# Does light create a gravitational pull?

**URL:** <https://boards.straightdope.com/t/does-light-create-a-gravitational-pull/548275>\
**Category:** Factual Questions\
**Created:** [July 28, 2010, 5:48am UTC](https://boards.straightdope.com/t/does-light-create-a-gravitational-pull/548275 "2010-07-28T05:48:30Z")\
**Posts on this page:** 4\
**Page:** 2

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**Author:** ![Asympotically\_fat](https://avatars.discourse-cdn.com/v4/letter/a/e47c2d/32.png) [@Asympotically\_fat](https://boards.straightdope.com/u/Asympotically_fat)\
**Post date:** [July 29, 2010, 11:36am UTC](https://boards.straightdope.com/t/does-light-create-a-gravitational-pull/548275/21 "2010-07-29T11:36:24Z")

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> [@Pleonast](#):
>
> A note on the system of parallel photons: such a system has a temperature of absolute zero and is thus as impossible to create as any other zero-temperature system. So in practice, any system of photons you find will have finite mass.

Except in the degenerate case of a single photon.

> [@Chronos](#):
>
> You don’t actually need a global definition of momentum; a local definition will work fine, so long as your locality is large enough

But you still have the problem though that whilst 4-vectors are well-defined in the tangent spaces, except in the Minkowskian case, there’s more than one way to define equivalence classes of 4-vectors in a decently sized hypervolume of spacetime.Plus there’s no guarantee that all defitnitons of equivalence classes of 4-momentum will lead to it being a conserved quantity.

Obviously in the case of a satellite orbiting a planet your generally well within the Newtonian limit and there must be some sort of functional conservation of momentum that can derived from a full general relativistic treatment. I’m not saying the same isn’t necessarily true of a single photon passing the planet, but the photon is highly relativistic so it doesn’t seem trivially true that there will be a sensible functional conservation of momentum.

> [@Old Guy](#):
>
> I’m pretty sure this is wrong. I believe the event horizon is the boundary of the region from which no light cannot escape to infinity regardless of its initial direction.

The qualifier you’ve added isn’t neccessary, it doesn’t matter which direction the flashlight is pointed, the light will not escape to infinity.

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**Author:** ![ricksummon](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ricksummon/32/3333_2.png) [@ricksummon](https://boards.straightdope.com/u/ricksummon)\
**Post date:** [July 29, 2010, 3:37pm UTC](https://boards.straightdope.com/t/does-light-create-a-gravitational-pull/548275/22 "2010-07-29T15:37:54Z")

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The gravitational field of a beam of light can be represented by the [Bonnor Beam](http://en.wikipedia.org/wiki/Bonnor_beam) metric in general relativity. The article also notes that if there is more than one beam, they will remain parallel if they are going in the same direction, but will attract each other if they are moving in opposite directions.

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**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\
**Post date:** [July 29, 2010, 4:05pm UTC](https://boards.straightdope.com/t/does-light-create-a-gravitational-pull/548275/23 "2010-07-29T16:05:03Z")

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**OldGuy** , I’m not sure what distinction you’re making there in your definition of the event horizon. It looks like you’re saying the same thing as the quote you’re disagreeing with.

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**Author:** ![OldGuy](https://avatars.discourse-cdn.com/v4/letter/o/3bc359/32.png) [@OldGuy](https://boards.straightdope.com/u/OldGuy)\
**Post date:** [July 29, 2010, 5:11pm UTC](https://boards.straightdope.com/t/does-light-create-a-gravitational-pull/548275/24 "2010-07-29T17:11:51Z")

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> [@These are my own pants](#):
>
> The qualifier you’ve added isn’t neccessary, it doesn’t matter which direction the flashlight is pointed, the light will not escape to infinity.

Yes I see I misread the original statement that we are inside a “collapsing shell” of radiation.

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