hole’s faint glow may ultimately reveal not just how black holes die, but how reality itself is built! I’ve said this before at the end of my articles, and I’m going to say it again. How amazingly awesome is that!!

Quantum Teleportation: Not Star Trek, But Still Pretty Wild

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Hello my lovely readers, you may be few, but I love you all the same.

Having had a read through my previous articles since I started this whole bringing physics to the masses journey, I noticed there’s a nice progression developing: Gravity > Relativity > Entanglement > Bell’s Proof, a quick side step into the footy  (let’s not talk about that), which leads naturally onto this week’s article:

Quantum teleportation.

Now then, there’s a misconception about teleportation, and I’ve touched on it with my Star Trek allegories in previous articles.

When most of us hear the word teleportation, we probably all think of Captain James T. Kirk demanding that Scotty gets the malfunctioning transporter fixed pronto and beams him up (or Benjamin Sisko making the same demand of Chief O’Brien, or Captain Janeway…. Sorry, got a bit carried away, on with the article). Quantum teleportation is both less dramatic and far more profound (every time I type the word profound, I can hear Prof Brian Cox saying it in my head).

Unfortunately, scientists have not discovered a way to transport humans, objects, genesis devices, or even particles from one place to another. What they have discovered is a way to transport the quantum state of a particle, that is a complete description of its quantum properties, from one particle to another, potentially over vast distances.

In other words, quantum teleportation does not move matter, it moves information.

And that distinction may reveal something fundamental about the nature of reality.

So, if it’s not Star Trek personnel being teleported, what is?

To understand quantum teleportation, we first need to understand a quantum state.

Again, as I mentioned previously, unlike a classical bit, which can be either 0 or 1, a quantum bit, or qubit to give it its correct term, can exist in a superposition of all states simultaneously: it can be zero, one, or both, at the same time. The state of a qubit contains all the information that can be known about it.

The remarkable achievement of quantum teleportation is that the exact state of one qubit can be recreated in another distant qubit without ever measuring and copying the original state. The original state disappears, and the new state appears elsewhere. There is nothing physical travelling between the two except for a teeny tiny amount of ordinary information.

Quantum teleportation relies on three extraordinary ingredients.

The first ingredient is quantum entanglement.

When two particles become entangled, they cease behaving as independent objects. Instead, they become part of a larger quantum system whose properties are linked regardless of the distance separating them. You know what’s coming, don’t you? Yep, you’ve got it, this is what our old mate Albert Einstein famously referred to as “Spooky action at a distance.”

You’ve read my previous article on Bell’s Inequalities, so you know this already, but I’m going to repeat myself again anyway.

Today, entanglement is one of the most experimentally verified phenomena in physics.

On to ingredient number two. The sender (traditionally called Alice, but who I have renamed Betty in honour of my dear old late Mum), performs what’s known as a Bell-state measurement on the particle she wants to teleport and her half of an entangled pair. This measurement destroys the original quantum state, this destruction is not a bug though, it’s a requirement.

Ingredient three is the classical communication.

Betty then sends Bob, the receiver, two ordinary classical bits describing the measurement result.

It is only after Bob receives those bits that he can perform the operation needed to reconstruct the original state on his entangled particle.

At that moment, the teleportation is complete.

Sound familiar? If you read my “Spooky Action at a Distance” series, it should. I’m about to reuse that same trick about entanglement and the speed limit on information. If you haven’t read it yet… well, what are you waiting for?

Now we have one of the biggest misconceptions concerning quantum teleportation. Why doesn’t it break the speed of light?

Because entanglement creates correlations that appear instantaneous, but usable information still cannot travel faster than light. Bob cannot reconstruct the teleported state until Betty’s classical message arrives. This keeps Einstein’s theory of relativity perfectly safe and intact. No matter how strange quantum teleportation appears, and I know I’m repeating myself here, it does not allow faster-than-light communication.

Let’s look at the rule that makes this all possible, which is a fundamental principle of quantum mechanics called the No-Cloning Theorem. This rule states that an unknown quantum state cannot be copied perfectly, essentially meaning teleportation doesn’t produce two versions of the same particle state, the original state is destroyed, and the distant state is created. Quantum teleportation is a transfer, not a duplication.

The theory of quantum teleportation was first proposed in 1993 by Charles Bennett, Gilles Brassard, Claude Crépeau, Richard Jozsa, Asher Peres, and William Wootters. Their groundbreaking paper showed how an unknown quantum state could be transferred using a combination of entanglement and classical communication. If you really want to have a deep dive into it, let me know and I’ll send you the paper. It took only four years for researchers to successfully demonstrate the effect experimentally using photons in 1997. I remember being really, REALLY, excited at the time, I still get goosebumps looking back to when I first heard the news.

What had once sounded like science fiction had become a laboratory reality.

To me, and I’m sure to many others, this has become one of the greatest ironies of modern physics. Poor Einstein, the phenomenon he disliked most of all, quantum entanglement, became the foundation of quantum teleportation. However, the universe turned out to be even stranger than Einstein imagined.

That’s where Bell came in with his theorem I tried to explain (and hopefully succeeded) in my previous article before the footy one.

For decades, some scientists hoped that hidden variables might explain away Einstein’s quantum weirdness. John Bell showed that certain predictions of quantum mechanics could be experimentally tested and the results repeatedly favoured quantum mechanics and ruled out large classes of local hidden-variable theories.

If it wasn’t for Bell, quantum teleportation might still be considered an interesting mathematical idea rather than a physical reality.

Ah, another question from you? How far have we been able to teleport?

Well, it’s actually a great deal further than you probably think. Quantum states have been teleported using photons, atoms, electrons, superconducting circuits, and solid-state quantum systems.

Researchers have demonstrated space-based teleportation using China’s Micius satellite over distances exceeding 1,000 kilometres. And more recent experiments have teleported quantum information between different physical systems and over telecom-compatible networks designed to support future quantum communications. Just this year, researchers reported teleporting a photon’s state between physically separate quantum dots connected across a 270-metre free-space link, another step toward practical quantum networking.

Looking to the future of this amazing technology, scientists believe quantum teleportation will one day become one of the foundational technologies of a future quantum internet.

Instead of simply sending data, future quantum networks may distribute entanglement between thousands or millions of devices. Quantum teleportation could be used to transfer qubits between quantum computers. It could also be used to construct ultra-secure communication links, enable distributed quantum computing, and build true global-scale quantum networks.

Can you imagine quantum processors in London, New York, and Tokyo acting like pieces of a single giant computer? Well folks, that’s the long-term vision.

Ok, ok, I know you’re desperate to ask the question everyone asks. Could we teleport a person?

Well, in principle, quantum mechanics doesn’t obviously forbid it, but in practice, it is almost unimaginably difficult.

A human body contains roughly 10²⁸ atoms, each participating in an immensely complex web of quantum interactions. Capturing every detail required to reconstruct a person would involve a truly humongously astronomical quantity of information. Oh, there’s also another problem.

Don’t forget the no-cloning theorem in the rules above. The original transported person would have to be destroyed during the process, which raises a somewhat disturbing philosophical question. If a perfect copy appears elsewhere while the original is destroyed, did the person actually travel? Or were they killed while an identical replacement appeared at the other end.

That is a question that science currently has no answer for.

And here we can link back to black holes and wormholes. Deep developments in modern theoretical physics suggest that quantum teleportation may connect to mysteries far beyond communication technology.

Researchers studying black holes discovered unexpected relationships between entanglement, information, spacetime, and wormholes.

There’s the famous proposal, known as ER = EPR, yep, that one from another of my previous articles, which suggests that entanglement and wormholes may be two descriptions of the same underlying phenomenon. Although this is still speculative, these ideas hint that teleportation may reveal something truly profound (there goes Prof B. Cox again. I didn’t know it was possible to have one person saying a single word as an earworm) about the structure of the universe itself.

And then we get to the biggest mystery of them all. Perhaps the most extraordinary aspect of quantum teleportation is that nobody fully understands why reality allows it.

The mathematics works flawlessly and the experiments work repeatedly. And the technology is rapidly advancing.

Yet the deeper philosophical question remains.

Why should information be able to move through the universe in this way?

Quantum teleportation sits at the crossroads of quantum mechanics, information theory, computing, cosmology, and philosophy. Beginning as a clever theoretical idea in 1993, today it is helping scientists build the foundations of the quantum internet.

Who knows, tomorrow, it may help explain nothing less than the nature of space, time, and reality itself! Wouldn’t that truly be profound!

The Magnus Effect: The Beautiful Games’s Most Beautiful Cheat Code

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I was in the pub Friday evening, having a sneaky pint after walking the pooch. As is inevitable at the moment, talk was on the World Cup. We were discussing England’s chances (I believe!), the Balogun/Infantino/Trump controversy, amongst other football related things. One of the guys, I shall call him Dave (because that is his name) said “what about that goal from Pavard for France in the 2018 final against Argentina.” We all nodded agreeing it is probably one of the finest goals ever scored in World Cup history (it really is). Before someone else (who probably doesn’t like the French, don’t worry France, we don’t like him!), piped up and said, “Pah, it was just a wonder goal, no skill in that, just luck. You never see defenders scoring goals like that. Even Beckham as good as he was in his day, wouldn’t have scored that.”

Now, this made me cross, for a number of reasons. Firstly, I don’t like the guy who said it very much anyway. He doesn’t know much about footy, just repeats other people’s opinions. And secondly, he was wrong. On many counts.

Number one:
Of course he intended to score, he took the shot for goodness’ sake!!
Number Two:
There was a lot of skill involved in that shot. As a professional footballer, he knew instinctively what he was doing     
Number Three:
Beckham so could have made that shot, even on whim, have you ever seen him hit a ball to switch play across field?
Number Four:
Yes, there was bit of luck in it, he hit it perfectly. That was the lucky bit.

The wonder bit came when physics took over.

Me being me, I then went on to try and explain how and why to all who would listen. That’s when everyone in the pub’s eyes started to glass over. So, I said, “I’ll tell you what, if you promise to read my blog, I’ll explain it in an article.”

And that is how I have found myself breaking one of my own rules and writing over the weekend.

Now then, after that massive intro are you ready?

Picture this:

A free kick is awarded 25 yards from goal. The defenders build a wall. The goalkeeper shuffles nervously across his line, pointing and shouting, the striker takes a few steps back, raises his hand (why do they do this?), runs up, and hits the ball.

For a split second the fans groan as it looks like the shot is heading straight into Row Z.

Then the ball changes its mind.

It bends around the wall, curls towards the top corner, and sends the goalkeeper flying through the air in pursuit of a ball that had already made its appointment with the net the second it was kicked. And the fans go wild!!!

Now then, this piece of football wizardry is the called the Magnus Effect after the dude who investigated this amazing phenomenon back in the 19th Century.

Most football fans often imagine a curling free kick as a battle between the player and the goalkeeper. Others are aware of the term Magnus Effect. And then you get people like me, who know far too much seemingly useless information that nobody wants to hear about in the pub when they’re discussing the footy.

It’s not a battle between the player and the keeper but against the air.

When a football is struck cleanly through the middle, it tends to travel fairly straight. But when it’s hit off-centre, it starts spinning, and as it spins, it drags air around with it, creating a pressure difference on either side of the ball. This results in one side of the ball ending up with lower pressure, and the other with higher pressure, which makes the ball get pushed sideways.

The result of which is you have a football that appears to have developed independent thought.

Players discovered this long before scientists explained that the spin is the power. If you add enough sidespin the ball curves left or right through the air.

This is why free-kick specialists are football’s equivalent of stage magicians. They know how to bend shots around walls, curl crosses into dangerous areas, shape passes around defenders, switch play so effectively as Beckham did and make highly skilled goalkeepers maybe question their career choices.

OK. There are three elements to this spin. There is sidespin, which is the classic freekick curve, hit the ball across its side and it swerves left or right through the air. It’s responsible for some of football’s most gorgeous goals. Think: David Beckham, Lionel Messi and pretty much every YouTube free-kick compilation that’s ever been made.

You have topspin, which makes the ball dive. The shot rises, clears the wall, and then suddenly plunges towards goal like it’s on its way to the World Cup and it remembers it’s left the oven on back home. This is the physics behind those infuriating shots that seem to drop out of the sky at the last possible moment.

And you have Backspin. It creates a lifting effect that helps the ball stay airborne longer, chip passes, floated crosses and delicate lobs often make use of it.

That was how Benjamin Pavard scored that amazing goal against Argentina back in 2018 when France beat them.

I don’t want to talk about that one though, I want to talk about another one, the one that Roberto Carlos hit against the French in 1997. This is the goal I asked you to imagine earlier, and no discussion of the Magnus Effect is complete without it.

In 1997, Roberto Carlos hit a free kick against France, in the Tournoi de France. A competition played in France with four international teams as a warm-up for the World Cup.

Now then, initially that free kick looked like it was going to go embarrassingly wide. It didn’t though, it turned out to be a 40 yard screamer into the back of the net, and is rightly considered one of the most famous and spectacular examples of the Magnus Effect in football.

For generations of football fans, this was the moment physics got a highlight reel, as it was all over the news with physicists explaining it. Just like I am now.

And this is why some players look like they have superpowers, as it gets stronger when players combine more spin, as this generally means more curve, more speed, as a faster moving ball experiences stronger aerodynamic forces, and technique.

And it is the technique that is the real secret here, and why elite players at the very top of their game don’t just hit the ball, they control exactly where, how, and what angle to strike it.

That’s why millions of Sunday-league footballers can understand the Magnus Effect, while the likes of Messi, can invoke it!

And there’s more! The Magnus Effect has a weird cousin, and that is the knuckleball which is where the fun really gets started.

Sometimes players try to do the opposite and hit the ball with almost no spin. So, instead of curving smoothly, the airflow becomes unstable and the ball wobbles unpredictably. The knuckleball is the famous free kick associated with players like Cristiano Ronaldo.

Where a Magnus free kick is a graceful ballerina, knuckleball is a shopping trolley with a faulty wheel. And that for me wins the ‘who’s better, Messi or Ronaldo’ debate.

The real magic involved with the Magnus Effect is that once you understand it, football somehow becomes even more impressive. Every curling free kick is a player manipulating air pressure, rotational velocity, aerodynamics, and fluid dynamics.

Where the crowd sees the magical, wizardly wondergoal, the physicist (and physics nerds like me) see pressure differentials, and the footballer sees a top corner that needs decorating with his mastery of the beautiful game.

And the ball? That’s the best bit, folks. The ball just follows the science.

The Magnus Effect is what turns football from a game of kicking a ball up and down the pitch into a game of mind-bending reality, just enough to make 60,000 people lose their minds!

Bell’s Inequalities: Spooky Action’s Reckoning

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Way, way back in 1935, Albert Einstein looked at the emerging theory of quantum mechanics and although he said a lot of things about it at the time, and a lot of things after that too, what it really boils down to is this. As I’ve mentioned in previous articles, Einstein had a teeny-tiny (did you see what I did there?) problem with quantum mechanics. To summarise, what he was basically declaring was…

“This cannot be the whole story.”

By the mid 1930’s quantum mechanics had already become hugely and enormously successful (LOL I did it again, didn’t I!). It correctly described atoms, light, and the microscopic world. Einstein, however, being Einstein, believed wholeheartedly that it contained a profound flaw. He couldn’t accept that physical properties might not exist until they had been measured.

As far as Einstein was concerned, the Moon was still there even when nobody looked at it. Which to him meant, a particle should possess a definite state before anyone measures it. If quantum mechanics suggested otherwise, then perhaps some deeper set of hidden facts and hidden variables remained awaiting discovery.

This argument seemed destined to remain philosophical for nearly thirty years, before a quiet physicist from Northern Ireland by the name of John Bell came along.

What Bell did was ask an astonishingly simple question. He didn’t ask whether Einstein or Bohr (brilliant Danish dude, one of many things he is most famous for is developing the Bohr model of the atom) were right. He asked a much more powerful question:

If Einstein’s intuition is correct, what measurable consequences would follow?

At the time this was revolutionary. In an instant, Bell had transformed a debate about the nature of reality into a scientific hypothesis that could be tested in a laboratory.

The result of which he published in 1964, which became universally known as Bell’s Theorem.

I hear one of those questions nagging at you again. You want to know what Bell’s Theorem is, don’t you?

Well, at its heart lies a family of mathematical relationships called Bell’s Inequalities.

Let’s assume for a moment, that the Universe trots along, as it does, according to good old-fashioned common-sense.

Let’s catch up with Bob and Betty again. Imagine that two particles, born together, have then been hurled in opposite directions.

One travels to Betty and the other one travels to Bob, so now two experiments are so far apart that even a light signal cannot travel between them quickly enough to coordinate their measurements.

Now you have to ask yourself, if that’s the case, how could the particles remain correlated?

And that is when the common-sense answer comes in and seems the most obvious. They must have left the source carrying matching instructions.

Like two naughty boys, who have already agreed on their response before getting told off severely for doing something stupid (middle school, my best mate Duncan and the green houses in the garden behind the Spar next door spring to mind, but that is 100% a different story and not for now). What if the particles already know how to respond to every possible measurement?

This view, however, rests on two assumptions:

Number one: Realism. That is the properties of the particles exist before measurement, and Number two: Locality. Nothing can influence something far away faster than light.

Put them together and these two ideas form what physicists call local realism (original, huh?).

Now then, to most people, like you and old me before I learnt all this stuff, local realism sounds less like a theory and more like plain old-fashioned common-sense.

And that was Bell’s perfectly set trap!

He wondered whether pre-arranged instructions could explain every possible pattern observed in entangled particles, and he imagined particles carrying complete answer sheets.

Here’s the clever bit though: Betty and Bob don’t just get asked one question each, they can each be asked one of two different questions. So each answer sheet needs a pre-written response ready for both, not just whichever one actually gets asked.

So, if Betty chooses measurement A, the particle will give a predetermined result.

And if Bob chooses measurement B, his particle will also give a predetermined result.

What’s so unreasonable about that? Sounds very reasonable to me. I hear you say.

The thing is, Bell had discovered something quite remarkable.

He discovered that no matter how cleverly those answer sheets are designed, they can only produce correlations up to a certain mathematical limit. And that limit, folks, is a Bell Inequality!

The most famous version of which is the CHSH inequality, which predicts that a quantity called S cannot exceed 2 if local realism is true.

Nature, however, had other plans. That’s right, you’ve hit the nail firmly on the head. It’s those pesky quantum mechanics breaking the rules again, isn’t it!

And you’re right! Quantum theory predicts that entangled particles can produce correlations stronger than Bell’s limit. Not infinitely stronger. Just strong enough. Instead of stopping at 2, quantum mechanics allows values as high as:

or approximately 2.828. This maximum quantum value is called the Tsirelson bound (you’ll have to take my word on this as it’s way to complicated for me to explain simply), and to Bell, this meant something extraordinary. (If you really wanna know about Tsirelson’s bound, have a look at Wikipedia here).

If experiments ever exceeded the classical limit of 2, then no theory based on local hidden variables could fully describe reality. That was when the argument ceased to be philosophical, as reality itself would have to choose a side.

And so the experiments began. In the 1970s, physicists John Clauser and Stuart Freedman conducted the first significant tests. The results of which agreed with quantum mechanics. Although a 1973 Harvard experiment by Francis Pipkin and Richard Holt initially produced opposite results before later experiments, such as Fry and Thompson in 1976, strongly violated Bell’s Inequalities, aligning perfectly with quantum mechanics.

There were still many sceptics who remained unconvinced though.

“Perhaps the experiments contained flaws!” they shouted

“Perhaps the particles somehow communicated!” they screamed

“Perhaps the detectors introduced biases?” the shyer ones whispered to each other.

Then, in the early 1980s, a French physicist called Alain Aspect and his collaborators performed more sophisticated experiments in which detector settings changed while the particles were already in flight. And, you’ve guessed it, once again, Bell inequalities were violated. And this time the evidence was growing harder to dismiss.

For decades, physicists searched for loopholes, but every escape route was closed.

Maybe detector inefficiencies could explain the results? Nope!

Or it could be experimental effects mimicking quantum correlations? Nope!

Ah, could the measurement choices somehow be known in advance? Nope!

Researchers attacked each loophole one by one, and every time the loopholes were firmly closed.

More recently in 2015, multiple groups began reporting “loophole-free” Bell tests. However, these experiments simultaneously addressed the major known objections and still found violations of Bell Inequalities exactly where quantum mechanics predicted them. The verdict was becoming increasingly unavoidable with everything they tried. Simply put, Nature does not obey local realism.

So, what did Bell actually prove?

We often hear popular science accounts say that “Particles communicate faster than light.”

Bell did not prove that. Nor did he prove that information travels instantaneously. Nor did he show that relativity is wrong. What he proved was way, way subtler.

Basically, Bell was saying that the world cannot be explained by a picture in which distant objects merely carry pre-existing local instructions, and that something about our classical understanding of reality must give way.

And so, the great mystery remains.

The awkward truth of the matter is that physicists still disagree about exactly what Bell’s theorem means, with some interpretations abandoning realism, and others abandoning locality.

Other physicists have attempted to rethink the measurement altogether.

But all serious interpretations must confront Bell’s result.  The experimental facts may no longer be in doubt, but the philosophical meaning remains fiercely debated.

And that leads us to why Bell matters. While many scientific discoveries tell us how the universe behaves, Bell’s theorem is fundamentally different. It tells us how the universe cannot behave.

It places a permanent limit on any explanation that can be built from everyday intuitions about objects carrying definite properties which are interacting only through local causes.

For centuries, philosophers wondered whether reality existed independently of observation. Bell found a way to ask the question experimentally.

The answer appears to be that the microscopic world is stranger than even Einstein imagined. And perhaps the most astonishing part of all is that a debate that began around blackboards and thought experiments ended with photons, detectors, laboratory equipment, and reality itself casting the deciding vote.

Hopefully, with my explanation of Bell’s Inequalities, my previous two articles will make a bit more sense now.

It’s mind-bogglingly amazing that with all the mind-bogglingly stuff all the clever physicists and quantum dudes have learned and discovered and now understand, that we have been going around in circles with this one for nearly a century. And that, my friends, is one of the many reasons I love physics!

Spooky Action at a Distance Part Two: Down the Rabbit Hole

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Hello again, lovely readers. If you missed Part One, here’s the short version: quantum entanglement lets two particles share a connection so deep that measuring one instantly tells you something about the other, no matter the distance between them. Einstein hated it, called it “spooky action at a distance,” and spent decades convinced something deeper had to be going on underneath. Turns out he was wrong about the “something’s missing” bit but, as we found out, also sort of right that something deeper was going on, just not in the way he expected. If you want a full recap of the article click here.

This one was a toughie folks, and I’m glad I took the extra week to get it right, and believe me, it gets weirder!

OK. Here we go. Now, where were we? Ah yes…

We left things with entanglement’s strangest party trick: the correlation between two particles is instant, but you still can’t use it to send a message faster than light. Relativity stays safe. Causality stays intact. Crisis averted.

So, if entanglement can’t be used to break physics, what is it actually good for? As it turns out, it’s good for quite a lot as from here on, things get genuinely practical, and then genuinely weird, in roughly that order.

We’re going to look at the real technology entanglement already powers, then follow the rabbit hole all the way down to black holes, wormholes, and the unsettling possibility that the universe itself might be built out of nothing but connections.

So, if you’re ready, buckle up, it’s time for an amazingly quantumly entangled ride…!

For decades, entanglement was just a philosophical curiosity, whereas these days it’s an actual working tool. Thanks to entanglement, quantum cryptography is possible. Entangled particles can be used to generate secure encryption keys, and the process appears almost magical! Any attempt to eavesdrop disturbs the system, so if you attempt to spy, you are immediately detected, clapped in irons, and locked in a dark room for ever (or spend hours and hours and days and days writing a blog piece trying to explain quantum entanglement as punishment. Obviously, I jest. I haven’t been spying. Honest. I wouldn’t know how to).

Something else that entanglement gives us is quantum computing, which again, is something else we are always hearing about, without actually hearing about it. Classical computers use bits, that is zeros and ones. Quantum computers use qubits (no, not Q*Bert, he’s the cute orange dude from the Nintendo game, which was originally an arcade game by someone else. I might be wrong on that one though, answer in the comments if you’re so inclined). Using qubits means each one can be zero, one, or both, at the same time, behaving as a single system with many possible simultaneous states, allowing problems to be solved dramatically faster.

And there’s more! Entanglement also gives us quantum teleportation; we can beam people from orbit to M class planets just like Star Trek!!! Sorry, got a bit carried away there, we sadly can’t do the Star Trek stuff, and it’s very likely we never will. Although it’s not anywhere near as exciting as Star Trek, quantum teleportation is still remarkably awesome as we can transfer the exact quantum state of a particle from one place to another without physically sending it.

Let me try and explain that with an analogy! Bob and Betty both share an entangled pair, Betty measures her particle in a way that reveals its information but destroys its original state in the process and sends the result to Bob as an ordinary classical message. Bob then uses that message to transform his own particle, the other half of the entangled pair, into the exact state Betty’s particle used to hold.

Hopefully that all makes sense, as we’re moving on to an even bigger idea now. Entanglement as the fabric of reality. How mind bendingly cool does that sound?

Let me try and explain that in my rambling way without rambling too much.

Some physicists suspect entanglement isn’t just a feature of the universe, but that it might be the thing that everything else is built on. Let’s take a look at black holes and entropy. You know what entropy is even if you don’t think you do. Rudolf Clausius, the German physicist, said (in 1850…?), when describing the second law of thermodynamics, that the universe’s natural state moves from order to disorder, which is known as entropy. You can see this in action just by watching an ice cube you have taken out of the freezer and left on the kitchen work top, as it naturally melts in a warm room, the puddle of water it leaves behind, however, never spontaneously freezes back into an ice cube. I have a feeling that I’m going to have to add thermodynamics and entropy to the list of future blog articles. At least I’ll be keeping myself busy!

Sorry, went off on a tangent there, back to black holes and entropy (you already know about black holes as you’ve read my Black Holes article, haven’t you?).

Black holes have entropy, but the strange thing is their entropy is proportional to their surface area, not their volume, which suggests something profound. It suggests that the information content of a region of space might live on its boundary and that space may be more like a hologram than a solid volume.

Let me try and explain that better.

When scientists analyse this, they find that the entropy of a black hole behaves like entanglement entropy, a bit like entanglement acting as a sticky information glue. The inside and the outside of a black hole are deeply entangled, which means the structure of spacetime within may emerge from these connections.

A modern theory suggests that entangled particles may be connected to tiny wormholes, not like the Star Trek wormhole near Bajor that you can travel through and start an intergalactic war, think of them more like mathematical bridges linking them together.

Right, here’s the bit I said in Part One I would attempt to explain later, this stuff is all relatively new, but I’m going to have a good stab at it.

To attempt to explain this, we’re going back to Einstein. ER = EPR (yes, another equation, this is new stuff though, real, current research and I love it!).

EPR is the Einstein-Podolsky-Rosen effect and refers to the bit in quantum mechanics where two particles become linked so that measuring one will instantly affect the other, regardless of distance.

ER is the Einstein-Rosen bridge (this is what actually got me interested in all this stuff way back in 1996 just after I bought my first house). The Einstein-Rosen bridge is a type of wormhole, a hypothetical tunnel connecting two different points in spacetime.

In simple terms, this means that when two particles are entangled, it’s as if they are connected by an invisible geometric link, in this case, think of it as a teeny tiny wormhole, not in the sense of travelling as I stated above with my Star Trek analogy, but just as a theoretical way to describe the connection. This is important because it tries to unify quantum mechanics with general relativity and may help to explain what happens in black holes.

The way it was first explained to me (by my father back in said nineties) is one of those things that you often see in sci-fi movies when the science boffin is trying to explain space travel (or something similar) to us regular folk. Imagine two particles as two dots left by a marker pen, on a piece of A4, one at the top of the page, the other at the bottom. Looking at the page, they are far apart, but you can fold the paper in half, so the two dots are touching. This “fold” is essentially the wormhole (the ER bit of the above equation. The Einstein-Rosen bridge).

This was Einstein’s description of quantum entanglement. His “Spooky action at a distance.” As I mentioned in my last article, he thought entanglement was weird and suspicious. These days the teeny tiny physics dudes (quantum physicists, not small people made out of physics, although that is technically not far off what we actually are, albeit all different shapes and sizes) have a different, more boring way of describing it.

They just say, “what if it’s not spooky, what if the particles are literally connected through spacetime?”

Don’t just take my word for it, this isn’t just theory, it has been engineered in labs and I have more equations to demonstrate coming up.

To create entanglement there is this simple recipe:

First off, you start with two qubits:

|0> |0>

Then you apply a Hadamard gate (a fundamental logic gate) which puts one qubit into a superposition. Next you apply a CNOT (Controlled not gate, another fundamental logic gate). This links the second qubit to the first. The result is:

(|00> + |11>) / √2

And that, folks, is an entangled state.

And it gives us real world hardware with different technologies that can achieve this physically. Superconducting qubits are controlled with microwave pulses, trapped ions use shared motion to couple particles and photons can be entangled through nonlinear optics. These processes, however, are precise, delicate and extremely sensitive to noise. Which is partly why we don’t see this in everyday use. With entanglement everywhere, why does the whole world look so normal? The answer to that is decoherence.

When quantum systems interact with their environment, such as air molecules, light, and heat, these interactions spread entanglement everywhere and destroy any clean quantum correlations, resulting in quantum weirdness fading as classical reality re-emerges. Because of entanglement, we are forced to rethink some deeply held assumptions. Here’s a little table to help visualise it:
 

What we expectWhat quantum physics says
Objects are separateSystems can be fundamentally inseparable
Properties exist alreadyThey may only form during measurement
Influences are localCorrelations can be nonlocal

This in turn leaves us with three uncomfortable truths.

  • Reality isn’t local.
  • It isn’t fully predetermined.
  • Information plays a central role in physics.

This forces us to think of the universe in a new way, which isn’t very easy if you are entrenched in the old ways. Instead of thinking of the universe in terms of things, think of it in terms of relationships. What entanglement is telling us is that what matters most isn’t what something is but how it is connected to everything else.

Poor old Einstein worried that entanglement meant that physics had gone off the rails, and it bothered him for years, when instead it revealed something so much deeper: the universe isn’t just a collection of isolated objects moving through space, it is a web of connections where space and separation may emerge from something more fundamental underneath.

Quantum entanglement shows us that reality is built not from independent pieces, but from shared states that link distant parts of the universe into a single, unified whole.

I suppose this could lead us into a discussion on dark matter and dark energy, but along with lots of other stuff I’ve mentioned while writing these articles, that is for another time. First, I need to crack on with some of the stuff I’ve teased already. Bell’s Inequalities next methinks, which will be another biggie!