Mass & Density: Unpacking Feynman's Idea

[Anko]

TL;DR

Do we know what mass is or just what it does in mechanics

What do you think of the idea that mass is a number?

This apparently derives from something Feynman said about energy. Apart from saying "nobody knows what energy is", he does go on to explain in the same lecture, what he knows about work energy. Is it more important to know how something is related to something else when both have physical units, or more important to know what the units "mean"?

Since having equations is also meaningful. Since in general equations imply a measurement?

 

[anuttarasammyak]

Mass of system is energy measured in the inertial frame of reference where the system has no momentum. I think it is the definition. Mass is reduced to energy in the definition.

 

[PeroK]

Summary:: Do we know what mass is or just what it does in mechanics

What do you think of the idea that mass is a number?

Mass in an invariant quantity. This means that the mass of an object is the same in all reference frames.

The Energy of a system, however, depends on the reference frame. E.g. kinetic energy depends on the speed of the object, which varies according to which reference frame you are using. And, GPE (gravitational potential energy) depends on an arbitrary reference point (e.g. zero at "infinity").

Only changes in energy are meaninful. E.g. if you move within a gravitational field, then the change in your GPE is the same, regardless of the reference point.

Note that in a closed system energy is conserved, which means it doesn't change over time.

 

[Dale]

Summary:: Do we know what mass is or just what it does in mechanics

This kind of question annoys me. Of course we know what mass is. Then we get the inevitable follow up questions like “But do we know what mass really is?” Or “Do we know what mass IS?” As though that changes the question.

 

[Vanadium 50]

But Dale, maybe we can know what mass is, but do what we know what it really really is?

<ducking and running for cover>