9 ms·

So correct me if I'm wrong, but if gravity is merely a curvature in space-time, does that mean there's no such thing as a graviton?

• raattgift 8y ago
  General Relativity is a classical field theory, so there's no graviton in it. However, there are various quantum gravity research programmes which have tried to quantize the (classical) tensor fields of General Relativity in one way or another, for instance by turning (classical) gravitational waves or position and momentum variables into quantum particles. In these theories the graviton is a spin-2 gauge boson and induce an attractive force on fermions with like gravitational charge. This compares with the photon, a spin-1 gauge boson that induces a repulsive force on fermions with like electric charge (and an attractive force on fermions with opposite electric charge). The spin also corresponds with the rank of the (classical) fields, where in General Relativity we have rank-2 tensors and in electrodynamics we have vectors (which are rank-1 tensors). Unfortunately this approach has a problem: particles other than fermions also have a gravitational charge, including gravitons themselves. (Photons do not have an electric charge, but they do have a gravitational charge). Quantum field theories use a variety of tricks to remain linear theories, but General Relativity's Einstein Field Equations are non-linear. That non-linearity manifests in graviton-graviton interactions in this sort of quantized General Relativity, and we do not know how to stabilize the resulting quantum field theories. In more concrete terms, let's "drop" an electron from higher potential to lower potential. The electron is a fermion with electric and gravitational charge. When we drop it from higher to lower electric potential, out pops a photon. When we drop it from higher to lower gravitational potential, out pops a graviton. But we can't move the photon around the electric field and get it to produce another photon, because the photon is not self-interacting. A graviton, on the other hand, can be dropped down a gravitational potential causing it to emit another graviton. That in turn influences the potential, making it easier for those two gravitons to each emit a further graviton, which influenes the potential, making it easier for those ... and so forth to infinity. Our original electron is in that potential too, so it in turn can emit further gravitons as well. Fortunately this explosion of gravitons only happens in strong gravity, and we only find that deep inside black holes (or in the very early universe), rather than somewhere we can outright observe (assuming the cosmic censorship principle is true), so we can still use this type of theory to describe most of the universe. Unfortunately, gravitational waves are hard enough to detect; when they are quantized, they turn into lots and lots and lots of gravitons, each of which has a tiny fraction of the detectability of a gravitational wave. Detecting an individual graviton experimentally is beyond the ability of present human technology, or anything reasonably likely to appear in the next few decades. So if we ever figure out how to stabilize this type of approach to quantum gravity, we will only be able to assess it against observations of many-graviton systems, likely at extreme events like black hole/neutron star mergers or asymmetrical hypernovae. This is just one broad example of quantum gravity; there are several other approaches, some with a "graviton" dynamics (e.g. string theory's gravitons are a string state) and some without. All these approaches share a common problem: General Relativity describes our universe exceptionally well in practice, and its fundamental components are among the best-tested of any scientific theory to date. Where there is weak gravity, barring significant masses in superpositions of position, any type of quantum gravity has to exactly reproduce the results of General Relativity. It is likely that we will be able to bring microgram masses into such superpositions in the next few years, and we will be able to directly probe one aspect of quantum field theory on (classical) curved spacetime (aka semiclassical gravity). However, if General Relativity fails in that situation, it would certainly allow some useful comparisons between competing theories (with and without gravitons) hoping to extend General Relativity as it in turn extended Newtonian gravity. If only graviton-containing theories match the experimental results, that would be evidence in favour of the existence of a graviton, even without direct detection; conversely, perhaps only non-graviton theories match, in which case the existence of a graviton would seem pretty dubious. Right now, though, we just don't know: in the absence of evidence for or against, gravitons are purely theoretical.
  • 6nf 8y ago
    Thank you
  • mrkstu 8y ago
    Is there any implication with the gravity wave detection, that since gravity has been verified to travel the speed of light that it is transmitted via particle? I would think that as inflation is not limited by the speed of light, gravity as a purely geometric phenomenon, would possibly function at supra-lightspeed.
    • raattgift 8y ago
      > [does gravitational wave detection imply the waves are particles] No, not really. However, if we deliberately quantize the gravitational waves, replacing the classical waves with large numbers of particles, the fact that the classical waves propagate at the speed of light means that the particles must be massless, like photons. There are a couple of families of alternatives to General Relativity that have "massive gravity", which requires that gravitational waves (or the particles, if we quantize gravitational waves) have a small nonzero mass. This means light will propagate faster than gravitational waves, and with some care, this can solve a couple of the handful of problems about the very early hot dense universe, namely in how it became so thermalized across huge regions of space: any sort of pressure density released lots of photons which carried the local energy away faster than gravitation could "react" and contain the heat locally. Unfortunately for such theories, the mass of gravitational waves has to decay away well before the formation of the cosmic microwave background, since massive gravity would leave unmistakeable markings in it that we do not see. Verifying that gravitational effects do not outrace optical effects effectively kills off all but the most complicated (early-decaying, thus dynamical) massive gravity theories, and those are rejected on parsimony grounds (the complexity is not needed to describe observations). You're right that matter embedded in an inflating universe is not limited by the speed of light. As with galaxies now, the matter remains more-or-less at the same comoving coordinates throughout the history of the universe, even during inflation. We don't see shear effects in distant galaxies even given the redshift (if all Doppler/motion related) implies ultrarelativistic recession velocities. The galaxies far away look pretty normal, moving slowly within clusters, with the clusters separating. However, lots of new space is being created between each and every galaxy cluster, but the galaxies do not "feel" acceleration. They continue merrily along their almost exclusively timelike worldlines. Inflation is essentially an extremization of this: lots of new space gets created in between the hot dense low-entropy matter of the pre-inflationary universe, and more new space gets created in less-dense/higher-entropy spots, which makes them even less dense / higher-entropy, which lets yet more space be created there, and so forth, for many "e-foldings". The pockets of gas do not "feel" like they are moving relative to one another, although if they had radars or the like, they could certianly see that during inflation they get much further away from each other (lots of radar signals will simply never return to the sender, or even be received at the target). At some point in the inflationary epoch the production of new space "cools off" and inflation ends, after which the accelerating expansion is driven by the residual small positive cosmological constant. Structure formation then collapses the relatively dense relatively low entropy matter into the first stars and galaxies, while the space between these areas gets ever less dense and ever higher entropy. Some time after inflation we switch to the modernized version of the expanding Einstein-de Sitter universe (1932), which in the late 1980s/early 1990s was the starting point for the current standard cosmology (\Lambda-CDM). \Lambda-CDM does a much better job describing observations relating to the early universe (before the first galaxies) and recent-epoch acceleration than the Einstein-de Sitter model. (Note, this is a broad stereotyping of cosmic inflation models; and for completeness, one can still get \Lambda-CDM from the hot dense phase (the "big bang") without inflation. Evidence from e.g. the Planck satellite has killed off some flavours of inflation and non-inflation proposals, and forthcoming evidence from Planck and other platforms will surely kill off many more.)