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In space, there is nothing quite so ominous as a black hole.
Perhaps it is the fear of our own mortality, the sure knowledge that these behemoths will
almost certainly outlast us.
Perhaps it is the unknown – these giants of the cosmos are still so poorly understood
by our science, and jealously guard their secrets.
Or perhaps it is their hunger, and the knowledge that once matter falls into them, it can never
leave.
That black event horizon marks a point of inescapable no-return, a guarantee of death
and destruction under the greatest pressure and gravitational force that can exist within
this universe.
One so great that not even light can ever be free.
But is it really true that nothing can escape a black hole?
I’m Alex McColgan, and you’re watching Astrum.
Join with me for the second part of our new series on black holes, as we attempt to understand
the weird science that might allow energy, and thus mass, to escape from the inescapable.
I hope by the end of the video to have earned your like and subscription.
For many years of black hole science, it was a given fact that nothing could ever get free
once past the event horizon.
That was sort of the point of them.
These celestial masses had so much gravity that light itself could not escape them past
a certain point.
And as Einstein laid out that nothing could move faster than the speed of light, then
nothing could overcome the crushing acceleration due to gravity that a black hole produces.
Any object that fell into it was doomed to remain there, lost to the rest of the universe
for all of time.
However, in 1974 Stephen Hawking came up with a theory that suggested a different outcome.
The title of his paper, “Exploding Black Holes?”
was certainly flashy.
In it, Hawking hypothesised that black holes would release energy slowly over time, in
initially tiny quantities.
As energy and mass were two expressions of the same thing according to Einstein’s famous
e=mc2 equation, this inevitably resulted in a reduction in the black hole’s mass.
However, as the black hole shrinks, the rate of energy release would speed up, getting
faster and faster until in the very last moments of the black hole’s life it would release
a burst of energy that was truly gargantuan in its scale, before vanishing entirely.
But how could both of these things be true?
If an event horizon was inescapable, how could radiation ever leave it, and eventually cause
such a black hole explosion?
The answer is a strange one, and relies on unintuitive ideas of quantum theory, that
completely go against our day-to-day experience.
But if it’s true, I hope you’re prepared for the universe to be a whole lot stranger
than you first thought.
But to begin understanding Hawking’s theory, we need to understand the idea of quantum
fields.
In our last video, we asked the question “What is nothing?”
We explained that light moved like a wave through even a completely empty patch of space,
which revealed that there must be something existing even in the nothing, or else light
wouldn’t be able to wave it.
If you want to go back and watch that video, go ahead and follow the link here for a refresher
on this idea.
But basically, scientists call this fundamental fabric of reality a quantum field.
In fact, they believe that there are several quantum fields, all overlapping each other
and all covering every single patch of the universe, be it past, present or future.
Each quantum field defines a particular type of something.
One field might define all of the electrons in existence, while another might define quarks
that make up an atom.
Where nothing can be found, the quantum field is relatively quiet.
Think of it like a guitar string that hasn’t been strummed, or a graph that has a zero
value.
But wherever in time and space mass or energy can be found, the quantum field is resonating
at that point, and when the resonance reaches a certain threshold or quantity, the universe
expresses that as, say, an electron or a photon.
It’s important to note that in this theory, the resonance is not just reacting to a piece
of matter, it is the matter.
An electron is nothing more than a resonating section of the quantum field that defines
electrons.
This is true for all energy, and all matter too – according to Einstein, energy and
matter are two sides of the same coin, after all.
All the universe you see around you is resonating quantum fields, and nothing more.
In this way, the theory portrays all of the universe as a song being played on these fields,
which I think is quite a beautiful image, if nothing else.
But why does this matter?
Why is it important to define the universe in this way?
Well, due to an idea of quantum physics called Heisenberg’s uncertainty principle, sometimes
the strings of the universe start strumming themselves.
Without going too deeply into this aspect of quantum physics, essentially when we’re
looking at really tiny objects on the atomic scale, it becomes impossible to know too much
about them.
You cannot know both the location and the direction of travel of an electron, for instance.
Because it is so small, as soon as you try to figure out the location of an electron,
it bounces off whatever you are trying to use to measure it so you can no longer be
sure of its direction of travel.
If you know its direction, according to this principle, you can’t know its location.
This is not just because our methods of measuring aren’t good enough, but because of some
fundamental laws about the nature of the universe itself.
According to Heisenberg’s uncertainty principle, you cannot know everything about particles
on a subatomic level.
But when you apply this principle to quantum fields, it gets weird.
Quantum fields fluctuate everywhere and by Heisenberg’s Uncertainty Principle, particle
and antiparticle pairs can actually pop in and out of existence.
The how and why get complicated, but basically the universe allows it as long as they only
exist for a very short period of time as ruled by uncertainty relations.
You might think this can’t possibly be a real thing.
Matter does not just pop into existence.
We would surely have noticed by now.
However, in an experiment done by Hendrick Casimir, evidence was found that suggests
that this might actually happen.
Casimir took two plates of conductive metal and placed them close enough together so that
only certain sizes of smaller virtual particles could pop into existence between them.
This limited the number of such particles that could pop into existence.
But because all types of particles could pop into existence on the outside of the plates,
this meant that there was a difference in pressure exerted on the two sides of each
plate.
Theoretically, the greater pressure by the larger number of virtual particles on the
outside the plates should push the two plates together, and in the test, this proved to
be the case.
You might think that particles appearing out of nowhere seems to defy the laws of conservation
of matter.
You would be right.
So, to balance the scales, whenever a virtual particle appears, a second particle also pops
into existence to pair up with the first particle.
But while one of the particles is matter, the other is anti-matter.
A “1” and a “-1” on our bar chart, thus keeping things overall at 0.
The universe is happy.
And on top of that, these fluctuations in the quantum field quickly crash into each
other and annihilate each other, removing them both from existence again, so we normally
don’t have to worry about them.
As a side note, there is a theory that anti-matter is simply matter that is moving in the opposite
direction through time, but that’s a level of weirdness that we don’t need to get into
here.
The important part is that the quantum fields are constantly resonating, and constantly
cancelling each other out.
This is why, for the most part, empty space is empty.
However, what would happen if you stopped only some of those fields resonating?
And that’s where black holes come in.
Black holes act a bit like putting your thumb on the guitar string of the universe.
Due to their event horizons, certain resonances in the quantum fields are dampened down while
others are not.
Hawking imagined sketching a line through time, in a patch of space where a black hole
was born.
He imagined a quantum field that resonated along this line, stretching from before the
existence of the black hole, into the future after it.
Before the birth of the black hole, all is normal.
Quantum fields are all resonating freely, and can cancel each other out.
However, the emergence of the black hole’s event horizon changed the curvature of space,
and outside it Hawking realised that certain pulses were now missing their opposite numbers.
As he looked at the math, he realised not everything was being cancelled out anymore,
after the black hole had formed.
Indeed, outside the event horizon, travelling away from the black hole, he found resonances
that perfectly matched the shape of thermal radiation flying away into space.
Radiation is energy, and energy cannot form from nothing.
As the black hole was creating this radiation, the black hole would have to pay the price.
Every piece of Hawking radiation would thus coincide with an equal amount of energy lost
from the black hole, which in time would eventually reduce it down to nothing.
If it exists, Hawking Radiation is kind of like money spontaneously appearing outside
of a bank, while inside the bank the money in its vault vanishes.
It’s also extremely difficult to prove, as Hawking predicted this radiation would
be colder than the background cosmic radiation that fills the universe, and would have a
wavelength as long as the black hole’s event horizon itself.
As some black holes have event horizons the size of solar systems, we have no way of detecting
this kind of radiation.
We’d only really see it once the entire universe had gone cold and dead, so there
was nothing else to get in the way.
Which would probably mean we weren’t around anymore to do the detecting.
However, in spite of the objections to it, the math behind Hawking radiation seems to
be sound.
And scientists have recently taken steps towards proving it in the lab.
In the Technion Israel Institute of Technology, researchers looking into Hawking radiation
came up with an idea to get around the difficulty of measuring a real-life black hole.
They did this by creating an analogue - a “sonic” black hole which would mimic the
properties of a real one.
They relied on the fact that sound moves much slower than light, so it’s much easier to
create a medium that moves faster than sound.
When it moves, any sound waves travelling in the same direction as it can never quite
escape it.
Interestingly, Hawking’s math worked for these sonic black holes just as well as it
did for gravity-based ones, and so Hawking radiation ought to be detected from it.
After repeating their experiment 97,000 times over 124 days of continuous experimentation,
the researchers detected multiple instances of Hawking radiation, and saw that it matched
Hawking’s model predictions of how his radiation might behave.
Although this does not prove that Hawking radiation is definitely real for actual black
holes too, the fact that Hawking’s math worked for this sonic analogue is a strong
implication that he might be onto something.
Hawking radiation might just be real.
So, to answer our initial question, if you fell into a black hole, could you ever escape?
Still probably not.
However, if you waited until almost the end of the universe, the black hole may just radiate
hawking radiation until the mass and energy that makes up your existence completely was
removed from inside the event horizon, and you would be unwritten from existence.
Does that count as escaping?
That’s probably not so appealing to you.
Probably best just to not go in.
In this video, I didn’t really discriminate between black holes, but actually they come
in all sorts of sizes, from theoretically very small, to black holes far larger than
our entire solar system.
On MagellanTV there’s a great documentary called “The Most Powerful Black Holes in
the Universe”, which even goes more in depth into Cygnus X-1 which we talked about in the
last episode, plus has fascinating insights into the supermassive black hole at the centre
of our own galaxy.
MagellanTV is a subscription streaming platform containing over 3000 documentaries, with more
being added weekly, covering a variety of topics like space and science.
If you want to check out this documentary I mentioned or any like it, you can watch
it for free by using my link in the description, which will give you MagellanTV for a month,
so why not give it a go?
Thanks for watching and for your support of the channel.
If you missed the last episode, you should definitely check it out here, it gives some
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All the best, and see you next time.