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Light so Fast it Travels Backward
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.
From a 1985 paper http://www.npl.washington.edu/av/altvw08.html/ :
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...
Dedicated Linux servers (root access) $45 p.M.
It has to explained out all over again every time an article of that type gets posted: phase velocity can exceed the speed of light in a vacuum, group velocity cannot.
Group velocity is what carries information, faster than light phase velocity is perfectly ok with Einstein's theory.
\u262D = \u5350
This is nothing new. Materials with negative indexes of refraction have been used in experiments before. Here is the abstract of their article in Science:
"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."
See original press release with animations.
Short answer: no Medium answer: There are a bazillion webpages explaining why. Google should be able to hand you half a jillion with the most obvious searches.
http://gregegan.customer.netspace.net.au/APPLETS/2 0/20.html
No information ever acutally travels faster than the speed of light.
Nice visual explanation anywho.
The short answer is "no." The long answer is Einstein's Theory of Relativity.
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 before Chuck sees your comments here and eats your entire family.
Well, the article says the light comes out the other end before the putting-in end has light going in
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.
"Spooky action at a distance" does NOT violate general relativity. There is no transfer of information. Yes, it is wierd that the particles are so connected; no, there is no transfer of information. It is not even theoretically possible to use this for communication, which is a direct result of the fact that there is NO information transfer. I am not claiming that this spookiness is not neat; it just does in no way violate general relativity.
If I understand Dirac correctly, his meaning is this: there is no God, and Dirac is his Prophet. -Pauli
Would not two photons/beams of light travelling in opposite directions be moving faster than the speed of light *relative* to one another?
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 -- so as to cover all of the bits of reality you and me.
If you were heading towards me at the same speed, and flashed a light at the same distance, it'd get to me at the same time -- but it would be shifted in the other direction, so as to cover all of reality between you and me.
Remember: the speed of light is instantaneous. What c measures is the speed of information-transmission through the fabric of reality -- or, more easily, c is the speed of an instant.
Someone correct me if I am wrong (and I know I take some dangerous assumptions, mostly involving divide by zero error) but I have a potential model of light that may help explain the phenomenon, based around the light as a particle theory.
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 all. But then we enter relativistic speeds, where essentially the fabric of space and time (and indeed matter as well) is torn through.
First, we will imagine the situation from a stationary observer watching an object with a small resting mass being accelerated. As it reaches higher and higher velocity, it's mass increases. If the object were to reach the speed of light, the mass would be multiplied by a factor of infinity. Obviously, as the object nears the speed of light the force required to accelerate it any further becomes greater and greater, and the object will never actually reach the speed of light.
Now, a photon has zero rest mass, so multiplying this by infinity wouldn't make sense, would it? Except we are not actually dealing with infinity for the factor, we are dealing with a number that APPROACHES infinity as the photon accelerates. This brings up limit math, so if we can assume that the resting mass of the photon is not actually zero but is simply so small that it approaches zero, then we are multiplying (the limit as the mass factor due to acceleration approaches infinity) by (the limit of a resting mass approacing zero.) This can be rearrange to (lim m->infinity)/(lim m-> infinity) which, as I recall, can be a real number. So, at some point, from the observer's point of view, the photon eventually goes so fast that it gains a mass. This means that the force accellerating the photon is no longer imparting an infinite accelleration, but a finite one. The math on all this would work out that, to an observer, the photon travels at... c, the speed of light.
Next, we will view the same situation, but instead of an obersver at rest, we will imagine that we are the object being accellerated. If we have a resting mass, our percieved mass does not increase. Instead, time and space contract to an extent that it does not take as long as Newtonian physics would predict to actually reach our final destination, but we also do not percieve that we are travelling as far. This has been experimentally proven with atomic clocks aboard really fast airplanes and whatnot.
The leap in imagination comes in imagining that we are the object being accelerated, except that we have zero resting mass. In such a case we are accellerated such that the time it takes us to get to the destination is zero, but space is compressed so much that we do not percieve having traveled at all, instead the distance between start and end simply compresses into zero. The end result: it takes no time for a photon to reach the final destination, from the photon's point of view. It as is it was simply knocked instantaneously from say, The Sun to The Earth. So, it is meaningless to compare the photons travelling relative to each other, as they indeed do not percieve themselves as travelling.
And the "what about a ship travelling near the speed of light turning on it's headlights" is equally meaningless, as any object with a rest mass can not actually reach the speed of light... either from an observer's fram of reference or from the ship's frame of reference. An observer would experience a time dilation effect, where it appears that the mass of the ship increases. The ship would experience a time constriction effect, where it appears that the distance traveled is compressed such that... the speed of light is always... the speed of light. Because in reality the photon is whizzing by at infinite speed, or not travelling at all.
I'll never make that mistake again, reading the experts' opinions. - Feynman
Yup actually when something goes faster than the speed of light in the medium, it creates what's called the Cherenkov radiation. As you pointed out that's still not about breaking the speed of light in vaccum.
You just got troll'd!
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 waves of many different frequencies extending out in both directions from the pulse maximum. If you create a region of space with a high gradient in the index of refraction, the different frequencies will change their relative phases, shifting the position of the pulse maximum. This can create the illusion of a change in the speed of light, since the pulse maximum appears to travel at a different speed. But for any information to be trasmitted, the whole pulse must be transmitted. It's not like a bullet--it's more like a vibrating string with a kink in it. When the string comes out the other side, the kink is in a different place, but the string moved the same speed. Since you can only send one whole string at a time, you can't send information faster than the speed of light. This is old hat.
Shouldn't you be doing something useful?
Imagine in a vaccuum that a pulse of light looks like a square wave (actually more like an impulse - a square wave of infinite thin-ness and therefore infinite magnitude). When you put this pulse of light through a material, the pulse is more like a bell curve (but not exactly). So they create different types of material that create wider and wider bell curves for the light that passes through them.
When a light pulse hits the material, the leading edge of the bell curve is observed in the material before the impulse (the "peak" of the curve) actually hits the material. The peak of the bell curve never travels anywhere faster than light, but the leading edge appears to happen BEFORE the cause.
So they create a material where the bell curve is so spread out that the leading edge starts to exit the other end of the fiber (and also reflects and goes back down the fiber) before the impulse hits the material at all. It's just a matter of how much you can stretch the "bell curve".
The thing is that the "information" can only be measured or used once the peak of the curve arrives because the pulse is a photon or made up of photons, and they need to interact with something to transmit information, and the interaction takes energy which requires all the energy from the pulse which requires at least the peak to arrive to effect some change on some other piece of matter, which means this is some novelty for some bored physics guys. (Just kidding).
I don't really know the theories, but after reading several articles about it, the above is my best stab at explaining it.
"I have never let my schooling interfere with my education." - Mark Twain
I'm a Physics grad student who just happens to be doing my Master's project on negative index materials (or commonly known as NIM). I'm not an expert in this subject but our reseach group actually discussed this same subject last week. The point here is that the individual photons are not moving faster than light. In fact (what I was told by my professors and others) is that the pulse going in is NOT the same pulse going out. It's the front end 'tail' of the pulse which 'piles up' to become a new pulse which is seen coming out the other end in the shape of the original pulse. The incoming 'peak' of the pulse collapses (actually a portion of it gets reflected which for some reason doesn't ever show up in these simulations of the phenomenon) so only a portion of it exists after going in (I see this in my 1D FDTD simulation all the time). There is actually alot of distortion of the pulse at the interfaces (and inbetween) to the point that it's hard to say what is the original pulse and what isn't. In fact, if you just send light in with no 'peak' you will still detect a 'peak' coming out.
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 of (mostly uninteresting) phenomena that travel "faster than light" by this definition. This is just one more. It is always a worthy effort to test established theory in regimes it has not been tested in before, but the odds of it producing any interesting results are staggeringly small. Absent the "faster than light" hook this story wouldn't be given any notice at all.
The honest headline would be, "Scientist tests well-established theory under extreme conditions and finds full agreement with predictions." Yawn.
Blasphemy is a human right. Blasphemophobia kills.
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 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".
"It is our blasphemy which has made us great, and will sustain us, and which the gods secretly admire in us." - Zelazny
"Group velocity" refers to the speed of a pulse on the wave, it's usually defined as (d\omega)/dk, where \omega is the temporal frequency and k is wavenumber. This can be adjusted to all sorts of crazy values with the right material. It's done all the time. Technically, group velocity exceeds c when light travels into many metals.
"Phase velocity" is c, the speed of the light itself, and the information it carries. It's never gone faster than 3e8 m/s (not that we know of), and still hasn't after this experiment.
So yeah, like everyone else says, this is a grabbing headline for a not-so-revolutionary story. The group velocity data confirms previous theory, and it's been difficult to check on this before, but no one has called the laws of relativity into question. No need to philosophize or write a freshman-experience 'thought-provoking' one-act play just yet.
This site has a nice illustrative applet on group velocity which helps to visualise some of the points in the parent posts: http://gregegan.customer.netspace.net.au/APPLETS/2 0/20.html
You can also use it to show why you can't transfer information faster than light.
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 example (it would even theoretically be possible using a mechanical system, if you had the time and inclination to build one).
See: http://www.rp-photonics.com/superluminal_transmiss ion.html
http://www.rp-photonics.com/group_velocity.html
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 - definition of group velocity
[2] http://www.rp-photonics.com/superluminal_transmiss ion.html - article on superluminal transmission, including a reference to situations where the group velocity is greater than c.
Interestingly, the governing wave equation for light, d'Alembert's equation, has two fundamental impluse solutions, or Green's functions.
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.
May the Maths Be with you!
You're mostly right. But the equation you're looking for is:
dx^2 + dy^2 + dz^2 - c^2*dt^2 = 0
That's actually the invarient you're looking for. As my physics teacher would say, always check your units.
I must say, I find the whole idea of time like, space like, and light-like separations intriguing, but I don't know enough about it to really comment much, yet.
I believe in the 1970s that Steven Hawking showed that at the event horizon of a black hole that certain particles had to travel faster than the speed of light. I don't have the details but it also doesn't violate Einstein's theory either. That's the funny thing about all this quantum crap; you can find out that something really does exist and not violate something that says it doesn't. :)
Cheers
The ENTIRE article was ripped from the university site. Not a single added value--in fact, it was negative value as I had to go to the original for the animations. In these cases, can we please bypass to blogospammers and just get the real deal? Pretty please?
http://www.rochester.edu/news/show.php?id=2544