# Thread: Is mass a thing? Or is it just a function of how much spacetime something warps?

1. ## Is mass a thing? Or is it just a function of how much spacetime something warps?

The more I think about it, the less it makes sense.

2. I am just an engineer, but I think the better question is about inertia. Is inertia a thing? This is at the heart of equivalence and is separated out by spin. Inertia is at the root of momentum and especially angular momentum. If mass were just for gravity and distorting spacetime, that’s easy to grasp. But angular momentum “ought” to involve another fundamental property. But it doesn’t. Inertial mass is the same as gravity mass in its behaviour. That is quite strange. IMO.

3. Originally Posted by parallaxicality
The more I think about it, the less it makes sense.
Yeah, an important question would be, are those really different or just two ways of saying the same thing?

4. Originally Posted by parallaxicality
Is mass a thing? Or is it just a function of how much spacetime something warps?

The more I think about it, the less it makes sense.
Is there a difference, or is it a semantic quirk? Mass is a property of substance, so it's both a "thing" and a function, in my layman's view. But I think the question is being debated at high levels even by the world's top physicists, so you're in good company, at least.

5. Is pregnancy a thing? Or is it just a function of a person growing a baby inside their uterus?

Grant Hutchison

6. I suppose what I'm asking is, if you could tug spacetime instead of pushing it down, could you create gravity without mass?

7. Originally Posted by parallaxicality
I suppose what I'm asking is, if you could tug spacetime instead of pushing it down, could you create gravity without mass?
How would that work? What could "tug" spacetime?

8. Originally Posted by parallaxicality
I suppose what I'm asking is, if you could tug spacetime instead of pushing it down, could you create gravity without mass?
Well, if you look at the Rindler metric for a continuously accelerating object, the shape of the spacetime "looks like" gravity, including an event horizon behind the observer. So the apparent gravity experienced aboard a continuously accelerating spaceship is associated with a spacetime that's compatible with being in an unusual gravity field.
But that's all just mathematics, in the same way the "shape of spacetime" around a mass is all just mathematics. The coordinates simply tell you what observers will experience, in terms of space and time, and different local observers will see different spacetimes if they extend their local coordinates--the Schwarzschild "gravity well" is not what is experienced by an infalling observer, for instance.
So I think you have to avoid thinking of spacetime as some sort of elastic entity in its own right, and think of it as a mathematical way of describing the consequences of mass and motion. You can't "tug on" mathematics.

Grant Hutchison

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Originally Posted by parallaxicality
I suppose what I'm asking is, if you could tug spacetime instead of pushing it down, could you create gravity without mass?
Gravity is tied to the stress energy momentum tensor. Mass is only a part of what is responsible for gravitational effects.

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