I am only 12 years old and I'm constantly wondering and trying understand how gravity really works. On YouTube everyone always talks about objects warping space time around themselves and uses the analogy of a trampoline. I still don't understand gravity because if space were like a trampoline, then earth would be spiraling in towards the sun along with all the other planets, right? So could someone explain to me how gravity really works without the trampoline analogy?
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5$\begingroup$ Duplicated on physics.SE: physics.stackexchange.com/questions/243317/… $\endgroup$– David HammenMar 14, 2016 at 9:42
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2$\begingroup$ The "real world" trampoline has friction acting on things on its surface, so they gradually lose energy and spiral inwards. In space there is no friction, so the planets stay orbiting for near-forever. $\endgroup$– pjc50Mar 14, 2016 at 11:32
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12$\begingroup$ Mandatory XKCD reference. $\endgroup$– Angew is no longer proud of SOMar 14, 2016 at 13:54
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3$\begingroup$ Mandatory Feynman reference. (Talks about magnets, but the lesson about how you think about things applies in any science.) $\endgroup$– jpmc26Mar 14, 2016 at 19:43
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3$\begingroup$ @Zaibis Don't feel too enlightened -- I think Luaan is simply wrong. If you brake an orbiting object it will indeed lose energy and altitude. In the new lowest position (the perigee) it will have a higher velocity, but at all times the sum of new velocity and "potential energy" from the gravity field is smaller than it was in the original orbit, demonstrated by the fact that it will be too slow at apogee to sustain the original higher orbit (we slowed it down there!). Cf. my reply to a comment Luaan made under my post below. He confuses the effect of two unrelated tidal forces on the moon. $\endgroup$– Peter - Reinstate MonicaMar 15, 2016 at 10:10
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5 Answers
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First of all: "How gravity really works" is a deep question, and any serious scientist would quickly concede that all we have is an incomplete working model. You certainly have heard about General Relativity; the first image on the page is your trampoline.
Our working model, General Relativity, is working because it explains a lot of observations very nicely. (Careful, here is another deep question lingering: "Explains" means that we can predict some observations from other observations with the model of gravity we have in our mind. It does not necessarily mean that we understand the "real nature" of the underlying issues.) But we are very confident that the model is working over a wide range of observations. One of the last "first-time" observations which followed the predictions and thus gave us more confidence in the model was the two black holes colliding lately. Lately? Well, billions of years ago. We just learned about it lately. Here is a link to a New York Times article with an impressive video. (I think one can still read a limited number of Times articles for free, so try it out.)
Our model of gravity is incomplete because it doesn't connect well to the model of nature we have for other things (elementary particles, quantum physics). For a while (like 70 years or so) it didn't connect at all; Einstein himself completely failed to connect the dots, which was probably not encouraging since he had received the Nobel Price for laying one of the foundations of quantum physics and was the obvious authority about gravity. If he couldn't do it, who could?
If I'm not mistaken, the physicists today are making progress, slowly. This connection between quantum physics and gravity is one of the main unresolved problems in modern physics.
Last, let me address your concern about the planets spiraling into the sun. This idea probably comes from actual balls on an actual trampoline spiraling in, I suppose. You probably know that the balls lose speed due to friction, much the same way you slow down on your bike when you stop pedaling. Some of the kinetic energy is transformed into heat.
And you know what? You are right. Given enough time, the planets would eventually fall into the sun. Low-flying satellites fall back to earth after a few years, because there are still traces of atmosphere slowing them down out there. The reason is that there is "friction" in the wider sense involved in all large-scale processes in the universe. That is actually one of the fundamental physical principles making up the world we know. It's just that the near-vacuum between the planets doesn't provide that much friction, and the planets are fairly massive bodies with an enormous mass and kinetic energy. It will take a long long time for them to lose enough energy that they'll be so close as to touch the sun. (Perhaps too long to happen at all.) In fact, over human life times the planets, moons and stuff are almost perfect examples for movement without friction. But in the astronomical time scale -- billions of years --, there certainly is friction. For example, the moon is showing us always the same side because friction slowed its rotation so that the rotation is now "locked" with its orbit.
Bottom line: The idea that gravity bends space and time "explains" all large-scale observations so far; the "trampoline" is a good model for a 2-dimensional "space", i.e. a surface, if you ignore friction.
answered Mar 14, 2016 at 10:22
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$\begingroup$ The Moon is also much further than it was in the past. Tidal friction decreased its orbital speed, which increases the orbital radius. The radius is increasing by about four centimeters a year, nowadays. $\endgroup$– LuaanMar 14, 2016 at 13:55
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6$\begingroup$ +1 for "We're not entirely sure, but here's some of our best guesses based on observations." $\endgroup$ Mar 14, 2016 at 14:18
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1$\begingroup$ Please correct the statement about moon. The moon is not synchronized ("locked") by accident but due to tidal forces of gravity - the closer side of moon suffers higher gravity than the other one. This force can actuall force object to increase its rotation, if it would rotate slower than the orbiting speed. $\endgroup$– libikMar 14, 2016 at 14:29
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2$\begingroup$ @libik I cannot see anything which would need correction (in particular, I did not say or imply "by accident" -- on the contrary, I mentioned friction as a cause) . One could mention tidal forces but I thought friction is good enough without detouring too much. You make an interesting point with a possible accelerating rotation due to tidal forces; but it's still a slowing down (to close to 0) relative to it's orbital reference frame. $\endgroup$ Mar 14, 2016 at 15:27
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2$\begingroup$ @Luaan There are two tidal forces in play. (1) The moon receives energy from earth's rotation by the tidal forces the rotating earth exerts on it, accelerating it in the direction of earth's rotation. This lifts it (slowly) higher in earth's gravity well, as you say correctly. (2) The moon's cyclical deformation ("kneading") caused by the moon's rotation in earth's inhomogeneous gravity field convert(ed) some of the rotational energy into heat, eventually synchronizing orbit and rotation, at which point there are almost no moon tides any more (short of those due to libration, I believe). $\endgroup$ Mar 14, 2016 at 15:37
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Why are objects not escaping?
Consider first an object with velocity, and no gravity in action:
Then, that blue object will become more and more distant, if it continues in the same direction.
But it does not continue in the same direction, after a while, the gravity of the big black object has changed its course:
That happens again, and again and again:
Your question is: Why does not the object spiral in? You are perhaps thinking that as it comes closer, the gravity becomes stronger, and therefore the object is forced to come even closer.
But when it falls closer, its velocity increases. As we have seen, the velocity of the objects tries to make it escape. So when it is closer, it has more velocity to counteract the increased gravity.
Edit: Just in case of a more literal interpretation of your question, the trampoline in the original analogy causes friction, and therefore spiralling, but space is a vacuum.
answered Mar 14, 2016 at 5:59
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1$\begingroup$ I think the key as to why it doesn't fall in is that in space we don't have friction - on a trampoline energy is constantly being removed from the ball via friction, whereas in space there's nothing to slow our planet down, so it just keeps going $\endgroup$– JeffMar 14, 2016 at 22:42
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4$\begingroup$ My physics teacher in high school said "the earth falls towards the sun all the time, but continues missing it because of its speed." $\endgroup$ Mar 15, 2016 at 8:32
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The trampoline analogy is useful if you understand gravity within a General Relativity framework. The conceptual problem there is that actually the space-time is wrapped in 4, not in 3 dimensions, i.e. including time.
In fact, when the Earth rotates around the Sun, it loses a very tiny amount of energy in form of gravitational waves. So, the Earth is actually spiraling towards the Sun. The thing is that this gravitational wave emission is so small, that by the time we observe any considerable spiraling, the Earth and the Sun would have already ceased to exist. Much before that, the Solar System becomes unstable due to chaotic effects already contained in classical Newtonian mechanics.
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Great question!
Have you heard of Newton's First Law? It says that an object in motion continues moving at the same speed and in the same direction unless acted upon by a force.
When we roll a ball along the ground, it will eventually stop. Before Newton, many people believed that everything slows down by itself. Newton's insight was that this is not true, and actually the only reason a rolling ball will slow down is because the ground and the air rub or push against the ball to slow it down.
On a trampoline, a ball will rub against the trampoline material and against the air, which slows it down. This is the only reason that the ball ends up spiralling towards the centre.
When there is nothing to slow the object down, it won't spiral towards the middle, it will just keep going round and round forever. In space there is (almost) nothing to slow an object down.
If you find this hard to believe, you can write a computer program to do all the calculations and see what happens! I have made an example simulation for you. You will see that without friction, the planet will end up where it started each time it goes around the sun. If you change the planet's initial yspeed from 20 to 40 and then click "Run" up the top you will see a more circular orbit. You can change other things and see what happens. I hope you find this useful!
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$\begingroup$ Nice simulation. (Although the planet escaped the sun after it got close. :-) ) $\endgroup$ Mar 15, 2016 at 8:30
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$\begingroup$ It's an easter egg ;) Actually it's a good discussion point---it reminds us that the simulation is ONLY simulating gravity, not collisions, and also that when the planet gets really close to the sun, the time-step of the simulation causes big inaccuracies. This can be reduced by more sophisticated numerical methods like Runge-Kutta, but now I'm well beyond the scope of the question! $\endgroup$– ArteliusMar 15, 2016 at 9:02
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$\begingroup$ I don't know if it's the same simulation anymore when you do this, but if you change the for-loop condition to
i < 1rather thani < 5and you change the timeout parameter to setInterval to10rather than100, the simulation gets a whole lot more pleasant to watch. It runs slightly faster, but the framerate is much higher, so the movement of the outer body isn't so jagged. $\endgroup$– AlexMar 16, 2016 at 11:34 -
$\begingroup$ Thanks Alex! Actually the timeout parameter should be 20 and then (assuming your CPU is fast enough) it is the same simulation. On my computer this slows the simulation down by 25%, presumably because my CPU isn't fast enough. Still, it does look smoother; here is a new simplified version: jsfiddle.net/0erknpk8/38 $\endgroup$– ArteliusMar 16, 2016 at 23:51
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Neutrino String Induction-Refraction is the cause of gravity. Some will say that neutrinos are insignificant, but Dirac, Hawking, and Tyson think otherwise and most discount the effect of a charged particle traveling at the speed of light. Keep in mind that no one can, or has proved that mass is a property of matter, more of an effect.
Go to www.themechanismofreality.com , this site explains exactly how gravity works. Every Physicist who examines this agrees that this is correct. From CERN to the University of Beijing's Physic Department agree that this is a 'Fantastic connection between graviton physics and string theory"! This was also confirmed , indirectly, by LIGO and the gravity wave announcement. Enjoy!
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$\begingroup$ This is a link only answer. (not encouraged), also, that paper seems a little weird towards the end. $\endgroup$ Mar 15, 2016 at 16:50
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$\begingroup$ I have the nagging feeling that the lack of math, references and collaboration indicates it's not revolutionary science but at most a popular science article. It's tough; one should, of course, not gratuitously add math just to appear serious. But this kind of lone paradigm-changing breakthrough (which, I think, is claimed here, since I never heard of it before) is exceedingly rare. $\endgroup$ Mar 15, 2016 at 17:50
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$\begingroup$ In order to make the theory in the article more palatable you could try to put it in context. Like, start with what the conventional theory (and its famous proponents) thinks a Neutrino is and how it interacts, and why a different assumption could explain gravity. $\endgroup$ Mar 15, 2016 at 17:53