Thank you to Displate for supporting PBS. For a while back there, we thought we might be able
to avoid the black hole. They'd been lurking as shadows in our theories of gravity forever. Enough
mass crammed into a small enough space would lead to a gravitational field from which not
even life could escape from the surrounding surface that we call the event horizon. The
event horizon generates paradoxes that worry physicists. And the singularity of infinite
density within the black hole worries them even more. And so many brave physicists have fought
for centuries to prove that these monsters don't exist. They hoped nature would step
in to save us from the theoretical horror of ultimate gravitational collapse. One of
our final hopes is the plank star, a ball of energy at the heart of the black hole,
like frozen shards of the Big Bang. Well, let's hope they're real for physics's sake.
it lights up after sun exposure, even in the depths of space. Now, on to the episode. First,
there were the dark stars of Mitchell and Lelass, constructed with only Newtonian gravity.
These things were gigantic, 500 times the size of the sun in Mitchell's mind. Happily,
they aren't possible. Giant clouds of gas fragment and collapse before a dark star can form. But any
matter collapsed far enough will have an event horizon. Those collapsing gas fragments would
form black holes themselves if they were not saved by the onset of nuclear fusion as internal
temperature and pressure spikes. The resulting outflow of energy counters the gravitational
crush, birthing a true star, which saves us from collapse until nuclear fuel runs out. Then the
collapse must continue. But happily, not to a black hole. Not yet. At extreme densities,
think an entire star crammed into the volume of the Earth, new strange quantum effects come to
the rescue. The electrons of the stellar core are crushed until all available quantum states
are filled and they cannot be forced together anymore. The resulting electron degeneracy
pressure halts collapse once again, giving us a white dwarf. Nature seems to have stepped in
to halt the absurdity of the black hole. So far so good for our hero physicists. We have Mitchell and
Lelass getting us into trouble in the first place with dark stars. Then Arthur Edington figured out
that stellar fusion halts collapse and it was Ralph Fowler applying the brand new field of
quantum mechanics that gave us white dwarfs. But then Subramanion Chandra Seca came along
and at only 19 years old on an ocean voyage from India to Cambridge to begin working with Fowler,
he proved that even his new bosses white dwarfs have a failure point. For any white dwarf 40% more
massive than the sun, gravitational crush will always exceed the outward electron degeneracy
pressure. In fact, by including Einstein's relativity to the quantum descriptions,
Chandra found that outward pressure no longer rises fast enough to resist the rising crush
as a white dwarf gains mass, leading to runaway collapse. Edington was famously very annoyed by
this result and disputed it. He was convinced that nature must prevent such absurdities as infinite
collapse. But Chandra was right about the white dwarf collapse. Maybe Edington will still be right
about the ultimate infinite collapse. There's one last resppite for collapsing stellar cores
when physicists realize that electron capture by protons could halt collapse as a neutron star. But
the salvation of the stellar core is limited in this case. The more mass of the neutron star, the
more compact it becomes. For neutron stars over a certain mass, the surface gravity stops light
from escaping and the dreaded event horizon forms. And that's it. We've lost our battle to stop black
holes a long time ago. In fact, they are real. We've seen them in their extreme gravitational
effects across the universe and in gravitational waves and now even in images. They are real. And
frankly, I think that's awesome. You might like them, too. I mean, you clicked on this episode.
So, what's this about physicists wanting to avoid the event horizon? Well, people were uncomfortable
with the idea of black holes for good reasons. And the best reason emerged in the 70s when
Steven Hawking and others showed that black holes slowly radiate away their mass, shrink,
and ultimately vanish. The main problem with this is that all information of everything swallowed by
a black hole is deleted in that process. This violates a core tenet of quantum mechanics,
information conservation. The other problem with the formation of an event horizon is that there
is no known process that can stop matter within it from collapsing into a point of infinite density
in the center. This singularity generates plenty of its own problems. Not least of which is that in
these conditions, the two theories that we use to get this far, quantum mechanics and general
relativity, are so conflicted that they can't be simultaneously true. Black holes point to
fundamental flaws in our theories of nature. Okay, so even if we couldn't prevent the event horizon,
maybe we can at least stop the formation of the theorybreaking singularity. New generations of
physicists took up the ancient battle to save us from this theoretical catastrophe. Most believed
that the solution must lie in a union of quantum mechanics and general relativity. For example,
string theory proposed fuzz balls in which matter unravels into its stringy weave filling the region
beneath the horizon. And we covered that already. Another possible solution is the plank star. a
star of near absolute collapse supported in the last instant only by the grainy structure of
spacetime itself. This comes from what has been called the main competitor of string theory loop
quantum gravity. In LQG, space at the tiniest scales is blocky. In particular, it's built up
of quantiz 2D area elements whose interplay looks like 3D space on larger scales. And here larger
means anything significantly bigger than the plank length around 10 theus 35 m. If LQG is right,
then it should give us the same spacetime as described by general relativity on larger scales.
And that needs to be true of any quantum gravity theory. But none have been completely worked out
and so there's some guesswork in connecting the plank scale to the scale of GR. One way to
do that is using so-called semiclassical gravity which guesses the pertabbations to the equations
of general relativity as we approach the plank scale. And this is how Carlo Revelian co got to
their first picture of the plank star. It actually came from an effort to describe what might happen
if the entire universe collapsed like a reverse big bang. As densities become extreme enough,
LQG predicts a semiclassical correction to the cosmological equations, the Freriedman equations,
in which an anti-gravity-like effect emerges, causing the collapse to bounce outwards. This
loop quantum cosmology is meant to describe an infinitely expanding and contracting universe with
loopy bounces between cycles. But in 2014, Ralli and Franchesco Vidatoto showed how this result
for a collapsing universe could also be used to approximate the end result of the collapsing star
inside a black hole. At a certain point, outward quantum pressure causes it to bounce. In a way,
it's analogous to the quantum degeneracy pressures that stopped our white dwarf and neutron stars
from collapsing further. In that case, it was quantum particles unable to occupy the same energy
levels. But with the plank star, it's the quantum elements of spaceime itself doing the work. If
LQG is right, this could handily stop the wicked singularity from ever forming. For a collapsing
sun mass star, the resulting object would be about 1 trillionth of a meter. That's small,
but it's not pointlike. It's not a singularity. In fact, it's 23 orders of magnitude larger than the
plank scale. So, relatively gigantic. This ultra compressed ball of matter is the plank star that
we've been teasing. The thing about plank stars is that they shouldn't last for long, at least in
this semiclassical description given back in 2014. The same space-time pressure that stops collapse
also triggers catastrophic rebound. Now, a similar thing happens when the core of a massive star is
halted by the formation of a neutron star. The rest of the star explodes outwards as a supernova.
But in the case of the plank star, the resulting rebound is a white hole. Basically, the opposite
of a black hole, the time reversal, space and the energy it contains rushing outwards and an
event horizon that reverses its direction. All of this takes place in about the time it takes light
to cross the size of the plank star, which is a a tiny fraction of a second. So, it sounds like our
plank star only exists for an instant after the black hole formation before exploding outwards.
But in that case, why do we still see black holes out there? And why don't we see the flashes of
extreme energy expected when a white hole forms? Maybe you've seen the film Interstellar and you
remember that time close to a black hole event horizon slows down from the point of view those
waiting for Matthew McConnA back on Earth. Hours can translate to years. Well, imagine how strong
that gravitational time dilation would be from deep below the event horizon. Strong. A rebounding
plank star would appear frozen in that state for billions of years for anyone but the plank
star. So there you have it. ultimate gravitational collapse foiled again in a 10 the -12 m wide ball
of energy that looks like the universe as it was essentially at the big bang but at least it's not
pointlike or even actually plank scale but we're not quite safe yet the description I just gave you
of the plank star is over a decade old it involves some serious approximations the quantum gravity
effects were approximated as a modification to the standard equations of general relativity and
the collapsing star was approx approximated as a collapsing universe, which really means that
its matter was smooth and infinitely extended. Not really what a collapsing star looks like.
Nonetheless, yet again, we have a mechanism by which the collapse is halted, reaching the
theoretical unpleasantness of the singularity. And in 2024, Ralli and Vidati updated the picture to
describe what the plank star eventually evolves into. Let's zoom back out to the event horizon
again. With the interior plank star frozen by time dilation, the event horizon itself slowly shrinks
as it leaks Hawking radiation. Remember that this is a problem if it causes the black hole to vanish
and take its precious quantum information with it. Now, just as loop quantum gravity arrests the
plank star collapse, it also stops the final stage of evaporation of the event horizon. In essence,
the surface area of the horizon becomes quantized and can't decay any further. That
leaves us with a plank relic, a plank length event horizon that's stuck that way forever.
And these are actually a possible prediction of quantum gravity in general. And of course,
we've talked about them before. But what about the frozen plank star within that plank relic event
horizon? Now remember that the internal plank star was much bigger than a plank length. So trillions
of times bigger than the plank relic that's supposed to contain it. But this is actually
what happens. Just as time dilation freezes the plank star rebound, the enormous stretching of
space within that near pointlike event horizon holds a plank star a trillion times larger. But
the weirdness doesn't end there. As the shrinking event horizon approaches the plank scale,
it is subject to strong quantum effects. And one of those is the possibility of quantum tunneling
from the black hole state into the white hole state. That same white hole can also transition
back into a black hole and the cycle can repeat indefinitely, leaving our plank relic and the star
it contains in a quantum superposition of black hole and white hole simultaneously. Okay, we've
come a long way since the gigantic dark stars of centuries ago. Now, nature seems pretty intent
on forming event horizons. But maybe we could narrowly avoid the singularity if we follow the
path of loop quantum gravity, the final stage of collapse may be simultaneously near point like a
knot, flickering eternally between being about to explode and about to collapse. As an added bonus,
quantum information is preserved in the relatively gigantic pocket within that infinite decimal spec.
Oh, and these things might explain dark matter, too, but that would require a terrifyingly large
number of these balls of big bang energy locked like genies in moes of frozen spacetime. Thank