To witness maximum pressure, peek inside a proton | Science News for Students

To witness maximum pressure, peek inside a proton

New data suggest nothing else in the universe matches the pressures inside this particle
Jul 2, 2018 — 6:45 am EST
an artist's illustration of a proton

A proton (depicted in this artist’s illustration) is way too small to see. But that hasn’t stopped physicists from exploring the properties of these subatomic particles.


Pity the proton: These little particles are under a lot of pressure. Their innards are squeezed harder than any other substance known, new data show. Within these subatomic particles, lots of force is concentrated over a very small area — a sphere whose radius is about a trillionth that of a poppy seed.

And how punishing is that pressure? Try a million trillion trillion times the strength of Earth’s atmospheric pressure.

“It’s really the highest pressure we have ever seen,” says Volker Burkert. He’s a physicist at the Thomas Jefferson National Accelerator Facility in Newport News, Va. He was part of a team that published the extreme number in the May 17 Nature.

Neutron stars used to hold the pressure record. These incredibly dense, dead stars can form when a massive star explodes, allowing its core to collapse. The process ends up squeezing a mass greater than the sun’s into a remnant the size of a city. But the proton? Its internal pressure is around 10 times that of a neutron star’s.

Scientists had predicted that such pressures might occur inside protons. The new number comes from the first experimental gauge of that pressure.

Physicist Peter Schweitzer works at the University of Connecticut in Storrs. A proton’s internal pressure, he says, is one of its fundamental properties. It is “as important as electric charge or mass,” he says. Yet until now, that pressure figure had remained unknown.  

Gauging the force

Protons are made up of smaller particles. These include quarks, which are electrically charged. Protons also contain gluons. These particles transmit the strong nuclear force that holds protons together. From the center of this ball of particles, Burkert and his team now report, an intense pressure pushes outward. But this record-breaking outward force is kept in check by an inward pressure from the outer regions of the proton.

This pressure pattern parallels what happens in much larger objects, says Oleg Teryaev. He’s a physicist at the Joint Institute for Nuclear Research in Dubna, Russia. “In some sense,” he says, “it’s looking like a star.” Stars also have pressures that push outward in their centers. Counteracting those pressures is the inward pull of gravity.

Stars are held together by gravity. Protons, however, are a different beast. They are held together by what’s known as the strong force. So “it’s natural, but it’s not completely trivial” that stars and protons would have similarities pressure-wise, Teryaev says.

To quantify the proton’s squeeze, researchers used data that had been collected three years ago in a particle detector at the Thomas Jefferson lab. It’s known as CLAS (short for the Continuous Electron Beam Accelerator Facility Large Acceptance Spectrometer). Scientists there beamed electrons with an energy of 6 billion electron volts at liquid hydrogen. Why hydrogen? It’s a rich source of protons. (Each atom’s nucleus is a single proton.)

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CLAS detector
To determine the pressure inside a proton, scientists used data from CLAS detector (shown), in which electrons are scattered off of protons.

As electrons slammed into the protons, those electrons ricocheted away together with particles of light known as photons. The researchers focused on how the electrons had interacted with the protons’ quarks. (They ignored the gluons, because the energy of the electrons had not been high enough to fully probe them.) To make their pressure estimate, the researchers assumed that the gluons’ pressure contribution was the same as the quarks’. In fact, that’s what some physicists do suspect.

By watching what happened as electrons interacted with the protons’ constituents, Burkert’s team was able to tease out, for the first time, the proton’s pressure.

Future particle accelerators, such as the planned Electron-Ion Collider, should allow for gauging the gluons’ contribution, too. That could offer an even more precise estimate of the crushing pressure that all tiny protons endure.

Power Words

(more about Power Words)

accelerator     (in physics) Also known as a particle accelerator, this massive machine revs up the motion of subatomic particles to great speed, and then beams them at targets. Sometimes the beams are used to deliver radiation at a tissue for cancer treatment. Other times, scientists crash the particles into solid targets in hopes of breaking the particles into their building blocks.

atmospheric pressure     The pressure exerted by the weight of the atmosphere.

atom     The basic unit of a chemical element. Atoms are made up of a dense nucleus that contains positively charged protons and uncharged neutrons. The nucleus is orbited by a cloud of negatively charged electrons.

collider     (in physics) Sometimes called an “atom smasher,” it is a type of particle accelerator that speeds up charged particles (ions) through an electric field inside a hollow tube or racetrack-shaped structure. Eventually the device will direct the ions to collide with an unmoving target or another beam of moving particles. The ensuing collisions force some particles to interact — and break apart or briefly bind. Some of smashed particle also may recombine, creating new particles. The biggest of these machines are used to hunt for the basic building blocks of all nature.

constituent     An ingredient or building block of some material. 

core     Something — usually round-shaped — in the center of an object.

electric charge     The physical property responsible for electric force; it can be negative or positive.

electron     A negatively charged particle, usually found orbiting the outer regions of an atom; also, the carrier of electricity within solids.

force     Some outside influence that can change the motion of a body, hold bodies close to one another, or produce motion or stress in a stationary body.

fundamental     Something that is basic or serves as the foundation for another thing or idea.

gauge     A device to measure the size or volume of something. For instance, tide gauges track the ever-changing height of coastal water levels throughout the day. Or any system or event that can be used to estimate the size or magnitude of something else. (v. to gauge) The act of measuring or estimating the size of something.

gluon     A subatomic particle believed to bind other particles together.

gravity     The force that attracts anything with mass, or bulk, toward any other thing with mass. The more mass that something has, the greater its gravity.

hydrogen     The lightest element in the universe. As a gas, it is colorless, odorless and highly flammable. It’s an integral part of many fuels, fats and chemicals that make up living tissues. It’s made of a single proton (which serves as its nucleus) orbited by a single electron.

innards     Slang term for internal organs, such as the stomach and intestines.

ion     (adj. ionized) An atom or molecule with an electric charge due to the loss or gain of one or more electrons. An ionized gas, or plasma, is where all of the electrons have been separated from their parent atoms.

liquid     A material that flows freely but keeps a constant volume, like water or oil.

mass     A number that shows how much an object resists speeding up and slowing down — basically a measure of how much matter that object is made from.

neutron star     The very dense corpse of what had once been a star with a mass four to eight times that of our sun. As the star died in a supernova explosion, its outer layers shot out into space. Its core then collapsed under its intense gravity, causing protons and electrons in its atoms to fuse into neutrons (hence the star’s name). Astronomers believe neutron stars form when large stars undergo a supernova but aren’t massive enough to form a black hole. A single teaspoonful of a neutron star, on Earth, would weigh a billion tons.

nucleus     Plural is nuclei. (in physics) The central core of an atom, containing most of its mass.

parallel     An adjective that describes two things that are side by side and have the same distance between their parts. In the word “all,” the final two letters are parallel lines. Or two things, events or processes that have much in common if compared side by side.

particle     A minute amount of something.

physicist     A scientist who studies the nature and properties of matter and energy.

pressure     Force applied uniformly over a surface, measured as force per unit of area.

pressure gauge     A device that measures pressure.

proton     A subatomic particle that is one of the basic building blocks of the atoms that make up matter. Protons belong to the family of particles known as hadrons.

quarks     A family of subatomic particles that each carries a fractional electric charge. Quarks are building blocks of particles called hadrons. Quarks come in types, or “flavors,” known as: up, down, strange, charm, top and bottom.

remnant     Something that is leftover — from another piece of something, from another time or even some features from an earlier species.

shard     A piece of broken pottery, tile or rock, or a hard, broken piece of anything that has an irregular shape.

spectrometer     An instrument that measures a spectrum, such as light, energy, or atomic mass. Typically, chemists use these instruments to measure and report the wavelengths of light that it observes. The collection of data using this instrument, a process is known as spectrometry, can help identify the elements or molecules present in an unknown sample.

star     The basic building block from which galaxies are made. Stars develop when gravity compacts clouds of gas. When they become dense enough to sustain nuclear-fusion reactions, stars will emit light and sometimes other forms of electromagnetic radiation. The sun is our closest star.

strong force      (in physics) A fundamental interaction in nature, one that binds quarks together to make other types of subatomic particles, such as protons and neutrons. This force also holds protons and neutrons together within an atom’s nucleus. Not all subatomic particles can “feel” this force. Electrons and other leptons, for instance, are immune to it.

sun     The star at the center of Earth’s solar system. It’s an average size star about 26,000 light-years from the center of the Milky Way galaxy. Also a term for any sunlike star.

trillion     A number representing a million million — or 1,000,000,000,000 — of something.


Journal:​ ​​ V.D. Burkert et al. The pressure distribution inside the protonNature. Vol. 557, May 17, 2018, p. 396. doi:10.1038/s41586-018-0060-z.