# Does gravity affect mass ?

• I
(This is to check my understanding )
I gather that gravity being a space-time distortion is thought to affect everything - mass, radiation, whatever - and can be thought of as an accelerational effect.
If it is an acceleration, even entities with mass = 0 are accelerated (?).
So is there any effect of gravitation on mass, specific to mass itself ?
(Obviously a mass creates a gravitational field, that is not the question.)

Dale
Mentor
2021 Award
I am not clear on what you are asking. Are you asking if the laws of gravity make a specific distinction between an object of 0 mass and an object of >0 mass with respect to passive gravity?

(sorry for being a little unclear)
On the basis of gravity being an acceleration via distortion in space time, any entity in such a field will be affected. So the entity does not necessarily have to have mass to be affected ( I think?), which makes me wonder if there is any specific gravity effect on mass, or is mass just something that is being affected because it is an entity that happens to be in the field. I guess the condensed question is, does gravity have an effect on mass, specific to mass as opposed to anything else.
(I hope that makes sense)

Ibix
On the basis of gravity being an acceleration
It isn't. A key realisation of relativity was that free-falling bodies aren't accelerating. It's those of us standing on the surface of the Earth or in a rocket under thrust who are accelerating - we are being pushed out of our free-fall paths by the floor, for one reason or another.
I guess the condensed question is, does gravity have an effect on mass, specific to mass as opposed to anything else.
Mass is a property of things, not a thing itself. So I think you mean "does gravity have a different effect on objects with non-zero mass compared to those with zero mass".

The short answer to that is yes and no. Qualitatively the effect is the same: the "straight line" that things travel in naturally becomes a geodesic, which generally has a curved spatial component, when there is a gravitational field. That is true for everything. When you get into the detailed maths, you find a couple of extra terms in the equations for the paths of massive bodies. That's not really surprising since massless objects must move at c and massive ones cannot, so of course their paths are different. It does have an effect on where orbits are possible - massless bodies can only have circular orbits around non-rotating black holes at 1.5 times the Schwarzschild radius, while massive bodies can have circular orbits at any radius above this (although they are only stable above 3 times the Schwarzschild radius.

Note that all the above is for test particles whose gravity is so much weaker than whatever they are orbiting that it can be ignored. The way that gravitational source terms other than mass enter the equations of general relativity is quite different from how mass does. I don't know a lot about that.

FactChecker
Gold Member
(This is to check my understanding )
I gather that gravity being a space-time distortion is thought to affect everything - mass, radiation, whatever
More precisely, it changes what we should think of as a straight path. The curvature of space-time due to gravity causes a bend in what is straight in space-time (a geodesic). So anything that moves, will move differently. It's like the surface of a globe changes the shortest path between two cities.
- and can be thought of as an accelerational effect.
It's really the opposite. What Newton thought of as acceleration due to gravity is thought of in GR as following a straight space-time line (geodesic) with no acceleration.
If it is an acceleration, even entities with mass = 0 are accelerated (?).
Anything following a geoesic may look curved in space by the old, Newtonian thinking.
So is there any effect of gravitation on mass, specific to mass itself ?
No. You should not think of it that way.
(Obviously a mass creates a gravitational field, that is not the question.)

Thanks !! excellent and very helpful replies