Speaker 1: Welcome to Bedtime Astronomy. Explore the wonders of the cosmos
Speaker 1: with our soothing Bedtime Astronomie podcast. Each episode offers a
Speaker 1: gentle journey through the stars, planets, and beyond, perfect for
Speaker 1: unwinding after a long day. Let's travel through the mysteries
Speaker 1: of the universe as you drift off into a peaceful
Speaker 1: slumber under the night sky.
Speaker 2: So I was actually walking through the Museum of Modern
Speaker 2: Art a few months ago.
Speaker 3: Oh nice, Yeah, trying to get.
Speaker 2: Some culture, you know it is, And I found myself
Speaker 2: just kind of standing in front of that small, almost
Speaker 2: unassuming canvas by Salvat or.
Speaker 3: Dolly the persistence of memory.
Speaker 2: Exactly, the melting clocks, right, you know, the pocket watches
Speaker 2: draped over dead tree branches and sliding off tables looking like,
Speaker 2: I know, slices of cheese left out in the sun.
Speaker 3: It is a very striking visual.
Speaker 2: It really is. And usually when you look at that painting,
Speaker 2: you think about the psychology of time. You think about
Speaker 2: how an hour in a dentist's chair feels.
Speaker 3: Like a week, Oh yeah, absolutely, and.
Speaker 2: An hour with a good friend feels like a minute.
Speaker 2: We did to think of memory and time as these
Speaker 2: purely internal fluid.
Speaker 3: Experiences, completely subjective, right.
Speaker 2: Completely subjective. But standing there looking at those warped watches,
Speaker 2: I started wondering, well, I started wondering if Dolly was
Speaker 2: accidentally stumbling onto a literal truth about physics.
Speaker 3: That is a fascinating leap, because we tend to think
Speaker 3: of the physical universe, you know, the hard vacuum of
Speaker 3: space as this rigid container.
Speaker 2: A clock is a clock, a meter is a meter.
Speaker 3: The stage where things happen exactly the stage.
Speaker 2: But what if the contenter itself? What if the stage
Speaker 2: actually melts? What if the universe has a memory and
Speaker 2: that memory looks like a permanent physical distortion.
Speaker 3: That is honestly the perfect mental anchor for what we
Speaker 3: are unpacking today.
Speaker 2: I thought it fit pretty well.
Speaker 3: It does because in the world of general relativity, specifically
Speaker 3: in the study of gravitational waves, there is a phenomenon
Speaker 3: literally called memory capital M memory capital M, and it
Speaker 3: suggests exactly what you just said. It suggests the universe
Speaker 3: isn't just a static stage that events happen on. It
Speaker 3: suggests the stage itself is recording the play.
Speaker 2: And just so you listen, are clear, we aren't talking
Speaker 2: about metaphor here, No, not at all. We aren't talking
Speaker 2: about history in the sense of writing it down in
Speaker 2: a physics textbook. We are talking about the actual fabric
Speaker 2: of space time getting bent by a catastrophic event and then,
Speaker 2: and this is the crazy part, never bending back.
Speaker 3: That is the core concept. It is a permanent scar
Speaker 3: on the geometry of reality.
Speaker 2: A permanent scar. So for this deep dive, we are
Speaker 2: looking at a brand new study. This is published in
Speaker 2: Physical Review Letters in February twenty twenty.
Speaker 3: Six rash off the presses, right.
Speaker 2: It's titled the Persistence of Gravitational Wave Memory, and it's
Speaker 2: led by Antonio Soccaros and his team.
Speaker 3: Who are out of the University of Illinois at Urbana
Speaker 3: Champagne and the Academy of Athens I believe, Yeah.
Speaker 2: Along with with collaborators for the University of Valencia and
Speaker 2: Montclair State. And they've basically been simulating what happens when
Speaker 2: two of the most violent objects in the cosmos slam
Speaker 2: into each.
Speaker 3: Other binary neutron stars.
Speaker 2: Right. And our mission today for this deep dive is
Speaker 2: to really understand how the universe records its own history.
Speaker 3: It is quite the mission.
Speaker 2: It is we need to figure out why neutron stars
Speaker 2: are so much more complicated than black holes, what role
Speaker 2: ghost particles like Neutrino's playing all this, and ultimately why
Speaker 2: this new study is considered such a massive stress test
Speaker 2: for Einstein's theory.
Speaker 3: It is the ultimate stress test, but it takes something
Speaker 3: we thought we understood, gravitational waves, and adds a layer
Speaker 3: of messiness that is frankly terrifying to simulate the mess.
Speaker 2: We love the mess, But before we get to the
Speaker 2: MESSI stress test, let's set the baseline good idea, because
Speaker 2: we need to understand the difference between a ripple and
Speaker 2: a scar. Most people listening have probably heard of gravitational
Speaker 2: waves by now. Ligo detected them back in twenty fifteen.
Speaker 3: Nobel prizes were handed.
Speaker 2: Out exactly we know that and big things crash space ripples.
Speaker 2: But to understand the new stuff, we need to know
Speaker 2: what exactly we are crashing together in this specific study.
Speaker 3: Right, so, we are looking at binary neutrons stars, two
Speaker 3: neutron stars orbiting each other.
Speaker 2: Give me the physical reality check here. I know they
Speaker 2: are dense, but help me visualize what we're actually dealing with.
Speaker 3: Sure, so imagine a star that started out much much
Speaker 3: larger than our own Sun. It burns through all its
Speaker 3: nuclear fuel over millions of years, and then the core collapses.
Speaker 3: It goes supernova, the big boom, the biggest. The outer
Speaker 3: layers blow off into space, but the core itself gets
Speaker 3: crushed inwards by its own gravity. It is squeezing so
Speaker 3: hard that the electrons and protons inside the atoms are
Speaker 3: literally forced together to become neutrons.
Speaker 2: So it's basically just a city sized atomic nucleus.
Speaker 3: That's the classic analogy, and it holds up perfectly. You
Speaker 3: have a perfect sphere, maybe twelve to fifteen miles across,
Speaker 3: the size of Manhattan or London. Yeah, but it contains
Speaker 3: up to two times the mass of our entire Sun.
Speaker 2: That density is just it's hard to even wrap your
Speaker 2: head around.
Speaker 3: It is. Try this. If you took a teaspoon of
Speaker 3: neutron star material, literally just a sugar cubesworth, it would
Speaker 3: weigh about a billion tons.
Speaker 2: A billion tons, Yeah, in a teaspoon.
Speaker 3: That's roughly the weight of Mount Everest compressed into a spoon.
Speaker 2: Okay, wow, So we are taking two of these Everest
Speaker 2: spoon stars, spinning them around each other at a fraction
Speaker 2: of the speed of light and just smashing them.
Speaker 3: Together exactly, And as they spiral in towards each other,
Speaker 3: they churn up the fabric of space time. They create
Speaker 3: gravitational waves.
Speaker 2: Now, the standard explanation, you know, the one I always
Speaker 2: see in headlines, is the ripple in the pond analogy, right,
Speaker 2: the classes You throw a rock into the water and
Speaker 2: ripples move outwards. If I'm a rubber duck floating on
Speaker 2: that pond, I just bob up and down as the
Speaker 2: ripple goes by.
Speaker 3: Right, And in this analogy, the rubber duck represents a
Speaker 3: detector here on Earth, like Lego or maybe two astronauts
Speaker 3: floating in space. Okay, As the gravitude wave passes through them,
Speaker 3: the actual space between them stretches and squeezes. They get
Speaker 3: closer than further, than closer than further, Bobby, exactly. That
Speaker 3: is the oscillatory part of the wave, the back and forth.
Speaker 2: And this is the key thing I want to focus on.
Speaker 2: Once the wave passes, the bond goes flat again, the
Speaker 2: rubbert duck ends up exactly where it started. The astronauts
Speaker 2: return to their original distance.
Speaker 3: That is what happens with a Spandard wave. Yes, it's temporary.
Speaker 3: The universe is assumed to be elastic, it snaps back.
Speaker 2: But the memory effect that this paper's about, yeah, says well,
Speaker 2: it says that's wrong.
Speaker 3: It says it's mostly right, but not entirely right. General
Speaker 3: relativity actually predicts that, in addition to that back and
Speaker 3: forth oscillation, there is a DC.
Speaker 2: Component DC like direct current.
Speaker 3: Yes, a direct current, a permanent offset.
Speaker 2: So to go back to the astronauts in space.
Speaker 3: The wave passes, they bob back and forth. But when
Speaker 3: the wave is completely gone and space is quiet again,
Speaker 3: they look at each other and realize they are now
Speaker 3: slightly further apart or slightly closer together than they were
Speaker 3: before the.
Speaker 2: Wave hit, and they stay that way forever.
Speaker 3: They do not snap back. The ruler itself has been
Speaker 3: lengthened or shortened. Space time has been permanently displaced.
Speaker 2: That is haunting. I mean, it implies that every single
Speaker 2: time two black holes or neutron stars merge anywhere in
Speaker 2: the universe, the geometry of the entire cosmos gets shifted
Speaker 2: a tiny bit.
Speaker 3: It does. It's a very very small shift, usually undetected
Speaker 3: by our current instruments because it's so subtle. But mathematically
Speaker 3: it has to be there the universe literally remembers the
Speaker 3: event by changing its shape.
Speaker 2: Okay, so that's the what. It's a permanent scar on reality.
Speaker 2: But I really want to get into the why, because
Speaker 2: looking at the history of this theory and the sources,
Speaker 2: it seems like physicists have been arguing about this for
Speaker 2: like fifty years.
Speaker 3: Oh easily. It's definitely not a new idea. Even if
Speaker 3: these specific supercomputer simulations are new. The foundation goes all
Speaker 3: the way back to the nineteen seventies.
Speaker 2: Nineteen seventy four with Zoldovich and.
Speaker 3: Polmer f yes Yakov Zeldovitch and Alexander palm Reft, two
Speaker 3: brilliant Soviet physicists. They were working with what we call
Speaker 3: linearized gravity.
Speaker 2: Linearized gravity, which is basically the light version of Einstein's equation.
Speaker 3: Exactly the light version, the simpler approximation, and even in
Speaker 3: that leet version, they realized that if you have a
Speaker 3: system of stars that permanently changes its arrangement, say a
Speaker 3: cluster of superdense stars suddenly flying apart, the gravitational field
Speaker 3: at a great distance creates this prominent shift.
Speaker 2: It was just a theoretical prediction at that.
Speaker 3: Point, purely theoretical The logic was simple, though. If masses
Speaker 3: move away to infinity, the metric of space time has to.
Speaker 2: Change, so that's what we call the linear memory. The
Speaker 2: stars moved, so the gravity changed basic cause and effect.
Speaker 3: Right, But then came the nineties and things got profoundly weird.
Speaker 2: Entered Demetrio's Christa d'lieu.
Speaker 3: Yes. In nineteen ninety one, he published a paper that
Speaker 3: is still considered a massive landmark in the field. He
Speaker 3: didn't use the light version. He looked at the full,
Speaker 3: unadulterated nonlinear Einstein equations.
Speaker 2: I actually want to pause on that word. Non linear.
Speaker 2: We hear it all the time, right, non linear dynamic
Speaker 2: is non linear storytelling? In this context? Does it just
Speaker 2: mean complicated?
Speaker 3: It means that the output of a system literally becomes
Speaker 3: an input for the system.
Speaker 2: Okay, give me an example.
Speaker 3: In a linear system, you have a choir singing. That's
Speaker 3: the source, and the sound waves are the output. The
Speaker 3: sound waves travel through the air to your ear. They
Speaker 3: don't really interact with each other.
Speaker 1: Right.
Speaker 2: If the choir sing's louder, the sound is just louder.
Speaker 2: It's a straight line from cause to effect exactly.
Speaker 3: But in a nonlinear system, imagine the choir sings so
Speaker 3: incredibly loud that the sound waves actually compress the air
Speaker 3: enough to heat it up. That heat changes the density
Speaker 3: of the air in the room, which then changes how
Speaker 3: the sound travels, which might create entirely new sound frequencies.
Speaker 3: The sound itself is changing the medium it travels through.
Speaker 2: The output messages of the input. So apply that to gravity.
Speaker 3: Well. Einstein's great insight was that mass equals energy E
Speaker 3: equals mc squared. You have mass, you have gravity. But
Speaker 3: if you have energy, you also have gravity.
Speaker 2: And gravitational wave they carry energy exactly.
Speaker 3: We know they carry energy because we can detect them.
Speaker 3: They physically move the heavy mirrors in the Lego detectors.
Speaker 3: That takes actual work. It takes energy. So Chris Dedooley realized,
Speaker 3: if gravitational waves carry energy, then gravitational waves must have
Speaker 3: their own gravitational feel Waitt.
Speaker 2: The gravity has gravity.
Speaker 3: The gravity has gravity.
Speaker 2: That sounds like a recursive loop that would just immediately
Speaker 2: crash a computer.
Speaker 3: It makes the math incredibly difficult, which is exactly why
Speaker 3: it took from nineteen seventy four until nineteen ninety one
Speaker 3: to mathematically prove it. Christ to Doos showed that as
Speaker 3: these waves ripple out from a collision, the massive energy
Speaker 3: in the waves themselves creates a secondary gravitational distortion.
Speaker 2: So it's not just the physical stars smashing together that
Speaker 2: creates the memory scar it's the waves themselves piling up
Speaker 2: as they travel.
Speaker 3: Yes, this is called the nonlinear memory, where the Christad memory,
Speaker 3: the waves effectively gravitate, They pull on space time as
Speaker 3: they travel through it, and this effect accumulates. It builds
Speaker 3: up over the entire duration of the wave signal.
Speaker 2: So we have the linear memory, which is just the
Speaker 2: stars moved, and the nonlinear memory, which is the energy
Speaker 2: of the scream scarred the air.
Speaker 3: That is a fantastic way to put it, the energy
Speaker 3: of the scream scarred the air. And for a long time,
Speaker 3: that combined picture was all we had, and we modeled
Speaker 3: this mostly using.
Speaker 2: Black holes because black holes are clean.
Speaker 3: They are the simplest macroscopic objects in the universe. A
Speaker 3: black hole has mass, it has spin, and it can
Speaker 3: have an electric charge. That's it. No surface, no atmosphere,
Speaker 3: no chemistry, just pure curved space time. So when you
Speaker 3: program a computer to simulate two black holes merging, you
Speaker 3: don't have to worry about fluid dynamics or magnetic fields
Speaker 3: or particle physics. It's pure gravity.
Speaker 2: But the universe isn't always clean, and that brings us
Speaker 2: to the actual meat of the new study by Socaros
Speaker 2: in his team. They didn't want the clean version. They
Speaker 2: wanted the full messy reality of neutron stars.
Speaker 3: And neutron stars are distinct because they are actual physical
Speaker 3: objects made of ultra dense matter. They bring extra ingredients
Speaker 3: to the party. They have magnetic fields, they emit light,
Speaker 3: and crucially they emit neutrinos.
Speaker 2: And so Kros is essentially asked, do these extra ingredients
Speaker 2: actually contribute to the permanent space time scar? Yes, which
Speaker 2: is a really fair question. Yeah, because if energy equals gravity,
Speaker 2: and neutrinos carry energy and magnetic fields hold energy, shouldn't
Speaker 2: they leave a mark too?
Speaker 3: Exactly? And to be fair, previous researchers had laid the
Speaker 3: groundwork for this, people like Lydia Bieri, Poning, Chen Shingtong Yao,
Speaker 3: and Dave Garfinkel. They had done the pen and paper
Speaker 3: math to prove that theoretically electromagnetic fields and neutrinos should
Speaker 3: contribute to the memory.
Speaker 2: I saw in the sources. This is sometimes called null memory.
Speaker 3: Why null Oh? It relates to radiation traveling along what
Speaker 3: physicists call null infinity, which is basically the mathematical boundary
Speaker 3: or path that light, rays and massless particles take to
Speaker 3: get infinitely far away from a source.
Speaker 2: Nold infinity has to say, physics has the best names.
Speaker 2: It sounds like an indie rock band.
Speaker 3: It really does. But the point is, while the theory
Speaker 3: for all memory was there, no one had successfully run
Speaker 3: a full scale numerical relativity simulation of a binary neutron
Speaker 3: star merger that actually quantified this.
Speaker 2: No one, if we put numbers to it, right.
Speaker 3: No oneted tract the neutrinos and the complex magnetic fields
Speaker 3: and said, okay, here's exactly how much of the memory
Speaker 3: they create until now.
Speaker 2: So let's get into the simulation. They built a digital universe,
Speaker 2: They put two massive neutron stars in it. They gave
Speaker 2: them realistic magnetic fields and neutrino emissions. What did they find?
Speaker 3: They found that the mess matters a lot.
Speaker 2: Put a number on it for me.
Speaker 3: The simulations suggest that the contributions from the extra ingredients,
Speaker 3: the magnetic fields, the neutrinos, and the buryonic ejecta, which
Speaker 3: is the physical matter thrown out of the crash. Account
Speaker 3: for anyone where from fifteen percent to fifty percent of
Speaker 3: the total gravitational wave memory.
Speaker 2: Fifty percent up to half of the scar on the
Speaker 2: universe is caused by the extras.
Speaker 3: Yes, if you were to simulate this event and just
Speaker 3: treat the neutron stars like black holes light, completely ignoring
Speaker 3: the neutrinos in the matter, you could be wrong by
Speaker 3: a factor of two. You'd be missing half the story.
Speaker 2: That completely changes the picture. It means you absolutely cannot
Speaker 2: just look at the gravity of the masses. You have
Speaker 2: to look at the particle physics.
Speaker 3: It forces a total marriage between gravity and nuclear physics.
Speaker 2: Take the neutrinos for example. I usually think of them
Speaker 2: as these ghostly little things that don't interact with anything,
Speaker 2: like there are millions passing through.
Speaker 3: My thumb right now, trillions actually trillions.
Speaker 2: And normally they are totally gravitationally insignificant, right.
Speaker 3: But a neutron star merger is not a normal environment.
Speaker 3: When these stars crash together, the temperature spikes to billions
Speaker 3: of degrees. The nuclear matter is literally being crushed and
Speaker 3: boiled at the same time.
Speaker 2: What does that do.
Speaker 3: It releases a flood of neutrinos that is frankly hard
Speaker 3: to comprehend. The paper mentions a total energy release of
Speaker 3: around ten to the fifty three ergs.
Speaker 2: Yeah, I saw that, and I know ergs is a
Speaker 2: unit of energy, but I have zero intuition for that
Speaker 2: is ten to the fifty three a lot.
Speaker 3: It's an astronomical number. For context, our Sun over its
Speaker 3: entire ten billion year lifespan will emit something like ten
Speaker 3: to the fifty one ergs of energy.
Speaker 2: Wait, so this single crash releases one hundred times more
Speaker 2: energy in just a few seconds just in neutrinos than
Speaker 2: the Sun does in its entire ten billion year life.
Speaker 3: Exactly. It is a sudden explosion of energy that briefly
Speaker 3: outshines the entire galaxy in neutrino radiation. And because E
Speaker 3: equals mc squared, that massive energy has a mass equivalence.
Speaker 2: So as these trillions of neutrinos stream out of the
Speaker 2: crash site in all directions, they're effectively carrying an immense
Speaker 2: amount of mass away with them.
Speaker 3: Right, imagine the crash site is losing weight incredibly rapidly.
Speaker 3: That sudden chain in the mass distribution that lightning of
Speaker 3: the central load causes a massive shift in the gravitational
Speaker 3: field that is the neutrino memory.
Speaker 2: It's like if you were standing on a trampoline holding
Speaker 2: a heavy bowling ball. The trampoline is curved down around you.
Speaker 2: If you suddenly throw the bowling ball away, the trampoline
Speaker 2: snaps to a new shallower shape.
Speaker 3: That's a very good analogy for the linear part of it.
Speaker 3: The mass has moved, so the curve changes. But there's
Speaker 3: also the nonlinear part we talked about. The neutrinos themselves
Speaker 3: are dragging space time as they fly outward, and the
Speaker 3: study showed this combined effect is a huge component of
Speaker 3: the final memory signal.
Speaker 2: And there's a specific visualization in the paper they mentioned
Speaker 2: that really caught my eye. A blue rectangle. Can you
Speaker 2: describe what we're looking at there?
Speaker 3: Sure? So this is a graph showing the strain, basically,
Speaker 3: the memory accumulation over time. On the horizontal axis you
Speaker 3: have time in milliseconds. On the vertical axis, you have
Speaker 3: the memory building up.
Speaker 2: And what exactly does the blue rectangle highlight?
Speaker 3: It highlights the time duration over which ninety percent of
Speaker 3: the post merger memory is accumulated. For a black hole merger.
Speaker 3: The memory happens in a literal snap. It's a step
Speaker 3: function on a graph. Nothing nothing s and app full memory,
Speaker 3: instant scarring. But for neutron stars, the blue rectangle shows
Speaker 3: that the memory builds up gradually. It takes tens of milliseconds,
Speaker 3: which sounds fast, but is an absolute eternity in this
Speaker 3: specific physics context for the memory to reach its full height.
Speaker 2: Why the delay? Why doesn't it just snap like a
Speaker 2: black hole.
Speaker 3: Because the neutrinos take time to leak out of the
Speaker 3: ultra dense core. The ejected matter, these massive clouds of
Speaker 3: gold and platinum dust, takes time to expand outward, and
Speaker 3: the remnant itself, the hyper massive blob of matter left
Speaker 3: over from the crash, wobbles and slashes around for a
Speaker 3: while before finally settling down.
Speaker 2: So it's a slow motion scar exactly.
Speaker 3: And that slope how fast or slow the memory builds
Speaker 3: up tells you immense amounts about the internal physics of
Speaker 3: the explasion.
Speaker 2: Now, speaking of the internal physics, we have to talk
Speaker 2: about the magnetic fields because this is where my own
Speaker 2: naive assumption totally failed me.
Speaker 3: While I was reading what was your assumption.
Speaker 2: Okay, so my logic was pretty straightforward. I know these
Speaker 2: stars are giant magnets. Magnetars are some of the scariest
Speaker 2: things in space, and I know energy equals gravity. Magnetic
Speaker 2: fields contain energy.
Speaker 3: Solid logic, so far right.
Speaker 2: So if I take two stars with super strong magnetic
Speaker 2: fields and smash them together, I'm adding more energy to
Speaker 2: the whole system than if they were unmagnetized. More energy
Speaker 2: should mean more gravity, which should mean a much bigger
Speaker 2: memory scar.
Speaker 3: That is entirely logical. It is what we call the
Speaker 3: naive expectation. And I don't mean that as an insult
Speaker 3: at all. It's exactly what many physicists expected too before
Speaker 3: they ran the numbers.
Speaker 2: But the simulation said no.
Speaker 3: The simulation showed that in some cases, adding incredibly strong
Speaker 3: magnetic fields actually reduced the final gravitational wave memory.
Speaker 2: How was that possible? Where did all that magnetic energy go?
Speaker 3: The energy is absolutely there, but it's not making gravitational waves.
Speaker 3: You have to remember a key rule. Gravitational waves are
Speaker 3: generated by asymmetry.
Speaker 2: Asymmetry, yes, they are.
Speaker 3: Generated by what we call the quadrupole moment. Basically, you
Speaker 3: need a lumpy, uneven wobbling distribution of mass to shake
Speaker 3: space time.
Speaker 2: A perfect sphere doesn't make waves.
Speaker 3: Correct. Even if the perfect sphere is spinning incredibly fast,
Speaker 3: it is symmetric, no waves are produced. You need the wobble. Okay, now,
Speaker 3: think about what a strong magnetic field does to a
Speaker 3: ball of highly conducting fluid, which is essentially what a
Speaker 3: hot neutron star is.
Speaker 2: It holds it together.
Speaker 3: It creates magnetic viscosity. It creates tension. Imagine stirring a
Speaker 3: cup of water with a spoon. It splashes everywhere, it swirls.
Speaker 3: It's highly turbulent and chaotic. Right now, imagine stirring a
Speaker 3: cup of cold honey or molasses.
Speaker 2: It moves way slower. It's stiff.
Speaker 3: Exactly. The strong magnetic field acts like a stiffener. It
Speaker 3: links the fluid elements inside the star together when the
Speaker 3: stars crash. If the magnetic field is strong enough and
Speaker 3: shaped a certain way, it actually suppresses the fluid turbulence.
Speaker 3: It prevents the massive sloshing and forces the resulting merged
Speaker 3: blob into a more symmetric, stable shape much faster.
Speaker 2: So the magnetic field acts like a straight jacket for
Speaker 2: the star. It stops the blob from slashing around as much.
Speaker 3: And less slashing means less asymmetry. Less asymmetry means less
Speaker 3: gravitational radiation. So even though you have physically more energy
Speaker 3: stored in the magnetic field itself, you have less dynamic
Speaker 3: shaking to create the memory signal.
Speaker 2: That is wildly counterintuitive. It's like saying that crashing a
Speaker 2: sports car at high speed might actually do less structural
Speaker 2: damage to the environment if the car is made of
Speaker 2: a much stiffer material.
Speaker 3: In a way, yes, the internal dynamics matter more than
Speaker 3: the raw energy count alone, and the study found this
Speaker 3: memory dampening depends heavily on the topology of the field.
Speaker 2: Copology that just means the shape of the magnetic field right.
Speaker 3: Yes, is the magnetic field aligned perfectly with the star's spin?
Speaker 3: Is it tilted perpendicular? Is it a toroidal field meaning
Speaker 3: it's shaped like a doughnut entirely inside the star. The
Speaker 3: study showed an intricate dependence on these shapes.
Speaker 2: Meaning you can't just guess the outcome based on energy.
Speaker 3: You absolutely cannot guess. You have to run the supercomputer
Speaker 3: code for every specific scenario.
Speaker 2: This brings us to the real face. So what of
Speaker 2: the episode We've got these amazing simulations. We know that
Speaker 2: neutrinos and magnets and gold dust all contribute up to
Speaker 2: half of this scar in the universe. But we haven't
Speaker 2: actually seen this yet, have we. Logo hasn't flashed a
Speaker 2: red light saying memory detected.
Speaker 3: Not yet. No, we have successfully detected the oscillatory waves,
Speaker 3: the chirp of the merger as they spiral in. But
Speaker 3: the memory is a very low frequency effect. It's a
Speaker 3: permanent DC offset, and current detectors just can't see that.
Speaker 3: Current ground based detectors like LIGO are not great at
Speaker 3: seeing things that happen slowly or just stay put.
Speaker 2: Why is that because of the Earth exactly.
Speaker 3: Think about how LIGO actually works. It's a series of
Speaker 3: heavy mirrors hanging on delicate glass threads. It's looking for
Speaker 3: high frequency vibrations. If the mirror moves and then just
Speaker 3: stays moved, it's incredibly hard for the computer to tell
Speaker 3: if that was a permanent gravitational wave memory, or if
Speaker 3: the tectonic plate under the building just shifted slightly, or
Speaker 3: if a truck drove by, or even if the laser
Speaker 3: slightly heated up the mirror.
Speaker 2: The permanent shift just gets lost in the everyday noise
Speaker 2: of the planet.
Speaker 3: It just looks like instrument drift. But with next generation
Speaker 3: detectors like the Einstein Telescope, which they are planning to
Speaker 3: build deep underground in Europe, or LYSA, which will be
Speaker 3: a triangle of satellites flying millions of miles apart in space,
Speaker 3: we should finally be able to separate this subtle memory
Speaker 3: signal from the noise.
Speaker 2: And when we do find it, what does it actually
Speaker 2: tell us? Why are so many physicists dedicating their careers
Speaker 2: to hunting for this scar.
Speaker 3: Because it is the ultimate forensic tool. If we can
Speaker 3: cleanly read the memory signal, we can essentially reverse engineer
Speaker 3: the dead star.
Speaker 2: This connects back to a term I saw all over
Speaker 2: the sources, equation of state. I really want to drill
Speaker 2: down this. What exactly is an equation of state?
Speaker 3: In physics? An equation of state is simply a mathematical
Speaker 3: rule that connects density to pressure. For the air in
Speaker 3: your car tire, it's simple physics. You can press it
Speaker 3: the pressure goes up linearly. But for the exotic matter
Speaker 3: inside a neutron star, we genuinely don't know the rule
Speaker 3: because we.
Speaker 2: Can't build a neutron star in a lab on Earth
Speaker 2: to test it right.
Speaker 3: We cannot squeeze normal matter to nuclear densities on Earth
Speaker 3: without it just becoming a bomb. So we don't know
Speaker 3: what happens at the very core. Is the core squishy,
Speaker 3: is it extremely stiff, Is it made of regular free neutrons,
Speaker 3: or does it melt down into a bizarre soup of
Speaker 3: free quirks. We just don't know.
Speaker 2: And whether it's stiff or squishy changes how it crashes drastically.
Speaker 3: A stiff star strongly resists collapsing it bounces. A soft,
Speaker 3: squishy star smushes together very easily. That physical difference changes
Speaker 3: how the whole merger happens, which changes the shape of
Speaker 3: the gravitational waves, which ultimately changes the slope and size
Speaker 3: of the permanent memory scar.
Speaker 2: So if we detect the memory scar with Lisa, we
Speaker 2: could look at its precise shape and say, Aha, that
Speaker 2: scar means the star was squishy. Therefore the core couldn't
Speaker 2: have been pure quark matter precisely.
Speaker 3: The space time memory inherently encodes the hidden nuclear physics
Speaker 3: of the core. It's a way to look deep inside
Speaker 3: an object that we can never ever visit.
Speaker 2: And beyond the forensics of the star itself, it's also
Speaker 2: a massive test for the fundamental theory of gravity.
Speaker 3: It is the stress test we mentioned at the start.
Speaker 3: General relativity is over one hundred years old now it
Speaker 3: has famously passed every single test.
Speaker 2: We've thrown at it, every single one.
Speaker 3: But the memory effect relies heavily on that nonlinearity. We
Speaker 3: discussed the wild idea that gravity creates its own gravity.
Speaker 2: So if we build Lisa and we look for the
Speaker 2: memory exactly where the simulations say it should be and
Speaker 2: it's just not there.
Speaker 3: Then Einstein was wrong, or at the very least his
Speaker 3: theory breaks down at these extreme high energies.
Speaker 2: That would be the biggest news in physics in a century.
Speaker 3: It really would. Or more likely, maybe we find the
Speaker 3: memory but it looks totally different than Sakarosa's simulations predict
Speaker 3: That would tell us we don't understand Neutrino's nearly as
Speaker 3: well as we thought, or that matter behaves in some
Speaker 3: entirely new way when extreme magnetic fields are involved.
Speaker 2: It's a win win for science. Either we definitively confirm
Speaker 2: the wildest prediction of the theory, or we break the
Speaker 2: theory and find new physics.
Speaker 3: That's the beauty of experimental astrophysics.
Speaker 2: I want to zoom out a bit as we wrap
Speaker 2: up here. We've been talking heavily about the math and
Speaker 2: neutrinos and fluid dynamics, but I keep coming back to
Speaker 2: that Dolly painting in the museum. And there was a
Speaker 2: specific quote from Sacroo's in the press release that really
Speaker 2: struck me.
Speaker 3: Ah, the one about the worldline.
Speaker 2: Yeah, he said, similarly to our own persistent memory, which
Speaker 2: is shaped by the worldline of our lives, binary compact
Speaker 2: objects also develop a persistent memory, as described by Einstein's
Speaker 2: theory of gravity.
Speaker 3: It's a beautifully poetic connection for a physicist to make.
Speaker 3: In relativity, a world line is just your path through
Speaker 3: four dimensional space time from the moment of your birth
Speaker 3: to your death. You trace a unique line.
Speaker 2: And he's essentially saying that, just like our lives leave
Speaker 2: physical memories in the neurons of our brains, the violent
Speaker 2: lives of these stars leave actual physical memories in the
Speaker 2: fabric of the universe itself.
Speaker 3: It implies that the deep history of the cosmos isn't
Speaker 3: just gone, it's physically recorded. The geometry of the universe
Speaker 3: right now today is the cumulative some of all the
Speaker 3: massive collisions and mergers that have ever happened since the
Speaker 3: Big Bang.
Speaker 2: Which leads to a sort of terrifying thought for me.
Speaker 2: If these mergers have been happening for billions of years
Speaker 2: across the whole universe, billions of black holes, billions of
Speaker 2: neutron stars, and each single one stretches space a tiny
Speaker 2: little bit and leaves a permanent scar.
Speaker 3: You're wondering if the universe is getting wrinkly, I.
Speaker 2: Am, is the universe smoother in the distant past and
Speaker 2: more discorded now, or we essentially accumulating cosmic trauma in
Speaker 2: a very.
Speaker 3: Real mathematical sense, Yes, the texture of the gravitational field
Speaker 3: is getting richer, more complex. It's constantly being worked over
Speaker 3: by these extreme events. We are living in a universe
Speaker 3: that is physically geometrically different than it was yesterday, precisely
Speaker 3: because of the events that happened today.
Speaker 2: We aren't just actors on a stage. We're on the
Speaker 2: states that is constantly warping and stretching beneath our feet, and.
Speaker 3: Thanks to studies like this one, we are just now
Speaker 3: learning how to read those warps.
Speaker 2: So what's next? For the team. They've proved the concept
Speaker 2: with this simulation.
Speaker 3: Sakaros calls this a first Foray. Now comes the real
Speaker 3: computational grind. They need to run hundreds, maybe thousands of
Speaker 3: these simulations, tweaking all the variables right, different masses, different
Speaker 3: magnetic field shapes, different equations of state for the core.
Speaker 2: That basically need to build a library.
Speaker 3: Exactly, a comprehensive catalog, so when Lisa or the Einstein
Speaker 3: Telescope finally turns on and catches a memory signal, the
Speaker 3: scientist can look it up in the library and say, oh,
Speaker 3: that matches simulation four hundred exactly. It was a one
Speaker 3: point four solar mass binary with a stiff core and
Speaker 3: ateroidal magnetic.
Speaker 2: Field, decoding the universe's scars.
Speaker 3: Multi messenger astronomy at its absolute finest.
Speaker 2: Well, my brain is suitably stretched. For one day, we
Speaker 2: went from melting clocks in New York to the density
Speaker 2: Mount Everest and at scone to the mind bending idea
Speaker 2: that gravity itself has weight.
Speaker 3: Just another day in astrophysics to.
Speaker 2: Recap for everyone listening out there, So you have this
Speaker 2: locked in one space. Time isn't rigid, It has a memory.
Speaker 2: Big crashes leave permanent physical scars on the geometry of
Speaker 2: the universe. Two neutron star mergers are incredibly messy. You
Speaker 2: absolutely cannot ignore the extra ingredients. Neutrinos and ejected matter
Speaker 2: account for up to half of that memory signal.
Speaker 3: It mass matters.
Speaker 2: Three magnetism is weird. It can actually reduce the toll
Speaker 2: memory signal by stiffening the star and making the resulting
Speaker 2: crash smoother and more symmetrical.
Speaker 3: The counterintuitive finding and four detecting this memory is how
Speaker 3: we eventually crack open the neutron star to see what's
Speaker 3: actually inside the core. A perfect summary, I want to leave.
Speaker 2: You with one final provocative thought. Today we talked a
Speaker 2: lot about the universe remembering about carrying the physical record
Speaker 2: of its own violent past. We usually think of the
Speaker 2: laws of physics as completely timeless, eternal. But if the
Speaker 2: actual structure of space itself is continuously evolving, if it's
Speaker 2: constantly accumulating these tiny permanent shifts, these memories, right, does
Speaker 2: that mean the universe we live in today is fundamentally
Speaker 2: structurally different from the universe of the Big Bang, not
Speaker 2: just because it's older or larger, but because it's scarred.
Speaker 2: Is the gravity holding you to the ground today exactly
Speaker 2: the same as gravity yesterday. Or are we all just
Speaker 2: floating in the microscopic wreckage of a billion cosmic car crashes,
Speaker 2: finally trying to make sense of the damage.
Speaker 3: That is a very heavy thought to end on.
Speaker 2: That's exactly why we call it the deep dive. Thanks
Speaker 2: for listening, everyone, keep looking up. See next time.
Speaker 3: Us SA