Exploring Relativistic Mass with pmb_phy: A Conversation on Its Relevance

[Aer]

Where are you going with this? Of course the energy of a photon contributes to the total energy and thus the inertial mass--if you have a box filled with radiation it will have more inertia than an empty box, that's what's predicted by the theory anyway.

So you are claiming a photon has inertia and thus mass?

 

[Aer]

Which paper are you referring to, and what specific quotes are you talking about? I think you've likely just misunderstood something here.

From pmb_phy's paper:

There is currently an unfortunate trend to ban the concept of
relativistic mass from physics. Why such a trend is occurring is difficult to
say for sure but is probably related to the various usages in certain
branches of relativity. Generally speaking, the concept of proper mass
finds more usage with the particle physics community while the concept
of relativistic mass finds more usage within general relativity and
cosmology.

And:

This is surely due, in part, to a debate regarding the
concept of mass in relativity that has lasted for several decades. 2-12 This debate
concerns the use of relativistic mass versus proper mass as being “the” mass in
relativity.

 

[JesseM]

NO! In quantum physics, there is no distinction between mass and energy.

That certainly isn't true, quantum physicists talk about the rest masses of particles all the time, and they don't talk about the rest mass of binding energy (which again, is just a type of potential energy) or of kinetic energy. It is true that in quantum field theory it is easy for kinetic/potential energy to be converted into mass or vice versa in reactions that create or destroy particles, but if we're talking about chemical reactions or the binding of a proton and a neutron into a deuteron, there is no creation or destruction of particles involved.

 

[Aer]

That certainly isn't true

So now you claim that the mass of a system is not the total energy divided by c^2? Make up your mind!

 

[JesseM]

From pmb_phy's paper:

There is currently an unfortunate trend to ban the concept of
relativistic mass from physics. Why such a trend is occurring is difficult to
say for sure but is probably related to the various usages in certain
branches of relativity. Generally speaking, the concept of proper mass
finds more usage with the particle physics community while the concept
of relativistic mass finds more usage within general relativity and
cosmology.

And:

This is surely due, in part, to a debate regarding the
concept of mass in relativity that has lasted for several decades. 2-12 This debate
concerns the use of relativistic mass versus proper mass as being “the” mass in
relativity.

As I thought, this is just your misunderstanding. The debate over whether to use the concept of relativistic mass is purely an aesthetic one, it's not like people who use relativistic mass will make any different physical predictions than people who don't, any statement involving relativistic mass can be translated into an equivalent one involving only concepts like rest mass, momentum and energy. Since everyone agrees on what relativity actually predicts physically, everyone agrees on the prediction about the resistance to acceleration of a compound object (ie the object's inertia)--no physicist would dispute the fact that relativity predicts the inertia of a compound object is proportional to its total energy.

 

[JesseM]

So now you claim that the mass of a system is not the total energy divided by c^2? Make up your mind!

No, I dispute the claim that "there is no distinction between mass and energy"--by "mass" I meant rest mass, as I made clear in my post. If you mean there is no distinction between the inertial mass of a compound object and its total rest energy, then I agree with that, but I'd say that all mainstream theories predict this is just as true of the macro-world as the micro-world.

 

[learningphysics]

A system can have multiple energies. As you've said, it can have kinetic energy and thermal energy to name a few. One of the energies a system has is mass. That is, mass is a form of energy. So when we add all the energies together of this puddy, we get total energy = mass energy + kinetic energy + thermal energy. In this case, the kinetic energy has been converted to thermal energy - notice that the mass energy is still there. Now answer my question: Does a photon have energy?

In the paper I referred you to... Einstein is defining rest energy of the body as total energy of the body in the center of mass frame. There are only two energies... the translation kinetic energy and the rest energy (this includes thermal energy...nuclear binding energy and anything that is not translation kinetic energy)

His paper shows that a change in rest energy is proportional to a change in rest mass.

Yes a photon has energy.

 

[learningphysics]

Thanks. Please note that I'm not ignoring all this because I'm lazy. I've had horrible back pain when I sit for more than a few minutes. It took a long time to figure out what it was. Turns out that I have a stone in my gall-bladder. It will be comming out when I have surgery in the near future.

I'm really sorry to hear about this Pete. Hope everything turns out well. Take care of yourself. Best wishes!

 

[Aer]

As I thought, this is just your misunderstanding. The debate over whether to use the concept of relativistic mass is purely an aesthetic one, it's not like people who use relativistic mass will make any different physical predictions than people who don't,

Just like I thought, you'd come up with another BS answer.

You can't even keep your arguments consistent! Pick a theory and stick with it. Either all energy contributes to an objects mass or it does not (and I am referring to the macroscropic world here). If you claim that quantum physics is the same regarding mass and energy as is on the macroscopic world, then the mass of an object in quanutm physics would be the total energy / c^2. I don't dispute the latter, it is the former that I dispute. That is - on the macroscopic level, other forms of energy exist other than mass energy.

 

[JesseM]

So you are claiming a photon has inertia and thus mass?

Physicists generally define "inertial mass" in terms of resistance to acceleration in the object's own rest frame, and you can't do this for a photon, although you can do it for a compound system which contains a photon. If you want to define the inertial mass of an object in a frame other than its rest frame, this is the same thing as using relativistic mass, and as you've pointed out many times, most physicists prefer to avoid using this concept.

 

[Aer]

Yes a photon has energy.

Does it have mass?

 

[Aer]

Physicists generally define "inertial mass" in terms of resistance to acceleration in the object's own rest frame, and you can't do this for a photon, although you can do it for a compound system which contains a photon.

OK - find all the inertial masses of the object's by themselves in their own rest frames.

 

[JesseM]

Just like I thought, you'd come up with another BS answer.

So do you disagree with me that the debate over "relativistic mass" is an aesthetic one rather than a matter of different predictions? Do you think someone using relativistic mass will make a different physical prediction than someone who doesn't?

You can't even keep your arguments consistent! Pick a theory and stick with it. Either all energy contributes to an objects mass or it does not (and I am referring to the macroscropic world here).

Did you read my post #146 from 12:39 PM? It depends on whether you use "mass" to mean rest mass or inertial mass, my arguments are consistent once you understand which one I'm talking about in which cases.

If you claim that quantum physics is the same regarding mass and energy as is on the macroscopic world, then the mass of an object in quanutm physics would be the total energy / c^2. I don't dispute the latter, it is the former that I dispute. That is - on the macroscopic level, other forms of energy exist other than mass energy.

Do you dispute the fact that our current theories of physics make a definite prediction about this, and that they say that the resistance to acceleration of a compound object is proportional to its total rest energy?

 

[Aer]

So do you disagree with me that the debate over "relativistic mass" is an aesthetic one rather than a matter of different predictions? Do you think someone using relativistic mass will make a different physical prediction than someone who doesn't?

What prediction would you like to make?

 

[JesseM]

OK - find all the inertial masses of the object's by themselves in their own rest frames.

You can't do this for a photon, but you can do this for any object moving slower than light. What's your point? The inertial mass of a compound object will not be the sum of the inertial masses of all the objects that make it up, according to relativity (assuming, again that you use the words 'inertial mass' to refer only to resistance to acceleration in the object's rest frame--if you allow the words 'inertial mass' to refer to resistance to acceleration in other frames, then the inertial mass of a compound object in its own rest frame is the sum of the inertial masses of all its parts in that frame, assuming there is no potential energy between the parts).

 

[learningphysics]

Does it have mass?

It has zero rest mass. It has relativistic and inertial mass = $$h\nu/c^2$$

 

[JesseM]

So do you disagree with me that the debate over "relativistic mass" is an aesthetic one rather than a matter of different predictions? Do you think someone using relativistic mass will make a different physical prediction than someone who doesn't?

What prediction would you like to make?

Personally I would expect the theory of relativity is correct in its prediction that the resistance to acceleration of a compound object is proportional to its total energy. Again, do you dispute that this is what the theory of relativity would predict? Please answer this question yes or no.

 

[Aer]

It has zero rest mass. It has relativistic and inertial mass = $$h\nu/c^2$$

Does gravity act on this inertial mass?

 

[learningphysics]

Does gravity act on this inertial mass?

Yes. Gravity bends light.

 

[Aer]

Personally I would expect the theory of relativity is correct in its prediction that the resistance to acceleration of a compound object is proportional to its total energy.

Let's talk about a single object first. What do you expect relativity to predict about a single object?

 

[Aer]

Yes. Gravity bends light.

The curvature of space bends light.

 

[JesseM]

Let's talk about a single object first. What do you expect relativity to predict about a single object?

A single particle? It would predict that its resistance to acceleration in its own rest frame (assuming it's a sublight particle) is proportional to its rest mass. Now will you answer my question about what relativity predicts for a compound object?

 

[Aer]

A single particle? It would predict that its resistance to acceleration in its own rest frame (assuming it's a sublight particle) is proportional to its rest mass.

If an object is moving relative to me at .9c, what would I predict it's mass to be?

Now will you answer my question about what relativity predicts for a compound object?

One step at a time. Look above.

 

[JesseM]

If an object is moving relative to me at .9c, what would I predict it's mass to be?

What do you mean by "mass"--rest mass? Relativistic mass? Inertial mass in the object's own rest frame? Inertial mass in your own rest frame? (physicists who prefer to avoid using 'relativistic mass' will want to avoid using this last concept of inertial mass too)

 

[Aer]

What do you mean by "mass"--rest mass? Relativistic mass? Inertial mass in the object's own rest frame? Inertial mass in your own rest frame? (physicists who prefer to avoid using 'relativistic mass' will want to avoid using this last concept of inertial mass too)

I just want what SR predicts. A force should be able to move this object - so what is the mass?

 

[JesseM]

I just want what SR predicts.

You can't make a definite prediction unless you define your terms. Different physicists may use the term "mass" differently but they're still making use of the same theory of relativity--the choice of terminology is a matter of tradition and aesthetics, it's not a physical question. Hell, we could interchange the meaning of "mass" and "length" if we wanted, theories of physics don't demand that you use language in a particular way, although if different physicists use different terms they must know how to map one set of terms to another to make sure they are not disagreeing about any physical predictions.

A force should be able to move this object - so what is the mass?

The amoung of force needed to accelerate the object by a small amount will depend on what frame you're in.

 

[Aer]

The amoung of force needed to accelerate the object by a small amount will depend on what frame you're in.

Good, but there is only one force acting on the body in reality and the only proper frame to measure this in is the rest frame of the object which is subjected to the force, correct?

 

[Aer]

This discussion is going nowhere - let's assume that relativity does say that mass is dependent on the total energy content of a system. Then we have the problem of showing experimental proof to say that this is true.

Let's just assume that it is true as you state it. Now can we find any experiments?

 

[Aer]

I think pervect summed it up with his post pointing to: http://arxiv.org/PS_cache/gr-qc/pdf/9909/9909014.pdf which states there is no experimental evidence to back up the assertion.

 

[learningphysics]

I think pervect summed it up with his post pointing to: http://arxiv.org/PS_cache/gr-qc/pdf/9909/9909014.pdf which states there is no experimental evidence to back up the assertion.

No evidence for what? In his last sentence before the Acknowledgments he writes:

"We can thus tell our students with confidence that kinetic energy has weight, not just as a theoretical expectation, but as an experimental fact."

Do you agree or disagree with this?

 

[Aer]

No evidence for what? In his last sentence before the Acknowledgments he writes:

"We can thus tell our students with confidence that kinetic energy has weight, not just as a theoretical expectation, but as an experimental fact."

Do you agree or disagree with this?

Does that not imply that the kinematic energy must be measured relative to the rest frame of the gravitational potential? Otherwise, what is the meaning of kinetic energy? The object must have motion relative to something to have kinetic energy.

 

[εllipse]

I think pervect summed it up with his post pointing to: http://arxiv.org/PS_cache/gr-qc/pdf/9909/9909014.pdf which states there is no experimental evidence to back up the assertion.

It also states that general relativity predicts kinetic energy is a part of gravitational mass. This is a relativity forum, and as such I think we should conclude that GR's predictions have the final say in this if such predictions have not been tested against experiment.

 

[Aer]

Perhaps the problem is we are trying to equate weight from gravity with mass.

From: http://www.conceivia.com/topics/not_quantum_physics.htm

At first, the assumptions made a certain amount of sense and were even believable. It wasn't until the assumption that kinetic energy has mass, that everything got all out of wack. I'm not saying that this assumption was incorrect, in fact I believe it was a valid assumption.

The mistake was incorporating this assumption into the formula for acceleration and decelleration. The mass gained from kinetic energy does not add to the kinetic energy. It adds to the gravitation of the moving object, but not to it's kinetic energy. The reason for this is that the mass of the object is relative to itself.

 

[JesseM]

Good, but there is only one force acting on the body in reality and the only proper frame to measure this in is the rest frame of the object which is subjected to the force, correct?

"Proper frame" by what criterion? You're free to analyze any situation in any frame you like according to relativity. But if you are asking about the force needed in the object's own rest frame, this will indeed be proportional to its rest mass for a single particle. Now can you answer my question about whether you agree or disagree that the theory of relativity predicts that for a compound object, the force needed to accelerate it a given amount in its rest frame will be proportional to its total rest energy?

 

[Aer]

"Proper frame" by what criterion? You're free to analyze any situation in any frame you like according to relativity. But if you are asking about the force needed in the object's own rest frame, this will indeed be proportional to its rest mass for a single particle.

Now wait - a certain amount of energy is known to be used to accelerate the object. Since the force changes depending on the frame, does that mean the energy changes?