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Author Topic: Light goes faster backwards  (Read 4369 times)

Offline DoctorBeaver

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Light goes faster backwards
« on: 20/05/2006 17:34:43 »
I won't write anything here, check out the link. http://www.livescience.com/technology/060518_light_backward.html

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Offline Soul Surfer

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Re: Light goes faster backwards
« Reply #1 on: 27/05/2006 12:06:48 »
There are lots of reports of experiments like this and it is important to understand about the difference between "group velocity" and "phase velocity" when electromagnetic radiation is propagating in confined spaces like waveguides or optical fibres.  Group velocity is the velocity with which information may be transmitted and is always less than or equal to the speed of light.  Phase velocity can be anything at all either backwards or forwards depending on the design of the waveguide and the frequency of the electromagnetic radiation however it cannot be used to transmit information so the general rule that nothing can go faster than the speed of light is still obeyed.

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Offline harryneild

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Re: Light goes faster backwards
« Reply #2 on: 27/05/2006 17:08:00 »
Hmm i don't understand..  Can you not shine a speeded up light wave on a light detector, using binary style flashes to transmit information faster than the conventional spped of light? What is it that stops this from happening? If you can't send information using these accelerated light waves then how do they know how fast they are going since they can tell if they have arrived yet?

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« Last Edit: 05/04/2007 21:54:20 by harryneild »
 

Offline ukmicky

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Re: Light goes faster backwards
« Reply #3 on: 27/05/2006 18:08:57 »
quote:
Originally posted by harryneild

Hmm i don't understand..  Can you not shine a speeded up light wave on a light detector, using binary style flashes to transmit information faster than the conventional spped of light? What is it that stops this from happening? If you can't send information using these accelerated light waves then how do they know how fast they are going since they can tell if they have arrived yet?

Thankyou

Harry Eakins

"Knowledge has to be improved, challenged, and increased constantly, or it vanishes." Peter F. Drucker

quote:
Originally posted by ukmicky

To another someone

Firstly Relativity is taking about information in light and not pure light. itís there fault why there is all this confusion because they donít explain themselves properly.

secondly all their measurements are group velocity and not front velocity.
They measure when the central fat part of the wave leaves the starting blocks and crosses the line,(group velocity) not the very front information part. (front velocity)

so yes what youíre saying is right
The wave basically changes shape squashing up like a spring as the central fat pure light part of the wave travels faster than c and catches up to and can even pass the front information part which is travelling at c.:)



Michael                                      


A PREVIOUS DISSCUSION ON THIS TOPIC
http://www.thenakedscientists.com/forum/topic.asp?TOPIC_ID=2775

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Basically the front part of the wave is the information part and is the part which would contain the information which you would use as a signal and cannot travel faster the c .

The part of the wave which normally follows on behind can however under certain circumstances be accelerated so it can travel faster than c overtaking the information part. This part you could recieve faster than c but as it cant hold any information you would still have to wait for the information part containing your binary flash which is travelling at C.


C = THE SPEED OF LIGHT

http://physicsweb.org/articles/world/13/9/3
Michael
« Last Edit: 27/05/2006 18:26:03 by ukmicky »
 

Offline realmswalker

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Re: Light goes faster backwards
« Reply #4 on: 28/05/2006 09:35:44 »
i dont get it...
anything at all being visible is all that is needed for a binary system to work isnt it?
 

Offline harryneild

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Re: Light goes faster backwards
« Reply #5 on: 28/05/2006 11:05:48 »
Yeh i just read an article that explained it. Picture a lighthouse with its revolving light beam shining onto the shore. If the shore was far enough away the spot of light at the end could be travelling faster than c. This however contains no information, and the only information is the light that travels from the lighthouse to the shore and this only travels at c.

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« Last Edit: 05/04/2007 21:52:21 by harryneild »
 

Offline Soul Surfer

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Re: Light goes faster backwards
« Reply #6 on: 29/05/2006 09:37:49 »
Thanks harry. I forgot about the lighthouse example.  That's probably the simplest example of light "travelling"  sideways much faster than the speed of light when all it's really doing is heading away from the lighthouse at the speed of light.  

waveguides are much more difficult to explain.  Inside them the group velocity is below the speed of light but the phase velocity is often greater than the speed of light and at cutoff becomes infinite

Waveguides are easiest to visualise in the microwave conext.  They are conducting rectangular tubes and the best way of imagining the waves proagating down them is to think of a air of plane waves travelling backwards and forward (at the speed pf light) reflecting off the shortest faces of the waveguide in a zig zag fashon  for the normal band of operation of the waveguide the waves are angled at about 45 degrees so they travel as a group at 1/sqroot 2 of the speed of light as a group (because they are going in zigzags) but the phase moves down the guide at the speed of light.  

As the frequency gets lower and the waveguide approches cutoff the angle gets smaller and the group velocity gets slower while the phase velocity gets faster.  If it was 30degrees  the group velocity is 1/ root3 of the velocity of light while the pgase is root three times the velocity of light.  (hope people understand the trigonometry involved in these examples)

At the cut off point the waves just travel backwards and forwards between the faces of the waveguide so the group velocity is zero but the phase velocity is infinite.

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Offline DoctorBeaver

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Re: Light goes faster backwards
« Reply #7 on: 29/05/2006 14:19:19 »
Was that last reply actually written in English? [xx(]

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Offline JimBob

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Re: Light goes faster backwards
« Reply #8 on: 29/05/2006 19:48:15 »
I second that confusion.

Perhaps I am dense, but I cannot follow this either.



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Offline DoctorBeaver

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Re: Light goes faster backwards
« Reply #9 on: 29/05/2006 20:26:14 »
All I know about waveguides is that we used to stick leaves on the end of them to frazz them

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Offline Soul Surfer

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Re: Light goes faster backwards
« Reply #10 on: 29/05/2006 23:25:36 »
Sorry if you can't understand it it probably best to stick to the rotating lighthouse analogy or possibly the optical fibre analogy wher the light goes down the fibre reflecting off the inside walls of the fibre.   To go much further into waveguide theory will requre several pages of text and diagrams.  One additional helpful piece if information is that all electromagnetic radiation with wavelengths siginificantly long er than the spaces between atoms reflect off conducting surfaces with a smoothness less than a fraction of a wavelength like light off a glass mirror.  I am also assuming that you know that light and all electromagnetic radiation travels in streight lines from the source.

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Re: Light goes faster backwards
« Reply #10 on: 29/05/2006 23:25:36 »

 

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