# Speed of light times 1.4

Tatheg
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Topic 189754

It appears that the speed of light isn't constant. At least it can be demonstrated that light can be manipulated by humans at room temperature.
http://physicsweb.org/articles/news/9/8/13/1
and on a similar vien:
http://www.telegraph.co.uk/connected/main.jhtml?xml=/connected/2005/08/24/ecfysw124.xml&sSheet=/connected/2005/08/24/ixconnrite.html

What now?
Yours,
Bob

klasm
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### Speed of light times 1.4

The title is a bit missleading, as is often the case with "slow" and "fast" light in popular science.
Quote from the first link
"others have demonstrated group velocities greater than the speed of light in vacuum."

The important thing here is that the group velocity is not the same as the speed of the photons making up the light. A group velocity can be ass high as you like since it is not an actual physical entity moving. The information content and energy of thelight pusle still moves no faster than the speed of light.

A simple analogue to this is the following. Take 60 lights with timers and put the on a straight line with 1 light years between each. Set the timer so that the first light start at 12.00, the next light 1 second later and so on. At 12.00 you will get a "pulse" of starting lights which moves with 1 ligth year per second for one minute, but nothing physical has actually moved that fast.

Czar Brent
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### The speed of light is

The speed of light is constant when the definition is:

"The speed at which electromagnetic radiation propagates in a vacuum; it is defined as 299 792 458 m/s (186,212 miles/second)."

When light passes through a medium such as fiber the light is refracted. The definition of the index of refraction is:

"The index of refraction is defined as the speed of light in vacuum divided by the speed of light in the medium."

As you can the speed of light is "variable" to us but is not variable by definition.

WARNING! DiHydrogen MonOxide kills!

MarkF
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### I reallllllly wished the

I reallllllly wished the first author had left out faster than c comment. The work can stand on its own merit without such claims.
I suggest klasm link.
The second article can be fully explained as a search light effect. The area illuminated by traversing beam of light can easily exceed the speed of light. The claim stated in the article is pure sensationalism. I doubt the original authors would have stated things in such terms.

ps sometimes I wish people who write such articles could be sued for slander and defamation.

Ben Owen
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### There are several different

There are several different things here:

It's the "phase velocity" that can be faster than c. The phase velocity basically tells you how fast a pure sine wave moves. But there's no such thing as a pure sine wave, because it extends infinitely far ahead and behind.

Realistic signals are made by summing sine waves of lots of frequencies, so that after some finite number of wavelengths they interfere destructively and you get a signal of finite width and duration. When you figure out how fast a real signal (group of sine waves) goes, that's the "group velocity." And that's always less than c.

And then there's the fact that light slows down in matter (as compared to vacuum). So if the speed of light in some medium is 0.5c, you can make matter particles go 0.6c, and if you want you can call them superluminal. You do get neat effects like Cerenkov radiation (that lovely nuclear reactor glow), but you're not violating causality because that's tied to the vacuum speed c. Same for the "searchlight effect" and others.

Nine times out of ten, when you hear a "faster than light" story it's the confusion about phase velocity and group velocity. The tenth time it's one of the other things.

There is a very human effect here: Reporters want people to read their stories, so they write headlines that grab attention. When they read or (more rarely) listen to the scientist's explanation, they pick out certain things that sound sensational. Even if the scientist says no, the point is something else entirely. I've seen my own work exaggerated pretty dramatically, and that was from a reporter I had a lot of contact with. It's not necessarily deliberate, just the effect of people being busy and not listening to everything. The combination of them wanting attention grabbers and us not explaining things very well is volatile.

I'll be interested to see the headlines when LIGO bags a pulsar. "Little Green Men emit gravity waves?" That's what they said about the first radio pulsar, after all.

And then everyone rushes to give SETI all their resource share instead of us....

Ben

Gary Roberts
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### RE: And then everyone

Message 15883 in response to message 15882

Quote:

And then everyone rushes to give SETI all their resource share instead of us....

Not everyone!! I currently support EAH/LHC/Seti in the ratio 70/25/5. When LHC next runs out of work it will be 95/5. There are a lot of Setians who are gradually waking up to the "value" of EAH ...

Constructive contributors to Science Board discussions do have some effect :).

Cheers,
Gary.

Ben Owen
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### Gary Roberts

Message 15884 in response to message 15883

Gary Roberts wrote:

Quote:

Constructive contributors to Science Board discussions do have some effect :).

Glad to hear it!

An enormous amount of time and effort (not to mention money) is going into improving the instruments and the data analysis techniques. Constantly. We'll have a fighting chance of detecting something within the next year. By the early 2010s, the instruments will be so sensitive and have so much data that if we don't detect anything, that will actually be a bigger "discovery" than if we do. More likely, we'll be doing a pulsar census at that point.

But it doesn't happen without you folks running the screen saver, and people are more enthused about doing it when they know what's going on.

We do appreciate that-
Ben

PeteBB
Joined: 24 Nov 05
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### RE: RE: And then

Message 15885 in response to message 15883

Quote:
Quote:

And then everyone rushes to give SETI all their resource share instead of us....

Not everyone!! I currently support EAH/LHC/Seti in the ratio 70/25/5. When LHC next runs out of work it will be 95/5. There are a lot of Setians who are gradually waking up to the "value" of EAH ...

Constructive contributors to Science Board discussions do have some effect :).

I too am adding my 2cents worth. I am currently running EINSTEIN, ROSETTA, and CLIMATE PREDICTION(since 2004) to my satisfaction.
I have been unable to reconnect to SETI; It seems like 'beating a dead horse', but it is still a PET project for me - sure wish they (you?) would get it up and running again.
I am giving all of my accounts equal time (33.33% each, at present) and would like to add SETI to the mix (ultimately 25% each).
I really DO NOT understand what is happening with ROSETTA, but so long as I am helping, that makes me happy. I probably have not researched deep enough.
Anyone wishing to contact me, personally, is welcome - - your E-mail will probably go into my SPAM Folder, but if you head it EINSTEIN, ROSETTA, CLIMATE PREDICTION, or SETI, I'll pick it up and add you to my 'Science' buddies.

PeteBB
Joined: 24 Nov 05
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Credit: 449,559
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### RE: Gary Roberts

Message 15886 in response to message 15884

Quote:

Gary Roberts wrote:

Quote:

Constructive contributors to Science Board discussions do have some effect :).

Glad to hear it!

An enormous amount of time and effort (not to mention money) is going into improving the instruments and the data analysis techniques. Constantly. We'll have a fighting chance of detecting something within the next year. By the early 2010s, the instruments will be so sensitive and have so much data that if we don't detect anything, that will actually be a bigger "discovery" than if we do. More likely, we'll be doing a pulsar census at that point.

But it doesn't happen without you folks running the screen saver, and people are more enthused about doing it when they know what's going on.

We do appreciate that-
Ben

Question: What is the advantage of running the screen saver? My thinking is - letting the program run of itself is sufficient. Please advise -

PeteBB
Joined: 24 Nov 05
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Credit: 449,559
RAC: 0

### RE: The speed of light is

Message 15887 in response to message 15880

Quote:

The speed of light is constant when the definition is:

"The speed at which electromagnetic radiation propagates in a vacuum; it is defined as 299 792 458 m/s (186,212 miles/second)."

When light passes through a medium such as fiber the light is refracted. The definition of the index of refraction is:

"The index of refraction is defined as the speed of light in vacuum divided by the speed of light in the medium."

As you can the speed of light is "variable" to us but is not variable by definition.

It would seem, to me, there is no such thing as 'constant'; Why? When you add variables such as temperature, pressure, and media these ALWAYS changes the parameters of the problem in question.

Mike Hewson
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### RE: It would seem, to me,

Message 15888 in response to message 15887

Quote:
It would seem, to me, there is no such thing as 'constant'; Why? When you add variables such as temperature, pressure, and media these ALWAYS changes the parameters of the problem in question.

Light travelling through a medium ( non-vacuum ) is a modelled as a process of absorbtion followed by re-radiation, primarily by interaction with the electrons in ordinary matter. There is vacuum propagation between each interaction, matter being mostly 'empty' space. This predicts well for the states of matter found hereabouts anyway. The 'c' constant appears as a factor in the mathematical 'propagator' of quantum electrodynamics, where the overall calculation involves Feynman's sum over histories or path integral to get the amplitudes, phases and then probabilies which yield the final prediction of behaviour. In the absence of matter this method predicts the classical straight line movement with classical speed c.
So yes, the distance that light travels, divided by the time it takes to do it is going to vary depending on what it passes through. I think the merit in claiming 'c' as a constant of nature is to separate out light's intrinsic behaviour ( when left alone, so to speak ) from it's behaviour during interactions by passing through matter.
One thing I've wondered about is whether relativistics needs to take into account indices of refraction etc. After all Einstein grounded alot of his conclusions by considering light signals between separated points in space. Now if the light is slowed because it has to 'stop and talk to the electrons' on the way, then that is going to have an effect. I can't imagine a GPS system not having to account for the timing change because the signal from the satellites is passing through the atmosphere, to achieve the required accuracy.
Also the propagation of other forces are based on 'c' too. The weak and strong nuclear forces are too short range to really worry about over everyday distances, but gravity isn't. I'd assumed that matter would affect gravitational radiation via it's mass/energy content, however small, but that we could ignore electric charge etc. Maybe this is the area which is not yet unified, but I'm not sure.

I have made this letter longer than usual because I lack the time to make it shorter ...

... and my other CPU is a Ryzen 5950X :-) Blaise Pascal

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