Good news for causality [Speed of Light]

PhysicsWeb ^ | 18 November 2004 | Belle Dumé

[PatrickHenry]

Physicists in Switzerland have confirmed that information cannot be transmitted faster than the speed of light. Nicolas Gisin and colleagues at the University of Geneva have shown that the "group velocity" of a laser pulse in an optical fibre can travel faster than the speed of light but that the "signal velocity" - the speed at which information travels - cannot (N Brunner et al. 2004 Phys. Rev. Lett. 93 203902).

Two types of velocity are used to describe the propagation of a wave in a dispersive medium: the phase velocity and the group velocity. The phase velocity is the speed at which light of a single wavelength moves. However, pulses of light contain a range of wavelengths, which all move at different speeds in a dispersive medium. The group velocity is the speed at which the pulse moves. In particular, the energy travels at the group velocity rather than the phase velocity.

In recent years, physicists have shown that both these velocities can exceed the speed of light in certain situations without breaking the laws of special relativity. This means, therefore, that neither the group nor the phase velocity can be used to describe the speed at which the information in the pulse travels, and we need to define it by another velocity - the "signal velocity". This is defined as the speed at which the "front" of the pulse travels. According to relativity, this speed can never exceed the speed of light in a vacuum because, if it did, it would be equivalent to sending the signal backwards in time, which would violate causality.

In their experiment, Gisin and colleagues sent a pulse of polarized photons into an optical fibre that was sandwiched between an input and an output polarizer. The fibre is birefringent, which means that it splits the pulse into two pulses that are orthogonally polarized to each other. By carefully choosing the correct input and output polarizations, the Geneva team found that it could obtain constructive interference for photons at the front of the pulse and destructive interference for photons at the rear. Since only photons at the front give rise to a signal, the team was able to measure the time it took for these to arrive at a detector.

Gisin and colleagues plotted counts against the time-of-arrival of the photons (see figure). Using this graph, they calculated a mean group velocity that was 1.76 times the speed of light in vacuum. More importantly, they measured the signal velocity for the first time and showed that the increase in the group velocity does not increase the speed at which information travels.

[PatrickHenry]

The figure the article mentions is at the original article, so you'll have to follow the link.

[PatrickHenry]

Science list Ping! This is an elite subset of the Evolution list.
See the list's description in my freeper homepage. Then FReepmail me to be added or dropped.

[longshadow]

Any bets on how many posts it will take for the Luddites to swarm?

[Tarpaulin]

"confirmed that information cannot be transmitted faster than the speed of light."

They obviously did not perform research on my ex-wife in this study.

[djf]

What does this say about Bell's work?

I thought all his stuff was confirmed by the Aspect experiments.

[PatrickHenry]

The first 10 posts will be of the "Bush did it" type.

[eagle11]

I'm curious about the "interval" between (1.0)C and (1.76)C. What vacuum property causes this (0.76) C difference? Interesting article! Ping.

[PatrickHenry]

What vacuum property causes this (0.76) C difference?

I donno. I just post 'em.

[billorites]

Ans. "a mean group velocity 1.76 times the speed of light in vacuum"
Ques. How long before the first Kerry insider writes a 'tell all" book?

[flashbunny]

Well there goes my flux capacitor design.

[cripplecreek]

You might be suprised at how many "luddites" the "I'm smarter than you" comments will draw. If insults are the cost of learning, I'll just keep my pride and my ignorance thank you.

[VadeRetro]

Within the first twenty posts there will be a limerick.

[djf]

A couple of related items:

http://math.ucr.edu/home/baez/physics/Quantum/bells_inequality.html

http://www.drchinese.com/David/EPR_Bell_Aspect.htm

[Pahuanui]

Shouldn't this more accurately read, "Physicists in Switzerland have confirmed that information cannot be transmitted faster than the speed of light when using light as the medium and/or reference point"?

[Joe Miner]

Since only photons at the front give rise to a signal...

But since a photon can be in two places at one time until you observe it, can you really say one was breaking the speed limit while on a cell phone?

[Tarpaulin]

"Within the first twenty posts there will be a limerick."

There was a young lady named Bright
Who traveled much faster than light
She started one day
In a Relative way
And returned on the previous night.

[VadeRetro]

Thanks for preventing me from having to fulfill my own prophecy!

[Gibtx]

and next week they will prove that it can be ...... like the sound barrier all this obsessing about something that is so obvious ..... Albert would be very disappointed...

[PatrickHenry]

Within the first twenty posts there will be a limerick.

When trying to undo causality,
Fritz removed himself from reality,
When he saw what he'd done,
He said: "This is such fun!"
"I never did really want to be."

It takes a minute or two to compose one of these things.

[RightWhale]

Yes. The special theory of relativity is based on electromagnetic phenomena and the photon. The general theory adds gravity. There are apparently two other kinds of fields. Is there another way to look at this?

[cloud8]

In recent years, physicists have shown that both these velocities can exceed the speed of light in certain situations without breaking the laws of special relativity.

"Both these velocities" being, namely, the "phase velocity of light" and the "group velocity of light." Right? I'm not trying to be a wise guy, but isn't that the same as saying "the speed of light can exceed the speed of light in certain situations?"

But surely I am misreading. Perhaps when the quoted passage of the article says "the speed of light," the author is referring to a particular speed, like one posted on a sign. In that case, the author might have achieved greater clarity had he written "the phase velocity and the group velocity can exceed 186,000 miles per second."

[JRandomFreeper]

Physicists in Switzerland have confirmed that information cannot be transmitted faster than the speed of light.

Tell that to my elder daughter. And all of her friends. Ooops. Too late.

/john

[PatrickHenry]

I somehow managed to get it into the first 20 posts, thus proving your prophecy.

[PatrickHenry]

"Both these velocities" being, namely, the "phase velocity of light" and the "group velocity of light." Right? I'm not trying to be a wise guy, but isn't that the same as saying "the speed of light can exceed the speed of light in certain situations?"

In a nutshell, the transmission of information can't exceed the speed of light.

[Jim_Curtis]

According to relativity, this speed can never exceed the speed of light in a vacuum because, if it did, it would be equivalent to sending the signal backwards in time, which would violate causality.

Someone explain how a signal going faster than the speed of light gets sent back in time. According to whose watch?

[VadeRetro]

My flashlight, it throws off a mighty beam.
To outrace it seems such a flighty dream.
But each time I try it,
The facts just deny it,
Because I get back and it turns out I haven't even started yet.

I have trouble with last lines.

[JRandomFreeper]

LOL! BRAVO!!! Encore!!!

/john

[PatrickHenry]

Don't give up your day job.

[PatrickHenry]

Someone explain how a signal going faster than the speed of light gets sent back in time.

I have an old post on this. If I have a chance I'll dig it out and post it.

[js1138]

You guys should do a few more prophesy experiments, then do a probability analysis.

[VadeRetro]

I prophesy I'm going to go out for spaghetti and sausage.

[Nick Danger]

The fibre is birefringent, which means that it splits the pulse into two pulses that are orthogonally polarized to each other.

Damn! They've found the secret of my diabolical Politics Ray.

[snarks_when_bored]

If bits traveled faster than light,
I'd whip out my keyboard and write
A message to me,
As I used to be,
Saying "Buy Microsoft, you asswipe!"

[steve86]

According to whose watch?

G_d's

[mental]

Is there another way to look at this?

I don't know but it looks like there's been another transporter accident...

[layman]

This confirms what I've been saying all along.

[Cornpone]

So does anyone have the answer to Roger Penrose's discussion of quasi-crystals and non-local causality (The Emperor’s New Mind)? He implies that information can travel faster than the speed of light and the seeming empirical evidence is quasi-crystal growth. The crystal grows from the outer boundary inward in perfect symmetry which implies communication of some sort.

[Trainfish]

Good news for causality, bad news for Kerry's "plans"...

[finnigan2]

"Well there goes my flux capacitor design."

- To hell with your flux capacitor - this news has just ruined all my plans for an Infinite Impossibility Drive based on the number 42. Now you tell me it's only 1.76!

[realpatriot]

"Shouldn't this more accurately read, "Physicists in Switzerland have confirmed that information cannot be transmitted faster than the speed of light when using light as the medium and/or reference point"?"

Exactly what I was thinking. If you had a long enough rod say from the earth to the moon, Morse code could be transmitted instantly.

(I know physical constraints and a multitude of other considerations make this unfeasible, but you get the idea.)

[Dallas59]

So....if we travel faster than light to someplace....we won't know we're there until our brains catch up with us?

[snarks_when_bored]

If you had a long enough rod say from the earth to the moon, Morse code could be transmitted instantly.

(I know physical constraints and a multitude of other considerations make this unfeasible, but you get the idea.)

Uh, nope. Electromagnetic forces hold physical rods together. Any pressure wave in a physical rod propagates courtesy of electomagnetism. Hence the speed of light is the upper limit for such propagation.

[delacoert]

I thought all his stuff was confirmed by the Aspect experiments.

I though this stuff was prime effacia evident by a little scientific sense that appreciates the difference between an abstract mathematic parameter (group velocity) and an actual physical quantity described by mathematics (wave speed). But hey, that's just me.

[snarks_when_bored]

'electromagnetism' in the previous post. Dang.

[theFIRMbss]

>You guys should do a few more prophesy experiments

Okay. How many of you found this thread oddly arousing even before you arrived at this post? Your subconscious knew it was coming . . .

[ijcr]

Someone explain how a signal going faster than the speed of light gets sent back in time. According to whose watch?

"I am John Kerry and I am reporting for duty."

Further instances of time travel are the sole domain of envoironmental climatologists.

[DBeers]

Fast-light and slow-light curves compared to the reference pulse, whose centre is marked by the dashed vertical line. The solid vertical line marks the position of the summit of a pulse that would have travelled at speed c. The fast pulse (in red) shows superluminal group velocity (image and text: Phys. Rev. Lett. 93 203902)

[DBeers]

Good news for causality

Assuming causality is limited to that which is only measurable in the causality frame referenced is error itself. Nothing has been proven.

[Gumption]

Gisin and colleagues sent a pulse of polarized photons into an optical fibre that was sandwiched between an input and an output polarizer. The fibre is birefringent

Well duh!

[Capitalism2003]

how can it travel faster than the speed of light? Wouldn't it be going backwards through time then?