3 ms·

Photons that travel in free space slower than the speed of light

58 points by mwc 12y ago

rpedela 12y ago
What is the effect of this result, if any, on established theories?
- dietrichepp 12y ago
  At a low level, none. This is an experimental validation of established theories.
- archgoon 12y ago
  None. This is predicted by existing theories. The setup they have here is basically a waveguide[1][2]. You can show classically, using Maxwell's equations, that the group velocity of a wave of light when confined to a waveguide is less than light in free space. Thinking classically, the actual wave that is transmitted is a superposition of a bunch of plane waves bouncing off the walls. The complete wave moves slower than the individual waves that make it up [3]. What the authors have experimentally verified here is that single photons also travel at the group velocity. This is mostly expected, but difficult to measure (the time difference is 30 femtoseconds after travelling a meter). It's somewhat counterintuitive because the superposition is caused by the photon interacting with itself; but that's pretty par for the course with single photon experiments. I'm mildly surprised we haven't done this until now; probably one of those things where it's been generally accepted but no one's bothered to test directly. Cool experiment. [1] http://en.wikipedia.org/wiki/Waveguide http://en.wikipedia.org/wiki/Waveguide [2] Technically, it's a Bessel beam http://en.wikipedia.org/wiki/Bessel_beam http://en.wikipedia.org/wiki/Bessel_beam ; but if you understand why this should be in a waveguide, you understand what's going on. They make this analogy directly in the paper. [3] http://en.wikipedia.org/wiki/Group_velocity http://en.wikipedia.org/wiki/Group_velocity
  - tjradcliffe 12y ago
    If there is a waveguide involved why does the headline talk about photons in free space? Photons in waveguides are not in free space. The title should be corrected to something truthful, like "Photons in waveguide-like configuration travel slower than light due to self-interference, as predicted". Anything else is deliberately, willfully, misleading click-bait (I am a physicist and clicked on the comments specifically to avoid clicking on a click-bait headline I knew was almost certainly a lie.)
    - ScottBurson 12y ago
      The last two sentences of the abstract are: Our work highlights that, even in free space, the invariance of the speed of light only applies to plane waves. Introducing spatial structure to an optical beam, even for a single photon, reduces the group velocity of the light by a readily measurable amount. So it appears there is no actual waveguide involved. I guess that means somehow they construct photons in a "waveguide-like configuration" but in free space?
    - cynicalkane 12y ago
      There isn't a physical waveguide. It's a Bessel beam.
aortega 12y ago
The paper actually has a nice one-sentence summary: "The group velocity of light in free space is reduced by controlling the transverse spatial structure of the light beam." The key word here is "group velocity"
- nilkn 12y ago
  As a non-physicist, though, that sentence is still mostly unintelligible to me. Could anybody explain it in simple terms? I don't really know what group velocity is supposed to mean, nor do I really know what the spatial structure of a light beam is.
  - grondilu 12y ago
    A light beam has an extension in space, and does so even if we're talking about the probability amplitude of a single photon. Therefore there are several ways to define the notion of velocity. One of them is the group velocity, which intuitively is the speed of propagation for the "shape" of the wave, as opposed to the phase velocity, which is the speed propagation of its "zeros". There is a quite extensive Wikipedia article on the subject, check it out: https://en.wikipedia.org/wiki/Group_velocity https://en.wikipedia.org/wiki/Group_velocity Basically this ArXiv article shows that when a light beam has an unusual spatial shape, its group velocity can become measurably slower than light. The group velocity is usually equal to the actual speed of photons, so if it's still the case here, the authors have shown that in certain conditions light can propagate in vacuum slightly slower than it usually does.
- sravfeyn 12y ago
  For titles such as these that seemingly contradict existing physics, I always open the comments section first here to see whether the article has anything real to offer or spells misconceptions on me. Thank you (HN).

58 points by mwc 12y ago

rpedela 12y ago

What is the effect of this result, if any, on established theories?

dietrichepp 12y ago
At a low level, none. This is an experimental validation of established theories.
archgoon 12y ago
None. This is predicted by existing theories. The setup they have here is basically a waveguide[1][2]. You can show classically, using Maxwell's equations, that the group velocity of a wave of light when confined to a waveguide is less than light in free space. Thinking classically, the actual wave that is transmitted is a superposition of a bunch of plane waves bouncing off the walls. The complete wave moves slower than the individual waves that make it up [3]. What the authors have experimentally verified here is that single photons also travel at the group velocity. This is mostly expected, but difficult to measure (the time difference is 30 femtoseconds after travelling a meter). It's somewhat counterintuitive because the superposition is caused by the photon interacting with itself; but that's pretty par for the course with single photon experiments. I'm mildly surprised we haven't done this until now; probably one of those things where it's been generally accepted but no one's bothered to test directly. Cool experiment. [1] http://en.wikipedia.org/wiki/Waveguide http://en.wikipedia.org/wiki/Waveguide [2] Technically, it's a Bessel beam http://en.wikipedia.org/wiki/Bessel_beam http://en.wikipedia.org/wiki/Bessel_beam ; but if you understand why this should be in a waveguide, you understand what's going on. They make this analogy directly in the paper. [3] http://en.wikipedia.org/wiki/Group_velocity http://en.wikipedia.org/wiki/Group_velocity
- tjradcliffe 12y ago
  If there is a waveguide involved why does the headline talk about photons in free space? Photons in waveguides are not in free space. The title should be corrected to something truthful, like "Photons in waveguide-like configuration travel slower than light due to self-interference, as predicted". Anything else is deliberately, willfully, misleading click-bait (I am a physicist and clicked on the comments specifically to avoid clicking on a click-bait headline I knew was almost certainly a lie.)
  - ScottBurson 12y ago
    The last two sentences of the abstract are: Our work highlights that, even in free space, the invariance of the speed of light only applies to plane waves. Introducing spatial structure to an optical beam, even for a single photon, reduces the group velocity of the light by a readily measurable amount. So it appears there is no actual waveguide involved. I guess that means somehow they construct photons in a "waveguide-like configuration" but in free space?
  - cynicalkane 12y ago
    There isn't a physical waveguide. It's a Bessel beam.

aortega 12y ago

The paper actually has a nice one-sentence summary: "The group velocity of light in free space is reduced by controlling the transverse spatial structure of the light beam." The key word here is "group velocity"

nilkn 12y ago
As a non-physicist, though, that sentence is still mostly unintelligible to me. Could anybody explain it in simple terms? I don't really know what group velocity is supposed to mean, nor do I really know what the spatial structure of a light beam is.
- grondilu 12y ago
  A light beam has an extension in space, and does so even if we're talking about the probability amplitude of a single photon. Therefore there are several ways to define the notion of velocity. One of them is the group velocity, which intuitively is the speed of propagation for the "shape" of the wave, as opposed to the phase velocity, which is the speed propagation of its "zeros". There is a quite extensive Wikipedia article on the subject, check it out: https://en.wikipedia.org/wiki/Group_velocity https://en.wikipedia.org/wiki/Group_velocity Basically this ArXiv article shows that when a light beam has an unusual spatial shape, its group velocity can become measurably slower than light. The group velocity is usually equal to the actual speed of photons, so if it's still the case here, the authors have shown that in certain conditions light can propagate in vacuum slightly slower than it usually does.
sravfeyn 12y ago
For titles such as these that seemingly contradict existing physics, I always open the comments section first here to see whether the article has anything real to offer or spells misconceptions on me. Thank you (HN).