Do Black Holes Die? | Unveiled

Do Black Holes Die?

There are some aspects of the universe that feel as though they’ll go on forever… and chief among them are black holes. These vast, sprawling, all-consuming entities help hold the universe together. But, will they always be there? Will these mysterious objects ever reach an end to their lives?

This is Unveiled and today we’re answering the extraordinary question; Do black holes die?

While there are various types, generally speaking black holes happen when a massive star goes supernova, causing a gravitational collapse in its core. In this way, a black hole is already a dead star. So, can something that’s already dead die again?

We didn’t have much of an answer to this question until Stephen Hawking came up with his theory of Hawking Radiation. It states that through quantum mechanical processes, black holes lose mass over time.

We knew that, at the subatomic level, positive particles and negative antiparticles occur all over the place all of the time… but that they typically cancel each other out. The closer you get to a black hole, though, the less and less typical things become. Hawking theorized that around a black hole we have negative antiparticles being pulled into the singularity while the positive particles manage to escape. What’s key, though, it is that over time this radiation diminishes a black hole’s mass and causes it to decay. And, suddenly, what seemed a cosmological ever-present is ticking toward its own demise just like everything else in the universe. So, yes, black holes do die.

But how long would that process take? Well, it almost completely depends on how massive the black hole is. The smaller a black hole the hotter it gets, the faster it loses energy and the quicker it dies. More massive ones, like the supermassive black holes at the centres of most galaxies, take far longer than smaller, stellar black holes to decay. But, that said, even smaller black holes take a long time to expire. According to astrophysicist Paul Sutter, speaking on his “Ask a Spaceman” podcast, a black hole the size of our own sun would take ten to the sixty-seven years to dissolve all of its mass - that’s much, much longer than the universe has even been around. An all but inconceivable length of time - and for what would ultimately be a fairly small black hole. For further comparison, Sutter also notes that a black hole the size of the Eiffel Tower would only take about a day to disappear.

Crucially, though, for a black hole to effectively evaporate, it has to have stopped absorbing matter itself. In their prime, black holes take in matter faster than they lose it, so there’s really zero prospect of it dying anytime soon. It’s only when the tables turn (and it expels more matter than it attracts) that a black hole’s brilliantly slow death can even start to play out. And there’s one major obstacle which prevents that from easily happening - the universe itself.

We already know that when black holes lose mass, they also get hotter. But we also know that the second law of thermodynamics says that heat always flows naturally from an object with a higher temperature to one with a lower temperature. Why does this matter? Well, the Cosmic Microwave Background radiation left over from the big bang registers a temperature of just 2.7 Kelvin (that’s minus-454 degrees Fahrenheit). According to the Stanford physicist Frank Heile, though, black holes the size of our moon or larger are inevitably even colder than the CMB… and this means that most black holes gain energy in the form of heat from the universe.

The energy gained via this cosmic transference is generally more than that lost through Hawking radiation, which means that any black holes larger than our moon should hit a standstill and won’t decay past that point. That doesn’t mean that they will unquestionably live forever, though. We think that the CMB temperature will continue to fall with time and will eventually reach zero Kelvin. At that point all of the black holes in the universe register as the hotter of the two objects and will finally evaporate.

But there comes the next mystery… Because we don’t know what exactly happens when a black hole fully disappears. For general relativity, the idea that it just dissolves into nothingness is problematic…. while quantum mechanics plain forbids that process from happening. Simply put, the energy that had existed in the dying black hole can’t then just be deleted at the quantum level.

One theory on “what happens next” comes from theoretical physicist Carlo Rovelli. He suggests that when black holes reach their smallest points, they actually form their theoretical counterparts, white holes. This is perhaps done through a process called quantum loop gravity, where former structure is rebounded outward - turning it into something which repels matter (rather than attracts it). We still can’t say with any degree of certainty that this is truly what happens, though. And that’s because white holes, while allowed by the same equations that prove black holes, have never actually been observed. It’s quite a significant catch, but an exciting prospect all the same!

In the meantime, however, while research into white holes is ongoing, another proposal for what happens during a black hole’s last moments is that there is an explosion. It’s the widely touted and slightly more conventional idea that, as with the star that formed the black hole in the first place, the black hole at death’s door spews matter out in one final burst of energy. Remember, because it gets hotter as it loses mass, it’s at this point that a black hole reaches its maximum temperature. For Stanford’s Frank Heile, the energy released from such an event could even equal that generated by five million megatons of TNT, or about 1,000 times the total nuclear capacity of Earth. Which is pretty momentous, though it still doesn’t rank amongst the most powerful explosions possible in the universe… we believe that they come from Gamma-Ray Bursts, which it’s commonly said release more energy in ten seconds than our sun does in its entire ten billion year lifetime!

But, back to black holes, and now that we know that they do eventually die, does that mean it’s also possible to instigate their dying - to deliberately kill them? It’s not something we ever really imagine needing to know… but say in a hypothetical, future reality, if Earth was on a crash course toward a black hole, would there be anything we could do to stop it? In short, no there really wouldn’t be. As black holes absorb all matter thrown at them, nothing currently known to science could ever stop one from doing its thing. So, if humanity ever did find itself bound for a black hole, then we’d either have to wait and hope that the black hole naturally expired before we reached it… or, seeing as that would take billions, possibly trillions of years, we’d be better advised to find a way off of Earth and out of the solar system as soon as possible!

Ultimately, black holes do die, but it takes them ages to do it. And what exactly occurs at their end… we’re still not sure about, because we’ve never actually seen it happen. According to some theories on the end of the universe, because black holes are so long-lived it’s possible that the cosmos will one day be filled with only black holes - with them having devoured all other forms of matter that had ever existed. It’s a bleak but fairly fascinating outlook. Fortunately for us, there’s still a long, long time between now and then.