The Dual Nature of Light: Exploring Light's Particle and Wave Properties

[Swetha.M.L]

why light shows both particle and wave nature?

 

[Drakkith]

why light shows both particle and wave nature?

We don't know. That simply appears to be the way nature works. Quantum Electrodynamics is the theory that fully describes our understanding of light at the quantum scale, but it is very difficult to explain if you know nothing about the basics of quantum physics. In any case, it only explains how light works, not why it is the way it is.

 

[bhobba]

See our FAQ:
https://www.physicsforums.com/threads/is-light-a-wave-or-a-particle.511178/

The wave particle duality is actually a myth:
http://arxiv.org/pdf/quant-ph/0609163.pdf

Unfortunately the way physics is taught they sometimes start out with things you later learn is wrong - but we all have to start somewhere. Feynman commented on it and lamented it is like that - but as he knew well, teaching is about what students need to eventually understand - which sometimes entails being a bit loose with the truth at the start.

You have however asked a question the correct answer to which means we can't be speak in 'half' truths.

Thanks
Bill

 

[muscaria]

The wave particle duality is actually a myth:
http://arxiv.org/pdf/quant-ph/0609163.pdf

I get the feeling myth is maybe a bit of a strong word no? Maybe misnomer is better? I know he didn't consider the FAQ but his question didn't ask about wave-particle "duality" strictly, he asked about why light had both wavelike and particle like qualities. Although we don't know why this is really the case we know from basic Electromagnetic theory that light is a transverse wave that carries energy, linear momentum and mass akin to the properties of traveling particles. The connection was made by Einstein who showed that the flow of energy associated with the translation of relativistic particles at the rate of the speed of light is precisely the flow of energy associated with these light waves.
EDIT: Apologies for the FAQ comment - I thought you were suggesting OP question didn't quite comply with rules, i.e didn't do enough background reading or something..

 

[bhobba]

I get the feeling myth is maybe a bit of a strong word no?

Its a myth.

Its perpetrated by the semi-historical approach most beginner level QM texts use. Here is a MUCH better way of looking at QM:
http://www.scottaaronson.com/democritus/lec9.html

Advanced texts like Ballentine don't even mention it because it was done away with when Dirac came up with his transformation theory:
http://www.lajpe.org/may08/09_Carlos_Madrid.pdf

I know this and other myths, like virtual particles are real, are very difficult to shake because its widely used in popularisations and beginner texts. This forum is likely the first place the truth is told for many posters. It leads to long threads where people wedded to the myth quotes this and that - but they really go nowhere because it doesn't change the facts.

Thanks
Bill

 

[bhobba]

The connection was made by Einstein who showed that the flow of energy associated with the translation of relativistic particles at the rate of the speed of light is precisely the flow of energy associated with these light waves.

That's not what he did.

He showed the photoelectric effect could be explained with light considered as particles.

Interestingly it is now known that model isn't required - but that really requires a whole new thread. Best to start another along those lines if you want to pursue it.

Thanks
Bill

 

[zonde]

Here is a MUCH better way of looking at QM:
http://www.scottaaronson.com/democritus/lec9.html

Are you sure it's (much) better? It doesn't seem so:
"Come read about why quantum mechanics, far from being a mysterious, arbitrary structure foisted on us by experiment, is something that mathematicians could easily have discovered without leaving their armchairs. (They didn’t? Minor detail…)"

 

[Nick666]

Its a myth.
Its perpetrated by the semi-historical approach most beginner level QM texts use. Here is a MUCH better way of looking at QM:
http://www.scottaaronson.com/democritus/lec9.html

Could you please elaborate a tiny bit more on this ?

[["More generally, if you want to flip over an N-dimensional object by a continuous motion, then you need to go into the (N+1)st dimension.

Exercise 6 for the Non-Lazy: Prove that any norm-preserving linear transformation in N dimensions can be implemented by a continuous motion in N+1 dimensions.

But what if you want every linear transformation to have a square root in the same number of dimensions? Well, in that case, you have to allow complex numbers. So that's one reason God might have made the choice She did."]]

It seems to me that the last paragraph says that we need complex numbers in order to have continuous motion in our 3d space , otherwise we'd need 4d space for that. Right ?

However, when first reading the first paragraph, I was thinking: well of course, that (3+1) must the time dimension... if that makes any kind of sense. So I'm a bit puzzled by this.

 

[julcab12]

..One variable is involved -- Observer, act or some form of interaction which breaks (in a sense) duality. Well, we can say ways about QM. As a layman. It boils down to broad approaches; QM has both particle/wave property, waves is a property/illusion of particle (QM), particle is a property/illusion of wave(classical) and so on. The most common is the copenhagen interpretation (majority of the QM textbooks). Others include, MWI and many of it's variations(BST) -- the most literal, simplest yet paradoxically rich and head on/direct approach to QM; Consistent histories -- It has both classical and schrodinger eqn; Epistemic interpretations; statictical interpretations; modal interpretations; transactional interpretation and my personal favorite RELATIONAL interpretation.

Anyways, at the end of the day. We can't really tell and to what extent which one is truer since most of the things i mentioned above has it's own flavors and some change in formalism -- mathematical theory. The only bottlenecks we have are our threshold in our experimental endeavors and consistency evaluations. If it's appears in superposition and known to decohere then it tells something about some interpretation. I accept the picture as 'threshold' and see how far we can go about it but i wouldn't go too far to say that it is the absolute way. Illusions/blackswan/phenomenon are apparent in nature but it doesn't matter. What matters most is what 'contributes' to that phenomenon to appear/show that way. It is a standard macroworld idealization that is lost in QM (Not all but most)..

https://www.sciencenews.org/blog/context/tom%E2%80%99s-top-10-interpretations-quantum-mechanics

 

[bhobba]

Are you sure it's (much) better? It doesn't seem so:
"Come read about why quantum mechanics, far from being a mysterious, arbitrary structure foisted on us by experiment, is something that mathematicians could easily have discovered without leaving their armchairs. (They didn’t? Minor detail…)"

Well, having waded through tons of beginner books its way way more transparent - but each to his/her own I suppose.

Thanks
Bill

 

[bhobba]

Could you please elaborate a tiny bit more on this ?

Sure.

Here is a more detailed argument along the same lines.

Suppose we have a system in 2 states represented by the vectors [0,1] and [1,0]. These states are called pure. These can be randomly presented for observation and you get the vector [p1, p2] where p1 and p2 give the probabilities of observing the pure state. Such states are called mixed. Now consider the matrix A that say after 1 second transforms one pure state to another with rows [0, 1] and [1, 0]. But what happens when A is applied for half a second. Well that would be a matrix U^2 = A. You can work this out and low and behold U is complex. Apply it to a pure state and you get a complex vector. This is something new. Its not a mixed state - but you are forced to it if you want continuous transformations between pure states.

QM is basically the theory that makes sense out of pure states that are complex numbers. There is really only one reasonable way to do it - by the Born rule (you make the assumption of non contextuality - ie the probability is not basis dependant, plus a few other things no need to go into here) - as shown by Gleason's theorem.

The following gives a more rigorous development of this idea:
http://arxiv.org/pdf/quant-ph/0101012.pdf

The key idea, and what distinguishes QM from standard probability theory, is you must have continuous transformations between pure states. But in modelling physical systems it pretty much what is required ie if a system changes to a another state after one second, it must go through another state after half a second.

Thanks
Bil

 

[bhobba]

..One variable is involved -- Observer, act or some form of interaction which breaks (in a sense) duality. Well, we can say ways about QM. As a layman. It boils down to broad approaches; QM has both particle/wave property, waves is a property/illusion of particle (QM), particle is a property/illusion of wave(classical) and so on.

Remember what I said before:

I know this and other myths, like virtual particles are real, are very difficult to shake because its widely used in popularisations and beginner texts. This forum is likely the first place the truth is told for many posters. It leads to long threads where people wedded to the myth quotes this and that - but they really go nowhere because it doesn't change the facts.

They have been discussed in many threads on this forum. Going over the same thing again will serve no purpose. If it is still unclear please go over the link I gave before about myths in QM:
http://arxiv.org/pdf/quant-ph/0609163.pdf

Thanks
Bill

 

[Brage]

I agree with bhobba the idea of a particle does not make sense from a modern quantum mechanical perspective, so why then do we observe particles in experiments? The best answer I have seen to date is that when we make an observation, we are actually entangling our own wave function with that of the photon, electron, whatever. This means what the particle we observe is actually just a reflection of our own state within the entangled wave function. This of course leads to interpretations of quantum mechanics like the Many Worlds Interpretation by Everet, which although not widely accepted has a lot of useful insight.
There are however other interpretations, such as the Many Interaction Worlds Approach, proposed recently, in which particles do exist in an infinity of semi-classical interacting "parallel" worlds (worth the read, very interesting perspective). The point is that we don't actually know why we observe particles but theories describes waves, and while there are many explanations, there has not been proposed one satisfactory one on which the scientific community agrees.

Hope this helps guys
Brage

 

[bhobba]

The point is that we don't actually know why we observe particles but theories describes waves,

We do - but its part of that rather difficult area known as Quantum Field Theory:
http://arnold-neumaier.at/physfaq/topics/position.html

Its true, however, ordinary QM doesn't explain why some things have position as an observable - it just assumes it does.

Thanks
Bill

 

[julcab12]

Remember what I said before:

Please read my whole statement and yes Bob i did read the article before but thanks anyways. I gave a broad statement. What i meant was It is difficult to determine what is a true interpretation, competing theory, and what is merely a change in formalism. An interpretation is technically an interpretation of the mathematical theory.

I don't take positions (realist, orthodox and agnostic in Griffiths QM) and always fascinated on the different solutions and interpretation of QM. In relational view they don't give much attention to real or not or even facts for that matter. It is a purely dynamic description formulation.

 

[bhobba]

Please read my whole statement.

I did - and you clearly mentioned the wave particle duality 'QM has both particle/wave property'.

Thanks
Bill

 

[julcab12]

I did - and you clearly mentioned the wave particle duality 'QM has both particle/wave property'.
Thanks
Bill

Ah. No. I only say duality in a broad way. In essence, standard QM is wave and particle is a projection or special case as was sited in the paper. I can also say the other way around.

 

[muscaria]

That's not what he did.

I just meant that Maxwell Calculated the energy and linear momentum of his E-M waves. Then Einstein came up with relativity and showed that relativistic particles traveling at c had the same energy and linear momentum.

 

[atyy]

why light shows both particle and wave nature?

"Because" matter also shows particle and wave properties :) For that matter "spacetime" also shows particle and wave properties. So it is natural in quantum mechanics for things to have particle and wave properties.

 

[zonde]

It is quite meaningful to talk about light as particles in QM. The reason is that energy of light comes in discrete packets. That is characteristic property of particles. Waves do not show such property.

 

[microsansfil]

Hi all,

Is it duality wave / particle or duality wave / corpuscle ?

We say Corpuscular theory of light which is different from Particle physics, isn't it ?

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

Although the word "particle" can be used in reference to many objects (e.g. a proton, a gas particle, or even household dust), the term "particle physics" usually refers to the study of "smallest" particles and the fundamental fields that must be defined in order to explain the observed particles.

Patrick

 

[bhobba]

We say Corpuscular theory of light which is different from Particle physics, isn't it ?

Particle physics is the domain of Quantum Field Theory which is most definitely NOT a particle in any usual sense.

Thanks
Bill

 

[jk22]

I thought in quantum mechanics the wave particle duality comes from the measurement : if we measure position it gives a peaked wavepacket, momentum will give a wave.

However energy gives a mix between particle and wave.

 

[andresB]

Sure.

Here is a more detailed argument along the same lines.

Suppose we have a system in 2 states represented by the vectors [0,1] and [1,0]. These states are called pure. These can be randomly presented for observation and you get the vector [p1, p2] where p1 and p2 give the probabilities of observing the pure state. Such states are called mixed. Now consider the matrix A that say after 1 second transforms one pure state to another with rows [0, 1] and [1, 0]. But what happens when A is applied for half a second. Well that would be a matrix U^2 = A. You can work this out and low and behold U is complex. Apply it to a pure state and you get a complex vector. This is something new. Its not a mixed state - but you are forced to it if you want continuous transformations between pure states.

QM is basically the theory that makes sense out of pure states that are complex numbers. There is really only one reasonable way to do it - by the Born rule (you make the assumption of non contextuality - ie the probability is not basis dependant, plus a few other things no need to go into here) - as shown by Gleason's theorem.

The following gives a more rigorous development of this idea:
http://arxiv.org/pdf/quant-ph/0101012.pdf

The key idea, and what distinguishes QM from standard probability theory, is you must have continuous transformations between pure states. But in modelling physical systems it pretty much what is required ie if a system changes to a another state after one second, it must go through another state after half a second.

Thanks
Bil

I don't quite get what you are actually saying so the following may be wrong, but IF you are saying that the real distinction from classical mechanic to quantum mechanics is that QM is it mathematics formalism that would be false because classical mechanics (at least classical statistic mechanics) can be described using the same formalism as QM.

EDIT: Uhm, it seems to me now that the important part of what you were saying was it apply to pure states (because there is a born rule for classical mechanics but it deals with statistical assembles not classical pure states)

 

[andresB]

The wave particle duality is actually a myth:
http://arxiv.org/pdf/quant-ph/0609163.pdf

I find that assertion to be too extreme, in fact it seems close to a straw man or a fight over just grammatical terms.

If you make an idealized (of course) position measurement with infinite precision you don't get an extended wave but a point like answer, in that sense you can say that under that measurement the particle behave more like a particle and less like a wave.

 

[bhobba]

I find that assertion to be too extreme, in fact it seems close to a straw man or a fight over just grammatical terms.

What dimension does the wave-function of two entangled particles propagate in? Why would you consider such in any sense a wave? A basis expansion is arbitrary - why do you want to enshrine the position basis?

At an even more basic level, for there to be a wave, what is the medium? What is physically waving?

The bottom line is in some contrived circumstances we have solutions that look like the mathematics of waves but really are nothing like it - and that is only in some circumstanes - in most it doesn't even look like it marthematically.

This whole thing is a hangover from De-Broglie and was superseded when Dirac came up with his transformation theory in 1926 - likely before then - but certainly at that stage:
http://www.lajpe.org/may08/09_Carlos_Madrid.pdf

De-Broglie's ideas led to Schroedinger's wave equation. It was first thought to be just that - an equation about waves - but after a bit of experience that changed rather quickly and Schroedinger lamented he ever became involved in it. Then Dirac combined it with matrix mechanics and it was obvious it had nothing to do with waves - the key thing was this abstract thing called a state.

Thanks
Bill

 

[bhobba]

I don't quite get what you are actually saying so the following may be wrong, but IF you are saying that the real distinction from classical mechanic to quantum mechanics is that QM is it mathematics formalism that would be false because classical mechanics (at least classical statistic mechanics) can be described using the same formalism as QM.

Its a less rigorous version of this:
http://arxiv.org/pdf/quant-ph/0101012.pdf

Thanks
Bill

 

[andresB]

What dimension does the wave-function of two entangled particles propagate in? Why would you consider such in any sense a wave? A basis expansion is arbitrary - why do you want to enshrine the position basis?

At an even more basic level, for there to be a wave, what is the medium? What is physically waving?

The bottom line is in some contrived circumstances we have solutions that look like the mathematics of waves but really are nothing like it - and that is only in some circumstanes - in most it doesn't even look like it marthematically.

This whole thing is a hangover from De-Broglie and was superseded when Dirac came up with his transformation theory in 1926 - likely before then - but certainly at that stage:
http://www.lajpe.org/may08/09_Carlos_Madrid.pdf

De-Broglie's ideas led to Schroedinger's wave equation. It was first thought to be just that - an equation about waves - but after a bit of experience that changed rather quickly and Schroedinger lamented he ever became involved in it. Then Dirac combined it with matrix mechanics and it was obvious it had nothing to do with waves - the key thing was this abstract thing called a state.

Thanks
Bill

You could be right here or not, but this last post from you seems to not have anything to do with the paper you quoted (it seems to me that the author of that paper say the opposite of what you just said)

 

[bhobba]

You could be right here or not, but this last post from you seems to not have anything to do with the paper you quoted (it seems to me that the author of that paper say the opposite of what you just said)

How you got that beats me.

Thanks
Bill

 

[andresB]

How you got that beats me.

Thanks
Bill

From this paper you quoted

http://arxiv.org/pdf/quant-ph/0609163.pdf

"electrons and photons always behave as waves, while a particle like behavior corresponds only to a special case"

 

[bhobba]

"electrons and photons always behave as waves, while a particle like behavior corresponds only to a special case"

Did you read the context he gave of what he means by waves 'Instead, such serious textbooks talk only about waves, i.e., wave functions'

Wave functions, except in some special circumstances, behave nothing like waves.

This has been thrashed out time and again on this forum - and our FAQ has an entry on it. A simple search will bring you all the detail you can want about it. Its a common misconception people find hard to shake and you get these long threads that basically go nowhere.

Thanks
Bill

 

[tallal hashmi]

I think that dual nature of light is its nature.

 

[bhobba]

I think that dual nature of light is its nature.

Scratching my head why anyone would believe that after reading this thread.

I could point to all sorts of stuff explaining why it isn't true, in fact I already have done that.

Now I think a different tack is required.

Here is our FAQ:
https://www.physicsforums.com/threads/is-light-a-wave-or-a-particle.511178/

After reading it can you explain, in your own words, not with links elsewhere, quotes from physicists, or popularisations, but your own reasoning why you think its true.

Thanks
Bill

 

[tallal hashmi]

hyperphysics.phy-astr.gsu.edu/hbase/mod1.htmlYou should check this link

 

[bhobba]

You should check this link

I can't understand why, when I asked for a reason in your own words, and not for a link, you did exactly the opposite, and gave me a link.

There was a reason for that - to get you to think about it and give your own reasons, not others views.

Unless you are willing to do that then you will not progress and learn.

This is a common misconception, its hard to break because many beginning texts and links (like the one you gave - it basic beginner stuff) promulgate it. But its still wrong.

Now please can you tell me why you would believe some link you gave, rather than the experts from the site you have decided to come to to discuss physics, and who prepared the FAQ link I gave?

I don't understand that. It happens a lot and you get long threads going though the same stuff over and over.

Thanks
Bill