Thank you to brilliant.org for supporting
PBS. What if every single black hole that formed
in our universe sparked the big bang of a new universe? Cosmological natural selection proposes exactly
this - but even better, it claims to be able to test the hypothesis. Physicists have been struggling for some time
to figure out why our universe is so comfy. Why, for example, are the fundamental constants
- like the mass of the electron or the strength of the forces - just right for the emergence
of life? Tweak them too much and life, stars, galaxies,
the universe as we know it wouldn’t exist. In recent episodes we explored one possible
explanation for this - the anthropic principle and the idea of the multiverse. If there are countless universes with different
fundamental constants, then it’s not surprising that a few exist with the right numbers for
life - and certainly not surprising that we find ourselves in one of those good ones. But if you don’t like the anthropic principle
- and many scientists don’t - then rest assured, there’s an alternative. You only need to accept two things: that our
universe formed inside a black hole, and that universes can evolve. Our universe appears, in some sense, designed. It has finely tuned parameters that seem deliberately
set for a particular outcome - life. There’s another example in nature where
the illusion of design has a perfectly natural explanation - and that’s life itself. We now know that the fantastic complexity
of living organisms is an inevitable consequence of evolution by natural selection. Inspired by biological evolution, theoretical
physicist Lee Smolin came up with Cosmological Natural Selection. It goes like this: the formation of a black
hole triggers the formation of a new universe “on the other side” in a new big bang. Those daughter universes go on to expand and
make their own black holes and hence their own daughter universes. But in their formation the fundamental constants
of the daughter universes are shifted slightly and randomly from their parent - mutations
are introduced. Some of those shifts improve the daughter
universe’s ability to form new black holes. Those universes have an advantage in propagating
their cosmic genetics, and so gradually the ensemble of all universes get better and better
at making black holes, just as biological organisms with helpful mutations can get better
at surviving and reproducing. Now by happy chance there’s a correlation
between making lots of black holes and making life - both require stars. The universe that is better at making stars
is better at making planetary systems is better at making us. Seems fair enough. But is it any more than a cool story? Bro? Let’s take this apart to ask two questions:
is it plausible, and is it testable? First up, for any of this to make sense black
holes need to create universes. This is by far the most speculative part. In fact we have no idea, and only very tentative
reasons to think so. The idea originated with one of Lee Smolin’s
mentors, Bryce deWitt, who postulated that when a black hole collapses, its mass doesn’t
all end up stuck in the central, infinitely dense singularity. Rather it sort of bounces - but unable to
exit the event horizon of the black hole, it forms a new region of spacetime, effectively
creating a new universe. The details of how this happens is presumably
buried in the as-yet-unknown theory of quantum gravity. There are various proposals for how such a
bounce might happen - all of which are massively speculative, and perhaps we’ll cover another
time. John Archibald Wheeler expanded on the procreating
black hole idea by suggesting that the fundamental constants of these new universes could be different
to their parents. This seems plausible - if fundamental constants
can change at all then surely it’s in the highest possible energy environments, which
is exactly the end of a black hole collapse. Perhaps the configuration of the geometry
string theory’s extra dimensions gets shifted - this would do the job. Inspired by this idea, Smolin added one thing:
what if, when universes reproduces, the constants aren’t randomly reconfigured but rather
change only slightly - analogous to a small number of genetic mutations. If that were the case, a sort of evolution
by natural selection would become as inevitable as biological evolution. We have no good reason to believe any of this
procreating universe stuff - and Lee Smolin has readily admitted that. The point is to instead ask: what if it’s
true? What are the consequences? And can we test them? The exponential nature of the proposed process
means that the ensemble of all universes should very quickly be dominated by ones that are
extremely good and making black holes. Any given universe may not be totally optimal
because its constants varied randomly from its parent - in the same way that any given
living organism isn’t the paragon of its kind. So there’s a prediction: the fundamental
constants that define black hole production should be close to optimal in a given universe,
at least for a given mechanism for making black holes. In our modern universe, black holes are made
when the most massive stars explode as supernovae. There are other ways to make black holes,
and we’ll come back to them. So we should expect our universe to be optimized
for producing as many of the most massive stars as possible. Well is it? It’s actually very hard to say, but it does
seem like there’s some fine-tuning there. Stars are formed when giant clouds of gas
collapse under their own gravity. But in order for that to happen the gas needs
to cool to just a few degrees above absolute zero, rather than the typical 200-kelvin temperature
of the typical interstellar nebula. That cooling is extremely slow if the gas
only contains the hydrogen and helium produced in the big bang. Heavier elements and molecules allow clouds
to cool and stars to form much more quickly, and of these, carbon monoxide is by far the most important coolant. In addition, gas needs to be shielded from
the heating effect of other stars - and that seems to require the presence of tiny particles
of ice and hydrocarbon dust. So without carbon, oxygen, water, and chemistry
in general, far fewer stars and so far fewer black holes would form - and of course these
factors also seem to be essential for life. But what about other sources of black holes? Theoretical physicist and cosmologist Alexander
Vilenkin proposed that if a universe lasts forever then in the distant future, quantum
fluctuations of that near vacuum will cause black holes to spontaneously appear - and
given infinite time these will eventually outnumber those produced by stars or stellar
black holes. If all this is true then the most black holes
would be produced by the biggest universes - more space means more chances for these
quantum fluctuations. That favours lots of dark energy generating
rapid expansion. And that is definitely not our universe. Lee Smolin has various arguments against this:
for example, we don’t know that our physics can really be extrapolated to the insanely
long timescales required for these quantum fluctuations to happen. I would also add that even if Vilenkin’s
argument holds, there are no doubt different regions in the landscape of possible fundamental
constants where different types of black hole are optimized. This would lead to multiple branches of the
cosmic genetic tree - some of which correspond to producing lots of stellar black holes. And naturally we’d find ourselves on one
of those branches because those also happen to be the ones that favour life. But, whoopsie, I just invoked the anthropic
principle, which is exactly what were trying to avoid with this whole idea. As speculative as all of this is, Smolin claims
there’s a concrete test for the idea. If cosmological natural selection is true,
then the fundamental parameters favouring black hole production should be optimized
completely independently to those that also favour the appearance of life. And he suggests there is one such parameter. But first some background. When massive stars die, they actually mostly
produce neutron stars - planet sized balls of neutrons so dense that they teeter on the edge
of collapsing into a black hole. Black holes only form when the neutron stars
is above a certain mass limit. Now it may be that in the cores of the most
massive neutron stars, some particles can convert into strange quarks. The resulting material is even denser than
the original neutron star, and so brings the star closer to collapse. And the lower the mass of the strange quark,
the easier it is to convert lighter particles into strange quarks. That in turn means less massive neutron stars
would be able to collapse into black holes. Surely, then, if universes evolve to maximize
the number of black holes, then the strange quark mass should be optimized to make the cutoff
between neutron stars and black holes as low as possible. Lee Smolin calculates that optimized cutoff
at around 2 times the mass of the Sun. So, if this universe is optimized for black
hole production then there should be no neutron stars more massive than 2 solar masses. And? Well, the most massive known neutron star is 2.17
solar masses, discovered just this year. Now perhaps the extra .17 can be factored into the uncertainties of the theory... Or perhaps this is the falsification we were
looking for. We await Smolin’s comments on this. I’d like to add my own objection: cosmological
natural selection is meant to explain the fine tuning in the fundamental constants,
which appear to be either set by design or by extreme luck. It tries to avoid the anthropic principle
by proposing a natural selection that favours black hole production, and it’s just a happy
coincidence that the same factors also favor life. But then do we really gain anything? It just so happens that carbon and oxygen
are good for both black hole production and organic molecules ... but what if it was,
I dunno, beryllium and boron that helped stars form - or other elements that were useless to life. If we causally disconnect the selection process
for cosmic reproduction from the emergence of life then it seems we still have to invoke
a good lot of good luck? Overall, cosmological natural selection is
an appealing idea because it seeks a natural explanation for fine tuning, and one that
parallels a known process in nature - biological evolution by natural selection. It also seems to give us predictions that
we can try to test and falsify. And even though this idea is probably not
true, it’s really important to remember that speculative ideas like this are exactly
how we probe the edges of science. No one of them is likely to be true, but they
help us explore the vast space of all possible realities - where somewhere is hidden the
true nature of our reality. Or, you know, our universe's momma might be a black hole, and we live in an endlessly evolving, proliferating space time. Thank you to brilliant.org for supporting
PBS. If you want to understand astrophysics you’re
going to need to have a solid understanding of relativity. Brillaint.org has a course on special relativity
that includes interactive challenges and problems to solve. A hands-on approach can guide you through
thinking strategies for challenging subjects like relativity. In this course you’ll
begin by understanding Einstein’s postulates and the Lorentz transformations, and as your
knowledge advances you'll learn how faster than light travel can break causality and
you'll even solve the famous twin paradox. To learn more about Brilliant, go to brilliant.org/Spacetime. Hey everyone, thanks for watching - your support each week is what makes this show possible. Now, totally optional, but one way to help out even more is to become a patreon contributor. Even 2 bucks a month gets you access to our
hopping discord channel. Thanks a ton if you’ve already joined us,
and today an extra special huge thanks to Big Bang supporter Craig Stonaha. Craig,
as a small token of our appreciation we’ve contacted our friends at the large hadron
collider - they’re going to make you a black hole universe. Please email us with the configuration of
fundamental constants you’d prefer and we’ll get it right out to you. Oddly enough it’s the same shipping company
as our Merch Store, which you can check out at pbsspacetime.com Last week we talked about the doomsday argument
- the unnerving idea that, statistically speaking, there are aren't likely to be vastly more generations
of humans ahead of us than there have been in the past. There were a lot of counter arguments - perhaps
good ones because apparently we're still here. Many people come up with a similar objection. I'll quote Mr Fantastic, who articulated it
well: A human born 2 million years ago would come to the conclusion that the end of the
world is nigh, and so would a human born 2 million years from now. If we accept the reasoning of the doomsday
argument, doesn't this just mean that everyone, for all of history, would come to the conclusion
that we're all going to die sooner rather than later? This point it totally valid - in fact a cro-magnon
should reach the same conclusion and predict doom long before the 21st century - and obviously they would be wrong. But the doomsday argument isn't saying that
every member of a species who employs this reasoning to predict their species doom is
going to be right. It says that the majority will be right - if
they predict that there will be a similar number of future generations as past generations. And by similar, I mean within a factor of
a few. So ancient philosophers would have got it
wrong - and perhaps we'll eventually also be wrong ancient philosophers to some very
distant future generation. The point is if any given individual assumes that we're randomly sample from all generations who thought about the doomsday argument then chances are they didn't come near the start of their species. Now, the real problem with the doomsday argument
isn't that users of it in the distant past would be wrong. Rather, it's that it's not at all clear that
it's reasonable to count ourselves as "randomly selected" from all of the users of the doomsday argument. Zahaqiel highlights this trickiness in defining
reference class with a great example: Step 1: Define reference class as "homo sapiens
sapiens existing concurrently with the internet". Step 2: Observe that the internet has existed for
approximately 30 years. Step 3: Assume self-sampling assumption makes
sense and internet access is bell curved over over the duration of its existence or skewed towards
late-phase access... Then the internet's going to cease to exist some
time in the next 30 years guys. No one highlights another misuse of this idea. To quote: "What are the odds that I'm born
as the prince of France?", asked the prince of France. Well, from the Prince of France's perspective, the answer to that question is a probability of 1. From everyone else's perspective, the answer is a probability of 0. And this really highlights the challenge in identifying our "reference class" for this sort of anthropic reasoning. Now, the Prince of France knows that he's the Prince of France, so from his perspective the likelihood that he's the Prince of France
is indeed 1. But imagine the Prince of France was raised secretly in a normal French family. He doesn't know who he is - all he knows is
that someone in the population is the Prince of France. If you ask him for the probability that he's
the Prince of France he should probably say 1 in 33 million, or whatever the male population
of France is. The point is that you need to take into account prior knowledge when you're defining your reference class. Nick Bostrom has another nice example of misusing
the doomsday argument. Adam and Eve really want to ... you know,
hook up. Except they're afraid of God's wrath if Eve
gets pregnant. The serpent comes along and explains that
according to the doomsday argument the chances of Eve getting pregnant are nearly zero. After all, it's incredibly unlikely that Adam
and Eve are the first two out of billions or trillions of future humans - therefore, odds are, they can have all the fun they want without risk of spawning an entire species. But an even better doomsday absurdity was
from thatisjustgreat who says "I was 30 seconds in when I realized the video is probably almost
over". By that logic, this video is only half way
through.
[removed]
Fun fact
Using the equation to determine the size of a black hole - the schwarzschild radius (the size of the event horizon of a black hole for a given mass) you can calculate the size of a black hole that has the mass of the observable universe, it's event horizon would be the size of the observable universe. Most Astrophysicists think this is just a coincidence, but we'll probably never know for sure.
Turtles all the way down?
Nope. Black holes all the way up!
One could just as easily say as soon as our universe reaches it's heat death the speed of expansion of the universe will start to stretch out black holes until they "explode" outward to create another birth of the universe. Kind of like a puddle rippling outward. But I'm just pulling things out of my ass.
I've digested and retained more psychics knowledge from SpaceTime and Matt O'Dowd than I ever did in high school or university.
Bless PBS
PBS Space Time has been going at some pretty fringe stuff lately and still come up with way too long videos about it.
I dont think we can ever trully know anything about it in our lifetimes, so i always end up reading all i can about the subject and then i pick the solution or explanation that suits me the most. Thats how i believe scientists create thesis like this one. To be honest when i read about some paper about how tame and space "flip" in black holes i thought about this idea in the back of my mind and then after i imagined how all the energy of our universe came out of "nowhere" and the space is stretching exponentially so if we imagine a black hole in critical mass that colapses in a instant and all sorts of forces involved, and if a big enough black hole forms with enough matter inside it, one might say it is a whole new universe inside it. I mean this thoughts of mine are corelation not causation to this article but it made me remember my imagination from years back.
While I personally believe this, I think this guy essentially doesn't answer anything.
Probably dumb questions but wouldn’t more black holes cause each consecutive set of universes to get smaller? And wouldn’t natural selection favor the fastest creation of new black hole universes possible? I like the idea of black holes creating universes but I’m still trying to wrap my head around this theory.