Mass & Density: Unpacking Feynman's Idea

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Discussion Overview

The discussion revolves around the concept of mass and its relationship to energy, particularly in the context of Feynman's ideas. Participants explore the definitions and implications of mass, its invariance across reference frames, and the significance of understanding what mass "is" versus what it "does" in mechanics.

Discussion Character

  • Exploratory, Debate/contested, Conceptual clarification

Main Points Raised

  • Some participants question whether mass can be understood merely as a number, referencing Feynman's assertion that energy is not fully understood.
  • One participant proposes that mass is defined as energy measured in an inertial frame where the system has no momentum, suggesting a reduction of mass to energy in this definition.
  • Another participant emphasizes that mass is an invariant quantity, remaining constant across all reference frames, while energy is frame-dependent, leading to different interpretations of kinetic and gravitational potential energy.
  • Concerns are raised about the philosophical implications of understanding mass, with one participant expressing frustration over the recurring question of what mass "really is."
  • A later reply humorously suggests that while we may know what mass is, the deeper inquiry into its true nature remains open-ended.

Areas of Agreement / Disagreement

Participants express differing views on the nature of mass and whether its definition is satisfactory. Some agree on the invariance of mass, while others challenge the depth of understanding regarding what mass fundamentally represents.

Contextual Notes

Discussions involve assumptions about reference frames and the definitions of mass and energy, which may not be universally accepted. The philosophical nature of the questions posed adds complexity to the discussion.

Who May Find This Useful

This discussion may be of interest to those exploring foundational concepts in physics, particularly in relation to mass, energy, and their interpretations in different reference frames.

Anko
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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?
 
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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.
 
Anko said:
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.
 
Anko said:
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.
 
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But Dale, maybe we can know what mass is, but do what we know what it really really is?

<ducking and running for cover>
 
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