Incredibly huge inconsistency of modern physics

[fuelair]

What is "orthodox" physics?

Higher than conservative physics, more higher even than reform physics.

 

[a_unique_person]

Somehow I get the impression that wogaga would be happier to stick with the idea that everybody else is stupid.

 

[zosima]

I'll admit my original statement was poorly phrased and I should have said 'rest mass', I apologize if my statement was confusing. Although, I don't think it was particularly problematic if taken in the context of the discussion.

 

[Beausoleil]

If you replace mass with rest mass, where is the inconsistency? A photon has mass, energy and momentum, but not rest mass. What's the problem?

 

[Reality Check]

If you replace mass with rest mass, where is the inconsistency? A photon has mass, energy and momentum, but not rest mass. What's the problem?

A photon has no rest mass or mass. It has energy and momentum. These can be considered to as "relativistic mass".

 

[a_unique_person]

A photon has no rest mass or mass. It has energy and momentum. These can be considered to as "relativistic mass".

I love this stuff, even if it is hard to wrap my head around it.

Isn't momentum mass * velocity? (OK, I'm going all the way back to my high school physics). I get the feeling from what has been said in other posts, my high school definition is lacking, but the definition in high school was p=mv.

 

[Reality Check]

I love this stuff, even if it is hard to wrap my head around it.

Isn't momentum mass * velocity? (OK, I'm going all the way back to my high school physics). I get the feeling from what has been said in other posts, my high school definition is lacking, but the definition in high school was p=mv.

You may want to read the link which explains all of this. p=mv is only for particles with mass and as the article states:

In empty space, the photon moves at c (the speed of light) and its energy E and momentum p are related by E = cp, where p is the magnitude of the momentum.

 

[Dancing David]

I think that it bother some people that a photon can have mass because it moves at the speed of light but no rest mass.

They want the universe to agree with their preconceived notions.

 

[a_unique_person]

I think that it bother some people that a photon can have mass because it moves at the speed of light but no rest mass.

They want the universe to agree with their preconceived notions.

Nothing bothers me, except my inability to understand it all.

 

[Fredrik]

Isn't momentum mass * velocity? (OK, I'm going all the way back to my high school physics). I get the feeling from what has been said in other posts, my high school definition is lacking, but the definition in high school was p=mv.

That's the pre-relativistic definition of the momentum of a massive particle. Special relativity changes that definition to

[latex]\footnotesize $\vec{p}=\gamma m \vec v$[/latex]​

where

[latex]\footnotesize \gamma=\frac{1}{\sqrt{1-\frac{v^2}{c^2}}}[/latex]​

Note that we recover the pre-relativistic definition in the limit [latex]\footnotesize c\rightarrow\infty[/latex].

The m above is the particle's mass. Some people prefer to call it the rest mass. They would call the quantity [latex]\footnotesize\gamma m[/latex] the mass (or relativistic mass). They would also write m₀ instead of m, and they would write m instead of [latex]\footnotesize\gamma m[/latex].

 

[The Man]

Those of us who prefer to let the word "mass" refer to the rest mass of a particle would prefer to say it's the other way round. I'm not saying that you're wrong. It's a matter of taste.

Oh, I agree completely and that was primarily my point, the matter of taste. wogoga taking the mass energy equivalence on one hand and then only considering invariant mass on the other. Kind of like playing classical music and rap at the same time and then wondering why it sounds so bad. Mass is simply a value having the units of (or equivalent) Newton Second² Meter^-1. As far as tastes go my taste varies depending on what I am tying to find out. However, although I agree that relativistic mass can seem irrelevant, as long as the math works I dislike discarding anything as irrelevant since you could miss an important relationship, but that is just my taste. It is in fact that aspect which can also show us a photon has a frame dependent mass. With wavelength (and thus momentum) dependent on relative motion between the receiver and the light source and with the speed of light frame invariant then with the p = m * V relationship (which may be technically incorrect for a photon but the math still works) we know that the mass (or that Newton Second² Meter^-1 something) of the photon must also be frame dependent.

 

[a_unique_person]

That's the pre-relativistic definition of the momentum of a massive particle. Special relativity changes that definition to

where

Note that we recover the pre-relativistic definition in the limit

.

The m above is the particle's mass. Some people prefer to call it the rest mass. They would call the quantity

the mass (or relativistic mass). They would also write m₀ instead of m, and they would write m instead of

.

So what happens when v=c?

 

[Reality Check]

So what happens when v=c?

It never does for a particle with mass.

 

[a_unique_person]

It never does for a particle with mass.

I was thinking of the photon, but I didn't realise that the equation was only for particles with a resting mass.

 

[Dave Rogers]

So what happens when v=c?

p becomes infinite. Since this means that an infinite impulse would be required to impart sufficient momentum for a particle to travel at c, this shows that a particle of non-zero rest mass cannot travel at the speed of light.

Just expanding a little on Reality Check's answer.

Dave

 

[wogoga]

I wrote:

Equivalence between mass and energy on the one hand, but energy without mass on the other hand.​

Dancing David brought forward this analogy:

Equivalence between fur and dogs on the one hand, but dogs without fur on the other hand.​

If we take seriously the equivalence between fur and dogs we conclude:

• a fur without corresponding dog or a dog without corresponding fur is impossible
• a fur corresponds nevertheless to a dog without a fur
• fur and fur are two different concepts

I'll admit my original statement was poorly phrased and I should have said 'rest mass', ...

Zosima's original statement:

Mass-Energy equivalence means that even though a photon is massless it has energy​

If we replace mass with restmass we get:

Restmass-Energy equivalence means that even though a photon has no restmass it has energy​

This version is obviously wrong, because there is no equivalence between restmass and energy. Equivalence only reigns between (relativistic) mass and energy. The alternative

Equivalence between (relativistic) mass and energy means that even though a photon has no restmass it has energy​

doesn't make a lot of sense, because of the confusion between two different mass-concepts (mass and restmass).


Cheers, Wolfgang

Comments on FAQ - Does light have mass? (post 1, post 2, post 3)

 

[Reality Check]

I wrote:

Equivalence between mass and energy on the one hand, but energy without mass on the other hand.
​

..snip..
Zosima's original statement:

Mass-Energy equivalence means that even though a photon is massless it has energy
​

If we replace mass with restmass we get:

Restmass-Energy equivalence means that even though a photon has no restmass it has energy
​

This version is obviously wrong, because there is no equivalence between restmass and energy. Equivalence only reigns between (relativistic) mass and energy. The alternative

Equivalence between (relativistic) mass and energy means that even though a photon has no restmass it has energy
​

doesn't make a lot of sense, because of the confusion between two different mass-concepts (mass and restmass).


Cheers, Wolfgang

Comments on FAQ - Does light have mass? (post 1, post 2, post 3)

There is no such thing as "restmass". A particle with mass has a "rest mass". A particle with mass that is moving has a relativistic mass that is its rest mass multiplied by the Lorentz factor.

The original posting by zosima was "Mass-Energy equivalence means that even though a photon is massless it has energy" and that is inexact since mass-energy equivalence is usually taken to be E=mc² and that is the rest energy of a particle. A photon is never at rest.

There is no confusion between mass and rest mass, just in the expected conventions of their their use.
A photon has energy. We know that through their interactions with matter, especially the photoelectric effect.

 

[Gravy]

I wrote:

Equivalence between mass and energy on the one hand, but energy without mass on the other hand.​

Dancing David brought forward this analogy:

Equivalence between fur and dogs on the one hand, but dogs without fur on the other hand.​

If we take seriously the equivalence between fur and dogs we conclude:

• a fur without corresponding dog or a dog without corresponding fur is impossible
• a fur corresponds nevertheless to a dog without a fur
• fur and fur are two different concepts

Zosima's original statement:

Mass-Energy equivalence means that even though a photon is massless it has energy​

If we replace mass with restmass we get:

Restmass-Energy equivalence means that even though a photon has no restmass it has energy​

This version is obviously wrong, because there is no equivalence between restmass and energy. Equivalence only reigns between (relativistic) mass and energy. The alternative

Equivalence between (relativistic) mass and energy means that even though a photon has no restmass it has energy​

doesn't make a lot of sense, because of the confusion between two different mass-concepts (mass and restmass).


Cheers, Wolfgang

Comments on FAQ - Does light have mass? (post 1, post 2, post 3)

Ah, now you make complete sense! Have you considered writing a book in which you use color to elucidate the principles of physics, like Oliver Byrne did with Euclidean geometry?

 

[Almo]

Educated people know that there is no inconsistency.

I have to say this isn't quite true. People educated in physics know there's no inconsistency. But I would say that at least many people educated in other fields would trust the physics dudes when they say there is none.

 

[The Man]

I think some of the problem and weirdness wogoga is experiencing comes from the applied units vs the actual meaning of what those units represent. Let’s take Energy for example measured in units of Newton Meters, or Joules as such it is a force multiplied by the distance that force is applied through. Torque also takes the same form of units Newton Meters but has a different meaning, in this case it is a force multiplied by the distance from the rotational center (or radius) that the force is applied. Same units different meanings, to convert torque to energy you would need the rotational angle during the force application in radians and multiply it by the radius to get the cord subtended by that angle or in other words the actual distance the force was applied then multiply that times the force.

A similar thing is occurring here, Mass is measured in units of Kilograms or Newton Second² Meter^-1, some values can take that form of units yet may not adhere to the specific aspects that generate those units and thus not technically be mass. The property of mass that defines these units is Newton’s second law of motion F = m*a or Force equals mass times acceleration. We can convert this to represent mass as m = F / a or mass equals Force divided by acceleration. With the units of mass as Kilograms, force as Newtons, and the inverse of acceleration as Second² Meter^-1 the result is Kg = Newton Second² Meter^-1. More importantly though, is the meaning of the units or Force per acceleration. As a photon does not have the capability of acceleration (changing velocity) this specific aspect of mass does not apply. The energy of the photon can still be represented in this form of units that we use for mass (since it is just a frame invariant conversion) and all the normal equations will work out mathematically, but since the photon lacks the ability to accelerate the true meaning of the units is not applicable. With the units of acceleration as Meter Second^-1 the “acceleration” of a photon becomes the speed of light times the frequency of the photon. So what we might represent as the units of “acceleration” for a photon is simply the Doppler shift of frequency due to relative motion and not the result of any real force applied to the photon.