If gravity was repulsive at subatomic distance?

[Daecon]

What might the Universe be like in a fictional reality where gravity was repulsive instead of attractive at subatomic distances?

 

I'd assume it wouldn't make much difference to the average atom or molecule as the masses involved would be too small for gravity to have an effect either attractive or repulsive?

 

However, I did wonder what the behaviour of black holes might be like in this hypothetical scenario. Attractive at regular, human-experience distances and repulsive at the subatomic and Planck scales.

 

Would the mass reach a stable equilibrium between the attractive and repulsive, would the black hole explode once the singularity reached a critical threshold, would the hole appear to perpetually inflate and deflate as each "side" asserts dominance, and so on... Would if even be possible to calculate the behaviour given the hypothetical nature of the speculation?

[imatfaal]

We can do very sophisticated modelling on the Strong - and that goes from being attractive to repulsive at shortening distances. But in the case of the strong we have experimental data to guide our hand - in the case of pure supposition there are so many variables that any modelling has very limited value.

You could maybe think of it like this; gravity is always attractive at very low distances you start getting fermionic exclusion which, for example, is manifest through degeneracy pressure in neutron stars. Are you saying that pauli exclusion is actually gravitic - or that it can be better modelled as such?

[studiot]

 

Are you saying that pauli exclusion is actually gravitic - or that it can be better modelled as such?

 

 

Gravity affects both fermions and other particles with mass.

 

Pauli only affects (applies to ) fermions.

[Daecon]

Well I was reading about how some people have speculated that gravity may be repulsive at significant intergalactic distances, and my mind went off on a tangent regarding the inflationary epoch just after the big bang. I thought I'd start off with some speculation about the implications of small scale repulsive gravity before applying that train of thought to things like Inflation and such.

[imatfaal]

Gravity affects both fermions and other particles with mass.

 

Pauli only affects (applies to ) fermions.

 

Has gravity ever be demonstrated or observed on a non-composite boson other than the photon? Or at short range? If we are wildly speculating (which this thread is) about changing relationships at the most fundamental level then that which has not be empirically verified is up for grabs

[Daecon]

Hmm, I'd not really thought about things such as the Pauli exclusion principle. Wouldn't that require gravity to be stronger at nuclear distances though, for example would having repulsive gravity being the force that's keeping these speculative protons and neutrons apart imply that real-life gravity is keeping protons and neutrons together in the real world?

 

We know that it's not gravity that's responsible for nuclear cohesion in the real world, so I think we should assume it's not repulsive gravity keeping the Pauli exclusion principle in play in this scenario. Trying to keep suspension of disbelief as small as possible, and all that.

 

I don't expect there's much gravitational attraction between individual protons and neutrons, so I'm not imagining a difference in strength, just a difference in attraction/repulsion. (I'm not sure what the term would be, "polarity" doesn't seem appropriate.)

 

As for neutrons stars and such, if we imagine that gravity was repulsive up to the distances of the atomic or molecular scale and not just subatomic/Planck, and so the mass and density of degenerate matter would be high enough for gravity to be noticeable, that could work. I suppose one of the bigger problems with this speculation would be deciding where to have gravity change from being repulsive to being regular old attractive gravity.

[elfmotat]

Disregarding higher-order effects, if the gravitational potential energy between two masses was [math]+GMm/r[/math] instead of [math]-GMm/r[/math] then gravity would behave akin to like charges repelling each other (but much weaker). The result of which would be a universe with no large structures, and definitely no black holes.

 

If you want to keep gravity the way it is, except have it be repulsive at very short distances, the formulation of such a theory would be much more complicated. (I suspect it's probably been done before, but I haven't seen it). However, the shorter the scale at which it becomes repulsive, the more irrelevant the effects would be due to how weak gravity is in the first place compared to the other forces.

Edited June 3, 2016 by elfmotat

[Daecon]

Another thing that occured to me would be if gravity went between attractive and repulsive depending on distance, would that imply a specific distance where gravity is neutral or zero? I imagine such an exact distance wouldn't be very easily maintained though, so it might not matter.

[Daecon]

I hope nobody minds me bumping this old thread, but I was curious as to whether it would be mathematically possible to model an equation where gravity is both simultaneously attractive and repulsive, so that there's a (inversely?) proportional value between the strength of gravity and the distance where the attractive or repulsive aspects of gravity assert dominance over the other?

[imatfaal]

Another thing that occured to me would be if gravity went between attractive and repulsive depending on distance, would that imply a specific distance where gravity is neutral or zero? I imagine such an exact distance wouldn't be very easily maintained though, so it might not matter.

 

If there were no other forces acting then it would be very easily maintained as it would be the bottom of the potential - ie it would require energy to push closer together and energy to pull further apart

[swansont]

I hope nobody minds me bumping this old thread, but I was curious as to whether it would be mathematically possible to model an equation where gravity is both simultaneously attractive and repulsive, so that there's a (inversely?) proportional value between the strength of gravity and the distance where the attractive or repulsive aspects of gravity assert dominance over the other?

 

 

Can you model this? Sure. Add in a positive term that dominates at small distances but drops off faster than r^2. Maybe something of the form r^-8, or e^-ar

[Daecon]

 

If there were no other forces acting then it would be very easily maintained as it would be the bottom of the potential - ie it would require energy to push closer together and energy to pull further apart

 

Ah, right. I was getting confused somewhere and was imagining a point of zero gravitational influence as a metastable point. I guess that wouldn't really make sense, though.

 

 

 

Can you model this? Sure. Add in a positive term that dominates at small distances but drops off faster than r^2. Maybe something of the form r^-8, or e^-ar

 

Is there any useful software that would let me put in a value and it'll produce a graph?

[Sensei]

 

Is there any useful software that would let me put in a value and it'll produce a graph?

 

Well, if you need 2D graph, you could simply use Open Office/Excel,

Make one column input data (f.e. distance)

and second column using 1st column as parameter,

like A1=0

A2=A1+0.01

Fill down whole column, (so they will be A3=A2+0.01,A4=A3+0.01 etc)

B1=1/A1

B2....Bx fill down whole column.

Select A and B columns, and Make Graph (or whatever it's called in English version of Open Office).

 

See how it's done here

Edited June 21, 2016 by Sensei

[Daecon]

Thanks for that, Sensei. I'll have to have a play around in Open Office and see if I can figure it out.

[swansont]

 

 

Is there any useful software that would let me put in a value and it'll produce a graph?

 

 

I think Wolfram Alpha will do that online.