# How does gravity truly work? [duplicate]

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 wrapping 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 spiralling 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.

• If I had a penny every time someone asked this question... :) There's a lot of very good answers here, for example: physics.stackexchange.com/q/3009 The main deal is that once you have non-flat space, you don't need any force to explain gravity. In mathematics, this is very well explored by the field of topology. In physics, general relativity is built on that. If you're not afraid of learning a lot as you go, tinyurl.com/gr-math is a great explanation - a fair warning: it's university-level material. – Luaan Mar 14 '16 at 10:15
• Possible duplicates: physics.stackexchange.com/q/3009/2451 , physics.stackexchange.com/q/222390/2451 and links therein. – Qmechanic Mar 14 '16 at 10:31
• If you think of something like coins rolling around on a slope, instead of rolling on a surface and moving through air (both of which are slowing them down), picture them in a frictionless environment (vacuum), where their speed would remain (more or less) constant. – DevSolar Mar 14 '16 at 10:55
• ...then earth would be spiralling in towards the sun... The Earth is spiraling in toward the Sun. It's just happening very, very, slowly. The reason why a rolling ball spirals in toward a heavy object at the center of a trampoline is because friction robs the ball of kinetic energy. With no friction, the ball would circle forever. There is almost nothing out there in the Solar system to rob the Earth of kinetic energy as it orbits the Sun. – Solomon Slow Mar 14 '16 at 13:40
• Also asked on Astronomy. – HDE 226868 Mar 14 '16 at 21:33

I personally prefer this visual explanation of Einstein's general theory of relativy from Brian Greene, a professor at Columbia University. https://www.youtube.com/watch?v=0jjFjC30-4A

Space (better space-time) is not like a trampoline. That's just a really poor image that is mostly used in pop-science, even by people who know better like Kip S. Thorne. I did hear him use a much better analogy once in a talk for physicists in which he compared gravity to a river:

If you want to stand still while swimming in a river (relative to the banks of the river), you have to keep swimming upstream (use your rockets...) or be anchored to something (stand on the surface of a planet). As long as you are swimming with the stream, the water doesn't exert any forces on you, but if you want to stand still relative to the banks (or swim upstream), then there will be a significant force against the flow of the water.

The analogy for a black hole then becomes a circular waterfall: you can drift all the way to the edge of the falls and keep swimming there until you are tired (your fuel runs out...) and even swim back upstream (if you brought enough fuel...), but as soon as you fall over the edge, there is no swimming back! Over the edge of the falls the world at the level of the river disappears and it's only down from there... towards the level of "the singularity", that's the bottom of the falls that will crush you.

The model is not perfect, of course (it doesn't work for orbits), but it motivates that spacetime (which is the real foundation of gravity) has a one way component: time.

Now... in reality all of this is a lot more complicated and you will have to learn general relativity, if you really want to understand the details. The good news is that the golden time of gravity research has just begun. If you decide to become a physicist, you will see absolutely amazing things in your lifetime and many may be related to research on space, time and gravity.

• "If you decide to become a physicist, you will see absolutely amazing things in your lifetime" nice line, I wish I´d heard that when I went to school – TheBlastOne Mar 14 '16 at 15:32
• @TheBlastOne: I would be glad to get a do-over, too... so far nobody has offered me one. :-) – CuriousOne Mar 14 '16 at 20:24
• If ya askin' if time travel is possible, please post a new question ;-) – TheBlastOne Mar 15 '16 at 0:19
• @TheBlastOne: More like total rejuvenation... I want to be seven years old again. :-) – CuriousOne Mar 15 '16 at 0:46
• Tipler promises, and believes to have proven, that this will be reality in the far, far, far (ultimate) future. See ya there! Looking forward to it. – TheBlastOne Mar 15 '16 at 5:42

I still don't understand gravity because if space were like a trampoline, then earth would be spiralling in towards the sun along with all the other planets, right?

The trampoline analogy is in two dimensions. A small comet does spiral and end up in the sun, because as it moves it is losing mass and therefore angular momentum. Angular momentum is what keeps the earth and planets from spiralling down to the sun, in stable orbits, because angular momentum is a conserved quantity. To decrease, the earth should be losing mass, which does not happen in the vacuum where the earth moves. On the trampoline, because of friction the little balls lose momentum and angular momentum and end up spiralling to the center, no stable orbits can be established.

People developed the theories of how gravity works over centuries. Observations showed that what goes up, must come down, and that created the concept that gravity is a force, similar to the force needed to pull or push somebody. Newton made the first mathematical formulation which explained not only falling apples and throwing javelins but how the planets revolve around the sun, by assuming a force felt between the mass of the sun and the mass of the planet. This works very well as a mathematical model of gravity. The trampoline goes as step further, trying to model the gravity as expressed in General Relativity; making the masses generate the space and time , everything then follows mathematically.

To really understand physical reality with the available models, one has to study hard, mathematics and physics, which seems a big task for your age group, although I know of people who started on the physics path at 14. Mathematics is important in order to be able to understand physics .