# Escape Velocity

**URL:** <https://boards.straightdope.com/t/escape-velocity/21512>\
**Category:** Factual Questions\
**Created:** [June 6, 2000, 1:49am UTC](https://boards.straightdope.com/t/escape-velocity/21512 "2000-06-06T01:49:15Z")\
**Posts on this page:** 3\
**Page:** 2

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**Author:** ![CurtC](https://avatars.discourse-cdn.com/v4/letter/c/ce73a5/32.png) [@CurtC](https://boards.straightdope.com/u/CurtC)\
**Post date:** [June 6, 2000, 6:06pm UTC](https://boards.straightdope.com/t/escape-velocity/21512/21 "2000-06-06T18:06:38Z")

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He must have been trying to pull a fast one on you to see if you were all asleep. There’s no way you can take the Newtonian gravity equation (F=m1_m2_G/r^2) and have the speed of light fall out of it. If one of the masses is infinite, there will be an infinite escape velocity (according to my bar-napkin calculations, escape velocity is V=SQRT(2_m2_G/d) ).

So this equation has nothing to do with the speed of light, unless he threw in some relativistic equation into the mix, and if he did this, he manages to come up with the wrong answer.

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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:** [June 6, 2000, 7:10pm UTC](https://boards.straightdope.com/t/escape-velocity/21512/22 "2000-06-06T19:10:27Z")

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Quoth Phobos:

> [@](#):
>
> What about black holes & light? Photons don’t “run out of fuel” (or do they through redshifting?) but they can’t escape the event horizon of a black hole because the escape velocity is greater than the speed of light.

That’s a bit of an oversimplification, which may be part of your confusion-- You’re right, though, that photons lose energy by redshifting. A better statement of what goes on at the event horizon of a black hole, is that every possible path ends up getting you closer to the singularity.

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**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\
**Post date:** [June 7, 2000, 2:04am UTC](https://boards.straightdope.com/t/escape-velocity/21512/23 "2000-06-07T02:04:36Z")

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> [@](#):
>
> the entire concept of escaping the earth’s gravity seems a bit simplistic to me. If I remember my physics correctly, you can NEVER actually entirely escape the gravitational influence of the earth or any other body, but you can get to the point at which the earth’s gravity becomes secondary to the combined gravitational pull of all the other bodies in the universe

You can do that, but that’s not what escape velocity is about. Consider an empty space with just the Earth floating around. If you throw a ball slowly, it will come back down to earth. The faster you throw, the farther it gets before earth’s gravity slows its upward velocity to zero and it starts falling back down. But if you throw it _really_ fast, the ball will travel away from the earth forever - it can go infinitely far away and still have an ‘upward’ velocity. The minimum speed you need for this is the escape speed.

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