# Antigravity, and the American Journal of Physics **URL:** \ **Category:** Factual Questions\ **Created:** [March 8, 2003, 2:27am UTC](https://boards.straightdope.com/t/antigravity-and-the-american-journal-of-physics/159839 "2003-03-08T02:27:02Z")\ **Posts on this page:** 7\ **Page:** 2
**Author:** ![Shade](https://avatars.discourse-cdn.com/v4/letter/s/2bfe46/32.png) [@Shade](https://boards.straightdope.com/u/Shade)\ **Post date:** [March 10, 2003, 11:30pm UTC](https://boards.straightdope.com/t/antigravity-and-the-american-journal-of-physics/159839/21 "2003-03-10T23:30:07Z")
> [@](#): > > \*Originally posted by Skogcat \* > \*\*Weighing nothing is totally different from not feeling the effects of gravity. A balloon feels all of the effects of gravity, it just happens to have an equal force opposing it. \*\* 😕 If I have no mas (wiegh nothing in a gravity field) gravity exerts no force on me. If gravity doesn’t affect me, gravity exerts no force on me. What’s the big difference? Is it that one way I can have mass? ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [March 11, 2003, 12:21am UTC](https://boards.straightdope.com/t/antigravity-and-the-american-journal-of-physics/159839/22 "2003-03-11T00:21:42Z")
Gravity exerts a force on the ballon because it has mass, there just happens to be another force (buoyancy) that exerts a greater force than gravity, and in the opposite direction. If a body had no mass, the slightest force that acted upon it would accelerate it instantaneously to the speed of light (probably). ---
**Author:** ![tonbo0422](https://avatars.discourse-cdn.com/v4/letter/t/b5e925/32.png) [@tonbo0422](https://boards.straightdope.com/u/tonbo0422)\ **Post date:** [March 11, 2003, 12:38am UTC](https://boards.straightdope.com/t/antigravity-and-the-american-journal-of-physics/159839/23 "2003-03-11T00:38:27Z")
On the subject of mass and gravity, I heard that even though you weigh far less on the moon, if you jumped from, say, eighty feet, you’d splat just as badly as on Earth, because of your mass. ---
**Author:** ![Richard\_Pearse](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/richard_pearse/32/13144_2.png) [@Richard\_Pearse](https://boards.straightdope.com/u/Richard_Pearse)\ **Post date:** [March 11, 2003, 4:21am UTC](https://boards.straightdope.com/t/antigravity-and-the-american-journal-of-physics/159839/24 "2003-03-11T04:21:21Z")
> [@](#): > > \*Originally posted by Shade \* > \*\*😕 > > If I have no mas (wiegh nothing in a gravity field) gravity exerts no force on me. If gravity doesn’t affect me, gravity exerts no force on me. What’s the big difference? Is it that one way I can have mass? \*\* Your balloon example didn’t have “no mass”. My “weighing nothing” was miss worded. Your balloon has mass and is affected by gravity just like anything else. Being buoyant and floating on the air is not comparible to locally turning off gravity. ---
**Author:** ![Ring](https://avatars.discourse-cdn.com/v4/letter/r/6a8cbe/32.png) [@Ring](https://boards.straightdope.com/u/Ring)\ **Post date:** [March 11, 2003, 4:58am UTC](https://boards.straightdope.com/t/antigravity-and-the-american-journal-of-physics/159839/25 "2003-03-11T04:58:22Z")
> [@](#): > > \*Originally posted by BrainGlutton \* > \*\* > I always wondered: If you’re in an acceleration-frame that has the same EFFECT as a gravity field (e.g., a really fast elevator such as construction crews use to reach the top of a skyscraper-in-progress), then relativity says it actually IS a gravity field, that is, it has the same time-dilation effect, etc.; there’s no fundamental difference, regardless of whether the field is created by accelerated motion or a large concentration of mass. But if that is true – then what role, if any, do “gravitons” play in an acceleration-frame? \*\* What you’re talking about is called the weak equivalence principal, and what you’re saying is basically correct, except it only applies to a point particle in a uniform gravitational field. General relativity and Quantum Mechanics cannot be reconciled with each other, and gravitons are the mediating particles of various theories of quantum gravity, but all of these theories are in the early development stage and no one really knows whether gravitons exist or not. ---
**Author:** ![Shade](https://avatars.discourse-cdn.com/v4/letter/s/2bfe46/32.png) [@Shade](https://boards.straightdope.com/u/Shade)\ **Post date:** [March 11, 2003, 3:04pm UTC](https://boards.straightdope.com/t/antigravity-and-the-american-journal-of-physics/159839/26 "2003-03-11T15:04:52Z")
OK, I think I’ve got it. THe difference is that a balloon is cusioned by air, but a man would go straight through it? ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [March 11, 2003, 3:56pm UTC](https://boards.straightdope.com/t/antigravity-and-the-american-journal-of-physics/159839/27 "2003-03-11T15:56:40Z")
In a vacuum, a helium balloon will fall like a rock (conveniently ignoring the irrelevant fact that it would actually _burst_ due to internal pressure) - the balloon rises in air because it experiences _upthrust_ - the volum of air displaced by the balloon has more mass than the balloon and contents. An air bubble or a cork rises to the top of a container of water for exactly the same reason, if that makes it any easier to grasp - the cork does not lose mass from being submerged in a more dense medium. [Previous page](https://boards.straightdope.com/t/antigravity-and-the-american-journal-of-physics/159839.md?page=1)