notsureanymore said:
This topic may have been covered but I am not sure how to search for it, what phrase would I use? So my apologies if I am going over old ground.
That's what this place is for. The fact that you tried on your own first is a good thing. :)
notsureanymore said:
I am have great difficulty with light and its nature.
First of all, you should understand that
everyone- including Nobel Prize-winning physicists- has great difficulty with the nature of light. Let me explain why.
You are used to a world of objects that you can see and touch, and that have many characteristics. The smallest thing that you can see or feel is, on the level of a photon, incredibly huge and complicated. Billions and trillions of photons could fit in the smallest speck of dust you can see or feel. As a result of this, you are used to things that have many characteristics. At minimum, they have billions of atoms in them. Because of this, their behavior is not the behavior of a single thing, like a person; it is a collective, group behavior like a huge crowd of people. You have never encountered an object that was like a single person in that crowd, and you never will.
So when someone describes the characteristics of a photon to you, what they are describing is something that you quite literally cannot imagine; you have never encountered anything like this before, and you never will. You can only make up models of it in your mind; it's like a wave, or it's like a little ball, or its like a kind of little patch of space that has something weird about it. But it isn't really any of those things- it's a photon, and we never have seen one and we never will. And this is not merely true of photons- the same is true of atoms, and electrons, and neutrons and protons; in fact, all of the little things that are flying around making everything around you up are like that.
So first of all, you must get rid of your preconceptions that are built on a lifetime of experience dealing with complex objects in the world around you, because if you continue to think in those terms, you will never understand this at all. Actually, you never will and never can completely understand or comprehend it; it's starkly impossible, because you have never encountered anything simple enough to be like a photon, and you never will. But at least you can make up a model of it that will help you understand how it behaves. And remember that
no one can understand it- all they can do is try to imagine it, just as you will do, and try to understand its characteristics by making models in terms of familiar things, just as you will.
So, now, empty your mind of preconceptions and we will see what we can do to give you an understanding of what things are like in the realm of quantum mechanics.
notsureanymore said:
Recalling some very basic physics, light behaves as both a particle and a wave.
true or recanted ?
Well, no, not exactly. Light behaves as if it were made up of waves under certain circumstances, and as if it were made up of particles under certain others, and in some very confusing ways that are something like an amalgam of both under some very special circumstances.
notsureanymore said:
If light is a wave what makes it take on a particle status?
Light doesn't take on "particle status" or "wave status." It is what it is, and what it is is indescribable. Sometimes, we detect light behaving in ways that we can interpret as being wave-like, and other times particle-like, and in a few special circumstances like nothing at all that we have another name for than "photon-like." But to think that it's "being a wave" or "being a photon" is wrong. It's just being light, all the time; we just are better able to understand what it does under different circumstances by using those analogies.
Now that you have no preconceptions, let's try to give a more substantive answer. Our best models of light when we deal with it in terms of very small amounts of it are particle models, and we call those particles "photons." We have proof that there is some sort of "granularity" to light; it isn't just a continuous wavelike thing all the time, we can see that there is some sort of minimum amount of light that we can emit or absorb, at least for anyone frequency. The "size" of one of these "granules" of light, which we call "photons," is very, very, very small. It is also dependent on the frequency of the light. Remember that radio waves, and microwaves, and infrared, and visible light, and ultraviolet, and X-rays, and gamma rays, are all this same thing; the only difference between them is their frequency. I have given you the names of the big parts of the electromagnetic spectrum in order from lowest frequency (radio) to highest (gamma). So the point is, that the radio photons are the smallest, and the gamma photons the biggest, at least in terms of the energy they carry.
If you want to know the proof of the existence of these photons, then you should read about Max Planck and the
quantum theory of black body radiation, and about Einstein's quantum theory explanation of the
photoelectric effect. There is a very good explanation in Isaac Asimov's
The History of Physics, Walker, 1984. You can also get a little information from the links above, which are to Wikipedia; in the black body article, its about the third and fourth paragraphs, and in the photoelectric effect article, look for Einstein's name, where the explanation of the effect by the quantum theory is described. It's worth noting that Einstein won his Nobel Prize in physics for his description of the photoelectric effect, not for the Theory of Relativity.
Getting back to our subject, when we deal with large quantities of photons, it's better to think of them as waves. And it turns out that the waves have real, physical existence as well- that's how we can speak of frequency above, because this is a characteristic of waves. It has wavelength to go with the frequency, as all waves do, and a speed of propagation, and in fact the relation among these is the same as it is for all waves. The most important reason that we know light is wave-like is because it exhibits a behavior that we know can only be shown by waves:
interference.
So here we have something that interferes like a wave, yet is emitted and absorbed like a particle. So: is it a wave, or is it a particle? Neither. It is light!
notsureanymore said:
Does the wave 'collapse' to a particle?
This is actually a rather astute question; you must have been reading about the "collapse of the wave function." I'm going to introduce you to another way to think about things, which will help you some with this if you can get your mind around it.
A light wave is an oscillation of the vacuum. We don't really know what vacuum is, but we know that it can oscillate, and that these oscillations represent the probability of finding something- an electron, a photon, or whatever, at a particular location. The speed at which these oscillations travel through the vacuum is the speed of propagation of that thing- but until something happens, an event of some kind, the thing isn't really there; it's got a probability to be there, but it also has a probability to be over here, or over thataway. Once an event occurs, then we know right where the thing is- but that's
all we know, and we only know it for
that moment when the event is happening. If the oscillations represent photons, then they travel at the speed of light. When they encounter an obstruction, that is an event- and because an event has happened, the thing that those oscillations represent, a photon in this case, suddenly has a precise position. This change from an oscillation to a thing is called the "collapse of the wave function," and the oscillations are "probability waves."
I know this seems kind of confusing, but that's as well as anyone understands it. Including nuclear physicists. What they have that you don't is a bunch of math that they can use to predict the exact probability that the photon will show up over here or over there when the event happens- but remember, they don't know until the event happens whether it actually will be over here, or over there, or maybe even over yonder; all they know is the probability of it winding up in those places. And one of the key things about quantum mechanics is that it says that that is not merely all that we
can know- but that those probabilities are
all there is to know. The photon
doesn't have a position until the event occurs- all it has is a
probability.
notsureanymore said:
Consider a far distant star. The light from it has traveled many trillions of kilometres. Why is it that the person next to me can see the same star as me.
You vastly underestimate the number of photons involved. "Trillions" is a number so large as to be unimaginable- but it is also a number so small compared to the photons the star is putting out every second that it isn't even big enough to be microscopic. It's like comparing a kitchen match to the Sun- or maybe even to a million suns. So out of that truly astronomical number of photons, maybe ten or fifteen per second are hitting your retina, and about the same for your friend (unless s/he is wearing dark glasses). There are plenty to go around. You have to get a lot farther away or be a lot dimmer than any star you can see in the sky is before there aren't enough photons hitting your eye for you to see them.
It might help you to know that there are about 6,000 stars in the sky that you can theoretically see- on a very dark, very clear night, far from the city lights or any lights at all, after you've let your eyes adapt for an hour or so in pitch blackness. And that's in the
whole sky; at anyone time, you can only see perhaps 2,000 of them. Well, within the sphere that those 6,000 stars make, out to the farthest one-
there are over five million stars! So you can only see about one star out of a thousand. Every star you can see is either unusually close or unusually bright.
notsureanymore said:
Surely as the light expands from the star large gaps must appear so that if a light particle hits my eye, my friends eye is missed.
No. The star makes so incredibly many photons that even at hundreds of light years away, there are plenty for both of you, and they are plenty close together.
notsureanymore said:
Draw a circle and then draw lines from it radiating out, the further from the circle the greater the distance between the lines.
Now try to draw enough lines so that there is no space between the lines
Now extend these lines a few trillion K's. without creating spaces.
Can anyone please explain this?
There are so many photons that there aren't spaces more than a few thousandths of an inch where no photons will fall in a second. Your analogy is bad, because you are thinking of photons as relatively large things; they are actually very, very, very, very, very small, smaller by far compared to you than you are compared to the whole Earth, or even the whole Solar System.
If you want to learn more about this pretty quickly, you can get a small, short, inexpensive book by one of the most brilliant physicists who has ever lived- Richard Feynman. But you don't have to be a genius to read it; Feynman wrote it for everyone, and it is very simple. That is part of
his genius. The name of the book is
QED: The Strange Theory of Light and Matter, and it was published by the Princeton University Press in 1985.
