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Science

Scientists Slow the Speed of Light 139

Posted by Soulskill from the lazy-particles dept.
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.
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Scientists Slow the Speed of Light More

Scientists Slow the Speed of Light

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  • But, what are the applications of this?

    • Re: (Score:3, Funny)

      by ihtoit ( 3393327 )

      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

    • Re: (Score:3)

      by grub ( 11606 )
      Slower photons in fiber optics means a slower, more relaxed pace for my web browsing.

    • Re: (Score:1)

      by Anonymous Coward

      If you can get everyone to bet on a particular photon, and then slow that photon down so that all the other photons beat it, then you can clean up at the photon track.

      • If you can get everyone to bet on a particular photon, and then slow that photon down so that all the other photons beat it, then you can clean up at the photon track.

        Shhh ... this is how the SSC scientists make their beer money -- tricking the locals into betting like that.

  • Really Neat (Score:5, Insightful)

    by weilawei ( 897823 ) on Friday January 23, 2015 @01:47PM (#48886033)

    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.

    • Re:Really Neat (Score:5, Interesting)

      by Orgasmatron ( 8103 ) on Friday January 23, 2015 @02:08PM (#48886311)

      My first thought is that this is based on information.

      ** Crackpot speculation alert **

      c seems to be a limitation on the speed of information more than anything else. When a random photon comes in, the information arrives at the same time as the photon. If the photon has been selected in some way that allows you to make predictions, the information would arrive slightly early. To prevent this, the photons need to slow down so that the early information doesn't arrive before it should.

      • I don't think that's crackpot speculation - it's not a crazy idea. More like amateur speculation, and a really good one at that imho. Wish I had some mod points for you.

        • In fact this was one of the first things about Einstein's Special Theory of Relativity that he and his colleagues realized: the implication that information could not travel faster than C.

          But now we have yet another recent claim from the same old supposedly-discredited source that neutrinos can actually travel faster than light.

          I suspect that eventually Special Relativity will go the way of Newtonian physics: it will be deemed a very good approximation under most circumstances, with certain edge-case

    • Re: (Score:1)

      by laing ( 303349 )
      All of the experiments you cite relate to slowing down light as it passes through some medium other than a vacuum (such as a bose-einstein condensate). AFAIK nobody (until now) has ever been able to slow down light as it travels through a vacuum.

      • That was the point of my comment. I stated what had been done previously and then moved on to the new work.

    • Re: (Score:3)

      by Nemyst ( 1383049 )
      Wait. They're slowing the group velocity, which isn't what most people think of when they read "velocity".

      Group velocity is the speed at which the signal carried by a photon propagates. Essentially, if you look at a moving sine wave, group velocity is the speed at which it's moving. We already know that this velocity can be altered and can even be faster than c. This is different from signal velocity, which is the speed at which the individual photons carrying the signal propagate. Each photon is also a w

  • Surely this wasn't intended behavior? The more we poke at reality, the more it seems like a simulation that works really well, but where you can see some artifacts once you get in close.

  • But then don't some have to go FASTER than light? (Score:2, Interesting)

    by Anonymous Coward

    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.

    • My read on this (probably wrong) is that c is the max speed limit and the slow photons are the stragglers of the peloton, which would mean c' for this beam is slightly less then c. Again, probably totally wrong.

    • Re:But then don't some have to go FASTER than ligh (Score:4, Informative)

      by ihtoit ( 3393327 ) on Friday January 23, 2015 @01:57PM (#48886183)

      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.

      • Comment removed based on user account deletion

        • Re: (Score:1)

          by ihtoit ( 3393327 )

          no, because c is a fixed and finite value. It would represent omnipresence and omniscience if c were infinite, but it's not. What it means is that in our frame of reference, where time is a rail which we have absolutely no conscious control over, the further away an event from the observer the longer it takes the light from that event to reach the observer. If the speed of light were infinite, we'd be burned, blind and dying from the sheer pressure of radiation hitting us from all directions.

    • Re: (Score:2)

      by Arkh89 ( 2870391 )

      May be, but only for a very short period of time, otherwise the average speed would be increasing or decreasing. So in average they might all move at c/n (in medium of index n) but on a very short time scale they might go slightly faster, or slightly slower, just not always faster or slower...

    • Re: (Score:1)

      by Anonymous Coward

      Yes, despite what the other person below said. Or above, who knows in these funny Slashdot times.
      But before you get your knickers in a twist, there isn't anything useful here to use.

      Lets go with the car analogy.
      A ferrari is peering its head around a corner as you sit waiting at the traffic lights.
      You see only the start of it and automatically assume that it is a ferrari. Who wouldn't? They are pretty unique in design.
      Suddenly, out of nowhere, a train flies around the corner at insane speeds instead.

      This

    • 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 [wikipedia.org] has a decent discussion of it and other things that go faster-than-light.

  • I disagree! (Score:5, Funny)

    by Kenja ( 541830 ) on Friday January 23, 2015 @01:56PM (#48886165)
    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.

    • Re: (Score:3, Funny)

      by Anonymous Coward

      Maybe the time cube was right all along!

      • the time cube was right, partially right, partially wrong, and wrong all at once, simultaneously, in four, 24-hour quadrants of rightness.

    • Re: (Score:1)

      by ihtoit ( 3393327 )

      the speed of light is codependent on the measurement of time.
      Is it 299.792458 metres per microsecond?
      Or is it 17987547.48 km/minute?
      Or is it 149597870km per 8 minutes 20 seconds?
      Or is it 1.08 billion kilometres per hour?
      Or is it 30.857 trillion kilometres per 3.26 years?

      (time is a constant: in our reference frame, 1 second is defined by the SI as the amount of time it takes light to travel 299792458m in vacuo. Conversely, 1 metre is the distance light travels in vacuo in 1/299792458 sec (also a defined SI c

      • Re: (Score:1)

        by ihtoit ( 3393327 )

        (1000 / 27.9769265325) x 6.02214179×10^23 atoms of 28Si, even. Bloody crappy slashdot markup.

      • 1 second is defined by the SI as the amount of time it takes light to travel 299792458m in vacuo.

        Actually, no. It is defined in terms of the period of radiation from the transition between two hyperfine states of Cesium 133.

        http://physics.nist.gov/cuu/Un... [nist.gov]
        http://en.wikipedia.org/wiki/S... [wikipedia.org]

        • Re: (Score:1)

          by ihtoit ( 3393327 )

          ..."at rest, at a temperature of 0K". (SI, 1997)

          Which would actually be impossible to attain*. No laboratory has ever managed it, never mind to do it for long enough to count a second. It is far easier to measure the speed of light in a laboratory vacuum (eg take a known gap (1m?) in a vacuum chamber and time how long it takes for a beam of light to jump across it 299792458 times). Easier still to take a mean solar day and divide it by 86,400.

          *since we're talking such miniscule energies, even the mere act o

        • Yes. The meter is defined as the distance light travels in 1/299792458 of a second in a vacuum, so GP was half right.

          • Yes. The meter is defined as the distance light travels in 1/299792458 of a second in a vacuum, so GP was half right.

            Half-right perhaps. But circular. S/he defined a second in terms of a metre and the speed of light, and then turned around and did the opposite, defining a metre in terms of a second and the speed of light.

  • With a prism from my telescope and a magnifying glass.

    • Also, I shouldn't reply to dumb-sounding science stories until I at least read the article first, in case it isn't at dumb as they usually are...like this one.

  • I don't think you are seeing the hidden implications of this report.

    They are tracking individual photons, implying they know the location of those particles.

    But at the same time, they are also keeping tabs on the SPEED of those photons at the same time.

    Now the Uncertainty Principle argues against that ever happening, except that's what the researchers* claim. Obviously these guys have invented the Heisenberg Compensator which - as we all know - is a key component to Star Trek teleportation devices. It's jus

    • Heisenberg's uncertainty limit does not say you can't know the speed and location at the same time, but rather there is a limit to the overall accuracy. So the more precisely you measure the speed, necessarily the amount of uncertainty on your measurement of location goes up. Heisenberg's limit it pretty damn small, FYI.

      • Heisenberg's principle says you can't know the position and the momentum simultaneously, not the position and the speed. However, a photon has zero rest mass, so I'd think it would have precisely zero momentum if not going at C, meaning it could be anywhere in the Universe for all we know.

    • 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."
    • They didn't say they measured the speed of the individual photon, only that it arrived later than the other photons.

    • It's just a matter of time now until we will be able to teleport to Alpha Centauri.

      The good news is you arrive on Alpha Centauri.

      The bad news is that you die screaming as the teleporter destroys your body on Earth.

  • ... but isn't Netfilx already doing something like this?

  • Ultimate conclusion (Score:3)

    by Virtucon ( 127420 ) on Friday January 23, 2015 @02:21PM (#48886421)

    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!

  • Speed of light is still constant (Score:3)

    by Underholdning ( 758194 ) on Friday January 23, 2015 @02:26PM (#48886463) Homepage Journal
    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.

    • Light travelling in a straight line isn't affected. Only light on a curve is affected.

      So you're saying that a photon coming off the foot of David Beckham, or a spit-photon thrown by Nolan Ryan, travels slower than a photon normally would?

  • Interference pattern (Score:4, Insightful)

    by Moof123 ( 1292134 ) on Friday January 23, 2015 @02:26PM (#48886465)

    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.

  • If some light can be slowed down, and other light can go faster then it, does that mean something can go faster then the speed of light? Einstien disproved?

    • Nope. Can't slingshot light faster than the speed of light. It'll leave the slingshot without any acceleration. Speed limit of light has not yet been broken in normal spacetime.

      • Maybe he meant that if as in this case the "speed of light" is modified, then light travelling at the normal rate would in fact be traveling faster than the speed of the before-mentioned light.

        I'm also wondering if this mean c will continue to be a constant?

  • Possibly the filter that altered the photon that kept a slower speed, may have created virtual viscosity around it that slowed the photon down as if it was traveling though a different medium. Otherwise, it should have sped up to full speed after leaving the filter, even with a changed waveform.
  • Does that meant here is a possibility that photons can be slowed during travel?

    As in does the speed stay the same for a photon traveling 13 billion years? (Is space/time expansion real?)

  • Photons travel the speed of light unless slowed by a medium. Since there was no medium involved, is what being observed still a photon or instead a photon like particle? Second, it would seem that conservation of mass/energy would indicate if this is a photon then something else must have changed. If there has been some other change, whether we detected it or not, would that not negate the experiment because of a state change (yes, the photon is going slower, but the system is not in the same state it was b

  • ... do you not understand?

  • Farnsworth: Of course not. That's why scientists increased the speed of light in 2208.

  • red? (Score:2)

    by Mirar ( 264502 )

    So, they didn't redshift the photon, they made it slower with the same wavelength?

  • Are they sure? Maybe they just lengthened the meter :P
  • So 0*K is absolute zero. It's a theoretical quantity. If you ever cooled something to 0*K, you'd discover it's temperature fluctuating subtly between some nonzero positive and negative fraction of a degree. This is where we get the ridiculous concept of "negative energy" (which isn't so ridiculous after all).

    c is ~299,997km/s. It also is a theoretical quantity. If you ever accelerated a mass to c, you'd discover its velocity fluctuates subtly between some miniscule quantity around c. I wonder if we c

  • I ask because we thought that light traveled faster one way than the other once, and it turned out to be a measurement error in the circuitry.

    Could the low temp also slow down the reporting devices? Things behave strangely when the temperature drops too much.

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