Gravity in the Universe: Is There a Weakest Force?

[Lino]

Hi, Is there anywhere in the Univerese where the strength of gravity is absolutely zero? Given its infinite range, I assume that the force tends toward zero, but never actually equals zero. Does anyone know where I could find information on what the weakest gravitational force that might (theoritically) be experienced in the (known) Univerese?

Regards,

Noel.

 

[phinds]

Hi, Is there anywhere in the Univerese where the strength of gravity is absolutely zero? Given its infinite range, I assume that the force tends toward zero, but never actually equals zero. Does anyone know where I could find information on what the weakest gravitational force that might (theoritically) be experienced in the (known) Univerese?

Regards,

Noel.

The bolded statement is correct.

 

[Lino]

Thanks phinds.

 

[D H]

Hi, Is there anywhere in the Univerese where the strength of gravity is absolutely zero? Given its infinite range, I assume that the force tends toward zero, but never actually equals zero. Does anyone know where I could find information on what the weakest gravitational force that might (theoritically) be experienced in the (known) Univerese?

The bolded statement is correct for a universe comprising a single object. The universe comprises a whole lot more than just one gravitating body. Note that your statement would be applicable if you had asked about gravitational potential rather than about force.

You didn't ask about potential, and you didn't ask about a single object. You asked about force in our universe. Gravitational forces can cancel; they're vectors. However, those nothing special about those places where this does happen. Much more meaningful are places such as the Lagrange points. But gravitational force isn't quite canceling at those points.

 

[phinds]

Gravitational forces can cancel; they're vectors.

Good point ... I completely forgot about that. I think my answer was correct in the spirit of the OPs post, but clearly was not actually/technically correct. Thanks for that clarification.

 

[Lino]

Thanks D_H. I understand the basics of what you are saying ... multiple objects in the Universe, vectors cancelling, Lagrange points are more interesting ... but I'm not so sure about the difference between gravitational potential and gravitational force. Are you saying that even with zero gravitational potential, there can still be gravitational force? (If it was the other way around (zero force but still potential) I could understand (like a book sitting on a table) more easiely. In case I am just mis-understanding what you are saying, would you mind confirming please?)

Regards,

Noel.

 

[D H]

but I'm not so sure about the difference between gravitational potential and gravitational force. Are you saying that even with zero gravitational potential, there can still be gravitational force?

It's the other way around. The gravitational potential on some object of mass m due to another object of mass M separated by some distance r from the first object is -GMm/r. Gravitational potential is a scalar rather than a vector, and it's always negative. The sum of a finite number of negative numbers is another negative number. Gravitational potential can never be zero.

The gravitational force due to that object is the negative of the gradient of the potential: $\vec F = -\Delta \frac{GMm}r$. Forces are additive, so the net (total) gravitational force is the gradient of the total gravitational potential. While potential is never non-zero, gravitational force can easily be zero.

 

[Lino]

Thanks D_H.

Regards,

Noel.

 

[twofish-quant]

One thing is that most modern physics thinks in terms of fields rather than forces, and often the important thing isn't the strength of the field but rather differences in field strength. The way that a lot of things work is that you can set any point to be zero, but the important thing is the difference between the field strength in one area and another.

 

[Lino]

Thanks twofiah. That actually makes sense (to me).

Regards,

Noel.

 

[andrewkirk]

I was trying to think of what a zero gravity point would look like in terms of various models. This is what I thought might be the case. I'd be grateful if anybody could confirm or correct it.

A spacetime point with zero gravity would be a point where:

1. The spatial gradient of the scalar field (over the 3D hypersurface of constant time) of gravitational potential is zero (this is just another way of saying that the vector sum of forces on a test particle is zero).

or

2. The curvature at that point of any geodesic passing through the point is zero (or need this only be one geodesic?).

If that's right it seems highly likely that such points will exist in a universe containing multiple bodies. Of course, because things are moving around, a spatial point that is zero gravity at one instant probably won't be zero gravity the next.