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Author Topic: can the speed of light go twice as fast?  (Read 15593 times)

another_someone

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Re: can the speed of light go twice as fast?
« Reply #25 on: 13/11/2005 01:03:59 »
quote:
Originally posted by Solvay_1927

As for my links in my last posting above ... has anyone looked at these?  I'm interested to know if they're valid. (Have the experiments been verified and repeated?)  Because I find them a bit confusing - I don't understand why the results DON'T violate relativity theory. They seem to suggest that FTL (faster than light) signalling) IS possible through certain mediums.

Can anyone shed more light (excuse the pun) on these experiments?

Solvay.


"Eagles may soar, but weasels don't get sucked into jet engines."



Had a brief look, but don't say they make a lot of sense to me either.

One comment one should be aware of:

http://www.abc.net.au/science/news/stories/s154610.htm
quote:

So, while the New Jersey researchers showed that a pulse can be shown to go faster than a speed of light, they have not managed show that a message can be transmitted at that speed.



On the other hand, there are already a number of quantum effects that seem to cause problems for speed limits imposed by C – one of them being particle entanglement.
 

Offline ukmicky

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Re: can the speed of light go twice as fast?
« Reply #26 on: 13/11/2005 01:52:45 »
Hi  Paul

As usual don’t quote me, but I think I’m sort of right.

If they were to send information along with the pulse then they would be violating the rules,however i think you will find they are sending a smooth pulse of laser light which cannot contain information. If they were to include information in the pulse like a semaphore the pulse would no longer be smooth and the frequency changes would be too sharp for the type of chamber they are using to process. Therefore einsteins relativity theory remains intact.:)

I think quantum entanglement also runs into the same information hurdle as effects moving at superluminal speeds run into the uncertainty principle making it impossible to control any signals, and because you dont know what your going to get untill you observe it




Michael                                      
« Last Edit: 13/11/2005 02:30:51 by ukmicky »
 

another_someone

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Re: can the speed of light go twice as fast?
« Reply #27 on: 13/11/2005 05:54:52 »
quote:
Originally posted by ukmicky

I think quantum entanglement also runs into the same information hurdle as effects moving at superluminal speeds run into the uncertainty principle making it impossible to control any signals, and because you dont know what your going to get untill you observe it



Looking at http://fergusmurray.members.beeb.net/Causality.html (and I only understand a fraction of what they are saying), it seems that you are partly right.

It seems, as far as I can understand, that there are some proposed mechanisms using entanglement that might be able to deliver superluminal information transfer, but nothing yet that has been demonstrated in practice.

quote:

Although it seems as if the Bell inequality, at least as it is usually presented, does not give us the convincing reasons for believing in the non-locality of physics which it has often been claimed to, it may be that there are other reasons for believing in non-local causes.  Greenberger, Horne and Zeilinger (1989) have presented a scheme which avoids the criticisms I have presented, and if correct really seems to leave no room for a local interpretation.  However, this scheme has not yet been tested experimentally.
The GHZ scheme uses three particles rather than two, and measurements of spin rather than polarisation.  The particles are sent out in different directions, and one of two sorts of spin measurement is made on each – call the first type X and the second type Y.  The measurements are made in one of four combinations:  Either every particle will be asked X, or two of the particles will be asked Y, and the other one X.  Quantum mechanics predicts a 100% probability that if only X measurements are made, an odd number of the particles will be found in the ‘spin-up’ state, whereas if two Y measurements are made, an even number of particles will be measured as ‘spin-up’.  The theory says nothing about whether the odd number will be 1 or 3, or the even number 2 or 4.
In this arrangement (assuming it is not eventually disproved, either by experiment or by the unlikely discovery of a theoretical error) it seems to be truly impossible to find a local explanation for the correlations; if each particle were to decide in advance what it will answer in response to the two questions, a single run of the experiment would stand at least a one in four chance of showing results that are in conflict with quantum mechanical predictions.
Another area of quantum theory which may be irreconcilable with locality is the Aharonov-Bohm effect, whereby particles travelling through a field-free region are influenced by a nearby magnetic field.  Richard Healey18 argues that the apparent non-locality of this effect is closely analogous to that manifested in violations of Bell’s inequality.

 

Offline Solvay_1927

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Re: can the speed of light go twice as fast?
« Reply #28 on: 13/11/2005 23:06:30 »
I understand what you're saying, Someone and Michael.  But I'm still not convinced.  Maybe I'm just misunderstnding the experiment described in the earlier links above.

My understanding is that you have a container (containing ultra-cold caesium gas) which is, say, 30cm thick.  If a person on the left side of this container switches on a laser (of the right wavelength) at time t=0, a person on the right of the container sees light starting to come out his side at t<1 ns (ie. less than 1 nanosecond later).  But light in a vacuum would take a full nanosecond to traverse 30cm.

So you've got a "signal" being communicated faster than light. (The signal isn't a complicated message, it's just a simple "the laser has been switched on".  Like a binary 0/1 communication.)

Like I said, maybe I've misunderstood the experiment.  That's why I'd be interested in whether anyone has a more detailed explanation of what this experiment was about. (And why can't I find any references to this experiment dated later than 2000? Did they realise they'd made a mistake and quietly brush this false result under the carpet?)

Help.

Solvay.


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another_someone

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Re: can the speed of light go twice as fast?
« Reply #29 on: 14/11/2005 00:49:33 »
quote:
Originally posted by Solvay_1927

I understand what you're saying, Someone and Michael.  But I'm still not convinced.  Maybe I'm just misunderstnding the experiment described in the earlier links above.

My understanding is that you have a container (containing ultra-cold caesium gas) which is, say, 30cm thick.  If a person on the left side of this container switches on a laser (of the right wavelength) at time t=0, a person on the right of the container sees light starting to come out his side at t<1 ns (ie. less than 1 nanosecond later).  But light in a vacuum would take a full nanosecond to traverse 30cm.

So you've got a "signal" being communicated faster than light. (The signal isn't a complicated message, it's just a simple "the laser has been switched on".  Like a binary 0/1 communication.)

Like I said, maybe I've misunderstood the experiment.  That's why I'd be interested in whether anyone has a more detailed explanation of what this experiment was about. (And why can't I find any references to this experiment dated later than 2000? Did they realise they'd made a mistake and quietly brush this false result under the carpet?)

Help.

Solvay.


"Eagles may soar, but weasels don't get sucked into jet engines."



As I understand the document, it says that the back of the pulse exceeds the speed of light, but not the front of the pulse.

There seems a strange comment that the back of the pulse overtakes the front of the pulse.  I'm not sure if this is meant literally, or just a very loose way of saying that the back of the pulse is moving faster than the front of the pulse, and so catches up with the front of the pulse (i.e. the pulse is compressed).  If it is just that the pulse is compressed, and the information within the pulse is consequently garbled, then neither using the pulse front as a signal (which is still only travelling only at the normal speed of light), nor looking at the pulse contents (which are garbled) will give you any meaningful information quicker than it would take light to travel through the medium.  It is only if they meant what they said literally, about the back of the pulse actually overtaking the front (i.e. the pulse arrives back to front) that one might ask whether actual information (the knowledge of the existence of the pulse of light)  is travelling faster than the speed of light.

Do you know of any document that clarifies whether the back of the wave arrives at the output before the front would have expected to have arrived under normal speed of light conditions?

Even if the pulse arrives back to front, if the back of the pulse does not arrive at the exit point before the front normally would have (it seems to be the case, from what I read, that the front of the pulse is also slowed down, just and the back speed up), it would still not allow information to travel faster than the normal speed of light, even though it would (as clearly is the case they are talking about) still mean that the back of the pulse has exceeded the speed of light in order to get to the front; but taking the pulse overall, and the information about the pulse, it would still not have travelled at superluminal speed.
« Last Edit: 14/11/2005 02:56:58 by another_someone »
 

Offline ukmicky

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Re: can the speed of light go twice as fast?
« Reply #30 on: 14/11/2005 04:40:38 »
To another someone

Firstly Relativity is taking about information in light and not pure light. it’s there fault why there is all this confusion because they don’t 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 you’re 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                                      
« Last Edit: 27/05/2006 17:45:20 by ukmicky »
 

Offline Solvay_1927

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Re: can the speed of light go twice as fast?
« Reply #31 on: 14/11/2005 13:37:10 »
Thanks michael & someone, that clears it up for me - I can stop worrying about it now and get back to worrying about more trivial things (like whether I can pay the mortgage this month, for example).

Solvay.

(P.S. I'm still interested if anyone can find any other web link references to this experiment, so I can understand it better.)


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Offline ukmicky

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Re: can the speed of light go twice as fast?
« Reply #32 on: 14/11/2005 20:24:03 »
Hi paul
The problem is over the years there have been many experiment along the same lines.
The first link is a long read but should awnser all

http://www.phy.duke.edu/research/photon/qelectron/pubs/SFLProgressInOptics.pdf
http://www.chtm.unm.edu/AA_NEWS_STORIES_DATA/NEWS_ITEMS/mojahedi_story.html




Michael                                      
« Last Edit: 14/11/2005 20:25:06 by ukmicky »
 

Offline Solvay_1927

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Re: can the speed of light go twice as fast?
« Reply #33 on: 14/11/2005 21:39:07 »
Thanks Michael, these look very helpful.

One more question.  I've not read all of the first link yet (as it looks like that may take some time!), so I don't know if this question is covered in there, but ...

Could the experiment (in principle, at least) be done with such low intensity lasers that you're only getting a few (or even a single) photon passing through the apparatus at a time?  And if so, would the result be different?  (Does group velocity vs phase velocity still apply to individual photons?)

Or should I just go away and read up on this because my question just proves that I don't understand physics yet? :)


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Offline ukmicky

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Re: can the speed of light go twice as fast?
« Reply #34 on: 14/11/2005 23:02:02 »
originally posted by solvay
Thanks Michael, these look very helpful.

One more question. I've not read all of the first link yet (as it looks like that may take some time!), so I don't know if this question is covered in there, but ...

Could the experiment (in principle, at least) be done with such low intensity lasers that you're only getting a few (or even a single) photon passing through the apparatus at a time? And if so, would the result be different? (Does group velocity vs phase velocity still apply to individual photons?)

Or should I just go away and read up on this because my question just proves that I don't understand physics yet?
 

-----------------------------
NO maybe I should just go away and read up on this because your question just proves that I don't understand physics yet? :D

I don’t think any of the articles do answer your question; however it is possible to release and measure the speed of a single photon. Whether or not it would be possible in this particular experiment I don’t know. A few years back I looked into this same subject and at that time that was one of the questions I believe scientists were asking. I believe there were unresolved problems in regards to how to interpret the results if it worked. I have no idea whether the experiment has been tried yet.:)


Michael                                      
« Last Edit: 14/11/2005 23:07:26 by ukmicky »
 

Offline ukmicky

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Re: can the speed of light go twice as fast?
« Reply #35 on: 14/11/2005 23:13:24 »
paul
Here's another article that a bit easier and quicker to read but still gives detailed info into the experiment.

Michael                                      
 

Offline Solvay_1927

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Re: can the speed of light go twice as fast?
« Reply #36 on: 15/11/2005 12:35:54 »
Er ... is it written in invisible ink? [:p]

(Which reminds me of a question that I heard some time ago: How do you tell if you're out of invisible ink?:D)
 

another_someone

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Re: can the speed of light go twice as fast?
« Reply #37 on: 15/11/2005 13:13:23 »
quote:
Originally posted by Solvay_1927

(Which reminds me of a question that I heard some time ago: How do you tell if you're out of invisible ink?:D)




Is the answer that you can see what's been written?[:o)]
 

Offline ukmicky

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Re: can the speed of light go twice as fast?
« Reply #38 on: 15/11/2005 19:42:00 »
arrrhh,sorry about that http://physicsweb.org/articles/world/13/9/3

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Offline Solvay_1927

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Re: can the speed of light go twice as fast?
« Reply #39 on: 16/11/2005 12:57:11 »
Michael - many thanks, this is really useful.

Someone - if only life were so simple.  The other problem I have with invisible ink is ... how do you know you're not writing on top of something you've written previously?


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Re: can the speed of light go twice as fast?
« Reply #39 on: 16/11/2005 12:57:11 »

 

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