# Why Does Gravity Work? [closed]

I'm trying to work out if energy has gravity, because if it does, then it will all get sucked into black holes, including dark energy, meaning that there will be no pushing effect left, and that the extra gravity of all the dark energy and dark matter in black holes will be enough to cause the big crunch.

It also means that black holes cannot evaporate, and that the ultimate fate of the universe will not be photons flying around for all eternity, it will be the collapse of the universe in a big crunch, which causes the next big bang and so on forever.

I was told that energy gets converted into mass by the Higgs effect, and that somehow means that the same energy now has gravity, where it had none before.

Why is that? Can somebody explain why this particle of energy causes other particles of energy to form into something which has a completely different property to the particles of energy by themselves?

Is there any real testing of the hypothesis that energy particles do not have gravity unless they are formed into mass?

I understand the warping of space time, to some degree, it's like a three dimensional curve, like rolling down a hill, except there's nothing there in between the objects, and rolling down a hill is caused by gravity.

http://simple.wikipedia.org/wiki/Higgs_field

I'm not a physicist, but I've been looking at pages like this, and half the time I understand it, and the rest of the time, it goes way over my head and I imagine that there are a handful of people in the world who can really understand it, or I think it's actually just nonsense.

It matters to me because I believe that the universe is eternal, and that the only way the big bang could have happened in the first place is if there was this eternal cycle, and that means that death is probably not the end, because the same energy will be formed in the same way over and over again, into my brain, and my consciousness.

## closed as too broad by Kyle Kanos, rob♦, Jim, Brandon Enright, John RennieJun 30 '14 at 18:16

Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. Avoid asking multiple distinct questions at once. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

• Are you asking us to explain to you general relativity, the Higgs mechanism and QFT in one question? And preferably without math, I suppose? (Let me point out that Higgs is not what converts mass and energy into each other. They are equivalent by the $E = mc^2$ of relativity) – ACuriousMind Jun 30 '14 at 14:03
• Yeah, maybe to just simplify it a little. From what I heard, there aren't that many people who can even grasp the general relativity part. I tried looking at it. – rowanman28 Jun 30 '14 at 14:05
• Seems to be a good question for layman like me, but not for the physicists who have downvoted. I appreciate your question. – Immortal Player Jun 30 '14 at 15:07
• @Godparticle: the downvoters are downvoting because this question asks several really broad questions whose answers take up literally half of the site. – Jerry Schirmer Jun 30 '14 at 15:24
• Indeed, the question is too broad. There are probably books written about each sub-question! You'll get good results here if you tell us what pages you've looked at, and at what point it goes over your head. – garyp Jun 30 '14 at 16:13

## 1 Answer

At school you learn that mass causes gravity, and the acceleration due to the gravity of a mass $M$ is given by Newton's equation:

$$a = \frac{GM}{r^2} \tag{1}$$

But Newton's law is an approximation that works well at low speed and densities, and gravity is actually far more complex than this. For a more accurate description we have to go to general relativity and use Einstein's equation:

$$G_{\mu\nu} = \frac{8\pi G}{c^4} T_{\mu\nu} \tag{2}$$

The left side of equation (2) tells you how a test mass is going to move, so it's sort of analogous to the acceleration in equation (1), and the $T_{\mu\nu}$ on the right side of equation (2) is sort of analogous to the mass $M$ in equation (1) - it's the same constant $G$ in both equations.

The object $T_{\mu\nu}$ is the stress-energy tensor, which is normally written as a 4 by 4 matrix, and in most circumstances only the top left entry, $T_{00}$, is important. The number is the energy density i.e. it is the energy per unit volume. The stress-energy tensor makes no distinction between matter and energy - if you want to include matter you convert it to energy using Einstein's famous equation $E = mc^2$.

So the answer to your first question is that yes energy does create a gravitational field. Indeed there is no difference between the gravitational fields created by energy and matter give or take a factor of $c^2$.

But that doesn't mean energy is all going to be sucked into black holes. In fact not even all the matter is going to end up sucked into balck holes, because black holes don't suck. It's a very common misconception that black holes suck in matter, but outside the event horizon the gravitational field from a black hole is just like the gravitational field from a normal star with the same mass. The black hole can't suck in matter because matter will orbit it just like matter orbits a normal star.

The large scale structure of the universe is described by an equation called the FLRW metric, and assuming our current measurements are correct the universe will carry on expanding and won't recollapse. Black holes don't make any difference to this because on the scale of the observable universe even supermassive black holes are small perturbations, and on this scale matter is evenly spread out.

And finally, dark energy is almost certainly not what you think it is. No-one knows what dark energy is, but the two leading candidates are a cosmological constant or a scalar field called quintessence. The cosmological constant is part of the geometry of spacetime, so it isn't something that could be sucked into a black hole. If quintessence exists then it's spread out so widely that only a tiny fraction of its energy could ever end up inside black holes. Either way you need not fear that black holes will cause the universe to recollapse.

• Mercury is going to fall in the sun one day, as is the moon going to fall into earth. I'm talking about many, many, billions of years, when all mass is black holes. Over that time period, dark energy could have a similar effect caused by cooling down, which stops it's pushing effect. I do not think that dark energy remains at constant density as the universe expands, it's not a product of the void, it's energy from the same big bang that all other energy is from. – rowanman28 Jun 30 '14 at 16:37
• @rowanman28: no. The sun will expand to swallow mercury, eventually. The moon is actually getting farther from the earth with time, and both have a fate of getting swallowed by the sun when it goes to red giant phase. – Jerry Schirmer Jun 30 '14 at 16:54
• Hmm, that's strange, I thought I heard somewhere that it was the other way. Either way, our solar system will probably crash into another solar system, and the closest galaxy is moving towards us. – rowanman28 Jun 30 '14 at 17:08