WEBVTT

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Thank you to Displate for supporting PBS. For 
a while back there, we thought we might be able  

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to avoid the black hole. They'd been lurking as 
shadows in our theories of gravity forever. Enough  

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mass crammed into a small enough space would 
lead to a gravitational field from which not  

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even life could escape from the surrounding 
surface that we call the event horizon. The  

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event horizon generates paradoxes that worry 
physicists. And the singularity of infinite  

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density within the black hole worries them even 
more. And so many brave physicists have fought  

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for centuries to prove that these monsters 
don't exist. They hoped nature would step  

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in to save us from the theoretical horror 
of ultimate gravitational collapse. One of  

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our final hopes is the plank star, a ball 
of energy at the heart of the black hole,  

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like frozen shards of the Big Bang. Well, 
let's hope they're real for physics's sake.

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it lights up after sun exposure, even in the 
depths of space. Now, on to the episode. First,  

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there were the dark stars of Mitchell and 
Lelass, constructed with only Newtonian gravity.  

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These things were gigantic, 500 times the 
size of the sun in Mitchell's mind. Happily,  

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they aren't possible. Giant clouds of gas fragment 
and collapse before a dark star can form. But any  

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matter collapsed far enough will have an event 
horizon. Those collapsing gas fragments would  

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form black holes themselves if they were not 
saved by the onset of nuclear fusion as internal  

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temperature and pressure spikes. The resulting 
outflow of energy counters the gravitational  

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crush, birthing a true star, which saves us from 
collapse until nuclear fuel runs out. Then the  

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collapse must continue. But happily, not to 
a black hole. Not yet. At extreme densities,  

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think an entire star crammed into the volume of 
the Earth, new strange quantum effects come to  

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the rescue. The electrons of the stellar core 
are crushed until all available quantum states  

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are filled and they cannot be forced together 
anymore. The resulting electron degeneracy  

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pressure halts collapse once again, giving us 
a white dwarf. Nature seems to have stepped in  

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to halt the absurdity of the black hole. So far so 
good for our hero physicists. We have Mitchell and  

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Lelass getting us into trouble in the first place 
with dark stars. Then Arthur Edington figured out  

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that stellar fusion halts collapse and it was 
Ralph Fowler applying the brand new field of  

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quantum mechanics that gave us white dwarfs. 
But then Subramanion Chandra Seca came along  

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and at only 19 years old on an ocean voyage from 
India to Cambridge to begin working with Fowler,  

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he proved that even his new bosses white dwarfs 
have a failure point. For any white dwarf 40% more  

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massive than the sun, gravitational crush will 
always exceed the outward electron degeneracy  

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pressure. In fact, by including Einstein's 
relativity to the quantum descriptions,  

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Chandra found that outward pressure no longer 
rises fast enough to resist the rising crush  

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as a white dwarf gains mass, leading to runaway 
collapse. Edington was famously very annoyed by  

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this result and disputed it. He was convinced that 
nature must prevent such absurdities as infinite  

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collapse. But Chandra was right about the white 
dwarf collapse. Maybe Edington will still be right  

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about the ultimate infinite collapse. There's 
one last resppite for collapsing stellar cores  

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when physicists realize that electron capture by 
protons could halt collapse as a neutron star. But  

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the salvation of the stellar core is limited in 
this case. The more mass of the neutron star, the  

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more compact it becomes. For neutron stars over 
a certain mass, the surface gravity stops light  

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from escaping and the dreaded event horizon forms. 
And that's it. We've lost our battle to stop black  

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holes a long time ago. In fact, they are real. 
We've seen them in their extreme gravitational  

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effects across the universe and in gravitational 
waves and now even in images. They are real. And  

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frankly, I think that's awesome. You might like 
them, too. I mean, you clicked on this episode.  

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So, what's this about physicists wanting to avoid 
the event horizon? Well, people were uncomfortable  

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with the idea of black holes for good reasons. 
And the best reason emerged in the 70s when  

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Steven Hawking and others showed that black 
holes slowly radiate away their mass, shrink,  

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and ultimately vanish. The main problem with this 
is that all information of everything swallowed by  

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a black hole is deleted in that process. This 
violates a core tenet of quantum mechanics,  

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information conservation. The other problem with 
the formation of an event horizon is that there  

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is no known process that can stop matter within it 
from collapsing into a point of infinite density  

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in the center. This singularity generates plenty 
of its own problems. Not least of which is that in  

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these conditions, the two theories that we use 
to get this far, quantum mechanics and general  

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relativity, are so conflicted that they can't 
be simultaneously true. Black holes point to  

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fundamental flaws in our theories of nature. Okay, 
so even if we couldn't prevent the event horizon,  

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maybe we can at least stop the formation of the 
theorybreaking singularity. New generations of  

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physicists took up the ancient battle to save us 
from this theoretical catastrophe. Most believed  

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that the solution must lie in a union of quantum 
mechanics and general relativity. For example,  

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string theory proposed fuzz balls in which matter 
unravels into its stringy weave filling the region  

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beneath the horizon. And we covered that already. 
Another possible solution is the plank star. a  

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star of near absolute collapse supported in the 
last instant only by the grainy structure of  

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spacetime itself. This comes from what has been 
called the main competitor of string theory loop  

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quantum gravity. In LQG, space at the tiniest 
scales is blocky. In particular, it's built up  

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of quantiz 2D area elements whose interplay looks 
like 3D space on larger scales. And here larger  

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means anything significantly bigger than the plank 
length around 10 theus 35 m. If LQG is right,  

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then it should give us the same spacetime as 
described by general relativity on larger scales.  

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And that needs to be true of any quantum gravity 
theory. But none have been completely worked out  

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and so there's some guesswork in connecting 
the plank scale to the scale of GR. One way to  

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do that is using so-called semiclassical gravity 
which guesses the pertabbations to the equations  

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of general relativity as we approach the plank 
scale. And this is how Carlo Revelian co got to  

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their first picture of the plank star. It actually 
came from an effort to describe what might happen  

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if the entire universe collapsed like a reverse 
big bang. As densities become extreme enough,  

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LQG predicts a semiclassical correction to the 
cosmological equations, the Freriedman equations,  

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in which an anti-gravity-like effect emerges, 
causing the collapse to bounce outwards. This  

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loop quantum cosmology is meant to describe an 
infinitely expanding and contracting universe with  

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loopy bounces between cycles. But in 2014, Ralli 
and Franchesco Vidatoto showed how this result  

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for a collapsing universe could also be used to 
approximate the end result of the collapsing star  

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inside a black hole. At a certain point, outward 
quantum pressure causes it to bounce. In a way,  

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it's analogous to the quantum degeneracy pressures 
that stopped our white dwarf and neutron stars  

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from collapsing further. In that case, it was 
quantum particles unable to occupy the same energy  

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levels. But with the plank star, it's the quantum 
elements of spaceime itself doing the work. If  

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LQG is right, this could handily stop the wicked 
singularity from ever forming. For a collapsing  

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sun mass star, the resulting object would be 
about 1 trillionth of a meter. That's small,  

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but it's not pointlike. It's not a singularity. In 
fact, it's 23 orders of magnitude larger than the  

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plank scale. So, relatively gigantic. This ultra 
compressed ball of matter is the plank star that  

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we've been teasing. The thing about plank stars 
is that they shouldn't last for long, at least in  

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this semiclassical description given back in 2014. 
The same space-time pressure that stops collapse  

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also triggers catastrophic rebound. Now, a similar 
thing happens when the core of a massive star is  

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halted by the formation of a neutron star. The 
rest of the star explodes outwards as a supernova.  

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But in the case of the plank star, the resulting 
rebound is a white hole. Basically, the opposite  

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of a black hole, the time reversal, space and 
the energy it contains rushing outwards and an  

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event horizon that reverses its direction. All of 
this takes place in about the time it takes light  

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to cross the size of the plank star, which is a a 
tiny fraction of a second. So, it sounds like our  

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plank star only exists for an instant after the 
black hole formation before exploding outwards.  

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But in that case, why do we still see black holes 
out there? And why don't we see the flashes of  

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extreme energy expected when a white hole forms? 
Maybe you've seen the film Interstellar and you  

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remember that time close to a black hole event 
horizon slows down from the point of view those  

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waiting for Matthew McConnA back on Earth. Hours 
can translate to years. Well, imagine how strong  

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that gravitational time dilation would be from 
deep below the event horizon. Strong. A rebounding  

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plank star would appear frozen in that state 
for billions of years for anyone but the plank  

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star. So there you have it. ultimate gravitational 
collapse foiled again in a 10 the -12 m wide ball  

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of energy that looks like the universe as it was 
essentially at the big bang but at least it's not  

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pointlike or even actually plank scale but we're 
not quite safe yet the description I just gave you  

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of the plank star is over a decade old it involves 
some serious approximations the quantum gravity  

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effects were approximated as a modification to 
the standard equations of general relativity and  

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the collapsing star was approx approximated as 
a collapsing universe, which really means that  

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its matter was smooth and infinitely extended. 
Not really what a collapsing star looks like.  

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Nonetheless, yet again, we have a mechanism 
by which the collapse is halted, reaching the  

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theoretical unpleasantness of the singularity. And 
in 2024, Ralli and Vidati updated the picture to  

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describe what the plank star eventually evolves 
into. Let's zoom back out to the event horizon  

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again. With the interior plank star frozen by time 
dilation, the event horizon itself slowly shrinks  

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as it leaks Hawking radiation. Remember that this 
is a problem if it causes the black hole to vanish  

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and take its precious quantum information with 
it. Now, just as loop quantum gravity arrests the  

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plank star collapse, it also stops the final stage 
of evaporation of the event horizon. In essence,  

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the surface area of the horizon becomes 
quantized and can't decay any further. That  

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leaves us with a plank relic, a plank length 
event horizon that's stuck that way forever.  

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And these are actually a possible prediction 
of quantum gravity in general. And of course,  

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we've talked about them before. But what about the 
frozen plank star within that plank relic event  

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horizon? Now remember that the internal plank star 
was much bigger than a plank length. So trillions  

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of times bigger than the plank relic that's 
supposed to contain it. But this is actually  

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what happens. Just as time dilation freezes the 
plank star rebound, the enormous stretching of  

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space within that near pointlike event horizon 
holds a plank star a trillion times larger. But  

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the weirdness doesn't end there. As the shrinking 
event horizon approaches the plank scale,  

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it is subject to strong quantum effects. And one 
of those is the possibility of quantum tunneling  

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from the black hole state into the white hole 
state. That same white hole can also transition  

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back into a black hole and the cycle can repeat 
indefinitely, leaving our plank relic and the star  

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it contains in a quantum superposition of black 
hole and white hole simultaneously. Okay, we've  

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come a long way since the gigantic dark stars of 
centuries ago. Now, nature seems pretty intent  

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on forming event horizons. But maybe we could 
narrowly avoid the singularity if we follow the  

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path of loop quantum gravity, the final stage of 
collapse may be simultaneously near point like a  

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knot, flickering eternally between being about to 
explode and about to collapse. As an added bonus,  

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quantum information is preserved in the relatively 
gigantic pocket within that infinite decimal spec.  

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Oh, and these things might explain dark matter, 
too, but that would require a terrifyingly large  

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number of these balls of big bang energy locked 
like genies in moes of frozen spacetime. Thank