Is Light a Particle or a Wave According to Quantum Theory?

[scarletpete]

In my opinion, "wave/particle duality" has a conceptually revealing element, and a conceptually obscuring element, so the key is to stress only the part that is useful and eliminate the other. The useful part is that when we thought classically, we used to think that particles and waves were two completely different things, with a firewall between them. When a homerun is hit in baseball, you had a particle flying out of the stadium, and a rush of sound waves following it as the fans cheered, and no connection whatsoever existed between those. So it is very enlightening to understand that actually everything that baseball is doing could be expressed in the mathematics of a well-localized wave with very short wavelengths, and everything that rush of sound was doing could be expressed in the mathematics of particles moving stochastically, and in turn the movement of those particles could be expressed as localized waves with very short wavelengths. So perhaps "wave/particle unity" would be a better phrase-- it just means that everything that we thought was a particle also involves the rules of waves, and everything we thought was a wave also involves the rules of particles. But I don't throw away wave/particle duality, I just think wave/particle unity every time I hear the phrase.

Which brings up the obscuring aspect of the phrase, which I believe is the part most of its detractors object to. They object, and rightly so, to the interpretation that "wave/particle duality" means "sometimes it's a particle, and sometimes it's a wave, depending on the question we put to it." That makes it sound disjoint and schizophrenic, when in fact it's the opposite of that, it's well unified into an elegant and aesthetically beautiful theory, just one that is not like we used to think when we thought waves and particles were two completely different things. So the problem is not with the "duality", it is with the whole idea that there are two separate things there that would need to be "dual" to be thought of as elements of the same thing. Shall we say that a coin exhibits "head/tail duality", or shall we just say there are "two sides to the same coin"?

I agree a wave and particle cannot be separated, but the wave function of electromagnetic radiation carried by photons is only present when we have a flow of electrons.

 

[scarletpete]

And yet, you have just described what "wave/particle duality" actually is! It is the statement that particles are what register in particle detectors, and waves are what exhibit magnitude and phase so can interfere, and many particle accelerator experiments are going to use both of those concepts to interpret their results. Maybe the worse university gave you the wrong way to think about "wave/particle duality", and there are better ways to frame that duality, but the duality is still right there in what you just said. I think the real problem is associating "duality" with "multiple personality disorder," though the two are not necessarily synonymous. Look at other dualities in physics, like the Maldacena duality-- no one says a conformal field theory sometimes acts instead like an anti-de Sitter space, they just say the two are opposite sides of the same coin under scale inversion.

I have only thought about wave particle duality recently, have to look up Maldacena duality. get back to you soon.

 

[atyy]

But I don't throw away wave/particle duality, I just think wave/particle unity every time I hear the phrase.

That's probably the best answer I have had so far on the matter.

Indeed. Detractors of the concept of wave-particle duality like to show they are more "high class" than standard textbooks. But the duality is deep in the subject, so much so that "quantum field theory" is the language of "particle physics"!

[I concede that at the most rigourous level, probably the fields are more fundamental. However, the informal presentation in books like Weinberg's or Nair's approach quantum field theory from the viewpoint of particles. Also, a lot of string theory is formulated in first quantized language.]

 

[Steve Lajoie]

There is no wave. The graph of the probability distribution looks like a wave, and follows the similar math as a wave (but with phase differences that require in the most general case complex and not real solutions), but there is no medium to this wave, it is a mathematical description of the probability of getting a "ping!" in a particle detector.

Would you call it a "Probability distribution/particle duality"? Nope.

Best to not even call it a wave, as it causes confusion.It is not a wave; waves are physical phenomena. Oceans waves and sound waves. It confuses beginning QM students. In QM, the Schrodinger equation's solutions are probability distributions, not "waves".

I'm pretty sure that Landau and Lifshitz is a standard textbook.

 

[atyy]

I'm pretty sure that Landau and Lifshitz is a standard textbook.

Do L&L dislike the term?

 

[scarletpete]

I have been reading Richard Feynmans Quantum Electrodyamics and quite early in the first chapter he asserts that Photons are particles. His reasoning that as you decrease the intensity of light incident on a photomultiplier the clicks which the multiplier make become less frequent but equally loud. He doesn't go into much more depth on this issue and assumes particles in there "corpuscular" interpretation. However I always imagined light as quanta. Packets of photons, so is it not the case that decreasing intensity just decreases the number of quanta?

Feynman states there are properties which support his assertion and I was hoping Physics Forums could shed some light on them, excuse the pun

I think of light as discrete photon wave packets. In the photoelectric effect the number of photoelectrons set free does not increase with the light intensity or wavelength of light hitting a surface, but is more dependent on the threshold frequency, below which no photoelectrons are emitted.

 

[Ken G]

To me, the defining feature of a wave is simply that it exhibits a phase that can create interference when you superimpose amplitudes. So you have linearity, phase, and magnitude, and any other details about whether the evolution equation is first order or second order, or even whether or not you need a medium, is a minor detail. Historically, a big "aha" about waves was that they don't require a medium, not that some things aren't waves if they don't have a medium. After all, do macroscopic treatments of light qualify it as a wave because it solves a second-order Maxwell wave equation, or not a wave because it doesn't require a medium? These are details, none are central to the "wave" notion that is invoked in the wave/particle duality phrase. Great hay is made about the inconsistencies of quantum mechanics and relativity, but probably the most important insight from either of those theories is what they share: the recent discovery that you don't need a medium to get wavelike behavior, and that is the crux of wave/particle unity.

 

[Steve Lajoie]

This discussion, and a lot of pop-fizixs textbooks, wouldn't exist if the probability distribution was not called a "wave", because the confusion wouldn't exist.

The probability distribution is no more the physical particle than a bookies' odds are the actual horse your betting on.

 

[atyy]

This discussion, and a lot of pop-fizixs textbooks, wouldn't exist if the probability distribution was not called a "wave", because the confusion wouldn't exist.

The probability distribution is no more the physical particle than a bookies' odds are the actual horse your betting on.

But if one is quibbling about terminology, then calling it a probability distribution is even worse. It is not a probability distribution, rather it is a probability amplitude.

 

[Steve Lajoie]

It's not really an amplitude, is it? You calculate the probability of the particle being found in a range from x to x + delta. Then you get a probability amplitude.

Is it really a quibble to make the distinction between a wave and a particle, if there are so many folks that think that the probability distribution is the particle itself?

 

[atyy]

It's not really an amplitude, is it? You calculate the probability of the particle being found in a range from x to x + delta. Then you get a probability amplitude.

Is it really a quibble to make the distinction between a wave and a particle, if there are so many folks that think that the probability distribution is the particle itself?

Well, let's say we take your narrow view of a wave. In the second quantized Heisenberg picture, isn't the equation for the field a wave propagating in physical space?

 

[Ken G]

I think those who like to think about the unity/duality of particles and waves never think the probability amplitude is a particle. If they did, they would advocate to retire one of the words, for we wouldn't need both! Instead, we see the role for both words, that's how you can tell we don't think they are the same thing, any more than we think the "heads" side of a coin is the same as the "tails" side.

 

[ynon]

Scarletpete: I think you nailed it. Wikipedia seems to agree with you and says:".Wave–particle duality is the fact that every elementary particle or quantic entity exhibits the properties of not only particles, but also waves. It addresses the inability of the classical concepts "particle" or "wave" to fully describe the behavior of quantum-scale objects. As Einstein wrote: "It seems as though we must use sometimes the one theory and sometimes the other, while at times we may use either. We are faced with a new kind of difficulty. We have two contradictory pictures of reality; separately neither of them fully explains the phenomena of light, but together they do".[1]"

There have been experiments that " prove" particles and those that "prove" waves for many years. Debates raged over and over about which was right. Consistant with QM they were both right and wrong. People try to get QM to reconcile with their view of the macroscopic universe. Good luck there. I prefer still the term wave- particle duality with the understanding that we arent talking about big waves or big particles as in salt or beach waves.
Hawking radiation I understand in terms of particles and Heisenberg. Casimir effect in terms of waves. Let's call them Warticles and be done with it. We shouldn't let little things bother us.
Gary

 

[jaropat]

There is no wave. The graph of the probability distribution looks like a wave, and follows the similar math as a wave (but with phase differences that require in the most general case complex and not real solutions), but there is no medium to this wave, it is a mathematical description of the probability of getting a "ping!" in a particle detector.

Would you call it a "Probability distribution/particle duality"? Nope.

Best to not even call it a wave, as it causes confusion.It is not a wave; waves are physical phenomena. Oceans waves and sound waves. It confuses beginning QM students. In QM, the Schrodinger equation's solutions are probability distributions, not "waves".

The particle is not a wave. Change the state of particles is carried out continuously. Even the motion of planets in the sky we see continuous.
Wave character of particles is determined by our ability to obtain samples for each state of the particles in a point in time. Our instruments we tune to the resonant frequency waves to obtain information on the rate of change of states of particles.

 

[ynon]

Jaropat:
Steve Lajoie said: ↑
There is no wave. The graph of the probability distribution looks like a wave, and follows the similar math as a wave (but with phase differences that require in the most general case complex and not real solutions), but there is no medium to this wave, it is a mathematical description of the probability of getting a "ping!" in a particle detector.
You offered:

Would you call it a "Probability distribution/particle duality"? Nope.

Best to not even call it a wave, as it causes confusion.It is not a wave; waves are physical phenomena. Oceans waves and sound waves. It confuses beginning QM students. In QM, the Schrodinger equation's solutions are probability distributions, not "waves".

Interesting post and possibly correct. I will give a different perspective and I don't see why one couldn't call it "Probability distribution/particle duality" Call them partiwaves and be done with it.You can describe waves as a probability distribution.Any waves. Any time energy is introduced into an elastic medium waves are produced. The medium in this case is space-time itself. More elastic than rubber and harder to tear than steel. These waves manifest themselves as the probability of getting that "ping"...much as this medium manifests itself as virtual particles a la Heisenberg uncertainy. The QM student can only hope to be no more confused than the professors at this early stage in QM.!

If it looks like a duck and chicken, sounds like a duck and chicken and smells like a duck and chicken; it's probably a duck and chicken.

 

[ynon]

Atyy: "Ocean waves can be counted using integers, would it be equally as proper to refer to an ocean wave as a particle?"

No, nor is that a valid analogy. A bowling ball has a wave function { no excuse for gutter balls} but it is not a particle.

 

[atyy]

Atyy: "Ocean waves can be counted using integers, would it be equally as proper to refer to an ocean wave as a particle?"

No, nor is that a valid analogy. A bowling ball has a wave function { no excuse for gutter balls} but it is not a particle.

Just to note that your quotation has the wrong attribution.

 

[Steve Lajoie]

Here is a bad analogy:
If you sit down and calculate the probability of rolling a 1, 2, 3, 4, 5 or 6 on a dice, you don't say that the probability is the number that comes up when you roll the dice.

The probability is the probability. The number you get when you roll is the actual number you get.
The "wave" is a probability, literally. The particle is like the number you get.

 

[Feeble Wonk]

I think those who like to think about the unity/duality of particles and waves never think the probability amplitude is a particle. If they did, they would advocate to retire one of the words, for we wouldn't need both! Instead, we see the role for both words, that's how you can tell we don't think they are the same thing, any more than we think the "heads" side of a coin is the same as the "tails" side.

Here is a bad analogy:
If you sit down and calculate the probability of rolling a 1, 2, 3, 4, 5 or 6 on a dice, you don't say that the probability is the number that comes up when you roll the dice.

The probability is the probability. The number you get when you roll is the actual number you get.
The "wave" is a probability, literally. The particle is like the number you get.

I think what this thread demonstrates more than anything else is the limitation of written (non-mathematical) language to convey an idea precisely, and the flaws in the mental imagery that gets tied up with the language.

Whatever "it" actually is that is being observed, and whatever the quantum state vector "is", or "represents", it seems clear that "it" is not composed of "particles" as the word is typically defined. Information localized at a specific point in space-time is not, by common definition, a "particle". On the other hand, other than certain mathematical similarities, the information changing dynamically over space-time is not, by common definition, a "wave".

You physicists need to bite the bullet and create new vocabulary to describe what it is that you are trying to describe. But, I suppose that would require a consensus conclusion as to what that "is". Good luck with that. But as a non-physicist looking to you guys (and girls) for answers, I'm eagerly awaiting that conclusion from those of you that can get past the "shut up and calculate" mentality.

 

[Feeble Wonk]

Sorry if the last post came across a little antagonistically. Perhaps my frustration with my lack of understanding got the best of me.

But seriously, if we are going to discuss whether the particle/wave duality concept is outmoded by current understanding, should we not have to define our terms? If we are going to suggest that this concept is more accurately described by the quantum field concept, shouldn't we have to do the same?

 

[Ken G]

You are certainly right that the meaning of the words is crucial. Many people interpret the meaning of "wave/particle duality" along the lines of "sometimes it's a particle, sometimes it's a wave, it depends on the question we ask." They reject that, for good reason, because quantum mechanics is not sometimes one thing and other times something different, it's a very consistent approach to answering questions. But when I talk about "wave/particle duality", all I mean is "when we think about what used to be thought of as particles and their trajectories, we often forget that waves can do that too. And when we think about what we used to think of as waves and what they do, we did not realize that particles can do all those things too. Hence, the discovery is that particles and waves were never two completely different things, that was wrong. They are different aspects of the same things, aspects that show up everywhere we look."

Of course, others are free to say "neither the particle concept, nor the classical wave concept, are correct in quantum field theory," and that's fine-- conceptual entities like "particles" and "waves" that are defined in one theory are never defined the same way in some very different theory. But the concepts themselves are still of great value, in any of these theories-- the idea that entities show up in discrete bundles of fixed mass or energy is all one needs to mean by the "particle" notion, and that outcomes can exhibit interference is all one needs to mean by the "wave" notion, for those notions to be tremendously valuable. Perhaps you are looking for more specific meanings of what these things are, but every theory defines them for its own purpose.

 

[Feeble Wonk]

Perhaps you are looking for more specific meanings of what these things are, but every theory defines them for its own purpose.

Thanks Ken. Yes, I was hoping (not realistically) for a more specific description. But you're right, of course. Each QT interpretation would give me a different answer. I realize that, and accept it... but not without a great deal of frustration.