Transcript

The Universe Itself Might Be Hiding the Gravity Particle From Us

From PBS Space Time · Episode page · Watch on YouTube

· 20 min · 2,808 words · 23 timestamped sections

This is the full, timestamped transcript of "The Universe Itself Might Be Hiding the Gravity Particle From Us", an episode of PBS Space Time published January 13, 2026 (20 min, about 2,808 words). It mentions 32 people, works and ideas, including quantum gravity, Freeman Dyson, Planck length, gravitational waves, gravitational waves.

What this episode is about

The discussion centers on the challenges of detecting gravitons, the theoretical quantum particles associated with gravity, and how the universe appears to inherently obstruct such detection. Citing insights from physicist Freeman Dyson, it explores the notion that both practical and fundamental barriers may prevent scientists from uncovering this key element of quantum gravity, raising questions about our ability to fully understand the universe's underlying structure.

Key takeaways

  • Gravitons are like unicorns in physics—everyone's chasing them, but they might just be a myth.
  • Detection could require a sensitivity 10^36 times beyond current technology—good luck with that!
  • The universe plays hardball, setting limits that make even the boldest physicist sweat.
  • Measuring a graviton means contending with Planck-scale uncertainties—where our understanding goes poof.
  • So, can we outsmart the universe? Or are we just ambitious dreamers with cosmic blinders?

Who speaks in this episode?

What is discussed, and when

  1. – 5:00: quantum gravity, gravitational waves, gravitational waves, Space Time
  2. – 10:00: Planck length, Heisenberg uncertainty principle, Large Hadron Collider, particle collider
  3. – 15:00: Planck length, Stephen Weinberg, Compton effect, gravito-electric effect
  4. – 20:00: gravitational waves, gravitational waves, Brilliant, neutrino

Notable quotes from "The Universe Itself Might Be Hiding the Gravity Particle From Us"

  • 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…
    about quantum gravity · at
  • Of gravity impossible. In some cases it seems to be impossible for all practical experiments—the experiments are just too outlandish to ever happen. But in other cases…
    about quantum vacuum · at
  • The same way that an electromagnetic field is made of a sea of virtual photons, spacetime is made of gravitons. At least, that’s true if spacetime has a quantum nature, as most…
    about loop quantum gravity · at
  • Beams traveling along 4km perpendicular arms. When those phases fail to line up, it indicates a relative change in the lengths of those arms—a possible signature of a passing…
    about gravitational waves · at
  • The limits of measurability of literally everything is defined by the Heisenberg uncertainty principle. The principal sets an absolute limit on how precisely we can know…
    about Heisenberg uncertainty principle · at
  • So in order to accurately measure the separation of the mirrors, the measurement has to happen fast—faster than the mirrors move out of position due to the act of measuring.
    about Planck length · at
  • The theoretical physicist Stephen Weinberg calculated that there is potentially a huge quantity of high-frequency gravitons being emitted by the Sun.These are produced by…
    about Stephen Weinberg · at
  • Let’s try one more method that on the surface looks promising. In the 1960s Mikhail Gertsenshtein showed that electromagnetic and gravitational waves, in a strong magnetic…
    about Mikhail Gertsenshtein · at

Who and what is mentioned in this episode?

People

  • Freeman Dyson — Person · Reference · 1 mention · first at
    Central reference for the episode’s core argument: Dyson’s 2012 Poincaré Prize lecture frames why single-graviton detection may be practically or fundamentally impossible (black holes, quantum limits).
    In 2012, the legendary physicist Freeman Dyson gave a talk...
  • Stephen Weinberg — Person · Reference · 1 mention · first at
    Provides a quantitative estimate for a natural graviton source: Weinberg’s calculation of high-frequency solar graviton production, used to argue detection remains implausible due to tiny cross-sections and neutrino background.
  • Mikhail Gertsenshtein — Person · Reference · 1 mention · first at
    Cited for the Gertsenshtein effect (EM–gravitational wave coupling in strong magnetic fields), presented as a promising-but-ultimately-blocked route to graviton detection due to vacuum polarization limits.
  • Robert Gould — Person · Reference · 1 mention · first at
    Physicist mentioned in the context of graviton emission from stellar remnants
    We probably actually need a stronger source of gravitons than a mere entire star. The best option might be the collapsed core of a dead star. Robert Gould estimates that a hot…

Companies & orgs

  • Laser Interferometer Gravitational Wave Observatory — Company · Reference · 1 mention · first at
    Full name of LIGO, the organization behind the gravitational wave observatory
    We’ll think about this in terms of the most sensitive gravity detectors ever built: the Laser Interferometer Gravitational Wave Observatory—LIGO.
  • Large Hadron Collider — Company · Reference · 1 mention · first at
    Discussed as a tool for particle detection, including gravitons
    The most obvious example of the particle collision method is, surprise, a particle collider. We discovered the Higgs boson by building a collider so large that its collision…

Products & tech

  • gravitational waves — Product · Reference · 5 mentions · first at
    Phenomenon detected by LIGO, related to gravitons
  • general relativity — Product · Reference · 1 mention · first at
    Fundamental theory discussed in relation to quantum gravity
    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…
  • quantum mechanics — Product · Reference · 1 mention · first at
    Fundamental theory discussed in relation to quantum gravity
    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…
  • LIGO — Product · Reference · 1 mention · first at
    Organization behind the gravitational wave observatory
    We’ll follow Dyson’s argument here. LIGO is able to detect gravitational waves with a strain of 10^-22—that’s the relative change in length it’s sensitive to. It corresponds to…
  • particle collider — Product · Reference · 1 mention · first at
    Discussed as a tool for particle detection, including gravitons
    The most obvious example of the particle collision method is, surprise, a particle collider. We discovered the Higgs boson by building a collider so large that its collision…
  • Higgs boson — Product · Reference · 1 mention · first at
    Example of a particle discovered using a collider, analogous to potential graviton detection
    The most obvious example of the particle collision method is, surprise, a particle collider. We discovered the Higgs boson by building a collider so large that its collision…
  • SLC — Product · Reference · 1 mention · first at
    Hypothetical collider discussed for graviton detection
    For the graviton-electron interaction the cross section is proportional to the square of the Planck length. So even if we can create a graviton in our interstellar-sized…

Concepts & topics

  • quantum gravity — Concept · Topic · 2 mentions · first at
    Main theoretical framework discussed in relation to gravitons
  • Planck length — Concept · Reference · 4 mentions · first at
    Key concept in discussing the limitations of detecting gravitons
  • gravitational waves — Concept · Reference · 5 mentions · first at
    Phenomenon detected by LIGO, related to gravitons
    Beams traveling along 4km perpendicular arms. When those phases fail to line up, it indicates a relative change in the lengths of those arms—a possible signature of a passing…
  • Einstein’s general relativity — Concept · Reference · 1 mention · first at
    Fundamental theory discussed in relation to quantum gravity
    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…
  • Heisenberg uncertainty principle — Concept · Reference · 1 mention · first at
    Fundamental principle limiting graviton detection
  • neutrino — Concept · Reference · 2 mentions · first at
    Discussed as a challenge in detecting gravitons
    Catch the occasional rare interaction. But by comparison to the shy graviton, the neutrino is a party animal. For any of our graviton sources, regardless of whether it’s our…
  • quantum vacuum — Concept · Reference · 1 mention · first at
    Discussed as a challenge in detecting gravitons
  • loop quantum gravity — Concept · Reference · 1 mention · first at
    Theory of quantum gravity mentioned
  • Gertsenshtein effect — Concept · Reference · 1 mention · first at
    Method discussed for detecting gravitons
    Strength than is needed for the absorption of a graviton by matter. This means a photon may be transformed into a graviton and vice versa. So, if we shoot a graviton through a…
  • hierarchy problem — Concept · Reference · 1 mention · first at
    Theoretical issue mentioned in the context of gravity's weakness
    Is needed to increase the chance of generating one. Gravity is the weakest force by a long, long way. It’s 24 orders of magnitude weaker than the weakest of the other…
  • photoelectric effect — Concept · Reference · 1 mention · first at
    Historical example of particle detection, analogous to potential graviton detection
    The OG example of discovering a particle this way is the photoelectric effect, in which an electron is ejected from a conducting plate by a single photon. Einstein used this…
  • gravito-electric effect — Concept · Reference · 1 mention · first at
    Hypothetical effect analogous to the photoelectric effect for gravitons
    The OG example of discovering a particle this way is the photoelectric effect, in which an electron is ejected from a conducting plate by a single photon. Einstein used this…
  • Compton effect — Concept · Reference · 1 mention · first at
    Historical example of particle interaction, analogous to potential graviton detection
    Similar test to detect gravitons? An equivalent gravito-electric effect? Well in principle, yes. A sufficiently high-frequency graviton will carry enough energy to kick an…

Places & events

  • neutron star — Place · Reference · 2 mentions · first at
    Discussed as a potential source of gravitons
    We probably actually need a stronger source of gravitons than a mere entire star. The best option might be the collapsed core of a dead star. Robert Gould estimates that a hot…
  • white dwarf — Place · Reference · 2 mentions · first at
    Discussed as a potential source of gravitons
    We probably actually need a stronger source of gravitons than a mere entire star. The best option might be the collapsed core of a dead star. Robert Gould estimates that a hot…
  • Sun — Place · Reference · 2 mentions
    Natural source of gravitons discussed
    The theoretical physicist Stephen Weinberg calculated that there is potentially a huge quantity of high-frequency gravitons being emitted by the Sun.These are produced by…

Media & podcasts

  • Space Time — Media · Host · 4 mentions · first at
    Podcast series hosting the episode
    For example, we may be able to combine the two broad approaches I mentioned: using a LIGO-like interferometer combined with absorption of gravitons, this time by a detector…

Sponsors

  • Brilliant — Company · Sponsor · 4 mentions · first at
    Sponsor of the podcast episode
    Thank you to Brilliant for sponsoring PBS.
  • Algorithmic Thinking — Book · Sponsor Content · 1 mention · first at
    Course offered by the sponsor Brilliant
    Your critical thinking skills through problem solving—not memorizing. Each lesson starts by helping you master the foundations, then levels you up to increasingly challenging…

Full transcript with timestamps

Thank you to Brilliant for sponsoring PBS. 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?

We’ve got a couple quick announcements before we start. First, the algorithm hasn’t been sharing our episodes, so the best way to help the community is to like and comment - doing both really makes a difference. And if you’re new here, subscribe, hit the bell, and introduce yourself in the comments. We’re friendly. We’re also offering 20% off the entire merch store until January 19th, 2026. So if there’s anything you had your eye on there’s a link in the description. Now onto the episode In 2012, the legendary physicist Freeman Dyson gave a talk in which he speculated on the possibility of ever detecting gravitons. He was not optimistic. In his Poincare prize lecture, he laid out how the universe seems to conspire to make the detection of the quantum particle of gravity impossible. In some cases it seems to be impossible for all practical experiments—the experiments are just too outlandish to ever happen. But in other cases,

graviton detection seems to be in-principle and fundamentally impossible—we’re thwarted by the appearance of black holes or by the quantum vacuum from ever glimpsing a graviton. So does the universe really prohibit us from ever seeing the building block of the fabric of spacetime? As a reminder, the graviton is the quantum particle of gravity, just as the photon is the quanta of electromagnetism, gluons of the strong force, etc. But the graviton is more than that—it’s the building block of the fabric of the universe. In the same way that an electromagnetic field is made of a sea of “virtual” photons, spacetime is made of gravitons. At least, that’s true if spacetime has a quantum nature, as most physicists seem to believe. All theories of quantum gravity require them, from string theory to loop quantum gravity. So, probably worth trying to spot one if we want to verify these theories.

Now in the past we’ve talked about indirect ways to test the quantum nature of gravity, but what about detecting the graviton itself? Well, there are two broad ways we can think about graviton detection. On the one hand, we can take what we know about detecting classical gravitational fields and try to apply it to a quantized field of gravitons—basically, to detect the gravitational effect of a single graviton. On the other hand, we can treat gravitons like every other particle and use the same techniques we once used to squeeze out photons from the electromagnetic field. By tackling both perspectives we can get a reasonably comprehensive sense of how impossible graviton detection really is.

Let’s start by trying to detect the gravitational effect of a single graviton. We’ll think about this in terms of the most sensitive gravity detectors ever built: the Laser Interferometer Gravitational Wave Observatory—LIGO. This pair of detectors tracks the subtle differences in phase of a pair of laser beams traveling along 4km perpendicular arms. When those phases fail to line up, it indicates a relative change in the lengths of those arms—a possible signature of a passing gravitational wave. We’ve used these observatories to spot gravitational waves from hundreds of merging black holes and neutron stars since the first detection in 2015.

So what would it take for a laser interferometer along these lines to detect a single graviton? We’ll follow Dyson’s argument here. LIGO is able to detect gravitational waves with a “strain” of 10^-22—that’s the relative change in length it’s sensitive to. It corresponds to one-one thousandth the width of a proton for the 300 back and forth reflections along LIGO’s 4km arms. If gravity is indeed quantum then gravitational waves are made of a coherent superposition of gravitons, in the same way that our lasers are made of photons. A wave at LIGO’s strain limit would contain at least 10^36 gravitons. We’re trying to see a single graviton, so in principle we’d need a detector that’s 10^36 times more sensitive.

The critical question we need to ask is how precisely do we need to measure the variation in the length of detector arms to pick up a single graviton event? It turns out that the answer is independent of the type of gravitational wave: for optimal sensitivity, we need to be able to measure a length difference of order a single Planck length. This should already raise concerns because the Planck length is the distance where our current understanding of space breaks down. But let’s proceed. The limits of measurability of literally everything is defined by the Heisenberg uncertainty principle. The principal sets an absolute limit on how precisely we can know complementary pairs of properties simultaneously, for example the position or momentum of something. Let’s start simple by imagining one LIGO arm as a pair of free-floating mirrors. A gravitational wave causes the distance between them to change,

and we want to measure that changing distance. That’s effectively measured by bouncing a photon between them. However that photon also imparts momentum on the mirrors, inducing an uncertainty in the mirror position. The more precisely we want to measure those positions, the higher the frequency of light we need, therefore the more momentum we have to impart. The position-momentum uncertainty relation can be used to relate the limit of the position measurement to the imparted mirror momentum. We can then convert this relation to one involving time.

So in order to accurately measure the separation of the mirrors, the measurement has to happen fast—faster than the mirrors move out of position due to the act of measuring. Long story short, we can maximize the precision of our measurement of the mirror separation in two ways: by reducing the distance between the mirrors—that reduces measurement time. And by increasing the mass of the mirrors, heavy mirrors move more slowly when bumped by the measuring photon. If we want a 1-Planck-length position precision due to the passage of a graviton, the mirrors need to be massive enough and close enough together that they form a black hole. Well, that sucks. By definition, the formation of an event horizon prevents our distance measurement.

Dyson shows that we get the same issue via a different argument even if we fix the mirrors in place. In general, any distance measurement on the scale of the Planck length gives us black holes, which means that direct measurement of the effect of a single graviton by a LIGO-like device is fundamentally impossible. Okay, let’s try something else. The gravitational wave approach tries to measure the actual gravitational effect of a single graviton. But maybe treating gravitons gravitationally is using a classical hammer to crack a quantum nut. Instead, maybe we should look for gravitons by the same method as we look for other quantum particles. We can summarize that method in very general terms: crash particles together and see what happens.

The physics of colliding particles is inherently different to the interactions of classical waves, so maybe that’s the right approach for spotting a particle that arises from a quantum field. The most obvious example of the particle collision method is, surprise surprise, a particle collider. We discovered the Higgs boson by building a collider so large that its collision energies could generate the massive Higgs particle. And that’s the 27 km diameter large hadron collider. So how large a collider do we need to generate the graviton?

Gravitons are massless, so energy isn’t needed to give them mass like with the Higgs. Rather, energy is needed to increase the chance of generating one. Gravity is the weakest force by a long, long way. It’s 24 orders of magnitude weaker than the weakest of the other fundamental forces, a fact which is itself a conundrum called the hierarchy problem, which obviously we’ve talked about before. In quantum terms, that weakness can be expressed as a very small coupling constant between the graviton and other particles. The probability of generating a graviton in a particle collision depends on that coupling constant. Despite the word “constant”, the coupling factor actually increases with the energy of the interaction At around a billion Joules, the coupling for gravity reaches the ballpark strength of the other forces. So that’s the energy we need to reach in our collisions to have a fair chance of producing gravitons. How big an accelerator

do we need to reach that energy? Well the LHC collisions reach a whopping millionth of a Joule. For a fixed magnetic field strength, collision energy scales directly with collider size. To get to the energy to detect a graviton, a collider with the same magnets as the 27km LHC would need to be around 3 light years in diameter—much bigger than our solar system. That sounds insanely difficult, but it’s in-principle possible. And once we’ve build a graviton-factory, we get to the real challenge— it's not creating gravitons but detecting them.

Now massless gravitons are stable, so we can’t rely on looking for their decay products. We’d need to see one via its gravitational effect, which we just saw may be impossible, or when it’s absorbed by or scatters another particle. The OG example of discovering a particle this way is the photoelectric effect, in which an electron is ejected from a conducting plate by a single photon. Einstein used this effect to demonstrate the existence of photons as quanta of the electromagnetic field. So can we use a similar test to detect gravitons? An equivalent gravito-electric effect? Well in principle, yes. A sufficiently high-frequency graviton will carry enough energy to kick an electron out of an atom or out of a conducting plate. We could also imagine a graviton equivalent of the Compton effect, in which electrons are ejected from atomic orbitals. Either way, we shift the problem from detecting gravitons to detecting electrons, which is pretty easy.

The probability of an electron absorbing a graviton to produce one of these phenomena depends on the coupling strength of gravity. So just as with graviton creation, it’s super unlikely. Another way to think about this interaction strength is in terms of the cross-section. It’s called a cross-section because it tells us how big an electron looks to a graviton—how close the graviton needs to get to the electron center in order to hit it and get absorbed. And for the graviton-electron interaction the cross section is proportional to the square of the Planck length. So even if we can create a graviton in our interstellar-sized collider, let’s call it the SLC—“stupidly large collider”, we can probably never detect it.

The other way we can increase the likelihood of a gravito-electric interaction is by increasing the number of gravitons in the right frequency range. So maybe we amp up the power of our SLC, or we find a natural source of gravitons. The theoretical physicist Stephen Weinberg calculated that there is potentially a huge quantity of high-frequency gravitons being emitted by the Sun.These are produced by electron-graviton interactions in the hot, dense core of our star. At the energy we need, the sun should produce about 10^24 per second. This amounts to about 4 per meter squared per second passing through the Earth’s surface. You probably don’t notice the several that passed through you during this sentence alone. The interaction cross-section for this is so small that these solar gravitons only interact with a particle of matter roughly once every billion years or so across the entire volume

of the Earth. Now we could build a star-sized detector and maybe get a detection every 1000 years rather than every billion, but that still feels like an unsatisfying yield. We probably actually need a stronger source of gravitons than a mere entire star. The best option might be the collapsed core of a dead star. Robert Gould estimates that a hot white dwarf or a neutron star would emit, respectively, 100 and 100,000 times more gravitons than the Sun, though over a shorter timescale. So placing a planet or star-sized detector near such a stellar remnant might net us a single graviton on human timescales. Of course, the path to building such an experiment is far beyond human timescales if it’s possible at all.

And if we were somehow able to pull this off, we’d face an even more basic issue: neutrinos. These ghostly particles are notoriously hard to spot. Most of the Sun’s neutrinos pass straight through the earth, and we’ve built detectors out of the Antarctic ice cap to catch the occasional rare interaction. But by comparison to the shy graviton, the neutrino is a party animal. For any of our graviton sources, regardless of whether it’s our Sun, a white dwarf, or a neutron star, our detector will interact with 10^34 neutrinos per single graviton.

And distinguishing that graviton from the neutrino noise seems practically impossible. Let’s try one more method that on the surface looks promising. In the 1960s Mikhail Gertsenshtein showed that electromagnetic and gravitational waves, in a strong magnetic field, can couple together. This is more promising than looking for interactions between gravitons and, say, electrons. EM and gravitational waves, which both travel at the speed of light, can experience wave resonance that allows energy to transfer between them at much lower coupling strength than is needed for the absorption of a graviton by matter. This means a photon may be transformed into a graviton and vice versa. So, if we shoot a graviton through a magnetic field, it will oscillate between graviton states and photon states. This is called the Gertsenshtein effect.

Now to do this, we need a long hollow tube threaded by a strong magnetic field passing and pointed towards a source of gravitons. The hope is that some of those gravitons will turn into photons that we can detect. And, as you may have guessed, the universe again conspires against us. The magnetic field needs to be quite strong for the Gertsenschtein effect to work. How strong? Strong enough that we have spontaneous creation of matter-antimatter pairs inside the tube. This vacuum polarization resulting from this limits the coherence of EM waves in the tube, preventing resonance with gravitational waves and shutting down the Gertsenschtein effect.

So we’re at strike three for detecting gravitons. It seems the universe really doesn’t want us to see its deepest structure. But is it really impossible, or just nearly impossible? In a couple of the cases we looked out the answer is … impossible. The barrier to graviton detection seems fundamental. The formation of black holes or the breakdown of the vacuum tells us that the laws of physics forbid graviton detection by these methods. But in other cases it’s just astonishingly difficult—we need star-sized graviton sources and detectors and some yet-unknown way to filter neutrinos.

But next to impossible is still possible. I’m saying there’s a chance. And maybe the problem is actually just that we haven’t come up with a smart enough experiment yet. Or maybe we have. Since Dyson’s 2012 lecture, we’ve detected gravitational waves directly and we’ve made major advancements in quantum technology. This has inspired a new generation of physicists to come up with a new generation of proposals. For example, we may be able to combine the two broad approaches I mentioned: using a LIGO-like interferometer combined with absorption of gravitons, this time by a detector with novel quantum properties. How does that work? Well that’s for an episode coming soon, when we’ll find out how we really may be able to detect the particle at the heart of spacetime.

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Questions about this episode

Who speaks in "The Universe Itself Might Be Hiding the Gravity Particle From Us"?

PBS Space Time Host (Host), Freeman Dyson (Guest), Stephen Weinberg (Guest), Robert Gould (Guest).

What is "The Universe Itself Might Be Hiding the Gravity Particle From Us" about?

The discussion centers on the challenges of detecting gravitons, the theoretical quantum particles associated with gravity, and how the universe appears to inherently obstruct such detection. Citing insights from physicist Freeman Dyson, it explores the notion that both practical and fundamental barriers may prevent scientists from uncovering this key element of quantum gravity, raising…

What is mentioned in this PBS Space Time episode?

32 people, works and ideas are mentioned, including quantum gravity, Freeman Dyson, Planck length, gravitational waves, gravitational waves, general relativity. Each one on this page links to its own page with every episode that mentions it.

How long is "The Universe Itself Might Be Hiding the Gravity Particle From Us"?

The episode runs 20 min; the transcript is about 2,808 words in 23 timestamped sections.

More from PBS Space Time

About this transcript: the episode's spoken words as captioned on YouTube, split into timestamped sections. Reclip.Me locates every person, work and idea in the text, keeps the time it is first named, and quotes the words around it verbatim. Last updated January 24, 2026.