What is an electromagnetic wave?

[epepke]

Now explain an electron and a proton attracting each other.

Someone should say something about virtual photons. Someone who knows more about them than I do, that is.

http://math.ucr.edu/home/baez/physics/Quantum/virtual_particles.html

 

[epepke]

Good question - if the field around an electron is always present, no matter whether another electron is nearby to be affected by it, that implies that it must always be spitting out photons.

Yes. It's called the Dirac Gas.

 

[Soapy Sam]

So we're stuck with the pesky wave-particle duality. It's not nice and neat, but who said nature was?

Cheers - Mattus

O nobody. Certainly not me. But there seem to be two rather different uses of the wave concept here.

One is the statistical effect which jj mentions- waves as statistical averages of photons acting in large numbers. This is analogous to the gas laws , where statistical effects like temperature and pressure emerge from the averaging of molecular movement. But at the end of the day, only the molecular movement is fundamentally real. I sometimes wonder whether the fact that late 19th century physicists were steeped in the model of gas laws and thermodynamics made their immediate successors a little too ready to accept a statistical model in quantum mechanics.

In the case of a single photon , it's hard to see how a statistical property can exist. This is where the second meaning of wave comes in- De Broglie's meaning- a particle actually has a wavelength and frequency equivalent , tied to Planck's Constant. This is the wave behaviour of the 2 slit experiment. In theory, an elephant also has a frequency. In the macroscopic world we tend to ignore this as meaningless or useless, but we take it seriously in the subatomic world. I have never clearly understood why.

But where does this notional field arise, in either picture? I have always been unhappy about the idea of fields. I find the whole notion offensive, whether used by Sheldrake or Maxwell- a sort of unholy half way house between matter and energy.

"Virtual" photons, eh? Now there's a kludge if ever I saw one.

ETA- I was checking the bearings on an auger this morning- a 10 metre long trough, with a rotating Archimedes screw which moves mud and rock slurry along a trough. Watching the screw it's impossible not to see a moving wave, propagating along the auger. Yet we know that the whole thing is just a rotating shaft. The screw "goes" nowhere. Just for a moment, I felt I hovered on the verge of grokking the oneness of it all. Then I said "bugger this "and went for coffee.

 

[davefoc]

I just wanted to say that I always enjoy Soapy Sam's questions about this stuff. Alot of the time it seems that he has struggled with exactly the same kinds of things that I have and has arrived at about the same level of non-understanding.

 

[Yllanes]

In theory, an elephant also has a frequency. In the macroscopic world we tend to ignore this as meaningless or useless, but we take it seriously in the subatomic world. I have never clearly understood why.

The bigger the momentum of a particle, the smaller its asssociated wavelength (l = h /p). Ondulatory phenomena such as diffraction do not take place unless the aperture and the wavelength are of a similar size (if the aperture is much bigger, nothing happens). Now, the de Broglie wavelength of an electron is about the same size as the interatomic spacing in crystal lattices. This means we can use the latter to perform a double slit experiment with electrons. (The speed of an electron is roughly alpha · c ~ c /173 and its mass is 9.1e-31 kg, this gives l ~ 10^-10 m = 1 Angstrom. I have neglected relativistic corrections). Consider now a billiard ball, of m = 0.5 kg and v = 0.4 m/s. Its de Broglie wavelength would be 10^-33 m. There's no slit small enough to ever appreciate this. Even for something such as a dust speck, moving with speed as low as those caused by the thermal agitation due to the microwave background (~3 K), l ~10^-19 m.

To further illustrate this, let us consider an universe where Planck's constant is macroscopic. We will estimate its value in an universe where the minimum increment in the speed of a bicycle wheel is 10 km/h.

Angular momentum is discrete. The angular momentum of the wheel around its axis is L = n·hbar, with hbar = h /(2*pi) and n an integer. So the minimum increment is dL = hbar. Now, if I is the wheel's moment of inertia and w=v/R its angular velocity, L=I·w. If we take, for simplicity, all the mass in the wheel to be concentrated in its perimeter, I~MR^2. This gives L~MRv. And the minimum increment in v is then dv = hbar / (MR). With M = 2 kg and R = 0.5 m, assuming dv = 10 km/h, we get hbar ~ 3 Js. (Its value in our universe is hbar = 1.054e-34 Js). So things would have to be quite different for angular momentum to be discrete for macroscopic particles. For electrons, however, the discrete value of L is one of the things that classifies the various orbitals.

 

[rehn]

A simple way to generate an electromagnetic wave.
Take a standard bar magnet. Swing it to and fro 1 / S.
And, viola, you are sending out an electromagnetic wave
of 1 Hz. The movement of the magnet is generating an
electric file that is generating a magnetic field aso.
And this is moving though space at C.

And a question: how do I calculate the number photons do I send each second this way ??
Let's assume a 1 m long bar magnet rotating around
it's middle at 60 rpm. The strength of the magnet must come
into this too, how do you measure the strength of a magnet ?

 

[Ziggurat]

And a question: how do I calculate the number photons do I send each second this way ??

Calculate the classical EM field generated from this. You can calculate an energy flux per unit area from the fields, and by integrating this over a surface surrounding your magnet (easiest to do by picking a sphere much larger than the area you're swinging your magnet around in), which will give you the power output. That part's purely classical. Now calculate the energy of a single photon with that frequency. Divide total classical energy by small quantum energy to get # of photons (should be a huge number)

The strength of the magnet must come
into this too, how do you measure the strength of a magnet ?

Practically speaking? You pass it through a conducting coil and measure the induced current, which is proportional to the strength of the magnet.

 

[Soapy Sam]

I just wanted to say that I always enjoy Soapy Sam's questions about this stuff. Alot of the time it seems that he has struggled with exactly the same kinds of things that I have and has arrived at about the same level of non-understanding.

Glad I'm not alone. I read and I read and occasionally I think I'm getting a handle on it. Then someone else explains it all and I'm back where I started. What I need is a really dumb physicist . Smart ones are incomprehensible.

 

[CurtC]

"Virtual" photons, eh? Now there's a kludge if ever I saw one.

Yep, it's a kludge. So is every other way of explaining QM to try to make it more understandable to us macro-minded people.

Do the virtual photons exist? Almost certainly not. It's a way someone came up with to make it seem more grokkable.

We know how QM behaves with a great degree of precision. Put in the conditions, crank through the math, and we can predict how things will behave (statistically). The problem is that this mathematical model of the behavior is strange to us, so we come up with these "interpretations." If you can come up with another interpretation that's both accurate and makes sense to you, you should probably publish it.

 

[MattusMaximus]

A good book for the layperson

Perhaps this book will serve to shed some light on *some* of these topics...

"Quantum Physics for Everyone" by Ford & Hewitt
http://www.amazon.com/gp/product/0674013425/103-0760335-1504649?v=glance&n=283155

Ah, the weirdness of QM, will it ever end?

Cheers - Mattus

 

[MattusMaximus]

One is the statistical effect which jj mentions- waves as statistical averages of photons acting in large numbers. This is analogous to the gas laws , where statistical effects like temperature and pressure emerge from the averaging of molecular movement. But at the end of the day, only the molecular movement is fundamentally real. I sometimes wonder whether the fact that late 19th century physicists were steeped in the model of gas laws and thermodynamics made their immediate successors a little too ready to accept a statistical model in quantum mechanics.

In the case of a single photon , it's hard to see how a statistical property can exist. This is where the second meaning of wave comes in- De Broglie's meaning- a particle actually has a wavelength and frequency equivalent , tied to Planck's Constant. This is the wave behaviour of the 2 slit experiment. In theory, an elephant also has a frequency. In the macroscopic world we tend to ignore this as meaningless or useless, but we take it seriously in the subatomic world. I have never clearly understood why.

If you *really* want to bake your noodle, consider this unsettling fact.

If you spray a large number of photons at a slit that is roughly the same width as the wavelength of those photons (use a laser to get a single wavelength), you will get what is known as a diffraction pattern. This is a common example of the wave property of light. It is often explained by noting that the light waves interfere with one another to create the pattern.

However, what happens if you repeat the experiment with a setup that allows only single photons through at a time?

Answer: You form exactly the same interference pattern as before.

Spooky, eh?

Read more about it here:
http://en.wikipedia.org/wiki/Double-slit_experiment#Quantum_version_of_experiment

Cheers - Mattus

 

[ernon]

Calculate the classical EM field generated from this. You can calculate an energy flux per unit area from the fields, and by integrating this over a surface surrounding your magnet (easiest to do by picking a sphere much larger than the area you're swinging your magnet around in), which will give you the power output. That part's purely classical. Now calculate the energy of a single photon with that frequency. Divide total classical energy by small quantum energy to get # of photons (should be a huge number)



Practically speaking? You pass it through a conducting coil and measure the induced current, which is proportional to the strength of the magnet.

Please forgive my ignorance, but if you are 'sending' photons they should be visible, as even single photons can be seen in a dark room. Yet swinging a magnet does not produce light. "Huge numbers" of photons should make quite a bit of light. Why the disparity?

 

[CurtC]

Please forgive my ignorance, but if you are 'sending' photons they should be visible, as even single photons can be seen in a dark room.

Your eyes see only photons within a fairly narrow frequency (wavelength) range, and the photons in this experiment would be way out of that range.

 

[Underemployed]

This example has always sounded bogus to me. Doesn't it just show that either

a) The equipment is not actually sending out a single photon

b) There is no such thing as a single photon to send

c) Single photons are being emitted, and can be split up into multiple ones#

If anything, why isn't it touted as conclusive proof that light is a wave? If you perform the experiment in 2D with water and watch 'single' waves propagate, you see the same patterns.

Edited to add: I'm talking about the double-slit experiment.

 

[jj]

Please forgive my ignorance, but if you are 'sending' photons they should be visible, as even single photons can be seen in a dark room. Yet swinging a magnet does not produce light. "Huge numbers" of photons should make quite a bit of light. Why the disparity?

Don't forget. Radio waves are photons. Lots of low-energy photons.

UV, Gamma, X-ray. All photons. Far IR is starting to get into "radio wave" range (well, almost)

 

[ernon]

This example has always sounded bogus to me. Doesn't it just show that either

a) The equipment is not actually sending out a single photon

b) There is no such thing as a single photon to send

c) Single photons are being emitted, and can be split up into multiple ones#

If anything, why isn't it touted as conclusive proof that light is a wave? If you perform the experiment in 2D with water and watch 'single' waves propagate, you see the same patterns.

Edited to add: I'm talking about the double-slit experiment.

I am going to attempt to attach a great clip of the double slit experiment recently conducted by Hitachi. They did it with electrons not photons, however, without going into detail, it is impossible for more than one electron to be present in their device at a time and the electron must pass on one side or the other of a biprisim. Yet after running for 1/2 hour distinct interference fringes can be seen.

(found the link --> http://www.hqrd.hitachi.co.jp/em/movie.cfm)

 

[ernon]

Don't forget. Radio waves are photons. Lots of low-energy photons.

UV, Gamma, X-ray. All photons. Far IR is starting to get into "radio wave" range (well, almost)

True, those are all photons, but are any of the ones you listed generated by swinging a magnet? Which type of photon does a swinging magnet generate?

 

[GodMark2]

If anything, why isn't it touted as conclusive proof that light is a wave?

It is, but that doesn't mean that photons are NOT particles, only that: at the level of QE, the diference of meaning between a particle and a wave becomes rather fuzzy.

 

[GodMark2]

True, those are all photons, but are any of the ones you listed generated by swinging a magnet? Which type of photon does a swinging magnet generate?

VLF radio waves.

 

[Art Vandelay]

This example has always sounded bogus to me. Doesn't it just show that either

a) The equipment is not actually sending out a single photon

b) There is no such thing as a single photon to send

c) Single photons are being emitted, and can be split up into multiple ones#

The photons aren’t being split into multiple photons, they’re being split into something, but those parts are not other photons.

True, those are all photons, but are any of the ones you listed generated by swinging a magnet? Which type of photon does a swinging magnet generate?

Radio waves, usually. Ultimately, photons come from accelerating charged particles. One way to do this is to swing a magnet. If you take a bunch of atoms and jostle them around a lot, that’s called “heat”. And if something is heated enough, it will glow with visible light. That light is from electrons jumping around between energy states.

GodMark2

It is, but that doesn't mean that photons are NOT particles, only that: at the level of QE, the diference of meaning between a particle and a wave becomes rather fuzzy.

It seems to me that the concept of a "particle" is actually an approximation, and there is no such thing as a "particle" as the term traditionally was used.