100 years ago, a bout of hay fever led to the birth of quantum physics

In 1925, a young German physicist fled to the treeless island of Helgoland in the North Sea to ease a severe bout of hay fever.

With nothing but daily walks and long swims to distract him, 23-year-old Werner Heisenberg had time to grapple with a conundrum.

The macro world — of apples falling from trees — behaved differently to the micro world — of atoms and their subatomic components. 

While the macro world could be explained by Sir Isaac Newton’s laws of motion, nature’s tiniest particles seemed lawless. 

As Heisenberg later wrote in his memoir, all attempts to make sense of their behaviour with “older physics” seemed doomed to fail.

And so he arrived to the bracing sea air of Helgoland — “far from blossoms and meadows” — determined to find a mathematical solution.

A month after this trip to Helgoland, on July 29, Heisenberg submitted a paper considered to be the advent of quantum mechanics.

In the years that followed, the greatest minds in physics wrestled with what it all meant.

As a consequence, they discovered some of the strangest pillars of quantum physics.

1925 was a ‘big year’ in physics

Heisenberg’s musings and subsequent writings were triggered by the concept of “quanta”, which was introduced at the end of the 19th century.

Quanta are discrete packets of energy, and their existence challenged the old view of energy as a continuous phenomenon.

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Heisenberg managed to come up with a mathematical formulation to make sense of this shift in 1925 with what he called his “matrix mechanics”.

It was the first consistent and logical formulation of quantum mechanics, but it was also incredibly dense. 

Meanwhile, Austrian-Irish theoretical physicist Erwin Schrödinger was also spending stretches of 1925 in seclusion, receiving treatment for tuberculosis at a high-altitude sanatorium in Switzerland. 

He was working on his own formulation of quantum mechanics that would later be known as the wave equation.

The wave equation was easier to grasp than Heisenberg’s matrices, and as a result it’s still used today to understand the behaviour of particles. 

“It was a big year,” mathematician and historian Robyn Arianrhod, an affiliate of Monash University, says.

“It was the year quantum mechanics became formalised … and then all sorts of consequences happened when trying to interpret those two different formalisms.”

That’s because it’s not always immediately clear what a written equation means when it’s applied to the physical world.

Schrödinger initially imagined the wave in his wave equation as a physical phenomenon, like a soundwave or an ocean wave.

But Schrödinger’s interpretation of his own equation was wrong. 

“Really what the waves are predicting are probabilities,” Dr Arianrhod says. 

If you picture a very basic drawing of a wave on a piece of paper, the peaks and troughs will indicate where a particle is more or less likely to be found.

But here’s the strange thing — until observed, the particle doesn’t have a precise location. It exists in all of those possible locations at once. 

This is called superposition.

This concept is often explained through the Schrödinger’s cat thought experiment, where the cat is both alive and dead at the same time.

“And that was a really interesting and strange idea,” Dr Arianrhod says. 

So strange it started a decades-long debate between two titans of physics: Albert Einstein and Danish physicist Niels Bohr.

The Einstein-Bohr debates

The world’s greatest physicists met to discuss this new quantum mechanics at the Solvay Conference on Physics in 1927.

Two camps had emerged, and it was on the sidelines of this conference that they battled it out. 

Bohr and his followers accepted we could only ever know statistical likelihoods when it came to the properties of particles. 

But Einstein could not accept this — he did not believe God was “playing dice” with the very building blocks of reality itself. 

So during mealtimes, or while walking between the hotel and the conference venue, the two men debated.

“Every morning Einstein was like a jack-in-the-box, jumping up with fresh new thought experiments, trying to show the limitations of quantum theory,” Dr Arianrhod says.

“And every time, often after sleepless nights, Bohr found a way of answering those objections.”

After the Solvay Conference, it was assumed Bohr had won the debate. After all, the equations of quantum mechanics worked. 

“Although everybody thought Bohr had won, Bohr himself kept puzzling over these ideas,” Dr Arianrhod says. 

For years the men swapped letters and thought experiments, trying to figure out how a particle could be in a superposition of every possible state until observed. 

How could observing a particle alter the particle? Don’t particles have inherent properties, whether they’re observed or not? 

It was this observer effect, and Einstein’s attempts to undermine it, that led us to the strangest phenomenon of all: entanglement. 

Accidental discovery of entanglement

There’s a famous paper in physics known as the Einstein-Podolsky-Rosen (EPR) paradox. 

In it, the authors present a thought experiment to demonstrate a problem with the observer effect. 

“Say you’ve got a red and a green jelly bean and each is in a sealed box,” Dr Arianrhod says.

“If observer one opens their box and finds a green jelly bean, then observer two knows the colour of their jelly bean in the other box will be red.”

Easy enough to understand. However, if these are quantum mechanical “entangled” jelly beans, things get more complicated. 

According to quantum mechanics, neither jelly bean has an inherent colour. They exist in a superposition of both red and green until they’re observed.

“All we can say for sure is that each jelly bean has a 50 per cent chance of being red and each has a 50 per cent chance of being green,” Dr Arianrhod says. 

If observer one looks inside their box and discovers a red jelly bean, observer two’s jelly bean will instantaneously be green.

“And this means that the second jelly bean’s colour is determined by the first observer. It’s not pre-existing,” Dr Arianrhod says. 

The EPR paper concluded: “No reasonable definition of reality could be expected to permit this.”

Bohr responded to the EPR paper, disagreeing with Einstein’s conclusion. And that was that.

“The question was essentially put aside for decades,” theoretical physicist Eric Cavalcanti of Griffith University says.

“Anyone who tried to ask questions about the foundations of quantum mechanics was told to shut up and calculate.

However, 30 years after the EPR paper was published, a physicist from Northern Ireland, John Stewart Bell, decided it warranted a closer look.

The test that proved Einstein wrong

Einstein could not accept what he called “spooky action at a distance”.

He thought there must be “hidden variables” that determine the colour of the jelly bean, not the observer who simply opened a box and looked inside. 

So Bell devised a theorem to test Einstein’s idea.

He found that if you held to Einstein’s view of the world, there would be a limit to how much you could know about an entangled pair of particles at any time. 

For example, you might be able to discover the colours of your jelly beans, but finding out their momentum would be a step too far.

The implication was if you breached this upper limit, you proved Einstein wrong.

It was doing this that snared Alain Aspect, John Clauser and Anton Zeilinger the 2022 Nobel Prize in Physics.

They broke the upper limit, proving that quantum mechanics — in all its weirdness — was sufficient to explain the behaviour of particles. 

Although Professor Aspect’s experiments proved Einstein’s view of the world wrong, he didn’t gloat about it. 

“When people say, ‘Oh, you showed Einstein wrong’, I say, ‘Come on, I showed Einstein was great,'” he said in response to the award. 

After all, if Einstein hadn’t asked all those follow-up questions, it’s unclear where we might be in our understanding of particle physics, and our use of entanglement in quantum technology. 

“Bohr’s instinct was right,” Dr Arianrhod says. 

What comes next?

This year physicists travelled to Helgoland, tracing Heisenberg’s footsteps to mark the 100-year anniversary of his fateful trip.

The United Nations declared 2025 the International Year of Quantum Science and Technology.

And yet the question that Einstein asked way back in the beginning — what does this all mean? — continues to nag theoretical physicists. 

What does it mean for particles to be in a superposition of states, or entangled? What does it mean for observers to alter a particle? 

The maths might work, but it can’t make meaning. 

Part of the problem, Dr Cavalcanti says, is that “we have a lot of answers, but we don’t know which one is right.

“And each paints a completely different picture of reality.” 

One is the Many Worlds theory, which argues the wave of probabilities doesn’t collapse after observation. All probabilities continue to exist, playing out in parallel universes. 

“There’s a branch in which you chose to quit your job and there’s a branch where you chose to keep your job,” Dr Cavalcanti says. 

Then there’s QBism, which puts the observer’s subjective beliefs at the heart of measurement. Your expectations influence the observations you make. 

And then the de Broglie-Bohm theory, which allows faster-than-light interactions between particles, breaking Einstein’s theory of relativity.

There are dozens of interpretations out there, each weirder than the next. Any one of them could be true.

“Will we ever know? To Bohr, it didn’t matter. He didn’t really need to know, but Einstein did,” Dr Arianrhod says.

“There will always be people who want to know. Whether or not nature is going to reveal those secrets is anyone’s guess.” 

Listen to ‘The centenary of quantum mechanics‘ and subscribe to The Science Show podcast for more mind-bending science.