# What is mass?

1. Apr 30, 2012

### CarlosLara

Good afternoon. I am wondering what exactly is mass. Some say, quite inaccurately, that it is a measure of how much "stuff" an object has (what do they mean by "stuff"?). Others say that it's a quantity that means resistance to acceleration. What is mass and where does the mass of particles come from? I read that it arises from spontaneous symmetry breaking of the electroweak force through interaction with 2 Higgs fields. Could anyone explain this in a very straightforward way?

2. Apr 30, 2012

### ZxcvbnM2000

Mass is the amount of matter that is confined within a shape.

i.e for a gas m = n*M

3. Apr 30, 2012

### tiny-tim

Hi CarlosLara!

Asking "what is …" is ambiguous.

It can mean either "How come … ?" or "So what … ?"

The "so what" of mass is that it is resistance to acceleration (m = F/a).

The "how come" of mass is unknown, but maybe it comes from the Higgs field …

compare it with charge: some particles do not interact with the electromagnetic field, and so we say they have zero charge: others do interact via a "coupling constant" which we call the charge …

similarly, loosely speaking, maybe the photon does not interact with the Higgs field and so has no mass, while other particles do interact, and so passing through the Higgs field slows them down, and we call the "coupling constant" the mass.

4. Apr 30, 2012

### cosmik debris

Mass is a property of matter. "Stuff" usually means matter. Mass can be measured in different ways. One is to see how much it resists acceleration, another is to see how it accelerates other "stuff" towards it. Ultimately is boils down to comparing a mass with a known standard mass that lives in Paris.

5. May 13, 2012

### camilus

Mass is condensed energy.

6. May 13, 2012

### Fightfish

Seconded.
To add on: that curves space-time.

7. May 14, 2012

### Staff: Mentor

Are you certain of this? Consider that you will never find energy "by itself", but only as a property of another particle or a field with respect to something else.

Also, please tell me how you can condense something that's definition is "the ability to perform work".

8. May 14, 2012

### cragar

This is probably not technically right but how about the ability to resist change in
velocity.

9. May 14, 2012

### harrylin

Quite so: I would say that mass is a property of bundled or condensed energy.

Formerly mass was defined as "amount of matter", but if taken literally then that only works well in approximation. For example, if you put several atoms together in a molecule, technically you still have the same amount of matter but the molecule has slightly less mass than the separate atoms.
- http://www.phys.unsw.edu.au/einsteinlight/jw/module5_binding.htm

10. May 14, 2012

### Fightfish

I meant to agree with the notion of mass being a form of "trapped" or "localised" energy. That's how I tend to see it; though to me it is also sometimes a proportionality constant of sorts in classical mechanics.

11. May 14, 2012

### Whovian

If you're asking what it is by trying to figure out what causes it, I think that's still an open question, and one of the theories is the Higgs Boson. But that's all speculation.

12. May 14, 2012

### cragar

light could be thought of condensed energy and it has no mass.

13. May 14, 2012

### Whovian

The key word here is rest mass. Light has mass when moving at speed c, it just has no rest mass. Anyways, anything with energy must necessarily have mass.

14. May 14, 2012

### cragar

ok very good. Ya I forgot about the difference between relativistic mass and rest mass and like effective mass .

15. May 15, 2012

### Staff: Mentor

It's better to just drop "relativistic mass" and use momentum instead. Energy contributes to gravity so everything still works fine. Photons have no mass, ever!

16. May 16, 2012

### kevinsetiono

mass is not constant , it depends on gravity . W = M.g so , i wanna ask , if we are on the vacuum room , is our mass becoming zero ?

17. May 16, 2012

### kevinsetiono

mass is not constant , it depends on gravity . W = M.g so , i wanna ask , if we are on the vacuum room , is our mass becoming zero ?

18. May 16, 2012

### Whovian

First, you're talking about weight, not mass, here. Assuming you haven't done any Special Relativity, we'll assume Newtonian Mechanics here.

Mass is constant in Newtonian Mechanics. Weight, $\displaystyle W=m\cdot g$, where $g$ is acceleration due to gravity. If we're in a room with no air in it (assuming we could survive this), our weight is not zero. Gravity can still affect people through a vacuum. Example: Astronauts on the Moon.

19. May 17, 2012

### kevinsetiono

how about if we are on room with no gravity ?

20. May 17, 2012

### Staff: Mentor

If you have no gravity then you have no weight, but you most definitely still have mass. It's still harder to push a 10 kg block than a 1 kg block while weightless.