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kevin1981
2010-Jul-17, 07:31 PM
As i understand it, light is a small part of the electromagnetic spectrum. Packets of light are called photons. Light does not travel in 'waves', but because photons can be in several places at once we say light has a wave function. Is that correct so far?

What i would like to understand first is, what are the individual units of the radio wave ? And for gamma rays, microwaves ect..

I hope you see where i am coming from, thanks

Geo Kaplan
2010-Jul-17, 07:47 PM
As i understand it, light is a small part of the electromagnetic spectrum. Packets of light are called photons. Light does not travel in 'waves', but because photons can be in several places at once we say light has a wave function. Is that correct so far?

What i would like to understand first is, what are the individual units of the radio wave ? And for gamma rays, microwaves ect..

I hope you see where i am coming from, thanks

It's more useful (and correct) to say that visible light is part of the electromagnetic spectrum. Visible light, infrared, UV, x-rays, gamma rays, microwaves, and other radio waves are all light. They are distinguished solely by frequency or wavelength alone. Differing frequencies cause differing interactions with matter, but the laundry list of interactions is the same (diffraction, refraction, reflection, etc.) for all. Because these are all light, they can all be described in the language of quantum theory in terms of photons, or in classical wave terms.

kevin1981
2010-Jul-17, 08:05 PM
This is where i was heading. What i do not understand is, if they are 'particles' then how do they have different wavelengths? I understand that wavelengths are shorter and longer depending on which part of the spectrum we are talking about, but as i say, i thought they are 'particles' not 'waves'.

Sam5
2010-Jul-17, 08:39 PM
This is where i was heading. What i do not understand is, if they are 'particles' then how do they have different wavelengths? I understand that wavelengths are shorter and longer depending on which part of the spectrum we are talking about, but as i say, i thought they are 'particles' not 'waves'.

Electromagnetic waves can be waves but can act like a train of particles through their “bump, bump, bump” effect on what they hit, as one follows another in a wave train. One bump of a certain wavelength can be called a single “particle”. A radio engineer taught me this as part of radio antenna theory. A train of radio waves causes a radio antenna to resonate, if the antenna is the same length as the wavelength as the waves, or a certain fraction of a wavelength, such as 1/2, 1/4, 1/8, etc. So, radio waves do not have to be hard little physical “particles” to cause the “bump, bump, bump” effect. The same with visible light.

Geo Kaplan
2010-Jul-17, 09:11 PM
This is where i was heading. What i do not understand is, if they are 'particles' then how do they have different wavelengths? I understand that wavelengths are shorter and longer depending on which part of the spectrum we are talking about, but as i say, i thought they are 'particles' not 'waves'.

It turns out to be bit of a false dichotomy to say that something is either a particle or a wave. There is a third option that is often left out of the discussion, but which turns out to be the way nature is. Light (and, for that matter, matter) can act either as a classical particle or as a classical wave, depending on the particular experimental setup. Linguistically and historically, it's easiest to pick one descriptive metaphor for a given experiment. But light and matter are more subtle than that, and thus one must be careful not to insist on an answer to "Is it a particle or is it a wave?"

Here's a way to think about it, poor as it is: Suppose you had ever only known two languages (say, German and Latin). One day, you encounter English. Some words seem to be Germanic, others Romantic, and you wonder "Is it German or Latin?" The problem turns out to be in the question. The language is what it is. And so it is with light and matter.

Andrew D
2010-Jul-18, 12:02 AM
As Dr. Rocket said:


Learn from the master:

Richard Feynman Video - The Douglas Robb Memorial Lectures (http://www.vega.org.uk/video/subseries/8)

kevin1981
2010-Jul-18, 12:31 AM
The analogy of dropping a pebble in a pond and the waves traveling outwards is the way most people think of how radio waves travel, i suspect it is incorrect but i don't know.

If a radio station has a listening circumference of 20 miles, how do the radio waves travel ?

Cougar
2010-Jul-18, 01:01 AM
Light does not travel in 'waves'....

This would be incorrect. We naturally perceive the wave nature of visible light as red, blue, etc.


....but because photons can be in several places at once we say light has a wave function.

Um, you're mixing concepts, and you seem oblivious to the concept of wave-particle duality. But a long radio wave can go right around a stand-up mirror, for example, due to its physically long wavelength.

Hornblower
2010-Jul-18, 01:18 AM
The analogy of dropping a pebble in a pond and the waves traveling outwards is the way most people think of how radio waves travel, i suspect it is incorrect but i don't know.
It is not a bad analogy in my opinion. At radio frequencies the quantized particle character is virtually negligible, and a classical wave treatment works very well for practical radio engineering purposes.

At higher frequencies such as visible light and beyond, the photons have enough energy apiece to make their presence known individually, for example by exciting or ionizing atoms. The wave nature remains significant, as shown by diffraction patterns, refraction in a lens or prism, etc.


If a radio station has a listening circumference of 20 miles, how do the radio waves travel ?

For short-wave signals such as television and microwave, the curvature of the Earth becomes a barrier at relatively short distances from the transmitter. As we move away from the tower, it appears to drop below the horizon, and the radio signal is blocked in the same manner as the light from the marker light on the top.

Long-wave signals such as the AM band have some wraparound capability and can be received at longer distances. At night the ionosphere further stretches the range.

kevin1981
2010-Jul-18, 01:42 AM
This would be incorrect. We naturally perceive the wave nature of visible light as red, blue, etc.
I thought that was to do with wavelengths, not the way light travels.


Um, you're mixing concepts, and you seem oblivious to the concept of wave-particle duality. But a long radio wave can go right around a stand-up mirror, for example, due to its physically long wavelength.
I thought that particles having a wave like nature was just a way of describing the weird characteristics of quantum theory. I thought that particles have a wave like nature but that they do not actually travel in waves.

So when we make a measurement for light waves, they are traveling like the pond analogy i described?

But when we make a measurement for photons, we detect the discreet photons?

What we can not do is measure for both those phenomena at once.

Is any of this correct ! Thanks

Nereid
2010-Jul-18, 10:57 AM
I thought that was to do with wavelengths, not the way light travels.


I thought that particles having a wave like nature was just a way of describing the weird characteristics of quantum theory. I thought that particles have a wave like nature but that they do not actually travel in waves.

So when we make a measurement for light waves, they are traveling like the pond analogy i described?

But when we make a measurement for photons, we detect the discreet photons?

What we can not do is measure for both those phenomena at once.

Is any of this correct ! Thanks
Electromagnetic radiation, and associated phenomena, is explained with astonishing accuracy by a theory called quantum electrodynamic (QED). The theory is full of math, and a complete description of how light (etc) does its things is necessarily also full of math.

It turns out that there are experiments you can do in which light (and any EMR) behaves just like a classical wave (pebble in a pond analogy), and others in which it behaves just like a classical particle (tiny bullets analogy).

The particle-wave aspects can be seen in double-slit (http://abyss.uoregon.edu/~js/21st_century_science/lectures/lec13.html), or two-slit (http://www.colorado.edu/physics/2000/schroedinger/two-slit2.html) experiments, of which there are many variations.

In these experiments, over the long run, an interference patterns builds up on the screen. Yet the experiment can be done so that no more than one photon is in the apparatus at any given time! So how can the light (photons) know that it must produce an interference pattern (behaving as a wave) when it is clearly just photons?

It gets more weird.

You can ask 'which slit did any given individual photon go through?' and try to figure that out by selectively blocking a slit (there are variations of this experiment which are truly astonishing in the ingenuity with which they try to 'catch' the photon). Trouble is, every attempt to nail down the photons will fail - if you can't nail them down, then the two-slit interference pattern will appear; if you can, you get a single-slit pattern!

I've used this before, maybe you'll find it amusing.

Is light
a) a particle?
b) a wave?
c) both a particle and a wave?
d) neither a particle nor a wave?
e) all of the above?
f) none of the above?

You can also switch the last two ...

kevin1981
2010-Jul-18, 02:02 PM
Electromagnetic radiation, and associated phenomena, is explained with astonishing accuracy by a theory called quantum electrodynamics (QED). The theory is full of math, and a complete description of how light (etc) does its things is necessarily also full of math.
I have read a little about QED and know that Feynman got his nobel prize for working on it. I guess, even though i have these questions and i am very interested about how nature works, language can only get me so far. I do realize that mathematics is the real language of physics but unfortunately i am hopeless at maths so there's only a certain amount i can learn. It is a little frustrating but i guess thats just life !


It turns out that there are experiments you can do in which light (and any EMR) behaves just like a classical wave (pebble in a pond analogy), and others in which it behaves just like a classical particle (tiny bullets analogy).
So light does travel in 'waves', but again, to get a proper understanding i would have to know the mathematics.


In these experiments, over the long run, an interference patterns builds up on the screen. Yet the experiment can be done so that no more than one photon is in the apparatus at any given time! So how can the light (photons) know that it must produce an interference pattern (behaving as a wave) when it is clearly just photons?

This is where i presume that the photons do go through the slit one at a time, but we observe an interference patten which would make us think they are 'waves'. But i thought that they have 'wave' like characteristics and still go through one at a time. The reason they build up an interference patten is because of the probability that they will hit a certain place more than another. So the photons are not traveling like waves, they are single 'particles' and we don't know where they are until they hit the background. But the end result 'looks' as if they do travel like waves. And that is one of the reasons why it is so weird and counterintuitive. ?

When we try to measure where the photon is, we collapse the wave function. So it is not in a superposition of places now, it is in a single position. And that is the heisenberg uncertainty principle, in a nutshell.

In layman's terms is that an ok description of the double slit experiment ?

Also thanks for your answer Nereid.

Nereid
2010-Jul-18, 09:25 PM
kevin1981, the double-slit experiment, in all its variations, is a wonderful tool for exploring the weird, counter-intuitive wave/particle aspects of light.

And you can do that exploration with only relatively simple math, like understanding how an interference pattern results if you have a plane wave, two slits, and a screen some distance away.

The part I really enjoy, because it's so mind-twisting, is that the interference pattern emerges even when only one photon is in the apparatus at a time, and that there's no such pattern if you block one slit, even if only temporarily! That each photon is, indeed, like a bullet is unambiguous (each 'hit' on the screen can be clearly, and unambiguously, detected); yet the pattern which emerges is also, unambiguously, exactly like the pattern a plane wave will create. Further, each photon cannot possibly 'know' what any other photon did, which slit it went through, where it hit the screen, etc. So how does the collective behaviour of the photons arise?!? The math of QED explains it, no sweat, but how to turn that math into words which make even the tiniest amount of sense?

kevin1981
2010-Jul-19, 12:03 AM
Darn mathematics :exclaim:

kevin1981
2010-Jul-19, 12:38 AM
In post number 12, is my description of the double slit experiment ok in laymans terms?

astromark
2010-Jul-19, 01:18 AM
As Dr. Rocket said:

http://www.vega.org.uk/video/subseries/8

It does clear the way for good understanding.... As this question has been answered very well and in another post as well...

We have such a wealth of knowledge that finding what it is you ask is more of an issue than the question itself.

We have said that its All or none of the above.... :eh: from harmonics to signal wave heights to particles...

Do attempt to understand the good Doctor... He's good.

I think the word Photon is just a generalization for any energy detectable by any means of the Electro Magnetic Signature...

We do Not seem to know what it actually is... other than... It is. Good luck.

Nereid
2010-Jul-20, 02:10 PM
In post number 12, is my description of the double slit experiment ok in laymans terms?
Let's take a look ...


[...]

In these experiments, over the long run, an interference patterns builds up on the screen. Yet the experiment can be done so that no more than one photon is in the apparatus at any given time! So how can the light (photons) know that it must produce an interference pattern (behaving as a wave) when it is clearly just photons?

This is where i presume that the photons do go through the slit one at a time, but we observe an interference patten which would make us think they are 'waves'. But i thought that they have 'wave' like characteristics and still go through one at a time. The reason they build up an interference patten is because of the probability that they will hit a certain place more than another. So the photons are not traveling like waves, they are single 'particles' and we don't know where they are until they hit the background. But the end result 'looks' as if they do travel like waves. And that is one of the reasons why it is so weird and counterintuitive. ?

When we try to measure where the photon is, we collapse the wave function. So it is not in a superposition of places now, it is in a single position. And that is the heisenberg uncertainty principle, in a nutshell.

In layman's terms is that an ok description of the double slit experiment ?
Are you familiar with the thought experiment which goes by the name Schrödinger's Cat?

It's about collapsing wavefunctions and stuff.

While the photon is in the apparatus, before it hits the screen (detector), its wavefunction goes to infinity, and it is everywhere, simultaneously. Upon hitting the screen, the wavefunction collapses, and a 'hit' is recorded, at a specific location. The probability of it hitting that specific location is given by the wavefunction, and the probability distribution is exactly like the interference pattern, so, over time (repeated photons) the distribution of hits comes closer and closer to that pattern.

So, you can think of the photon, inside the apparatus before it hits the detector, as being a wave which interferes with itself - it really does 'go through' both slits! And it's only when the photon hits the detector does it stop behaving like a wave, and behaves like a particle.

BTW, this has nothing - directly - to do with the HUP.

Does that make things a little clearer?

kevin1981
2010-Jul-31, 06:24 PM
Hello all, my internet was playing up so i could not reply. Thanks everyone, great answers as always.

Hi Nereid, i have heard of the Schrödinger's Cat experiment, but have only read bits and bobs and i don't fully understand it. I had no idea it deals with wavelengths. Am i right in thinking that the HUP states that the more you no the momentum of a particle the less you know it's position and vice versa ?

So what i have learnt from this thread so far is that, electromagnetic radiation such as light and radio waves, do actually travel in waves. But when we make a detection the wave function collapses and we observe single photons

Also, all electromagnetic radiation is made out of the same thing, photons. But we only see light because it has a wavelength that allows us to see it.

Are the last two paragraphs correct? As i said, i am just trying to understand this sort of phenomena at a basic level, because i will remember what i have learnt.

Edit; oh, so does the Schrödinger's Cat experiment basically state that a particle is everywhere, until we make a detection? What does that have to do with the cat being dead or alive?

Ta.

Strange
2010-Jul-31, 07:01 PM
Am i right in thinking that the HUP states that the more you no the momentum of a particle the less you know it's position and vice versa ?

Exactly.


So what i have learnt from this thread so far is that, electromagnetic radiation such as light and radio waves, do actually travel in waves. But when we make a detection the wave function collapses and we observe single photons

That is one interpretation. Or, you could say that they (photons) are things that sometimes behave in a way that makes them appear like waves and sometimes behave in a way that makes them appear more like particles. But really they are neither; although they are discrete/quantized. (And when I say "behave" I guess I reallly mean, the results we get when we make measurements of them in particular ways).


Also, all electromagnetic radiation is made out of the same thing, photons. But we only see light because it has a wavelength that allows us to see it.

Yes.

(I'll leave someone else to comment on The Cat.)

Shaula
2010-Jul-31, 07:45 PM
The cat thing is very head spinning. It basically says that once the system is isolated (box is closed) the cat can exists as a superposition of all outcomes (dead and alive). It remains in this indeterminate state until a measurement is made at which point it 'choses' a state to be in probabalistically. Or in QM terms the wavefunction of the system exists as a superposition of all outcomes until a measurement collapses it into a well defined state.

Nereid
2010-Jul-31, 07:47 PM
kevin1981, I'll give you a more considered answer later, but for now this: don't confuse wavelength with wavefunction. If you'd like more on the difference, and, specifically what a wavefunction is (and is not), just holler! :)

neilzero
2010-Jul-31, 08:05 PM
My guess is we don't know exactly. Mainstream opinion is photons, and neutrinos are not particles, because they travel at the speed of light, but in some other respects behave sort of like particles.
We can use the term photon for individual units of radio waves, Xrays and gamma rays, but we rarely do as the equivelent to other forms of energy varies with frequency and wave length. Neil

kevin1981
2010-Jul-31, 08:20 PM
Or in QM terms the wavefunction of the system exists as a superposition of all outcomes until a measurement collapses it into a well defined state.

This is why i like to learn about the quantum world, because that is so bizzaire and weird. And to my human brain makes no sense at all, and yet as far as we can tell, at the fundamental level, that is how nature is !

I remember a quote that goes like this, "quantum mechanics is what happens when no one is looking." (I am sure you have heard that too)

Thanks Nereid, when you have time to write it, i would like to read about specifically what a wavefunction is (and is not), because as i have said, i find it interesting and i enjoy learning about nature.

To the best of my knowledge, i thought a wavefunction has something to do with the probability of where a particle will be, when we make an "observation"

And wavelength is to do with the peaks and troughs of different kinds of EMR. The shorter the wavelength the closer the peaks and troughs are, which also means they have more energy.

Strange
2010-Jul-31, 10:11 PM
Mainstream opinion is photons, and neutrinos are not particles, because they travel at the speed of light, but in some other respects behave sort of like particles.

I'm not sure about that. Feynman, for instance, (as mainstream as you can get) said very specifically that photons are particles; it's just sometimes they do things that we interpret as like waves. I don't know why travelling at the speed of light would mean they are not particles? Also, neutrinos seem very likely to have rest mass and so travel at something less than the speed of light. Photons are just as much particles (and waves) as are electrons or neutrons.


We can use the term photon for individual units of radio waves, Xrays and gamma rays, but we rarely do as the equivelent to other forms of energy varies with frequency and wave length.

The term photon is used for all wavelengths of electromagnetic radiation including radio and gamma wavelengths.

Delvo
2010-Aug-01, 02:37 AM
For the question about how photons can have wavelengths if they're particles, not waves: they can't and don't. The particle model is one thing, and the wave model is another, and you can't use elements of one model to describe another model. They do, however, both need to have some counterpart for each other's basic elements. Wave length is obviously a part of the wave model. So the question you really needed answered when you asked about particles having wave lengths is: What is the particle model's counterpart to the length of a wave in the wave model?... or: What distinguishes one photon (particle) from another in the same situations where wavelength distinguishes one wave from another? And the answer then is: energy level. A high-energy photon corresponds to a short-wavelength (high-frequency) wave, and a low-energy photon corresponds to a long-wavelength (low-frequency) wave. The particle doesn't have a wave length, it just has something else that plays the same role in the particle model that length plays in the wave model, and that is its energy level. Waves that we see as red have a longer wave than waves that we see as blue; photons that we see as red have lower energy than photons that we see as blue.

Similarly, the other basic dimension of a wave, its amplitude, also corresponds to something else with an analogous role in the particle model: number of particles in the group/beam/transmission. The particle model doesn't have waves so it can't have wave height any more than it can have wave length, just like the wave model doesn't have particles so it can't have numbers of particles or particle energy levels.

When two models exist for the same phenomenon, you need to be very aware of which ideas are part of which model and whether they can apply to the other one, or if not, then what else would take their place. Translating back and forth between the two might be simple in some cases, but you still have to do it and be aware of which one you're using at any given time. You can't just start talking about one in the other's context, just like you can't start talking about "x" & "y" on a polar/radial coordinate plane or distance & azimuth on a Cartesian/rectangular one.

Strange
2010-Aug-01, 09:13 AM
...

Nice explanation.

kevin1981
2010-Aug-01, 11:15 AM
Thanks Delvo, that has helped my confusion out loads, and it's not to technical that i do not understand any of what you wrote. I did'nt realize that there were different models for the wavelength nature of EMR and another for the particle nature. That makes a lot of sense and as i said, clears up a lot of confusion.

So basically, when we are talking about wavelengths, we are talking about a wave with a certain energy level. And you use the wave model to describe it.

The wave however, is made up of photons with certain energy levels. I did'nt know that photons had different energy levels. Is this right so far?

And obviously (now anyway!) when we want to describe what is happening with photons we use a different model.

So we have two models to describe the different aspects of wavelength and photons. Does the same go for all the other sub atomic particles, and even atoms as well?

Shaula
2010-Aug-01, 12:11 PM
Yup. Everything is both a wave and a particle. Both descriptions are idealised models of how things are and not how things really are. Photons are the quanta of the EM field which can be approximated as a light wave or a stream of particles. Both are just convenient, simple models we use to avoid having to solve the full on QM description of the system every time. A decent analogy is that you can describe all colours as mixtures of RGB (or CMKY for the picky!). That doesn't mean that yellow light really is at some fundamental level a mixture of red and green, just that you can describe it as such.

Not just atoms BTW I think they recently did the double slit experiement with Buckyballs (C60 molecules). Everything is both but the wavelength of the object decreases as its energy goes up. So a photon can act mostly wave like at the radio end of the spectrum and more particle like at the gamma end. To diffract somthing the slit has to be about the same size of its wavelength so a radio wave gets diffracted (a wave behaviour) through macroscopic gaps which it takes a sub-mocron gap to diffract light well. To diffract a cat you'd need a slit smaller than most nucleons IIRC!

Strange
2010-Aug-01, 12:15 PM
So basically, when we are talking about wavelengths, we are talking about a wave with a certain energy level. And you use the wave model to describe it.

It may be better to say: when we are talking about wavelengths, we could talk about a photon with a certain energy level. The difference is that the waveform view is a "continuous" thing (rather than quantized) so the wave can have have any frequency and any amplitude (i.e. energy level).


The wave however, is made up of photons with certain energy levels. I did'nt know that photons had different energy levels. Is this right so far?

I'm not sure that I would say the wave is "made up of" photons; that seems to be mixing the two different models. But we can alternatively view/model/consider the wave as a bunch of photons, where the energy of each individual photon corresponds to the wavelength, and the number of photons corresponds to the total energy (amplitude) of the wave.


So we have two models to describe the different aspects of wavelength and photons. Does the same go for all the other sub atomic particles, and even atoms as well?

Pretty much, yes.

kevin1981
2010-Aug-01, 01:31 PM
The difference is that the waveform view is a "continuous" thing (rather than quantized) so the wave can have any frequency and any amplitude (i.e. energy level).

So the wave/particle duality term is a little bit misleading, because to me at least, it sounds as if particles are one or the other. But to understand waves we use the wave model to describe what is going on and how EMR travels.

And to understand particles we use the particle model which has nothing to do with the nature of waves. So we have waves or particles, one or the other, never both at the same time. So it seems we are talking about two different types of phenomena.

So to understand how light travels we use the wavelength model but when we try to observe what is going on we see particles and have to use another model to describe what is happening.

Is there something underlying that connects the two. Or is this just another example of the strangeness of QM. Where, when light, electrons, atoms; are traveling we use wave mechanics to understand what is going on and it has nothing what so ever to do with particles.

But when we observe what is going on at these tiny distant scales we come into contact with particles and have to use a whole new model to describe what is going on and that has nothing to do with waves.






I'm not sure that I would say the wave is "made up of" photons; that seems to be mixing the two different models.
I actually thought that as i was writing it. But if you dont ask then you don't get an answer :)

Strange
2010-Aug-01, 01:47 PM
And to understand particles we use the particle model which has nothing to do with the nature of waves. So we have waves or particles, one or the other, never both at the same time. So it seems we are talking about two different types of phenomena.

Thats about right. The important word there is "seems".


So to understand how light travels we use the wavelength model but when we try to observe what is going on we see particles and have to use another model to describe what is happening.

Actually, everything that can be explained by the "old" wave model, can also be explained (well, mathematically, if not really "explained") using the photon model. However, many observations (e.g. photoelectric effect) can only really be explained by the photon model. So, in a sense, photons rule.


Is there something underlying that connects the two.

Good question. The creaking you can hear is a can of worms opening. I'm sure Ken G will have something to say about this.

I would say: yes, because why else would we see consistent behavior in the world. However what that underlying thing is, is probably unknowable in the sense that it isn't something we are familiar with in the everyday world: it isn't a particle and it isn't a wave. It is a photon which has characteristics of both, but also of neither. We may get a deeper understanding of what underlies QM at some point. Or maybe we never will....

kevin1981
2010-Aug-01, 04:16 PM
Are you familiar with the thought experiment which goes by the name Schrödinger's Cat?

It's about collapsing wavefunctions and stuff.

While the photon is in the apparatus, before it hits the screen (detector), its wavefunction goes to infinity, and it is everywhere, simultaneously. Upon hitting the screen, the wavefunction collapses, and a 'hit' is recorded, at a specific location. The probability of it hitting that specific location is given by the wavefunction, and the probability distribution is exactly like the interference pattern, so, over time (repeated photons) the distribution of hits comes closer and closer to that pattern.

So, you can think of the photon, inside the apparatus before it hits the detector, as being a wave which interferes with itself - it really does 'go through' both slits! And it's only when the photon hits the detector does it stop behaving like a wave, and behaves like a particle.

Does that make things a little clearer?

I am a little confused. If a photon is a particle, and not a wave, then how can it travel like a wave. What is a wavefunction ?

Delvo
2010-Aug-01, 08:14 PM
Is there something underlying that connects the twoThey are both ways of describing exactly the same thing. The behavior of electromagnetic phenomena is just one set of laws, not two, and is quite internally consistent and could even be described as simple (it doesn't take very many variables or formulas to fully describe any electromagnetic situation or event).

It's us humans who made it seem all weird and complicated by coming up with a couple of partially wrong and mutually contradictory models to describe it. (Silly us, always thinking in macroscopic scale just because that's usually been the most useful way to think in our daily lives!)

Shaula
2010-Aug-01, 08:37 PM
I am a little confused. If a photon is a particle, and not a wave, then how can it travel like a wave. What is a wavefunction ?
A photon is neither. Particle and wave are both classical description of objects, ones that came about because that is how the universe seemed to be divided up. A wavefunction is a Quantum Mechanical description of how a system or component of a system works. The wavefunction is not actually the probability of finding a particle somewhere, it is actually the wavefunction squared that gives this. There is no direct simple way to describe what waves in a wavefunction. It is simply a mathematical description of a system that can be operated on by quantum mechanical operators to give the expectation value (kind of like the average) of any measurement you make.

Again - a photon is a quantum mechanical object. Particle and wave are not distinguished in this theory. It is totally wrong and artificial to try to divide the world up into these two categories in QM terms. The photon is the quanta of the EM field. It is only because macroscopically there seems to be two categories of object, particle and wave, that we try to make the same distinction on a smaller scale where QM effects are more obvious.

Sorry there is no simple answer. Getting your head around this is basically the first six months of studying QM! I was lucky, I started reading stuff on this as a teenager so I never got into the whole particle and wave mindset.

Strange
2010-Aug-01, 08:40 PM
A photon is neither.

Exactly, you need to get over the "duality" idea (as promoted by popular science articles) and get to grips with the fact that a photon is just a photon.

Shaula
2010-Aug-01, 09:05 PM
I normally avoid analogies but I'll try for one. It is like someone only knows about birds and mice. What is a bat? Well it can fly so it is a bird. It has fur so it is a mouse. It eats bugs so it is a bird. It has ears, it is a mouse....

In both cases different observations give different answers and sooner or later you have to accept that it is neither and that more importantly it is not a hybrid of the two. It has attributes all of its own (e.g. Sonar, Quantum numbers like intrinsic spin).

kevin1981
2010-Aug-01, 10:58 PM
Ok, cheers for your answers. Shaula, when i said particle, what i really meant was, a single unit of quanta/energy/light. But to be fair, i did'nt ask that question very well at all. I will try to answer it myself.

We look at light waves and radio waves through one model and look at particles using another, right?

But, light waves and photons are different manifestations of the same thing, right?

If a single unit of quanta is fired at a detector, then it should act like a particle not a wave. But it only acts like a particle when we make a detection. But common sense says that, if we fire a single quanta then is should act like one and not a wave. But QM says that, if we fire a single quanta and do not make a detection then it acts/travels as a wave. Is that a correct statement?

If we made a detection before the particle hit the main detector, then would that be collapsing the wave function ?

Because it seems as the Schrödinger's Cat experiment and the double slit experiment are closely related ?

Shaula
2010-Aug-02, 12:21 AM
You cannot fire a single quanta with any great certainty. All you can do is reduce the emmission rate far enought that the probability of you getting one hit in a finite time is very high.

QM always says that the entities you are describing as particle and waves behave the same way. Quantum mechanically. We describe their interactions as particle like or wave like but they are descriptions, and bad ones at that. Making a measurement can let you determine some properties of the entity and make it behave more like a particle than a wave but that is not because it magically transforms it into one or the other. It is because you have destroyed the superposition of possible states that the wavefunction is related to. I am sorry this all sounds very complex but there is not really a simple way to put it that does not lead to more confusion.

If you measure the position of a photon before it hits a detector it does collapse the wavefunction. In the double slit experiment it also dectroys the fringes you normally see. But this is not because the entity we call a light wave/photon has turned from one to the other - it is just that we have affected the system in such a way that one or other of our models is more applicable. It may sound like logic chopping but you have to understand that quanta is not just the same thing as a particle.

The cat and the double slit are very closely related - they are both linked to the idea of a superposition of states. In some interpretations it is this superposition that is what interferes. The photon effectively interferes with itself as it is in a superposition of all possible paths through the slits. Making a measurement forces it to choose one and destroys this superposition, which destroys the fringes.

kevin1981
2010-Aug-02, 12:54 PM
So an atom for example, has properties we call wave like and particle like, but really they are just descriptions using language. But actually they are acting like neither, they are acting like atoms. In a closed system, an atom is everywhere at once due to its wavefunction.

It's not that complex, i know when you really want to understand QM properly with the mathematics then yes, it is ultra complex, mathematically. But i just want to be able to understand the basic concepts properly.

So does this mean that, all the atoms in my body are in superpositions of states until we make a detection?

Or even, are all elementary particles in superpositions of states until we make a detection/measurement?

Shaula
2010-Aug-02, 01:46 PM
First paragraph = bang on!

The superposition thing is tricky - observation in this case does not mean it is prodded by a physicist in their lab! If the system is isolated then it will enter a superposition - but as soon as it interacts with something else it collapses. That is the basic view of it. A slightly more complex view is called quantum decoherence. In essence this says that an isolated system of particles can enter into a coherent state (described by one wavefunction) but as soon as a thermodynamically irreversible interaction occurs (i.e. information leaks out of it through hitting something or being prodded by someone) it decoheres into a cluster of wavefunctions again which, to anyone on the outside, looks like it magically 'choosing' one possible state of the system. This is why large objects (like humans) never really show quantum behaviour. They are always interacting with the environment and so the coherence time (amount of time they are in superposition for) is essentially zero. Simpler, smaller systems interact less with the rest of tghe world so do appear to be in superpositions more.

Sorry if that is more confusing than helpful! QM is not tricky per se but it does require you to throw away a lot of 'common sense' and just accept that things do behave like that, no matter how much we may find it hard to believe.

kevin1981
2010-Aug-02, 02:20 PM
So the superposition of states only happens in a closed system at the atomic level, the micro world. To us humans in the macro world the superposition of states has decohered because particles are interacting with each other. This is why we see things nice and smooth because the wavefunction has collapsed and the particles are in one place/state?

When we talk about a closed system, does that mean it only happens when we create the closed system in a lab. Or are these tiny 'particles' in superpositions all the time everywhere but at some point decohere into the macro world that we observe ?

Shaula
2010-Aug-02, 07:55 PM
Yup - that is why we don't see 'Quantum spookiness' in our day to day life and why it is so hard to picture it!

Closed systems occur all the time - not just in the lab. The ones we create in the lab just tend to be cleaner (last longer). So you see quantum effects due to superposition happen but as the particle size goes up it becomes rarer. For example neutrino fluxes exist as a mixture of states most of the time as they very rarely interact with anything. Beams of atoms need to be chilled and in a vacuum to be put into a state of superposition reliably. Beams of tennis balls never behave quantumly.

I will add that I think everything I have said here is correct but to complicate things there are at least five ways to explain QM, all of which are equally valid. QM is a mathematical theory that predicts results (measurements). It is not really a natural language theory or something that is interested in what 'really' happens in words.

kevin1981
2010-Aug-03, 01:04 PM
So obviously there is a threshold were, the micro world becomes the macro world. But before reaching that threshold are all the particles in superpositions of all possible states; so could we say they are everywhere at once before decoherence ?

Shaula
2010-Aug-03, 01:26 PM
Not quite everywhere. They are bounded by a light cone or their own speed or energy. Superposition combines all possible states but not, for example, one in which a particle magically appears somewhere it cannot have gotten to in the time the system has been evolving from.

The thing to remember is that superposition is fragile. It is very, very easily broken and so systems are unlikely to remain coherent for long enough for their position to be that delocalised. Even for very small systems superposition is not their 'natural' state. We do see it but it is not a threshold below which everything is always in superposition.

kevin1981
2010-Aug-03, 03:09 PM
So quantum coherence does not last very long due to other interactions with other particles. So if a photon hit an electron would both there wavefunctions collapse. No more superposition ?

If that statement is right, then im guessing it would still happen without conscious observers.

Nereid
2010-Aug-03, 03:25 PM
So quantum coherence does not last very long due to other interactions with other particles. So if a photon hit an electron would both there wavefunctions collapse. No more superposition ?
Sorta.

As you've worded it, both "photon" and "electron" as not QM entities ("hit"). How an electron and a photon interact (not "hit") is extremely well described by QED, frighteningly well (to 12? 14? significant digits!). The details of the photon-electron interaction determine the nature, and duration, of any quantum coherence. And don't forget that it's quite difficult to create a situation in which only an electron and a photon exist, wrt interactions and coherence ...



If that statement is right, then im guessing it would still happen without conscious observers.
Decoherence certainly happens without conscious observers! However, handling this, in a consistent manner, is quite tricky.

Shaula
2010-Aug-03, 03:42 PM
You are chipping away at some of the deeper bits of QM now. I have an idea how they work but may not be right - just warning you.

As Nereid says it is all down to the sort of interaction between the photon and the electron. They could form a system of their own which is a superposition of all possible outcomes so long as the possible outcomes are thermodynamically reversible. If they are not then the system decoheres. This is because essentially what thermodynamically irreversible means is that information escaped the system. In essence this is what could be observed and therefore triggers decoherence/wavefunction collapse (these are both ways of describing the same phenomenon).

Sorry, you have kind of hit the level where there are no even nearly correct simple answers!

Boratssister
2010-Aug-03, 09:30 PM
Brilliant thread !its as weird as quantum entanglement. One question though -how do you generate single photons? As in the double slit experiment.

Hornblower
2010-Aug-03, 10:21 PM
Brilliant thread !its as weird as quantum entanglement. One question though -how do you generate single photons? As in the double slit experiment.

I would use a dim light shining through a pinhole in a long tunnel that can be blacked out. I will have to review my calculations, but I think I estimated that a star that is barely visible to the unaided eye puts just a few thousand photons per second through our eye pupils. I will try to get back later with some numbers.

Shaula
2010-Aug-04, 01:10 AM
Yup, you just turn down the intensity so that statistically there will only be one photon at a time in the system (or for safety less). You cannot say for sure that is the case (that'd involve measurements!) but on average it will be.

Hornblower
2010-Aug-04, 01:20 AM
With the transit time of light through the apparatus on the order of nanoseconds, I would describe the situation of a few thousand photons per second as one at a time.

Boratssister
2010-Aug-04, 05:30 PM
With the transit time of light through the apparatus on the order of nanoseconds, I would describe the situation of a few thousand photons per second as one at a time.

So the interference patern could be photons bouncing of the side of the slits? forgive me I need to do more reading.

Shaula
2010-Aug-04, 06:41 PM
So the interference patern could be photons bouncing of the side of the slits? forgive me I need to do more reading.
Nope. The fringes are a wave phenomenon. Not due to any sort of particle dynanics. You can throw tennis balls at a pair of gaps in the wall and they will never produce nice neat fringes. You can broaden the slits in the light based experiment and the fringes vanish because they are not acting as 'point sources' for the light any more.

Boratssister
2010-Aug-04, 09:39 PM
Nope. The fringes are a wave phenomenon. Not due to any sort of particle dynanics. You can throw tennis balls at a pair of gaps in the wall and they will never produce nice neat fringes. You can broaden the slits in the light based experiment and the fringes vanish because they are not acting as 'point sources' for the light any more.

if I throw thousands of tennis balls at two slits just wide enough for the balls to fit through then I would not expect all the balls to hit the same spot, I would expect to see a pattern developing though.
My reason for posting on this thread is that its been claimed the double split experiment works with just one photon in the apparatus and I don't understand how you isolate a photon.

Also with the schrodingers cat thought experiment - why can't the cat be dead if a particle triggers the poison and if not it stays alive. Being alive and dead just seems silly. Of course you can't tell until you open the box which is just common sense.

Nereid
2010-Aug-04, 10:19 PM
if I throw thousands of tennis balls at two slits just wide enough for the balls to fit through then I would not expect all the balls to hit the same spot, I would expect to see a pattern developing though.
They do develop a pattern ... just not a wave interference pattern!

There are quite a few simulations/animations of this experiment, on the web, some of them quite good. Have you seen/used any? If not, I will dig up a few (unless someone else beats me to it, which happens a lot these days).


My reason for posting on this thread is that its been claimed the double split experiment works with just one photon in the apparatus and I don't understand how you isolate a photon.
Do you understand now?

Doing the double slit experiment with just one electron/photon/sodium atom/etc in the apparatus at any given time is entirely feasible, if somewhat tricky. However, I wouldn't be the least bit surprised to learn that a version of this 'single photon' experiment is routinely done in science labs in high schools throughout the world.


Also with the schrodingers cat thought experiment - why can't the cat be dead if a particle triggers the poison and if not it stays alive. Being alive and dead just seems silly. Of course you can't tell until you open the box which is just common sense.
Unfortunately, in quantum physics common sense is frequently wrong, or at best misleading.

Schrödinger's cat is a thought experiment which illustrates things like superposition of states, decoherence, the role of the observer, and so on. At the microscopic level - single atoms, say - "Schrödinger's cat" has lived, died, and been half-dead and half-alive millions of times, in thousands of experiments. Indeed, IIRC, there was, some years ago, a macroscopic example (it involved a tiny circuit, perhaps 1 mm in size, very close to absolute zero, rather than a cat). At one level, Schrödinger's cat is the basis for quantum computers ... and these are very real, if not yet very powerful.

Boratssister
2010-Aug-05, 01:15 AM
They do develop a pattern ... just not a wave interference pattern!

There are quite a few simulations/animations of this experiment, on the web, some of them quite good. Have you seen/used any? If not, I will dig up a few (unless someone else beats me to it, which happens a lot these days).


Do you understand now?

Doing the double slit experiment with just one electron/photon/sodium atom/etc in the apparatus at any given time is entirely feasible, if somewhat tricky. However, I wouldn't be the least bit surprised to learn that a version of this 'single photon' experiment is routinely done in science labs in high schools throughout the world.


Unfortunately, in quantum physics common sense is frequently wrong, or at best misleading.

Schrödinger's cat is a thought experiment which illustrates things like superposition of states, decoherence, the role of the observer, and so on. At the microscopic level - single atoms, say - "Schrödinger's cat" has lived, died, and been half-dead and half-alive millions of times, in thousands of experiments. Indeed, IIRC, there was, some years ago, a macroscopic example (it involved a tiny circuit, perhaps 1 mm in size, very close to absolute zero, rather than a cat). At one level, Schrödinger's cat is the basis for quantum computers ... and these are very real, if not yet very powerful.

having looked at the wikipedia site about the double split experiment and generating single photons , it states that true single photon experiments didn't occur until 1986 and this was something to do with Fock states....... Clicking on the link to Fock states I soon realised that I shouldn't have.......
So the answer to your question Nereid is , NO*I*STILL*DO*NOT*UNDERSTAND
and I don't like it!!!!!

Shaula
2010-Aug-05, 02:49 AM
Then ignore the Fock state bit. It is merely a note that to get a true single photon source you technically need an emitter that can only emit one photon at a time and do so on demand. See this paper (http://www.rle.mit.edu/eap/other/single_photon_source_article.pdf). Otherwise while the mean numer of photons per pulse may be one the number of photons in each pulse can vary a fair bit. And they could be in a superposition of states such that they have mixtures of one, two or three photons per pulse. Hence the need for a Fock state emitter.

Boratssister
2010-Aug-05, 02:10 PM
Then ignore the Fock state bit. It is merely a note that to get a true single photon source you technically need an emitter that can only emit one photon at a time and do so on demand. See this paper (http://www.rle.mit.edu/eap/other/single_photon_source_article.pdf). Otherwise while the mean numer of photons per pulse may be one the number of photons in each pulse can vary a fair bit. And they could be in a superposition of states such that they have mixtures of one, two or three photons per pulse. Hence the need for a Fock state emitter.

Thankyou Shuala,that link sheds light on the subject.
Single photon generation is pretty complicated stuff and should allow for quantum teleportation and quantum computing. One of the main applications will be secure communication. A good read indeed.......
Also I have completely forgot about that thingy state, thanks again.

Staticman
2010-Aug-05, 03:03 PM
Another angle that I think deserves to be explored with respect to the 2-slit experiment is regarding the photogrphic film (in the old experiments) or the electron detector where either photons or electrons interact, first of all even if you actually manage to send one photon at a time, which as it's been discussed is really hard, certainly every dot in the screen requires more than a photon. For the film probably thousands photons are needed to to make the silver cystals react, for an electronic detector certainly much less (photomultipliers do just that, multiply the light source) photons are needed but I'd bet more than one photon is needed to make a dot shine. So it is always prudent to take all those claims of "one single photon-one dot in the screen" or "one photon interacts with itself and seems to know where the previous photon went" of popular science accounts with a grain of salt..
But what I am more curious about is the role the detector screen plays, if any, in forming the interference pattern, we know matter is discrete and made of atoms constantly oscillating and with electrons changing of state in a random manner, I wonder how much of the pattern might be due to the way matter interacts with waves and how much to the way photons themselves behave.

Shaula
2010-Aug-05, 04:20 PM
Individual photons can affect photographic plates - it only takes one photon to produce a silver atom. The trick is to integrate for long enough that the pattern becomes visible.

Google Single photon avalanche diodes for just one way to detect single photons.

Given that the same patterns seems to appear no matter what detector you use and that you can destroy the pattern by taking a measurement of the photon my money is on it being a property of the photons, not their interaction with the detector. Add to that the fact that we have a very powerful theory that predicts that pattern, and that we also see it with electrons, atoms and molecules and I see no support for the idea that it is somthing to do with the detector.

Strange
2010-Aug-05, 04:34 PM
for an electronic detector certainly much less (photomultipliers do just that, multiply the light source) photons are needed but I'd bet more than one photon is needed to make a dot shine.

Photomultipliers can detect single photons. This uses the photoelectric effect (for which A Einstein got his Nobel) and a cascade of secondary emitters to multiply the signal.


But what I am more curious about is the role the detector screen plays, if any, in forming the interference pattern, we know matter is discrete and made of atoms constantly oscillating and with electrons changing of state in a random manner, I wonder how much of the pattern might be due to the way matter interacts with waves and how much to the way photons themselves behave.

Good question. However, if was due to the characteristics of the detector, then you would expect to see differences depending on the detection material. But photons, however they are detected have the same wavelength-energy relationship. And however you do the double slit experiment, the interference pattern is determined by the wavelength of the source and the difference in path lengths.

ETA: I see Shaula has said the same. (So it must be true!)

kevin1981
2010-Aug-05, 11:43 PM
Thanks for everyones answers in this thread. I am just going to go over what i have learnt as talking about them will help me remember, though i don't think i will forget. I have already noticed a difference reading past comments as i understand them better now.

I knew photons were discreet units of energy and massless- hence being a constant speed and nothing can travel faster.

What i did not know was, photons have different energy levels. Photons are the electromagnetic quanta of the electromagnetic field. Radio photons are the same thing as light photons, fundamentally, but have different energy values- hence we can see light but not radio waves. Also, this is if you are looking at them through the particle model of QM.

Radio and light can also travel in "continuous" waves and be looked at using the wave model of QM. The two are basically describing the same phenomena but from different perspectives, this goes for all types of EMR too.

Quantum coherence and decoherence is also something i did not understand before this thread. I like to think i at least understand the basics now. Quantum coherence is where a system is closed and has not been interfered with, so a quanta has a wavefunction and is in all possible states at the same time. This is called superposition.

Decoherence is when the wavefuction collapses, decoherence and wavefunction collapse are basically the same thing. When a system decoheres, due to information leaking or interactions with other phenomena, the particle/wave/quanta appears to be in just one place/state.

I do have some more questions, but would like to see if what i have wrote(written?) above makes sense first.

Shaula
2010-Aug-06, 05:15 AM
Looks like a pretty good summary. Either you've done this before or you are a quick learner! I don't mean that in a patronising sense - wish I could still pick up new stuff quickly.

kevin1981
2010-Aug-06, 10:36 AM
It's probably a bit of both i think, it's just a shame i can't apply myself the same way when it comes to maths. Because i am REALLY bad !

When we say, information leaks from a closed system, what does it mean. Even though i wrote it, i don't understand.

Strange
2010-Aug-06, 10:50 AM
Good summary. I think the most important (fundamental) part is the relationship between photon energy and wavelength.

Just being picky:

Radio and light can also travel in "continuous" waves and be looked at using the wave model of QM.

I would say: Radio and light can also be considered to travel in "continuous" waves ...

kevin1981
2010-Aug-06, 11:04 AM
In my head, i see radio waves traveling from there source the same way water waves do from a pebble using the pond analogy. Are they not actually continuous?

Strange
2010-Aug-06, 11:15 AM
In my head, i see radio waves traveling from there source the same way water waves do from a pebble using the pond analogy. Are they not actually continuous?

Well, you can treat them as if they are (classical electromagentic theory) in most cases. But "really" they are quantised as photons. But this is more a matter of interpretation than "fact" so stick with the model that works for you (at least for the time being). I don't want to introduce more confusion or go round the same loop again when you have just got it straight in your head!

Shaula
2010-Aug-06, 01:49 PM
When we say, information leaks from a closed system, what does it mean. Even though i wrote it, i don't understand.
The answer to that question is probably more complex than I am going to make it sound (because the only explanations I have are ones I have trimmed to fit into my head!). There is speculation that underlying a lot of quantum theory is some form of information theory, whether is is just an interpretation of QM or something deeper I don't think has been answered.

That caveat made... Any interaction with a system that allows some inferences to be made about the state of the quantum system essentially leaks information. If your detector flashes you know your electron hit it. So instead of a superposition of all possible wavefunctions it can now only be in those where the electron was in a certain place at a certain time. Through its interaction you have essentially 'forced' the electron to throw away all those other potential states and 'pick one'. These are bad ways of describing it, making it sound like the electron has free will, but you get what I mean, I hope. Information leakage is basically that - going from a system where you know nothing about its state to one where you know something.

Strange
2010-Aug-06, 01:58 PM
Information leakage is basically that - going from a system where you know nothing about its state to one where you know something.

And, just to be explicit, in order for that to happen, the internal state will have chnaged as a result. So what you know isn't necessarily what the state was before.

kevin1981
2010-Aug-06, 02:27 PM
Ok thanks, it sounds like another example of Schrödinger's cat to me. If the system is closed and has'nt been interfered with then the quanta is in all possible states and we have no clue to what is going on, information leaking affects that state and changes it.

Shaula
2010-Aug-06, 03:35 PM
In a nutshell, yes. That is what is actually meant by something being observed. Too often the observations thing is used as a sort of proof that intelligence or conciousness is something magically important to the way the quantum universe works. That nothing happends without a mind to observe it. They miss out the fact that anything capable of interacting in a thermodynamically irreversible way with an isolated system is techncailly an observer!

kevin1981
2010-Aug-06, 03:55 PM
I would like to try and understand the Electromagnetic field a bit better now. I thought that the EM field was distributed all through space, and particles excited it.

But i also read that, particles with positive charges create there own EM field, and thats where the EM field comes from, moving positively charged particles/quanta ?

If someone could help me understand the EM field then i feel i would be catching up on Ken G :D

Shaula
2010-Aug-06, 09:19 PM
Have a read through the Wikipedia page (http://en.wikipedia.org/wiki/Quantum_field_theory) on quantum fields (skipping the maths!) - it seems to explain fairly well what QFT is. Shout if you get questions from it. I'll try to post more fully later.

Delvo
2010-Aug-06, 09:44 PM
A field is simply a part of space where one thing (the field's source or generator) can influence other things because they're close enough. An asteroid drifting through space enters a planet's gravitational field when it gets close enough for the planet's gravity to affect the asteroid's movement. It's the same for electromagnetism: Object C enters object D's field when it's close enough for object C's movement to be affected by object D's charge(s). This is the case regardless of whether the force is carried by particles, waves, distortions in space, or tiny magical spirits: if you're close enough for it to affect you, you're in its field, so the field is wherever you'd be close enough for it to affect you. In particle theory, that means a field is where you're close enough to get hit by particles. In wave theory, that means a field is where you're close enough to get hit by waves.

In a technical mathematical sense, there's no upper limit on interaction distance, which would make all fields infinite, but in real life, we can often find some way to identify a threshold or field boundary for ourselves, because we don't care about effects that are really really really small and can simply be treated as zero.

One thing making electromagnetic fields slightly more complicated than gravitational ones is that particles' electrical charges, unlike their masses, include two different factors instead of just one. For gravity, the only question you need to answer about a particle is "how massive is it?"; for electromagnetism, the two questions are "how much positive/negative charge does it have, and which way is it 'spinning'?". And both of those two aspects of charge have separate effects on how charged particles interact; while charge magnitudes cause the particles to be attracted or repelled straight toward or away from each other, the spins cause them to twist around to align their spins in parallel, which tends to push or pull the particles off to one side or the other in a torque-like effect. That leads to an electromagnetic field having components that might sometimes seem like two separate things, an electrical one (direct attraction/repulsion) and a magnetic one (torque 90° off from the attraction-repulsion axis). But they're really the same field; it just looks electrical when you look at it in one direction and looks magnetic when you look at it in another direction. And the same field can be pretty strong in one component but pretty weak in the other, so we might call it only an electrical or magnetic field, not because the two can ever really be separate, but because the "other" one is just not powerful enough to pay attention to in the given case. But either way, the field is still just the region of space in which the given effect (electricity or magnetism) can happen because things in that region of space are close enough to be affected.

kevin1981
2010-Aug-06, 11:42 PM
Let me start again. Simply, where does the EM field come from ?

Delvo
2010-Aug-06, 11:45 PM
From charged particles sending out EM waves or photons which interact with other charged particles.

kevin1981
2010-Aug-06, 11:56 PM
What i am trying to understand is, how do photons travel from the sun through the vacuum of space. Do the photons interact with each other creating a field to travel through? Or is the EM field already in space waiting to be excited by the photons. I think the former is closer, but i do not know

Len Moran
2010-Aug-07, 05:37 AM
What i am trying to understand is, how do photons travel from the sun through the vacuum of space. Do the photons interact with each other creating a field to travel through? Or is the EM field already in space waiting to be excited by the photons. I think the former is closer, but i do not know

Take measurements and observations very seriously - that is empirical reality, but take the bit that lay between and beyond direct measurements less seriously.

Shaula
2010-Aug-07, 07:51 AM
Photons are the particles we observe when we interact with a field. The field is fundamental but it is not generated by the photons and not already there. I simply do not know enough about QFT to give you a correct, coherent answer on this. My vague ideas are that the motion of charges generated the time varying field which propagates. We never actually interact directly with the field but with the quanta of it that we call photons. How this works in detail I do not know. I am more at home thinking in terms of forces and potentials - static fields rather than oscillating propagating ones. The maths probably gets too tricky for my little brain at this point!

Sorry about that but I'd rather admit ignorance than fill your head with confused babbling.

kevin1981
2010-Aug-07, 09:59 AM
Ahh no worries ! I have enjoyed reading and learning from this thread, but i guess each answer always has another question and at some point they are going to get harder to understand. End of thread for me i think. hahaha :)

mugaliens
2010-Aug-08, 05:51 AM
Let me start again. Simply, where does the EM field come from ?

It's a combined field comprised of both electrostatic and magnetic fields. One can have either a static electric field, or a static magnetic field, but when you start combining the two, the only way to do that is by means of moving electric charges, and as soon as you start moving charges, particularly with varrying rates of motion (acceleration), you wind up with combined EM fields.

Visible light is an EM field. So are x-rays, IR, UV, gamma, ham radio...

"Where does the EM field come from?" It comes from moving electric charges...

kevin1981
2010-Aug-10, 06:25 PM
"Where does the EM field come from?" It comes from moving electric charges...

Ok cheers for that. I will get back to this at some point. I only have one more question to ask.(i think!)

I would like to learn about 'locality' and 'non locality', as it has come up in another thread, but i would like to keep QM type questions here. Basically is locality the fact that we can observe things in one place or another in the macroworld? Because of 'space' ?

And is 'non locality' the fact that we can not do this in the microworld because there is no space and 'everything' is in a coherent state?

So we only experience 'locality' through decoherence ?
Thanks

Shaula
2010-Aug-10, 06:54 PM
The principle of locality simply says that only local things can affect an object. Non-locality implies that a system can comprise widely separated parts that can still have an effect on each other instantly. Note that this is not in violation of the light speed limit as no information is transferred by soemthing like decoherence/wavefunction collapse. Simply put the principle is something like "If A and B are spatially separated then they cannot affect each other instantaneously".

Most interpretstions of QM imply that decoherence breaks the principle of locality, entangled systems collapse together no matter how far apart the components are. This is a philosphical headache but so far no experiements have shown that it actually causes and theory breaking results. We just have to accept that QM describes what we measure and that the interpretation we have of 'how' it does this breaks the principle of locality.

kevin1981
2010-Aug-10, 07:52 PM
Is non locality, basically, action at a distance. So Entanglement happens in a cohered state. The reason being QM does not treat time in the same manner as we think of it classically?

Locality states that we can affect something, but not spontaneously. The quickest way we can affect something is by light speed?

Shaula
2010-Aug-10, 09:07 PM
I'd say QM doesn't treat objects as we would classically. We see two electrons, a long way apart and we assume that they are and have always been separate objects. QM says that they can have been part of the same system (until you measured their positions and wrecked their entanglement). QM has little to say about time AIUI, relativity is hotter on that.

Locality says two separated objects cannot affect each other faster than the speed of light, yes. But it is more than that. It states that there cannot be a connection, something done to one cannot affect the other unless a signal propagates. That is the underlying concept - QM doesn't assume one electron signals the other to say "Hey! A physicist just prodded me, better be spin up to conserve momentum or the universe will get upset". Both electrons are part of a spatially distributed system affected in total by your actions on it.

So in one way non-locality is action at a distance, or to put it another way it is a statement that spatially separate mesurements can be correlated.

kevin1981
2010-Aug-15, 11:59 AM
Hi Shaula, i have another question that i have been thinking about for a few days. When a 'particle' is in a closed system and we are not doing measurements the 'particle' is in a superposition of all possible states. But how do we know that, if we are not 'looking' at it. What experiments are done to show the 'weirdness' of QM.

I know of the double slit experiment, so i am guessing that, we can't say where the 'particle' will hit the detector. But there is some sort of probability factor involved.

When does probability start to enter QM? I don't now much about it at all but i do know probability is important in QM. Go easy on me though ! Ta :)

Shaula
2010-Aug-15, 03:10 PM
The probability is fairly easy to give a simplified explanation for. Basically when a wavefunction collapses it does so probabalistically. If you take a system and make a measurement then somehow put it back in its original state (or more easily of you do the same measurement on lots of identical systems) then you are not guaranteed to get the same answer. Instead you get a range of values with an expectation value (mostly likely result) equal to what the measurement 'should' be. Reality is fuzzy on a small enough scale...

Some superposition experiments I Googled:
Resonating bar that is moving and not moving at the same time (http://www.newscientist.com/article/dn18669-first-quantum-effects-seen-in-visible-object.html)
Quantum computing (http://www.newscientist.com/article/dn17736-codebreaking-quantum-algorithm-run-on-a-silicon-chip.html)

Sorry I am not a huge experimentalist! I know they do a lot of photon entanglement experiments but the details of how they prove superposition are hairy to say the least. All to do with correlated measurements.

kevin1981
2010-Aug-15, 03:37 PM
So basically when we set up identical experiments for firing a photon at a detector, classically we would expect the same result each time.

But in QM, even though the experiment is identical, the outcome is different due to it being probabilistic.(not deterministic)

Shaula
2010-Aug-15, 03:46 PM
So basically when we set up identical experiments for firing a photon at a detector, classically we would expect the same result each time.
But in QM, even though the experiment is identical, the outcome is different due to it being probabilistic.(not deterministic)
Nail, head! You are quite right and that disconcerted a lot of people to start with.

kevin1981
2010-Aug-15, 04:46 PM
Cheers, though i knew about probability's in QM, i could'nt think of a definition of one. But the identical experiment with different results is perfect, as it's not too complicated to remember or even explain to other people.

Yes, i know a little about Einstein's day when people thought everything could be explained by cause and effect and then came along the counterintuitive world of QM which some people, mainly physicist's i would imagine, could'nt get there head around.

nokton
2010-Aug-15, 06:19 PM
As i understand it, light is a small part of the electromagnetic spectrum. Packets of light are called photons. Light does not travel in 'waves', but because photons can be in several places at once we say light has a wave function. Is that correct so far?

What i would like to understand first is, what are the individual units of the radio wave ? And for gamma rays, microwaves ect..

I hope you see where i am coming from, thanks
Yes Kevin, but you are mislead, light can be a wave at one time, and a discrete particle at the same time,
consider this Kevin, many texts tell us that light slows down when passing thro glass, then resumes its speed when free
of it, truth is, light passing thro glass does not slow down, but, if you will, has to cope with the many reflections within
the glass which make its passing longer, am unaware of the properties of light described by quantum theory, please
enlighten me.
Nokton

kevin1981
2010-Aug-15, 06:50 PM
Yes Kevin, but you are mislead, light can be a wave at one time, and a discrete particle at the same time

Yes i am aware of that now, due to this thread. As i understand it, a photon is a photon so it behaves like a photon. It does not behave like a "wave" or a "particle" but it has certain properties that we call 'wave' like or 'particle' like, depending on how we measure the EM radiation.

Though thanks for pointing out the light passing through the window never slows down. It's a nice reminder of light being a constant speed. Though photons are constant, they do have different energy levels which give rise to what part of the EM spectrum they belong too. Again, i have learnt that due to this thread.

Shaula
2010-Aug-15, 08:35 PM
The extreme example of this being the Sun. Energy is created in the core and takes ten thousand to a hundred thousand years to make it to the surface... Then eight minutes to get to us! The photons travel at c all the time but are absorbed, re-emitted and generally messed about by matter until the hard gamma rays from the fusion process are smeared out (thermalised) to the relatibely begign spectrum we see at the top of the atmosphere.

caveman1917
2010-Aug-15, 08:49 PM
Radio and light can also travel in "continuous" waves and be looked at using the wave model of QM.


I would say: Radio and light can also be considered to travel in "continuous" waves ...

I think this is a misconception, in the sense that radiation can only be considered to travel in continuous waves.
It is only at times when we interact with the photon that it will localize and give a particle-like nature. The time it spends travelling between those interactions it acts solely as a wave.

This is how i visualize it anyway - travel as a wave, interact as a particle.

ETA: 'interaction' would also apply when it doesn't interact but could have. Consider one leg of a standard interferometry setup where we put a detector. If we don't detect the photon (no interaction) we still interacted with the wave passing by - only that it got localized somewhere else. So it will behave as a particle in the other leg of the setup - we interacted with it without 'interacting' with it.

caveman1917
2010-Aug-15, 08:53 PM
Though photons are constant, they do have different energy levels which give rise to what part of the EM spectrum they belong too. Again, i have learnt that due to this thread.

Technically a photon doesn't have any energy level. The energy ascribed to a photon is purely observer dependant. One observer will say it's a radio-photon while another will say (about the same photon) that it's a gamma-photon, both are correct. There is no inherent energy in a photon.

kevin1981
2010-Aug-15, 11:18 PM
What about the rainbow then? The different colors are because the photons have different wavelengths. Or quantitatively because the photons have different energy values.

caveman1917
2010-Aug-16, 07:39 AM
The different colours are because we ascribe different frequencies to the photons, and by "we" one can take humans in general - we're basically all in the same reference frame, so we'll all see the same rainbow.

Consider a racecar passing by at very high speed, he will not see the same rainbow. It is the doppler effect that interferes.

For example, we see the photons of some star in Andromeda as blue (somewhat blueshifted at least) due to the relative speed of Andromeda vs us. We'll say the photons have high energy.
However someone on a planet next to that star will see the same photons as red (or somewhat less blueshifted). He'll say the photons have low energy.
Both are correct. It depends on the relative speed of the observer vs the object that emitted the photons.

Frequency is not defined relative to the photon itself (inherent in the photon) because there is no valid FoR for the photon. Every observer will ascribe his own frequency to the same photon, and all are correct. It is observer dependent.