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

[Math Is Hard]

I have trouble understanding and visualizing this concept because I want to think of something tangible. I am not sure if I should be thinking of something tangible, though.
Are photons energy, or are they a just a by-product of electrons jumping between energy levels?
Sorry for such a rudimentary question. I just started my first physics class this week and wanted to make sure I don't start off with any wrong assumptions.

 

[repugno]

Measure of the ability of a system to do work.

 

[Math Is Hard]

Measure of the ability of a system to do work.

So it's a measurement only?
This is what got me curious:
The other night I watched that program "The Elegant Universe" and they talked about string theory. It was stated that matter at it's basest level was composed of vibrating strands of energy.
Doesn't make sense. Will I understand this later on?

 

[Les Sleeth]

So it's a measurement only?
This is what got me curious:
The other night I watched that program "The Elegant Universe" and they talked about string theory. It was stated that matter at it's basest level was composed of vibrating strands of energy.
Doesn't make sense. Will I understand this later on?

I think I know why it doesn't make sense to you. When one hears energy is the capacity to do work, and then one hears matter is energy . . . somehow it is hard to connect the two. I am not a physicist, but I have thought about how to explain what energy is.

As a term energy was probably first applied in the seventeenth century (borrowed from Aristotle’s energeia) to help explain the quality of motion, or “vis viva” in things. Today people who are spiritually inclined may speak of energy as well when referring to properties of consciousness, life, God, soul and ethereal peculiarities. Unfortunately the popularization of the energy concept has led to considerable misconceptions about it. Science writer Paul Davies writing in his book Superforce explains, “What made it appealing was that energy is always conserved, never created or destroyed.” Davies goes on to say, “When an abstract concept becomes so successful that it permeates through to the general public, the distinction between real and imaginary becomes blurred. . . . This is what happened in the case of energy. . . . Energy is . . . an imaginary, abstract concept which nevertheless has become so much a part of our everyday vocabulary that we imbue it with concrete existence.”

If we are to be accurate with our terms, then it must be understood that science claims first rights to the word energy and assigns it a very specific meaning, which is the capacity to do work. In science, energy is more of a mathematical and measuring tool of movement than anything actual. The thermodynamic law that states energy is “never created or destroyed” is really meant to support calculations in physics that gauge and record the path of movement power.

For example, the energy concept can help describe what happens in a mechanical system, say when electricity moves an electric motor. Fuel is burned and produces energy, which is transferred to a generator, which is transferred to electrical energy, and then back to kinetic energy as it turns the motor. If one adds up all the movement power used, plus that lost to heat and friction, it will total the amount of movement power, or energy, started with. So there was no energy created in the process and none was destroyed by the process; the power of movement was simply transferred from one thing to another.

And then there is me, who is afflicted with a disturbing need to understand things , and so has to wonder:

When the fuel is burned up and the generator stops, when electricity no longer flows and the motor comes to a halt, where is all that energy just used? Maybe it wasn’t destroyed, but it is gone from the system and it is gone for good. If it did survive,where did it go, and what was that movement power in the first place?

 

[turin]

So it's a measurement only?

The term measure is does not mean measurement in this context. A different way to say it would be, "energy is the ability/potential to do work." Here, two synonyms are suggested. But, as pointed out by LW Sleeth, the physics concept of energy was never intended to be defined in words.

If one adds up all the movement power used, plus that lost to heat and friction, it will total the amount of movement power, or energy, started with.

This is inaccurate (i.e. ambiguous). If one believes that E = mc², then I've never heard of a macroscopic process in which this statement could be considered true. Certainly, this is not true in the stated example.

 

[Les Sleeth]

This is inaccurate (i.e. ambiguous). If one believes that E = mc², then I've never heard of a macroscopic process in which this statement could be considered true. Certainly, this is not true in the stated example.

To be honest, I made my post to test it for accuracy. So I am very open to being corrected if someone finds a problem with what I said. I am going to debate you now to see if I can make my case, or if I need to adjust how I think of energy.

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," or was it substracting heat and friction, or was it neither, both or . . . If energy is defined as the capacity to do work, how do we know work is done? Isn't it only when we observe movement? 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.

It seems to me there are two ways of looking at energy. One is for everyday measurement; that is where we say energy is the capacity to do work. Then there is the theoretical aspect which recognizes a lot of energy is packed into an atom. However, it also seems more than energy is there because "c²" tells us light is present too. Hmmmmmmm . . .

 

[quantumdude]

LW Sleeth: If one adds up all the movement power used, plus that lost to heat and friction, it will total the amount of movement power, or energy, started with.

Turin: This is inaccurate (i.e. ambiguous). If one believes that E = mc², then I've never heard of a macroscopic process in which this statement could be considered true.

That's quite a nitpick there, Turin. If the energy is low enough, we don't have to take relativity into account. The change in mass of the fuel during burning could not be more than a tiny fraction of an eV per bond broken.

Certainly, this is not true in the stated example.

It would be true enough for any mechanical engineer.

 

[quantumdude]

When the fuel is burned up and the generator stops, when electricity no longer flows and the motor comes to a halt, where is all that energy just used?

The flow of power will not be simultaneous with the emptying of the fuel tank. That last bit of electricity will continue to power the motor until it is used up. And as will all electromechanical loads, some will be converted to motion and some to heat. The temperature of the whole system will be (for a time) higher than it was before operation.

Maybe it wasn’t destroyed, but it is gone from the system and it is gone for good. If it did survive,where did it go, and what was that movement power in the first place?

Don't forget that a "system" is an abstract concept that only exists in the minds of people. If we can draw an imaginary boundary around what we want to call a "system", and no matter or energy is transferred across that boundary, then we can apply the same conservation laws to that "system" that we believe hold for the universe in total. So, the energy is only gone from "the system" if we define "the system" not to include the surrounding air, or anything else to which the heat may escape.

 

[Les Sleeth]

The flow of power will not be simultaneous with the emptying of the fuel tank. That last bit of electricity will continue to power the motor until it is used up. And as will all electromechanical loads, some will be converted to motion and some to heat. The temperature of the whole system will be (for a time) higher than it was before operation.

Thanks, I hadn't thought of that.

Don't forget that a "system" is an abstract concept that only exists in the minds of people. If we can draw an imaginary boundary around what we want to call a "system", and no matter or energy is transferred across that boundary, then we can apply the same conservation laws to that "system" that we believe hold for the universe in total. So, the energy is only gone from "the system" if we define "the system" not to include the surrounding air, or anything else to which the heat may escape.

I see that, however . . .

(. . . and I am still "testing" my understanding) when I say it is gone from the system and gone for good, I mean once energy has passed through everything we understand as a system. If I am to take conservation as absolute, which it seems you are saying I should, and we consider the limits of the universe the extent of all systems, then I don't see how energy is necessarily conserved except as it passes through the relative subsystems that compose the overall universe. I say that because the universe is expanding, and the current thinking seems to be it is energy (of one sort or another) which is causing that. To me that means expansion must expend energy, and then that energy becomes unavailable not only to relative situations, but to the only absolute situation we know of: the universe.

So in the end, isn't energy (if we rely on current thinking) "disappearing"?

 

[quantumdude]

If I am to take conservation as absolute, which it seems you are saying I should, and we consider the limits of the universe the extent of all systems, then I don't see how energy is necessarily conserved except as it passes through the relative subsystems that compose the overall universe.

We saw in our motor example that the end product (after the motion has stopped) is heat, right? So, where does the heat go? If our system boundary is finite, the heat will eventually cross it and be transferred (by conduction, convection, or radiation) to a larger system, one which includes the air in the room.

But as the heat spreads, it will approach the boundary of that system and eventually cross it, and so on. But we will at some point reach an end to all this, such that the heat cannot cross any more boundaries, simply because there are no more boundaries for it to cross. Each subsystem will come to thermal equilibrium with every other subsystem, and their common temperature will be just a little higher than it was before I ran my motor.

The energy isn't being lost, the universe is getting hotter.

I say that because the universe is expanding, and the current thinking seems to be it is energy (of one sort or another) which is causing that. To me that means expansion must expend energy, and then that energy becomes unavailable not only to relative situations, but to the only absolute situation we know of: the universe.

So in the end, isn't energy (if we rely on current thinking) "disappearing"?

I'm a bit out of my depth here with general relativity, but what I do know is that the energy-momentum tensor, which is the source term for the tensor that describes the structure of spacetime, is written in terms of energy and momentum densities, not energy and momenta themselves. If I've got that right, then it's not that the energy-momentum is disappearing, it's that it is becoming more sparse. But when integrated over the whole of space, the total is a constant.

 

[Les Sleeth]

We saw in our motor example that the end product (after the motion has stopped) is heat, right? So, where does the heat go? If our system boundary is finite, the heat will eventually cross it and be transferred (by conduction, convection, or radiation) to a larger system, one which includes the air in the room.

But as the heat spreads, it will approach the boundary of that system and eventually cross it, and so on. But we will at some point reach an end to all this, such that the heat cannot cross any more boundaries, simply because there are no more boundaries for it to cross. Each subsystem will come to thermal equilibrium with every other subsystem, and their common temperature will be just a little higher than it was before I ran my motor.

The energy isn't being lost, the universe is getting hotter.

This confuses me a bit because what I've read tells me the universe is getting cooler. The worry over the universe's heat death, for instance . . . see

http://www.physlink.com/Education/AskExperts/ae181.cfm

I think I see what you mean however -- that the energy which seems to be disappearing is merely participating in "equilibrium." Yet I still have a logical concern . . .

I'm a bit out of my depth here with general relativity, but what I do know is that the energy-momentum tensor, which is the source term for the tensor that describes the structure of spacetime, is written in terms of energy and momentum densities, not energy and momenta themselves. If I've got that right, then it's not that the energy-momentum is disappearing, it's that it is becoming more sparse. But when integrated over the whole of space, the total is a constant.

Energy is commonly thought of as a distinct property. Yet even if it sacrifices itself to equilibrium, it really does in the end disappear as a distinct property. To me that suggests that either energy has left the system we call our universe, or energy was never really and truly distinct from the universe to begin with. In other words, whatever it is that created the universe might be some monistic property which some set of unrecognized conditions cause to appear as distinct and separate properties.

I know this is the classic physics area, and I shouldn't be talking so speculatively. But I can't resist pointing out that one cannot have it both ways. If energy is totally conserved, doesn't it imply unity at the root of everything?

 

[quantumdude]

This confuses me a bit because what I've read tells me the universe is getting cooler. The worry over the universe's heat death, for instance . . . see

http://www.physlink.com/Education/AskExperts/ae181.cfm

I think I see what you mean however -- that the energy which seems to be disappearing is merely participating in "equilibrium." Yet I still have a logical concern . . .

Right, what I meant was that the universe is just a little hotter than it would be if the motor had not run. It does dump heat into the surroundings, but the expansion of the universe (which is a competing effect) is winning, and so the universe is cooling overall.

Energy is commonly thought of as a distinct property. Yet even if it sacrifices itself to equilibrium, it really does in the end disappear as a distinct property. To me that suggests that either energy has left the system we call our universe, or energy was never really and truly distinct from the universe to begin with.

I don't know what you mean by "distinct", but maybe this will answer your question:

Heat is a form of energy.

So, when the temperature of a system rises or falls, its thermal energy content has changed. When (edit: if?) the so-called "heat death" of the universe occurs, all the energy that has ever existed will be unusable for work, and in theory that amount of energy will be equal to the total amount of energy (in all its forms) that has ever existed. It hasn't gone anywhere, it has just changed form.

Is that what you were getting at?

edit: re-wrote a part I didn't like too much

 

[Les Sleeth]

I don't know what you mean by "distinct" . . .

I wrote an answer and then deleted it after deciding this probably isn't the best place to be theorizing about energy losing its distinctness.

But I am still wondering about turin's criticism when I said, "if one adds up all the movement power used, plus that lost to heat and friction, it will total the amount of movement power, or energy, started with." Turin responded that "If one believes that E = mc2, then I've never heard of a macroscopic process in which this statement could be considered true."

Your reply seemed to be that if I were to be precisely correct, I'd need to take into account relativity. Would you (or anyone) explain why?

 

[zoobyshoe]

Hear Einstein explain E=mc² in his own words:

Einstein Exhibit -- Voice of Einstein
Address:http://www.aip.org/history/einstein/voice1.htm

-Zooby

 

[Les Sleeth]

Hear Einstein explain E=mc² in his own words:

Einstein Exhibit -- Voice of Einstein
Address:http://www.aip.org/history/einstein/voice1.htm

-Zooby

Thanks Zooby. Actually I understand that much already. What I am trying to understand now is turin's criticism. Is it because when I spoke of all the energy adding up in my example, I didn't include the energy still remaining in the burned fuel mass? If so, it is because I was only referring to the energy released for work as fuel is burned. So I am trying to understand if there is a problem with that latter idea or not. As I understand it, the energy released when fuel is burned moves through the system and does work, and the amount of work done plus what heat is lost equals the amount of energy originally released by the burned fuel.

 

[quantumdude]

Your reply seemed to be that if I were to be precisely correct, I'd need to take into account relativity. Would you (or anyone) explain why?

In chemical reactions (such as burning fuel), the main source of energy comes from the breaking fo chemical bonds. It turns out that there is also a very slight mass difference after the reaction, and the lost mass is converted to energy a la Einstein (E=mc²). But the contribution of this mass to the total energy is so minute, that there is not a chemist or engineer in the world who uses E=mc² in any theoretical model of chemical reactions. It is such a low-order effect that it is just not necessary, and the correction is below the threshold of measurement error anyway.

 

[Les Sleeth]

In chemical reactions (such as burning fuel), the main source of energy comes from the breaking fo chemical bonds. It turns out that there is also a very slight mass difference after the reaction, and the lost mass is converted to energy a la Einstein (E=mc²). But the contribution of this mass to the total energy is so minute, that there is not a chemist or engineer in the world who uses E=mc² in any theoretical model of chemical reactions. It is such a low-order effect that it is just not necessary, and the correction is below the threshold of measurement error anyway.

I see, thanks. I am happy to know that.

I was looking at Math Is Hard's posts and realized no one had answered what had confused her. She pointed out the fact that in string theory matter is believed to be little vibrating strings of pure energy seems to conflict with the idea that the term "energy" is mostly an abstraction used for the purpose of measurement.

I have the same problem with how energy (as a word) is used, such as when someone says a photon is pure energy, yet light seems to maintain its integrity, at least as vibration, whether it gains or loses energy. In the way I described it to her, calling it "movement power," it is hard to imagine a little string of movement power, or a little string of capacity to do work.

 

[quantumdude]

I was looking at Math Is Hard's posts and realized no one had answered what had confused her. She pointed out the fact that in string theory matter is believed to be little vibrating strings of pure energy seems to conflict with the idea that the term "energy" is mostly an abstraction used for the purpose of measurement.

Actually, it's an abstraction that is used for the purpose of theorizing. No one actually measures energy. Rather, experimentalists measure state variables that are used in mathematical functions which we call the different forms of energy.

Velocity is measured, from which kinetic energy is calculated.

Temperature is measured, from which thermal energy is calculated.

Position relative to a mass is measured, from which gravitational potential energy is calculated.

Frequency of a photon is measured, from which photonic energy is calculated.

Energy isn't measured, it's calculated.

I have the same problem with how energy (as a word) is used, such as when someone says a photon is pure energy, yet light seems to maintain its integrity, at least as vibration, whether it gains or loses energy.

That's a misuse of the term "energy". A photon is not to be identified with energy, a photon has energy, which as I noted above is calculated from its frequency.

In the way I described it to her, calling it "movement power," it is hard to imagine a little string of movement power, or a little string of capacity to do work.

Just as with photons, the same can be said of strings: They aren't to be identified with energy, they have an energy.


edit: fixed some tags

 

[quantumdude]

I have trouble understanding and visualizing this concept because I want to think of something tangible. I am not sure if I should be thinking of something tangible, though.

First thing to note is that energy is not a tangible substance. In fact, energy doesn't exist anywhere in the universe!

Energy has no reality apart from its (mathematical) functional dependence on state variables that do have reality (see my post to Les above).

Are photons energy, or are they a just a by-product of electrons jumping between energy levels?

As I just remarked above, photons are not to be identified with energy. Energy is a mathematical invention, but photons are real. Having said that, deexcitation of atoms is not the only way to produce them. They can be produced from any deexciting system (semiconductor, molecule, nucleus, nucleon, etc.) or by accelerating charges.

 

[Math Is Hard]

Thank you. I appreciate all the responses. This was the clarification I was looking for.

 

[jdavel]

Tom Mattson siad:

"Velocity is measured, from which kinetic energy is calculated.

Temperature is measured, from which thermal energy is calculated.

Position relative to a mass is measured, from which gravitational potential energy is calculated.

Frequency of a photon is measured, from which photonic energy is calculated."

I don't understand the distinction you're making. What's special about velocity? Why can't I say, "Distance and time are measured, from which velocity is caculated"?

In fact, can't I even work everything back to just length and say that's the only state variable, and everything else is just a definition or a mathematical abstraction based on length?

But why would I want to do that?

 

[PRyckman]

photons are both a particle and an energy wave. Depending on the circumstances

 

[quantumdude]

I don't understand the distinction you're making. What's special about velocity? Why can't I say, "Distance and time are measured, from which velocity is caculated"?

In fact, can't I even work everything back to just length and say that's the only state variable, and everything else is just a definition or a mathematical abstraction based on length?

But why would I want to do that?

Actually, I do think that length is really the only things that is ever measured. But I was trying to explain that energy isn't really a physical "thing" the way it is typically portrayed.

 

[Moonrat]

I suggest, in layman's terms, that energy is simply motion. It is not the ability of motion, but rather the motion itself.

Matter/Motion comprise the physical universe, and they are so intertwined, one does need to turn to 'art' or 'artistic expression' to gain a full picture, I don't think scientific literature can cover it enough for the layman without the layman stumbling upon paradox.

Motion and energy seduces us to discover it's source, so it takes us into the quantum realm, which becomes even more paradoxical..

Motion is everywhere the center, while matter is the circumfrence nowhere can be found...(and that's poetic ;-0)

Moonrat

 

[Math Is Hard]

I suggest, in layman's terms, that energy is simply motion. It is not the ability of motion, but rather the motion itself.
Moonrat

hmmm...I rather like thinking about it that way - and it makes sense. Motion's not tangible, but things definitely have (or can have) motion. Potential motion and kinetic motion seem to work by definition, too.

Thank you for the suggestion.

 

[Moonrat]

hmmm...I rather like thinking about it that way - and it makes sense. Motion's not tangible, but things definitely have (or can have) motion. Potential motion and kinetic motion seem to work by definition, too.

Thank you for the suggestion.

thanks for the question! kinda cool too when you think that all physical objects have motion affected by gravity, which 'motions' everything toward a center, or center source. (our moon centers around our planet, our planter centers around the sun, and our sun centers around the galatic core)

perhaps this motion can be quantified, perhaps, by 'omni-direction', uni-direction, and non-direction? (ternary quantifications are always pretty)

 

[zoobyshoe]

I suggest, in layman's terms, that energy is simply motion. It is not the ability of motion, but rather the motion itself.

I can't think of an example of energy that doesn't involve relative motion, so I think this is a good way to look at it to unify all the kinds of energy we speak of into one thing.

 

[quantumdude]

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.

 

[PRyckman]

If motion is energy then a PARticle of light traveling light speed would have infinite energy? I think I just figured out why there is an uncertainty principle. Because if you know the exact position of that photon and know its exact speed(light) then it is disobeying the laws of physics! Score one for d=E(t)

 

[chroot]

PRyckman:

A) The energy of a particle always given by $E = \sqrt{p^2 c^2 + m_0^2 c^4}$. Photons have no rest-mass, so $m_0$ is 0. That collapses the equation down to just $E = pc$, where p is the momentum carried by the photon. Photons do not have infinite energy.

B) You cannot simultaneously know the position and momentum of a photon to a precision that would violate the uncertainty relation.

C) Do not post your personal theories in the general physics forum.

- Warren

 

[PRyckman]

ok sorry, got ahead of myself. But if you could measure that photon's momentum and place at the same time without interacting with it. Would it's M make it an impossibility?

 

[chroot]

You can't measure a photon's momentum and position to infinite precision at the same time.

- Warren

 

[PRyckman]

What are the observations of it's speed, I ask you ,You know it must exist somewhere in there in probability. So If it were possible, what would your calculations show, objevtively

 

[chroot]

Are you asking "how do you know light travels at c?"

- Warren

 

[PRyckman]

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