Black holes might have a temperature | Science News for Students

Black holes might have a temperature

A lab-made mock black hole spit out radiation, suggesting Stephen Hawking was right
Jun 25, 2019 — 6:45 am EST
an illustration of a black hole

A black hole (illustrated) isn’t completely black. It probably emits a faint haze of particles. Scientists have now taken the temperature of the haze around a lab-made black hole and confirms these objects are only mostly dark.

NASA’s Goddard Space Flight Center; Background: DPAC/Gaia/ESA

Black holes are huge voids in space that trap light inside of them. Because they take in energy but supposedly don’t give any off, black holes should be dark and cold. But they may not be totally black and absolutely cold. At least that’s according to a new study. In it, physicists took the temperature of a black hole. Well, sort of. They measured the temperature of a pseudo black hole — a black hole simulated in the lab.

This simulated version traps sound, not light. And tests with it now appear to offer evidence for an idea first proposed by the famous cosmologist Stephen Hawking. He was the first to suggest that black holes aren’t truly black. They leak, he said. And what flows out of them is an extremely tiny stream of particles.

Truly black objects emit no particles — no radiation. But black holes might. And if they do, Hawking had argued, they wouldn’t be truly black.

The stream of particles that leak from a black hole is now referred to as Hawking radiation. It’s probably impossible to detect this radiation around true black holes, the ones in space. But physicists have spotted hints of similar radiation flowing from simulated black holes that they created in the lab. And in the new study, the temperature of the lab-made, sound-based — or sonic — black hole is similar to what Hawking suggested it should be.

This is a “very important milestone,” says Ulf Leonhardt. He is a physicist at the Weizmann Institute of Science in Rehovot, Israel. He was not involved with the latest study, but says of the work: “It’s new in the entire field. Nobody has done such an experiment before.”

If other scientists do similar experiments and get similar results, that could mean Hawking was right about black holes being not totally black.

a photo of Jeff Steinhauer in his lab
Jeff Steinhauer (shown here) and his colleagues created a sonic black hole in the lab. They used it to study famous predictions about black holes in space.
Technion-Israel Institute of Technology

Making a lab-based black hole

To take a black hole’s temperature, physicists first had to make one. That was the task Jeff Steinhauer and colleagues took on. Steinhauer is physicist at Technion-Israel Institute of Technology. It’s in Haifa, Israel.

To make the black hole, his team used ultracold atoms of rubidium. The team chilled them to nearly the point at which they would be absolutely still. That’s called absolute zero. Absolute zero occurs at -273.15 °C (-459.67 °F) — also known as 0 kelvin. The atoms were in gas form and very far apart. Scientists describe such a material as a Bose-Einstein condensate.

With a little nudge, the team set the chilled atoms flowing. In this state, they prevented sound waves from escaping. That mimics how a black hole prevents the escape of light. In both cases, it’s like a kayaker paddling against a current too strong to overcome.

But black holes can let a bit of light slip out at their edges. That’s because of quantum mechanics, the theory that describes the often weird behavior of things at the subatomic scale. Sometimes, quantum mechanics says, particles can appear in pairs. Those particles appear out of seemingly empty space. Normally, the pairs of particles immediately destroy one another. But at a black hole’s edge, it’s different. If one particle falls into the black hole, the other can escape. That escaping particle becomes part of the stream of particles that comprise Hawking radiation.

In a sonic black hole, a similar situation occurs. Sound waves pair up. Each tiny sound wave is called a phonon. And one phonon can fall in the lab-made black hole, while the other escapes.

Measurements of phonons that escaped and those that fell into the lab-made black hole allowed the researchers to estimate the temperature of the simulated Hawking radiation. The temperature was 0.35 billionths of a kelvin, just the tiniest bit warmer than absolute zero.

Concludes Steinhauer, with these data “we found very good agreement with the predictions of Hawking’s theory.”

And there’s more. The result also agrees with Hawking’s prediction that the radiation would be thermal. Thermal means that the radiation behaves like the light emitted from something warm. Think of a hot electric stovetop, for example. The light coming from a hot, glowing object comes with certain energies. Those energies depend on how hot the object is. The phonons from the sonic black hole had energies that matched that pattern. That means they, too, are thermal.

There’s a problem with this part of Hawking’s idea, however. If Hawking radiation is thermal, then it causes a conundrum called the black hole information paradox. This paradox exists because of quantum mechanics. In quantum mechanics, information can never really be destroyed. This information can come in many forms. For instance, particles can carry information, just as books can. But if Hawking radiation is thermal, information might be destroyed. That would violate quantum mechanics.

The information loss happens because of the particles escaping the black hole. When they escape, the particles take tiny bits of a black hole’s mass with them. That means a black hole is slowly disappearing. Scientists don’t understand what happens to the information when a black hole finally disappears. That’s because thermal radiation doesn’t carry any information. (It tells you how warm the black hole is, but not what fell into it.) If Hawking radiation is thermal, information can’t be carried away by the escaping particles. So the information could be lost, violating quantum mechanics.

Unfortunately, lab-made, sonic black holes may be no help in understanding if this violation of quantum mechanics actually happens. To know if it does, physicists will probably need to create a new theory of physics. It will probably be one that combines gravity and quantum mechanics.

Creating that theory is one of the biggest problems in physics. But the theory wouldn’t apply to sonic black holes. That’s because they’re based on sound and aren’t created by gravity. Explains Steinhauer, “The solution to the information paradox is in the physics of a real black hole, not in the physics of an analog black hole.”

Power Words

(more about Power Words)

absolute zero     The coldest possible temperature, also known as 0 kelvin. It is equal to minus 273.15 degrees Celsius (minus 459.67 degrees Fahrenheit).

analog     Something that resembles, stands in for, or is similar to another thing.

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.

behavior     The way something, often a person or other organism, acts towards others, or conducts itself.

black hole     A region of space having a gravitational field so intense that no matter or radiation (including light) can escape.

Bose-Einstein condensate     A state of matter seen when atoms (or subatomic particles) have been cooled almost to absolute zero and now behave as a community, becoming almost a “superatom,” with quantum behaviors. Predicted in the 1920s, the first Bose-Einstein condensate was not made until 1995.

celestial     (in astronomy) Of or relating to the sky, or outer space.

colleague     Someone who works with another; a co-worker or team member.

cosmology     The science of the origin and development of the cosmos, or universe. People who work in this field are known as cosmologists.

field     An area of study, as in: Her field of research was biology. Also a term to describe a real-world environment in which some research is conducted, such as at sea, in a forest, on a mountaintop or on a city street. It is the opposite of an artificial setting, such as a research laboratory.

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.

Hawking radiation     The particles emitted from the event horizon on the outer edges of a black hole. Energy can be converted into a pair of particles. If that happens very close to outer edge of a black hole, one of those particles can tunnel out and become detected — providing the only direct physical clue to the black hole’s presence. These emissions are called Hawking radiation for Stephen Hawking, the famous British physicist who came up with the idea that black holes can emit particles.

information paradox     (in physics) A problem created by two conflicting ideas about how black holes work and how the universe works. Black holes eventually disappear, and presumably, the information they contain about what’s in them also disappears. But this disappearance breaks a law of quantum mechanics, which says that information is never “lost” to the universe.

kelvin     A temperature scale that has units the size of those on the Celsius scale. The difference, 0 kelvin is absolute zero. So 0 kelvin is equal to -273.15 Celsius. That means 0 Celsius is equal to 273.15 kelvins. NOTE: Unlike with the Celsius and Fahrenheit scales, there is no use of the term “degrees” for numbers on the kelvin scale.

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.

milestone     An important step on the road to stated goal or achievement. The term gets its name from the stone markers that communities used to erect along the side of the road to inform travelers how far they still had to go (in miles) before reaching a town.

paradox     An idea or a statement that is true, but that seems logically impossible.

particle     A minute amount of something.

phonon     The basic unit of sound.

physics     The scientific study of the nature and properties of matter and energy. Classical physics is an explanation of the nature and properties of matter and energy that relies on descriptions such as Newton’s laws of motion. Quantum physics, a field of study that emerged later, is a more accurate way of explaining the motions and behavior of matter. A scientist who works in such areas is known as a physicist.

quantum     (pl. quanta) A term that refers to the smallest amount of anything, especially of energy or subatomic mass.

quantum mechanics     A branch of physics dealing with the behavior of matter on the scale of atoms or subatomic particles.

radiation     (in physics) One of the three major ways that energy is transferred. (The other two are conduction and convection.) In radiation, electromagnetic waves carry energy from one place to another. Unlike conduction and convection, which need material to help transfer the energy, radiation can transfer energy across empty space.

rubidium     Number 37 on the periodic table, German chemists discovered this element in 1861. Its presence created a deep red spectral line that showed up on an instrument (called a spectroscope). It had been used to analyze a mineral that contained this element. Rubidium’s name reflects the spectral surprise: It’s Latin for the deepest red.

simulate     To deceive in some way by imitating the form or function of something. A simulated dietary fat, for instance, may deceive the mouth that it has tasted a real fat because it has the same feel on the tongue — without having any calories. A simulated sense of touch may fool the brain into thinking a finger has touched something even though a hand may no longer exists and has been replaced by a synthetic limb. (in computing) To try and imitate the conditions, functions or appearance of something. Computer programs that do this are referred to as simulations.

sonic     Of or relating to sound.

sound wave     A wave that transmits sound. Sound waves have alternating swaths of high and low pressure.

subatomic     Anything smaller than an atom, which is the smallest bit of matter that has all the properties of whatever chemical element it is (like hydrogen, iron or calcium).

theory     (in science) A description of some aspect of the natural world based on extensive observations, tests and reason. A theory can also be a way of organizing a broad body of knowledge that applies in a broad range of circumstances to explain what will happen. Unlike the common definition of theory, a theory in science is not just a hunch. Ideas or conclusions that are based on a theory — and not yet on firm data or observations — are referred to as theoretical. Scientists who use mathematics and/or existing data to project what might happen in new situations are known as theorists.

thermal     Of or relating to heat. (in meteorology) A relatively small-scale, rising air current produced when Earth’s surface is heated. Thermals are a common source of low level turbulence for aircraft.

void     An empty space or cavity.

wave     A disturbance or variation that travels through space and matter in a regular, oscillating fashion.


Journal:​ ​​ J.R.M. de Nova et al. Observation of thermal Hawking radiation and its temperature in an analogue black hole. Nature. Vol. 569, May 30, 2019, p. 688. doi:10.1038/s41586-019-1241-0.