# A question and theory about light

Hey guys! I'm pretty new to physics, so please forgive me if this is a dumb question. Anyway, I was thinking yesterday, "What the heck is light made of?" So I looked it up on the internet and found out light is made of particals and waves.(or something like that.) So I just kinda sat down and watched the discovery channel and saw a show that was talking about the displacement of water when something falls into it. Then I thought, "Does light displace water?" Since its particals and all. Can you guys tell me the answer to this question?

actually light is nothing but electromagnetic waves and they travels in the form of small bundles called "Photons"

Hi Mato. Although it's possible to think of light as partcles, they are not made of matter and have no mass or weight in the usual sense. So they wouldn't displace water. Light can travel through water, as you can see and gets deflected in the process.

Hi Mato. Although it's possible to think of light as partcles, they are not made of matter and have no mass or weight in the usual sense. So they wouldn't displace water. Light can travel through water, as you can see and gets deflected in the process.

This isn’t exactly true, light can displace water. Optical tweezers for example can move a single molecule of water. Because light has momentum it can move water, given the right constraints. Albeit seeing "light" (EM waves) move water is not going to be observable on our macroscopic scale.

Hurkyl
Staff Emeritus
Gold Member
So I looked it up on the internet and found out light is made of particals and waves.(or something like that.)
The third thing you say here is right.

Before quantum mechanics, we had classical mechanics. In classical mechanics, we talked about things we called "particles" and "waves". Light was confusing because it behaved a little like a classical particle, and a little like a classical wave.

Now, we have quantum mechanics, which says that classical particles and classical waves simply don't exist. Instead, everything is this new quantum mechanical "stuff". And even if we continue to use the word "particle", we really mean a "quantum mechanical particle" which is not a "classical particle".

Wizardsblade, please check the definition of 'displace' as used in the OP. It does not mean 'move'.

But you're certainly right about the optical tweezers.

Mentz114,

Could you displace water with an inverted optical tweezers, i.e. what if intensity goes as R (radial distance)? Would that push/pull the water out of place?

The third thing you say here is right.

Before quantum mechanics, we had classical mechanics. In classical mechanics, we talked about things we called "particles" and "waves". Light was confusing because it behaved a little like a classical particle, and a little like a classical wave.

Now, we have quantum mechanics, which says that classical particles and classical waves simply don't exist. Instead, everything is this new quantum mechanical "stuff". And even if we continue to use the word "particle", we really mean a "quantum mechanical particle" which is not a "classical particle".

Hurkyl, what exactly does this mean? I have been trying to study quantum mechanics for a few years now and honestly it sounds more like a religion than something based on facts. Not saying there are no facts, just that when you attempt to locate them you are bombarded with a "You just have to believe" sort of argument. So is it a particle? Or is it a wave? quantum mechanics talks of a bunch of strange things and then never really seems to answer anything. Just raises more questions.

Please forgive my statement as I'm not a scientist so a lot escapes me I'm sure... :-)

glenn

Glenn,
This will sound odd but light is both a wave and a particle. If you do a particle expiroment on light it will act like a particle, but if you do a wave expiroment on light you the light acts like a wave. In other words a wave and a particle are the same thing. As far as understanding how/why this is true QM does not go there, it just states the math. There are some theories on what it all means but nothing that is proven.

Hurkyl
Staff Emeritus
Gold Member
Hurkyl, what exactly does this mean?
It means that quantum mechanics is not a kind of classical mechanics -- so it is wrong to insist on understanding quantum mechanics in terms of classical ideas.

I have been trying to study quantum mechanics for a few years now and honestly it sounds more like a religion than something based on facts. Not saying there are no facts, just that when you attempt to locate them you are bombarded with a "You just have to believe" sort of argument.
What do you mean? There are tons of facts; QFT is an extremely well-tested theory.

It suspect you mean that QFT does not explain things in terms of things that are "intuitively obvious". That is not a failing of QFT -- it simply means you have not yet developed the necessary intuition, which should be one of the aims of your study.

So is it a particle? Or is it a wave?
Neither. Light is not a classical object. It is wrong describe light in terms of these classical ideas.

(Of course, for many purposes, these classical ideas are often sufficient descriptions, even if they aren't exactly correct. And since the classical ideas are simpler, it would be right to use the classical ideas for such purposes)

Glenn,
This will sound odd but light is both a wave and a particle. If you do a particle expiroment on light it will act like a particle, but if you do a wave expiroment on light you the light acts like a wave. In other words a wave and a particle are the same thing. As far as understanding how/why this is true QM does not go there, it just states the math. There are some theories on what it all means but nothing that is proven.

But see, this is where I think things are off. Specifically what experiment determined that light was a wave? The only experiment that I've seen that supposedly proves this as fact is the way light behaves when it is split and how it interferes with itself causing an interference pattern on the wall if you will.

However, a little deeper thought may produce another explanation for what you see during this transaction so I'm not totally convienced the earlier statements of light being a wave were accurate.

Would make a very interesting conversation though... :-)

glenn

It means that quantum mechanics is not a kind of classical mechanics -- so it is wrong to insist on understanding quantum mechanics in terms of classical ideas.

But doesn't this make quantum mechanics some sort of black magic? I don't deny that it predicts certain things based on probability, but does that in and of itself justify people taking it to the extreme in human comprehension?

For example: Based on the results of the two slit experiment and the "wave" functions that quantum mechanics uses to predict things, people have gone off on multi-world universes where nothing exists until someone looks upon it and it becomes measured. Whereupon it snaps into 1000 different universes with all the possible solutions to the measurement along with 1000 different measurers that were conducting the test.

So instead of looking at the fact that quantum mechanics is made up of wave functions that make predictions (not absolute predictions, but best guess solutions), it seems we instead try to give it a magic quality with religious intent. Something to strive to understand because you can't possibly understand it without great study and reflection... Just seems we've gone too far here...

It suspect you mean that QFT does not explain things in terms of things that are "intuitively obvious". That is not a failing of QFT -- it simply means you have not yet developed the necessary intuition, which should be one of the aims of your study.

Neither. Light is not a classical object. It is wrong describe light in terms of these classical ideas.

I'm not sure what a classical measurement is anymore. Taking the claims of quantum mechanics, one would seriously have to backup and look at all aspects of past science and question if we've done anything right at all.

glenn

Hurkyl
Staff Emeritus
Gold Member
But doesn't this make quantum mechanics some sort of black magic?
No. At least -- I don't see why one would think so.

does that in and of itself justify people taking it to the extreme in human comprehension?
Undergraduate linear algebra is hardly the "extreme in human comprehension," and is all you need to understand most of quantum "weirdness". Well, I suppose you also need to admit you may need to learn something new, which does appear to be extraordinarily difficult for some people. But that's only a limitation for those people, not for humanity as a whole.

I think it's extremely arrogant for a person to insist that everything can be explained in terms of things that he already knows. (And even more so for him to insist that if he doesn't understand it, then nobody can) However, I do have empathy for those who simply find it difficult to learn new things.

So instead of looking at the fact that quantum mechanics is made up of wave functions that make predictions (not absolute predictions, but best guess solutions), it seems we instead try to give it a magic quality with religious intent.
Who is this "we" of which you speak? As far as I can tell, you are the only one here who is making any claims that quantum mechanics is "magic" or "religious".

I will be taking linear algebra next spring so am happy to hear that it will help me to grasp the concepts of quantum mechanics. Not sure why you feel the need to take questions to personal insults, but then thats just were some people live. If I didn't enjoy learning new things I wouldn't even be here asking questions to begin with.

I'll keep studying and try not to ask too many questions so as to not affend anyone.

For those of you that offered suggested reading materials, thank you very much. I am looking through them and will continue to study.

Thanks,

glenn

There's nothing weird about quantum mechanics. I don't know where people get this idea. The popular press has made a fortune pushing this notion so maybe that's it.

I've studied several books on quantum mechanics and quantum field theory and not one of them has a label on it saying 'warning - this stuff is weird'.

Nor do they say things like - 'please toss a coin and choose formula A if it's heads or formula B if it's tails'. Now that would be magic.

In fact far from being weird, QM is completely deterministic.

Hurkyl
Staff Emeritus
Gold Member
I'm not nearly as offended as I sound. I love questions, and so do most of the people here -- don't feel that you should ask fewer questions.

It's the other little things that are troublesome -- for example, many people will insist that if we cannot explain {fill in the blank} to them in a few paragraphs over the internet, then we must not understand it, or it must be obviously wrong, et cetera, et cetera.

When you asked loaded questions like "does that ... justify people taking it to the extreme in human comprehension?" it sounded like you were beginning to go down that road. Thus the response. Maybe talking about arrogance is too much, but I think it really puts things in perspective.

As a caveat, I don't know where you can actually learn about quantum weirdness, from just linear algebra. I was sort of lucky: there was a brief "quantum computing for mathematicians" in one of my classes, where we didn't do any physics at all... we just got an overview of how to program a quantum computer. But because that involves mathematically manipulating quantum "weirdness" (and this required only basic linear algebra), including a prototypical "measurement" (a CNOT-gate), it resulted in my biggest epiphany regarding quantum mechanics. Alas, the result is that I can puzzle my way through the weirdness of Bell tests, counterfactual computation, et cetera... but I can't actually do any physics.

I'm not nearly as offended as I sound. I love questions, and so do most of the people here -- don't feel that you should ask fewer questions.

It's the other little things that are troublesome -- for example, many people will insist that if we cannot explain {fill in the blank} to them in a few paragraphs over the internet, then we must not understand it, or it must be obviously wrong, et cetera, et cetera.

When you asked loaded questions like "does that ... justify people taking it to the extreme in human comprehension?" it sounded like you were beginning to go down that road. Thus the response. Maybe talking about arrogance is too much, but I think it really puts things in perspective.

Thats fine. no hard feelings here either. I understand what you're saying and apologize if I misled you. I actually have a very open mind and love learning new things. I look forward to the day where this stuff makes sense.

Also, keep in mind that I've only read those off the shelf books about how mysterious it is and how there are lots of people using it but none that can explain why it works (hence the wierd comments). They have done a good job of making me feel its a strange science indeed but perhaps not in the correct sense.

I'll keep asking questions as long as you guys don't kick me off. Just don't think I'm being hard headed, I am just the type person that has to ask questions until it sinks into my head.

Thanks,

glenn

jtbell
Mentor
There's nothing weird about quantum mechanics.

You must not yet have encountered Bell's Theorem and the experiments that test its predictions.

If that is the case, you might want to read the following article which describes a thought experiment that encapsulates the "weirdness" demonstrated by these experiments:

http://qt.tn.tudelft.nl/~lieven/qip/extra/mermin_moon.pdf [Broken]

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You must not yet have encountered Bell's Theorem and the experiments that test its predictions.

If that is the case, you might want to read the following article which describes a thought experiment that encapsulates the "weirdness" demonstrated by these experiments:

http://qt.tn.tudelft.nl/~lieven/qip/extra/mermin_moon.pdf [Broken]

Thank you very much. that was some very interesting reading. However, it has only served to bring up more questions.

All of the experiments that I read about seem to have a lot of unknown things that were not tracked. In the specific article you sent me on Bell's Theorem and the test with the two boxes not linked, it would seem that because both pieces of particles that were seperated from a single source took on the opposite spins when they left. Because the boxes were both detecting in opposite ways then it would appear to me that whatever got registered at one would always register opposite at the other while the magnets were set equally.

On the other hand, with them set on different settings then it would also appear to me that they'd get fairly equal responses based on whatever the setting was and the orientation of the spins as they left their initial point.

I would think the only way to truely rule out classical thinking in an experiment such as this would be to seperate two particles and then somehow take them miles apart and reuse the same particles. Changing ones orientation and showing how it effects the one at the other end. But it seems that you have to be very careful not to get mixed up between thought experiments and actual experiments as things seem to get mixed up quite a bit.

I agree that if proven, this would seem to seal the cap on Einstein's old argument against Quantum Mechanics, but I have to say that I've not read about an experiment in which I haven't questioned what was really being observed compared to what was being assumed.

Thanks,

glenn

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According to the Compton effect, the photon has a mass. It is in the order of approximately 3.3 x 10-31kg(?). So in theory, enough light in water would increase the mass. The mass of the photon is a concept included in Einstein's thought experiments.

The problem with light is that no one really knows what it is. Is is a particle, or is it a wave? Well, this is and will continue to be a question for some time to come. The solution could be very simple. Perhaps it is a particle which emits wave-like phenomenon. You know, like a boat that creates a wake. Unfortunately, the idea of a 'ethereal' medium has been disproved.

You have to understand that there is an immense universe between atoms. Think of it on a universe scale. The sun, the earth, the moon. Does a plane travelling through the sky displace the moon? Not really. ]

There are approximately 1000 cosmic rays falling into the earth per cubic meter per second- none of which you can see. How many photons do you think rain down on us from the sun. We are virtually bathed in visible, ultraviolet, cosmic, all kinds of radiation all of the time. The space between water is not empty of light at any time.

You have to understand that there is an entire universe between the atoms. Consider the sun, earth, and moon. Does a plane displace the moon-not really.

Yes a photon has math as proven by the Compton Shift experiment. The problem is that the photon presents two phenomenon, one particle, and one wave. It could be as simple as a boat, which is a particle, creating a wake, which is a wave. The problem is that the only model of an intervening medium that connects us all together, was dismissed. (The Mikaelson-Moreley experiment)

The problem with light is that no one really knows what it is. Is is a particle, or is it a wave? Well, this is and will continue to be a question for some time to come. The solution could be very simple. Perhaps it is a particle which emits wave-like phenomenon. You know, like a boat that creates a wake. Unfortunately, the idea of a 'ethereal' medium has been disproved.

You bring up another interesting topic though. Einstein, I do not believe, ever said the ether doesn't exist, just that you didn't need it to explain a planets motion. To make matters worse, we've disproved something because we can't prove it, not because we've proven definatively that it doesn't exist. Einstein also stated that it'd be impossible to detect something in which we exist. You can't measure it because depending on your position things change length and shape. You don't and can't detect them because you're a part of the same system.

So could you explain to me why we KNOW that the ether doesn't exist? I personally think that it explains quite a few things about what we see and how light moves and so on, and have been trying to find something that proves me wrong so I could move on... :-) So far I see plenty of material stating for fact that its wrong, but none define why or how this is know as fact. Its just another assumption made based on "non" evidence from what I have seen.

Anyone know of an interesting article on this subject I can read?

I'd also like to say, that while you guys are probably annoyed with my questions, I am happy I've found this forum...

Thanks,

glenn

jtbell
Mentor
According to the Compton effect, the photon has a mass. It is in the order of approximately 3.3 x 10-31kg(?).

Can you give a reference or citation for this? That mass is about 1/3 the mass of the electron. The kinematics of Compton scattering are explained very well by assuming conservation of relativistic energy and momentum, and an invariant mass (often called "rest mass") of $9.11 \times 10^{-31}$ kg for the electron and 0 for the photon.

Hi jtbell, thank you for the link. As it happens I've been as baffled as the next man for years about 'locality' and the Aspect et al. results. And anti-particles propagating backwards in time, the 'doppler' effect in photons etc.
I suppose my sense of weirdness has been overworked.

In case you haven't seen this paper, it's worth a long look -

#### Attachments

• Clifford alg values and Bell quant-ph. 0703179.pdf
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So could you explain to me why we KNOW that the ether doesn't exist?... So far I see plenty of material stating for fact that its wrong, but none define why or how this is know as fact. Its just another assumption made based on "non" evidence from what I have seen.

We can't be certain there's no aether, despite none having been observed.

BUT why introduce something you don't need ? All the phenomena we have dealt with so far can be explained with theories that agree with experiment and do not need an aether.

Keep studying. This is a good article -

http://en.wikipedia.org/wiki/Luminiferous_aether

We can't be certain there's no aether, despite none having been observed.

BUT why introduce something you don't need ? All the phenomena we have dealt with so far can be explained with theories that agree with experiment and do not need an aether.

Keep studying. This is a good article -

http://en.wikipedia.org/wiki/Luminiferous_aether

Thanks, I'll take a look

glenn

DrChinese
Gold Member
...I would think the only way to truely rule out classical thinking in an experiment such as this would be to seperate two particles and then somehow take them miles apart and reuse the same particles. Changing ones orientation and showing how it effects the one at the other end. But it seems that you have to be very careful not to get mixed up between thought experiments and actual experiments as things seem to get mixed up quite a bit.

I agree that if proven, this would seem to seal the cap on Einstein's old argument against Quantum Mechanics, but I have to say that I've not read about an experiment in which I haven't questioned what was really being observed compared to what was being assumed.

Thanks,

glenn

There have been such experiments. One was performed at Innsbruck with over a kilometer of separation (and other tests have gone even farther):

http://http://arxiv.org/abs/quant-ph/9810080 [Broken]

Here is my web page devoted to Bell's Theorem and related links, which you may find useful:

Bell's Theorem: An Overview with Lotsa Links

Please note that local realistic theories have very specific requirements which are NOT present for orthodox quantum theory. Your idea of the spin being related even after the particles are separated is agreed to by both quantum theory AND classical theory. But what is not agreed is that a specific observation on one is related to the specific observation on the other.

Classical thinking actually leads to very different predictions than quantum theory. A pure classical view consistent with your idea actually leads to a prediction of correlation of .25+(cos^2/2) while the quantum prediction is cos^2. Guess which is actually observed?

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I would think the only way to truely rule out classical thinking in an experiment such as this would be to seperate two particles and then somehow take them miles apart and reuse the same particles. Changing ones orientation and showing how it effects the one at the other end. But it seems that you have to be very careful not to get mixed up between thought experiments and actual experiments as things seem to get mixed up quite a bit.

I agree that if proven, this would seem to seal the cap on Einstein's old argument against Quantum Mechanics, but I have to say that I've not read about an experiment in which I haven't questioned what was really being observed compared to what was being assumed.

Thanks,

glenn
Let's not forget about physical results that only have a quantum explanation. Classically, an accelerated charge emits electromagnetic waves, losing energy, so the orbit of an electron as it circles around a proton would be unstable. It'd spiral into the nucleus. Quantum mechanically, you find the position of the wavefunction of the electron by solving Schodinger's equation, substituting the Coulomb potential of the proton into the Hamiltonian in the equation, and you solve for all possible wavefunctions. It turns out that only some wavefunctions are allowed, each with its average distance from the proton and its energy. Furthermore, there is a lowest possible energy, a closest distance the electron can be to the proton. We call it the ground state. According to quantum mechanics, an electron can only emit radiation when it moves between states. Thus quantum mechanics yields the correct answer where the classical prediction fails.

Hurkyl
Staff Emeritus
Gold Member
Let's not forget about physical results that only have a quantum explanation. Classically, an accelerated charge emits electromagnetic waves, losing energy, so the orbit of an electron as it circles around a proton would be unstable.
For the record, this may be a very poor choice of example: the SED people seem to have shown that, classically, an electron in a hydrogen atom may remain in (an erratic) orbit about the proton, because, on average, it absorbs as much background radiation as it radiates.

Of course, there are many other examples that do, so far, defy classical explanation.

I have another question. I'll be finishing up CalcIII in the fall and afterwards need to start either physics classes or chemistry classes. I was wondering which would be more useful towards a better understanding of QM? I'm thinking Chemistry but wanted some input.

Thanks,

glenn

reilly
In particle physics, the quantum version of course, we talk about particles -- including photons-, and not much about waves. that's simply because many of the the measuring techniques stem from old classical-physics based instruments. Toward the bottom of the energy chain, we describe electrons going through crystals in terms of waves -- diffraction and all that. The reality, some say, is that sometimes electrons are waves, sometimes particles.However, it is far more appropriate to say that sometimes electrons BEHAVE like particles, sometimes like waves -- as determined by experiment. We really don't have a clue about the structure of electrons; we know a bit about their behavior. (Is water liquid, solid or gas?)

However, the very structure of Quantum Field Theory, directly involves both wave and particle features. QFT is a big word subject, and is considerably more abstract and mathematically sophisticated than ordinary QM. A key difference is that QFT is about particle transformations -- A->B + C --,

electron->electron + photon.

(Yes, I know about energy conservation; it's not important here and now.)

When you work through the problem you find the following: before radiation there was no photon in the system, then there was a photon. How can we deal with this creation process? What physicists did was to invent the answer. Why not invent an operator -- like position or spin - that changes the number of photons or whatever? Thus we talk about a creation operator and a destruction operator. The Lord giveth and the Lord taketh away.

Phew. The deal is that the standard natural mathematical expression of these transformations involves these creation and descruction operators to create or destroy particles in the system. But they do this in association with the particle's wave-function; both notions are there from the beginning. Ultimately, a creation operator creates a particle and a wave together -- there is a great deal of poetic license here.

Nature is weird, and does not always conform to human notions. Sometimes wave behavior, sometimes particle behavior; crazy making. Who knows from?

Why? -- Lot's of history.

Regards,
Reilly Atkinson

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DrChinese
Gold Member
In particle physics, the quantum version of course, we talk about particles -- including photons-, and not much about waves. ...

Regards,
Reilly Atkinson

Nice post, reilly!

reilly