# Light and Gravity

**URL:** <https://boards.straightdope.com/t/light-and-gravity/141797>\
**Category:** Factual Questions\
**Created:** [December 10, 2002, 5:44pm UTC](https://boards.straightdope.com/t/light-and-gravity/141797 "2002-12-10T17:44:36Z")\
**Posts on this page:** 4\
**Page:** 2

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**Author:** ![William\_Ashbless](https://avatars.discourse-cdn.com/v4/letter/w/a87d85/32.png) [@William\_Ashbless](https://boards.straightdope.com/u/William_Ashbless)\
**Post date:** [December 10, 2002, 10:58pm UTC](https://boards.straightdope.com/t/light-and-gravity/141797/21 "2002-12-10T22:58:15Z")

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> [@](#):
>
> \*Originally posted by Ring \*  
> \*\*Well, here’s a hand-waving answer.
> 
> In the presence of a gravitating mass an object that travels slowly wrt to c experiences a lot more time than space, and so the deflection is almost completely from the “curvature of time,” and this matches the curvature predicted by Newton.
> 
> A photon, travelling at c, experiences equal measures of space and time and therefore is equally deflected by both “time curvature” and “space curvature,” or twice what Newton’s theory predicts. \*\*

Mm, seriously handwaving. 😃 I think I have to see this in actual math. I understand the idea behind the analogy, though.

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**Author:** ![BZ00000](https://avatars.discourse-cdn.com/v4/letter/b/47e85d/32.png) [@BZ00000](https://boards.straightdope.com/u/BZ00000)\
**Post date:** [December 10, 2002, 11:13pm UTC](https://boards.straightdope.com/t/light-and-gravity/141797/22 "2002-12-10T23:13:23Z")

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How can I see something that has no mass? How does a wall stop light?

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**Author:** ![BZ00000](https://avatars.discourse-cdn.com/v4/letter/b/47e85d/32.png) [@BZ00000](https://boards.straightdope.com/u/BZ00000)\
**Post date:** [December 10, 2002, 11:20pm UTC](https://boards.straightdope.com/t/light-and-gravity/141797/23 "2002-12-10T23:20:20Z")

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Those last questions look kind of dumb. I guess I am asking is how can something have no rest mass (which would also mean no energy?) have mass when it moves faster? Is it just nothing moving really fast?

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**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\
**Post date:** [December 10, 2002, 11:35pm UTC](https://boards.straightdope.com/t/light-and-gravity/141797/24 "2002-12-10T23:35:04Z")

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> [@](#):
>
> \*Originally posted by BZ00000 \*  
> \*\*How can I see something that has no mass? How does a wall stop light? \*\*

You see things the same way a wall stops light. Your eye absorbs the photon (or the wall absorbs the photon). Now energy has been added to your eye (or wall) and it heats up a bit. Your brain registers the absorbtion of this energy and interprets it givng you a picture of what you’re seeing.

Think of energy like a sound wave. Does sound have mass? Nope but it causes the air to compress and decompress which vibrates your ear drum which you interpret as sound. This is an imperfect analogy since light needs no medium (such as air) to propogate but maybe it helps give you a sense of it.

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