Ask Ethan: Can Weber bars detect gravitational waves?

Here in our Universe, we have many different species of energy populating the spacetime we inhabit. There’s matter: both normal matter and dark matter, in all the forms they can take, from plasmas to stars to black holes to diffuse, fluffy haloes. There’s dark energy: a mysterious form of energy that appears to be inherent to the fabric of space itself, consistent with Einstein’s cosmological constant. And there’s also radiation: like photons of all different wavelengths and neutrinos when they move nearly at the speed of light. But gravitational waves, even though they aren’t represented by any component of the Standard Model of elementary particles, also exist in our Universe, and also behave as a form of radiation: an inherently gravitational one.

Originally theorized by Einstein way back when general relativity was first put forth, gravitational waves were finally successfully detected, for the first time, back in 2015: a full 10 years ago. In the time since, we’ve expanded our total to several hundred detections, with more being discovered all the time in the current (fourth) observing run of the LIGO, Virgo, and KAGRA detectors. But way back in the day, going all the way back to the 1950s and 1960s, there was a completely different attempt made to see gravitational waves: through a series of bar detectors developed by Joseph Weber. Given all we know now, could Weber’s original methods be revived, and could they pay off? That’s what Jon Joseph wants to know, asking:

“[I] sat in on a Kip Thorne lecture at the University of Minnesota. Among other things, Dr. Thorne discussed the impressive improvements in his (and others) LIGO experiment in order to reach the necessary sensitivity. Given what we now know about gravity waves and what is needed to measure them, could any form of Weber bars be made to work?”

It turns out there’s more than one way to detect a gravitational wave, but one particular aspect of Weber bars — their small size — is what critically works against them. Here’s the full story.

As masses move through spacetime relative to one another, they cause the emission of gravitational waves: ripples through the fabric of space itself. These ripples are mathematically encoded in the metric tensor of Einstein’s general relativity, and carry energy away from co-orbiting systems like this, leading to orbital decay and an eventual merger.

When Einstein first put forth general relativity, he was already familiar with Maxwell’s electromagnetism: the classical theory of electric charges, their motion, and their relationship to electric and magnetic fields. Whenever you had an electric charge accelerating in an electromagnetic field, that charge would emit radiation: electromagnetic waves, in the form of light. Those electromagnetic waves carry energy away from the system of charges and fields, conserving the total energy of the system (including rest mass energy through Einstein’s E = mc) as well as the system’s total momentum.

He recognized that a great many phenomena in electromagnetism must have analogies in general relativity, and the gravitational analogy of electromagnetic waves was also a prediction within his new theory of gravity: gravitational waves. Einstein himself viewed it as a mathematical curiosity that likely had no physical meaning, but others disagreed. Decades later, in 1957, the main topic of GR1 — the first American conference on general relativity — Richard Feynman made the key argument that showed that those waves should be physically real, and must carry energy: the sticky bead argument.

The “sticky-bead” argument, put forth by Richard Feynman, was that gravitational waves would move masses along a rod, just as electromagnetic waves moved charges along an antenna. This motion would cause heating due to friction, demonstrating that gravitational waves carry energy. The principle of the sticky-bead argument would later form the basis of the design of LIGO.

According to Feynman, you can imagine having two thin, perpendicular rods: each with beads on the end, as you see illustrated above. On each rod, one bead is fixed: it’s attached to the rod and isn’t allowed to move. But the other bead is free to slide along the rod, and should do exactly that if a gravitational wave passes through the system, causing the distances between the two beads to expand-and-contract in oscillatory fashion. But now, Feynman argued, let’s allow that movable bead and the rod to experience friction, or to allow them to be “sticky” in some sense. What happens now? The motion of the bead against the rod causes the atoms/molecules/electrons to rub against one another, producing heat through friction, and thereby providing a method for extracting energy from the gravitational waves.

So, then, the next question to ask is this: what is it that makes these gravitational waves that we hope to detect?

One answer is simple: the same answer that Einstein gave. Whenever you have masses that move and accelerate through curved spacetime, the fact that you have a gravitational source moving in a gravitational field mandates that you’ll induce ripples in spacetime itself: gravitational radiation, or gravitational waves. The more massive you are, and the more strongly space is curved, the greater the amplitude of your gravitational waves. This means that some of the best sources for generating gravitational waves are the densest objects of all: black holes, neutron stars, white dwarfs, the cores of core-collapse supernovae, etc.

Gravitational waves span a wide variety of wavelengths and frequencies, and require a set of vastly different observatories to probe them. The Astro2020 decadal offered a plan to support science in every one of these regimes, as they are the best methods for detecting the waves we have at the frequencies we calculate they should be at. “Weber bars” are not part of this arsenal, although here in 2025, none of these future observatories seem likely to materialize within the United States.

There’s a “fun fact” about black holes that physicists greatly appreciate, but that surprises most people. If you ask, “where, in the Universe, does spacetime experience the greatest amounts of curvature,” you might suspect that the answer would be “right next to the event horizon of a black hole.” And that’s true! Spacetime may be more strongly curved inside of a black hole’s event horizon, but anything that happens in there can’t get out; nothing can escape a black hole’s interior, even at the speed of light. (Which is also the speed of gravitational waves.) But it’s the smallest, lowest-mass black holes that have the greatest curvature at their event horizons, not the largest ones.

And this is how modern gravitational wave observatories — like the LIGO, Virgo, and KAGRA detectors — actually successfully detect these gravitational waves. The amplitude of gravitational waves arising dense masses (black holes and/or neutron stars) inspiraling into a low-mass black hole’s event horizon is the greatest in the known Universe. By building long detector arms of a few kilometers, and then reflecting light up-and-down those arms thousands of times, we can make detectors that are sensitive to these events: the inspirals and mergers of compact objects up to a couple of hundred solar masses. The combination of large amplitudes and fast frequencies means that a sensitive enough detector of “only” a few kilometers in size can pay tremendous scientific dividends.

This “Eagle” plot shows the black holes and neutron stars, as a function of mass, that were detected by either gravitational wave events (blue and orange) or with electromagnetic signatures (red and yellow). With the advent of the first part of LIGO’s Observing Run 4 data, we now have 218 robust gravitational wave events, with approximately 100 more in the pipeline.

But it isn’t like there were other attempts to detect gravitational waves in between 1957 and LIGO’s first direct detection success in 2015. First off, there are other places where gravitational waves can be generated: in supernovae, from stellar cataclysms, and all the way back in the earliest stages of cosmic history: from the hot Big Bang or even, prior to that, from cosmic inflation. In fact, the first earnest attempts at a gravitational wave detector didn’t look anything like LIGO’s laser arm interferometers, but instead were based on human-scale bars that were shielded inside precisely calibrated electromagnetic cavities: detectors known as Weber bars.

Weber bars were named for Joseph Weber, a physicist with an electrical engineering background, who specifically studied microwave radiation, and had a strong interest in astronomy and astrophysics. Back in the mid-1950s, Weber went to George Gamow — frequently known as the “father” of the Big Bang — and asked if there were any interesting problems that could potentially benefit from someone with a background in electrical engineering in general and microwave radiation in particular. Gamow, who was the first to predict the existence of a leftover background of radiation for the Big Bang, told Weber, “No,” and so Weber pursued other research avenues. Alas, we would have to wait until the mid-1960s for the cosmic microwave background to be detected, and even that was by accident.

This image shows Arno Penzias and Robert Wilson, co-discoverers of the cosmic microwave background (CMB), with the Holmdel Horn Antenna used to discover it. Although many sources can produce low-energy radiation backgrounds, the properties of the CMB, including its perfectly blackbody nature and uniform temperature in all directions, confirm its cosmic origin. As time goes on and the leftover glow from the Big Bang continues to redshift, larger telescopes sensitive to longer wavelengths and smaller number densities of photons will be required to continue to detect it.

And so, instead, Weber went to work on a different set of problems. He happened to be an attendee at that 1957 conference where Feynman put forth his sticky-bead argument, and was captivated. In 1959, Weber published his first paper on how one could possibly detect such gravitational waves, and in 1961 began building such a detector. These conducting cylinders, made of aluminum, were engineered to vibrate at specific frequencies: 1660 and 1220 hertz. These frequencies, Weber argued, had to be large, as gravitational waves passing through them would only induce a size change of a fraction-of-a-proton’s width. Weber made them as large as possible: 1.5 meters long, and up to 0.6 meters in diameter.

If a gravitational wave passed through these bars, Weber argued, the lengths of the cylinders would change very slightly, and so he used extremely sensitive piezoelectric sensors: sensors that could detect a change in bar length of as little as 10^-16 meters. Because gravitational waves cause space to alternatively compress-and-expand as these waves pass through them, the bar length would change, the waves would deposit energy into them, and the sensors could then read out the amplitude and frequencies of these changes as the bars “rang” from gravitational radiation passing through them. That’s the idea, and the setup, of a Weber bar experiment.

A resonant bar detector, also known as a Weber bar, was the first experimental device developed in the attempt to directly see gravitational waves. This one, at the University of Glasgow, wasn’t one of the originals of Joseph Weber, but rather of a scientist who attempted to, and failed to, replicate Weber’s alleged results.

The underlying principle is the same one that’s at play in the modern LIGO, Virgo, and KAGRA detectors, but with a much smaller-scale set of experiments. Weber installed bars in multiple disparate locations, and although the bars themselves did display signs of significant amounts of noise, Weber looked for correlations in these noisy signals between bars in different locations. By 1968, he began presenting evidence that supported similar signals in different locations appearing at the same time. On statistical grounds, Weber argued that these apparent signals were unlikely to be noise, and instead represented the first directly detected gravitational wave events, with accompanying speculations about what sort of astrophysical events could have generated them.

At a conference in 1969, when other physicists were debating the very existence of gravitational waves, Weber presented evidence supporting their direct detection. Other groups attempted to reproduce Weber’s results but couldn’t do so; they only saw null results: no detection, to the limit of the random noise that researchers could not eliminate within their detectors. But Weber was always convinced that what his apparatuses were seeing was a real signal. He defended his interpretation of the data in a popular 1971 article, and one of his detectors was even brought to the Moon in 1972. For years, Weber claimed to continue to detect these waves, including when our Local Group’s closest and most recent supernova went off in 1987.

This photograph from the University of Maryland archives shows Joseph Weber at work on one of his largest-diameter Weber bar experiments, seeking to detect gravitational waves. Although many attempted to reproduce Weber’s results, no one else saw what he claimed to see. Today, Weber’s results are largely dismissed and discarded, and rightfully so, based on the full suite of modern evidence.

And this was the case for decades: Weber asserting and reasserting the robustness and validity of his detections, and the rest of the community doubting him. It wasn’t that they were doubting him because they refused to accept overwhelming evidence; they doubted because, despite repeated and precise independent attempts, no one else could replicate Weber’s results. Not Vladimir Braginsky, not Ron Drever, not Richard Garwin, not Tony Tyson. Weber was the only one “seeing” these signals, where everyone else was only seeing noise. Many suspected that Weber was yet another common victim of wishful thinking: where he was “massaging the data” until it aligned with his preferred results.

There were big troubles over on the theoretical side, too: the real problem is the signal amplitude and frequencies where Weber was claiming to have detected these waves. If what Weber was seeing was a series of real gravitational wave signals, then three pathologies would have arisen.

1. First, the high frequencies that Weber was claiming a detection at indicated that events were being generated with sub-millisecond frequencies: too quick to arise from realistic astrophysical sources like black holes.
2. Second, the large amplitudes of the gravitational waves needed to generate the events that Weber was claiming a detection of would provide more energy than could possibly cosmically exist in any-and-all forms of radiation combined; the Universe as a whole ruled his interpretation out.
3. And third, Weber’s bars were simply too short to be sensitive to the types of gravitational waves that the Universe would actually generate. The only realistic possibility would arise from the Big Bang itself, and Weber’s claimed detections would again be internally inconsistent with how much of a gravitational wave signal could have been generated in those early cosmic stages.

This series of eight large aluminum bars, at the University of Maryland, is named the Weber Memorial Garden in honor of Joseph Weber’s early attempts to experimentally detect gravitational waves. To date, none of Weber’s claimed detections could have been correct, as other observations have shown their inconsistency with the Universe we actually have.

So now, we have to go back to the original question and ask whether there are any ways that Weber bar detectors could ever conceivably give a robust detection of gravitational waves. Remember, the large-amplitude signals that LIGO, Virgo, and KAGRA detect are very fast — very short-period — signals, generated by the lowest-mass black holes as they inspiral and merge, where the curvature at their events horizons is maximal: the greatest anywhere in the Universe. So what else could make an even faster signal?

• Stellar cataclysms, where white dwarfs or neutron stars explode, could have core asymmetries that make very quick “bursts” of gravitational waves.
• Core-collapse supernovae, where the core of the star collapses while forming a neutron star or black hole, could potentially make a detectable, short-period burst if it occurred near enough: within the Milky Way or Local Group.
• Or from the earliest periods in the cosmos: the Big Bang (if there was no inflation) would generate a thermal spectrum of gravitational waves, while inflation (if our models are correct) could also generate an interesting spectrum of gravitational waves, albeit only at low amplitudes.

However, you again have to look at the amplitude and frequency of what a Weber bar would be sensitive to. At the small sizes of Weber’s original bars, no, there isn’t anything that could be detected; the amplitude of these events — even if they were to occur in our own cosmic backyard — would still be too low to detect. If we could detect signatures from gravitational waves with such a setup, then that would imply a cosmic gravitational wave energy density that was far greater than our observations of the Universe permit.

If there’s a strongly first-order electroweak phase transition, then in addition to the gravitational wave background produced by inflation (black line), there should be a new spectrum of gravitational waves (red peak) produced that upcoming gravitational wave observatories, such as the upcoming LISA, will be sensitive to. Even at the maximum allowable amplitude, however, a Weber bar would not be able to practically reveal the presence of such waves.

But this doesn’t mean that the Weber bar approach couldn’t possibly work for detecting gravitational waves. All we’d need is a bar that was large enough: one that was the same size as the speed of light divided by the gravitational wave’s frequency. For the large-amplitude waves that LIGO, Virgo, and KAGRA see, this would take a Weber bar of a few thousand kilometers in size: approximately the diameter of the Moon. For the gravitational waves that space-based detectors, such as LISA, are designed to see, you’d need a Weber bar that was somewhere around the length of the Sun’s diameter: an even more challenging task.

Sure, you can always look for the short-period gravitational waves that would be detectable with a bar detector that was only a couple of meters long, but that’s where the small amplitude of these waves crushes your hopes. Weber claimed his experiments were sensitive to changes as small as 10^-16 m in length. The smallest distance scale we’ve ever probed with our highest-energy particle physics experiments is around 10^-19 m, or 1000 times better than Weber’s assertions. But to see the gravitational waves the Universe actually produces, you’d need sensitivities of around 10^-24 meters at best, and down to around 10^-31 meters in a more conservative scenario: something far beyond any practicalities.

For me, both personally and professionally, Weber’s work was colossally important in that it paved the way for modern gravitational wave detectors to be built and to succeed at directly detecting these ripples in spacetime. But as far as the future is concerned, the methods we’re using now are far more precise, and require far less infrastructure at the relevant scales of interest, than any Weber bar (or Weber bar analogue) could hope to achieve. While using such a bar as a gravitational wave detector isn’t a physical impossibility, we know enough about the Universe today to be confident that our resources would be far better spent elsewhere if the scientific goal is to understand the Universe.

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