Whether you're a paleontologist digging for fossils, or a cosmologist staring into the depths of the universe, some knowledge is simply lost to time.

Like, no matter how much we look at dinosaur bones, we'll just never see a dinosaur sneeze. And we'll never truly know just how many would-be planets Jupiter flung out of our solar system.

Those moments have passed and your feelings of FOMO are valid. But it turns out, that thanks to gravity the universe itself has, like, a sort of memory. Massive objects leave subtle but lasting marks in the fabric of spacetime, like grooves on a vinyl record. So far, we can't play those memories back like a record, but scientists do have a starting point for where to look for them.

And as out technology improves, these messages carved in spacetime could help us get to the bottom of some of the universe's biggest mysteries.

Just like a bunch of other brain-bendy cosmological phenomena, the idea that the universe has some kind of memory traces its roots back to Einstein's theory of general relativity. In 1916, Einstein predicted that an object with any amount of mass should give off gravitational waves as it accelerates through spacetime. In other words, it should pinch and stretch the fabric of space itself, much like the ripples moving across the surface of a pond. Except the kind of pinching and stretching we can detect here on earth is a far, far more subtle effect.

I'm talking, like, subatomic scales. So scientists didn't actually detect gravitational waves until 2015, but that didn't stop them from trying way earlier than that. Take the physicist Joseph Weber; back in the 1950s and 60s, he constructed giant aluminum bars that were supposed to act like tuning forks, and were meant to start vibrating as soon as a strong enough gravitational wave washed over them. In 1969, Weber declared that he'd finally detected the signals he was looking for, which set the physicists scrambling.

People all over the world built their own giant tuning forks to see if they could reproduce Weber's cosmic hum, but there was nothing but silence. No gravitational waves, nothing to be heard, except for Weber's insistence that his signals were real. So a few years into this particular scientific tea, one pair of physicists were like, "Okay, let's ignore Weber's results for a moment and lets just calculate how sensitive would a device like this need to be in order to detect gravitational waves?"

They used Einstein's general relativity to run the math on some hypothetical scenarios involving objects like black holes that could send out some of the most powerful gravitational waves in the universe. And well basically, there was just no way that Weber's device picked up what he claimed it had. Those super-powerful gravitational waves were far, far too weak for his fancy little tuning fork to pick up. But while they were calling Weber's bluff, there was another little nugget that turned up in their findings. The math revealed that as a gravitational wave passes through two objects that are at rest relative to each other, they don't return to the exact same positions they were in before, and instead, they're slightly just displaced. That meant that gravitational waves didn't just wrinkle up spacetime and then return it to its previous state, like ocean waves rolling under a boat - they changed spacetime in a lasting way. In other words, the universe remembers every gravitational wave that moves through it, whether it's from two black holes spiraling towards each other or you waving at your neighbor. But not every gravitational memory forms the same way. Physicists recognize a few different origin stories. One reason has to do with the way objects with mass curve spacetime around them. Up in space, you might imagine a scenario where two black holes briefly swing by each other and fling each other off in different directions. And two things are happening here: they radiate gravitational waves because they're accelerating, but by merely existing, each also creates like this sort of dip in spacetime like a marble sitting on a mattress. This curving of spacetime is their gravitational field, and as they move through space, like accelerating or not, that dip moves with them. So to ground this, let's pretend that you're at the beach, floating just offshore in your favorite inflatable, when a wave rolls through. That wave picks you up and puts you right back down, so you'd expect to end up in the exact same place where you began, except this is the ocean and right now the tide is going out. So in the time that the wave took to roll underneath you, the water level has dropped ever so slightly, which means you're not quite in the same position as where you began. Now in the case of gravitational waves, the motion that creates the wave is also what creates this shift in the fabric of spacetime. 

So, back to our beach analogy, we'd technically want a single event that created the wave and lowered the water level, but ultimately this is what matters: that slight offset in your position tells you a gravitational wave passed through, whether or not you saw the wave move you.

Now if those two black holes aren't moving fast enough to fly off in opposite directions after that encounter, they'll get trapped in a binary system and start swirling around each other. And when this happens, we've got another way that gravitational waves leave a lasting mark on space. And that is, the gravitational waves themselves.

See, gravitational waves carry energy. And as Einstein taught us before he got into all the gravity stuff, energy and mass are equivalent. So gravitational waves themselves? They kinda have mass? Which means they kind of have their own gravity. And that gravity warps spacetime just a little bit. This warping interferes with the warping being done by the gravitational wave, kind of like if a wave rippling through a pond also created a splash that then gave off its own little ripples. That extra interference leaves a mark on spacetime that lingers after a gravitational wave moves through.

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So, physicists lump both of these lasting effects under the term "Gravitational Memory." And they're both super small, like, we're talking about detecting something that's just a fraction of the entire gravitational wave signal, which is already really weak to begin with- exhibit A: Weber's tuning forks.

But detecting gravitational memory isn't out of the question, because scientists have managed to detect about 100 gravitational waves in the past decade. That's thanks to experiments like LIGO: The Laser Interferometer Gravitational-Wave Observatory. Interferometers are used to study a bunch of different astrophysical phenomena, so they don't all look the same. But LIGO features two long arms that are normally the exact same length, but they alternately stretch and contract anytime a gravitational wave passes through. And by bouncing a laser beam down the arms, scientists can measure that minuscule change in the length.

LIGO has already made dozens of confirmed detections, and each one of those signals should also include tiny displacements from gravitational memory too. But, because of the way LIGO is designed to account for Earth's gravity when making these detections, all of those little displacements automatically get reset, erasing all trace of a memory after a wave has passed through. But even if it didn't do that, LIGO still isn't sensitive enough to tease all those extra subtle signals out of the data. At least, not yet, because some researchers think that by combining the data from hundreds or thousands of gravitational wave detections, they might be able to pick out the faint signal of gravitational memory.

Meanwhile, scientists are also working to build even better gravitational wave detectors. The European Space Agency is planning to launch a space-based interferometer called LISA in 2035. It'll be made of three separate spacecraft, forming the points of an otherwise invisible triangle just beyond Earth's orbit. Because LISA will be up in space, the parts that get shifted due to gravitational waves won't need to be held in place by anything the way they do in LIGO, and that means they'll stay displaced after the event. So, LISA is expected to be able to detect the signals of gravitational memory in just one event- not one hundred, not one thousand- one. But you might be wondering, "Why even bother hunting for this teeny-tiny needle in a slightly less tiny haystack that doesn't look nearly as cool or informative as, like, fossilized dino tracks?"

Well, scientists think that there could be all kinds of information in the Universe's "memory." More importantly, it could help us understand one of the most fundamental questions in physics: how accurate is general relativity really? 'Cause, sure, it's one of the most successful theories we've ever had in physics and it's the theory that led us to gravitational memory in the first place, but it doesn't have all the answers. For one thing, it doesn't agree with quantum mechanics- that other incredibly useful theory that describes how the Universe works- about what happens in extreme gravitational fields. 

So, physicists want to figure out just how right general relativity is and where it breaks down, so gravitational memory might give us a way to answer that. And since black holes are super extreme gravitational environments where general relativity gets pushed to its limits, they are an obvious target to study.

One aspect that physicists are especially interested in is a special event called "ringdown." It's what happens just after two black holes merge into one larger black hole, when the whole thing does a special little wobble as it settles down. It only lasts for a fraction of a second, but, in that moment, the system sends out powerful gravitational waves. And these gravitational waves carry information about the objects that made them, like, how much mass they had to start out with and how fast they were spinning. But, in order to extract this information from a ringdown signal, physicists need to be able to decypher it really precisely and they're still working on that. But, one day, they might be able to tease out the memory signal from a gravitational wave signal, and, if so, they'd be able to confirm whether or not black holes perfectly match what's described by general relativity or if anything is a little bit off.

One clue that something might be wrong with general relativity would be if any real black holes out there in the Universe seem to have any defining features other than their mass and their spin. Because, according to general relativity, that's all you really need to know to describe a black hole. It's as if a person's only features were their height and weight, and, in that case, two people that were 170 cm tall and weighed 82 kg would be 100% identical; forget about any defining features, like eye color or hairstyle. This is called the "No-Hair Theorem," as in black holes don't have anything like hair to set them apart from each other.

But, maybe gravitational memories could suggest they have some other features, some "hair," as it were. And, if black holes have "hair," that's a sign that general relativity needs an update.

Gravitational memory might also reveal how we might unite general relativity and quantum mechanics into some quantum theory of gravity. And it just might come in the form of a solution to another decades-old conundrum called the "Black Hole Information Paradox." Once again, the paradox boils down to the fact that quantum mechanics and general relativity just don't play nice when you plop them both onto a black hole jungle gym. 

General relativity says that any information that falls into a black hole can never get out. And then quantum mechanics says that information can't be created or destroyed. And the paradox pops up when you realize that quantum mechanics also predicts that black holes slowly shrink over time, shedding completely random particles until one day they disappear entirely. In other words, the information that was contained within them gets destroyed, unless it somehow gets saved somewhere before the black hole goes "poof!" And that's where the gravitational memory could come to the rescue, maybe.

Imagine you have a shoe falling into a black hole and, as it falls in, it emits some gravitational waves and these should contain a memory component. But, while that means you get some information about the shoe encoded into the space-time outside the black hole, we haven't solved the black hole information paradox just yet. Because none of that information describes the ultimate quantum nuances of the shoe's existence, its "Quantum State," if you want to use the appropriate jargon. That all went into the black hole with the shoe, or so we thought.

This next part is still controversial, but, back in 2016, Stephen Hawking and some of his colleagues proposed that as matter falls into a black hole, the corresponding gravitational memory also contains information about the matter's quantum state. Their reasons for proposing this are pretty complicated, so we won't go into the details, but, if they're right, anything falling into a black hole could give a black hole a kind of "hair" that would hold quantum information, which Hawking and his colleagues called "soft hair." In which case, the information we thought was lost had just been saved in a new format and there's actually no paradox after all. If we could eventually paint a precise picture of black holes based on the information in their ringdown signal, we might be able to detect "soft hair."

Like any other kind of black hole hair, it could show up as a discrepancy between real-world observations and models based on general relativity.

Of course, all of this is hypothetical until we actually manage to detect these gravitational memory signals and figure out what's actually in them. For now, it's as if we think someone launched a message in a bottle written in code from far, far away. So, first we have to find it, and then we have to get to work decoding it and seeing what kind of secrets it holds. It's a long shot, but, all things considered, we're not that far off from seeing what the Universe remembers.

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