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?
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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.
What do you think?
Is there anything we missed?
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