# Speed of light.

1. Sep 12, 2004

### tumor

Can speed of light increase?

2. Sep 12, 2004

### 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.

3. Sep 13, 2004

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

4. Sep 13, 2004

### 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.

5. Sep 15, 2004

### Nereid

Staff Emeritus
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).

6. Sep 15, 2004

### 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 recieved this can vary between not arriving at all up to an approximate speed of one and a half times faster than just travelling through the substance. Someone please correct me if this is wrong

7. Sep 15, 2004

### 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.

8. Sep 15, 2004

### 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.

9. Sep 16, 2004

### Phobos

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

10. Sep 16, 2004

### Nereid

Staff Emeritus
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?

11. Sep 16, 2004

### geometer

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.

12. Sep 16, 2004

### geometer

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.

13. Sep 16, 2004

### Gonzolo

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.

14. Sep 16, 2004

### Nereid

Staff Emeritus
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?

15. Sep 16, 2004

### meteor

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

16. Sep 17, 2004

### 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?

17. Sep 17, 2004

### JV

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".

18. Sep 17, 2004

### GOD__AM

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.

19. Sep 17, 2004

### Nereid

Staff Emeritus
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?

20. Sep 17, 2004

### GOD__AM

Well, there well be a number of photons released in one revolution. If you devide 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.