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Scientists Slow the Speed of Light
lightbox32 sends news that scientists have found a way to slow individual photons within a beam of light. Their work was published today in Science Express (abstract, pre-print). The researchers liken a light beam to a team of cyclists — while the group as a whole moves at a constant speed, individual riders may occasionally drop back or move forward. They decided to focus on the individual photons, rather than measuring the beam as a whole. The researchers imposed a particular pattern on a photon, then raced it against another photon, and found that the two arrived at their destination at slightly different times.
The work demonstrates that, after passing the light beam through a mask, photons move more slowly through space. Crucially, this is very different to the slowing effect of passing light through a medium such as glass or water, where the light is only slowed during the time it is passing through the material—it returns to the speed of light after it comes out the other side. The effect of passing the light through the mask is to limit the top speed at which the photons can travel.
This is incredibly cool. Previous work has managed to fully stop light, but this is quite a finding (that light can travel slower through a vacuum).
The old stuff, from Wiki:
In 1998, Danish physicist Lene Vestergaard Hau led a combined team from Harvard University and the Rowland Institute for Science which succeeded in slowing a beam of light to about 17 meters per second,[1] and researchers at UC Berkeley slowed the speed of light traveling through a semiconductor to 9.7 kilometers per second in 2004. Hau later succeeded in stopping light completely, and developed methods by which it can be stopped and later restarted.
However, now we can alter the structure of the beam of light and measure a slowdown (from the abstract):
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.
Details from the pre-print:
We use an ultraviolet laser incident upon a beta-barium borate (BBO) crystal to produce photon pairs with central wavelength at 710 nm. The photons, called signal and idler, pass through an interference filter of spectral bandwidth 10 nm and are collected by polarization-maintaining, single-mode fibers. One fiber is mounted on an axial translation stage to control the path length (Fig. 2A). The idler photon goes through polarization maintaining fibers before being fed to the input port of a fiber-coupled beam splitter (Fig. 2B) (17). Instead of going straight to the other beam splitter input, the signal photon is propagated through a free-space section (Fig. 2C). This consists of fiber-coupling optics to collimate the light and two spatial light modulators (SLMs). SLMs are pixelated, liquid-crystal devices that can be encoded to act as diffractive optical elements implementing axicons, lenses and similar optical components. The first SLM can be programmed to act as a simple diffraction grating such that the light remains collimated in the intervening space, or programmed to act as an element to structure the beam (e.g. axicons or lenses with focal length ). The second SLM, placed at a distance 2, reverses this structuring so that the light can be coupled back into the single-mode fiber that feeds to the other input port of the beam splitter. The output ports of the fiber-coupled beam splitter are connected to single-photon detectors, which in turn feed a gated counter (Fig. 2D). The coincident count rate is then recorded as a function of path difference between the signal and idler arms. The position of the HOM dip is recorded as a function of the spatial shaping of the signal photon.
No, this is progress! One way to travel at the speed of light is to slow down that speed.
I don't think it does.
The Lorentz equations use the constant c, which happens to be the same as the maximum speed of light in a vacuum. Tricking some light into going slower doesn't change the constant, and it isn't a big deal to go faster than some particular light (see Cherenkov Radiation), but it would be a big deal to go faster than c.
See that "Preview" button?
Clearly they didn't slow the speed of light, but sped up time. The speed of light is a constant, the flow of time is not.
"Have you ever thought about just turning off the TV, sitting down with your kids, and hitting them?"
no, c is the top speed of your paceman - in fact, c is the only speed of your paceman. Every other rider can only travel at the same speed or *slower*. Switching pacemen means that your current paceman must drop back (ie slow down) rather than the column speeding up to overtake (thus breaking c). The average speed of the entire column must necessarily be less than c at all times, the guy at the front (doesn't matter who it is) is always the fastest man on the field unless he is dropping back to let the column overtake him - without the column having to speed up.
In cycling, the pace rider may travel at a certain speed (let's call it 40km/h), that may be the designated pace for the event. His replacement may do a short burst at 41km/h to assume the pace position. This breaks the model.
Political debates have me rolling my eyes so much I think I got optical whiplash. I should sue. - Foamy The Squirrel
I read TFA and could not specifically find where they showed they adjusted the speed and not just added an initial delay. They ran it through a mask, then onto a ~1 meter long "race track" to compare. I really wanted a clear explanation that they ran the test over 2 lengths to factor out any static delay caused by the propagation through the mask itself.
Somehow it reminds me about US educational system.
The US Senate is planning to vote on changing the speed of light. The old speed was a "hoax" . . .
Schroedinger's Brexit: The UK is both in and out of the EU at the same time!
The good news: we've got a fix ready for deployment.
The bad news: this fix will force a system restart.
At least you didn't write "Serves me right for reading coffee before drinking my morning Slashdot."
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