Find Gravitational Force of Photons: Equations & Potential Energy

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    Gravity Photons
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Discussion Overview

The discussion centers around the gravitational force produced by photons, exploring the theoretical implications of massless particles and their interaction with spacetime. Participants examine equations related to gravitational potential energy and the challenges of defining gravitational effects in the context of general relativity and quantum mechanics.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants propose that photons, despite being massless, must warp spacetime due to their energy, leading to questions about the gravitational force they exert.
  • Others argue that a single photon lacks a defined location in spacetime, which complicates its ability to have a local gravitational effect.
  • A few participants reference the Aichelburg–Sexl ultraboost as a model for the spacetime produced by a brief pulse of light.
  • One participant suggests using mass-energy equivalence (E=mc²) to derive the gravitational force between a photon and a material particle, but questions the validity of this approach given the massless nature of photons.
  • Another participant challenges the use of Newton's law for massless radiation, emphasizing the need for a stress-energy tensor and the complexities of defining gravitational potential energy in general relativity.
  • Some participants note that while the stress-energy tensor for a photon includes components that contribute to gravity, the absence of a quantum theory of gravity leaves many questions unanswered.
  • There is a discussion about the implications of pressure and momentum in the context of gravitational effects, with some arguing that pressure contributes to gravitational interactions.
  • Several participants express uncertainty about the applicability of certain equations and concepts, highlighting the ongoing debate about the nature of gravity and photons.

Areas of Agreement / Disagreement

Participants express a range of views on the gravitational effects of photons, with no consensus reached. Some agree on the need for a more nuanced understanding of the stress-energy tensor, while others maintain differing opinions on the validity of using classical equations to describe these effects.

Contextual Notes

Participants acknowledge limitations in their understanding, particularly regarding the lack of a quantum theory of gravity and the challenges of applying classical physics to massless particles. The discussion reveals dependencies on definitions and unresolved mathematical steps related to gravitational interactions.

  • #31
Finally, in very early Universe the density of matter was negligible in comparison with density of radiation. So at some époque, most of the gravity of the Universe came from photons, slowing down the expansion.
 
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  • #32
Ich said:
Agreed to all, but here's the relativity forum. Please understand my statements as referring to a wave packet with negligible spatial extension, not zero extension. I think that's what is meant by a "photon" in a classical relativistic thought experiment (a null geodesic).
For such a pulse of light the correct spacetime is the http://en.wikipedia.org/wiki/Aichelburg%E2%80%93Sexl_ultraboost" that I mentioned earlier.
 
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  • #33
Hi.

A group of photons surely generates gravity. e.g. electromagnetic radiotion closed in a hollow body has energy called black body radiation. Its electromagnetic energy momentum tensor appear in the right hand side of Einstein's gravity equation. QM says that photon wavelength lamda gives its position uncertainty. So the gravity source position has uncertainty of wavelength.
Schwartzshild's radius of photon is 2Gh / (c lamda). In the case the disance between the two photons are much larger than both lamda and Schwartzshild's radius, I expect a photon motion influenced by the gravity generated by another photon can be evaluated approximately at least. In other cases e.g. two photons are in the same wave train of a laser beam we must face unknown quantum gravity.

Regards.
 
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