# Why is gravity attractive?

Why is matter attracted to other matter and not pushed away by it?
Does this attraction apply to antimatter and/or some other exotic kind of matter also?

Years ago, when I asked my high school physics professor the first question (and mind you, she wrote standard textbooks for my country), she just gave me her trademark smirk and said “Because.”
I am now somewhat older and nonethewiser.

Is there a better answer than that I was already given?

Lots of cosmetic surgery.

The question seems a bit strange - gravity is the attraction of matter to other matter from what I gather so it would be nothing if not attractive - but one cop-out answer would be that if it wasn’t for gravity’s attractive force, planets would not form, we would never have evolved, and so we would not be in the position to pose the question…

Mass bends space in such a way that attempting to travel in a straight line actually moves you in a curve toward the object with mass. Something like that, anyway. Ultimately, at the end of all the ‘why’, it has to boil down to ‘it just does’ - maybe not here, but if you dig deep enough, there has to be a point where things ‘just are’.

Not quite it “just does”, there is some reasoning behind it.

OK, a slightly easier way to think about gravity, rather than it being a force, per se, that its more a function of geometry (despite the fact that idea of gravity being a function of geometry coming from general relativity) .

Think of the universe as a big flat rubber sheet (or plastic carrier bag). Now, imagine that this plastic sheet is stretched taut. Now roll a marble across it. It travels in a straight line, yes?

Now, put a larger marble in the middle of the sheet; it makes an indentation. Now roll the small marble again; its path will be perturbed, following the indentation slightly. That’s kind of like how gravity works.

The marbles following straight lines in the absence of a large mass, and then following the curve of the sheet is only really technically correct for light, but it sort of works for matter too.

I always found the ‘rubber sheet’ analogy really unhelpful; the marble only rolls down the indentation in the rubber sheet because of gravity - the force you’re trying to explain.

It’s more like a sheet of graph paper with a ‘pinch’ in the grid lines (actually more like a ‘swirl’ than a pinch, but that’s harder to explain)

Well, yes, but it gets the basic idea across. I’m fully aware that there are flaws with the analogy, but its one I’ve been using for a while with my “gifted and talented” groups, and it goes a long way towards visualising an otherwise abstract concept for them.

What is needed is a theory which predicts gravity as an attractive force far weaker than the other three (electromagnetic, strong and weak). Otherwise, we will only ever be describing it as it is, hence the teacher’s smirk (and, IMO, dishonesty - the honest but brave answer is “I don’t know”).

There is a theory which predicts a fundamental massless spin 2 particle which would provide the macroscopic warping of spacetime which bends distant starlight (visibly during a solar eclipse): String theory.

So ask a mind-bending question, get a mind-bending answer. Why is gravity attractive?

Because the universe is made of music.

For me, describing an item by an analogy that itself explicitly contains and requires the item represents a fatal flaw; but maybe that’s why I never got invited into the gifted and talented groups…

Yeah, to me things like the rubber-sheet system only really made sense in the context of multiple-field theories, that is, building up a system of modelling the universe where a single and simple force like uber-gravity… (that is, gravity from a perspective outside the rubber sheet,) can explain multiple force effects within the rubber-sheet universe. But that’s just me.

That’s because you’re thinking of using it to explain a particle already moving towards the center. The real use of the rubber sheet analogy is to show that paths passing near the center and travelling as straight as they can are curved from our point of view.

If you’ve seen those curved funnels you drop coins into and they spiral around, that’s another example – the coin follows a straight line along the surface, which to us looks like a spiral.

Maybe I’m misreading but I think the analogies and to-date discussion is missing the point of the OP.

What is the mechanism behind gravity. WHY is the rubber sheet indented? That’s a question worth getting answered.

I believe SentientMeat has answered the question about as well as it can be answered:

But, as he noted, the most truthful answer is “I don’t know”. People are trying to find out, and as I understand it, String Theory is the best they’ve got so far.

We start with the observation that unsupported things fall toward the earth. That is, if you hold a rock in your hand and let go it falls to earth. Follow up with astronomical observations that the planets circle the sun in elliptical orbits. Both of these can be explained by a universal attractive force between objects and not by a repellant force between such objects.

Science does’t answer basic “why” questions such as the one you ask or why like electrecal charges repel and unlike charges attract. It starts with the observation that that is what happens and tries to explain that.

This depends on how you’re trying to explain things. A particle physicist will say that he understands the other three forces, in terms of quantum field theories, but that he doesn’t understand gravity. A general relativist will say that he understands gravity, in terms of pseudo-Riemannian geometry, but he doesn’t understand the other three. In any explanation, of anything, no matter how simple or complicated, you have to fall back on something else which you assume you already understand. Which means that some things must be “understood” without explanation, to use as the foundation. So ultimately, the answer to an endless stream of "Why?"s must just be “because”.

As for the OP’s other question, so far as theory and experiment can determine, antimatter behaves exactly like matter, with respect to gravity. One sometimes hypothesizes some sort of exotic matter with negative mass (which would be repelled by gravity), but so far as anyone can tell, no such substance exists or can exist in the Universe.

Which gives me an opportunity to bring up my favorite illustration of this uncomforable truth. If we insisted that every single word be defined without reference to some words that are simply understood without any definition, how would anyone ever produce a dictionary?

Except — isn’t the coin spiraling because it’s steadily losing energy? If friction weren’t present, the coin’s “straight line” trajectory would be some sort of precessing orbit.

Just because two concepts happen to share the same name doesn’t mean they have anything in common. The analogy purports to explain einsteinien gravity in terms of a generic downward force. Theres no requirement for the downward force to be inverse square of masses or to be able to bend space time. Just that it pushes things downwards such the ball stays in contact with the sheet.

The way you phrased the question renders it unanswerable by mortal man. If you had left it at “Why is matter attracted to other matter?”, it could conceivably be explained in terms of some underlying mechanism. But when you ask why gravity is attractive and not repulsive, you are really asking why the universe is the way it is, i.e., what the thoughts of the Creator were as he was drawing up the plans. Only he, supposing he exists, could answer that.

And the anthropic principle would say that if it weren’t so, you wouldn’t be here to ask the question. Or, in it’s strongest sense, it would say that it must be so in order for you to be here to observe it.