Light so Fast it Travels Backward 415
An anonymous reader writes "Slowing down light used to be considered a neat trick for physics wonks. But researchers in New York now say they've pushed light into reverse. And as if to defy common sense, the backward-moving light travels faster than light." While there's not much use to come of it yet, it will be interesting if Einstein himself is proved wrong.
Slashdot is like Charlie Brown (Score:4, Funny)
Stupid Science Stories ==> Lucy & the football
Re:Slashdot is like Charlie Brown (Score:5, Funny)
AARRGGH!!
Re:Slashdot is like Charlie Brown (Score:5, Funny)
Re:Slashdot is like Charlie Brown (Score:3)
(It's a West Wing quote).
Re:Slashdot is like Charlie Brown (Score:3, Informative)
This nonsense depends on an equivocation on "velocity". It is easy to get phase velocities that are not just faster than light, but infinite. It is impossible to get group velocities that are faster than c (the speed of light in a free vacuum, a universal constant.) Information travels with the group velocity.
For a scientist to report this as "faster than light" is simply dishonest, a means of grabbing headlines and attention in the hopes that it will bolster the next grant application.
The world is full
Parent needs to read up on modern optics (Score:5, Informative)
This statement, and your criticism of the experiment, is based on out of date (or simply ill-researched) information, and it worries me that it got modded up to 5.
In this case, the group velocity is indeed faster than the speed of light - the form of the wavepacket peak (the speed of which is the definition of the group velocity [1]) travels through the fibre almost instantaneously, much faster than c. This is one of the two things about this experiment is interesting, as by the old-fashioned definition you are championing, information has just been transmitted faster than the speed of light (as has been done before [2], although I believe it was generally in quantum-tunneling type situations, rather than something as normal-seeming as a optical fibre.)
The significant point to take home from that part is that the "It's the group velocity that carries information" mantra is not strictly true. In this case, the leading edge of the pulse is all that is needed to reconstruct the whole thing, and then suddenly we're faced with a battle between our definition of information transportation at the group velocity (with the wave peak) and causality. Causality obviously wins, and information transportation needs a more complex definition than is covered in introductory optics courses.
References, cos I like that sort of thing:
[1] http://www.rp-photonics.com/group_velocity.html [rp-photonics.com] - definition of group velocity
[2] http://www.rp-photonics.com/superluminal_transmiss ion.html [rp-photonics.com] - article on superluminal transmission, including a reference to situations where the group velocity is greater than c.
Re:Slashdot is like Charlie Brown (Score:4, Informative)
You can also use it to show why you can't transfer information faster than light.
Re:Slashdot is like Charlie Brown (Score:3, Informative)
No, it'd be you who has the two terms reversed. Group velocity is the one which defines the rate of information travel (usually), and is the one which is tricky (but not impossible) to get above c.
Phase velocity is not the 'speed of light itself', but the speed of an individual point on the wave profile, and it is trivially simple to get values greater than c - they arise naturally during X-ray propagation in metals, for exa
Reverse? (Score:5, Funny)
quote (Score:5, Insightful)
Way to read the article, CowboyNeal.
Re:quote (Score:2)
Re:quote (Score:2)
Re:quote (Score:3, Insightful)
Re:quote (Score:3, Insightful)
This: http://gregegan.customer.netspace.net.au/APPLETS/2 0/20.html [netspace.net.au] is what is happening.
LIGHT IS NOT MOVING BACKWARD! Only the "pulse" is.
Look at the simulator and just imagine changing the waves slightly so that the pulse moves backward instead of forward.
The "science" here is not new at all, and the real kicker is this piece of nonsense: "Boyd is already working on ways to see what will happen if he can design a pulse without a leading edge."
He sort of redeems himself by
/Obvious (Score:5, Funny)
Re:/Obvious (Score:2)
Darkness quicker than light! (Score:5, Funny)
Re:Darkness quicker than light! (Score:5, Funny)
I wonder, could this be used to make an unlightbulb? I've always wanted one of those. Too bright in the room? Don't walk over to pull the curtains, just switch on the unlight and voila, light just gets sucked into it and darkness speads into every corner of the room. Even better you get paid by the grid for the electricity you generate. Goodbye suncream, I've got my unlight with me, no need for trees to make shadows just hang it up and relax back in the shade. Imagine the tricks you could play on people with a 3000W unspotlight! Mwaahahaha. Who said science was boring?
Re:Darkness quicker than light! (Score:3, Informative)
One, the retarded Green's function, is a wave front emitted from the source and travelling outwards in time. The other, known as the advanced or acausal Green's function, is a wave front travelling inwards in time, which is absorbed by the source.
The unlightbulb is not as fanciful as you might think.
Re:Darkness quicker than light! (Score:3, Funny)
Re:Darkness quicker than light! (Score:3, Funny)
Well, why don't you go tell Phil, the Prince of Insufficient Light [unitedmedia.com] about it ?
Speed of what? (Score:3, Interesting)
I hate it when headlines use the semantics of "the speed of light" to sound sensational. "The speed of light" is just used to refer to the maximum speed of information propagation because light in a vacuum travels as that speed. I can change the speed of light by wearing glasses; while experiments similar to the one in TFA are much more complex and interesting, the point is that neither one is affecting the speed of information at all.
Re:Speed of what? (Score:2)
So, yes - The speed of the information is being affected.
Re:Speed of what? (Score:2)
Re:Speed of what? (Score:3, Informative)
No. In fact neither the speed of light nor the rate of trasmission of information exceed the speed of light in a vacuum. It is only the position of a relative maximum along the length of a light pulse that is accelerated or slowed. A light pulse (gaussian, say) consists of the sum of wav
Re:Speed of what? (Score:5, Informative)
Special relativity starts with the notion that you will measure light as going C no matter how fast you're going, or what direction you're going. (Why? Because that's what experiments showed [virginia.edu] when they tried to find an absolute frame of reference - if there were one, you could find it by looking at how light behaves). Briefly, something going faster than C means that you can find a reference frame in which cause follows effect - time travel.
The way the math shakes out, all of special relativity is based upon the notion that light in a vacuum travels along the geodesic:and that simultaneity happens along those geodesics. C, the "speed of light in a vacuum" is critical as the normalizing factor for distance and time (in doing SR and GR, velocities are best expressed as fractions of C - so half the "speed of light in a vacuum" is the unitless 0.5 - unitless because time and distance have the same unit).
Now, if light travels slower than C in any particular medium, even in a vacuum, as long as it's consistent in all reference frames that's no great shakes for special relativity per se - it just means that light isn't as special to space-time as we thought and that the M-M experiments seemed to show. If light travels faster than C, *that* is what breaks special relativity and the definition of simultaneity. In essence, it means that you can define a reference frame in which an effect will preceed its cause.
If you want to learn more about it, google on terms (along with "special relativity") like "light cone," "simultaneity," "absolute past," "absolute future," and "absolute elsewhere." For the history of special relativity, start with the link I included earier, or "Michelson" and "Morley".
Negative time was the subject of an Asimov novel (Score:5, Informative)
When advanced-wave light travels from point A to point B it arrives at point B earlier than the time it left point A. Shortly after World War II, when radar was still new, a pulsed radar beam was first bounced off the Moon and reflected back to Earth. Measuring the round trip time of the radar pulse (about 2.5 seconds) became a very precise way of determining the Earth-Moon distance. If the same measurement were done with advanced radar waves the reflection from the Moon's surface would arrive back at the Earth 2.5 seconds before the pulse was transmitted.
From there, it isn't much of a trick to lengthen the interval with automatic repeaters which bounce the advanced waves many times, lengthening the look-ahead time from seconds to minutes or hours or even days. A computer could be hooked up to broadcast ASCII-coded advance-wave messages to the past and to receive and decode them when received. Such messages could be used in any number of schemes for fun, profit, or military preparedness. The reader who is interested in possible applications is referred to Isaac Asimov's pseudo-science-fact articles in the Astounding SF's of the 1950's concerning "thiothimoline", a kind of soluble organic crystal with the unique property that it dissolved slightly before water was added.
Guess we are almost there now.
Just think of the applications:
Knowing any stock price swing several minutes (OK, just give me one minute!) in advance.
Ah, the possibilities...
Asimov sez: (Score:4, Interesting)
Course, as it's been said - this was fiction, so it had to make sense. :)
Re:Negative time was the subject of an Asimov nove (Score:2, Insightful)
However, I believe it would be safe to assume that the prior to sending beam could only appear if you were in fact going to send the beam, as in whatever dimension that allows this to happen, the beam is a single thing moving all at once through space and time, and not travelling unusually at all. It still has to be sent from the one time point to appear at the next. Or previous in this case. I think what
Faster than light! (Score:5, Funny)
Practical Uses (Score:2, Funny)
Perhaps this interesting effect could be used somehow to cause light-speed spam to reverse upon itself, causing spammer inboxes to convert to pure energy, which in turn annihilates the spammers.
Hey, a fellow can dream, can't he now?
what if you change your mind? (Score:5, Funny)
What if you are about to put the photon in, and it comes out of the fiber at the other end, but you change your mind and don't put it into the going in end?
Re:what if you change your mind? (Score:2)
Re:what if you change your mind? (Score:5, Informative)
False, if you read the article nothing comes out the output end until the proceding edge of the light enters the input. The proceeding edge contains all the information about the light pulse, so causality is never violated and your thought experiment would never work.
Re:what if you change your mind? (Score:3, Interesting)
Re:what if you change your mind? (Score:5, Interesting)
I read a great short story on that theme once (really short; I believe it was less than two full pages). A researcher built a time machine, and sent a brass cube five minutes back in time during a demonstration. An audience member, looking at the "two" brass cubes on the desk asked what would happen if he never sent the original cube. They tried - and the universe, except for the brass cube, ceased to exist.
Re:what if you change your mind? (Score:2, Funny)
Re:what if you change your mind? (Score:5, Interesting)
The thing is, there were other universes where everything was fine. The scientist put the cube into the machine and everything was okay, or the scientist never put the cube into the machine and the demonstration failed. Nobody died, and the whole of everything didn't suddenly end, they just continued along one of the consistent timelines. The versions of the people in the dead-end timeline didn't know what happened (because they ceased to exist) and the people in the continuing timeline were unaware of the existence of any others (except in a "I wonder what would have happened if..." sense).
I'll concede that this is kind of fatalistic, but if you want to allow time travel, then you really have to give up on the idea that the "forward" direction of time is special. If the second brass cube was on the table then someone must have put it there in exactly the same way that someone must have put the first one there. Cause and effect become indistinguishable because the causal relationship can run in either direction.
Re:what if you change your mind? (Score:3, Interesting)
My take on it is that maybe you aren't locked into sending the photon in, but a photon with the right properties will end up going in.
FP! (Score:5, Funny)
Re:FP! (Score:2)
Dupe (Score:5, Funny)
Ahem (Score:3, Funny)
Phase velocity vs. group velocity (Score:5, Informative)
Re:Phase velocity vs. group velocity (Score:5, Informative)
Once an for all (Score:2, Informative)
Obvious how they did that (Score:3, Funny)
Re:Obvious how they did that (Score:2)
Yeah, but the cool thing is that he did it in the future after he were dead!
Re:Obvious how they did that (Score:2)
the actual search term is (Score:2)
Re:the actual search term is (Score:2)
Re:Obvious how they did that (Score:2)
Re:Obvious how they did that (Score:2, Informative)
Since we're on the subject of light...Chuck Norris invented black. In fact, he invented the entire spectrum of visible light. Except pink. Tom Cruise invented pink.
Anyways, sounds like you better get a humor transplant and head over to http://www.chucknorrisfacts.com/index.html [chucknorrisfacts.com] before Chuck sees your comments here and eats your entire family.
Re:Obvious how they did that (Score:2)
You didn't get it? OMG!! LOL!!! But it waz soo ferneeeeeeeeeeeee!!! Ok ok ok... I'd explain it to you, but there's a Carrot Top marathon on, so I'll have to get back to you laterz.
Bow Tie... hehehe where does this guy come up with these things? Ahahahahaa.
my head just exploded (Score:2, Insightful)
Advanced fiberoptics. (Score:2)
I guess we can expect less lag in the future. But as a side effect you find yourself getting fragged in quake than reciving the data of the rocket that hit you moments later.
Nothing new (Score:3, Informative)
"Simultaneous Negative Phase and Group Velocity of Light in a Metamaterial"
"We investigated the propagation of femtosecond laser pulses through a metamaterial that has a negative index of refraction for wavelengths around 1.5 micrometers. From the interference fringes of a Michelson interferometer with and without the sample, we directly inferred the phase time delay. From the pulse-envelope shift, we determined the group time delay. In a spectral region, phase and group velocity are negative simultaneously. This means that both the carrier wave and the pulse envelope peak of the output pulse appear at the rear side of the sample before their input pulse counterparts have entered the front side of the sample."
Re:Nothing new (Score:2)
Overflow (Score:3, Funny)
Putting on a show... (Score:2, Insightful)
Two photons travelling in opposite directions (Score:3, Interesting)
Would not two photons/beams of light travelling in opposite directions be moving faster than the speed of light *relative* to one another?
I'm sure I'm missing something... so please, rip apart the above over-simplified statement. I hope to learn something by observing the process.
Re:Two photons travelling in opposite directions (Score:2)
Re:Two photons travelling in opposite directions (Score:2)
Re:Two photons travelling in opposite directions (Score:2, Insightful)
This is because of time dilation, which would result in making everything, from the flashlight holder's perspective, appear to be moving really really fast (including the light). A velocity that would normally be measured as 0.001 c will instead be measured as c due to this time dilation.
The stationary observer would, of course, measure the s
Re:Two photons travelling in opposite directions (Score:2, Informative)
Re:Two photons travelling in opposite directions (Score:2, Interesting)
Re:Two photons travelling in opposite directions (Score:2)
Re:Two photons travelling in opposite directions (Score:2)
Re:Two photons travelling in opposite directions (Score:5, Informative)
Re:Two photons travelling in opposite directions (Score:5, Funny)
If I had mod points, I'd be unsure whether to mod this up +1 Informative or +1 Funny.
Re:Two photons travelling in opposite directions (Score:3, Informative)
Technically, yes. But it doesn't matter, because nothing travels between those two photons -- they're unrelated.
If you were on a ship traveling at 0.5c and flashed a bright light at me from 1 light-hour away, that light would still get to me in one hour--even though you might think that it'd get to me in more time. The light would, however, be "stretched" -- red-shifted
Re:Two photons travelling in opposite directions (Score:3, Informative)
Basically, light is comprised of photons, which are particles which have zero mass when at rest. If a force is applied to the photon, it will experience infinite acceleration. Infinite acceleration means... infinite velocity if that force is applied for any time at al
Original Press Release has animations (Score:5, Informative)
Dupe! (Score:5, Funny)
Hmmm... (Score:2)
Wasn't it called the Ferris Bueller Ferrari Effect?
Will be fixed in the next upgrade (Score:5, Funny)
Further dossier that Einstein is still the geezer (Score:5, Informative)
No information ever acutally travels faster than the speed of light.
Nice visual explanation anywho.
Re:Further dossier that Einstein is still the geez (Score:3, Insightful)
Another Stupidly Confusing Physics Story (Score:5, Interesting)
It is perfectly possible to get *effects* from light that appear to travel at faster than the speed of light. Just take a flashlight in a super huge room and whip it around really fast. The spot of light on the wall may very well 'travel' faster than light but no actual photons traveled faster than light so this isn't a problem.
While this experiment is somewhat different I believe a similar confusion makes it sound way more interesting than it really is. In particular there are two different speeds one needs to talk about when you are talking about how fast light goes. There is the speed at which a crest of the wave advances and then there is the speed that a photon travels (probably some other ones too than I'm forgetting). I believe all this experiment is doing is making it so the crest of the wave appears to travel faster (or with negative speed?) than light even though all the photons in the light are not moving faster than light.
Thus it is a big analagous to the flashlight case where you have some effect (in this case the crest of the light wave) which appears to move faster than light even though no actual photons or information is really doing so.
To give an idea of how this could happen (though not the mechanism here) imagine a bunch of rods in a row like this:
_____ (time 0)
Now suppose we put activators under these rods to raise them at prearranged times. If we did this right we could get a 'wave' moving like this:
-______ (time 1)
--_____ (time 2)
_--____ (time 3)
__--___ (time 4)
Now if we timed the activators right we could make this 'wave' travel down the line arbitrarily fast (in principle even faster than the speed of light) even though no information or particle is actually being moved that fast.
While clearly the mechanism is different in this case I believe this is all that is happening. Namely the peak of electric field moves faster than light (or negative?) even though no real thing is doing so.
Hard to concieve of. (Score:3, Insightful)
is that how they teach optics today? (Score:2)
Effect and Cause? (Score:2)
That's why you can play with a cat and a hand-held laser pointer and the dot can move faster then the cat, but smack a cat
Wierd idea (Score:2, Interesting)
Misconceptions! (Score:3, Insightful)
previously termed 'Reflection' (Score:4, Funny)
Ah, when I were a lad, back in the days before this backwards superluminal light was deeply researched it was known more commonly as reflection.
There exist some mediums (Score:3, Interesting)
Wait... (Score:2, Funny)
The pulses aren't the same. (Score:3, Informative)
Re:The pulses aren't the same. (Score:3, Informative)
1) Their experiment detected the "other" solutions to maxwell's equations - in other words, the advanced wave. It's counter-intuitive, but a viable mathematical solution to the equations.
2) Maybe the "backwards pulse" is a not-yet-understood result of a superluminal group velocity in the fiber materials studied. Superluminal group velocities happen quite often. And by definition the group velocity is the speed at which the modulation of the wave's ampli
!@$%ing useless blogs. (Score:3, Informative)
http://www.rochester.edu/news/show.php?id=2544 [rochester.edu]
Re:Einstein was proven wrong in his lifetime. (Score:2, Insightful)
Re:Einstein was proven wrong in his lifetime. (Score:2, Informative)
Re:In a related idea... (Score:2)
Re:In a related idea... (Score:2)
Re:In a related idea... (Score:3, Interesting)
Also consider this, what's the temperature in a vacuum where there are *no* molecules to be moving at all?
Tom
Re:In a related idea... (Score:2)
I know
Also consider this, what's the temperature in a vacuum where there are *no* molecules to be moving at all?
Well, IANAPSWWBPOIBAM (I Am Not A Physicist, Something Which Will Bebome Painfully Obvious In But A Minute), but:
I vaguely remember "heat" to have a wider definition today, related to the energy state of particles more generally. And since vaccuum is never actually empty (the lack of real particles gives room for lots of virtual par
Re:FTL speeds... (Score:2)
Re:FTL speeds... (Score:2)
Re:FTL speeds... (Score:2)
Re:Okay, I'm stupid. Enlighten Me. (Score:3, Informative)
When a light pulse hits the material, the leading edge of the bell curve is observed in the material before