# Evaluate: The universe must be finite, because if it were infinite...

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Eric Newlandposted 5 years ago

...there would be no gravity.

My reasoning comes from the Shell Theorem:

http://en.wikipedia.org/wiki/Shell_theorem

Simply put, a spherical shell exerts no net gravitational force on objects inside it. Likewise, there will be no net gravitational force on a point at the very center of a solid sphere.

Now consider an infinite universe, with infinite size and containing an infinite amount of matter. It's safe to say that at any point in this universe there will be an infinite amount of mass in all possible directions, exerting an infinite gravitational force. Gravity decreases exponentially with difference, but that doesn't matter: infinite mass = infinite gravity.

However, because this force is exerted in all directions at once it must cancel itself out. Effectively, an infinite universe behaves like a solid sphere of infinite radius, and any point might as well be the center.

Local variations in density of matter, therefore, would be negligible. Nearby bodies (if bodies of matter even existed, with no gravity to pull matter together into planets and stars) with finite mass could not counteract the effect of infinite mass even by their proximity.

Thus, an infinite universe would mean no gravity. Right?

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lobobrandonposted 5 years ago in reply to this

Hmm Good point, will have to do a bit of thinking about this. I'll be back to reply Tomorrow when I'm at University during some boring classes I've got something to think about. Thanks Eric.

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lobobrandonposted 5 years ago in reply to this

What if from the very beginning, the universe didn't expand symmetrically about a point?

Before the big bang all matter was dense at a tiny point right - not sure abt the size. If at all the expansion was not symmetric it would never form a sphere until some parts slow down and others accelerate so as to form a unison (what's the probability?). But, if this happens would the universe come to a stand still or would it result in the great crush?

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lobobrandonposted 5 years ago in reply to this

Addition: It won't result in a complete standstill as everything is already in motion so gravity won't disappear altogether. In comparison to the size of the universe I guess you can consider stellar systems to be electrons revolving around the nucleus (the center of their respective galaxies and neighbouring galaxies as other atoms in the vicinity). Gravitational forces in such a case is negligible, but nuclear forces are rampant - I'm considering the masses even though they may seem uniform throughout the universe would be disproportionate in the galaxy and hence gravity would exist; but, on the scale of the entire universe it would be negligible. But, it does exist!

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kirstenblogposted 5 years ago in reply to this

So inside a singularity there would be no gravity? I assume a singularity is going to have a spherical shape as its the most efficient shape in nature and it would have to be under those pressures.
I don't actually think we understand what gravity is well enough to deduce this. Is gravity an effect of the accumulation of mass or is it to do with the relationship between mass and motion?

I am not saying you are right or wrong, just that we don't know enough for something like this to be anything more then a bit of fun food for thought

How would this theory explain gravity inside a sun? I ask because it is a sphere and as far as I knew it does have gravity inside it, but maybe I am not understanding the theory there?

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lobobrandonposted 5 years ago in reply to this

Good point and just as you say it's just a thought to ponder on. The sun isn't exactly a sphere and nothing can be perfectly a sphere I guess. It would be spherical instead.

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kirstenblogposted 5 years ago in reply to this

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lobobrandonposted 5 years ago in reply to this

Yup

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Eric Newlandposted 5 years ago in reply to this

Likely. A singularity could probably be considered a sphere of zero radius, and besides, any net force would have to project matter outward from the singularity, and you know that's not happening.

As you travel "downward" into a spherical body gravity decreases, as the "shell" formed by the matter "above" you no longer exerts a net force on you. Of course, you still have gravitationally affected matter pushing down on you, which is why pressure would increase. At the very core there is no net gravity, but the "weight of the world," in a fairly literal sense, would be pressing in on you from all directions, so unless you could construct an ultra-strong room with air at a livable pressure you couldn't exactly enjoy the experience of floating.

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kirstenblogposted 5 years ago in reply to this

I am still trying to work out in my mind what the conditions at the very center of a sphere might be like. When I think of our planets core, I just am not sure that there is a weightless center. Our core of liquid metals and other elements is super hot due to the pressures from everything above and more importantly spins. The engine of the planet as it were. I am trying to zoom in (in my minds eye) on that spinning mass to a point where, it doesn't spin anymore? How big could that space be? A single atom of space? If it were anything bigger surely one atom is going to have ever so slightly different pressures from the mass around it as its an atom off-center as it were. Next, I am going to be asking how long a piece of string is!!!

I of course also have to consider that in my thinking, using a planet to help me visualize this zero point in spherical mass for gravity, I am working with inaccurate tools. The planet of course is not a perfect sphere, it's spheroid but oblongs because of outside forces like the sun and moon, hence our tides etc.
In relation to your question this could parallel to the theory of multiple big bangs (creating outside forces to distort the sphere of our universe). There is a body of thinking that considers that the universe might be doughnut shaped, to explain how it might be impossible to get 'outside' the universe because it would allow you to travel in a straight line yet bend without ever reaching the 'end' or boundary that defines our universe as infinite.
Something in a sphere universe should in theory be able to escape the outer boundary of the sphere without being deflected back around.

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Jesus was a hippyposted 5 years ago in reply to this

Is it possible that everything in this universe has an equal counterpart (antimatter). Maybe gravity is reversed with anti matter. If the total amount of matter is equal to antimatter, then the gravity caused by the two would equal zero.

Maybe there is no gravity?

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agvulpesposted 5 years ago

From what I can remember you are right up to a point!  Outside of the Earths gravitational pull there is no 'gravity'
The gravity on Earth is created by it's own rotational forces (or something to that effect) Correct me if I am wrong I am always willing to learn something new?

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Eric Newlandposted 5 years ago in reply to this

Gravity is exerted by all bodies on all other bodies as a function of mass and distance. Well, distance squared, technically.

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lobobrandonposted 5 years ago in reply to this

If you test a bomb to explosion, does it explode spherically? I'm not sure just asking. The BB would be the same

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kirstenblogposted 5 years ago in reply to this

So when you are in a elevator and it suddenly goes up, you get that momentary feeling of being heavier, is that not gravity? Likewise, when the elevator suddenly drops from under you and you get that second of almost weightlessness is that not the reduction of gravity by another force? What I am getting at is that mass and distance are just two factors but motion does seem to be an equal factor to consider when understanding gravity.

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Eric Newlandposted 5 years ago in reply to this

Gravity is constant. When other forces (such as an elevator) change your motion you merely perceive it as a change in gravity. But earth is still accelerating you toward its center at 9.8m/s^2 regardless of anything else affecting your motion.

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kirstenblogposted 5 years ago

Another question, when you say 'the universe' are you only talking about the matter that came from the big bang but not the 'space/emptiness' it banged into?

I am not aware of the big bang still spewing matter into the universe so the matter originating from it would indeed be finite

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lobobrandonposted 5 years ago in reply to this

I still can't imagine the big bang. Wonder what the density was

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kirstenblogposted 5 years ago in reply to this

I sometimes wonder, could it be the collective density of all the black holes that will be when entropy sees the last star burn out and either become a black hole or be consumed by one. When there is nothing but black holes left would they all pull at each other, collide and become massive super black holes and could enough of them join to create a force capable of creating a new big bang? I think maybe

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lobobrandonposted 5 years ago in reply to this

Never thought of it that way Good point. I guess there should be black holes being consumed by others and as I type this I'm sure there's one happening somewhere out there.

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Eric Newlandposted 5 years ago in reply to this

Yes, I'd say that Big Bang Theory necessarily supposes finite mass, in which case the exponential decay of gravity with distance means most bodies don't affect us in a noticeable way

But yes, if the universe is finite and roughly spherical then it must exert gravitational force effectively originating at its center of mass, wherever that is. It stands to reason that the closer you get to that center the less that universal gravitational force would be.

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lobobrandonposted 5 years ago in reply to this

At the center there wouldn't be any gravity then and I guess time would remain still?

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kirstenblogposted 5 years ago

As much fun as I am having with this thought line I have a computer that is trying to do a back up and I will mess it up if I don't let it get on with it

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lobobrandonposted 5 years ago in reply to this

Will wait for you to reply as well I have to go for my dinner and complete my assignments too. Doing them atm

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Druid Dudeposted 5 years ago

We are in a black hole, within a black hole, within a black hole, within a black hole, and one day, we will enter another black hole. Do you know how many holes it takes to fill the Albert Hall?

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IWantFroyoposted 5 years ago in reply to this

The volume of a black hole (not counting the event horizon) is theoretically 0. Therefore, that's dividing by zero, or infinity.

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Druid Dudeposted 5 years ago

This universe is inside a black hole, inside a black hole, inside a black hole, and one day will slip inside a black hole. Do you know how many holes it takes to fill the Albert Hall?

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knolyourselfposted 5 years ago

Mr. Memory would know, Hitchcock: 39 Steps

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Druid Dudeposted 5 years ago in reply to this

A line from a Beatles song?

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Druid Dudeposted 5 years ago in reply to this

sory about repeating myself. I'm having another weird day. Hohum!

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knolyourselfposted 5 years ago

Hitchcock film, his first good one, 1938. The spies steal the plans and they hide them in Mr.
Memories mind.

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Druid Dudeposted 5 years ago in reply to this

This is where Hawking was before he published his latest theory. He stil refuses to acept that science has found God and re-labeled 'him'(it).

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scottcgruberposted 5 years ago

This would only make sense in a Newtonian conception of gravity, in which gravity is instantaneous. However, gravity obeys the same speed limit as light. The gravity affecting us is not the gravity of the entire universe - just the portion in a 13.7 billion light year radius around us. The gravity of objects farther away has not reached us yet. The same would apply to every other point in the universe, so there cannot be infinite gravity. It has to be finite, allowing the inverse square law to apply.

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