# Can someone explain me gravity? **URL:** \ **Category:** Factual Questions\ **Created:** [August 15, 2010, 6:53am UTC](https://boards.straightdope.com/t/can-someone-explain-me-gravity/550314 "2010-08-15T06:53:41Z")\ **Posts on this page:** 20\ **Page:** 2
**Author:** ![Clothahump](https://avatars.discourse-cdn.com/v4/letter/c/51bf81/32.png) [@Clothahump](https://boards.straightdope.com/u/Clothahump)\ **Post date:** [August 20, 2010, 4:05pm UTC](https://boards.straightdope.com/t/can-someone-explain-me-gravity/550314/21 "2010-08-20T16:05:49Z")
[quote=“komolono, post:13, topic:550314”] > [@Chronos](#): > > Now, if you move the magnet the frog moves - yes? This should be easily tested. Could this be another form of transportation in the future? Let the sorry Maglex-frog earn it’s name in history! I believe that would be Frog-lex. :D:D ---
**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:** [August 20, 2010, 4:08pm UTC](https://boards.straightdope.com/t/can-someone-explain-me-gravity/550314/22 "2010-08-20T16:08:49Z")
> [@Clothahump](#): > > I believe that would be Frog-lex. > > :D:D Report this post:D ---
**Author:** ![Grey](https://avatars.discourse-cdn.com/v4/letter/g/b782af/32.png) [@Grey](https://boards.straightdope.com/u/Grey)\ **Post date:** [August 20, 2010, 5:16pm UTC](https://boards.straightdope.com/t/can-someone-explain-me-gravity/550314/23 "2010-08-20T17:16:27Z")
> [@Grey](#): > > The only one I can think of recently (aside from a Jupiter/star transit from a few years back) would be [Gravity Probe B](http://en.wikipedia.org/wiki/Gravity_Probe_B). The Jupiter/star experiment had a thread in GQ though I can’t find it now. Found [it](http://boards.straightdope.com/sdmb/showthread.php?t=262369&highlight=jupiter+relativity+gravity+speed) but it was only tangential to the “speed of gravity” discussion. ---
**Author:** ![komolono](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/komolono/32/532_2.png) [@komolono](https://boards.straightdope.com/u/komolono)\ **Post date:** [August 21, 2010, 11:47am UTC](https://boards.straightdope.com/t/can-someone-explain-me-gravity/550314/24 "2010-08-21T11:47:56Z")
[quote=“Bad\_Astronaut, post:16, topic:550314”] > [@komolono](#): > > I’m pretty sure the magnetic force on the frog only acts in the direction pointing from the magnet to the frog. So if you move the magnet forward, the frog will fall as soon as the magnet is not directly beneath it (and the frog will even be pushed backward a little). I just watched a “classic” video on youtube where a scientist moves a supercooled magnet and the other magnet follows above. Well there isn’t much of acceleration, but anyways… ---
**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:** [August 21, 2010, 10:45pm UTC](https://boards.straightdope.com/t/can-someone-explain-me-gravity/550314/25 "2010-08-21T22:45:06Z")
Oh, backing up to an older question: > [@](#): > > Why aren’t there any tides of moon dust? The dominant tidal forces on the Moon are exerted by the Earth, but since the Moon always faces the same side towards the Earth, that wouldn’t give you any motion of dust around. It just slightly changes what “level” is on the Moon. The Moon also gets tidal forces from the Sun, and if the Moon had oceans of water, they would show tides from that. But dust doesn’t flow as easily as water does. Basically, from the point of view of the dust, the surface it’s sitting on is slightly tilted, and the tilt changes over the course of a month, but it’s always only a slight tilt. And dust, unlike water, can stay put on a slight tilt. ---
**Author:** ![Cheshire\_Human](https://avatars.discourse-cdn.com/v4/letter/c/5f8ce5/32.png) [@Cheshire\_Human](https://boards.straightdope.com/u/Cheshire_Human)\ **Post date:** [August 22, 2010, 1:25am UTC](https://boards.straightdope.com/t/can-someone-explain-me-gravity/550314/26 "2010-08-22T01:25:44Z")
The local level on the moon changes according to “lunar libration” (Google for details) which only amounts to about 5% maximum, or approximately 1/2 inch per foot of apparent slope (5 cm / meter). That’s trivial as far as dust flowing is concerned. It wouldn’t cause any movement on any slope that’s not already just about to give way. Which after as many billions of years the Moon has been tidally locked with Earth means “nowhere on the Moon”. ---
**Author:** ![Ike\_Witt](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ike_witt/32/283_2.png) [@Ike\_Witt](https://boards.straightdope.com/u/Ike_Witt)\ **Post date:** [August 22, 2010, 3:04am UTC](https://boards.straightdope.com/t/can-someone-explain-me-gravity/550314/27 "2010-08-22T03:04:17Z")
Is gravity a particle or a wave or perhaps some sort of feature of quantum mechanics? ---
**Author:** ![ethansiegel](https://avatars.discourse-cdn.com/v4/letter/e/2bfe46/32.png) [@ethansiegel](https://boards.straightdope.com/u/ethansiegel)\ **Post date:** [August 22, 2010, 3:51am UTC](https://boards.straightdope.com/t/can-someone-explain-me-gravity/550314/28 "2010-08-22T03:51:21Z")
> [@Mijin](#): > > > [@komolono](#): > > > > Einstein predicted the speed of gravity to be what is now measured, that is the speed of light. Can you tell me why not slower/faster? > > Has it really been measured? I thought that was still a huge technical challenge? Hey all, we _do_ know that gravity travels at the speed of light! If you look at systems of binary pulsars, such as the Hulse-Taylor pulsar, PSR 1913+16, you’ll find that the orbits are decaying. (Wikipedia, here, [Hulse–Taylor pulsar - Wikipedia](http://en.wikipedia.org/wiki/PSR_B1913%2B16), actually gives a pretty good writeup.) Now, when an object passes through spacetime, it causes distortion in that spacetime, which propagates outwards at a certain speed. By measuring the orbital properties of these pulsars over some time, we can determine what that speed is, and it turns out to be the speed of light, c. This is an _indirect_ measurement, however. We’d like to be able to measure it directly. We are optimistic that sometime in the next couple of decades we’ll be able to detect the gravitational radiation that must be carried away from this system… LISA should be able to get it. And that would allow us to know for sure. An old (1998) semi-technical writeup by Steve Carlip (who’s totally credible) is available here: [Does Gravity Travel at the Speed of Light?](http://www.xs4all.nl/~johanw/PhysFAQ/Relativity/GR/grav_speed.html) ---
**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:** [August 22, 2010, 5:29am UTC](https://boards.straightdope.com/t/can-someone-explain-me-gravity/550314/29 "2010-08-22T05:29:12Z")
> [@](#): > > We are optimistic that sometime in the next couple of decades we’ll be able to detect the gravitational radiation that must be carried away from this system… LISA should be able to get it. And that would allow us to know for sure. Eh, in a couple of decades, LISA will only be ten years away from launch. The sorts of sources LIGO will see are less likely to have electromagnetic analogues, but I’d still bet on LIGO getting a direct measurement of the speed of gravity before LISA does. > [@](#): > > Is gravity a particle or a wave or perhaps some sort of feature of quantum mechanics? Static gravity (like what keeps the Earth in orbit around the Sun) is not currently described as a particle or a wave. Current theories of gravity do predict the existence of gravitational waves, and we’ve seen indirect evidence of them, but we haven’t yet directly detected them. LIGO and LISA, mentioned above, are two instruments intended for this. LIGO is ground-based and is already running, but is not yet sensitive enough (they’re upgrading it, and it should be sensitive enough in a couple of years). LISA will be space-based, and has been “ten years away from launch” for at least the past ten years. It is presumed that quantum mechanics can be extended in such a way as to describe gravity, but nobody has yet figured out how. Such a quantum theory of gravity, if we had it, would presumably describe a gravitational wave as a stream of particles called gravitons, much the same way that an electromagnetic wave can be described as a stream of photons, and would also presumably describe static gravity in terms of virtual gravitons being exchanged between the objects, much as static electric forces are described in terms of virtual photons. ---
**Author:** ![komolono](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/komolono/32/532_2.png) [@komolono](https://boards.straightdope.com/u/komolono)\ **Post date:** [August 22, 2010, 5:43am UTC](https://boards.straightdope.com/t/can-someone-explain-me-gravity/550314/30 "2010-08-22T05:43:59Z")
Has anyone noticed that that some of the quotes in this thread are getting mixed up? Or maybe I have an identity issue I wasn’t aware of… Anyway, what about frog collisions? Would a levitating frog be safer in an traffic accident, if the carrier were larger than the frog. Only the vehicles would crash:confused: ---
**Author:** ![komolono](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/komolono/32/532_2.png) [@komolono](https://boards.straightdope.com/u/komolono)\ **Post date:** [August 22, 2010, 6:42am UTC](https://boards.straightdope.com/t/can-someone-explain-me-gravity/550314/31 "2010-08-22T06:42:46Z")
My memory capacity is about one byte (compressed). Everyone can see what happens here: [![](https://img.youtube.com/vi/M5xVPJUdTHk/hqdefault.jpg "Who can explain this?") ](https://www.youtube.com/watch?v=M5xVPJUdTHk) The hand is moving the levitating magnet (not vice versa) Although the levitating magnet is seemingly weightless, the net weight must be magnet#1+magnet#2 or otherwise a silly question might arise. ---
**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:** [August 22, 2010, 9:25am UTC](https://boards.straightdope.com/t/can-someone-explain-me-gravity/550314/32 "2010-08-22T09:25:09Z")
> [@komolono](#): > > Has anyone noticed that that some of the quotes in this thread are getting mixed up? > Or maybe I have an identity issue I wasn’t aware of… It’s because people aren’t editing nested quotes with enough care. ---
**Author:** ![Ike\_Witt](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ike_witt/32/283_2.png) [@Ike\_Witt](https://boards.straightdope.com/u/Ike_Witt)\ **Post date:** [August 22, 2010, 2:44pm UTC](https://boards.straightdope.com/t/can-someone-explain-me-gravity/550314/33 "2010-08-22T14:44:37Z")
> [@komolono](#): > > Anyway, what about frog collisions? Would a levitating frog be safer in an traffic accident, if the carrier were larger than the frog. Only the vehicles would crash:confused: I don’t think so. You seem to be applying the same logic that people use when they say that if you are in a falling elevator you should jump up just before hitting the bottom. You are still moving in relation to what you hit regardless of what your relation is to the thing you are riding in. ---
**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:** [August 22, 2010, 5:52pm UTC](https://boards.straightdope.com/t/can-someone-explain-me-gravity/550314/34 "2010-08-22T17:52:41Z")
Actually, I do think that the levitated frog would be safer. Effectively, the magnetic field is providing really good padding. ---
**Author:** ![komolono](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/komolono/32/532_2.png) [@komolono](https://boards.straightdope.com/u/komolono)\ **Post date:** [August 25, 2010, 12:55pm UTC](https://boards.straightdope.com/t/can-someone-explain-me-gravity/550314/35 "2010-08-25T12:55:22Z")
You cannot put a price tag on people’s lives, but I’m under the impression that maintaining low temperatures is not costly after the system is built. How about 10 Teslas to your system? ---
**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:** [August 25, 2010, 2:51pm UTC](https://boards.straightdope.com/t/can-someone-explain-me-gravity/550314/36 "2010-08-25T14:51:08Z")
If you’re suggesting that we put systems like this in cars as crash prevention, yes, you can put price tags on human lives, and it’s done all the time. For the cost of such a system, you could do a heck of a lot more to make cars a heck of a lot safer, just using straightforward established technologies. The only reason they don’t already is because it would cost too much compared to the number of lives it would save. And I’ve never heard of adverse health effects of high magnetic fields on humans, but then, there hasn’t been much study of it. ---
**Author:** ![komolono](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/komolono/32/532_2.png) [@komolono](https://boards.straightdope.com/u/komolono)\ **Post date:** [August 25, 2010, 8:45pm UTC](https://boards.straightdope.com/t/can-someone-explain-me-gravity/550314/37 "2010-08-25T20:45:59Z")
Is it just a space legend that you grow in lenght in zero gravity? Well, at least hospital beds for coma patients (bedsores). Any more? ---
**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:** [August 25, 2010, 8:51pm UTC](https://boards.straightdope.com/t/can-someone-explain-me-gravity/550314/38 "2010-08-25T20:51:29Z")
Aside from the unanswered health questions, nobody’s ever built a strong enough magnet big enough to fit a human in the bore, and even a frog-sized one is still hugely expensive. Maybe once someone figures out how to make a room-temperature superconductor, it’ll be easier. ---
**Author:** ![komolono](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/komolono/32/532_2.png) [@komolono](https://boards.straightdope.com/u/komolono)\ **Post date:** [August 27, 2010, 7:24pm UTC](https://boards.straightdope.com/t/can-someone-explain-me-gravity/550314/39 "2010-08-27T19:24:15Z")
Can someone really explain me gravity (as something relative to electromagnetism)? ---
**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:** [August 27, 2010, 7:32pm UTC](https://boards.straightdope.com/t/can-someone-explain-me-gravity/550314/40 "2010-08-27T19:32:53Z")
> [@komolono](#): > > Can someone really explain me gravity (as something relative to electromagnetism)? How do you want it explained, Newtonian gravity is an easier theory to comprehend than classical electromagetism. General relativity is the current best theory of gravitation, but it takes quite a different tact than Newtonian gravity or classical electromagnetism. [Previous page](https://boards.straightdope.com/t/can-someone-explain-me-gravity/550314.md?page=1) [Next page](https://boards.straightdope.com/t/can-someone-explain-me-gravity/550314.md?page=3)