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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.
if you can slow the speed of a photon, then you bring the energy required to travel faster than light below infinity. FTL travel thus becomes feasible.
o.0
Political debates have me rolling my eyes so much I think I got optical whiplash. I should sue. - Foamy The Squirrel
Not a physicist, but a cyclist and an engineer--
If the population travels as 'c' on average, and they have proven that some photons slow down... Doesn't that mean other photons MUST be traveling faster than c? My impression is the relativity has no bearing here--by traveling at 'c' they are already breaking that equation. The peloton works because some move back while others move up. This blurb seems to only discuss the "back" part.
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.
Slower photons in fiber optics means a slower, more relaxed pace for my web browsing.
Trolling is a art,
Umm... This sounds like Physcis 101... Something traveling through a medium vs a vacuum will always be slower was one of the first lessons I learned
Hence "speed of light in a vacuum".
Oliver's law of assumed responsibility: If you're seen fixing it, you will be blamed for breaking it.
If it moves, tax it. If it keeps moving, regulate it.
By slowing light down, the government will be able to tax and regulate light, dramatically decreasing budget deficits and changing the economic landscape! Of course people with solar panels will be assessed charges based on the amount of light they're using unlike the rest of us who use good old coal fired electricity. Light will now be regulated into special light speed and slower than light speed lanes on the highways with of course, toll booths.
Stop taxing and regulating light now!
Harrison's Postulate - "For every action there is an equal and opposite criticism"
Well I did Read The Fine Article... in what must be the worst ever case of Casting Physics Pearls Before Illiterate Swine.
But I did get this from the abstract, and it summarizes the point of their results:
"Our work highlights that, even in free space, the invariance of the speed of light only applies to plane waves."
Pretending this is my office full of bitter coworkers..
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!
While this is an interesting read, a lot of the above comments talks about this as it is a general slow down of light. It is not. A light beam emerging from a flashlight still has the same velocity as always. Light travelling in a straight line isn't affected. Only light on a curve is affected.
Underholdning.info
If they indeed can do this, I would have like to have seen a demonstrate interference pattern showing the beat note between the normal beam and the "slowed" beam. It should be roughly as simply as using a beam splitter, one though their mask, then back into a beam combiner. If coherent laser light is pump in the slower photons should create an interference pattern along the length of the beam that any crummy detector should be able to pick up.
Instead they compared time of arrival over a single distance (as best I can tell from TFA), which is subject to systematic offsets, such as the fixed delay to get through the mask.
The good news: we've got a fix ready for deployment.
The bad news: this fix will force a system restart.
"National Security is the chief cause of national insecurity." - Celine's First Law
But this process is too young for a reboot! It's only 36,288,000 second old!
Broadcast message from root@u-vers3 ...
(/dev/pts/2) at 13:47
The system is going down for reboot NOW!
Connection to 192.168.0.3 closed.
$
At least you didn't write "Serves me right for reading coffee before drinking my morning Slashdot."
Get free satoshi (Bitcoin) and Dogecoins
Not a physicist, but a cyclist and an engineer--
If the population travels as 'c' on average, and they have proven that some photons slow down... Doesn't that mean other photons MUST be traveling faster than c? My impression is the relativity has no bearing here--by traveling at 'c' they are already breaking that equation. The peloton works because some move back while others move up. This blurb seems to only discuss the "back" part.
Try reading about phase and group velocities. In fact some EM waves have velocities above c, but these can't convey information so aren't a problem for relativity. This article has a decent discussion of it and other things that go faster-than-light.
"For every expert, there is an equal and opposite expert"
You read, but you did not comprehend. Yes, their experimental setup accounted for this.
The short form is that they played with the wave. Think of it as giving a (planar) sine wave a sideways tug. At any given moment it's still traveling at c, but it's taking a curvier path to get there.
“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.”
How they made their light fly in curlicues is definitely interesting, but as for the results, I can only say: Go home, photons, you're drunk.
At least you didn't write "Serves me right for reading coffee before drinking my morning Slashdot."
thanks, I was wondering why it tasted so bad