Exploring the Physics of Photons - A New Learner's Journey

[zoobyshoe]

The problem with Moonrat's identification is that the term "motion" is already attached to a very different concept, namely that of velocity. Energy is not the same as velocity. The two have different units, and are described by different dynamical laws.

Yes, I see the problem. It isn't exactly accurate to say energy is motion. However, I don't see that the term motion is attached to velocity in a completely exclusive way. We don't say motion is velocity. I think it would be possible to arrive at a way to accurately describe all the forms of energy such that the relative motion in each is viewed as the unifying concept.

This would be a big help to those people who get hung up on grasping the concept. It explains a lot about the interconvertability of the different forms, and hence about the conservation of energy. The motion of one thing gets transferred to another and to another: the kinetic motion of a coil relative to a conductor causes the motion of electrons that cause the motion of their electric fields which results in the motion of photons. Is it not safe to say that all things which are in motion have energy, and that all things that have energy are in motion? (Potential energy = potential motion, of course.)

 

[chroot]

No I am asking, If you knew Where a photon was at a given time, and knew it's momentum to be c would that violate the laws of physics, other than the uncertainty principle

No, it would violate just that one law. And by the way, the momentum of a photon is not c; that's it's speed. It's momentum is

$$p = \frac{h \nu}{c} = \frac{h}{\lambda}$$

where $\nu$ is it's frequency and $\lambda$ is its wavelength.

- Warren

 

[PRyckman]

It wouldn't violate E=mc^2 ?

 

[chroot]

How does E=mc^2 say anything at all about a photon's position and momentum? I don't see either of those variables in the equation.

- Warren

 

[PRyckman]

But a photon has E, E by definition represents all forms of energy does it not?

 

[PRyckman]

Doesn't E=mc^2 say that something which has mass can't travel at the speed of light?

 

[chroot]

The full equation is (I've already shown you this): $E = \sqrt{p^2 c^2 + m_0^2 c^4}$.

Photons have no rest mass, so the energy is just $E = pc$.

And no, E=mc^2 deals with the interconvertibility of mass and energy, which are basically the same thing.

The equation:

$T = (\gamma - 1) m_0 c^2$, where T is the kinetic energy, $m_0$ is the rest mass, says that something with non-zero rest mass acquires infinite kinetic energy when going the speed of light. That equation says that nothing with non-zero rest mass can go the speed of light.

- Warren

 

[PRyckman]

So question from post 35, it would violate that equation, and the uncertainty principle? Also conservation of energy right?

 

[PRyckman]

and to zoobyshoe, I think without mass motion can't give energy

 

[zoobyshoe]

and to zoobyshoe, I think without mass motion can't give energy

If you're talking about photons I guess you've never gotten a suburn.

edit: Come to think of it, if you're talking about photons, I guess you aren't even reading this post.

 

[chroot]

So question from post 35, it would violate that equation, and the uncertainty principle? Also conservation of energy right?

As I've already said quite clearly, knowing the photon's position and momentum simultaneously would violate only the Heisenberg uncertainty principle, nothing else.

- Warren

 

[PRyckman]

if the photon interacted with your skin didn't it have mass? If a photon is kept in a box of perfect mirrors, it has mass.

 

[chroot]

Photons do not have rest mass. They do have energy, which is "equivalent" to mass, but they do not have mass.

- Warren

 

[PRyckman]

In post 42 you say mass and energy are somewhat interchangeable
In your Equation for kinetic energy you say mass must be non zero to travel the speed of light. But we actually know light has mass, if you put it into a box with perfect mirrors the photon would add mass to the box. Therefor We still know that photon is traveling light speed, And we know it has mass.

So if you bring that into E=mc^2 energy is infinite.
Thats what it would be, correct?

 

[chroot]

Photons do not have rest mass. They do have energy. There is really no room to argue this issue. Gravity couples to both energy and mass, so a box of mirrors with a photon inside weighs more than one without, but that does not mean photons have rest mass.

- Warren

 

[zoobyshoe]

Photons do not have rest mass. They do have energy, which is "equivalent" to mass, but they do not have mass.

- Warren

Photons are always moving relative to everything, aren't they? It isn't really possible to find an inertial frame relative to which any photons are at rest, is it?

 

[PRyckman]

Well I think Gravity is defined by time. What do you think of this

If electron A exists only in probability to be found 50% of the time on the left of a line and 50% on the right of a line. Then just like everything in our universe time is relative, so if time is running at a higher frame rate on the right of the line it may actually have spent more than 50% of the time on the right of the line. Therefor the energy it exerts on the rest of the atom is shifted to the right, and the whole atom shifts in such a method.

Ok now what if you put a photon in a box of perfect mirrors and there was only one Planck length of space to bounce around in.

 

[chroot]

Photons are always moving relative to everything, aren't they? It isn't really possible to find an inertial frame relative to which any photons are at rest, is it?

Quite right, zoob.

- Warren

 

[chroot]

Well I think Gravity is defined by time. What do you think of this

I think this is third time I've told you not to post your personal theories in the general physics forums. I really don't want to have to restrict your posting; please just follow our rules.

- Warren

 

[PRyckman]

Isn't this the theory development one? sorry I'm getting tired. I think I'll just stick to talking in my own thread

 

[chroot]

It's okay -- just please help us keep the place in order.

- Warren

 

[quantumdude]

However, I don't see that the term motion is attached to velocity in a completely exclusive way. We don't say motion is velocity. I think it would be possible to arrive at a way to accurately describe all the forms of energy such that the relative motion in each is viewed as the unifying concept.

To refresh my memory, I did a search in my old dynamics books for the word "motion". It turns out that the term is actually identified not with the velocity, but with the position as a function of time. "Find the motion of particle P under the given forces" really means "Find r(t) for particle P under the given forces."

This would be a big help to those people who get hung up on grasping the concept. It explains a lot about the interconvertability of the different forms, and hence about the conservation of energy. The motion of one thing gets transferred to another and to another: the kinetic motion of a coil relative to a conductor causes the motion of electrons that cause the motion of their electric fields which results in the motion of photons.

Hmmm...Something seems to be lost in this equivocation though. When a mass is raised from a height y₁ to a height y₂, there particle is not in motion with respect to the Earth in either state. But it's energy most definitely has changed.

I don't think we can accurately say that energy is motion when there is an instance in which the motion of a body experiences no net change, while the energy does.

Is it not safe to say that all things which are in motion have energy, and that all things that have energy are in motion? (Potential energy = potential motion, of course.)

Yes, but here's the kicker: It's also safe to say that all things which are in motion have momentum, and that all things that have momentum are in motion.

Equating motion and energy not only blurs the distinction between those two concepts, but it also blurs the distinction between energy and momentum.

 

[turin]

I appologize for the lateness of my response. (For some reason, my notification setting had been changed to weekly update.)

LW Sleeth,
I apologize if I seemed obsesively contrary. It was not my intention. I basically agree with everything you said and that you gave a good explanation.

I can't say I fully understand why you find a contradiction between E = mc² and what I said. Was it me describing energy in terms of "movement power,"

Yes.

how do we know work is done? Isn't it only when we observe movement?

That is my understanding.

Matter may contain a lot of energy, but the only way we know that is when we release it and observe how it moves things.

This characterizes my objection (which I admit was picky) precisely.

That's quite a nitpick there, Turin.

Yes, I agree. I guess I should emphasize that I basically agree with everything LW said. I just wanted to point out that the definition was not a general one.

It would be true enough for any mechanical engineer.

I would ask, how does a mechanical engineer define the initial amount of energy? It is this definition with which I contend.

 

[Moonrat]

The problem with Moonrat's identification is that the term "motion" is already attached to a very different concept, namely that of velocity. Energy is not the same as velocity. The two have different units, and are described by different dynamical laws.

so correct me if I am mistaken, you cannot have velocity WITHOUT motion, correct, velocity without energy?
Velocity is motion *n* mass in combination and in harmony, am I mistaken with this understanding?

 

[quantumdude]

so correct me if I am mistaken, you cannot have velocity WITHOUT motion, correct, velocity without energy?
Velocity is motion *n* mass in combination and in harmony, am I mistaken with this understanding?

No, what you have written here is not mistaken, but the problem is that it does not imply that "motion is energy".

Yes, motion implies that a body has energy. But to make the identification "motion is energy", it has to work both ways. That is, it must be true that having energy implies being in motion, and it doesn't.

Also, if you see my post to zoobyshoe, the identification "energy is motion" is too loose, because the reasons for applying it could just as easily apply to the statement "momentum is motion". Indeed, the latter statement would be more accurate, because you truly cannot have momentum without motion, and vice versa.

 

[Math Is Hard]

so correct me if I am mistaken, you cannot have velocity WITHOUT motion, correct, velocity without energy?
Velocity is motion *n* mass in combination and in harmony, am I mistaken with this understanding?

well, let's see... I can be sitting here completely still but you can still come up to me and calculate my velocity. It's zero. So I do have a velocity. But to have positive or negative velocity there would have to be some motion involved. I have to move in a direction.
If I sit in the chair and do nothing I still have energy, because I have the ability to do some work.
rats - I can't remember where I was going with this!

 

[quantumdude]

well, let's see... I can be sitting here completely still but you can still come up to me and calculate my velocity. It's zero. So I do have a velocity. But to have positive or negative velocity there would have to be some motion involved. I have to move in a direction.

And this brings up another problem with the identify "motion is energy". Motion is directed, and energy is not. You can have two people moving in opposite directions with the same mass and the same speed. They have the same energy, but not the same motion.

 

[Math Is Hard]

And this brings up another problem with the identify "motion is energy". Motion is directed, and energy is not. You can have two people moving in opposite directions with the same mass and the same speed. They have the same energy, but not the same motion.

That makes sense - it will always be a scalar quantity. I tried to look up the units that energy is expressed in and I came back with several:

joules - the official energy unit of the International System of Units
watt-hours or kilowatt hours ( one kilowatt hours = 1000 watt hours)
BTUs - British Thermal Units
Therms (100 Therms = 1 BTU)

from http://www.uccs.edu/~energy/courses/energy_units.html

The one about Joules bugs me because I think of Joules as a measure of Work. If Joules are units of energy, then are Foot-Lbs units of energy also?

 

[quantumdude]

The one about Joules bugs me because I think of Joules as a measure of Work. If Joules are units of energy, then are Foot-Lbs units of energy also?

Energy has the same units as work by definition. When solving problems involving nonconservative forces using energy methods, you add the nonconservative work directly to the energy, which means they must have the same units.

The really annoying thing is not that energy and work have the same dimensions (indeed, they must), but that torque has the same dimensions as both energy and work (but we don't report torque in Joules, we report it in Newton-meters).

 

[zoobyshoe]

Hmmm...Something seems to be lost in this equivocation though.

I have spent about an hour scouring what I said to see why you got the impression I was equivocating. I suppose it was this:"Yes, I see the problem. It isn't exactly accurate to say energy is motion." To avoid the impression of equivocation I should perhaps have been more definite, and said: "Yes, I see the problem: it isn't accurate to say energy is motion.

Ans so, when I said: "I think it would be possible to arrive at a way to accurately describe all the forms of energy such that the relative motion in each is viewed as the unifying concept," I was saying I thought such a description was possible, and not that it had already been accomplished. (Certainly not by saying "energy is motion". I agree with you that this can't be used.)

When a mass is raised from a height y₁ to a height y₂, there particle is not in motion with respect to the Earth in either state. But it's energy most definitely has changed.

I don't think we can accurately say that energy is motion when there is an instance in which the motion of a body experiences no net change, while the energy does.

I agree that we can't say "energy is motion."

However, I can use your example of a change in potential energy to demonstrate how following the trail of motion helps to understand the conservation of energy: in changing from the state of lower potential energy y₁ to the state of higher potential energy y₂ the particle had to be moved by an outside agent which had to transfer some part of its own kinetic energy to the particle to move it. If you subtract motion from the situation the particle will never change position. There will never be a higher position y₂ and, consequently, no net change in energy.

Yes, but here's the kicker: It's also safe to say that all things which are in motion have momentum, and that all things that have momentum are in motion.

I think you have an excellent point here, because if my goal is to clarify things by describing energy with respect to motion, there is the momentum/kinetic energy confusion that seems to crop up quite often in the very same people who ask "What is energy?"

Everything you have said in this thread has been quite helpful in helping me to sharpen up the edges of my concept of energy. Every time this question comes up I am thrown back to remembering the massive Gale17 "What is Energy?" threads, and I am just realizing now that she may have been completely thrown off by the preconception you pointed out that some people have, that energy is some kind of substance or perhaps fluid. I am glad you thought to specify that it isn't. That kind of science fiction notion of "pure energy" as a substance or fluid may be the preconception that most needs to be eliminated when people are confused by the concept of energy.

Zooby

 

[Moonrat]

No, what you have written here is not mistaken, but the problem is that it does not imply that "motion is energy".

Yes, motion implies that a body has energy. But to make the identification "motion is energy", it has to work both ways. That is, it must be true that having energy implies being in motion, and it doesn't.

.

cannot something have internal motion? as a cheeky example, me. I can sit complelty motionless and still, even in a coma. Yet I still have internal motion, organ systems and what not.

So even an object, say a asteroid. It is composed of elements that too are 'motioning' against one or the other..

am I mistaken in this?

thank you for your time

 

[quantumdude]

I have spent about an hour scouring what I said to see why you got the impression I was equivocating. I suppose it was this:"Yes, I see the problem. It isn't exactly accurate to say energy is motion." To avoid the impression of equivocation I should perhaps have been more definite, and said: "Yes, I see the problem: it isn't accurate to say energy is motion.

Sorry, I saw where you had approved of Moonrat's equivocation "energy=motion", and I read something into your post that wasn't there. Sorry you wasted an hour on it!

 

[quantumdude]

cannot something have internal motion? as a cheeky example, me. I can sit complelty motionless and still, even in a coma. Yet I still have internal motion, organ systems and what not.

Yes, there is always internal motion, but there's no apparent motion that can account for the difference between particles in different positions in a gravitational field. Also, there is the problem of direction that I mentioned.

So even an object, say a asteroid. It is composed of elements that too are 'motioning' against one or the other..

am I mistaken in this?

No, you're not mistaken, it's just that what you have presented is not enough to justify the identification "motion is energy".

 

[zoobyshoe]

Sorry, I saw where you had approved of Moonrat's equivocation "energy=motion", and I read something into your post that wasn't there. Sorry you wasted an hour on it!

Now I think there is a different problem here. I see you referring to "energy=motion" as an "equivocation". My impression is that you think the word "equivocation" means "to relate by equasion" .

However "equivocation" is from the verb "equivocate" which means: to use equivocal language, esp with intent to decieve 2: to avoid committing oneself in what one says syn see LIE-equivocation.

In other words, by using the word "equivocation" you are either calling something a deception, or calling it non-committal.

I hope you can see why I was somewhat distressed.

 

[quantumdude]

Now I think there is a different problem here. I see you referring to "energy=motion" as an "equivocation". My impression is that you think the word "equivocation" means "to relate by equasion" .

However "equivocation" is from the verb "equivocate" which means: to use equivocal language, esp with intent to decieve 2: to avoid committing oneself in what one says syn see LIE-equivocation.

In other words, by using the word "equivocation" you are either calling something a deception, or calling it non-committal.

I hope you can see why I was somewhat distressed.

Ach, no, that's not what I meant. I was using the word "equivocation" in the specialzed sense of logic. A "fallacy of equivocation" for example is the use of words that are not univocal in such a way as to make an unsound argument appear sound. It is done by equating two things that are not in fact equivalent.

No malintent is assumed in a fallacy of equivocation. Indeed, the "trickster" is not the person forwarding the argument, but language itself.

From Fallacy Files:

Of course, most words are ambiguous, but context usually makes a univocal meaning clear. Also, equivocation alone is not fallacious, though it is a linguistic boobytrap which can trip people into committing a fallacy. The Fallacy of Equivocation occurs when an equivocal word or phrase makes an unsound argument appear sound. Consider the following example:

All banks are beside rivers.
Therefore, the financial institution where I deposit my money is beside a river.

And that's what I meant. So you can think of my use of "equivocation" as "mis-identification".

Sorry for the misunderstanding.