PBS Space Time

To progress to the next level in understanding reality, we need to combine quantum mechanics and Einstein’s general relativity. And to do that, most physicists believe we need a theory of quantum gravity .. which means we need gravitons. But it also seems like the laws of physics make it impossible to ever detect this quantum particle of gravity. Almost like the universe is set up to keep the final answer forever out of our reach. So, can we outsmart the universe, catch a graviton, and finally solve physics?

The universe thrums with quantum fields; and the particles of matter and force emerge as vibrational manifestations of the deep symmetries of these fields. The layers and reflections of those symmetries give us the wonderful richness of what we call the standard model of particle physics. Except there seems to be something missing: the sterile neutrino.

Physicists have long believed that detecting the particle of gravity—the graviton—was fundamentally impossible, with the universe itself seeming to block every direct attempt. This episode explores a new generation of clever experiments that may finally let us detect gravity’s particle, and why even succeeding wouldn’t quite mean what we think it does.

We've known that the universe is expanding since 1929, and that its expansion is accelerating since 1998. The culprit behind the acceleration is unknown, so we live with a stand-in term "dark energy". Our modern cosmological model assumes that dark energy has a constant density--always the same amount of the outward-shoving stuff per volume. But there's recent evidence to the contrary--which may be why our primary efforts to measure the expansion rate of the universe disagree with each other.

Some people worried that the large hadron collider would smash particles together so hard it would make black holes that would swallow the earth, open wormholes to other dimensions. It didn’t and won’t. But it may be making a different kind of portal. A portal to the dark sector. This isn’t the Rather, it’s a hypothetical family of elementary particles that exists in parallel to the familiar particles of the standard model, but are invisible to it. Invisible to us, and so could be the answer to the dark matter conundrum. And what is this portal? It’s the particle that the LHC was built to find in the first place - the Higgs boson.

Kurt Gödel discovered a solution to General Relativity that allows time travel without any exotic physics, revealing that the theory doesn’t actually guarantee a consistent chain of cause and effect . His “Gödel universe” shows that under certain conditions, the structure of spacetime itself can loop back on itself—blurring the line between past and future and exposing a deep limitation in our understanding of reality.

Is Earth’s core a solid or a liquid? Yes. The mysteries of our own planet’s interior have, in many ways, been harder to crack than those of the rest of the cosmos. We can send probes to the edge of the solar system, and the 42 billion light years to the cosmic horizon are largely transparent—a big enough telescope can see the most distant galaxy. But the 6400km to Earth’s center are both opaque to light and far beyond the reach of any conceivable drill. The best we can do for most of our planetary depths is to listen to the faint rumblings of distant earthquakes and then try to piece together how those seismic waves bounce around the interior.

The James Webb Space Telescope found galaxies that are too ancient-looking for our young universe. You may have heard that, but it keeps finding them, and our recent efforts to solve this conundrum point in wildly different directions. Have we found galaxies older than the universe, or did we just learn something incredible about how galaxies form?

For a while back there 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 at the surface from which not even light could escape from a 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 Planck star—a ball of energy at the heart of the black hole like frozen shards of the Big Bang. Let’s hope they’re real, for physics’ sake.

We’ve been looking for messages from the stars ever since Frank Drake pointed the Green Bank radio telescope at Tau Ceti and Epsilon Eridany 65 years ago. He saw nothing that couldn’t be explained by natural causes. Nor have the much more extensive SETI surveys conducted since. So, maybe there are no alien signals to see. Or maybe we need to update how we search for them. We have, after all, learned an awful lot since 1960—both about the galaxy and about observing the galaxy.

Don’t panic, but it's possible that an event horizon is forming right behind you right now. Don’t bother looking—you won’t see it. You won’t know that it’s growing to envelop you until it’s way too late. And it definitely won’t look like the classic picture of a black hole that you’re probably familiar with.

Modern physics is weird. In quantum mechanics we have the Schrodinger’s cat thought experiment, a cat in a quantum superposition of being simultaneously alive and dead. And in relativity the twin paradox thought experiment, a space-faring twin ages less than her stay-at-home sibling. But what if we could combine these and send our quantum twin around the galaxy. Could we be simultaneously old and young? What would that tell us about the quantum nature of time? And would we need schrodinger birthday candles?

Last year, the Dark Energy Spectroscopic Instrument—DESI—made headlines around the world. Its enormous map of the Universe hinted that dark energy—the mysterious something causing the expansion of the Universe to accelerate—might not actually be constant. Last month, another giant cosmology experiment released its final results. The Dark Energy Survey. DES. Its results are intriguing. But did they confirm the DESI measurement? We’ll, let’s see, but there’s also a much more interesting question—Why are completely different scientific studies so much more powerful in combination than any one study on its own? The answer brings us to the heart of what science really is.

LIGO may have detected a black hole that shouldn't exist, potentially revealing the first evidence of primordial black holes formed in the earliest moments after the Big Bang. But the bigger story is how a decade of gravitational-wave astronomy has transformed from proving Einstein right into a mature science capable of uncovering entirely new physics.