[Jacques]

Photon are considered a wave in some case like interference and as a particule in other case like the photo-electric effect.
I imagine a wave of light leaving from a distant star like a sphere expanding in all directions. When that wave hit a photo detector or a CCD the behavior of that wave is like a particule. Does it mean that all the energy spread out on the sphere having many lights years of diameter is instanly concentrated on one pixel of my CCD ???
Is my image of a wave bad ???
Thanks

[mezarashi]

No, the sphere of light does not "instantly concentrate" on your one pixel. It obeys classical power dissipation laws as with any other wave. It is difficult to give light a universal visualization.

The way I like to look at it is to imagine it to be "localized possibility". I consider them to be more particle than wave. They are called photons after all. As for their wave and quantum properties, imagine a small probability wave function. Each little wave function is like a photon of light. Because the wave smears out at the edges, it is never certain where exactly this photon of light is. It also interacts with itself (or possibly the electromagnetic field) in such a way that it behaves like a wave around corners, tight spaces, or when changing mediums (refraction, diffraction).

[ydoaPs]

i think the only wave in light is that it is an EM wave. i THINK it doesn't have a probability wave because a photon's momentum is different enough that the HUP doesn't apply to photons in the sense of p vs x.

[Tom Mattson]

About this whole "wave-particle duality": It doesn't exist.

That expression is just a way to attempt to make quantum theory understandable to laymen, in terms of familiar concepts. If you look at QM or QFT you do not see a dual treatment of either particles or photons. You see a single, unified treatment of them.

[Jacques]

My brother told me to visualize the photon like a packet of energy, some kind vibrating point moving at the speed of light. Is it a better analogy ?
It is a vibration that doesn`t disperse like the spherical model of electro-magnetics waves . I think that model can be used to calculate the collective behavior of photonssss.
I am trying to understand expressions like "the wave function colapse" something like this...
Thanks for your answers

[swansont]

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I am trying to understand expressions like "the wave function colapse" something like this...

There's a thread on that, cryptically called Collapse the wave function

[Cashio]

it is a good question jacques, don't believe any one because whatever they say, they don't know! :)

this is one the major problems with quantum mechanics, it has a more famous form as "the cat of shredinger". it goes like this - if you close a cat in box it will eventually die. the question is when. the problem is that to know that, we have to open the box. saddly, acording to quantum mechanics this is the moment the "wave function" of the cat collapses and therefore it is the only moment we can conclude whether the cat is alive of dead. it is logical, but we also know as a fact that the cat will die eventually and this will have nothing to do with us opening the box. so the answer is - we don't know when the cat will die - and in relation to your question - we don't know where the photon is, or what it's "wave radius". we could only assume. and acording to quantum mechanics, it could be anywhere in the universe.

[reverse]

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it is a good question jacques, don't believe any one because whatever they say, they don't know! :)
and acording to quantum mechanics, it could be anywhere in the universe.

and no where else? :D

[Severian]

Cashio said:

this is one the major problems with quantum mechanics, it has a more famous form as "the cat of shredinger". it goes like this - if you close a cat in box it will eventually die. the question is when. the problem is that to know that, we have to open the box. saddly, acording to quantum mechanics this is the moment the "wave function" of the cat collapses and therefore it is the only moment we can conclude whether the cat is alive of dead. it is logical, but we also know as a fact that the cat will die eventually and this will have nothing to do with us opening the box. so the answer is - we don't know when the cat will die - and in relation to your question - we don't know where the photon is, or what it's "wave radius". we could only assume. and acording to quantum mechanics, it could be anywhere in the universe.

I have never understood why this is a problem. If we hypothesise that the cat can collapse wavefunctions then the cat is always either dead or alive (ie. in an 'alive eigenstate' or a dead one). If we hypothesise that it cannot, it is in a superposition. Where is the problem? It isn't even a physically well defined issue, since we cannot experimentally distinguish the two cases (unless we are a cat).

[Saint]

Severian said:

I have never understood why this is a problem. If we hypothesise that the cat can collapse wavefunctions then the cat is always either dead or alive (ie. in an 'alive eigenstate' or a dead one). If we hypothesise that it cannot, it is in a superposition. Where is the problem? It isn't even a physically well defined issue, since we cannot experimentally distinguish the two cases (unless we are a cat).

I always thought that this was brought up to illustrate a particular point of view. Kind of a Copenhagen-ish view. That is - knowledge of reality collapses the wave function, reality alone does not.

[Severian]

Saint said:

I always thought that this was brought up to illustrate a particular point of view. Kind of a Copenhagen-ish view. That is - knowledge of reality collapses the wave function, reality alone does not.

Yes, I agree. It is a philosophical objection rather than a scientific one. It is the misuse of the word 'paradox' which makes people think it is some kind of problem. There is no paradox.

[Severian]

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No, the sphere of light does not "instantly concentrate" on your one pixel. It obeys classical power dissipation laws as with any other wave. It is difficult to give light a universal visualization.

The way I like to look at it is to imagine it to be "localized possibility". I consider them to be more particle than wave. They are called photons after all. As for their wave and quantum properties, imagine a small probability wave function. Each little wave function is like a photon of light. Because the wave smears out at the edges, it is never certain where exactly this photon of light is. It also interacts with itself (or possibly the electromagnetic field) in such a way that it behaves like a wave around corners, tight spaces, or when changing mediums (refraction, diffraction).

I think your post is misleading, because I don't think you are answering what was asked.

I would say that yes, the sphere of light does "instantly concentrate" on your one pixel. The collapse of the wavefunction in QM is a nonlocal phenomena. However, the evolution of the wave itself is local (so as you put it, it evoles like a 'classical' wave). This means that the possible places a photon can be are still restricted by relativity, but until you measure the position the probability of it being anywhere in on the light cone is non-zero. Since measuring the position of the photon at any time will collapse the wavefunction (and pick a position for the photon) you cannot see anything 'weird' happening. So you cannot use this to pass information faster than light-speed for example.

[Cashio]

Severian said:

I have never understood why this is a problem. If we hypothesise that the cat can collapse wavefunctions then the cat is always either dead or alive (ie. in an 'alive eigenstate' or a dead one). If we hypothesise that it cannot, it is in a superposition. Where is the problem? It isn't even a physically well defined issue, since we cannot experimentally distinguish the two cases (unless we are a cat).

there is a problem. according to logic the cat can not be both dead and alive at the same time - it must be in one of those states. but to determine in what state he is we must interfere with the experiment - eventually causing the "wave fucntion" of the cat to collapse. if we follow quantum mechanics properly, we must assume that until the wave function collapses the cat is BOTH dead and alive (superposition) and that is a contradiction to the first assumpsion.

Let me remind you that quantum mechanics works and that this specific feature (superposition) has been proven (in the Aharonov-Bohm effect for example). now do you see the problem? :)

[Severian]

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there is a problem. according to logic the cat can not be both dead and alive at the same time - it must be in one of those states.

There still is no problem, because your 'logic' is in error. If the cat is not an observer, then it is a superposition of dead and alive. How can 'logic' say otherwise? Since you cannot observe the cat without collapsing the wavefunction, you cannot prove my statement wrong, and therefore there is no contradiction. You are basing your logic of assumptions on the classical world around you, which is not valid quantum mechnically.

[swansont]

Severian]There still is no problem, because your said:

is[/B'] a superposition of dead and alive. How can 'logic' say otherwise? Since you cannot observe the cat without collapsing the wavefunction, you cannot prove my statement wrong, and therefore there is no contradiction. You are basing your logic of assumptions on the classical world around you, which is not valid quantum mechnically.

Which, if I may point out, is the whole point of the exercise. What makes sense classically does not, necessarily, on the quantum level.

[J.C.MacSwell]

Severian]There still is no problem, because your said:

is[/B'] a superposition of dead and alive. How can 'logic' say otherwise? Since you cannot observe the cat without collapsing the wavefunction, you cannot prove my statement wrong, and therefore there is no contradiction. You are basing your logic of assumptions on the classical world around you, which is not valid quantum mechnically.

My unproveable thought would be that even though the superposition is not disproveable it cannot be proven either. ;)

[ydoaPs]

post 13 seems to dissagree with you.

[ecoli]

What I don't understand, is how the "third" state of being can be taken advantage of using quantum computers. a qubit have 3 states, right? 1, 0 and 1 and 0. Is the state atcually one and zero? If not, then how can we use it?

[Jacques]

The fact that we don`t know if the cat is alive or dead, does not imply that he is both. Simply we don`t know. We can tell that the probability of being dead is 50% and 50% alive. That is just a probability... Maybe I am wrong but I think that the successes of quantum physic is mostly a succes of applying probability to physic problem.

[ecoli]

Right, but given that, if the cat isn't both, how do we apply the probability that it's both to practical applications?