What is the evidence for light being particles instead of waves?

[CLourensS]

Hi to all
This may be a stupid question.

1.How much evidence is there proving light to be particles rather than waves? Like the double slit experiment, using a single photon. Are there other experiments with a similar outcome?
And 2. How sure are we that we are actually firing a single photon in the said experiment? (it could, for example, be just the smallest amount of light that could still be considered light.. )

 

[Simon Bridge]

Welcome to PF;
Those are actually decent questions.

1. Usually the photoelectric effect is cited as pretty definitive for this - a telling trait is that sometimes you get an ejected electron the second you turn the instrument on (i.e. the light wave has not had time to deliver enough energy.)

What distinguishes the (classical) wave from the particle model is that the particles deliver their energy in one sharp hit.
QM accounts for things like interference patterns by having the wave-behavior in the statistics rather than the actual object.

2. You don't need to fire a single photon ... just like you don't need to isolate a single electron to prove that charge is quantized.
http://www.cnrs.fr/Cnrspresse/en25a4.html

"The smallest amount of light that can be considered light" is the definition of "photon".

But I think you are imagining switching a light sourch on and off fast enough that only one photon gets fired out? In fact, the light source is so dim that only individual photons traverse the apparatus in one time ... in a typical experiment, the average distance between successive photons is over a kilometer (i.e. the probability that two or more do so can be made vanishingly small). The experiment is repeated many time to collect statistics (in case two or three did make it through sometimes) and the photon detectors always register light energy in randomly spaced lumps rather than as a smooth delivery ... these days we can use detectors good enough to register a single photon hit.
http://qubit.nist.gov/qiset-PDF/Nam.QISET2004.pdf

There is a semi-classical formulation that tries to account for this.

I understand there is a purely wave theory as well ... also not classical waves.
However, I think it is most useful when you are learning about particle physics to start thinking of everything in terms of fundamental particles: just to make the break with classical waves and particles and any lingering ideas about "duality".

The following lecture series often helps people understand the concepts:
http://vega.org.uk/video/subseries/8
... Richard Feynman on QED and wave-particle duality.

 

[f95toli]

And 2. How sure are we that we are actually firing a single photon in the said experiment? (it could, for example, be just the smallest amount of light that could still be considered light.. )

There are well established methods for doing this that are based on measuring correlation functions.

It is imporant to understand that single-photonics is "everyday science" nowadays, and is nothing special as long as you are talking about photons with large energies (things get a bit trickier in the infrared and microwave regimes because of technical problems).

You can get single photon sources that can generate photons "on demand" ("machine guns" ) and detectors that can register single events (without much need for statistics). It is true that neither the sources not the detectors are perfect (problems with dark counts etc), but that is true of all experimental equipment and it is the job of the experimentalist to understand and work around those problems.

Also, note that single photon sources/detectors have been used in commercial quantum cryptography equipment for a number of years.

My main point here is that this is extremely well-established and well-understood science which has already resulted in relativly mature commercial technology.

 

[CLourensS]

Thanks for the answers. The reason for my question, in essence, is; I find the wave particle duality, perhaps in ignorance, as sloppy results. As if Occam's razor alarms should sound.

I can visualize the wave, moving, as with light, as radiation at a certain frequency.

Traveling at the speed of light is easy to visualize for a wave but hard for a particle.

I struggle to see how energy can be converted to a particle (and back) at will. For example, if stars were emitting particles, shouldn't the mass of the universe be affected at the very least?

Most of all, I am baffled by the "single photon" going through 2 slits, except if observed or rather detected.

Possible explanations I can think of:
1.That it was never one photon particle that was fired, rather the smallest amount of radiated light wave that still has the original properties.
2.That our understanding of the fabric of space is too limiting and that the propagation of light through the medium of space is hiding the answer *
3.That the method of observing/detecting the light at the slit affects the experiment

*Imagine light warped space in a similar way matter does, causing a tiny gravity-well in the space around it or even just a mere disturbance in the fabric of space; how would it affect the experiment when one photon is fired through through the two slits. The way I have it, we don't know much about the propagation of light, in the way we understand that sound is pressure waves in atmosphere.

 

[ZapperZ]

Thanks for the answers. The reason for my question, in essence, is; I find the wave particle duality, perhaps in ignorance, as sloppy results. As if Occam's razor alarms should sound.

https://www.physicsforums.com/showthread.php?t=511178

As for evidence, we have plenty. Practically all of the which-way experiments (see http://people.whitman.edu/~beckmk/QM/grangier/Thorn_ajp.pdf and B.J. Pearson and D.P. Jackson, Am. J. Phys. v.78, p.471 (2010).) and photon antibunching experiments are evidence for QM's photon picture of light. And note that this picture can also arrive at the so-called wave description of light without having to switch gears.

Zz.

 

[CLourensS]

https://www.physicsforums.com/showthread.php?t=511178

As for evidence, we have plenty. Practically all of the which-way experiments (see http://people.whitman.edu/~beckmk/QM/grangier/Thorn_ajp.pdf and B.J. Pearson and D.P. Jackson, Am. J. Phys. v.78, p.471 (2010).) and photon antibunching experiments are evidence for QM's photon picture of light. And note that this picture can also arrive at the so-called wave description of light without having to switch gears.

Zz.

Thanks, I shall look up the given. Good stuff,

 

[f95toli]

I struggle to see how energy can be converted to a particle (and back) at will. For example, if stars were emitting particles, shouldn't the mass of the universe be affected at the very least?

You've missunderstood what the "duality" is all about. No one belives that light is "converted" from one form to the other, the duality referes to the fact that light has wave-like and particle-like features, and which one "dominates" depends on the experiment you are doing. But light is neither a "particle" (in the Newtonian sense) nor a wave (in the "water" sense), it is something else we don't have a good intuition for.
Moreover, this is only an issue when you try to interpret youre results, the mathematical description of light is consistent and never requires you "choose" between waves and particles if you use the full formalism.

 

[CLourensS]

You've missunderstood what the "duality" is all about. No one belives that light is "converted" from one form to the other,...

What I meant was: use energy to make a wire glow with heat (light bulb). The heat cause photon particles to appear, travel at just under 300 000km/s, hit an object causing heat on object. Converting energy into particle and back.

Your explanation did shed more light on the subject though.

 

[Bill_K]

I struggle to see how energy can be converted to a particle (and back) at will.

E = mc². Happens all the time.

For example, if stars were emitting particles, shouldn't the mass of the universe be affected at the very least?

Total mass is not conserved. Total energy (which includes mass) is conserved.

 

[Simon Bridge]

http://vega.org.uk/video/subseries/8
Feynman addresses wave-particle duality in an accessible way.
I think the main conceptual challenge concerns the habit of thinking in terms of classical particles and waves.

Energy can get changed from one form to another ... the particles are another form of energy. For light $E=hf$, and for massive particles $E=\gamma mc^2$.

 

[HomogenousCow]

In a classical context they are waves since we describe them with maxwells equations.
The "particle" side only becomes useful in QFT, even in QM we never speak of photons in a rigorous way.

I've always wondered how the term "particle wave duality" came to being, the schrodinger equation has nothing to do with waves.

 

[Simon Bridge]

There are endless histories of the term online :)
I think it came about with that matter-wave chap where is it, oh yes:

de Broglie L. (1924) Recherches sur la théorie des quanta [PhD Thesis! Sorbonne]

But the idea goes back to Newton's time.

 

[phinds]

... I can visualize ...

If you are going to study quantum mechanics, you HAVE to get over the idea that concepts such as "I can visualise ... ", "It seems intuitively ... ", and the like are in any way helpful. They will just lead you astray.

Our human experience is based on scales that do not include cosmological scales or quantum scales, so our "common sense" / "intuition" etc are not designed to be helpful at those scales.

 

[HomogenousCow]

We really should stop using the term wave, QM has nothing to do with waves

 

[CLourensS]

So in essence it is more of an abstract description of it's behavior, rather than what it actually is.. This I can visualize again, to some limited extend. It's hard to try to not visualize. And I suspect that the terminology wave particle duality must originate from an attempt to visualize the damn concept.

 

[HomogenousCow]

Well particles are represented by states, if you project the state in a position basis you get this wave looking thing for some hamiltonians

 

[CLourensS]

Is the propagation mechanics of light moving through space known? How it is "conducted" and how does it affect space. It is mass/energy after all = gravity or bent space? And the speed of light, perhaps a max amount of change possible in relation to mass and energy within certain constraints to conserve energy in the universe. (Almost e=mc2) This is probably not QM but certainly super relevant to QM. Forgive my possible blundering, I am untrained in this discipline.

 

[Simon Bridge]

You mean "does light have gravity?" or, "do we know how a photon got from A (source) to B (detector)?"

I think the speed that light travels at is understood as a property of space.

https://www.physicsforums.com/showthread.php?t=224574
... light has gravity.

(Off the Feynman lectures prev.)
... we consider the exact path of a particular photon to be uncertain. The classical trajectories are what you get on average.

There is a relativistic formulation of quantum mechanics - usually covered at post-grad level.

Does that help?

 

[CLourensS]

You mean "does light have gravity?" or, "do we know how a photon got from A (source) to B (detector)?"

Both. How does it move from A to B; There must be a form of conductance to carry the information.
If light (a photon (I struggle to say a photon because in my mind it is counter intuitive)) is/has energy and a mass, (to my understanding e and m is almost interchangeable but not quite) it must affect space and have a gravity of sorts.

Now I wonder how this gravity or affected space around a photon could affect what we would observe at the two slits. For example what if the two slits were further apart?

I suspect (probably in ignorance) that the way light travels and gravity could tell us a lot about the structure of space.

 

[Simon Bridge]

Well then - both questions have already been answered for you :)
I have provided a "tldr" version in my previous post with a link for a more detailed version. You should read/view those links in order to better frame further questions.

Meantime:

Both. How does it move from A to B; There must be a form of conductance to carry the information.

In the EM-wave model for light, the electric and magnetic fields sustain each other through space with no need for a conductor or a medium.
In the standard (particle) model, photons get from A to B the same way as any particle. Do you have any trouble with the idea that an electron can travel through a vacuum without having a conductor there? The details of how light, or any particle, gets from A to B, is covered in the Feynman lectures linked to earlier.

If light (a photon (I struggle to say a photon because in my mind it is counter intuitive)) is/has energy and a mass,

Light does not have mass.

(to my understanding e and m is almost interchangeable but not quite) it must affect space and have a gravity of sorts.

Mass and energy are interchageable through the mass-energy relation.

Now I wonder how this gravity or affected space around a photon could affect what we would observe at the two slits.

We know how the 2-slit experiment is affected - you can look and see!

For example what if the two slits were further apart?

You know that - just move the slits further apart and see.

I suspect (probably in ignorance) that the way light travels and gravity could tell us a lot about the structure of space.

The way light travels does tell us a lot about the structure of space ;)

You need to check out those links before you reply again.
;)

 

[CLourensS]

Meantime:In the EM-wave model for light, the electric and magnetic fields sustain each other through space with no need for a conductor or a medium.
In the standard (particle) model, photons get from A to B the same way as any particle. Do you have any trouble with the idea that an electron can travel through a vacuum without having a conductor there?

Space is the medium, the way I see. Vacuum for the least interference. Anything not vacuum would affect the structure of space as medium. To our current model, everything inside our expanding universe is space. And as we know, it is affected by energy and mass. And through which light travels. How it is conducted or transferred is a different question. Light could for example, since it has no mass, be a ripple in space caused by an energy source. (Just an example)

I'm working away from home and office for a bit and have only a Blackberry for internet access, which is challenging for research. Pardon though, and I will read up as soon as I'm back.

 

[Simon Bridge]

When we say that something is a medium for a wave - we mean that the wave is composed of the substance of the medium. eg. water waves are composed of water molecules, sound waves air, and so on.

To say that vacuum is the medium for EM waves is to say that EM waves are composed of bits of vacuum somehow - bits of nothingness. Fundamental particles are often described as being ripples in a field associated with that particle ... but not ripples in space.

What education level are you approaching this at?

 

[CLourensS]

Meantime:In the EM-wave model for light, the electric and magnetic fields sustain each other through space with no need for a conductor or a medium.
In the standard (particle) model, photons get from A to B the same way as any particle. Do you have any trouble with the idea that an electron can travel through a vacuum without having a conductor there?

Space is the medium, the way I see. Vacuum for the least interference. Anything not vacuum would affect the structure of space as medium. To our current model, everything inside our expanding universe is space. And as we know, it is affected by energy and mass. And through which light travels. How it is conducted or transferred is a different question. Light could for example, since it has no mass, be a ripple in space caused by an energy source. (Just an example)

I'm working away from home and office for a bit and have only a Blackberry for internet access, which is challenging for research. Pardon though, and I will read up as soon as I'm back.

 

[HomogenousCow]

Light, in classical electrodyanmics is simply the electromagnetic field, when you accelerate a charged particle a part of the field detaches and goes off to infinity (Physicists linguo for "goes off to wherever it might go off to")

 

[CLourensS]

Sorry for the duplicate; not only do I have to use a Blackberry, but I also have to content with intermittent reception. The last one appeared to have not gone through.

 

[CLourensS]

When we say that something is a medium for a wave - we mean that the wave is composed of the substance of the medium. eg. water waves are composed of water molecules, sound waves air, and so on.

To say that vacuum is the medium for EM waves is to say that EM waves are composed of bits of vacuum somehow - bits of nothingness. Fundamental particles are often described as being ripples in a field associated with that particle ... but not ripples in space.

What education level are you approaching this at?

I would argue that space isn't nothing. It has structure and behavior and affects everything.
The ripple in space was more of an analogy for visualization of an abstract concept.
If matter and energy cause gravity and if gravity is space behaving in a particular way; be it bend or curve, and if it affects the path light travels in. It can be said that light travels through space and is affected by the structure of space. It still doesn't mean space is the medium, although... What limits the speed of light to what it is in a vacuum?

My education? In this field, nothing much more than interest and self study.

 

[harrylin]

[..] I think the speed that light travels at is understood as a property of space. [..]

Agreed. However:

[...] To say that vacuum is the medium for EM waves is to say that EM waves are composed of bits of vacuum somehow - bits of nothingness. [..]

To say that space has properties implies that it is not nothingness.

 

[Simon Bridge]

I would argue that space isn't nothing.

Argue all you like. Can you substantiate these arguments?

It has structure and behavior and affects everything.

... this would be a metaphorical structure and behavior?

The ripple in space was more of an analogy for visualization of an abstract concept.

Analogies are not physics. Space does not behave remotely analogously to a medium for EM waves though ... so I don't think it is a useful analogy.

If matter and energy cause gravity and if gravity is space behaving in a particular way;

Well - it sort-of is and isn't. eg. a uniform gravitational field does not curve space-time.

be it bend or curve, and if it affects the path light travels in. It can be said that light travels through space and is affected by the structure of space. It still doesn't mean space is the medium, although...

Quite.

What limits the speed of light to what it is in a vacuum?

Why do you think that light needs something external to itself to limit it?
Apart from that - it looks like a "why" question in disguise. "Why does c have that particular value and not, say, some other particular value?" We don't do "why" questions ;)

Light as able to go so fast because it does not have any mass... so it is not actually being "limited". A much more interesting question to ask what is limiting all the massive particles - a question subject to a lot of expensive study.

My education? In this field, nothing much more than interest and self study.

The reason I ask is so I know where to pitch my replies ... you seemed to be drawing ideas from all over the place and generally mixing models up.

Let us know when you gone through the references I gave you.

 

[CLourensS]

The speed of light a property of space, kind of allows for it to ba a medium?
Sound, for example, although not QM, can travel through various substances. Wouldn't those be mediums? And those also limit it's seed. I know it's not like that but..

 

[CLourensS]

Argue all you like. Can you substantiate these arguments?... this would be a metaphorical structure and behavior?Analogies are not physics. Space does not behave remotely analogously to a medium for EM waves though ... so I don't think it is a useful analogy.Well - it sort-of is and isn't. eg. a uniform gravitational field does not curve space-time.Quite.Why do you think that light needs something external to itself to limit it?
Apart from that - it looks like a "why" question in disguise. "Why does c have that particular value and not, say, some other particular value?" We don't do "why" questions ;)

Light as able to go so fast because it does not have any mass... so it is not actually being "limited". A much more interesting question to ask what is limiting all the massive particles - a question subject to a lot of expensive study.

The reason I ask is so I know where to pitch my replies ... you seemed to be drawing ideas from all over the place and generally mixing models up.

Let us know when you gone through the references I gave you.

I will be home tonight late; at airport now. I shall look at the references soonest in the morrow.
Out of ignorance: isn't a uniform gravitational field already curved space?
Thanks for the patience. I'll be switching my phone off for flight. Apparently they make planes drop out of the sky. I think it is because of the uncertainty principle: They are uncertain why we may not have our phones on. And that is why mine shall be in my pocket, merely on silent. As a silent protest for reason.

 

[Dali]

To touch on your original question:

1.How much evidence is there proving light to be particles rather than waves?

I think the best example when light really behaves like particles is the Compton effect, see http://en.wikipedia.org/wiki/Compton_scattering . This experiment can easily be done as an undergraduate lab on a tabletop setup today. To demonstrate the effect one uses an x-ray source that shines x-rays directly on one scintillating crystal detector. Every now and then, a photon hits an electron in the crystal, and the energy of that electron can be measured. At the same time (well, after the photon traveled to your next detector!) you can measure the deflected photon at a specific angle from its original line of propagation. The deflected photon has lost the precise amount of energy that was transferred to the electron it collided with. I.e. the deflected photon has a longer wavelength than it originally had.

This effect it really impossible to explain with light being any kind of classical wave. Firstly, a wave would never kick just one electron like that. And a wave loosing energy should decrease its amplitude, not change its wavelength. Secondly, one can calculate the wavelength-shift as a function of the deflection angle using a simple billiard-ball collision model. This show that light (or x-rays) - in this particular case - really is behaving very similar to a classical particle, and not at all like a wave.

(The two-slit experiment is the other extreme example where light - in that particular case - displays its wave-like properties!)

 

[Simon Bridge]

The speed of light a property of space, kind of allows for it to ba a medium?

No.

Sound, for example, although not QM, can travel through various substances. Wouldn't those be mediums? And those also limit it's s[p]eed. I know it's not like that but..

... but it is a compelling analogy? Analogies are all very well - but physics has to have a better connection to reality than that. You keep making analogies that lead you in unhelpful directions.

Sound waves are composed of the substance they move through ... if air, then they are the motion of air molecules, if a solid, then the atoms of the solid are shifted from their equilibrium positions.

Sound waves travel faster through a solid than through air because the component parts are more tightly bound to each other - so a displacement of parts in one place strongly affects the other parts nearby.

Light is usually thought of as a disturbance in an electromagnetic field ... the exact kind of disturbance depends on the model being used. None of the models propose light waves composed of

I think all your questions so far have been answered accurately, if not to your satisfaction, and you have a lot of reading to do. You have yet to show that you have started any of that reading. Get back to us when you have.

Cheers and happy learning :)

 

[harrylin]

Hi to all
This may be a stupid question.

1.How much evidence is there proving light to be particles rather than waves? Like the double slit experiment, using a single photon. Are there other experiments with a similar outcome?
And 2. How sure are we that we are actually firing a single photon in the said experiment? (it could, for example, be just the smallest amount of light that could still be considered light.. )

It's not a stupid question at all. There is a joke that light is particles during the week but waves during the weekend. Particles can be modeled with waves; however common waves are modeled as consisting of vibrating particles. Consequently you quickly face the philosophy of those who happen to be reply your question. It may be more constructive to ask how successful the existing models handle the cases that you bring up in your questions.

 

[CLourensS]

It's not a stupid question at all. There is a joke that light is particles during the week but waves during the weekend. Particles can be modeled with waves; however common waves are modeled as consisting of vibrating particles. Consequently you quickly face the philosophy of those who happen to be reply your question. It may be more constructive to ask h ow successful the existing models handle the cases that you bring up in your questions.

Ah. This is good news. I have this concept or idea but need to learn more. My knowledge is woefully lacking, especially the math. I have some leads to follow now and material to study. This forum is the bees knees. Hopefully I can do a bit more than ask irritating questions.