Exploring the Possibility: Can Light Speed Change?

[tumor]

Can speed of light increase?

 

[mathman]

The speed of light in a vacuum is constant. There is no way to change it. In material, the speed of light (slower than vacuum) depends on the material, so you can change it by varying the medium.

 

[Nenad]

you cannot increase the speed of light in a vacuum but you can have light move faster than other light, like mathman stated above.

 

[Gonzolo]

Well... there is a faster-than-light effect that can be done with light pulses. From what I recall, certain media will cause a pulse to be partly reflected and partly transmitted such that the distance between the peaks of the incident and transmitted pulses will have increased faster than c. This is explained with the fact that the phase velocity of light that can be greater than c in such media. The group velocity however, which carries energy and information can absolutely never pass c, so all is compatible with relativity.

 

[Nereid]

There have been several observational tests of the constancy of the 'fine structure constant' over the observable history of the universe (i.e. ~12 billion years). The best results are that it hasn't varied by more than a few ppm (IIRC). If the fine structure constant is constant, then c is also constant.

There are some cosmological/physical theories in which the speed of light does vary with time, but we cannot test those at present, as the era when the variations occurred are way before the earliest time we can observe (of course, it may be that the indirect effects could be observed, e.g. in the wrinkles of the CMBR).

 

[Ba]

Also the speed of light if you consider photons can travel at different speeds through the same materials (at least as I understand it) by a 'tunneling' effect where the photon travels through a substance without actually appearing inside it but from the information I received this can vary between not arriving at all up to an approximate speed of one and a half times faster than just traveling through the substance. Someone please correct me if this is wrong

 

[geometer]

There are some everyday things that can travel faster than light. Consider a seawall and a single water wave impinging on the wall at some angle greater than 0 with respect to the wall. The point at which the wave first impacts the wall moves down the wall with some velocity that is dependant on the wave's velocity and the angle the wave makes with the wall. As this angle approaches 0, the point of contact moves faster and faster until at 0 it can be considered to have infinite velocity.

 

[Matrixman13]

I've read Joao Magueijo's book...Faster Than The Speed of Light. His theory is that in early stages of the universe...the fifth dimension was "unraveled", allowing light to travel faster than the modern speed limit. As the universe cooled, the fifth dimension curled up, slowing the speed of light. I think this may have been vaguely mentioned earlier.

 

[Phobos]

geometer - those waves bend (do not remain rigid down the line)

 

[Nereid]

I've read Joao Magueijo's book...Faster Than The Speed of Light. His theory is that in early stages of the universe...the fifth dimension was "unraveled", allowing light to travel faster than the modern speed limit. As the universe cooled, the fifth dimension curled up, slowing the speed of light. I think this may have been vaguely mentioned earlier.

The connection between what we can observe and what happened in the early universe is somewhat indirect; the earliest we can 'see' is ~300,000 years after the BB (the surface of last scattering, the CMBR), and other, earlier footprints (e.g. primordial abundance of nuclides, large scale structure, ISW effect) require good models to interpret.

What observable consequences of his FSL theory did Magueijo propose?

 

[geometer]

geometer - those waves bend (do not remain rigid down the line)

Not in my experience, and even if they did, I think that would only change the rate at which the point of contact accelerates as the angle decreases. I haven't done the math on that one yet, so it's just a guess.

This would still be consistent with SR; as far as I can tell, there is no information being carried by the wave's point of contact with the wall.

 

[geometer]

Not in my experience,...

hmmmmmm - quoting myself; could that be considered narcissistic?

Anyway, another way to look at it - if the length of the seawall is short compared to the length of the wave front, the wave front will be "straight" with respect to the wall.

 

[Gonzolo]

There are some everyday things that can travel faster than light. Consider a seawall and a single water wave impinging on the wall at some angle greater than 0 with respect to the wall. The point at which the wave first impacts the wall moves down the wall with some velocity that is dependant on the wave's velocity and the angle the wave makes with the wall. As this angle approaches 0, the point of contact moves faster and faster until at 0 it can be considered to have infinite velocity.

The same can be said about the edges of two sheets of paper brought together. There is no limit to the speed at which an intersection can travel. A zero degree angle (parallel) implies infinite speed. But it comes down to comparing the time at which two independent events occur. If a Canadian and an Australian drop a sandwich on the floor at nearly the same time, nothing prevents the Earth-Diameter / Time-interval ratio to be greater than c. Similarly, the arrival times of each wave molecule at the intersection are independant of each other.

 

[Nereid]

Think of one of those laser pointers, spinning; the 'red dot' so beloved of Hollywood action movies will move across a canvass 100 million light years away at a speed many times c. For a more 'down to Earth' example, what about the radio (or X-ray) pulse from a pulsar? We can clearly 'see' these pulses, even from 10,000 light-years away; if the pulsar is a milli-second one (let's say period 10 ms), how fast does the pulse sweep across radio telescopes on Earth?

 

[meteor]

The group velocity however, which carries energy and information can absolutely never pass c, so all is compatible with relativity.

But Nicolas Gisin claims that he has observed a group velocity faster than c, and Gisin is a very prestigious optician
http://arxiv.org/abs/quant-ph/0407155

I remember that recently was performed an experiment in which a quantity called front velocity was measured to be faster than c

 

[Gonzolo]

Interesting article meteor, I superficially went through that. Gisin says that both phase velocity and group velocity can exceed c, but talks about a distinct "signal velocity", defined as the front of a square pulse. It is this that cannot exceed c. The front velocity is that of small excitatations that can apparently preceed the signal front, but not define it (Gisin's 5th reference).

So then generally speaking, there are many (apparently 4) related speeds one can associate to a single light pulse, but one of them cannot exceed c.

The article is very recent, and perhaps subject to further review, what journal is it submitted to?

 

[JV]

Think of one of those laser pointers, spinning; the 'red dot' so beloved of Hollywood action movies will move across a canvass 100 million light years away at a speed many times c. For a more 'down to Earth' example, what about the radio (or X-ray) pulse from a pulsar? We can clearly 'see' these pulses, even from 10,000 light-years away; if the pulsar is a milli-second one (let's say period 10 ms), how fast does the pulse sweep across radio telescopes on Earth?

That speed would be much larger than c. However, still nothing goes faster than the speed of light. The photons still travel with c. The first photons hit one side of the earth, the next set of photons hit the other side no time later. But there is no relation between these photons, so nothing did go faster than c. Maybe a better definition is: "there cannot be an information transfer faster than c".

 

[GOD__AM]

Think of one of those laser pointers, spinning; the 'red dot' so beloved of Hollywood action movies will move across a canvass 100 million light years away at a speed many times c. For a more 'down to Earth' example, what about the radio (or X-ray) pulse from a pulsar? We can clearly 'see' these pulses, even from 10,000 light-years away; if the pulsar is a milli-second one (let's say period 10 ms), how fast does the pulse sweep across radio telescopes on Earth?

I think that when the distance gets great enough for the "red dot" to be propagating across the surface, at near the speed of light, that it would start to be redshifted to the point that you would observe nothing. Maybe redshifted isn't the right word, but I know this "red dot" will not be observed going above c.

 

[Nereid]

I think that when the distance gets great enough for the "red dot" to be propagating across the surface, at near the speed of light, that it would start to be redshifted to the point that you would observe nothing. Maybe redshifted isn't the right word, but I know this "red dot" will not be observed going above c.

Let's say the laser pointer spins at 1 rps (revolution per second). At a distance of 1km from the centre of the pointer, the beam will trace a circle, $$\pi$$ km long, so the red dot moves across a circular screen 1 km in radius, at $$\pi$$ km/sec.

What if we set up a white screen 100 km away? 10,000 km? 1 million km? Where we set the screens up has no effect whatsoever on the laser pointer! Indeed, setting up one screen makes no difference to another screen (unless one blocks the other)!

So, why do you think the red dot wouldn't be observed going faster than c?

 

[GOD__AM]

Let's say the laser pointer spins at 1 rps (revolution per second). At a distance of 1km from the centre of the pointer, the beam will trace a circle, $$\pi$$ km long, so the red dot moves across a circular screen 1 km in radius, at $$\pi$$ km/sec.

What if we set up a white screen 100 km away? 10,000 km? 1 million km? Where we set the screens up has no effect whatsoever on the laser pointer! Indeed, setting up one screen makes no difference to another screen (unless one blocks the other)!

So, why do you think the red dot wouldn't be observed going faster than c?

Well, there well be a number of photons released in one revolution. If you divide the number of photons by the circumfrence you will get a distance. As the circumfrence increases so do the distance between photons. My guess is that this will decrease the freequency.

 

[Gonzolo]

Well, there well be a number of photons released in one revolution. If you divide the number of photons by the circumfrence you will get a distance. As the circumfrence increases so do the distance between photons. My guess is that this will decrease the freequency.

The frequency of what? Certainly not the one related to photon energy $$E = hf$$ Why would you guess that?

 

[GOD__AM]

The frequency of what? Certainly not the one related to photon energy $$E = hf$$ Why would you guess that?

From a position on the canvass looking back at the laser, if it weren't spinning, we would measure a steady freequency of photons. Now if it starts spinning at a constant rate we can measure a number of photons received during the time the light is visible. If it starts spinning faster the number of photons we measure during the time we can see the laser decreases. Isn't that a change in frequency or am I thinking wrong? Eventually the frequency will drop below 1. What do we detect then?

 

[Gonzolo]

Ok, I see what you mean, but this isn't a frequency, it is simply the number of photons hitting your detector. This number will decrease as the spinning increases. The number of photons is related to intensity, which will also decrease. (Something that is bright shoots out many photons.). If we put the word "intensity" instead of "frequency" in your last post, it all makes sense. When it is below 1, we detect nothing.

Each photon has its frequency, which is related to its wavelength and energy (colors, channels). This value does not change in this context.

There is also the mechanical spinning of the laser. 1 rotation per second means a mechanical "frequency" (totally distinct from photon frequency above).

 

[turbo]

From a position on the canvass looking back at the laser, if it weren't spinning, we would measure a steady freequency of photons. Now if it starts spinning at a constant rate we can measure a number of photons received during the time the light is visible. If it starts spinning faster the number of photons we measure during the time we can see the laser decreases. Isn't that a change in frequency or am I thinking wrong? Eventually the frequency will drop below 1. What do we detect then?

The frequency of the photons your screen receives will depend on the properties of space-time between your screen and the emittor, not on the speed with which the beam traverses your receptors.

The speed of the beam's traverse across your screen will be detectable only if you can collect enough photons to model the traverse, and if you are smart enough to "put this together" with dissociated but similar observations on a long enough baseline to make the observations fit some model that you have regarding light. Problem is, the Earth's diameter is not a very big baseline to measure pulsar arrival rates from an object that is VERY far away.

 

[GOD__AM]

Yes, I'm sorry. I shouldn't have used the word frequency. So what this means basically is that there is just more distance between events (photon detection). Not that anything is observed traveling faster than light. I mean if we made a strip of lights 600,000 Kilometers long and set timers to trigger the lights in sequence so that they all go off over one second, we have gotten a chain of events to happen faster than the speed of light, but nothing moved faster than the light.

 

[Gonzolo]

Yes, I'm sorry. I shouldn't have used the word frequency. So what this means basically is that there is just more distance between events (photon detection). Not that anything is observed traveling faster than light. I mean if we made a strip of lights 600,000 Kilometers long and set timers to trigger the lights in sequence so that they all go off over one second, we have gotten a chain of events to happen faster than the speed of light, but nothing moved faster than the light.

That's quite exactly how I see it too. Nice example.

 

[meteor]

..."signal velocity", defined as the front of a square pulse. It is this that cannot exceed c.

Hi, I've doing some investigation today. I've found this article by Nimtz
http://arxiv.org/abs/physics/9812053
He says that superluminal signal velocity of evanescent modes is possible. It gets me very confused
BTW, are you spanish gonzolo? Your nick seems a deformation of Gonzalo, a spanish name

 

[Gonzolo]

Gonzo + Han Solo, the muppet acting as the Star Wars character called himself Gonzolo in an episode of the 15 year-old cartoon "Muppet Babbies" which also featured Kermit as Luke Skyhopper, Miss Piggy as Princess Piggy, etc. Unfortunately, I have never seen the episode, I just remember the clever name from a friend's description. Any hints as to how I could finally see it are welcome...not that I still watch cartoons or anything...

I also find myself to be somewhat confused as to the various light speeds. I always imagined these were clearly understood and resolved. Although your arxiv site seems to come from Cornell Univ. I am unfamiliar with it and a bit skeptical as to how the articles are reviewed. References from JOSA, or Physical Review would be much more solid IMO.

 

[meteor]

the front velocity is defined as
$$\lim_{k\rightarrow\infty} \frac{w(k)}{k}$$
according to this site
www.uni-koeln.de/~abb11/workshop/announce4.html[/URL]
w is the angular frequency and k the wave number

So we have 3 velocities that can exceed c (group velocity, phase velocity and signal velocity) and one that cannot (front velocity). Interesting.

I've noticed that this is my post 666, he,he,he

 

[Gonzolo]

"So we have 3 velocities that can exceed c (group velocity, phase velocity and signal velocity) and one that cannot (front velocity). Interesting."

Yet, in that last site, it states that vgr < vs < vfr ...

And in Gisin's article, it says that vfr cannot exceed c.

Just for the record, I am not convinced about vgr being able to be vgr > c.

 

[meteor]

Just for the record, I am not convinced about vgr being able to be vgr > c.

Just to add more to your confussion (not that I want to make you suffer, but...):
take a look at this experiment of Lijun Wang, where he reports a negative group velocity of -310c
http://physicsweb.org/articles/news/4/7/8/1

 

[Gonzolo]

Well I was just about to call 911 until I read :

"There is no widespread agreement among physicists about the speed at which information is carried by pulses in such experiments."

and :

"The team intend to analyse this further..."

Whenever I get the chance, I'll look into this further. Cool stuff.

 

[meteor]

I've been thinking today about this oddity called negative group velocity, then i',ve searched if there was such a thing called negative phase velocity and I've found this:
http://www.innovations-report.de/html/berichte/physik_astronomie/bericht-32117.html
"Whether viewed dimly through the haze and lights of a city or in all their glory in a pristine wilderness, the stars that surround the Earth are magnificent, and one day Earthlings will travel to some of the new planets that astronomers are locating. However, the stars we see are not necessarily where we think they are, according to an international research team.

"We know that the light from distant stars takes a very long time to reach the Earth," says Dr. Akhlesh Lakhtakia, distinguished professor of engineering science and mechanics, Penn State. "But, taking into account the distance a star will have moved while that light travels, we still may not be able to accurately locate the star.

Negative phase velocity media or materials with negative refractive index may be responsible for this locational uncertainty. Recently, materials researchers at the University of California San Diego, working with micro and nano materials, developed a metamaterial that had a negative refractive index for microwaves, proving that negative phase materials could exist at least in the microwave part of the electromagnetic spectrum. Their requirements for this material were that both the relative permittivity, a measure of the charge separation in a material, and the relative permeability, a measure of how electrons loop in materials, of a substance must be less than zero."
I'm glad that at least there's no such a thing called negative front velocity or negative signal velocity. at least according to google