First ever photograph of light as a particle and a wave

Mar 4, 2015

Credit: Fabrizio Carbone/EPFL

By Science Daily

Light behaves both as a particle and as a wave. Since the days of Einstein, scientists have been trying to directly observe both of these aspects of light at the same time. Now, scientists at EPFL have succeeded in capturing the first-ever snapshot of this dual behavior.

Quantum mechanics tells us that light can behave simultaneously as a particle or a wave. However, there has never been an experiment able to capture both natures of light at the same time; the closest we have come is seeing either wave or particle, but always at different times. Taking a radically different experimental approach, EPFL scientists have now been able to take the first ever snapshot of light behaving both as a wave and as a particle. The breakthrough work is published in Nature Communications.

When UV light hits a metal surface, it causes an emission of electrons. Albert Einstein explained this “photoelectric” effect by proposing that light — thought to only be a wave — is also a stream of particles. Even though a variety of experiments have successfully observed both the particle- and wave-like behaviors of light, they have never been able to observe both at the same time.

A new approach on a classic effect

A research team led by Fabrizio Carbone at EPFL has now carried out an experiment with a clever twist: using electrons to image light. The researchers have captured, for the first time ever, a single snapshot of light behaving simultaneously as both a wave and a stream of particles particle.

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19 comments on “First ever photograph of light as a particle and a wave

  • 1
    RandyPing says:

    This is so cool. Some people say that removing the mystery of things removes the wonder, but I think that just the opposite is true.

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  • I will be interested in the comments of some of our resident physicists. If the technical details stand up to scrutiny, then this truly is an amazing result. This was thought to be impossible.

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  • How do you interpret this?

    I see a wavy upper surface. I assume the false colour on it is purely decorative, nothing to do with anything.

    I assume the lower surface is a superimposed output from some other measuring device. Its position relative to the top surface is arbitrary.

    If you have a standing wave, does that imply there is an ever-changing photon at the same spot at some frequency?

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  • 4
    NearlyNakedApe says:

    I’m guessing that the color probably expresses wavelength (the wave part of the observation) and the flat bottom part of the chart has spots matching the crests of the waves like it’s some sort of “shadow” or cross-section of the wave. Maybe this is the portion of the chart that expresses the particle or energy packet aspect of the photon.

    Just pulling stuff out of my (rather large) butt BTW… But thank goodness there are real physicists here to set me straight on this. 🙂

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  • ScienceDaily uses a diagram due to Fabrizio Carbone that lacks explanation. The Nature paper explains its diagrams, but Carbone’s diagram is not there. The paper’s diagrams use colour to represent electron counts, with the fewest electrons at the blue end of the rainbow (or, for some diagrams, the green end of a green-red spectrum). Carbone’s diagram shows that something unspecified grows at low electron counts.

    Wave-particle duality occurs because, fundamentally, nature is about entities other than waves or particles. These are what physicists call fields – the Higgs field, the electromagnetic field, the electron-positron field etc. Each has a certain value at each point in spacetime. You can measure different aspects of fields in experiments, usually favouring waves over particles or vice versa if you attempt to interpret what you see in terms of classical physics. This experiment looks at the electromagnetic field, simultaneously displaying “quantisation” (in the sense that it looks like you can count classical particles and discuss their momentum) and spatial interference (in the sense that it looks like classical waves are interfering in a location-dependent way, as they are wont to do).

    Light consists of photons. It is usually defined as visible light, i.e. photons human eyes can see. Photons that are in general invisible quantise the electromagnetic field, but this experiment is actually concerned with another aspect of the field, surface plasmon polaritons. These are electromagnetic waves that travel along the interface of a metal with an insulator such as air.

    I hope this is helpful.

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  • Yes Jos, it does help.

    I’ve recently been re-reading about radio active clocks, decay rates of elements in the Periodic Table, and how photons continually replenish Carbon-14 in the upper atmosphere enabling radio carbon dating to be carried out continuously.

    Light doesn’t just prevent the sighted from bumping into things, it’s useful stuff in other ways; for stuff that isn’t stuff that is.

    Or does it count as stuff, and I’m just talking stuff and nonsense?

    Enough, I hear you cry.

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  • 7
    NearlyNakedApe says:

    Yes thank you Jos. I guess now I have to read up on plasmon polaritons. This is heady stuff. So much to learn, so little time…

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  • Although “stuff” isn’t a technical term, a physicist might interpret it as referring to matter, as contrasted with radiation. The difference is based on rest mass. Photons are massless in a vacuum, but not in any medium that slows them. I suppose slower photons could be called “stuff”, but I’m not really sure…

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  • 9
    Lorenzo says:

    Photons are massless in a vacuum, but not in any medium that slows them.

    I’ve never come across massive real photons, so far… Besides, photons don’t actually slow down in media, they undergo a series of scattering events on (mostly) electrons, don’t they? This means that the average speed through media is less than c, but that’s just because the photons hang around in the material instead of flying straight through it.
    Macroscopically, and if the photons aren’t too energetic and the material is transparent to their wavelength, the process is described by diffraction… isn’t it?

    Perhaps you’re referring to an effect that makes them appear to have a mass, even though they are massless? I can’t recall any such effect but my memory isn’t reliable…

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  • There’s a bit more to it than that. The charged particles in a medium contribute to the local electromagnetic field, and solutions to Maxwell’s equations in that context dictate photonic paths.$ To conserve energy and momentum at the boundary of two media (one of which may be a vacuum) requires that the light is partially transmitted into the second medium, but also partially reflected back into the first one. A single photon effectively splits into two photons. You can show that energy-momentum conservation is consistent with the mass interpretations that explain the observed speeds. You can also show that the energy-momentum that goes each way on this view matches the reflection and transmission coefficients we observe, and can alternatively obtain using Maxwell’s equations at the boundary (together with appropriate continuity requirements). The great thing about such approaches is the explanation of Snell’s law, or equivalently Fermat’s principle (which is a special case of geodesic motion in general relativity). “Photons at speed c keep changing direction due to collisions” doesn’t explain that.

    Incidentally, media can also adjust neutrino masses, which in a vacuum have small nonzero values as shown empirically by neutrino oscillation. Were it not for that fact, we could still entertain the idea that neutrinos, like photons, are massless (indeed, an early variant of the modern Standard Model imagined that neutrinos were massless). So let’s take the massless-neutrino approximation for the moment. Why do media then adjust their masses? It can’t be due to the very rare neutrino-matter interactions that destroy the neutrino. Is it due to species-preserving weak-interaction forces? No; it’s due to a distortion in the Higgs field. This modifies the mass that the Higgs field gives to the neutrinos. You can interpret what happens to photons in the same way. The electromagnetic/weak split is due to the fact that, unlike the W/Z bosons, photons don’t gain a nonzero mass from the Higgs field in a vacuum. But again, in media that can all change.

    $ In case you’re wondering how an electrically neutral particle can be affected by electromagnetic fields, the answer is that, while an electrically neutral particle with a definite classical time-dependent position in space doesn’t see that position accelerate due to electromagnetism, the spacetime-dependent photonic field, a 4-vector in relativity, is a solution to an inhomogeneous differential equation that depends on the charge distribution of matter. As I said in a previous post, thinking of fields as fundamental, rather than particles or waves, is useful.

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  • 11
    Lorenzo says:

    You can show that energy-momentum conservation is consistent with the mass interpretations that explain the observed speeds.

    Yes, I get that. But that looks more like a description rather than “the real deal” -actually, taking in account of how Special Relativity is formulated, photons traveling at a speed lower than c are required to have a mass. But my point is that photons don’t actually slow down, but they “hang around”. Now that “hanging around” isn’t due to particle-like scattering on electrons: as far as the photons we are talking about is concerned, their energy is too low to engage in Compton-like processes. This is why their wavelength is much to large to actually “knock” onto something the scale of an atomic orbital and change direction.
    I’ve been nosing around the polariton yesterday, after posting, which exhibits a non-zero effective rest mass and that seems to account for the effect.

    photons don’t gain a nonzero mass from the Higgs field in a vacuum. But again, in media that can all change.

    They don’t gain mass by the vacuum field for good reasons: you want U(1) gauge invariance and (thus) charge conservation. Even before coming to the Higgs field, those condition require a massless boson field. When the Higgs field is introduced into the mix, great care is taken so that, in the vacuum, photons stay massless -the result of that is a lack of coupling between the photon and the Higgs field… which, in turn, might mean that an effective Higgs potential in media is not possible; unless you find a way to save local and global gauge invariance for the EM Lagrangian by introducing such a coupling and keep it dormant in the vacuum. Does it make sense?

    It can’t be due to the very rare neutrino-matter interactions that destroy the neutrino.

    Well, only charged current events destroy the neutrino, while NC events don’t, at the tree level. Furthermore, CC events concern only electronic neutrinos and not the others. This would lead you to expect a different effect of media on neutrinos depending on their flavor. And that’s exactly what’s being observed.
    These interactions can be described with an effective potential on neutrinos, which then goes into their Hamiltonian and, then, leads to modifications over what you expect to see in the vacuum.

    it’s due to a distortion in the Higgs field. This modifies the mass that the Higgs field gives to the neutrinos.

    This is a very interesting hypothesis: do you have some paper on the subject? Or some place where the maths is done anyway. Anything, really.
    Anyhow: I can see how an effective Higgs potential (thinking along the same line as above) can lead to an effective different coupling of the neutrinos mass eigenstates but, due to the really tiny cross section, I’d expect this effect to be at least not dominant.

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  • that’s cute, cant wait for WLC’s explanation for what is really going on 😉

    thanks for the explanation. I am constantly aware of how everything that furthers my understanding of the natural world enhances my perception of how beautiful and wonderful it all is. what a great way to start the weekend.

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  • I’m sure I’m not the only one here who hates how little comment nesting is allowed here. The following is a reply to Lorenzo’s latest post.

    Lorenzo, I’ve double-checked a few aspects of both photons and neutrinos, so I’ll now summarise (and to an extent correct) points so far.

    (1) Both the photon effect and the neutrino effect on phase velocities in a medium are examples of “coherent forward scattering”. This is to do with the electromagnetic field effects of local charges, as I said before. We’re agreed that this isn’t a case of particle-like scatterings.

    (2) My point about photon mass was that, whereas it’s important that neither the Higgs field nor anything else gives photons in vacuo mass the way that W/Z bosons in vacuo gain masses from the Higgs field, circumstances for photons in other media are different in general.

    (3) So we’re agreed that the mass change for neutrinos isn’t due to CC events, and the question is whether it’s down to NC events or Higgs effects, both of which have small cross-sections with neutrinos. You’re naturally curious about what research I had in mind. My poor memory had caused me to mix up to completely different physical effects. After this message, I’ll be embarrassed but all issues will be thoroughly clarified.

    (i) The effect on neutrino effective masses in media that I had in mind is the MSW effect, a neutrino analogue of the coherent forward scattering responsible for photons’ refraction. Wolfenstein showed in 1978 that the neutrino masses that cause oscillation are medium-sensitive (due to NC effects), e.g. oscillation is possible in media even if neutrinos in vacuo are massless and hence immune to oscillation. When I brought in the Higgs field, I was thinking of something else that affects masses, which will make for an interesting read too:

    (ii) When Cooper pairs form in a superconductor$, the gauge bosons which the paired fermions exchange all experience the same increase in their squared rest mass (equivalently, the gauge fields to which the paired fermions couple experience such an increase), and this increase is proportional to the squared binding energy for Cooper pairs. That really is due to the Higgs field, and is called the Anderson-Higgs mechanism. Virtual photons then become a short-range potential, just like W/Z bosons in vacuo. (Of course, the effective photon mass is proportional to the Cooper pair binding energy, and is on the order of meV rather than GeV.)

    So, in a general medium, mass corrections can happen due to both (i) and (ii), and hence due to both NC effects and Higgs. Of course, anything but a vacuum causes (i), whereas (ii) happens less often.

    $ This includes something esoteric such as colour superconductivity, for which what I’ve said about photons applies also to gluons.

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  • Thanks for all that information Jos.

    Now a question: we all know why light travels faster than anything else in the universe, but am I right in thinking that under specific conditions neutrinos travel faster than light?

    And that unlike light, neutrinos pass through opaque matter; including the planet and us?

    I do hope this is true, because I think they’re cheeky little things; they’re my favourite particles!

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  • Faster-than-light neutrinos have never been confirmed (a famous experimental mistake notwithstanding, and every physicist (including those involved) expected it was a mistake anyway). (It also wasn’t the first time.) As for the theoretical scope for it, see here.

    And neutrinos do pass through matter (except for rare interactions). Indeed, some neutrino experiments point the source at a detector on the other side of the planet because they will easily pass through the core!

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  • Oh yes, now I remember; when I heard about it I thought it must be wrong, and soon after that I heard Laurence Krauss saying in his inimitably calm manner that he didn’t believe it; or if memory serves, that he was convinced there had been a mistake in the experimental methodology, and that all we had to do was wait for an announcement to be made to that effect.

    Why, or rather how, did I forget that?

    Anyway, thanks for the correction.

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