Light-like scattering in quantum gravity

Bjerrum-Bohr, N. E. J.; Donoghue, John F.; Holstein, Barry R.; Planté, Ludovic; Vanhove, Pierre

doi:10.1007/jhep11(2016)117

Cited by 118 publications

(188 citation statements)

References 62 publications

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“…Moreover, the case can be made that all gravitational theories display some causal uncertainty, due to their spacetime fluctuations. As an example of this we note that even in the effective theory, quantum corrections to the trajectories of massless particles are spin dependent and the particles no longer follow geodesic motion [8]. This makes the concept of a lightcone uncertain.…”

Section: Pacs Numbersmentioning

confidence: 98%

Arrow of Causality and Quantum Gravity

Donoghue

Menezes

2019

Phys. Rev. Lett.

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Causality in quantum field theory is defined by the vanishing of field commutators for space-like separations. However, this does not imply a direction for causal effects. Hidden in our conventions for quantization is a connection to the definition of an arrow of causality, i.e. what is the past and what is the future. If we mix quantization conventions within the same theory, we get a violation of microcausality. In such a theory with mixed conventions the dominant definition of the arrow of causality is determined by the stable states. In some quantum gravity theories, such as quadratic gravity and possibly asymptotic safety, such a mixed causality condition occurs. We discuss some of the implications. PACS numbers:When caught making a sign mistake in a phase, a colleague would say: "Physics does not depend on whether we use +i or −i", and happily change the sign. At first sight, this phrase seems true. Classical fields are real. The probabilities of quantum mechanics are absolute values squared. Measurements in physics do not seem to care if we defined √ −1 as +i or −i. On second thought, the sign in front of i often does make a major difference. We define time development byThis results in "positive energy" being defined via e −iEt/ . Canonical quantization is defined viaThe path integral treatment of quantum physics is defined using e iS , not e −iS (in units of = c = 1), with S being the action. The Feynman propagator has very important sign conventions in both the numerator and the denominator, withwith infinitesimal and positive. We see that the specific signs in front of i are important in the formalism of quantum mechanics. On third thought, we can see that these signs are a convention, although they do tie in with another feature of our physical description -in particular the time direction (arrow) of causality.In somewhat colloquial language, we would describe causality as "there is no effect before the cause". In relativistic quantum theories of course, one must be careful about what one means by "before". As a simple example, consider the Feynman diagram shown in Figure 1. The Feynman propagator in coordinate space includes both forward and backward propagation in timeFIG. 1: The simple Feynman diagram on the left is decomposed into two time ordered diagrams. In one of the time orderings the final particles emerge before the initial particles have annihilated.withwith E q = q 2 + m 2 and D back F (x) = (D for F (x)) * . What we commonly refer to as positive frequency, or e −iEt , is propagated forward in time and negative frequency backwards in time. However, we see that in one time ordering the final state particles (the effect) emerge before the initial particles have interacted (the cause). So our colloquial notion of causality is inadequate.We note in passing that the time advance of the final state vertex is not observable -due to the uncertainty principle. The time difference of the vertices is of order ∆t ∼ 1/E, where E is the center of mass energy. The time localization of the initial state ...

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Section: Pacs Numbersmentioning

confidence: 98%

Arrow of Causality and Quantum Gravity

Donoghue

Menezes

2019

Phys. Rev. Lett.

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“…Alternatively, there exist many techniques for using scattering amplitudes to compute classical and quantum corrections to gravitational potentials [38][39][40][41][42][43][44][45][46][47][48][49][50][51], and there has been significant recent progress in obtaining such corrections in the PM approximation [52][53][54][55][56][57].…”

Section: Introductionmentioning

confidence: 99%

Observables and amplitudes for spinning particles and black holes

Maybee¹,

O’Connell²,

Vines³

2019

J. High Energ. Phys.

252

272

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We develop a general formalism for computing classical observables for relativistic scattering of spinning particles, directly from on-shell amplitudes. We then apply this formalism to minimally coupled Einstein-gravity amplitudes for the scattering of massive spin 1/2 and spin 1 particles with a massive scalar, constructed using the double copy. In doing so we reproduce recent results at first post-Minkowskian order for the scattering of spinning black holes, through quadrupolar order in the spin-multipole expansion.

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“…The only clear information that we have about DM is that very likely it is there and interacts gravitationally with the ordinary matter. If this is the only type of coupling with the ordinary matter or fields, the scattering cross section would be of the infinitesimal order of magnitude O m 2 χ /M 4 P with M P = 1.22 × 10 19 GeV/c 2 is the Planck mass [70]. The simple WIMP model of DM primordial production in the Hot Universe before the Baryogenesis appears to be at the moment seriously questionable, because both the limits from the Fermi-LAT gamma ray telescope and the HEP Collider, that are compatible only with a mass largely in excess of 100 GeV/c 2 ( or ∼400 GeV/c 2 for the compatibility of the LHC Run 2 upper limit with the WIMP's thermal production).…”

Section: Discussionmentioning

confidence: 99%