Do Photons Have a Gravitational Effect?

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Discussion Overview

The discussion revolves around whether photons, which have energy but no mass, create a gravitational effect according to General Relativity (GR). Participants explore the implications of the stress-energy tensor and its components, the role of electromagnetic fields, and the gravitational effects of light in various contexts, including theoretical and experimental perspectives.

Discussion Character

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

Main Points Raised

  • Some participants assert that according to GR, energy contributes to gravity, suggesting that photons, despite having no mass, may still create gravitational effects due to their energy.
  • Others clarify that the stress-energy tensor has multiple components beyond energy, and while photons have energy and momentum, their role in creating gravity is complex and not fully resolved.
  • A participant mentions that electromagnetic fields have a stress-energy tensor, which implies they should contribute to gravitational effects, although the physical justification of certain solutions remains uncertain.
  • Some participants reference laboratory tests confirming that static electromagnetic fields produce gravitational effects, while noting a lack of direct empirical tests for electromagnetic radiation.
  • There is a discussion about the gravitational force produced by photons and whether it can be calculated using the equation E=mc², with some arguing that the gravitational field of light cannot be simply derived without solving the Einstein field equations.
  • One participant describes a hypothetical experiment involving a hollow sphere filled with light, suggesting that the pressure from the light would create a gravitational field greater than that of cold matter, although this scenario assumes certain ideal conditions.

Areas of Agreement / Disagreement

Participants express differing views on the gravitational effects of photons, with some supporting the idea that they can create gravity due to their energy, while others argue against this based on the mass-energy relationship in GR. The discussion remains unresolved, with multiple competing perspectives presented.

Contextual Notes

Limitations include the dependence on the definitions of energy and mass in the context of GR, the complexity of the stress-energy tensor, and the unresolved nature of quantum gravity theories. The discussion also highlights the challenges in applying classical equations to scenarios involving light and energy.

  • #91
DaleSpam said:
No. According to GR the source of gravity is the stress-energy tensor. There are 10 independent components in the stress-energy tensor. Energy is only one of those 10 components.

http://en.wikipedia.org/wiki/Stress-energy_tensor

We do not have a working theory of quantum gravity at the time so I cannot answer your question wrt photons, however I can answer it wrt classical pulses of light. Pulses of light have energy, they also have momentum, so several of the components of stress energy tensor will be non-zero. So light can be a source of gravity.

I wonder what you think of the following thought experiment by Dimitry67 that seems to show that parallel beams of light will not converge. Thus it is as though light can't be a source of gravity.

Consider two massive objects, separated by some distance, flying in the same direction at velocity v according to an observer. In their inertial system they collide, say, in 1s. For the observer this process takes longer because of the time dilation. The faster the two objects are flying the longer it takes. In the limit where v --> c they never converge according to the observer.
 
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  • #92
johne1618 said:
I wonder what you think of the following thought experiment by Dimitry67 that seems to show that parallel beams of light will not converge. Thus it is as though light can't be a source of gravity.
Dimitry67 is correct, but you should not take the quote out of context. The bolded conclusion above does not follow, as you can clearly see by considering the full quote:

Dmitry67 said:
This is correct, light beams create gravity.

However, when it was discussed here about 1 or 2 y ago, I remember that someone (with much deeper knowledge of GR - I am just a layman) told me that:

2 parallel light beams going in the same direction do not attract (even they attract the surrounding objects)
2 parallel light beams going in opposite directions do attract.

The first fact might be clear if we look at 2 massive objects, separated by some distance, flying in the same direction. In their inertial system they collide, say, in 1s. For an external observer, this process would take longer because of the time dilation. The faster 2 objects are flying the longer it takes. You can think about the case N1 as a limit where v --> c (it takes forever)
 
  • #93
So,

What's the verdict? Threadmark has no idea, or Threadmark is ahead of his time?

What percentage of the universe's net gravitational field would be due to electromagnestism?

GrayGhost
 
  • #94
DaleSpam said:
Dimitry67 is correct, but you should not take the quote out of context. The bolded conclusion above does not follow, as you can clearly see by considering the full quote:

I stand corrected - I did take Dimitry67's argument out of context.

I did it for two personal reasons I guess.

1/ I like the argument.

2/ I have a "pet theory" that the inertia of a particle with rest mass is caused by retarded gravitational waves impinging on the particle from all the other massive particles in the Universe. I believe that initially massless particles respond to this by following circular orbits whose rotation energy gives half the mass/energy of the particle (the rest being in the mutual gravitational energy between the particle and the rest of the Universe). I am trying to formalise Mach's Principle. I want to argue that light is different so that it does not pick up an inertia. I probably need to think about my theory more before I can decide what it says about light. I'm not using GR itself but an approximation to it called gravitomagnetism that is like Maxwell's theory. In fact I should probably just stick to trying to understand electromagnetically induced inertia for the moment as i feel on safer ground with EM. (I only have at best an undergraduate understanding of physics).
 
Last edited:
  • #95
johne1618 said:
I have a "pet theory" ... I only have at best an undergraduate understanding of physics
Hmm.
 
  • #96
GrayGhost said:
What's the verdict? Threadmark has no idea, or Threadmark is ahead of his time?
Threadmark is wrong. Was there any ambiguity?
 
  • #97
GrayGhost said:
What percentage of the universe's net gravitational field would be due to electromagnestism?

If you mean the gravitational field as defined in Newtonian mechanics, that isn't an unambiguously well defined thing in GR. By the equivalence principle, the gravitational field at a given point can be anything you like, depending on your frame of reference. In the frame of an observer at rest with respect to the cosmic microwave background, the gravitational field is zero by symmetry (in a homogeneous and isotropic cosmological model).

Interpreting your question more loosely, the answer is that the universe was radiation-dominated at one time, then matter-dominated, and is now vacuum-dominated.
 
  • #98
No,

All I meant was that if photons gravitate, then they produce gravitation. The cosmos has some net collective gravitational field. Given photons are everywhere, I was just wondering what percentage of the cosmic gravitational field would owe to EM?

GrayGhost
 
  • #99
GrayGhost said:
The cosmos has some net collective gravitational field.

That's incorrect, for the reasons given in #97.
 
  • #100
bcrowell said:
That's incorrect, for the reasons given in #97.

hmm.
 
  • #101
GrayGhost said:
No,

All I meant was that if photons gravitate, then they produce gravitation. The cosmos has some net collective gravitational field. Given photons are everywhere, I was just wondering what percentage of the cosmic gravitational field would owe to EM?

GrayGhost

Does a photon displace?
Does a photon have mass?
Can you create mass from a photon?
Observation shows us the sorce point of a photon. The relative position of objects in the universe change after the photon is originated. If photons gravitated we would see a blur in the universe.
 
  • #102
Hi Great Richard, welcome to PF!

Please read through the above discussion and ask if you have questions. Light does gravitate, and mass can be created from a pair of photons.
 

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