Hawking radiation, which is particles predicted
by Stephen Hawking that are emitted by black
holes, are at the center of a huge debate
among physicists.
Unfortunately, we’ve only recently been
able to take a black hole’s picture, let
alone study the quantum particles it might
be giving off.
So for clues on how real black holes behave,
scientists are creating stand-ins in a lab
and taking their temperature.
What could be analogous to a black hole, you
ask?
Well, black holes curve space-time so much
that the fastest thing in the universe, light,
cannot escape.
Once it crosses what’s known as the event
horizon, it can’t come back.
So instead of making something light can’t
escape, what if we made a medium another wave
cannot escape?
Like, sound.
A fluid moving at supersonic speeds could
do just that.
Amazingly in 1981, it was shown that the exact
same equations that describe event horizons
can also be used to describe sonic horizons
in a system like that.
The math even predicted vibrations called
phonons, which can act as the sonic equivalent
of Hawking radiation under the right circumstances.
In empty space, virtual particles are popping into
existence all the time, and when they meet,
they immediately annihilate each other again.
When these virtual particles form straddling
the cusp of an event horizon, however, one
of them gets sucked into the black hole, while
the other escapes.
In the same way, quantum units of sound called
phonons can arise in fluids.
In normal circumstances, these tiny vibrations
will meet and cancel each other out, but if
one phonon forms where the fluid is moving
slower than the speed of sound and it’s
opposite forms where the fluid is supersonic,
they should be separated and made permanent.
It took until 2009 for scientists to actually
make one of these sonic black holes.
They supercooled rubidium atoms until they
formed a Bose-Einstein condensate and got
them flowing.
By zapping the moving fluid partway along
its path with a laser, that section of fluid
was accelerated to supersonic speeds, creating
a sonic event horizon.
Sure enough, the scientists observed entangled
phonons that were consistent with sonic Hawking
radiation.
Since that experiment scientists have got
even more crafty with their black hole analogs,
creating multiple sonic horizons that would
bounce phonons back and forth, amplifying
them and making them easier to detect.
And in 2019, they finally measured the temperature
of the phonons, which could prove Hawking
right on a controversial prediction about
his radiation.
You heard me, the quantum sounds also gave
off a bit of heat, about 0.35 billionths of
a kelvin.
Hawking predicted his radiation would also
have a temperature, but those predictions
are a huge sticking point for quantum mechanics.
If Hawking radiation is thermal, it would
be a random spread of energies.
That would mean it carries no information
about what it once was before falling into
the black hole.
But quantum mechanics treats information as
indestructible, and says that the past state
of the universe can always be determined if
you rewind from the present.
Either Hawking is wrong or we need to rethink
quantum mechanics.
Unfortunately for quantum mechanics, the fact
that these phonons have a temperature consistent
with Hawking’s calculations points to Hawking
being right.
That is of course assuming that a sonic black
hole is a perfect stand in for a black hole,
and even the researchers behind this experiment
admit it may very well not be.
Until we can take the temperature of a real
black hole or come up with a theory of quantum
gravity that combines gravity with quantum
mechanics, Stephen Hawking’s prediction
will remain unresolved.
Thanks for watching be sure to subscribe to
learn all the weird ways we’re trying to
untangle the mysteries of black holes, like
how they might be like fuzz balls.
Pun absolutely intended.
Maren spins a yarn in this video here.
The disagreement between Hawking’s predictions
and quantum mechanics’ rules about information
is known as the information paradox.
That’s all for me, see you next time on
Seeker!
