If light has momentum then wouldn't it have force?

[ZapperZ]

As you gain energy you gain mass. So why is it unreasonable to say that mass is just energy? Why would I even think it is something else? What else is there in this universe other than the fundamental fields and the energy in the permutation of these fields?

Because of ALL the reasons that I've told you already!

Besides the fact that there are already tons of threads discussing the fact that "relativistic mass" is really a misleading concept[1] that even Einstein abandoned it[2], there is already an issue on why you refuse to address the points that I already brought up. Your arguments now has nothing to do with physics, but rather based on TASTES. The FACT that dimensionally, E and NOT the same as m should end this whole discussion. The FACT that "energy" cannot produce charge and the spin required, should also end this whole discussion.

You only focused on one conservation issue while ignoring others. In other words, a cow is a watermelon.

Zz.

[1] L.B. Okun Am. J. Phys. v.77, p.430 (2009).
[2] E. Hecht, Am. J. Phys. v.77, p.799 (2009).

 

[brainstorm]

How could light have force unless it accelerates? F=MA. Doesn't light always travel full speed according to the limits of its medium? It never accelerates within the same medium, correct?

 

[diazona]

A couple things: first of all, acceleration does not come from a change in speed, it comes from a change in velocity, which has both a magnitude and a direction. So if an object's speed of motion changes, it accelerates, but if its direction of motion changes, that's also acceleration.

Besides, ΣF=ma is actually a special case of Newton's second law that only applies to objects of constant, nonzero mass. The more general equation is
\vec{F} = \frac{\mathrm{d}\vec{p}}{\mathrm{d}t}
in other words, force = rate of change of momentum. And like acceleration, momentum is a vector quantity, so it changes when either its magnitude or direction changes. (Or both, of course)

 

[brainstorm]

A couple things: first of all, acceleration does not come from a change in speed, it comes from a change in velocity, which has both a magnitude and a direction. So if an object's speed of motion changes, it accelerates, but if its direction of motion changes, that's also acceleration.

Besides, ΣF=ma is actually a special case of Newton's second law that only applies to objects of constant, nonzero mass. The more general equation is
\vec{F} = \frac{\mathrm{d}\vec{p}}{\mathrm{d}t}
in other words, force = rate of change of momentum. And like acceleration, momentum is a vector quantity, so it changes when either its magnitude or direction changes. (Or both, of course)

Doesn't this suggest that light only has force when it is interacting with matter, either to change direction or get absorbed and re-emitted? Light seems to translocate momentum more so than it actually carrying momentum the way matter does. An object acted upon by force can accelerate and increase its kinetic energy but can light's energy ever increase because it is acted upon by an external force (gravity, for example)? It seems to me that this would violate the conservation of energy whereas matter accelerating/decelerating can't because it can store and release energy in terms of its position and direction within a gravity field. Am I grasping straws here?

edit: radiation seems more like a form of energy storage between matter than a thing that is itself energized. I don't see how it could change momentum when it just seems to deliver momentum from emitting particles to others.

 

[diazona]

Doesn't this suggest that light only has force when it is interacting with matter, either to change direction or get absorbed and re-emitted?

Light doesn't have force, it exerts and experiences force when it interacts with matter. Same applies to matter, by the way, it doesn't have a force, it exerts and experiences force when it interacts with anything else.

Light carries momentum, just like moving matter does. No idea what you're getting at with this "translocation" idea, but that's a totally unnecessary concept.

The energy of a light wave can certainly increase or decrease when it interacts with something else. You might consider gravitational redshift to be an example of this: gravitational potential energy is converted into electromagnetic energy (or vice-versa). The total energy is conserved.

edit: radiation seems more like a form of energy storage between matter than a thing that is itself energized. I don't see how it could change momentum when it just seems to deliver momentum from emitting particles to others.

Well then you have the wrong idea about what radiation is. It has momentum and energy, just like matter does; and that momentum (and energy) can change.

 

[LostConjugate]

No you don't.

Besides, to rehash an example I think I saw earlier in this thread, on the Earth's surface, as you gain height, you gain energy. So why is it unreasonable to say that height is energy?

height is known as potential energy.

 

[LostConjugate]

Because of ALL the reasons that I've told you already!

Besides the fact that there are already tons of threads discussing the fact that "relativistic mass" is really a misleading concept[1] that even Einstein abandoned it[2], there is already an issue on why you refuse to address the points that I already brought up. Your arguments now has nothing to do with physics, but rather based on TASTES. The FACT that dimensionally, E and NOT the same as m should end this whole discussion. The FACT that "energy" cannot produce charge and the spin required, should also end this whole discussion.

You only focused on one conservation issue while ignoring others. In other words, a cow is a watermelon.

Zz.

[1] L.B. Okun Am. J. Phys. v.77, p.430 (2009).
[2] E. Hecht, Am. J. Phys. v.77, p.799 (2009).

I did not address it because I did not understand what you meant by your references to spin.

 

[diazona]

height is known as potential energy.

No, it isn't. Height and potential energy are different things.

 

[LostConjugate]

No, it isn't. Height and potential energy are different things.

In a gravitational field, height is energy, its called potential energy. Since your example was giving in a gravitational field, height is energy.

 

[diazona]

In a gravitational field, height is energy, its called potential energy. Since your example was giving in a gravitational field, height is energy.

No, that's not true. Regardless of whether there's gravity or not. Height is a distance, not a form of energy.

 

[ZapperZ]

I did not address it because I did not understand what you meant by your references to spin.

You mean that you have never heard of the spin quantum number for particles? You've never heard of spin quantum number of 1/2 for electrons and 1 for photons?

What about charge? Have you heard of them? Does the fact that electrons have charges, but "energy" does not, come into any this? Do you merely consider only conservation of energy as the over-riding conservation law, but not charge?

If you are not aware of these properties of matter, then how in the world are you able to make such sweeping statements in the first place relating matter and energy? Do you not see that you are making assertion based on something you don't even fully understand? Do you think this is rational?

Zz.

 

[LostConjugate]

You mean that you have never heard of the spin quantum number for particles? You've never heard of spin quantum number of 1/2 for electrons and 1 for photons?

What about charge? Have you heard of them? Does the fact that electrons have charges, but "energy" does not, come into any this? Do you merely consider only conservation of energy as the over-riding conservation law, but not charge?

If you are not aware of these properties of matter, then how in the world are you able to make such sweeping statements in the first place relating matter and energy? Do you not see that you are making assertion based on something you don't even fully understand? Do you think this is rational?

Zz.

Hold on Hold on. I am familiar with spin and charge. So, my fault for not addressing what you said, I didn't understand it so I didn't try to respond. If this is the impediment to mass being nothing but energy then I would be interested in talking further about it however.

Here is what you said:

" It's spin must also be taken into consideration, and here, a one-to-one conversion of energy into mass cannot occur without other external factors coming into play."

So what do you mean here? Do you mean because it is only measurable in the presence of an external magnetic field?

" The same with a photon being converted into mass. You can't just take a photon and completely turn it into just one particle."

I am lost again, a photon was defined as a particle, its just definitions, there is no "thing" to turn into anything. A photon was termed in the photoelectric experiment where it was found that there was a minimum energy in an EM wave that could interact with matter. Never heard of turning photons into anything.

However I do think a photon could be mistaken for a piece of mass as it has energy. And things that have energy give the illusion of mass. For example in the solar sail experiment you might conclude that because it exhibits a force and F = ma that there is a massive object hitting the sail. This is exactly how we mistake energy for mass in our every day lives.

 

[ZapperZ]

Hold on Hold on. I am familiar with spin and charge. So, my fault for not addressing what you said, I didn't understand it so I didn't try to respond. If this is the impediment to mass being nothing but energy then I would be interested in talking further about it however.

Here is what you said:

" It's spin must also be taken into consideration, and here, a one-to-one conversion of energy into mass cannot occur without other external factors coming into play."

So what do you mean here? Do you mean because it is only measurable in the presence of an external magnetic field?

There are more than just ONE conservation laws. You claim that energy = mass based on the Einstein equation. You ignore OTHER conservation laws that prohibit the idea that mass is nothing more than just energy! Re-read what I've written earlier! If you say that an electron is nothing more than just energy, then I ask you how do you account for the fact that energy has NO CHARGE, AND DOES NOT HAVE A SPIN OF 1/2?

This is why simply stating that E=m is faulty based not on just dimensional analysis, it also violates several conservation laws! All my argument that you've ignored all the conservation law and simply have a narrow view of JUST the energy balance somehow have went over your head!

" The same with a photon being converted into mass. You can't just take a photon and completely turn it into just one particle."

I am lost again, a photon was defined as a particle, its just definitions, there is no "thing" to turn into anything. A photon was termed in the photoelectric experiment where it was found that there was a minimum energy in an EM wave that could interact with matter. Never heard of turning photons into anything.

Read "pair production".

However I do think a photon could be mistaken for a piece of mass as it has energy. And things that have energy give the illusion of mass. For example in the solar sail experiment you might conclude that because it exhibits a force and F = ma that there is a massive object hitting the sail. This is exactly how we mistake energy for mass in our every day lives.

Then this is even sillier than I first thought. You went through a rather "laborious" argument using the Einstein equation, and made the simplification of E=m. What "E" do you think this is? Gravitational potential? If it is, then you're trying to formulate something "new" (and faulty) here. If you want to claim that other forms of energy are equivalent to "mass", then the violation of conservation laws are even MORE apparent. Show how kinetic energy, potential energy, etc. can have charge or spin to account for an electron.

And if you're not referring to photons, then you are OFF TOPIC. Read the OP.

Zz.

 

[Redbelly98]

In a gravitational field, height is energy, its called potential energy. Since your example was giving in a gravitational field, height is energy.

Gravitational potential energy is mgh, which is not equivalent to the height h.

 

[LostConjugate]

Zapper,

What do you mean by Energy does not "have" spin? Spin is just a slight difference in an angular momentum measurement and angular momentum is just a measurement of energy. Spin has energy. Spin is energy.

If Energy is permutations in fields then it is not surprising that energetic objects are associated with fields and to an observer appear to manifest fields.

 

[LostConjugate]

Gravitational potential energy is mgh, which is not equivalent to the height h.

He said:

"on the Earth's surface, as you gain height, you gain energy. "

The Earth's surface means there is a g and when he says you he is referring to an energetic (massive) object m, it goes without saying.

 

[LostConjugate]

In physics, mass–energy equivalence is the concept that the mass of a body is a measure of its energy content. In this concept the total internal energy E of a body at rest is equal to the product of its rest mass m and a suitable conversion factor to transform from units of mass to units of energy. If the body is not stationary relative to the observer then account must be made for relativistic effects where m is given by the relativistic mass and E the relativistic energy of the body.

For example, in many systems of natural units, the speed (scalar) of light is set equal to 1 ('distance'/'time'), and the formula becomes the identity E = m'('distance'^2/'time'^2)'; hence the term "mass–energy equivalence".[2]

The equation E = mc2 indicates that energy always exhibits mass in whatever form the energy takes.[3] Mass–energy equivalence also means that mass conservation becomes a restatement, or requirement, of the law of energy conservation, which is the first law of thermodynamics. Mass–energy equivalence does not imply that mass may be ″converted″ to energy, and indeed implies the opposite. Modern theory holds that neither mass nor energy may be destroyed, but only moved from one location to another.

In inertial reference frames other than the rest frame or center of mass frame, the equation E = mc² remains true if the energy is the relativistic energy and the mass the relativistic mass. It is also correct if the energy is the rest or invariant energy (also the minimum energy), and the mass is the rest or invariant mass. However, connection of the total or relativistic energy with the rest or invariant mass requires consideration of the system total momentum also, in systems and reference frames where this has a non-zero value. The formula then required is the extended version of Einstein's equation, called the relativistic energy–momentum relationship:

Here the (pc)2 term represents the square of the Euclidean norm (total vector length) of the various momentum vectors in the system, which reduces to the square of the simple momentum magnitude, if only a single particle is considered. Obviously this equation reduces to E = mc² when the momentum term is zero.

Nuff said.

 

[diazona]

He said:

"on the Earth's surface, as you gain height, you gain energy. "

The Earth's surface means there is a g and when he says you he is referring to an energetic (massive) object m, it goes without saying.

If it's true that "as you gain X, you gain Y", it is not necessarily true that X and Y are the same thing.

Nuff said.

Wikipedia, huh? Not good enough. If you look on the discussion page you'll notice an ongoing debate about the accuracy of the article, and besides it doesn't cite any sources with which the statements can be verified.

As it stands, that quote is apparently trying to make some compromise between the older convention which recognizes two concepts of "relativistic mass" and "rest mass," and the modern convention which defines "mass" as the invariant (rest) mass.

 

[ZapperZ]

Zapper,

What do you mean by Energy does not "have" spin? Spin is just a slight difference in an angular momentum measurement and angular momentum is just a measurement of energy. Spin has energy. Spin is energy.

If Energy is permutations in fields then it is not surprising that energetic objects are associated with fields and to an observer appear to manifest fields.

Er.. this makes no sense at all. What is the quantum number of "kinetic energy"? And note that who said that "energy has no spin"? I already stated that photons (which IS energy, after all) has a quantum spin of 1, or did you completely missed that?

Furthermore, what is in question is the CONSERVATION rules. While energy can be converted into mass an vice versa, there are OTHER conservation rules that must be considered. Therefore, claiming that energy is mass IGNORES other conservation rules. Energy ALONE cannot account for the presence of CHARGE and the EXACT SPIN of, say fermions! Photons have spin of 1, fermions have spin of 1/2. You can convert one into the other exactly via one-to-one conversion!

It is too bad that your primary source of info is, of all things, Wikipedia. I could easily go in an edit such a thing.

I think I've done everything I can in this discussion. It appears to be going around and around in circles, and you seem to be unable to comprehend what is really basic physics. I no longer have any desire to correct your faulty knowledge. However, I will strongly advise you to refrain from offering this same "explanation" again here as a response to other members.

This thread is done. If the OP has other questions relevant to this thread, please contact a Mentor to have it reopened.

Zz.