Is Relativistic Mass Still Relevant in Modern Physics Discussions?

[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?

 

[Aer]

the force needed to accelerate it a given amount in its rest frame will be proportional to its total rest energy?

"total rest energy" is ambiguous.

 

[pervect]

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?

Definitely. This is true in Newtonian mechanics as well.

It takes much more energy to go from 10 m/s to 11 m/s than it does to go from 0 m/s to 1 m/s.

 

[Aer]

Definitely. This is true in Newtonian mechanics as well.

It takes much more energy to go from 10 m/s to 11 m/s than it does to go from 0 m/s to 1 m/s.

So when the acceleration is constant in the frame of our object, you are saying the energy required to accelerate it constantly increases in the frame of our object?

 

[Aer]

I wish I had the thread on hand, but the "relativistic mass" for a force from one frame to another was γ³m was it not? I think it was you who came up with this, though it may have been someone else - I forget. Anyway, in this thread, it was established that "relativisitic mass" was simply γm, where m is rest mass, was it not?

 

[jtbell]

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?

To elaborate: in classical physics, we can associate to any object a number m that has the following useful properties:

(a) we can use it to calculate the object's acceleration in response to a given force, \vec a = \vec F / m, regardless of how the object is moving to begin with, and regardless of the direction of the force. ("inertial mass")

(b) we can use it to predict the gravitational force that the object exerts on another object; also how the object responds to the gravitational influence of another object. ("gravitational mass")

(c) for any particular object, m is constant, and an intrinsic property of the object, so long as we're not adding pieces to the object or chipping pieces away from it. ("invariant mass")

(d) if we combine two objects together to form a single object or system, we can simply add m_1 + m_2 = m to get a number that plays the same role for the composite object.

In relativistic physics, no single number (or even a single formula that caculates a number as a function of speed) fills all of these roles. In particular, since nobody has mentioned it yet, I'd like to point out that (a) is especially problematical. Not only does an object's acceleration in reponse to a given force depend on how fast the object is moving to begin with, it also depends on the direction of the force relative to the object's direction of motion! The familiar formula for "relativistic mass" works only if the force is perpendicular to the direction of motion. If the force is parallel to the direction of motion, we have to use a different "relativistic mass". Some books call these "transverse mass" and "longitudinal mass". (And then of course, we have a "45-degree mass" and a "72-degree mass", etc. )

So, if you want to talk about the "mass" of an object in relativity, you have to specify, or at least have it already be understood from context, which of these properties you really want to deal with. You can't have them all.