Scientists have argued that light cannot escape the pull of a black hole. But something might. That escapee would be a kind of energy called Hawking radiation. To date, no one has ever witnessed Hawking radiation. But one scientist thinks he may have recorded evidence of the next best thing: energy escaping an experimental type of black hole in the lab. If other scientists can repeat his findings, this would offer evidence that Hawking radiation exists.
Daniele Faccio calls the new experiment “amazing, groundbreaking work.” A physicist at Heriot-Watt University in Edinburgh, Scotland, he did not take part in this research. The new work, he says, “demonstrates something that everyone thought was impossible.”
A black hole is a place in space where a lot of mass is packed into a small volume. A supermassive black hole has such intense gravity (owing to its great mass) that its attractive force could play a big role in holding an entire galaxy together. Scientists used to believe that nothing — not even light — could escape a black hole. But in the 1970s, physicist Stephen Hawking at the University of Cambridge, in England, introduced a radical new idea. He suggested that some particles could, in fact, escape.
His idea came from the world of quantum physics. Its rules govern the motion and behavior of particles that are smaller than atoms. According to quantum physics, pairs of particles are always popping into existence. But once they collide, they vanish again.
What would happen if these particles formed at the edge of a black hole and only one fell in, Hawking wondered. The other particle might escape, he concluded. And that would look like this particle was coming from the black hole. Over time, enough of these surviving particles — eventually called Hawking radiation — could make an entire black hole evaporate.
For decades, scientists have been looking for Hawking radiation. Getting experimental proof, however, has been tricky. After all, it would only show up at black holes, and physicists have no access to them. (The nearest is several thousand light-years away.) Hawking radiation also would be so faint that even telescopes couldn’t pick it up.
Jeff Steinhauer is a physicist at Technion-Israel Institute of Technology in Haifa. For his new experiment, Steinhauer didn't build a real black hole. Instead, he built an analog, or a device that mimics some properties of the real thing. Instead of light and matter, his lab-built black hole traps sound.
Similar to light, sound travels as a wave. To understand how Steinhauer’s black hole works, imagine a jet flying faster than the speed of sound. Now imagine that the jet makes a noise. Perhaps the pilot knocks on the cockpit window. Because the jet is flying so fast, sound waves from that knock can't move ahead of the jet. All stay behind it.
Steinhauer’s experiment created a similar situation. It accelerated a stream of ultracold atoms to superfast speeds. The point where atoms were moving faster than sound became a sort of point of no return. That would make it similar to a black hole’s event horizon — the point at which no light nor matter should escape. Any sound waves created behind the lab’s event horizon should similarly be trapped.
However, some sound waves did escape, Steinhauer found. He concludes this is evidence of Hawking radiation. He described his findings October 12 in Nature Physics.
Physicist William Unruh at the University of British Columbia in Vancouver, Canada, has spent decades studying black holes and Hawking radiation. The new experiment is “probably the closest anyone has come” to finding Hawking radiation, he told Science News. At the same time, he says that the escaping sound waves might be coming from somewhere else. So for now, he argues, more experiments are needed: “I would not say that the case is proven.”
A sonic black hole is different from one in space. Finding radiation in a lab-built black-hole analog “does not prove it would occur in [true] black holes,” Unruh told Science News. “However, it sure increases my confidence that it does.”
analog Something that resembles 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 neutrally charged neutrons. The nucleus is orbited by a cloud of negatively charged electrons.
black hole A region in space with a lot of mass packed into a small volume. The gravity is so strong that not even light can escape.
event horizon An imaginary sphere that surrounds a black hole. The more massive the black hole, the bigger the sphere. Anything that happens inside the event horizon is invisible, because gravity is so strong that under normal circumstances even light can’t escape. But according to some theories of physics, in certain situations small amounts of radiation can escape.
galaxy A system of millions or billions of stars, together with gas and dust, held together by gravitational attraction. Most galaxies are believed to have a black hole at their center.
gravity The force that attracts any body with mass, or bulk, toward any other body with mass. The more mass there is, the more gravity there is.
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.
light-year A unit of measurement equal to the distance light can travel in a year. It equals about 9.5 trillion kilometers (6 trillion miles).
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.
mechanics The study of how things move.
particle A minute amount of something.
physicist A scientist who studies the nature and properties of matter and energy.
quantum physics A branch of physics that uses quantum theory to explain or predict how a physical system will operate on the scale of atoms or sub-atomic particles.
radiation The emission of energy as electromagnetic waves or as moving subatomic particles.
sonic Of or relating to sound.
star Thebasic 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.
telescope Usually a light-collecting instrument that makes distant objects appear nearer through the use of lenses or a combination of curved mirrors and lenses. Some, however, collect radio emissions (energy from a different portion of the electromagnetic spectrum) through a network of antennas.
A. Grant. “Hawking radiation spotted within sonic black hole.” Science News. Oct. 12, 2014.
S. Ornes. “Black hole mysteries.” Science News for Students. May 29, 2013.
Original Journal Source: J. Steinhauer. Observation of self-amplifying Hawking radiation in an analogue black-hole laser. Nature Physics. Published October 12, 2014. doi: 10.1038/nphys3104.