Faster than light

[Lancer]

http://www.telegraph.co.uk/earth/main.jhtml?xml=/earth/2007/08/16/scispeed116.xml

Got this from CG.

[Provost Harrison]

A shadow can move faster than light

[Lancer]

Nah uh.

[Provost Harrison]

Move an object in front of a point light source at a speed approaching the speed of light - if the wall is the same distance from the wall as the point light source, the shadow will appear to move twice as fast as the object and thus approaching twice the speed of light - although I think that could get complicated if the object is moving linearly parallel with the wall...but I digress

[Lancer]

****.

But original ****, I'll give you that.

[Provost Harrison]

Precisely, I don't want to think about that kind of stuff, especially not just before bed

[Lancer]

I've just napped, I'm fully rested, recording a show for Dolores (who is working) that's definately girl TV, so I can't sleepor watch TV or hang with the wife.

Grim.

[Nostromo]

A paper submitted to the physics arXiv has been picked up by a number of major news outlets (e.g., the Daily Mail) because the paper suggests that its authors have measured something traveling faster than the speed of light. Unfortunately, the claim is worse than weak; it is silly. I'll talk about why that is after briefly discussing their research.

The paper in question has no data at all so; although it asserts that it has measured superluminal velocities, it offers nothing to back that up. It also has very little in the way of experimental detail, so we can't determine with certainty what they are measuring, making it very difficult to evaluate their claims. We'll take as close a look as we can, given these limitations.

The researchers make use of the property called total internal reflection (brief discussion). When light is above a certain angle of incidence on an interface between two materials—say, at the face of a prism—it can be totally reflected, provided it is arriving at this interface from the higher refractive index material. However, near the boundary, something called an evanescent wave forms that does not propagate like normal light (technically it does not propagate at all) and quickly decays away to nothing. If you take a second prism and place it very close to the interface where total internal reflection occurred, then some light from this evanescent wave will leak across the interface and exit the second prism. The prisms have to be no further than the wavelength of light involved for this to work.

Now the interesting questions are: where did the energy in this light come from? How fast did it travel across the boundary? The first question is interesting because the evanescent field has no energy in it. This is because the electric and magnetic fields that make up the field are phased in such a way that the product is always zero. The second question is interesting because the speed of light is not defined in a way that is intuitive to non-physicists. Suffice it to say that, for the evanescent wave, the speed of light is zero, and therefore any measurable speed is faster than the speed of light.

So, how are these authors measuring an excessive speed of light? In practical terms, most experiments measure light in terms of what is called the group velocity, which is how fast a pulse propagates along an underlying carrier frequency. This can, in some circumstances, lead to the pulses traveling faster than the speed of light in the medium they're in, but not faster than light in vacuum. Although the setup in the new paper is not entirely clear, they were measuring the arrival time of pulses, which means we're talking about group velocity rather than the actual speed of light.

Another problem that occurs in these experiments comes from determining when the pulse actually arrived. If you analyze a pulse of light, you find that it is made up of a huge number of frequencies that, as you move away from the fundamental frequency, get lower and lower in amplitude. Once you look at the experimental set up in detail, you find that it is triggering on the pre-pulse noise generated by these high frequency components.

Separate from the whole speed of light issue, the answer to the energy question in this experimental setup is interesting. Once the two prisms are close to each other, the evanescent wave is partially reflected from the second prism back to the first prism. When this happens, the total electric field and total magnetic field are no longer such that their product is always zero—there is energy in the field. Furthermore, if you analyze the components of the fields that contain the energy, you find that they do have a non-zero speed of light and it is—you guessed it—the same c that applies everywhere else in the universe.

So although this makes for an interesting physics lecture—or at least I thought it was interesting—it is not new physics and not a breakdown of special relativity.

http://arstechnica.com/news.ars/post/20070816-faster-than-the-speed-of-light-no-i-dont-think-so.html

[Joseph]

Some years ago Lawrence Livermore Lab in Livermore, Calif., located on I - 580 just east of the Bay area, was testing some partical and one them when 1.2 light years. They did this in the late 80s or early 90s. Maybe it was the early 2000s.

[Eli]

Stupid sensationalist reporting.

[BeBMan]

If you're faster than light, aren't you always in the dark then?

Sounds anti-enlightenment to me

[Dauphin]

Originally posted by Provost Harrison
Move an object in front of a point light source at a speed approaching the speed of light - if the wall is the same distance from the wall as the point light source, the shadow will appear to move twice as fast as the object and thus approaching twice the speed of light - although I think that could get complicated if the object is moving linearly parallel with the wall...but I digress

You should have stuck with the joke.

Light thinks it travels faster than anything but it is wrong. No matter how fast light travels, it finds the darkness has always got there first, and is waiting for it.

[Straybow]

Originally posted in The Telegraph
For instance, an astronaut moving faster than it would theoretically arrive at a destination before leaving.

What crap. No, an astronaut would arrive before EM signal news of his departure reached the destination.

[LordShiva]

Originally posted by Straybow

Originally posted in The Telegraph
For instance, an astronaut moving faster than it would theoretically arrive at a destination before leaving.

What crap. No, an astronaut would arrive before EM signal news of his departure reached the destination.

No