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This made me wonder how far we are from being able to create and detect gravitons. The Wikipedia page on gravitons [0] addresses this question: Unambiguous de

This made me wonder how far we are from being able to create and detect gravitons. The Wikipedia page on gravitons [0] addresses this question: Unambiguous detection of individual gravitons, though not prohibited by any fundamental law, is impossible with any physically reasonable detector. The reason is the extremely low cross section for the interaction of gravitons with matter. For example, a detector with the mass of Jupiter and 100% efficiency, placed in close orbit around a neutron star, would only be expected to observe one graviton every 10 years, even under the most favorable conditions. [...] However, experiments to detect gravitational waves, which may be viewed as coherent states of many gravitons, are underway (such as LIGO and VIRGO). Although these experiments cannot detect individual gravitons, they might provide information about certain properties of the graviton. For example, if gravitational waves were observed to propagate slower than c (the speed of light in a vacuum), that would imply that the graviton has mass [...]. Fascinating! I take it that the question of whether the graviton could have mass is now considered to be well answered in the negative. [0] https://en.wikipedia.org/wiki/Graviton https://en.wikipedia.org/wiki/Graviton

sandworm101 11y ago
If gravitons have mass, then the universe is too strange to exist. Gravity is an interaction that defines the presence of matter (see dark matter). For the object that transmits that force between masses to itself have mass ... how can a black hole then project gravity? Imho whatever is carrying gravity between masses cannot itself have a mass.
- TrainedMonkey 11y ago
  Gravitation does not technically interact with light either, but rather bends the spacetime the light travels through. So question is, what makes gravitons different?
  - noobermin 11y ago
    I'm a physicist, but in a different field, but my (possibly incorrect) impression is what a quantized particle is is a bit mysterious when the field is strong, for example, with lensing. Best example: the Hydrogen atom is supposedly quantum, but if it is quantum, where are the photons? the q^2/r potential is a mean field that one finds from classical electrodynamics, it isn't formed by the summation of photons. [Another mental poker, photons are momentum eigenstates, so how can potential be described in position space? You'd need to sum up an infinite number of them! (For EM students, recall how to represent 1/r in spherical harmonics or in terms of sines and cosines)] What happens, as I understand it, is with strong fields, one tends to use a semi-classical description because in the strong field limit, one deals with many photons, which should approach the classical limit. Basically, quanta are like "pertubations" of the fields from their "free" solutions, as they are in GR (linearization of the GR field eqns) and as they are in EM. Free essentially means in the absence of sources, like charges, or masses for GR. So trying to explain general phenomena in terms of "pertubations", which are basically the solutions for "free" fields, is not always fair. One doesn't always face this in high energy physics because in HEP, most of the incoming and outgoing states in a problem are these "free" solutions. For example when doing scattering off a hydrogen atom, the incoming states are "free" (a free nuclei, a free electron), so one can use photons for that phenomena, and one finds that the scattering is like scattering against a (mean) 1/r potential. But in the case where the strong fields don't turn off, like when you are bound to a Hydrogen atom, or when considering nucleons in nuclei in the low energy limit, one turns away from the pertubative, photon/gluon model and either solving the problem numerically or treats the fields as semi-classical, as with the Hydrogen atom. For my field of laser-plasma physics, this shows up in the so-called "Volker-state", rather than treating the strong laser field as a sum of innumerable (ie., not-simulatable) photons, one treats the Laser field as a semi-classical background for the quantum guys (electrons, ions). I think lensing is like strong static fields in EM. One wouldn't really think of them in terms of quanta of the field.
- kingkawn 11y ago
  maybe a black hole projecting gravity is how it sends information back out
- contravariant 11y ago
  Force carrying particles in general don't have mass. Except that some of them seem to do, which was rather puzzling for some time, but was solved using the Higgs mechanism. I can't think of an obvious reason the Higgs mechanism wouldn't work for gravitons, but I could be mistaken, it's not exactly the most intuitive area of physics. Also, keep in mind that the strong force transmits the force between colour charges while also having a colour charge itself, so it isn't entirely inconceivable for the force transmitting the attraction between masses to have a mass. No clue if a massive graviton would allow for black holes, but it's not entirely sure what black holes even are (especially quantum mechanically). At the very least it's presumably possible for some particles to escape it (e.g. as Hawking radiation).
  - sandworm101 11y ago
    If the massive gravitron was leaving a black hole it would be slowed by the black hole's gravity. (1) We should see this as some inconsistency in how gravity scales with the mass of a black hole. The larger ones would have proportionately greater 'drag' on leaving gravitrons, pulling more in. (2) If they are massive, and therefore subject to slowing, shouldn't gravity waves leaving a black hole be subject to some sort of doppler effect? Should we be looking for red/blueshifts in these waves? (3) If gravitrons have mass and are subject to gravity, what brings that gravity? What sub-gravitron particle regulates gravity going in/to/out of the gravitron? This would require a new set of particles be created by non-gravitron massive objects (ie black holes) alongside the gravitrons. Like I said, too strange to exist.
    - contravariant 11y ago
      >(1) We should see this as some inconsistency in how gravity scales with the mass of a black hole. The larger ones would have proportionately greater 'drag' on leaving gravitrons. Perhaps, no clue how a quantum mechanical gravity would interact with a black hole. >(2) If they are massive, and therefore subject to slowing, shouldn't gravity waves leaving a black hole be subject to some sort of doppler effect? Should we be looking for red/blueshifts in these waves? The Doppler effect happens even for light, which isn't massive at all (that we know of). >(3) If gravitrons have mass and are subject to gravity, what brings that gravity? What sub-gravitron particle regulates gravity going in/to/out of the gravitron? There's no reason they couldn't interact with themselves, in fact I guess that's probably the most likely case.
- SaberTail 11y ago
  What do you think of the strong nuclear force? Gluons are the force carriers between color-charged particles. And gluons themselves have a color charge. So gluons transmit force between each other. Does that mean the universe is already too strange to exist? By the way, this is why the strong nuclear force has such a short range. Gravity has infinite range as far as we can tell, so that makes it unlikely that the graviton, if it exists, has mass.
  - thfuran 11y ago
    How does the mediating particle being affected by the force translate to a shorter range?
    - scentoni 11y ago
      No--the mediating particle having mass translates to a finite range.
    - pyre 11y ago
      I'm assuming it's because they interact with each other.
    - SaberTail 11y ago
      There are two aspects to it. And I'll be very hand-wavey because QCD is not my field. Suppose you have two quarks and you start to pull them apart. The gluons that transmit the force between the two quarks tend to "bunch" together because they have their own charge. You can think of it roughly like a rope of gluons trying to pull the quarks back together. If you keep pulling on the quarks, you might expect the gluons to eventually "break". But this doesn't happen, because the gluons act on each other. If there were ever a break, more gluons would join in, tugging the break back together. Eventually, you end up with so much energy density in all these gluons that they start forming new quarks and other particles. These new particles will bind with your quarks and each other to form color-neutral particles. So I spoke a little imprecisely. Gluons, being massless, have infinite range. But you won't ever see the strong force acting over any large distance because anytime you try to get color-charged particles far enough apart, you'll end up making more particles. Wikipedia has a brief write-up that provides an illustration. https://en.wikipedia.org/wiki/Color_confinement https://en.wikipedia.org/wiki/Color_confinement
- Manishearth 11y ago
  > Imho whatever is carrying gravity between masses cannot itself have a mass. Forget gravitons, this already exists in pure general relativity. Spacetime is curved around a massive object, and that curvature contains energy. That's just another word for mass, so the curvature itself exerts gravity. This creates more curvature (...etc etc). This is one of the reasons as to why Einstein's equations are nonlinear. Note that the concept of having mass is separate from the gravity force. Interaction with the Higgs field gives rise to mass, whereas gravitons are the force carrying particle for gravity.
  - cyphar 11y ago
    Only electrons get mass from the Higgs mechanism. Most of your mass comes from your protons and neutrons (or rather the energy "stored" in the bonds between the quarks that make them up).
    - Manishearth 11y ago
      You're talking about binding energy, and that's not where the majority of mass comes from. All massive particles get their mass from the Higgs field.
      - rotorblade 11y ago
        All _fundamental_ particles gets their mass from the Higgs (up to some issues with the neutrinos). Composite particles, say the Proton, is a strongly coupled system (that is, can not be described by perturbation theory) and it does not have the mass being the sum of its constituents (not even most of it). Hence, it is not known what gives most of the mass of particles such as the Proton.
        Manishearth 11y ago
        Er, sorry, implicitly was talking about fundamental particles. IIRC we do know where protons get their mass. The internal color field has some energy, thus some mass, which comes from that field interacting with Higgs.
dbranes 11y ago
Gravitational waves are a prediction of classical linearlized gravity - gravitational waves are to (classical) gravitons what light waves are to photons.
- smegel 11y ago
  But we have direct experimental confirmation of light behaving light both a wave and as a particle. Do we have any such evidence to show gravity exhibiting particle like behavior?
  - sjburt 11y ago
    No, but we predict it to be essentially impossible to observe a graviton. http://arxiv.org/abs/gr-qc/0601043 http://arxiv.org/abs/gr-qc/0601043
dbranes 11y ago
Gravitational waves are a prediction of classical linearlized gravity - gravitational waves are to (classical) gravitons what light waves are to photons.
jug 11y ago
Related to this... According to Kip Thorne, this recent discovery alone places an upper bound of maximum mass for a graviton, if it exists and has mass, at no more than 10^-55 grams. Since otherwise the observed waves would have been distorted. It's cool that we are already seeing conclusions drawn from this observation. This single obsrrvation. https://starguyspeaks.wordpress.com/2016/02/11/on-gravitational-waves-2/ https://starguyspeaks.wordpress.com/2016/02/11/on-gravitatio...
pbhjpbhj 11y ago
>how far we are from being able to create and detect gravitons // Surely we create gravitons whenever we move a mass just like we create photons whenever we move a body that interacts electromagnetically? Isn't the point that we're constantly exchanging gravitons with all matter as they mediate gravitational attraction.
- ScottBurson 11y ago
  I think you must be right -- but maybe there's some other way to create them, without needing to move a mass. The ability to manipulate gravity itself might be the key to space travel.
  - other_herbert 11y ago
    Flying cars man! Think of the flying cars! And maybe real hover boards
    - undersuit 11y ago
      First we need to find out how to create repulsion. Right now I'm pretty sure a graviton generator would just be a novelty device that weighs more than what it's mass would lead you to think it weighs. Maybe we could make orbital graviton beam generator that could literally suck an object off the face of the Earth.
      - celticninja 11y ago
        That sounds like a tractor beam to me.
      - pavel_lishin 11y ago
        > I'm pretty sure a graviton generator would just be a novelty device that weighs more than what it's mass would lead you to think it weighs Which would make space travel a lot easier - no more worrying about bone density loss!
      - 11y ago
thedatabase 11y ago
Here's a quote from John Baez's G+: "Gravity is a weak force, in the sense that the gravitational force between two protons is about 10^33 times weaker than the electric force between them. And I'm using protons rather than electrons here to make the gravity stronger - with electrons gravity would be almost 10^40 times weaker. This has various consequences, but one is that gravitational waves are absorbed by matter much less than electromagnetic waves. It would be fun to estimate the amount of energy absorbed by the Earth as this particular gravitational wave came through, but it would be absurdly small. Gravitational waves make neutrinos look like rampaging gorillas." Rephrasing, and assuming waving commutes with rampaging, gravitons make neutrinos look like WAVES of rampaging gorillas. I'm no physicist but to answer the original question I'd hazard a guess: quite far!

sandworm101 11y ago

If gravitons have mass, then the universe is too strange to exist. Gravity is an interaction that defines the presence of matter (see dark matter). For the object that transmits that force between masses to itself have mass ... how can a black hole then project gravity? Imho whatever is carrying gravity between masses cannot itself have a mass.

TrainedMonkey 11y ago
Gravitation does not technically interact with light either, but rather bends the spacetime the light travels through. So question is, what makes gravitons different?
- noobermin 11y ago
  I'm a physicist, but in a different field, but my (possibly incorrect) impression is what a quantized particle is is a bit mysterious when the field is strong, for example, with lensing. Best example: the Hydrogen atom is supposedly quantum, but if it is quantum, where are the photons? the q^2/r potential is a mean field that one finds from classical electrodynamics, it isn't formed by the summation of photons. [Another mental poker, photons are momentum eigenstates, so how can potential be described in position space? You'd need to sum up an infinite number of them! (For EM students, recall how to represent 1/r in spherical harmonics or in terms of sines and cosines)] What happens, as I understand it, is with strong fields, one tends to use a semi-classical description because in the strong field limit, one deals with many photons, which should approach the classical limit. Basically, quanta are like "pertubations" of the fields from their "free" solutions, as they are in GR (linearization of the GR field eqns) and as they are in EM. Free essentially means in the absence of sources, like charges, or masses for GR. So trying to explain general phenomena in terms of "pertubations", which are basically the solutions for "free" fields, is not always fair. One doesn't always face this in high energy physics because in HEP, most of the incoming and outgoing states in a problem are these "free" solutions. For example when doing scattering off a hydrogen atom, the incoming states are "free" (a free nuclei, a free electron), so one can use photons for that phenomena, and one finds that the scattering is like scattering against a (mean) 1/r potential. But in the case where the strong fields don't turn off, like when you are bound to a Hydrogen atom, or when considering nucleons in nuclei in the low energy limit, one turns away from the pertubative, photon/gluon model and either solving the problem numerically or treats the fields as semi-classical, as with the Hydrogen atom. For my field of laser-plasma physics, this shows up in the so-called "Volker-state", rather than treating the strong laser field as a sum of innumerable (ie., not-simulatable) photons, one treats the Laser field as a semi-classical background for the quantum guys (electrons, ions). I think lensing is like strong static fields in EM. One wouldn't really think of them in terms of quanta of the field.
kingkawn 11y ago
maybe a black hole projecting gravity is how it sends information back out
contravariant 11y ago
Force carrying particles in general don't have mass. Except that some of them seem to do, which was rather puzzling for some time, but was solved using the Higgs mechanism. I can't think of an obvious reason the Higgs mechanism wouldn't work for gravitons, but I could be mistaken, it's not exactly the most intuitive area of physics. Also, keep in mind that the strong force transmits the force between colour charges while also having a colour charge itself, so it isn't entirely inconceivable for the force transmitting the attraction between masses to have a mass. No clue if a massive graviton would allow for black holes, but it's not entirely sure what black holes even are (especially quantum mechanically). At the very least it's presumably possible for some particles to escape it (e.g. as Hawking radiation).
- sandworm101 11y ago
  If the massive gravitron was leaving a black hole it would be slowed by the black hole's gravity. (1) We should see this as some inconsistency in how gravity scales with the mass of a black hole. The larger ones would have proportionately greater 'drag' on leaving gravitrons, pulling more in. (2) If they are massive, and therefore subject to slowing, shouldn't gravity waves leaving a black hole be subject to some sort of doppler effect? Should we be looking for red/blueshifts in these waves? (3) If gravitrons have mass and are subject to gravity, what brings that gravity? What sub-gravitron particle regulates gravity going in/to/out of the gravitron? This would require a new set of particles be created by non-gravitron massive objects (ie black holes) alongside the gravitrons. Like I said, too strange to exist.
  - contravariant 11y ago
    >(1) We should see this as some inconsistency in how gravity scales with the mass of a black hole. The larger ones would have proportionately greater 'drag' on leaving gravitrons. Perhaps, no clue how a quantum mechanical gravity would interact with a black hole. >(2) If they are massive, and therefore subject to slowing, shouldn't gravity waves leaving a black hole be subject to some sort of doppler effect? Should we be looking for red/blueshifts in these waves? The Doppler effect happens even for light, which isn't massive at all (that we know of). >(3) If gravitrons have mass and are subject to gravity, what brings that gravity? What sub-gravitron particle regulates gravity going in/to/out of the gravitron? There's no reason they couldn't interact with themselves, in fact I guess that's probably the most likely case.
SaberTail 11y ago
What do you think of the strong nuclear force? Gluons are the force carriers between color-charged particles. And gluons themselves have a color charge. So gluons transmit force between each other. Does that mean the universe is already too strange to exist? By the way, this is why the strong nuclear force has such a short range. Gravity has infinite range as far as we can tell, so that makes it unlikely that the graviton, if it exists, has mass.
- thfuran 11y ago
  How does the mediating particle being affected by the force translate to a shorter range?
  - scentoni 11y ago
    No--the mediating particle having mass translates to a finite range.
  - pyre 11y ago
    I'm assuming it's because they interact with each other.
  - SaberTail 11y ago
    There are two aspects to it. And I'll be very hand-wavey because QCD is not my field. Suppose you have two quarks and you start to pull them apart. The gluons that transmit the force between the two quarks tend to "bunch" together because they have their own charge. You can think of it roughly like a rope of gluons trying to pull the quarks back together. If you keep pulling on the quarks, you might expect the gluons to eventually "break". But this doesn't happen, because the gluons act on each other. If there were ever a break, more gluons would join in, tugging the break back together. Eventually, you end up with so much energy density in all these gluons that they start forming new quarks and other particles. These new particles will bind with your quarks and each other to form color-neutral particles. So I spoke a little imprecisely. Gluons, being massless, have infinite range. But you won't ever see the strong force acting over any large distance because anytime you try to get color-charged particles far enough apart, you'll end up making more particles. Wikipedia has a brief write-up that provides an illustration. https://en.wikipedia.org/wiki/Color_confinement https://en.wikipedia.org/wiki/Color_confinement
Manishearth 11y ago
> Imho whatever is carrying gravity between masses cannot itself have a mass. Forget gravitons, this already exists in pure general relativity. Spacetime is curved around a massive object, and that curvature contains energy. That's just another word for mass, so the curvature itself exerts gravity. This creates more curvature (...etc etc). This is one of the reasons as to why Einstein's equations are nonlinear. Note that the concept of having mass is separate from the gravity force. Interaction with the Higgs field gives rise to mass, whereas gravitons are the force carrying particle for gravity.
- cyphar 11y ago
  Only electrons get mass from the Higgs mechanism. Most of your mass comes from your protons and neutrons (or rather the energy "stored" in the bonds between the quarks that make them up).
  - Manishearth 11y ago
    You're talking about binding energy, and that's not where the majority of mass comes from. All massive particles get their mass from the Higgs field.
    - rotorblade 11y ago
      All _fundamental_ particles gets their mass from the Higgs (up to some issues with the neutrinos). Composite particles, say the Proton, is a strongly coupled system (that is, can not be described by perturbation theory) and it does not have the mass being the sum of its constituents (not even most of it). Hence, it is not known what gives most of the mass of particles such as the Proton.
      - Manishearth 11y ago
        Er, sorry, implicitly was talking about fundamental particles. IIRC we do know where protons get their mass. The internal color field has some energy, thus some mass, which comes from that field interacting with Higgs.

dbranes 11y ago

Gravitational waves are a prediction of classical linearlized gravity - gravitational waves are to (classical) gravitons what light waves are to photons.

smegel 11y ago
But we have direct experimental confirmation of light behaving light both a wave and as a particle. Do we have any such evidence to show gravity exhibiting particle like behavior?
- sjburt 11y ago
  No, but we predict it to be essentially impossible to observe a graviton. http://arxiv.org/abs/gr-qc/0601043 http://arxiv.org/abs/gr-qc/0601043

dbranes 11y ago

Gravitational waves are a prediction of classical linearlized gravity - gravitational waves are to (classical) gravitons what light waves are to photons.

jug 11y ago

Related to this... According to Kip Thorne, this recent discovery alone places an upper bound of maximum mass for a graviton, if it exists and has mass, at no more than 10^-55 grams. Since otherwise the observed waves would have been distorted. It's cool that we are already seeing conclusions drawn from this observation. This single obsrrvation. https://starguyspeaks.wordpress.com/2016/02/11/on-gravitational-waves-2/ https://starguyspeaks.wordpress.com/2016/02/11/on-gravitatio...

pbhjpbhj 11y ago

>how far we are from being able to create and detect gravitons // Surely we create gravitons whenever we move a mass just like we create photons whenever we move a body that interacts electromagnetically? Isn't the point that we're constantly exchanging gravitons with all matter as they mediate gravitational attraction.

ScottBurson 11y ago
I think you must be right -- but maybe there's some other way to create them, without needing to move a mass. The ability to manipulate gravity itself might be the key to space travel.
- other_herbert 11y ago
  Flying cars man! Think of the flying cars! And maybe real hover boards
  - undersuit 11y ago
    First we need to find out how to create repulsion. Right now I'm pretty sure a graviton generator would just be a novelty device that weighs more than what it's mass would lead you to think it weighs. Maybe we could make orbital graviton beam generator that could literally suck an object off the face of the Earth.
    - celticninja 11y ago
      That sounds like a tractor beam to me.
    - pavel_lishin 11y ago
      > I'm pretty sure a graviton generator would just be a novelty device that weighs more than what it's mass would lead you to think it weighs Which would make space travel a lot easier - no more worrying about bone density loss!
    - 11y ago

thedatabase 11y ago

Here's a quote from John Baez's G+: "Gravity is a weak force, in the sense that the gravitational force between two protons is about 10^33 times weaker than the electric force between them. And I'm using protons rather than electrons here to make the gravity stronger - with electrons gravity would be almost 10^40 times weaker. This has various consequences, but one is that gravitational waves are absorbed by matter much less than electromagnetic waves. It would be fun to estimate the amount of energy absorbed by the Earth as this particular gravitational wave came through, but it would be absurdly small. Gravitational waves make neutrinos look like rampaging gorillas." Rephrasing, and assuming waving commutes with rampaging, gravitons make neutrinos look like WAVES of rampaging gorillas. I'm no physicist but to answer the original question I'd hazard a guess: quite far!