Gravitational Forces
February 1, 2020 11:42 AM   Subscribe

 
Whoa! Cool stuff!!!
posted by Don.Kinsayder at 12:22 PM on February 1


I wish the Guardian would link to this arxiv link when linking to the 2010 paper. The wikipedia article on Massive Gravity is a fair summary as well.
posted by vacapinta at 12:31 PM on February 1 [2 favorites]


So... does anyone have the TD/DU (too dumb/ didn’t understand?) ? Thanks
posted by From Bklyn at 1:58 PM on February 1 [5 favorites]


"There must always be some quantum jiggle."

T-shirt of the week.

Nice primer on gravitational waves.

does anyone have the TD/DU (too dumb/ didn’t understand?) ? Thanks
I wing but then my friends brother and some others found a black hole in the center of our galaxy.
posted by clavdivs at 2:28 PM on February 1


does anyone have the TD/DU (too dumb/ didn’t understand?) ? Thanks
I'm no physicsmatt, but I can give it a shot. (With apologies in advance for any egregious errors).

So. Every phenomenon in the universe requires a quantum carrier, a "medium of exchange". Examples of these include photons, the force carrier of electromagnetism, which produce light and radio waves, and the Higgs boson, which provides mass. Given this model, it makes sense that we need a "messenger particle" that communicates gravity, called gravitons. While we can see their effects (the ordinary, everyday attraction of the Earth, the movement of planets in orbit), gravitons themselves remain entirely theoretical, since no-one has yet detected any. But - given that the rest of the quantum field works - it makes sense that they should exist.

However, there's a problem. At very, very large distances, it looks like things like galaxies are flying apart at an accelerating rate. After the "push" provided by the expansion of the Big Bang, something appears to be putting its foot on the cosmic gas pedal. Gravity acts as a retardant force to this. It's not enough to cancel out the acceleration, but it does act to slow it down.

It was assumed that gravitons, like photons, are massless: that's what allows light to move as fast as it does. But if gravitons have even the tiniest amount of mass, gravity - again, at unimaginable distances - would "peter out" faster than the inverse square law would imply, meaning that objects further away would be "allowed" to accelerate quicker from the Big Bang's impulse, providing the momentum we see as dark energy.

I should mention that this is hardly the only theory. Another possibility is that the "standard candles" we use as one method of estimating distances in the universe aren't so standard after all, meaning that our interstellar measurements may be way off, and "dark energy" may be equivalent to the aether of the 19th century: a way of explaining a phenomenon that doesn't actually exist. Personally I think that's unlikely, not least because it would overturn or invalidate a number of Nobel prizes, but the possibility should be considered.

I hope this helps!
posted by Bora Horza Gobuchul at 3:47 PM on February 1 [24 favorites]


At very, very large distances, it looks like things like galaxies are flying apart at an accelerating rate.

How can this be distinguished from "at shorter distances, it looks like things like galaxies are flying apart at a decelerating rate"? I.e., that the rate of expansion is slowing?
posted by Joe in Australia at 4:06 PM on February 1


How can this be distinguished from "at shorter distances, it looks like things like galaxies are flying apart at a decelerating rate"? I.e., that the rate of expansion is slowing?

by comparison with events at human scale - which is the basis for intuitive understanding of large or small scale events (which don't always play along!)

the picture that we expect to see from an explosion is that first there is a great acceleration of mass (including the mandatory one wheel which must always roll away from any explosion), and that those objects would subsequently... not accelerate outward from the explosion anymore.

ie: that they would slow and stop if on earth or that they would continue at a constant velocity if in space.

If things appear to be accelerating after the apparent cause is no longer happening... the models that are commonly used to visualize this are that the fabric of spacetime is expanding (some youtube videos will have this as an expanding balloon) or that there is something else either pushing from the 'center' or pulling from outside.

Given that there is no privileged frame of reference in physics, any distinctions like "it seems slower close up" or "it seems faster far away" are largely meaningful only in helping us to make a mental model of the event.

(which is fun and natural, but I don't ever expect to have a 'real' picture in my mind.)
posted by Acari at 5:30 PM on February 1 [1 favorite]


to be a little more clear, it is more 'natural' to act as if we're at the center where things are normal and that things far away are the weird ones. Mostly because we're only in one place (which is not the center of the universe) and that there is more 'out there' than there is 'nearby'.

also that the 'at constant velocity in space' bit is not what we'd actually expect (because gravity seems like it ought to be pulling all masses back towards each other), but that even if we ignored that, things seem to be weird anyway.

all of this has nothing to do with the article - just a layman's overview of the situation that the article is talking about

posted by Acari at 5:44 PM on February 1


The thing I can't get my mind around is how the carrier of a force can also be a source of that force.

A photon doesn't emit photons.
posted by hypnogogue at 6:48 PM on February 1


It's worth mentioning that the evidence for dark energy rests on more than just supernova measurements (cosmic microwave background, baryon acoustic oscillations from large-scale structure, to name a few). There are several current and soon-to-be-current projects dedicated to making more precise measurements of cosmic expansion like the Vera C. Rubin Observatory (used to be Large Synoptic Survey Telescope) or the bonanza of astronomical collaborations with "Dark Energy" in their name like the Dark Energy Survey or the Dark Energy Spectroscopic Instrument.

As this article shows, a pretty wide variety of theories can be crafted to explain the current observations. "Dark Energy" as a vacuum energy which is not diluted by the expansion of the universe is just the simplest model that can accommodate the current observational evidence, where "simple" probably involves some bias/groupthink in the culture of physics to minimally deviate from Einstein. (It's also arguably the most boring, which means we live in an exciting world!)
posted by getao at 10:33 PM on February 1


Also meant to include new confirmation of gravitational "frame dragging".
posted by blue shadows at 11:01 PM on February 1


The speed of gravity has too been measured. There was a whole focus issue in the Astrophysical Journal Letters on the first combined measurement of gravitational waves and an electromagnetic counterpart; since the signals arrived here on Earth near-simulatenously, the speed they travelled at has to be similar. Many of the articles therein are open access.

Including, for example, Gravitational Waves and Gamma-Rays from a Binary Neutron Star Merger: GW170817 and GRB 170817A. Among other results, they constrain the speed of gravity to be the same as the speed of light, to better than 10^{-14}. Really, if someone wants to write a breathless article about modern advances, they should be including the relevant advances! The article even talks a bunch about the importance of LIGO and upcoming detectors like LISA, so it seems extra weird to just claim that the speed hasn't been measured.

(Of course, it isn't really measuring directly the speed; this is assuming that the gamma ray burst and gravitational wave signal found by LIGO/Virgo and Fermi actually are from the same source, but that's really very likely; insert normal scientific dissembling here).

Anyhow I actually would be very interested in the response of the massive gravity community to the constraints placed on it by LIGO observations, and particularly by GW170817/GRB170817A combined detection. If someone knows where a summary of such response exists, please do let me know -- it wasn't in this article. (And I have enough things to read in my own field, so I haven't gone wading into the massive gravity literature much in the years since 2017).
posted by nat at 11:16 PM on February 1 [1 favorite]


Are there worm holes in this theory? We were promised wormholes.
posted by sammyo at 6:55 AM on February 2 [1 favorite]


this is so super dope

also i really just desperately want one of the hugest puzzles in the universe, possibly THE hugest puzzle we currently know of, to be solved by a 41 year old woman. we will be able to power more than one of the sensors she'll want to keep doing research using only the tears of shitty old dudes
posted by lazaruslong at 7:13 AM on February 2


A photon doesn't emit photons.
posted by hypnogogue

I get that. But dark matter (theory) has gravitational force but does not interact with photons. Here's an old but good video on 'Detecting Dark Matter.'
posted by clavdivs at 9:58 AM on February 2


Would this limit the effects of the hypothesized Big Rip to separating galaxies, but not ripping atoms apart?
posted by joeyh at 12:11 PM on February 2



Would this limit the effects of the hypothesized Big Rip to separating galaxies, but not ripping atoms apart?
As I understand it, yes. Big Rip scenarios rely on dark energy constantly increasing. If gravity decays at a rate faster than expected, then dark energy would never affect baryonic matter directly, only the spacetime between objects that had slipped the bounds of gravitational attraction.

At a practical level, any Big Rip that could affect the universe at the level of individual atoms in measurable ways would likely occur long after the Degenerate Age 100 trillion years from now, perhaps even after protons decay (assuming that they do). If the universe degenerates to a random soup of dark matter, electrons, and positrons, there wouldn't be any atoms left to tear apart.
posted by Bora Horza Gobuchul at 10:51 PM on February 2


A photon doesn't emit photons.

No but light is affected by gravity even though photons are massless. So gravitons themselves would follow the curves of spacetime too, thus also affected by gravity. What nat posted above, the LIGO results, show not only that gravitons mass would have to be tiny but also that it appears that gravitons followed pretty much the same curved path through spacetime as the light.

How a graviton-graviton interaction would work in the case of gravitons having mass is a question a theory of quantum gravity would have to answer, which we are still far away from.
posted by vacapinta at 2:38 AM on February 3


Worth pointing out that gluons can self interact.
posted by edd at 2:44 AM on February 3


Quantum fluctuations sustain the record superconductor: "when atoms are treated like quantum objects, which are described with a delocalized wave function, the energy landscape is completely reshaped: only one minimum is evident... Somehow, quantum effects get rid of everybody in the mattress but one person, who deforms the mattress only in one single point."

Observing localisation in a 2D quasicrystalline optical lattice: "It's well-known that when the potential wells in a crystal get strong enough, its electrons 'localize': instead of having spread-out wavefunctions, they get trapped in specific locations as shown here. This is called 'Anderson localization'. But when a Bose-Einstein condensate localizes, all the atoms get trapped in the *same place* - because they're all in exactly the same state!"

Researchers demonstrate optical backflow of light: "If a special superposition of waves, all propagating forward, is constructed, the overall wave can realize what's called 'optical backflow.'"

Optical vortex: "An 'optical vortex' is a beam of light that turns like a corkscrew as it moves. It's dark at the center. There's one type of optical vortex for each integer m. You can use an optical vortex to trap atoms! They move along the dark tube at the center of the vortex."

Polaritons: "First, when an electron in a crystal is knocked out of place, it leaves a 'hole'. This hole can move around -- and it acts like a positively charged particle! Since electrons are negative and holes are positive, they attract each other! An electron orbiting a hole acts like a hydrogen atom. It's called an 'exciton'. It can move around! But after a while, the electron falls into the hole. Finally, an exciton can attract a photon! They can stick to each other a form a new particle called a 'polariton'! Polaritons are exciting to me because they're made of an electron, an *absence* of an electron, and light."

Liquid Light: "Scientists have made *liquid light* by mixing the light with matter."

Physicists Discover New Quasiparticle: "there are several more complex quasiparticles: excitons, phonons (particles derived from the vibrations of atoms in a solid), plasmons (particles derived from plasma oscillations), magnons (collective excitations of the electrons' spin structure in a crystal lattice), and polarons (electrons dressed by a phonon cloud)."

Making high-temperature superconductivity disappear to understand its origin: "The team believes that something similar to electron-phonon coupling is going on in this case, but instead of phonons, another excitation gets exchanged between electrons. It appears that electrons are interacting through spin fluctuations, which are related to electrons themselves."

Scientists discover hidden symmetries, opening new avenues for material design: "Fruchart and Vitelli imagined using this approach to take a particle such as a phonon—essentially a particle of heat—and give it properties that it doesn't usually have. Electrons have a property called 'spin' that is used as a basis for some of the latest high-tech electronics. Phonons don't have a spin, but if scientists could shape the structure of materials to give phonons a 'fake spin', they could potentially use them in phononic devices—similar to electronics, but with different abilities, such as heat control."

Physicists Have Identified a Metal That Conducts Electricity But Not Heat: "the team looked at the way that electrons move within vanadium dioxide's crystal lattice, as well as how much heat was being generated. Surprisingly, they found that the thermal conductivity that could be attributed to the electrons in the material was 10 times smaller than that amount predicted by the Wiedemann-Franz Law. The reason for this appears to be the synchronised way that the electrons move through the material."
posted by kliuless at 11:42 PM on February 7 [3 favorites]


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