LUX Searches In The Deep For Dark Matter

The Homestake Mine started yielding gold in 1876. If you had asked George Hearst, the operator at the time, if the mine would someday yield the secrets of the universe I bet he would have laughed you out of the room. But sure enough, by 1960 a laboratory deep in the mine started doing just that. Many experiments have been conducted there in the five and a half decades since. The Large Underground Xenon (LUX) experiment is one of them, and has been running is what is now called the Sanford Underground Research Facility (SURF) for about four years. LUX’s first round of data was collected in 2013, with the experiment and the rest of the data slated to conclude in 2016. The method, hardware, and results wrapped up in LUX are utterly fascinating.

It’s All About Noise Filtering

Detecting dark matter is exceedingly hard. So much so that we’ve never actually done it. A sufficiently large detector like the LUX can hope to catch just a few interactions per year. To make sure that these aren’t missed, it’s important to filter out as many non-events as possible. This is the reason behind the underground location; 4,850 feet of earth stand between the detector and open air to reduce cosmic rays by a factor of 1 million. That doesn’t get rid of everything but it becomes possible to discern false readings.

The ultra pure Xenon further filters this noise in the system. What is left is a very dark environment waiting for Weakly Interactive Massive Particles (WIMPs) to pass through it. Theory tells us that somewhere between millions and billions of the WIMP dark matter particles move through one square centimeter of space every second. But one square centimeter on a subatomic scale is a vast and empty space. With the noise filtered out, researchers are just waiting for a WIMP to collide with a Xenon nucleus.

Detecting Light

Xenon is a scintillator; when the nucleus is struck by a fast-moving subatomic particle, it gives off a photon. Ionizing electrons are also a result of the interaction and they in turn create secondary scintillation. The pattern of primary and secondary light is specific to the type of interaction and, if measured properly, can be used to distinguish a dark matter interaction from other events. The good news is that we’re really good at measuring light. In fact, you can probably already guess what mechanism is used in the measurement: a Photomultiplier Tube (PMT).

PMTs are used in all kinds of scientific measurement equipment, and also appear often in medical devices and photography equipment. We seen this last example as a source for a PMT that Kerry Wong used to demonstrate the speed of light.


LUX has 122 PMTs in its sensor array. A fair amount of signal conditioning sets the levels before being fed into the custom triggering system. Then things really start to get interesting. Readings are summed into 16 groups of PMTs which are then processed by the triggering system to establish if the pattern is that of a possible dark matter interaction. If you need to do a lot of very fast, parallel processing, what kind of hardware do you choose? You’re right, you reach for an FPGA and build up a system around it.

If you’re on the edge of your seat for details, the research team has come through in a big way. In November they published a paper entitled FPGA-based Trigger System for the LUX Dark Matter ExperimentAnd even if you don’t want to dig that deep, Professor Frank L.H. Wolfs at the University of Rochester has a concise set of pages dedicated to the triggering hardware which was designed by Wojtek Skulski.

The current iteration of the trigger pulse digitizer board goes by the part number DDC-8DSP. Two of these boards use a Xilinx Spartan-3A to capture the signals using 14-bit resolution at 64 MHz. Each of these boards collects and processes the signals, then send the data to a Trigger Builder board via HDMI cables. This board gets its name because it is responsible for inspecting for a characteristic pattern of primary and secondary scintillation that indicates a WIMP interaction (and differentiates from other interactions). The diagram on the left is from the published paper and illustrates the signal pattern that the hardware is looking for.

Did it Work?

LUX-ZEPLIN diagram

Ah, be careful what you ask. Yes, LUX worked, delivering the first round of data in 2013 as anticipated. The real question is did it detect dark matter? Evidence of dark matter has not been proven in that data. But this, the most sensitive detector ever build for this purpose, hasn’t gone to waste. The research team has been able to further establish what properties WIMPs do not possess.

The experiment is currently running another round, having been further configured based on the 2013 data. This data set is expected to be ready in June of this year and will be the last run for this iteration of LUX. In the works is the LUX-ZEPLIN project which will use a much larger detector apparatus with vastly more liquid xenon, 488 PMTs in the sensor array, and several other improvements.

Experiments that Deserve Celebrity Status

When first hearing about LUX, you might be reminded of similar experiments to detect neutrinos because both experiments need to be located underground to filter out cosmic noise. Neutrino experiments have a higher public profile, although they’re probably not at celebrity status like the Large Hadron Collider. And there will be an exciting neutrino detection experiment sharing he SURF facility with LUX before too long.

With one billion dollars committed to build the new Deep Underground Neutrino Experiment (DUNE), planned to start searching for neutrinos in 2022, it is easy to look at these types of experiments as the new space race. Surely, investment in experimentation will yield technological advancements similarly transformative as those which can be attributed to mankind making our way into space. The missing piece of the puzzle is widespread public awareness and excitement for scientific discovery.

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38 thoughts on “LUX Searches In The Deep For Dark Matter

  1. Woah, woah, woah. You *cannot* mention Homestake without mentioning Ray Davis. That’s a big, gigantic physics foul.

    For those who don’t know, Ray Davis set up the first experiment in Homestake – the Homestake Experiment, the first solar neutrino detector. It consisted of 100,000 gallons of dry-cleaning fluid, which they bubbled helium through to find a few *atoms* of argon every few weeks. And this was in the late 1960s! The results were the first to show that the solar electron neutrino flux was way below theoretical calculations, and won Ray Davis the Nobel Prize.

    In fact, the mine itself shut down in the early-2000s because it just wasn’t profitable as a mine anymore. So pretty much the only reason why it even still exists is Davis, which is why it’s fantastic that it was chosen to be the home for what became SURF, since it’s a big part of physics history.

    1. There’s plenty of theoretical research on this, that’s how dark matter was “discovered”, on paper. They’ve still to actually find some. That’s when you have to go out into reality and actually build something. Academics don’t waste money, they usually know exactly what they’re trying to find when they eventually build the instrument.

  2. Seven tonnes of Xenon! Woah. That’s a substantial chunk of the entire world’s annual production of the stuff. We used to pay about $6 per *gram* ($35/L medical grade gas) in kilobuck lots. I wonder how much this hyper-pure tank of it will cost?

    1. If that’s metric tons, then 7,000,000g (seven tonnes)/131 g/mol = 54,000 moles => /22.4 mol/l = 2410 litres.

      According to Wikipedia, worldwide production of xenon in 1998 was estimated at 5,000–7,000 m^3.

      So about half the 1998 production amount.

      At the prices you listed, that would be $35*2410 = about $85,000

      Curiously, a market research paper I found noticed an uptick in the demand for Xenon in recent years. Perhaps this is why.

        1. Absolutely right: I misread the units in the world production value, cubic meters instead of liters, 1 cubic meter is 1000 liters.

          So 2400 litres is about 2.4 cubic meters, or about 0.05% of the 1998 production value.

          Also, I can’t tell if the Wikipedia production value is in cubic meters of gas, or cubic meters of liquefied gas. I’m assuming it’s gas-phase.

          (Feynman once quipped that about a third of all physics is doing conversions.)

      1. PWalsh-
        22.4 Liters per mol is true for a gas at STP, but this is a liquid and has a much higher density (around 3.1g/cm^3). On the Lux webpage they say they use 370Kg of liquid xenon, which is around 120 liters.

        I’m having trouble finding a good source, but one webpage says liquid xenon goes for around $1.20/g, so that would be a total cost of around $440,000.


  3. I am a skeptic.
    I think they are having so much trouble finding dark matter because it does not exist and they should spend at least a little of the billions on testing the some of the alternate theories that equally well account for the observations without requiring over 80% of the universe to be things that have not been observed.

      1. What do you mean? Some portion of all modern science funding is going into discovering as much as possible about gravity and how it works. There is endless theorizing, debate, experimentation and so on.


        1. Perhaps it has to do with my google skills but when I look for who’s getting research grants I don’t see anything getting funded that isn’t mainstream. I enjoy reading about different theories and the debate that goes with it but it seems that anything that would contradict dark matter doesn’t get much positive attention or money.

          “A new generation of students raised to believe in dark matter often assumes I must be some kind of crackpot.” – Stacy McGaugh

          1. There’s no serious money because no one’s got a viable alternative. Most “modified GR” theories just can’t explain all of the data as well as lambda-CDM (dark energy + cold dark matter). Theorists would love alternative ideas. They’d get to write more papers! The problem is they just don’t exist.

            In fact, the alternative theory to dark matter that gets the most attention (MOND) is really just getting it because, well, it’s been there a long time, and there’s nothing better. Even the original proponent of MOND admits that even by ad-hoc modifying gravity, you can’t account for all the matter in galaxy clusters. So what’s his suggestion? Undetected normal matter. Oh, not dark matter, of course. That would be crazy. This is just matter that hasn’t been detected because it’s not luminous. Because it’s, y’know. Dark.

            Lensing results (e.g. the Bullet Cluster) make it really, really hard for any other theory. Dwarf galaxy morphology is just going to make it worse in the coming years.

      2. Not taking about gravity, but E.Hoyle proponent of the Steady State Universe was labeled as “Marxist”, but on the other side a half time mathematician AND PRIEST made the Big Bang Theory without any BIAS…
        Sure things looked better for the BBT, but now 94% of dark energy/matter and the CBR loosing it’s credibility…

        1. Lemaitre didnt ‘make’ the big bang theory. He just was one of many physicists with similar ideas emerging at the time.

          For obvious political reason, Lemaitre was a good vessel.

          How is CMB loosing credibility? I read that before in a comment section of a woo article – and followed it to some crack pot website where an italian guy was claiming he, himself alone, had proven CMB is false. His alternative position was essentially electric universe.. He said planets are giant capacitors….

          CMB has been measured from earth and from space.. It isnt the heat from the oceans, as some have claimed.

          1. That has nothing to do with the CMB itself, and actually not much to do with the Big Bang, but with right after (inflation). A group looking for polarization whirls in the CMB (which can be produced by the CMB photons *passing through space dust*) thought they had found whirls that inflation had imprinted on the CMB. Turns out the whirls in the CMB were mostly space dust.

            The CMB itself is still fine.

    1. Alternative theories are tested. I think you will find many ‘alt theories’ such as ‘aether’ and ‘electric universe’ type theories were tested long in the past.

      Cosmology is always quite tenuous. It changes massively at unpredictable times. Have you ever read about modern cosmology? Amazing things have happened when they test what now might be called “alt. theories” – new, better theories emerge, and so do new and better lab techniques, mechanisms for research, and apparatus..

      Read about the genesis of interferometry and it’s relationship to aether debunking. This is a perfect and easily understood example. This is the way in which science has moved past some of the theories you might be thinking of, and an example of how basic science research can result in major breakthroughs that effect everyone, like the Michelson Interferometer and ability to measure very minute physical changes represents.

    2. Um, we *do* spend money testing those alternate theories. And basically all of them fail (*).

      Dark matter explains galaxy rotation curves and lensing. Dark matter + dark energy explains the previous, cosmic acceleration, and the cosmic microwave background. There are no generally-accepted examples of a viable non-standard cosmology that can explain all of those. The CMB is particularly difficult since it very clearly breaks up the components that contribute to the attractive (gravitational) and repulsive (pressure) portions.

      Plus, as a point of note: many of those “non-standard cosmology” theories are purely empirical, in that they say “let’s modify gravity by magic” – there’s typically a free parameter (or a function) which is fit to explain the data, with no explanation as to where that function arises. Lambda-CDM has some of that (in dark energy), but the dark matter portion of it has a phenomenological model in terms of WIMPs. And WIMPs have a reason to exist besides dark matter – they’re predicted in basically all supersymmetric theories. So searching for WIMPs is more than just looking for dark matter, it’s also looking for supersymmetry.

      *: Some kinds of ‘alternate theories’ are, in fact, so general that’s it’s basically impossible to disprove them, because they have enough freedom to include basically any data. It’s also important to distinguish between alternate versions of dark energy, because that’s just a *completely* ad-hoc thing in lambda-CDM, and alternate versions of dark matter.

      1. I find it interesting how often any article on dark matter has comments from people who very obviously aren’t physicists positing that it’s all BS, and accusing researchers of blindly chasing one hypothesis to the exclusion of others. Sometimes they drop a link to supposedly better explanations, but there’s never any meaningful analysis on what makes them better. I always end up wondering whether Dunning-Kruger has struck again, or if it’s just run-of-the-mill anti-intellectualism.

        1. I actually blame science reporting. In the past 40-50 years or so, reporting on physics and astrophysics has gotten so completely ridiculous that the average person really has no idea what the heck they’re reporting on.

          And that’s not because it’s hard to understand. Really, it’s super-easy to understand. But they just make it sound increasingly amazing so people think they *can’t* understand it. When people say “oh, it’s crazy to think that 80+% of the universe is invisible,” that’s because there are slogans out there that *make it sound crazy* so that people will be interested in it (like ‘99% of the universe is unknown – come help us find it’, etc.).

          In truth, the idea that you can’t see the vast majority of matter/energy in the Universe is absolutely nothing new. When a supernova explodes, you can’t see 99% of the energy that gets released. Neutron star binaries spiral inwards for no apparent reason whatsoever, since they radiate energy that you can’t see. It’s not a big deal.

          In order to “see” those things, we need to build crazy big and sensitive detectors. This is the same thing.

        2. The only evidence for the existence of Dark Matter/Energy I have been exposed to boils down to “The part universe we can see does not behave the way our theory says it should, therefore there must be a lot of stuff we can’t see affecting it.”
          If there are better facts I would be interested to see them.

          My understand of the scientific method is observe the world, come up with theories about how the world works, test those theories against the world by trying to observe things the theory says should be there or not there, revise theory to account for the new information and repeat.

          It seems to me they are failing on the revise part after the observations failed to support the theory.

          1. “It seems to me they are failing on the revise part after the observations failed to support the theory.”

            You’re actually very wrong. Which is, again, a failing of science reporting. In fact, it’s the other way around. They haven’t *had* to revise the idea of dark matter, because it’s worked so well. It’s actually the only explanation to the observations that hasn’t needed major revision. Dark energy has less “credit” at this point, since it’s so new, but the CMB results were “predictable” based on the idea of dark energy, so it has that going for it. So I won’t mention dark energy, but it’s separate from dark matter anyway.

            Dark matter started out as the “missing mass problem,” discovered by Zwicky in the 30s. The idea here was easy – you estimate the mass in the galaxy cluster based on their speeds, and then based off of the galaxies themselves, and say holy crap, these motions don’t make sense. More and more examples of this started coming in, famously in galaxy rotation curves. So you postulate “there’s more matter here that isn’t radiating.” The idea of “dark” matter came because for galaxy rotation curves, you see the amount of light fall off as you go out from a galaxy. So the “radiating” matter is falling off. But the rotation speed isn’t. So the ratio of “radiating” to “non-radiating” (or ‘dark’) matter must not be constant.

            Let me stress: the modern idea that dark matter is “invisible” is *not* what people started off thinking. Astronomers can only see stuff that glows. We know of *lots* of things that (theoretically) don’t glow: black holes without accretion disks, planets without a star, black dwarf stars, etc. Even non-luminous *particles* aren’t surprising – neutrinos don’t couple to light, and have mass. The idea that there’s matter that’s separate from luminous (radiating) matter is not controversial. In fact, correlating “amount of light” to “amount of matter” is itself non-trivial.

            But obviously there are other ways to explain galaxy rotation and galaxy clusters, and that’s where modified gravity came in. Which is fine, you want alternate theories, so you can compare them. But at this point, you’re now in the 1990s or so. What’s happened in the past 20 years?

            First, the CMB anisotropy measurements. The CMB, unlike what a lot of people say, isn’t really a “picture of the Big Bang.” It’s a picture of when the Universe cooled enough to not be a plasma. And the cool thing about that is that you can look at the acoustic modes in the CMB to figure out lots of stuff.

            1: You can figure out how much stuff there is (regardless of what it is).
            2: You can figure out how much stuff with positive pressure couples to radiation.
            3: You can figure out how much stuff with positive pressure *doesn’t* couple to radiation.

            So you can see that from the CMB, the ‘dark matter’ theory would say “#3 is going to be way bigger than #2.” Naively modifying gravity on large scales likely wouldn’t do that the same way. And guess what? The CMB anisotropy experiments matched *very* well to what dark matter predicted.

            Second, gravitational lensing measurements. If there’s really just “stuff” out there that’s bending light, then we should be able to look at gravitational lenses, back out the effects, and create a “map” of that stuff. Again, modifying gravity on large distance scales wouldn’t do that the same way: for one thing, dark matter densities could be totally random compared to luminous matter densities. And guess what? When you do the calculations, the lensing experiments match very well to dark matter, and *not* to modified gravity.


            So the idea of “lots of matter that doesn’t glow” started off as an idea in the 30s, and its predictions have been matched by experiment over and over. The theory isn’t the problem. The problem is that the ‘best’ theory left for what dark matter *actually is* makes it really stupidly hard to measure them. But that’s an economic problem, not a fundamental one.

    3. Appears to me funding for science is similar to funding for petroleum. The majority of the money to drilling where the consensus is where oil will discovered, because the conses has served well in the past. Wild cat operations have to scout for funding because wild catting has a spotty success history. Dark matter may very well be there. Diverting funding from that research to other research could delay the discovery of dark matter with the chance the other research can fall short. Money goes to the consensus when the money likes the track record of the consensus. Wildcatting has always had to find alternate sources of funding. I’m pretty damned glad I’m not the one charged to make the funding decisions.

      1. Your analogy does not really work because scientific knowledge is not petroleum, what wild cat petroleum explorers discover has no effect on the established drillers operations. In Science one wild cat discovery can change what all scientists are doing not just what the discoverers are doing, its as if the wild cat well could drain every other well on earth. This means that scientific wild cat operations offer rewards far beyond what petroleum wild cat operations do, if wild cat petroleum operations could potentially take the business away from all other petroleum operations combined then established petroleum operators would have a big incentive to impede independent wild cat operations and to slow their own discovery of new wells which then might disenfranchise some of their other operations, much like the diamond industry which functions as a monopoly with express interest in suppressing new discoveiries. Where money is concerned science is usually compromised.

    4. I agree with your criticism, designating the goal of the experiment as an “attempt to find dark matter” is scientifically inappropriate, if this were a murder trial its as if the jurors were instructed by both the prosecuting and defending attorney that the trial is an “attempt to establish presumed guilt” with no mention made of potential innocence! If Wimps have not been found the scientific outlook must presume the universe to be innocent of producing them, this should be labled an attempt to see if wimps exist in the first place not an attempt to “find wimps”.

      Scientifically this should be framed as an attempt to falsify the wimp theory and not an attempt to prove the wimp theory, so far the wimp theory has been falsified. Since the wimp theory continues to be falsified by every experiment done so far the scientific response should be to grant more funding to develop ALTERNATIVE THEORIES, but by framing the goal as it has been framed one effectively denies the role of theory in science which protects the architecture of the current funding regime, but threatens the architecture of scientific discourse.

      1. “Scientifically this should be framed as an attempt to falsify the wimp theory ”

        No, that’s not right at all.

        There’s no “WIMP theory” that’s being tested here. The idea that dark matter is made of cold, weakly interacting massive particles does not, in *any way*, require a mass or an interaction cross section that any of these experiments could ever reach. Nature doesn’t have to be nice to us.

        Second, the idea that dark matter is made of cold WIMPs does not “continue to be falsified.” It fits a lot of the data. Its biggest problems have nothing to do with the fact that they haven’t been seen. The biggest problem for cold WIMPs are the missing satellites problem and the core/cusp problem, which both have to do with the structure and number of dwarf galaxies. But the solution to those will most likely just have us break the idea that WIMPs are cold and don’t interact (even with themselves) at all.

        The parameter space that LUX and most other dark matter experiments are carving out have to do with minimal supersymmetric models, not WIMPs. If they don’t find dark matter, it’s not WIMPs that are the problem, it’s the MSSM models that they’re probing.

  4. I always had a feeling that gravity and the Casimir effect were one in the same, with gravity just being the effect on a large scale.

    If familiar with the Casimir effect, you’ll know that it’s quite strong; yet considered to fall off at very short distances, and therefore isn’t considered to be responsible for what we know as gravity. It’s true that in a lab, two small masses have to be very close together for the effect to be measurable.

    But rapid fall-off doesn’t mean the force becomes zero, at any distance. It falls off according to the inverse square law, just like any other force. And soon falls below our ability to measure, at least for such small masses. Which are also surrounded by much larger masses also exerting effects (the test chamber, the lab, and ultimately the whole planet), with these effects becoming more dominant by proportion as the distance between test masses increases.

    Thinking on a cosmological scale, a galaxy is a few billion small masses (stars), which are very close together; at least relative to the vast expanse of nothing in which they are surrounded. It’s an enormous version of the Casimir experiment. If you were to shrink a galaxy equally in both size and mass, right down to the size and mass of a typical Casimir experiment, would you not fully expect to be able to measure a strong Casimir force? And in reversing this shrinkage, the distance between masses increases, but so does the mass; so why is it assumed the Casimir force is irrelevant?

    Can’t prove anything of course, and I admit it may be silly to indulge in such speculation. But no sillier in my opinion than to assume that 95% of the universe is dark matter/energy because gravity doesn’t work like we expect, when we don’t even know what gravity is.

    1. The Casimir effect in fact falls off as 1/r^4, not 1/r^2.

      Moreover it only applies to objects with an electric charge (that can interact with an electromagnetic field), whereas gravity interacts with everything with mass (.. presumedly).

      1. “The Casimir effect in fact falls off as 1/r^4, not 1/r^2.”

        True, but if I’m not mistaken, aren’t those formula measuring force in a different way?

        Gravity = falls off as 1/r^2, the force generated on *one* mass by another
        Casimir = falls off as 1/r^4, the *total* force generated by two masses *on each other*

        1. You’re mistaken. All forces are reciprocal. Newton’s third law, and all that. You pull on the Earth with exactly the same force that the Earth pulls on you. The Casimir force is exactly the same.

        2. Actio = Reactio, Newton’s third law

          The force exerted on one object by the other is exactly the same as the other way ’round.

          Also adding two numbers that fall of like 1/r^2 gives you another number that falls like 1/r^2.

  5. I agree with the criticism, designating the goal of the experiment as an “attempt to find dark matter” is scientifically inappropriate, if this were a murder trial its as if the jurors were instructed by both the prosecuting and defending attorney that the trial is an “attempt to establish presumed guilt” with no mention made of potential innocence! If Wimps have not been found the scientific outlook must presume the universe to be innocent of producing them, this should be labled an attempt to see if wimps exist in the first place not an attempt to “find wimps”.

    Scientifically this should be framed as an attempt to falsify the wimp theory and not an attempt to prove the wimp theory, so far the wimp theory has been falsified. Since the wimp theory continues to be falsified by every experiment done so far the scientific response should be to grant more funding to develop ALTERNATIVE THEORIES, but by framing the goal as it has been framed one effectively denies the role of theory in science which protects the architecture of the current funding regime, but threatens the architecture of scientific discourse.

    1. It’s a search for a black swan, and unfortunately is gigantic and expensive cos that’s the sort of swans they’re looking for. They won’t disprove the theory until another theory is proven that specifically rules this one out.

      I can only assume that dark matter theory, and the associated maths, has impressed those in the know enough to pay for building this detector.

      Maybe there’s a hegemony, there probably is. Science needs to be conservative, but all scientists are aware that they could be wrong, about anything and everything.

      Personally I don’t think dark matter smells right, seems like a kludge, filling the Universe with some unlikely particle just to make the maths work. But I don’t know enough about it to say that with any sort of authority, so if people who know what they’re talking about more than I do want to put photomultiplier tubes down mines, it doesn’t bother me.

      1. “Personally I don’t think dark matter smells right, seems like a kludge, filling the Universe with some unlikely particle just to make the maths work.”

        Particle physics has worked like this for years. You see energy that’s missing, you hypothesize a particle.

        Besides, the Universe *is* filled with weakly interacting particles with mass – neutrinos. Annoyingly, it looks like they just don’t have *enough* mass. So it’s not like the idea is totally crazy. There’s precedent.

        1. Sure but hypothesising, what, the majority of mass in the Universe, and dark energy being even more prolific than that? It’s not some teeny, humble little particle, it’s supposed to be what EVERYTHING is made of! And strangely enough it has bugger-all effect on anything detectable, except on a galactic scale. Most other particles aren’t supposed to dominate the Universe!

          And it all comes about because gravity isn’t quite adding up. Extraordinary claims require extraordinary evidence. Sure, we might well see if this detector project, or others like it, turn something up.

          It’s just such an enormous claim to make. But that said I’m sure I wouldn’t understand the equations if I saw them, so. I’ll withhold judgement in any meaningful way. But it still doesn’t smell right!

          I’m not sure what dark energy is supposed to actually be, from what I’ve read it seems science isn’t too sure either. Is it, again, just something that’s affecting gravity?

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