How Do Black Holes Evaporate?

Article Updated: 23 Dec , 2015

Nothing lasts forever, not even black holes. According to Stephen Hawking, black holes will evaporate over vast periods of time. But how, exactly, does this happen?

The actor Stephen Hawking is best known for his cameo appearances in Futurama and Star Trek, you might surprised to learn that he’s also a theoretical astrophysicist. Is there anything that guy can’t do?

One of the most fascinating theories he came up with is that black holes, the Universe’s swiffer, can actually evaporate over vast periods of time.

Quantum theory suggests there are virtual particles popping in and out of existence all the time. When this happens, a particle and its antiparticle appear, and then they recombine and disappear again.

When this takes place near an event horizon, strange things can happen. Instead of the two particles existing for a moment and then annihilating each other, one particle can fall into the black hole, and the other particle can fly off into space. Over vast periods of time, the theory says that this trickle of escaping particles causes the black hole to evaporate.

Wait, if these virtual particles are falling into the black hole, shouldn’t that make it grow more massive? How does that cause it to evaporate? If I add pebbles to a rock pile, doesn’t my rock pile just get bigger?

It comes down to perspective. From an outside observer watching the black hole’s event horizon, it appears as if there’s a glow of radiation coming from the black hole. If that was all that was happening, it would violate the law of thermodynamics, as energy can neither be created nor destroyed. Since the black hole is now emitting energy, it needs to have given up a little bit of its mass to provide it.

Let’s try another way to think about this. A black hole has a temperature. The more massive it is, the lower its temperature, although it’s still not zero.

From now and until far off into the future, the temperature of the largest black holes will be colder than the background temperature of the Universe itself. Light from the cosmic microwave background radiation will fall in, increasing its mass.

Viewed in visible light, Markarian 739 resembles a smiling face.  Inside are two supermassive black holes, separated by about 11,000 light-years. The galaxy is 425 million light-years away from Earth. Credit: Sloan Digital Sky Survey

Viewed in visible light, Markarian 739 resembles a smiling face. Inside are two supermassive black holes, separated by about 11,000 light-years. The galaxy is 425 million light-years away from Earth. Credit: Sloan Digital Sky Survey

Now, fast forward to when the background temperature of the Universe drops below even the coolest black holes. Then they’ll slowly radiate heat away, which must come from the black hole converting its mass into energy.

The rate that this happens depends on the mass. For stellar mass black holes, it might take 10^67 years to evaporate completely.

For the big daddy supermassive ones at the cores of galaxies, you’re looking at 10^100. That’s a one, followed by 100 zero years. That’s huge number, but just like any gigantic and finite number, it’s still less than infinity. So over an incomprehensible amount of time, even the longest living objects in the Universe – our mighty black holes – will fade away into energy.

One last thing, the Large Hadron Collider might be capable of generating microscopic black holes, which would last for a fraction of a second and disappear in a burst of Hawking radiation. If they find them, then Hawking might want to the acting on hold and focus on physics.

The LHC. Image Credit: CERN

The LHC. Image Credit: CERN

Nothing is eternal, not even black holes. Over the longest time frames we’re pretty sure they’ll evaporate away into nothing. The only way to find out is to sit back and watch, well maybe it’s not the only way.

Does the idea of these celestial nightmares evaporating fill you with existential sadness? Feel free to share your thoughts with others in the comments below.

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23 Responses

  1. Steven says:

    I have a memory that the conservation of matter and energy required the in falling particle to have a “negative mass-energy” – n new category of sorts. Not anti-matter, by negative matter. And that when it meets normal matter (on the inside of the event horizon) it neutralizes (not annihilates – that’s what anti-matter does) and the total mass of the black hole decreases.

  2. Plenum says:

    That anti-particle that escapes out to space will have to have an enormous amount of energy to escape the black hole’s gravity. Where’s that energy coming from, please?

    • pearsonsjp says:

      Um, don’t be snide unless you actually know what you’re talking about “Plenum”.

      Escape velocity is dependent on distance. If the pair of particles appear at the exact right spot (Which is specifically what this article is describing), then one will be within the pull of gravity and be consumed, while the other will be exactly on the other side of that imaginary line, and will escape – without the need for your “enormous amount of energy”.

      This is a well known, scientifically accepted phenomena….so read a book, please?

    • Ivan3man_At_Large says:

      According to Wikipedia, Hawking Radiation:

      Physical insight into the process may be gained by imagining that particle-antiparticle radiation is emitted from just beyond the event horizon. This radiation does not come directly from the black hole itself, but rather is a result of virtual particles being “boosted” by the black hole’s gravitation into becoming real particles. As the particle-antiparticle pair was produced by the black hole’s gravitational energy, the escape of one of the particles takes away some of the mass of the black hole.

  3. Wayne Roberts says:

    Stephen Hawking an actor best known for cameo performances but also an astrophysicist?!?!?!?!?!???????????????
    Does that make me an air-breather who’s also a human?

  4. This begs a question I have often pondered, but not ever found the answer to . . . . . perhaps someone can answer . . .
    If a black hole is emitting matter, what becomes of that matter? Does it turn into hydrogen, which in essence would mean fuel for future stars?

    • FarAwayLongAgo says:

      No, it will just beam off in all directions, extremely redshifted because of the curvature of space around the black hole. Not a better building material than the radiation which has left the stars, stealing their energy. Sunshine doesn’t make new stars.

  5. johnsankey says:

    The vanishing paradigm assumes that black holes aren’t quantized. But everything else in our universe is, why not black holes? Then they might not totally vanish but end up in a ground state. Might that ground state be one of the Higg’s bosons, or dark matter, or something else essential to the structure of our universe?

  6. Weinstein says:

    Blackholes should be visible by a surround of light — charged particles radiating as they accelerate due to gravity.

    Gravity, itself, is emitted from a blackhole. That should be an energy leakage. Mass travels by and is dragged due to the gravity of the blackhole, etc.. (Force X distance = energy.)

    Besides, who says blackholes are black? OK, everybody. But before it is black it is “infrared.”

  7. btraymd says:

    [Comment deleted by moderator – reason: this is not a blog for your personal viewpoints! ]

    • Ivan3man_At_Large says:

      So, according to you, Anthony Peratt – using early 1980s computer simulations based on plasma laboratory experiments conducted in the 1950s! – knows more than all the physicists that are currently working with the Large Hadron Collider to find out the secrets of the Universe?!

      • btraymd says:

        At least he shows laboratory experiments to confirm observations and has shown a workable model for spiral galaxy formation. Why would one ignore this work in the face of the current lack of understanding clearly demonstrated by the standard model.

      • Ivan3man_At_Large says:

        For the simple reason that the “Birkeland currents” of the magnitude needed – 10^18 amps over scales of megaparsecs – for galaxy formation do not exist!

    • btraymd says:

      Suppression of information relative to your topic that takes a different vantage point….exactly the point I was making. Very unscientific and begs the question…. Why are you afraid of something that makes common sense????

  8. nswanberg says:

    What about when a pair materialize and the particle escapes to space and the anti-particle falls in to the black hole? Wouldn’t the conversion of anti-matter to energy warm the black hole? Perhaps that makes the rate of pair production near the black hole increase? This could be a feedback loop? My head hurts.

  9. Meaning no disrespect, the evaporation of black holes hypothesised by the eminent physicist Professor Hawking is just that, a hypothesis. There is no evidence of the phenomenon available at this time that should see it elevated to a theory (in the technical sense of the word).

  10. TraKtorman says:

    So does the theory say what happens when the mass of the evaporating black hole decreases below the minimum required to create a black hole in the fist place?

    • mewo says:

      There is, AFAIK, no lower limit on the mass of a black hole. The processes that form them, like supernova explosions and such, may impose a lower limit to the mass of the BH that is produced, but that doesn’t mean it can’t later on evaporate to a smaller mass.

      It’s a bit like a factory that produces boiled lollies in a set range of sizes. If you buy a big one and start sucking on it, it gets smaller, but you don’t ask what happens if it shrinks to a size smaller than the smallest lollies that the factory produces. It just gets smaller and smaller until it’s all gone.

  11. Wayne Roberts says:

    I’m comfortable with the scenario of a pair of virtual particles materialising near the event horizon, and one being consumed while the other escapes.
    My understanding falters with regards to the next step in the black hole evaporation process – it seems to posit that the virtual particle with negative energy is invariably the one consumed, thus decreasing the mass of the black hole.
    Why should not the particle with positive energy be that which crosses the event horizon?

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