The Electron Is So Round That It’s Ruling Out New Particles | Quanta Magazine (2023)

Imagine an electron as a spherical cloud of negative charge. If that ball were ever so slightly less round, it could help explain fundamental gaps in our understanding of physics, including why the universe contains something rather than nothing.

Given the stakes, a small community of physicists has been doggedly hunting for any asymmetry in the shape of the electron for the past few decades. The experiments are now so sensitive that if an electron were the size of Earth, they could detect a bump on the North Pole the height of a single sugar molecule.

The latest results are in: The electron is rounder than that.

The updated measurement disappoints anyone hoping for signs of new physics. But it still helps theorists to constrain their models for what unknown particles and forces may be missing from the current picture.

“I’m sure it’s hard to be the experimentalist measuring zero all the time, [but] even a null result in this experiment is really valuable and really teaches us something,” said Peter Graham, a theoretical physicist at Stanford University. The new study is “a technological tour de force and also very important for new physics.”

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Poaching Elephants

The Standard Model of Particle Physics is our best roster of all the particles that exist in the universe’s zoo. The theory has held up exceptionally well in experimental tests over the past few decades, but it leaves some serious “elephants in the room,” said Dmitry Budker, a physicist at the University of California, Berkeley.

For one thing, our mere existence is proof that the Standard Model is incomplete, since according to the theory, the Big Bang should have produced equal parts matter and antimatter that would have annihilated each other.

In 1967, the Soviet physicist Andrei Sakharov proposed a possible solution to this particular conundrum. He conjectured that there must be some microscopic process in nature that looks different in reverse; that way, matter could grow to dominate over antimatter. A few years before, physicists had discovered such a scenario in the decay of the kaon particle. But that alone wasn’t enough to explain the asymmetry.

Ever since then, physicists have been on a hunt to find hints of new particles that could further tip the scale. Some do so directly, using the Large Hadron Collider —often touted as the most complicated machine ever built. But over the past several decades, a comparatively low-budget alternative has emerged: looking at how hypothetical particles would alter properties of known particles. “You see footprints [of new physics], but you don’t actually see the thing that made them,” said Michael Ramsey-Musolf, a theoretical physicist at the University of Massachusetts, Amherst.

One such potential footprint could appear in the roundness of the electron. Quantum mechanics dictates that inside the electron’s cloud of negative charge, other particles are constantly flickering in and out of existence. The presence of certain “virtual” particles beyond the Standard Model — the kind that could help explain the primordial supremacy of matter — would make the electron’s cloud look slightly more egg-shaped. One tip would have a bit more positive charge, the other a bit more negative, like the ends of a bar magnet. This charge separation is referred to as the electric dipole moment (EDM).

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The Standard Model predicts a vanishingly tiny EDM for the electron — nearly a million times smaller than what current techniques can probe. So if researchers were to detect an oblong shape using today’s experiments, that would reveal definitive traces of new physics and point toward what the Standard Model might be missing.

To search for the electron’s EDM, scientists look for a change in the particle’s spin, an intrinsic property that defines its orientation. The electron’s spin can be readily rotated by magnetic fields, with its magnetic moment serving as a sort of handle. The goal of these tabletop experiments is to try to rotate the spin using electric fields instead, with the EDM as an electric handle.

“If the electron’s perfectly spherical, it’s got no handles to grab onto to exert a torque,” said Amar Vutha, a physicist at the University of Toronto. But if there’s a sizable EDM, the electric field will use it to tug on the electron’s spin.

In 2011, researchers at Imperial College London showed that they could amplify this handle effect by anchoring the electron to a heavy molecule. Since then, two main teams have been leapfrogging one another every few years with increasingly precise measurements.

One experiment, now at Northwestern University, goes by the name of Advanced Cold Molecule Electron EDM, or ACME (a backronym inspired by the old Road Runner cartoons). Another is based at the University of Colorado’s JILA institute. The competing teams’ measurements have jumped in sensitivity by a factor of 200 in the last decade —still with no EDM to be seen.

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“It is sort of a race, except we have no idea where the finish line is, or whether there is a finish line, even,” said David DeMille, a physicist at the University of Chicago and one of the leaders of the ACME group.

A Race to the Unknown

To keep trekking ahead, researchers want two things: more measurements and a longer measurement time. The two teams take opposite approaches.

The ACME group, which set the previous record in 2018, prioritizes quantity of measurements. They shoot a beam of neutral molecules across the lab, probing tens of millions of them every second, but only for a few milliseconds each. The JILA group measures fewer molecules, but for longer: They trap a few hundred molecules at a time, then measure them for up to three seconds.

The ion-trapping technique, first developed by Eric Cornell, a physicist at the University of Colorado, Boulder who directs the JILA group, was “a big conceptual breakthrough,” DeMille said. “Many people in the field thought this was nuts. Seeing it come to fruition is really exciting.”

Having two distinct experimental setups that can cross-check one another is “absolutely crucial,” Budker said. “I don’t have words to express my admiration of this cleverness and persistence. It’s just the best science there is.”

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Cornell’s technique was first showcased in 2017 with hafnium fluoride molecules. Since then, technical improvements have allowed the group to surpass ACME’s record by a factor of 2.4, as described in a recent preprint led by Cornell’s former graduate student Tanya Roussy. The team declined to comment while their paper is under review at Science.

Probing the electron’s roundness with increased precision equates to looking for new physics at higher energy scales,or looking for signs of heavier particles. This new bound is sensitive to energies above roughly 1013 electron-volts — more than an order of magnitude beyond what the LHC can currently test. A few decades ago, most theorists expected that hints of new particles would be discovered significantly below this scale. Each time the bar rises, some ideas are discredited.

“We have to keep wrestling with what these limits imply,” Ramsey-Musolf said. “Nothing’s killed yet, but it’s turning up the heat.”

Meanwhile, the electron EDM community forges ahead. In future experimental iterations, the dueling groups aim to meet somewhere in the middle: The JILA team plans to make a beam full of ions to increase their count, and the ACME team wants to extend the length of their beam to increase their measurement time. Vutha is even working on “some totally crazy” approaches, like freezing molecules in blocks of ice, in the hope of jumping several orders of magnitude in sensitivity.

The dream is that these EDM experiments will be the first to detect signs of new physics, prompting a wave of follow-up investigations from other precision measurement experiments and larger particle colliders.

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The shape of the electron is “something that teaches us about totally new and different pieces of the fundamental laws of nature,” Graham said. “There’s a huge discovery waiting to happen. I’m optimistic that we’ll get there.”


The Electron Is So Round That It’s Ruling Out New Particles | Quanta Magazine? ›

The Electron Is So Round That It's Ruling Out Potential New Particles. If the electron's charge wasn't perfectly round, it could reveal the existence of hidden particles. A new measurement approaches perfection. If an electron were the size of Earth, the experiment could detect a bump the size of a sugar molecule.

How round is the electron? ›

Electrons are extremely round, and some physicists are not pleased about it. The symmetrical roundness of the electrons suggested that unseen particles aren't big enough to skew electrons into squashed oblong shapes, or ovals.

Why are electrons round? ›

The electron gets its shape from the way that positive and negative charges are distributed inside the particle. The best theory for how particles behave, called the standard model of particle physics, holds that the electron should keep its rotund figure almost perfectly.

Who said that electrons act like particles and waves? ›

French physicist Louis de Broglie proposed (1924) that electrons and other discrete bits of matter, which until then had been conceived only as material particles, also have wave properties such as wavelength and frequency.

What is the shape of the electron? ›

This means that the electron's mass is not literally squeezed into an infinitely small volume. Rather, in certain cases where the electron looks somewhat like a particle, it interacts as if it were completely located at a single point. Therefore, in the sense of particle-like interactions, an electron has no shape.

Is an electron perfectly round? ›

Physicists from the Advanced Cold Molecule Electron Electric Dipole Moment (ACME) Collaboration have examined the shape of an electron's charge with unprecedented precision to confirm that it is extremely round.

How many times does the electron go round? ›

The answer is 6.59 ×10^15.

Is an electron a circle? ›

The most accurate measurement yet of the shape of the electron has shown it to be almost perfectly spherical.

What keeps an electron moving in a circle? ›

Electrostatic force provides necessary centripetal force for circular motion of electron.

What is a circle of electrons called? ›

In chemistry and atomic physics, an electron shell may be thought of as an orbit followed by electrons around an atom's nucleus.

Is light a particle or a wave? ›

Light can be described both as a wave and as a particle. There are two experiments in particular that have revealed the dual nature of light. When we're thinking of light as being made of of particles, these particles are called “photons”. Photons have no mass, and each one carries a specific amount of energy.

Can an electron behave like a wave? ›

Electrons behave as a particle because they have a mass. Electrons also behave as particles because when they collide, they'll bounce off each other. Electrons behave like waves because they produce an interference pattern in the double-slit experiment the same way light does, which travels as a wave.

Can electrons exist between energy levels? ›

Electrons can jump from one energy level to another, but they can never have orbits with energies other than the allowed energy levels. Let's look at the simplest atom, a neutral hydrogen atom. Its energy levels are given in the diagram below.

What are the 4 shapes of electrons? ›

There are four basic types of orbitals: s, p, d, and f. An s orbital has a spherical shape and can hold two electrons. There are three p orbitals, each of which has the same basic dumbbell shape but differ in its orientation in space.

What is inside an electron? ›

Right now, our best evidence says that there are particles inside of neutrons and protons. Scientists call these particles quarks. Our best evidence also shows us that there is nothing inside of an electron except the electron itself.

Is An electron A particle? ›

An electron is a negatively charged subatomic particle that can be either bound to an atom or free (not bound). An electron that is bound to an atom is one of the three primary types of particles within the atom -- the other two are protons and neutrons.

What shape are protons? ›

If the quarks are moving more slowly, the surface indentations of the peanut shape fill in and the proton takes on a form something like a rugby ball, or a beehive. The slowest quarks produce the spherical shape that physicists generally expected to see.

Are protons perfect spheres? ›

We usually think of electrons, protons and the like as being perfectly spherical, like tiny marbles. Yet while this seems to be pretty accurate in the case of electrons, studies of protons have revealed that they are constantly changing shape.

What is electron pair geometry vs molecular geometry? ›

Difference Between Electron Geometry and Molecular Geometry

The arrangement of atoms in a molecule, usually relative to a single central atom, is known as molecular geometry. The arrangement of electron pairs around a central atom is known as electron geometry.

Do electrons move in circles? ›

With all of this in mind, an electron in a stable atomic state does not move in the sense of a solid little ball zipping around in circles like how the planets orbit the sun, since the electron is spread out in a wave.

Do electrons ever run into each other? ›

No. Like rmhleo said, the Pauli exclusion principle prevents this. For an analogy, think of whirlpools. They don't overlap, and they don't crash into each other like billiard balls.

Do electrons move through time? ›

A calculation shows that the electron is traveling at about 2,200 kilometers per second. That's less than 1% of the speed of light, but it's fast enough to get it around the Earth in just over 18 seconds.

Are electrons matter or light? ›

Electrons are particles of matter that form the outer shells of atoms in the everyday objects around us.

What shape do electrons travel in? ›

The electrons can be found at any point in their orbital. The orbitals can be shaped as a sphere, as lobes — which kind of look like two squashes put together at the small ends — or in the shape of a doughnut around the nucleus.

Do electrons spin or not? ›

But many physicists will tell you that electrons are not really spinning—they merely act like it. For example, electrons have angular momentum, which is the tendency of something to keep rotating—like a moving bicycle wheel or a spinning skater—and because they have this property, one might conclude they are spinning.

Can you stop an electron from moving? ›

No, it's not possible to stop an electron. because of the simple fact, it has to obey the Heisenberg uncertainty relation with respect to place and momentum. In the extreme case (theoretically) we can measure the electron's momentum with absolute certainty.

What triggers electrons to move? ›

This force is called electromotive force, EMF, or voltage (V). Sometimes it is convenient to think of EMF as electrical pressure. In other words, it is the force that makes electrons move in a certain direction within a conductor.

Do electrons actually orbit the nucleus? ›

Electrons do not orbit a nucleus in the manner of a planet orbiting the Sun, but instead exist as standing waves. Thus the lowest possible energy an electron can take is similar to the fundamental frequency of a wave on a string.

What are the 4 quantum numbers? ›

In atoms, there are a total of four quantum numbers: the principal quantum number (n), the orbital angular momentum quantum number (l), the magnetic quantum number (ml), and the electron spin quantum number (ms).

Is light a quantum wave? ›

Light also behaves as a quantum particle in a number of important ways. Its energy is quantized into individual packets called photons, where each photon contains a specific amount of energy.

Is light a matter or an energy? ›

Light exists in tiny packets called photons. Photons have no rest mass and they do not occupy any volume. So light is not matter. It is the radiation of energy.

How does light have energy but no mass? ›

In addition to being a particle, light is also a wave. This allows it to carry momentum, and therefore energy, without having mass.

What is the quantum of light? ›

Quantum Theory: The quantum theory of light was proposed by Einstein, It states that light travels in bundles of energy, and each bundle is known as a photon. Each photon carries a quantity of energy equal to the product of the frequency of vibration of that photon and Planck's constant.

Can a proton act like a wave? ›

Atoms, electrons, protons, and neutrons also behave like waves! In other words, matter is just like light in that it has both wave-like and particle-like properties.

Do electrons create electromagnetic waves? ›

Charged particles—such as electrons and protons—create electromagnetic fields when they move, and these fields transport the type of energy we call electromagnetic radiation, or light.

What happens when an electron loses energy? ›

Answer and Explanation: a) When an electron loses energy, there is an emission of electromagnetic radiation in the form of the wavelength of a photon (a particle) that is equal to the difference between two energy levels.

Can two electrons have same energy? ›

Of course two electrons can be in the same energy level. Pauli's principle only states that two fermions (in this case, electrons) cannot occupy the same state. This means two electrons can still have different spins and occupy the same energy level, while having different states.

Where do electrons get their energy from? ›

For example, as electrons gain energy from photons (small bundles of energy), they might move from the second to the third energy level shell. Then, as they lose energy by emitting photons, they might move back to the second energy level shell or even to the first energy level shell.

How many electrons can go into each shape? ›

The first shell (closest to the nucleus) can hold two electrons. The second shell can hold 8 electrons. The third shell can hold 32 electrons. Within the shells, electrons are further grouped into subshells of four different types, identified as s, p, d, and f in order of increasing energy.

How many electrons can be in each orbital? ›

Each orbital can hold no more than two electrons. Two electrons in the same orbital must have opposite spins (the Pauli exclusion principle).

Where are electrons located? ›

Where Are Electrons? Unlike protons and neutrons, which are located inside the nucleus at the center of the atom, electrons are found outside the nucleus. Because opposite electric charges attract each other, negative electrons are attracted to the positive nucleus.

Has an electron ever been seen? ›

Yet, all physicists believe in the existence of electrons. An intelligent but superstitious man advances this analogy to argue that 'ghosts' exist even though no one has 'seen' one.

Are humans made of protons and neutrons? ›

At a pretty basic level, we're all made of atoms, which are made of electrons, protons, and neutrons. And at an even more basic, or perhaps the most basic level, those protons and neutrons, which hold the bulk of our mass, are made of a trio of fundamental particles called quarks.

What does quark look like? ›

Quark is a dairy product made by warming soured milk until it curdles, then straining it. The finished product is firm, creamy, and looks a bit like cottage cheese.

Are electrons in the universe? ›

Electrons occupy every atom in the universe. There is one in the hydrogen atom, two in the helium atom, three in lithium and so on. The electron content of an atom completely governs how likely it is to form a bond with another atom, making a molecule. So an electron has great power really, but only a very tiny mass.

Are electrons made of matter? ›

We learn in school that matter is made of atoms and that atoms are made of smaller ingredients: protons, neutrons and electrons. Protons and neutrons are made of quarks, but electrons aren't. As far as we can tell, quarks and electrons are fundamental particles, not built out of anything smaller.

Is A quantum A particle? ›

A quantum particles is the smallest physical particle that can exist without being broken down. Electrons are quantum particles as they cannot be broken down further. Quantum particles like electrons have properties that make them unique such as their spin and energy level.

How spherical is an electron? ›

Scientists have made the most accurate measurement yet of the shape of the humble electron, finding that it is almost a perfect sphere. The experiment suggests that if the electron were magnified to the size of the solar system, it would still appear spherical to within the width of a human hair.

Do electrons have a radius? ›

Using the best available values for the wave-length and the scattering by matter of hard X-rays and γ-rays, the radius of the electron is estimated as about 2 × 10−10 cm.

Are electron shells circular? ›

Electron shells and the Bohr model

The Bohr model shows the atom as a central nucleus containing protons and neutrons, with the electrons in circular electron shells at specific distances from the nucleus, similar to planets orbiting around the sun.

Why are all atoms spherical? ›

Answer and Explanation: "Even though the orbitals present in an atom are not spherically shaped, atoms are usually depicted as spheres". This is because of the superimposition of the orbitals. The electrons present in the atoms will occupy various orbitals.

Are all atoms spherical? ›

It is not true that all atoms exhibit spherical shape; only the simplest of the atoms have that shape. Heavy atoms have more complex shapes. Examples of atoms that are not spherical are those of iron and carbon. In quantum physics, the time independent Schroedinger s equation gives an insight into the shape of atoms.

Who said that atoms are spherical? ›

According to Thomson model, an atom possesses a spherical shape in which the positive charge is accumulated at the centre.

Do electrons have dimension? ›

In physics, it is already well-known that electrons behave very differently in three dimensions (cube), two dimensions (sheet) or one dimension (wire).

What are electrons made of? ›

Protons and neutrons are made of quarks, but electrons aren't. As far as we can tell, quarks and electrons are fundamental particles, not built out of anything smaller.

Does an electron experience gravity? ›

Yes, Electrons have mass and everything with mass is affected by gravitation(gravity).

Where do electrons circle? ›

Electrons revolve around the nucleus in fixed orbits or shells called energy levels. Electrons rotate around the nucleus in one or more energy levels.

Are protons actually round? ›

But in time, the findings proved to be right, and led physicists to the conclusion that protons aren't always spherically shaped, like a basketball. “Some physicists thought they did the experiment wrong,” said Miller, a University of Washington physics professor.


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2. Something Weird Just Happened At CERN That No One Can Explain
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3. How Does Light Actually Work?
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4. Quantum Field Theory: What is a particle?
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5. All Fundamental Forces and Particles Visually Explained
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6. The Simple Question that Stumped Everyone Except Marilyn vos Savant


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