Could Muonium Reveal a Fifth Fundamental Force?

Welcome to the Quarkside Quantum Physics Podcast, an exploration of the fundamental structure of reality, where quantum laws govern matter, energy, and information. Here, uncertainty is a feature, not a flaw, and understanding begins at the smallest scales.

I want you to picture something, just for a second. Um, Hold an imaginary apple in your hand.

Okay.

Now, you know, just let it go. It falls to the ground.

Right, as it always does.

Exactly. It's like the most mundane, everyday observation in the world. It's something you've expected to happen since you were a toddler dropping food from a high chair.

Yeah, it's basically hardwired into our brains from day one.

But that simple action, the universality of free fall, it actually forms the bedrock of our entire understanding of the cosmos. I mean, it dates all the way back to Galileo rolling spheres down winter ramps and Isaac Newton watching fruit fall in an orchard.

Right. And ultimately, it is the absolute core of Albert Einstein's theory of gravity, the equivalence principle.

Which is an incredibly simple rule, right? Everything falls the exact same way.

It is comforting in its absolute certainty, honestly. Whether you drop a feather or a solid lead hammer, if you remove air resistance, you know, drop them in a vacuum, they hit the floor at the exact same instant.

Right.

They fall together.

Yeah. And we base pretty much all of modern physics and our entire understanding of how the universe is glued together on this one giant assumption.

But this is where it gets incredibly fascinating. What if we're wrong?

Well, that's the question.

Or I guess more specifically, what if that foundational, seemingly unbreakable rule of the universe is only true for like some of the ingredients in the cosmos? What if we've just been testing the wrong stuff this whole time?

It's a profound question, and it takes us way beyond the physics we learned in high school. I mean, when you start poking at the edges of the equivalence principle, you're entering a space where the rules of reality might actually fracture.

Wow.

Yeah. We assume gravity is a universal constant because we've only ever been able to test it on a tiny mundane fraction of what the universe is actually made of.

So today, our mission is to explore a massive groundbreaking experiment currently underway in Switzerland that is putting Einstein-Vries theory to the ultimate test. And they are doing it using exotic, artificially created matter.

It is genuinely a journey to the absolute cutting edge of modern experimental physics.

We are going to explore how scientists are basically freezing time, building what they call atomic cannons, and potentially hunting for a mysterious fifth fundamental force of nature.

Because the reality is we don't actually know if all matter falls at the same rate. Proving it one way or the other requires an absurd level of engineering.

So to figure out if we've been testing the wrong stuff, we first kind of have to understand the weird family tree of the universe's building blocks, right? What exactly is the right stuff we should be testing?

To understand that, we have to look at the standard model of particle physics. Think of it as the ultimate recipe book for the universe.

Okay.

According to this model, matter is divided into three distinct generations.

And the first generation is basically our entire world.

Exactly. The first generation makes up you, me, the chair you're sitting on, the Earth, the sun, all the stars you see. It includes protons, neutrons, and everyday electrons. It is the visible, tangible, stable universe.

But then nature decided to make sequels.

Yeah, that is essentially what happened. There is a second and even a third generation of matter.

Just sitting out there.

Right. And you can think of them as exact copies of the first generation, but massively scaled up in weight. These generations contain heavier, highly unstable particles.

OK, so for this experiment in Switzerland, what are they using?

The star of the show here is a second generation particle called the muon.

OK, I've heard the word, but what exactly is a muon?

So a muon behaves almost identically to an everyday electron. It has a negative electrical chart, it spins the same way, but it has about 200 times more mass.

Wow, so it's just really heavy.

Yeah, it is the electron's much heavier, much weirder sibling. And here's the thing that keeps physicists up at night. We still have absolutely no idea why these extra heavier generations exist in the first place.

But why do we need a heavier electron?

Exactly. Why did nature need three versions of these building blocks? It's a complete mystery.

So the question is, if this heavier sibling, the muon, falls into the influence of gravity, does it fall at the exact same rate as the everyday electron we are used to?

That is the multi-million dollar question. Does the increasing mass of a second generation particle interact with the fabric of space-time exactly like the lighter first generation particles?

I see.

Because if we can test the equivalence principle on a muon, we are testing gravity on a fundamentally different type of matter for the very first time in human history.

That is incredible. But I imagine testing gravity on a tiny subatomic particle is ridiculously frustrating.

Oh, it is an absolute engineering nightmare, primarily because of electrical charge.

Right, because you just said the muon has a negative charge, just like an electron.

And gravity is unimaginably weak compared to electromagnetism.

Like how weak?

We're talking about a force that is 10 to the power of 36 times weaker.

That is a number I can't even process.

It's massive. So if you try to drop a negatively charged particle to see how gravity pulls on it, the tiniest stray electromagnetic field will violently yank that particle around.

Oh, so it ruins the trop?

Completely. Even the static electricity from a scientist's shirt across the room would overpower the delicate pull of Earth's gravity. It would be like trying to listen to a single whisper while standing on the runway next to a screaming jet engine.

You'd never hear the whisper. The jet engine completely drowns it out.

Exactly.

So to measure gravity on this particle, it has to be electrically neutral.

You need a perfectly neutral package. If the total charge is zero, electromagnetism just ignores it, turning off the jet engine and leaving only the whisper of gravity to act on the mass.

And this is where the Swiss team comes in with what I can only describe as a Frankenstein atom.

Yeah.

To get this neutral package, they assemble an exotic artificial atom called muonium.

Yes. It's an incredibly elegant solution, actually.

That's all?

Well... To make muonium, you take a positively charged antimuon and you bind it with a negatively charged everyday electron. The positive and the negative perfectly cancel each other out, giving you a neutral exotic atom that is ready to be dropped.

Hold on. My brain just short-circuited a bit.

Yeah.

You just said we're trying to test a second-generation particle, but now you're saying we're using an antimuon, like antimatter?

Yes, an antimuon is antimatter.

Doesn't throwing antimatter into a gravity test... Completely ruin the experiment. I mean, does antimatter even fall down?

That is actually a very common point of confusion. Antimatter responds to gravity the exact same way regular matter does. It falls down, not up.

Wait, really?

Yes. Gravity only cares about mass and energy, and an antimuon has the exact same mass as a regular muon. The only difference is that antimatter has the opposite electrical charge of its regular matter counterpart.

Oh, I see.

So we have to use the tantamuon because its positive charge is the exact required puzzle piece to perfectly balance the negative charge of a normal electron.

So it creates that neutral package without altering how gravity will interact with the particle.

Exactly. It zeroes out the electrical charge perfectly.

Okay, so it's essentially a clever accounting trick to zero it out. We have our perfect test subject. The Frankenstein atom is ready. But there is a massive ticking clock here that makes this entire experiment sound literally impossible.

The lifespan of a muon.

Right. It lives for just 2.2 microseconds before it decays and vanishes entirely.

Yeah, that is 2.2 millionths of a second.

That is absurd.

It is.

That is the entire window you have to create the atom, stabilize it, drop it, and measure how gravity affected it before it literally ceases to exist.

I genuinely don't understand how you even track something that disappears that fast. Especially because, from what I gather, when scientists previously tried to make muonium, the atoms were produced in what they call a hot state.

Right.

And hot in particle physics basically means chaotic and highly energetic.

So they were moving fast.

Extremely fast. In previous experiments, when scientists successfully created these atoms, the muonium would just fly off in random directions at wildly different, blindingly fast speeds.

Like kicking a beehive. Yeah. You can't precisely measure how a single bee is falling if a billion bees are aggressively swarming in every direction for two millionths of a second.

You absolutely cannot. You need order. You need to be able to track the trajectory with microscopic precision. And that requires a cold state where the atoms are moving slowly and uniformly.

And this is where physics professor Anna Soder, lead author Jesse Zhang, and their teams at ETH Zurich and the Paul Scherer Institute, or PSI, step in.

Yeah, the Paul Scher Institute in Switzerland is arguably the only place in the world equipped for this. They house a massive particle accelerator that generates the world's most intense, continuous muon beams.

How do they even do that?

They achieve it by taking protons and accelerating them to massive speeds before violently smashing them into a carbon target. The resulting debris from that massive collision creates the intense stream of muons needed to even attempt this.

But that's still a violent, chaotic stream, right? Their breakthrough was figuring out how to take that chaos and force it into this cold state where the muonium atoms aren't swarming randomly, but are moving parallel to one another at similar speeds.

Like a perfectly orderly marching band.

Exactly. And the way they achieve this is, I mean, it's one of the most fascinating pieces of experimental physics engineering I've heard of.

It really is. To cool these highly energetic particles down and line them up, They fire the antimuons into a thin layer of superfluid helium that has been cooled to near absolute zero.

Which is what, minus 273 degrees Celsius?

Yes. It is incomprehensibly cold.

But they don't just call it a cold liquid. They call superfluid helium a quantum fluid, which honestly sounds like something out of a superhero movie. What does that actually mean?

Well, in a normal liquid, like water in a glass, the atoms are bouncing around and interacting individually. But when you cool helium down to just a fraction of a degree above absolute zero, it undergoes this bizarre phase change. The individual helium atoms actually lose their distinct identities. They merge into a single collective quantum state.

Like they all give up their individuality and become one giant super atom.

Essentially, yes. And because it exists in this strange singular quantum state, The superfluid becomes incredibly rigid in its purity.

Right.

It tolerates absolutely no impurities or foreign bodies whatsoever.

Which turns out to be the secret weapon for the experiment.

Exactly.

So the accelerator fires an energetic antimion into this absolutely pure quantum fluid. The antimion slows down in the cold. It encounters a free electron, grabs it, and becomes our neutral muonium atom. Right. But the moment it forms, the superfluid helium basically realizes there is an impurity in the pool.

Yes, the scientific term is that the newly formed muonium atom has a positive chemical potential relative to the fluid.

In layman's terms.

In layman's terms, the muonium wants to exist in the lowest possible energy state, but being trapped inside the pure helium forces its energy levels too high, so the quantum fluid rejects it violently.

Jesse Zane calls this an atomic cannon. Because the fluid violently rejects the muonium, it converts that chemical potential into kinetic energy, shooting the atom vertically out of the surface of the liquid.

It's quite a literal description.

It's exactly like trying to hold an inflated beach ball underwater. You push it down, but the water desperately wants to reject that pocket of air, and the second you let go, it shoots straight up to the surface.

That is a brilliant analogy. The helium pool literally ejects the muonium atoms upward in a highly controlled, slow-moving beam.

But wait, I have a logistical question here that is kind of bothering me. Sure. We just said the helium is cool to absolute zero, or nearly there. Doesn't cooling something to absolute zero mean that all atomic motion stops? Like, it freezes.

Usually, yes.

So how does the muonium move through a frozen liquid before its 2.2 microseconds run out? Wouldn't it just get stuck in the ice?

That brings us back to the bizarre nature of superfluidity. It doesn't act like a solid block of ice at all. A quantum fluid has zero viscosity.

Zero viscosity, meaning like zero friction?

Absolutely none.

Wow.

Yeah, so when the muonium atom is moving through this incredibly cold substance, it experiences zero resistance, doesn't drag, it doesn't get stuck. It can move completely unhindered, gliding perfectly through the fluid so it can escape the surface well before its microscopic lifespan expires.

That is mind-blowing. We are literally using a frictionless quantum fluid to shoot artificial exotic atoms out of an atomic cannon.

It is science fiction turned into reality.

Okay, so now that the Swiss team has successfully fired this cold, orderly atomic beam, how do they actually watch the particle fall? I mean, we are still dealing with something that only exists for a fraction of a fraction of a second. You can't just set up a high-speed camera.

No, definitely not. The scale is far too small, and the time is much too short for anything resembling traditional photography. This is where Anna Soder and doctoral student Paul Wegman are implementing cryogenic silicon detectors... and a highly specialized tool called an interferometer.

Why do the silicon detectors need to be cryogenic?

Because they are trying to measure impossibly tiny signals. If the detector is at room temperature, the atoms inside the silicon are vibrating with thermal heat, which creates a lot of electronic noise.

So it drowns out the signal.

Right. By freezing the detector, you quiet down all that background vibration so you can actually capture the delicate signal of the muonium.

Okay, that makes perfect sense. But what about the interferometer? If we aren't watching the atom as a physical object dropping like an apple, what exactly are we measuring?

We are actually measuring the wave properties.

Of the atoms. Wait, you're measuring a physical particle by looking at its wave? I thought we were dropping a physical thing with mass.

Welcome to wave-particle duality.

Oh boy.

Yeah, at the quantum level, everything including you, me, and this muonium atom exists as both a physical particle and a wave of probability.

Okay.

The interferometer takes advantage of this. It takes the beam of muonium atoms and physically splits their path into two separate routes, then recombines them at the end. When the paths recombine, the waves interact with each other, creating an interference pattern.

Sort of like throwing two pebbles into a perfectly still pond at the exact same time. You get those expanding rings of ripples. And where the ripples crash into each other, they create a distinct, highly predictable pattern of peaks and valleys.

That is a perfect visualization. Yes. Now imagine gravity is pulling downward on those atoms as they travel through those two separated paths inside the interferometer.

Right.

That microscopic gravitational pull will subtly alter their route. When the waves recombine, that tiny change in their path will cause a measurable shift in the interference pattern of those ripples.

But to stick with a pond analogy, measuring a gravitational drop over 2.2 microseconds... That sounds like trying to measure if gravity is slightly tilting the pond itself while you're only allowed to look for the duration of a lightning flash.

It is incredibly challenging.

Is that actually realistic? It feels like trying to measure the growth of a blade of grass during a single blink.

Of an eye.

It is phenomenally difficult, and your skepticism is totally warranted. But that extreme sensitivity is exactly why traditional dropping fails. We cannot visually see the particle drop.

Right, because it's just too fast.

Yes. But an interference pattern is extraordinarily sensitive to even the slightest change in the environment. A tiny shift in gravity causes a massive, easily readable shift in the wave pattern.

Oh, I see.

Measuring quantum wave interference is literally the only way to catch gravity in the act on this microscopic timescale.

It is such a clever workaround.

Yeah.

And the timeline for this is moving surprisingly fast, right? The team plans to test the atomic beam method this year, in 2026.

Yes, and if that goes well, the actual gravity experiment will happen in just two to three years.

And it's worth noting that this pristine, cold muonium beam isn't just good for the gravity drop. It will also allow for entirely new laser spectroscopy experiments.

Yes, that's a huge bonus.

Meaning they can probe the atom with lasers to measure its internal structure.

Exactly. Because they now have this highly controlled, slow-moving beam... They can hit it with lasers to precisely measure the mass of the muon and other fundamental physical constants with far greater accuracy than we ever have before.

So even before we test gravity, we are getting a high-definition upgrade to our understanding of the standard model.

It's a win-win for physics.

But let's get back to gravity, because this is where the stakes get astronomically high. All of this incredible multi-million dollar engineering, the particle accelerators, the quantum fluids, the interferometers... It all leads to a binary outcome.

It really does.

Either the muonium falls at the exact same rate as ordinary matter, or it doesn't.

And if it doesn't, I mean, if this exotic Frankenstein atom falls even a fraction of a percent slower or faster than an everyday electron, it would be one of the most significant discoveries in the history of physics. No. Currently, physics accepts four fundamental interactions, gravity, electromagnetism, the strong nuclear force, and the weak nuclear force.

And those four forces govern literally everything that happens in the visible universe.

Yes.

But if gravity acts differently on the second generation of particles, it cannot be explained by our current understanding of those four forces.

So what would it mean?

It would point directly to the existence of a highly theoretical fifth force of nature.

A fifth force. That is wild. But despite this absolutely earth-shattering potential... Anna Soder maintains a remarkably grounded view of the whole thing.

She does very much so.

She says she isn't setting out to break physics. She just wants to be the first to measure if the equivalence principle applies to these heavier generations. She actually said, you know, this alone is quite an inspiring piece of work.

And she's entirely correct to frame it that way.

But I have to question her restraint there. Isn't proving a fifth force the holy grail of modern physics? Why would the team downplay that and just focus on making a measurement?

Because that is the philosophy of rigorous fundamental science. You don't build an experiment wanting a specific answer.

Right. Because of bias.

Exactly. If you design an experiment desperately trying to find a fifth force, you risk introducing bias into your methodology. You might subconsciously tweak the data or the sensors to find what you want to find. You build the experiment to find the truth.

You just want to set up the apparatus and see what nature actually does when you ask it the right question.

Breaking physics is just a spectacular byproduct of doing the math correctly. The goal is an accurate measurement. The implementation is what follows.

That makes sense. So what does this all mean for you listening to this right now? Why should you care if a microscopic particle in a Swiss lab falls at bit differently?

Well, it affects you more than you'd think.

Right. Because your entire modern life relies on the assumption that gravity is a fixed universal constant.

Take the GPS satellites your phone uses to navigate your car.

Okay.

They orbit thousands of miles away from Earth's center of mass, which means they experience slightly weaker gravity than we do on the surface. Because of Einstein's theories of relativity, we know that this difference in gravity actually warps space-time, changing how time flows for those satellites.

The clocks on the satellites literally tick at a fundamentally different rate than the clocks inside our phones.

And the algorithms in your phone and in the satellites have to constantly calculate and correct for that gravitational time dilation.

Or your map app would just be completely wrong.

Exactly. Now, if gravity isn't universal, if different types of matter respond to the fabric of space time differently, then our fundamental formulas for how the universe operates are incomplete.

So our understanding of how the universe is expanding, how black holes form, how galaxies hold together... It all relies on Einstein's equivalence principle being the ultimate law of the land.

It is the load-bearing pillar of modern cosmology.

It is absolutely staggering to think about. We've gone from Galileo dropping lead spheres down wooden ramps, just watching them hit the dirt, to firing atomic cannons out of absolutely pure, absolute zero quantum fluids.

It's quite the leap.

We are orchestrating these unimaginably complex choreographed ballets of exotic particles. just to see if something that lives for two millionths of a second falls the way a guy a hundred years ago predicted it would.

It's a testament to human curiosity. We simply cannot leave a stone unturned, especially when that stone belongs to an entirely different generation of matter that we still don't fully understand.

And before we wrap up, I want to leave you with one final cascading idea to mull over, something we haven't even touched on yet, but that flows naturally from all this.

Oh, there are so many cosmological implications if the equivalence principle breaks.

Right. Think about the hunt for dark matter.

That's a huge one.

For decades, astronomers have been looking out at the galaxies, and they see them spinning way too fast. There isn't enough visible stuff, enough first-generation matter, to create the gravity needed to hold those spinning galaxies together.

Right, so we assumed there must be this invisible dark matter providing extra gravity. We assume the heavy mass is there, creating a gravitational pull, but we just can't interact with it visually.

But? What if we take everything we just talked about and apply it to the cosmos? What if exotic matter simply doesn't play by the same gravitational rules we experience here on Earth?

That changes everything.

Right. If gravity acts differently on the heavier second generation of matter, what about the heaviest, most invisible things in the universe?

If a fifth force exists, or if gravity behaves wildly differently for non-ordinary matter, it could completely rewrite our cosmological models.

Exactly. Maybe the dark matter isn't hiding from us. Maybe the mass isn't missing. Maybe we've just been looking at the cosmos with a localized, first-generation set of rules this entire time.

If the equivalence principle breaks in that Swiss laboratory, the lens through which we view the entire universe shatters with it. We would have to rebuild our understanding of reality from the quantum scale all the way up to the galactic scale.

Yeah. It all comes back to that apple. Next time you drop something, just watch it fall. It seems so simple, so incredibly predictable. but down at the very bottom of reality in the freezing quantum depths of a laboratory. That simple downward motion might just hold the secret to an entirely new universe.

Could Muonium Reveal a Fifth Fundamental Force?

Kapitelklicken springt direkt hin

Shownotes

Transkriptvom Podcast bereitgestellt · Zeitmarken springen hin