[QUOTE]Light can change speed, even in a vacuum, a new paper reports. The discovery could change the way we think about one of the constants of the universe.The importance of the speed of light to physics can hardly be overstated. The number 2.997 x 108 m/s governs our lives, even if we seldom notice it. It forms the [URL="http://zidbits.com/2011/04/why-cant-anything-go-faster-than-the-speed-of-light/"]speed limit of the universe[/URL]; the c in the famous equation [URL="http://www.1728.org/einstein.htm"]e=mc2[/URL], and also defines the way we measure distance. High school physics teaches us that this speed is not quite universal – when travelling through water or glass, light slows down; an effect put to good use in [URL="http://www.physicsclassroom.com/class/refrn/Lesson-5/Refraction-by-Lenses"]lenses[/URL] and [URL="http://mintaka.sdsu.edu/GF/explain/optics/prisms.html"]prisms[/URL].
For this reason, c is correctly referred to as “the speed of light in a vacuum.” However, [URL="http://arxiv.org/ftp/arxiv/papers/1411/1411.3987.pdf"]in a paper on [/URL][URL="http://arxiv.org/ftp/arxiv/papers/1411/1411.3987.pdf"]arXiv[/URL], Miles Padgett from the University of Glasgow has shown that even this needs a rethink. He manipulated the wave structure of some photons and sent them on a path of the same length as unaltered packets of light. The manipulated photons arrived later, indicating they were travelling more slowly.
The manipulation occurred by twisting a [URL="https://www.princeton.edu/~achaney/tmve/wiki100k/docs/Plane_wave.html"]plane wave[/URL] (one where the wave front is a parallel plane at right angles to the direction of travel) into a [URL="http://ieeexplore.ieee.org/xpl/login.jsp?tp=&arnumber=1451581&url=http%3A%2F%2Fieeexplore.ieee.org%2Fxpls%2Fabs_all.jsp%3Farnumber%3D1451581"]conical wave front[/URL], which is analogous to focusing a wave from a spread-out source onto a single point.
[B]The slowing occurs at a rate of about one part in a hundred thousand. So in the time it takes unmodified light to travel a meter, the adjusted light makes it 0.01 millimeters less.[/B] With some understatement, Padgett and his co-authors note, “Measuring the arrival time of single photons with femtosecond precision is challenging.” The team achieved this by producing strongly correlated photon pairs and having them meet at the destination so that tiny variations in their arrival times would be revealed as [URL="http://www.physics.wisc.edu/undergrads/courses/spring08/208/Handouts/InterferenceFAQ.pdf"]phase differences[/URL]. [/QUOTE]
[URL="http://www.iflscience.com/physics/speed-light-can-vary-vacuum"]Source[/URL]
Rather interesting, however I'm curious if the discrepancy is due to amplitude invariance, considering the bessel beam works off of interference thus it may have been below the detector's threshold when it arrived IIRC.
Another fascinating look into slowing down light in a vacuum, this time with [URL="http://www.iflscience.com/physics/could-we-be-wrong-about-speed-light"]electron/positron pair production/destruction![/URL]
Im over here with barely any knowledge of this. Still interesting.
[quote]2.997 x 108 m/s[/quote]
That seems like a slightly awkward way to refer to the speed of light. It's like it's sorta kinda making the effort to go scientific notation, but then does it's own thing.
Edit: I am aware that the period is used as the thousands seperator.
But hasn't it basically always been known that the speed of light changes depending on the medium through which it travels?
[QUOTE=helpiminabox;46995103]That seems like a slightly awkward way to refer to the speed of light. It's like it's sorta kinda making the effort to go scientific notation, but then does it's own thing.[/QUOTE]
Usually in most macro-scale engineering you can just simplify it down to 3x10^8 with negligible effect.
[QUOTE=helpiminabox;46995103]That seems like a slightly awkward way to refer to the speed of light. It's like it's sorta kinda making the effort to go scientific notation, but then does it's own thing.[/QUOTE]
It's a formatting issue due to copy-pasting
[editline]23rd January 2015[/editline]
[img]http://puu.sh/eUXv3/235c4e1456.png[/img]
[QUOTE=EvacX;46995120]It's a formatting issue due to copy-pasting[/QUOTE]
Oh, duh. Ignore me then.
[QUOTE=helpiminabox;46995103]Edit: I am aware that the period is used as the thousands seperator.[/QUOTE]
It's not, it's a decimal point
[QUOTE=Jcorp;46995108]But hasn't it basically always been known that the speed of light changes depending on the medium through which it travels?[/QUOTE]
thats the speed of sound
[QUOTE=Jcorp;46995108]But hasn't it basically always been known that the speed of light changes depending on the medium through which it travels?[/QUOTE]
In actual matter yes, in free space it's a constant. The permittivity (E0) & permeability(μ0) of free space are constant (And these values vary based upon the relative permittivity(Er) and permeability(μr), or the dielectric constant, of the matter the light propagates through):
A vacuum has a relative permeability/permittivity of 1.
V_light = 1 / sqrt( (Er*E0) * (μr*μ0) )
I remember there was a lab experiment where light passing through some form of matter(I forgot what material it was) slowed down.
[QUOTE=AlienFanatic;46995158]thats the speed of sound[/QUOTE]
No, it's both
[QUOTE=LoneWolf_Recon;46995163]In actual matter yes, in free space it's a constant. The permittivity (E0) & permeability(μ0) of free space are constant (And these values vary based upon the relative permittivity(Er) and permeability(μr), or the dielectric constant, of the matter the light propagates through):
A vacuum has a relative permeability/permittivity of 1.
V_light = 1 / sqrt( (Er*E0) * (μr*μ0) )[/QUOTE]
Ah, fair enough. Pardon my ignorance.
I went to see Miles (the paper's author) give a talk about this a couple of months ago. He didn't realise that it was a public talk and started being a dick to people because they didn't understand university-level optics.
Still, it's an interesting discovery. I'd love to join the optics lab at Glasgow once I graduate.
Isn't it more common to use dot / decimal point instead of comma?
I always hated how for example in excel you have to write 1,03 instead of 1.03.
All because my brain says "," is separator for different values in a list etc. and "." is for decimal/float number.
Pls american units, why are you always doing everything more difficult and illogical?
[QUOTE=creec;46995194]Isn't it more common to use dot / decimal point instead of comma?
I always hated how for example in excel you have to write 1,03 instead of 1.03.
All because my brain says "," is separator for different values in a list etc. and "." is for decimal/float number.
Pls american units, why are you always doing everything more difficult and illogical?[/QUOTE]
Fun fact: American standard is a dot as decimal marker and comma as thousand seperator and it's the opposite in Finland.
[QUOTE=LoneWolf_Recon;46995088][URL="http://www.iflscience.com/physics/speed-light-can-vary-vacuum"]Source[/URL]
Rather interesting, however I'm curious if the discrepancy is due to amplitude invariance, considering the bessel beam works off of interference thus it may have been below the detector's threshold when it arrived IIRC.
[/URL][/QUOTE]
Uhh.. yeah, just what I was thinking.
[QUOTE=creec;46995194]Isn't it more common to use dot / decimal point instead of comma?
I always hated how for example in excel you have to write 1,03 instead of 1.03.
All because my brain says "," is separator for different values in a list etc. and "." is for decimal/float number.
Pls american units, why are you always doing everything more difficult and illogical?[/QUOTE]
It's really not hard to tell which one it is based on context...
Also, on topic, presumably the effect is/will be small enough that it can be simplified to what we all know it as in most circumstances? I don't see anything in the article which suggests under what circumstances the speed might change, just that they noticed some apparent difference in speed...
[QUOTE=EvacX;46995170]No, it's both[/QUOTE]
afaik pretty much everything slows down in some circumstances
so just make the vacuum 100000 times longer and it still stop light completely, right?
right?
:\
[QUOTE=Jcorp;46995108]But hasn't it basically always been known that the speed of light changes depending on the medium through which it travels?[/QUOTE]
You didn't read the article, this is about the speed of light changing in a vacuum from what it's suppose to be to very slightly slower while still in a vacuum.
This has some major implications if it is proven to be accurate. For one, some stars could be thousands of light years closer than previously established, for while .01 mm/m doesn't sound like much, in terms of light years, that's a pretty substantial difference.
Not super surprising. Slowdown of light in a medium is caused by the material emitting its own electromagnetic radiation that interferes with the original light and changes its group velocity. It's neat that they can do it without other material but that's really all they're doing here: changing the group velocity of the light. It's not breaking relativity or the known laws of the universe or anything. I don't understand why science media needs to make everything sound like the breakthrough of the century.
[editline]23rd January 2015[/editline]
[QUOTE=valkery;46996194]This has some major implications if it is proven to be accurate. For one, some stars could be thousands of light years closer than previously established, for while .01 mm/m doesn't sound like much, in terms of light years, that's a pretty substantial difference.[/QUOTE]
Not really. Mostly the methods we use to measure stellar distances won't be affected by this.
[QUOTE=JohnnyMo1;46996434]Not super surprising. Slowdown of light in a medium is caused by the material emitting its own electromagnetic radiation that interferes with the original light and changes its group velocity. It's neat that they can do it without other material but that's really all they're doing here: changing the group velocity of the light. It's not breaking relativity or the known laws of the universe or anything. I don't understand why science media needs to make everything sound like the breakthrough of the century.
[editline]23rd January 2015[/editline]
Not really. Mostly the methods we use to measure stellar distances won't be affected by this.[/QUOTE]
Nevermind then. :v:
It might just be the flawed human perspective.
Wake me up when we can manipulate gravitational fields
[QUOTE=Impact1986;46999435]Wake me up when we can manipulate gravitational fields[/QUOTE]
But we already can, man.
Just pack on more mass.
Sorry, you need to Log In to post a reply to this thread.