I know that Gravity keeps things in the Sun’s orbit (or other stars/planets), but how do they have gravity?
In: Physics
Everything with mass has gravity. Includes you. In space you could have your own little moon.
Gravity is a property of mass. Everything that has mass has gravity.
There are two possible explanations that may or may not be compatible. Scientists are still trying to figure that out.
In quantum field theory, there are quantum *fields* that permeate all of spacetime. *ALL* of space has these fields, and all particles are packets of energy within those fields. So, for example, electrons are big packets of energy in the electron field. Seems simple! These fields can trade energy and affect each other. So, for example, the electron field trades energy with the electromagnetic field. Photons are packets of energy within the electromagnetic field. When electrons trade energy with the electromagnetic field, it creates photons.
And that trading of photons between charged particles is what creates the electromagnetic *force*. Electrons repel each other by trading photons that impart momentum, and in the case of two electrons that momentum moves them away from each other.
Three of the four forces – electromagnetism, the strong force, and the weak force – all have particles that scientists have observed, proving the existence of those fields. One more field that has been proven is the Higgs field through finding the Higgs boson. The Higgs field is related to mass and it *seems* like all particles that have mass interact with the Higgs field, which gives them inertia.
So what about gravity? Well, we know that a gravitational field must exist, because we observe that gravity exists. We also observe that gravity propagates at the speed of light, just like photons. We’ve observed gravitational waves, which proves that gravity “radiation” exists. The idea would be that things with mass trade particles which imparts energy and momentum like photons do. Things with mass have a sort of “gravity charge” like negatively charged electrons, except with gravity all things have the same one charge, and that charge always attracts.
The problem is that no gravity *particle* (which would probably be called a *graviton*) has *not* been observed. Without that particle, there’s no conclusive proof that gravity has the same kind of field as the other forces, or at least that gravity can be quantized like other forces (which is a different ELI5 concept, but it would be very weird if gravity can’t be quantized). That obviously means that gravity doesn’t exist, but it could mean that gravity works in a fundamentally different way.
So what is that other way? Einstein theorized that mass warps spacetime, causing it to curve. Things like photons traveling through space follow a “straight” line, except in the presence of mass those lines are curved towards the mass. This is visualized [like this](https://wtamu.edu/~cbaird/sq/images/spacetime_curvature.png) and is often explained [like this](https://www.youtube.com/watch?v=MTY1Kje0yLg). Thus, when not acted upon by another force, things move towards other things with mass. Thus, the Moon is trying to follow a straight line through space, but that line is curved around towards the Earth. Likewise, the Earth is trying to go in a straight line, but that line is curved around the Sun. However, the Moon is going fast enough that its line doesn’t actually intersect with the Earth, it just keeps going around (and the same is true about the Earth around the Sun, and everything else orbiting everything else).
Why do things with mass warp spacetime? They just…do, it’s a property of how reality works. Electrons have a charge that repels other electrons and attracts protons, nothing can go faster than 299,792,458 m/s, the ratio of a circle’s circumference to its diameter is equal to 3.1415926…, and mass warps spacetime.
Einstein’s theory has been experimentally proven to work. It makes predictions about things like [gravitational lensing](https://www.roe.ac.uk/~heymans/website_images/Gravitational-lensing-galaxyApril12_2010-1024×768.jpg) and time dilation. So we know it *works* and makes accurate predictions at large scales. It does *not* make predictions about what happens at quantum scales, and unfortunately there’s no good theory of quantum gravity that is compatible with Einstein’s theories of Relativity.
In relativity, everything that has mass or energy warps and stretches space (and time so it’s called spacetime but don’t worry about that for now). If you think about a bowling ball on a trampoline, it “attracts” other things on the trampoline. It’s really hard to imagine in more dimensions than that but everything makes a little (or big) dimple in spacetime. So, things naturally “fall” towards each other.
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