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Author Topic: what is Mass?  (Read 23230 times)

paul.fr

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what is Mass?
« on: 11/05/2007 03:48:01 »
all things have mass, but what is mass and can it be manipulated? IE, reduced or increased.

another_someone

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what is Mass?
« Reply #1 on: 11/05/2007 04:19:40 »
There are at least two answers to that.

Most simply, mass is just a measure of how difficult (how much force is required) to cause an object to change speed or direction - i.e. a measure of just how stubborn an object is going to be about continuing to do what it is doing.

More technically, mass is just another form of energy (E = mc2), and thus can be interchanged with other forms of energy (e.g. in an atomic bomb, where a small decrease in the mass of the bomb releases a huge amount of kinetic energy).

lightarrow

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what is Mass?
« Reply #2 on: 11/05/2007 13:21:44 »
all things have mass, but what is mass and can it be manipulated? IE, reduced or increased.
You need to loose weight?

another_someone

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what is Mass?
« Reply #3 on: 11/05/2007 16:34:10 »
Another technical definition of mass can be a quantum definition, in terms of the wavelength of a quantum wave:

where
h is Planck's constant, and
p is the momentum of the object.
λ is the DeBroglie Wavelength.

Where, at non-relativistic speeds, p = mV

where m = mass, and V = velocity.

Thus the above becomes:

λ = h/mV.

Thus:

m = h/λV

It gets more complex for relativistic speeds, but it still boils down to the mass being a function of the quantum wavelength of the particle.
« Last Edit: 11/05/2007 16:49:21 by another_someone »

another_someone

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what is Mass?
« Reply #4 on: 11/05/2007 17:21:34 »
Most simply, mass is just a measure of how difficult (how much force is required) to cause an object to change speed or direction - i.e. a measure of just how stubborn an object is going to be about continuing to do what it is doing.

OK, leading on from here, I suppose the logical question is to define the object (i.e. matter) and the nature of force.

I suppose, if we go back to the earliest experiments with mass, the experiments that lead us to our most basic understanding of what mass is, it is really about two issues.

The simplest is we weigh objects, which tell us about the relationship between mass and gravity.

Secondly, we observe objects collide - but the objects we see collide are (in the early days) all composed of atoms, and what we are really seeing is the electrons around one group of atoms repelling the electrons around another group of atoms, so the force we see is really an electric force.

Thus, when we look at the relationship between mass when it is weighed, and mass in a collision, we are looking at the relationship between electrical and gravitational forces.

But, even when we are weighing an object, what we are really measuring is a form of of electrostatic collision - since the object whose weight we are measuring is composed of atoms which are repelling other atoms in order to apparently resist the force of gravity.

Thus, all of our modern concepts of mass ultimately derive from a notion of the interaction of electrostatic forces, and the inertia associated with the focii of these electrostatic forces.

We do now have more complicated ideas of subatomic particles that react to strange forces like the strong and the weak force, but these are all attempts to fit more recent observations into these early ideas that were born purely from the observations of electrostatic forces.

Batroost

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what is Mass?
« Reply #5 on: 11/05/2007 19:27:09 »

What you have pointed out is that the everyday concept of a solid object is flawed. What we precieve as a solid is , as you quite rightly say, merely electrostic repulsion between its atoms and ours.

But I don't think this invalidates our concept of inertial mass. A 'solid' ball of one isotope of lead could be constructed with the same number of atoms as one of a different isotope. The balls would appear physically identical and would certainly have the same number (rougghly!) of electrons and protons - both being lead - so the electrostatic forces between it and us would be the same when we pushed them. But we would be able to percieve/measure the difference in force that would have to be applied in order to give a specific acceleration.

another_someone

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what is Mass?
« Reply #6 on: 11/05/2007 20:25:49 »

What you have pointed out is that the everyday concept of a solid object is flawed. What we precieve as a solid is , as you quite rightly say, merely electrostic repulsion between its atoms and ours.

But I don't think this invalidates our concept of inertial mass. A 'solid' ball of one isotope of lead could be constructed with the same number of atoms as one of a different isotope. The balls would appear physically identical and would certainly have the same number (rougghly!) of electrons and protons - both being lead - so the electrostatic forces between it and us would be the same when we pushed them. But we would be able to percieve/measure the difference in force that would have to be applied in order to give a specific acceleration.

Not sure that I was trying, as such, to invalidate anything - just to understand the limits of what we perceive and understand.  I was not trying to say that our model is right or wrong (although inherently all models are to some extent wrong, which is why we can so confidently predict that at some time in the future all models will be superceded by a new model).

That different atoms have different masses does not invalidate what I was saying, that our perception of solid matter is only through our perception of electrostatic forces (even the light we see is a manifestation of the electrostatic force).  That the electrostatic force of a the electrons around a lead atom may behave with different inertial characteristics, which we interpret as meaning that they are carrying different amounts of baggage (i.e. have different numbers of neutrons in the atoms) does not alter the fact that this is merely an interpretation of why the electrons are behaving as they are - we cannot actually observe the neutrons (o.k., we cannot actually observe the electrons - only the electric force they exert).

Batroost

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what is Mass?
« Reply #7 on: 11/05/2007 23:30:11 »
Ok - I see what you mean.

I was tempted to talk about the mass of non-atomic particles - or at least the observed masses calculated from experiment but then most of these experiments involve electromagnetic forces in one way or another.

Perhaps it's worth asking why electromagnetic effects have such a big effect on what we percieve? I think the answer is probably their high strength and long range compared with the other fundamental forces (at least as imagined by physicists):

Strong Force  - Short range  .. but eh the strongest!
Electromagnetic  about 1% as strong as the 'Strong' force but much longer range
Weak Force - 1/1,000,000,000 as strong and very Short range
Gravitational Force - Long range but very, very weak (10-38 as strong as the stronng force).

So in everday life it is the electromagnetic effects we notice?

another_someone

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what is Mass?
« Reply #8 on: 12/05/2007 00:11:48 »
Another interesting issue about electrostatic forces, they are the only force that comes in two polarities (i.e. the only force that is able to repel as well as attract).  If there was not this ability to repel, everything in the universe would collapse in on itself (excepting the issue of having to overcome inertia - but there would not be any force able to hold objects apart that have no inertial force pulling them apart).

another_someone

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what is Mass?
« Reply #9 on: 12/05/2007 01:42:45 »
Strong Force  - Short range  .. but eh the strongest!
Electromagnetic  about 1% as strong as the 'Strong' force but much longer range
Weak Force - 1/1,000,000,000 as strong and very Short range
Gravitational Force - Long range but very, very weak (10-38 as strong as the stronng force).

The point is that the strong force has a range of less that 10-14 metres, and the weak force has range of about 10-18 metres.  At this distance, we do not even know for sure that the gravitational force exists, and I am not sure that we know how consistently the laws of mass and inertia behave.  So it may even be (although maybe you will say there are experiments to the contrary - I don't know what does exist there) that mass only exists (in the form we understand it) at ranges of greater than the effect of the strong and weak force, and thus mass is only really pertinent to the electrostatic and gravitational forces.

jolly

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what is Mass?
« Reply #10 on: 14/05/2007 01:30:50 »
More technically, mass is just another form of energy (E = mc2),

Sorry to be annoying but the maths Of E=MC2 is designed to give you the answer for the energy content of an object or whatever- not the mass!!!! To do the equation you need- the mass!

another_someone

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what is Mass?
« Reply #11 on: 14/05/2007 03:42:38 »
More technically, mass is just another form of energy (E = mc2),

Sorry to be annoying but the maths Of E=MC2 is designed to give you the answer for the energy content of an object or whatever- not the mass!!!! To do the equation you need- the mass!

The point is that the equation provides an equivalence between mass and energy, such that you can create energy by reducing mass (as is done in an atomic bomb) and by inference can create mass from energy - thus the two are interchangeable.

jolly

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what is Mass?
« Reply #12 on: 14/05/2007 14:28:05 »
More technically, mass is just another form of energy (E = mc2),

Sorry to be annoying but the maths Of E=MC2 is designed to give you the answer for the energy content of an object or whatever- not the mass!!!! To do the equation you need- the mass!

The point is that the equation provides an equivalence between mass and energy, such that you can create energy by reducing mass (as is done in an atomic bomb) and by inference can create mass from energy - thus the two are interchangeable.
More technically, mass is just another form of energy (E = mc2),

Well this is just not true mass is another form of energy!

The equation is E=MC2! right so under that equation mass and energy are considered distint and seperate! to work out the Mass you would have to do the equation

M=E/DIVIDED BY C2

The best bit is really... That you still dont know how to work out the mass or the energy of an object! and until you do you wont be able to check the maths in this equation...

but when you do you will realise it is utterly incorrect!!!
« Last Edit: 14/05/2007 14:30:59 by jolly »

Bored chemist

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what is Mass?
« Reply #13 on: 14/05/2007 15:04:40 »
Jolly,
If a=b then b=a
you don't need to specify which direction you are going in. If 2 things are the same it doesn't matter if you say this is the same as that or that is the same as this.
Rearanging the equation in terms of mass doesn't change it; it's still Einstein's relation between mass and energy.

And what, exactly do you mean by the last bit
"The best bit is really... That you still dont know how to work out the mass or the energy of an object! and until you do you wont be able to check the maths in this equation...

but when you do you will realise it is utterly incorrect!!!
"?

another_someone

• Guest
what is Mass?
« Reply #14 on: 14/05/2007 16:06:48 »
Well this is just not true mass is another form of energy!

The equation is E=MC2! right so under that equation mass and energy are considered distint and seperate! to work out the Mass you would have to do the equation

M=E/DIVIDED BY C2

Yes, it is correct that m = E/c2.

We have particle accelerators that can impart massive energy to a particle, and observe the increase in mass associated with that – so we know it works.

The best bit is really... That you still dont know how to work out the mass or the energy of an object! and until you do you wont be able to check the maths in this equation...

but when you do you will realise it is utterly incorrect!!!

Mass is merely a quantity that links energy, momentum, and force.

There are many ways of measuring force, energy, and momentum; and so calculate the mass.

I cannot believe you seriously think that we cannot work out how much energy it takes to lift a 1lb weight 1 foot in the air, or how much force your bathroom scales might have upon it when you stand upon it.

lightarrow

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what is Mass?
« Reply #15 on: 15/05/2007 14:00:22 »
m = h/λV
It gets more complex for relativistic speeds, but it still boils down to the mass being a function of the quantum wavelength of the particle.
For relativistic speeds:
m = Sqrt[1 - (v/c)2]*h/λV
So, if you make the limit for V --> C, m --> 0.

jolly

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what is Mass?
« Reply #16 on: 22/05/2007 11:33:11 »
Well magnetism and electricity are clearly linked! but I do not agree that mass and energy are! As a mass can hold varying amounts of energy, which flux; so under those circumstances how can you relate them!

Mass I do not believe realitively afects the amount of energy an object can hold!

The problem really is we still dont know what a mass is; or rather we need better understanding of what is classified/constiutues a mass!

another_someone

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what is Mass?
« Reply #17 on: 22/05/2007 16:17:57 »
Well magnetism and electricity are clearly linked! but I do not agree that mass and energy are! As a mass can hold varying amounts of energy, which flux; so under those circumstances how can you relate them!

Mass I do not believe realitively afects the amount of energy an object can hold!

We are not actually saying that the mass in any way limits the amount of energy matter can hold, but rather the contrary, that the amount of energy a particle contains is manifest in the apparent mass of the particle, and that even at rest, that mass contains some energy.  Neither the mass nor the energy are limited (just as neither electricity nor magnetism limit each other, but they are nonetheless directly linked to each other).

The problem really is we still dont know what a mass is; or rather we need better understanding of what is classified/constiutues a mass!

I don't doubt this, but all that we can talk about is the current state of knowledge based upon experimental results and theories that try to explain those results.  There is no doubt that at some time in the future, new experiments will necessitate a reinterpretation of our understanding of mass, but the problem is that without having performed those experiments (and we don't even know what the experiment is likely to be until we try it, and suddenly find it returns a result not explained by current theory), then we have no basis upon which to make that reinterpretation.  Until that time, we can only work with what we know, not with what we do not know.

syhprum

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what is Mass?
« Reply #18 on: 24/05/2007 09:10:55 »
My priest says its where we celebrate the Eucharist

paul.fr

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what is Mass?
« Reply #19 on: 24/05/2007 09:42:18 »
My priest says its where we celebrate the Eucharist

Mass is a place!

BenV

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what is Mass?
« Reply #20 on: 24/05/2007 10:35:50 »
Well magnetism and electricity are clearly linked! but I do not agree that mass and energy are! As a mass can hold varying amounts of energy, which flux; so under those circumstances how can you relate them!

Mass I do not believe realitively afects the amount of energy an object can hold!

We are not actually saying that the mass in any way limits the amount of energy matter can hold, but rather the contrary, that the amount of energy a particle contains is manifest in the apparent mass of the particle, and that even at rest, that mass contains some energy.  Neither the mass nor the energy are limited (just as neither electricity nor magnetism limit each other, but they are nonetheless directly linked to each other).

Mass and energy are linked by the equation of mass-energy equivalence given in Einstein's theory of special relativity, e=mc2.

Mass is a place!

Wouldn't that be Mass. ?
« Last Edit: 24/05/2007 11:09:30 by BenV »

lightarrow

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what is Mass?
« Reply #21 on: 24/05/2007 21:18:45 »
An interesting feature of mass is this:
one photon has exactly zero mass.
Two photons, no!
(It's not a joke!).

Edit: two photons have mass if they don't move exactly in the same direction.
« Last Edit: 10/06/2007 12:26:57 by lightarrow »

another_someone

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what is Mass?
« Reply #22 on: 25/05/2007 01:05:05 »
An interesting feature of mass is this:
one photon has exactly zero mass.
Two photons, no!
(It's not a joke!).

First I have ever heard about this.  Would you like to say (preferably in words of one, maybe two at most, syllables) how this comes about, and what it means?

Do you mean that two photons that are somehow bound, and it is the binding energy that has mass; or do you mean that even two independent photons are measured to have mass?

lightarrow

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what is Mass?
« Reply #23 on: 25/05/2007 14:20:40 »
An interesting feature of mass is this:
one photon has exactly zero mass.
Two photons, no!
(It's not a joke!).
First I have ever heard about this.  Would you like to say (preferably in words of one, maybe two at most, syllables) how this comes about, and what it means?

1. Simple explanation.
If you know that a system has energy but no momentum, using simple relativistic considerations, you should easily conclude that it has rest mass, isnt'it?
Ok. Now, consider the system of two photons of exactly the same energy, travelling in opposite directions. This system has clearly non-zero energy but zero momentum (total p = p1 - p1 = 0). So, the system mast have a rest mass!

2. Just a bit less simple explanation.
The exact formula relating rest mass m, energy E and momentum p of a relativistic system of any kind, is:
(E/c)2 - |p|2 = (mc)2
So, if p = 0, then (E/c) = mc --> m = E/c2 ≠ 0 because E ≠ 0 (in this case, E = 2hν).

Quote
Do you mean that two photons that are somehow bound, and it is the binding energy that has mass; or do you mean that even two independent photons are measured to have mass?

Very good question! Don't know. I would say even two indipendent photons, but I'm not sure.
The fact is that if you observe the system of two photons I've described up, from a moving ref. frame, the two photons don't move in the described way, anylonger: you will see the two photons travelling away from each other forming an angle < 180°; but the rest mass of the system is invariant, so it's exactly the same in this new frame, so, I assume you can pick up any pair of travelling photons in the universe, with the prescription they are not travelling exactly in the same direction and sense (that is, their trajectories form a non-zero angle), and you'll have a non-zero rest mass system.

What all this could exactly mean, is a mystery to me.
« Last Edit: 25/05/2007 14:23:52 by lightarrow »

another_someone

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what is Mass?
« Reply #24 on: 25/05/2007 16:29:34 »
1. Simple explanation.
If you know that a system has energy but no momentum, using simple relativistic considerations, you should easily conclude that it has rest mass, isnt'it?
Ok. Now, consider the system of two photons of exactly the same energy, travelling in opposite directions. This system has clearly non-zero energy but zero momentum (total p = p1 - p1 = 0). So, the system mast have a rest mass!

2. Just a bit less simple explanation.
The exact formula relating rest mass m, energy E and momentum p of a relativistic system of any kind, is:
(E/c)2 - |p|2 = (mc)2
So, if p = 0, then (E/c) = mc --> m = E/c2 ≠ 0 because E ≠ 0 (in this case, E = 2hν).

It sounds to me simply to be a consequence of the notion that light can have energy, and momentum, and thus mass, while in motion, while having zero rest mass in the first place.  More specifically, it is down to the inevitable paradoxes one gets when one deals with infinities (the existence of light implies an assumption of zero rest mass multiplied by an infinite Lorenz factor because it is travelling at the speed of light - so don't be too surprised if you get quirky results).

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what is Mass?
« Reply #24 on: 25/05/2007 16:29:34 »