FTL Communication

[Overdog]

No, compression in a rod or tension in a string will not travel faster than light. Even when pushing a "rigid" rod the compression wave will propagate through the rod at less than light speed.

 

Probably the easiest way to think of this is the string or rod being made of atoms which must push on one then another down the line sequentially. It is not then exactly rigid and will not send information simultaneously.

 

-modest

 

Ok, I see the problem with the mechanical device, but I think idea of the speculation was about the possibility of a junction, or the point of intersection of something, (say of two electromagnetic waves that are intersecting from a great distance apart), to proceed along the intersecting wave fronts FTL.

 

EDIT: I'm not suggesting in any way that this allows information or causality to propagate > c

[modest]

Ok, I see the problem with the mechanical device, but I think idea of the speculation was about the possibility of a junction, or the point of intersection of something, (say of two electromagnetic waves that are intersecting from a great distance apart), to proceed along the intersecting wave fronts FTL.

 

EDIT: I'm not suggesting in any way that this allows information or causality to propagate > c

 

You lost me a bit and maybe sanctus will know better where you're coming from. I don't know of any theory of intersecting waves proposed to travel faster than light. Do you have a link?

 

Also, a description of the scissors thought experiment as well as a refutation is here:

 

The Superluminal Scissors

 

-modest

 

//EDIT

 

EDIT:lol modest you beat me by one minute :)

 

:)

[sanctus]

Maybe it is inked to phase- and group-velocity? But I have not looked at this in a long time, but here is a link that describes it group_velocity

the group velocity can be bigger than the speed of light, but as usual not a signal velocity...

[Overdog]

From your link, this is what I was trying to communicate, and merely as an interesting curiosity...

 

Caveat

The contact point where the two blades meet is not a physical object. So there is no fundamental reason why it could not move faster than the speed of light, provided that you arrange the experiment correctly. In fact, it can be done with scissors provided that your scissors are short enough and wide open to start, very different conditions than those spelled out in the gedanken experiment above. In this case it will take you quite a while to bring the blades together -- more than enough time for light to travel to the tips of the scissors. When the blades finally come together, if they have the right shape, the contact point can indeed move faster than light.

[Overdog]

Maybe it is inked to phase- and group-velocity? But I have not looked at this in a long time, but here is a link that describes it group_velocity

the group velocity can be bigger than the speed of light, but as usual not a signal velocity...

 

Yes....

 

Quote from

Sound Past Speed of Light? - The Science of Sound - tribe.net

 

 

In recent years, it has been shown experimentally that the group velocity of a laser pulse can exceed the speed of light in vacuum -- 300,000,000 metres per second -- in certain situations. However, special relativity is not violated in these experiments because they do not involve the transfer of information, matter or energy.

 

 

Measuring group velocity (pic: physicsweb.org/objects/ne...051101.jpg)

Mobley has now calculated that the group velocity of a pulse of high-frequency sound waves could be increased by five orders of magnitude by sending it through a small chamber that contains about 8 millilitres of water and some 400,000 tiny plastic spheres. This means that the group velocity would exceed the speed of light in vacuum. The spheres have diameters of about 0.1 mm and account for about 5% of the volume of the water-bead mixture.

 

The increase in speed is caused by dispersion -- the phenomenon that causes different wavelengths to move at different phase velocities. When the pulse enters the mixture it experiences severe dispersion, which causes the different wavelengths that make up the pulse to travel at very different speeds. This changes the shape of the pulse and can result in the pulse itself moving faster than the speed of light. However, the dispersion also significantly reduces the intensity of the pulses.

 

"It has long been recognised that such velocities should be possible with acoustic waves," Mobley told PhysicsWeb. "My work shows that it can be done in a specific and very simple system and that extreme conditions are not necessary."

[modest]

Ok, different examples of non-physical things traveling at FTL speeds. A simpler example would be a shadow. As you say: not useful in FTL communication. Interesting thought experiments though.

 

-modest

[Moontanman]

A point of light, like a laser dot, can be made to travel faster than light. If you projected a dot of light on a distant object you could move the projector at it's source slower than light but the dot it projects could move faster then light.

[freeztar]

A point of light, like a laser dot, can be made to travel faster than light. If you projected a dot of light on a distant object you could move the projector at it's source slower than light but the dot it projects could move faster then light.

 

I'm not sure I understand. Can you elaborate?

 

If I'm reading this correctly, it seems no different than the luminal scissors idea. :)

[Moontanman]

I'm not sure I understand. Can you elaborate?

 

If I'm reading this correctly, it seems no different than the luminal scissors idea. :shrug:

 

See this link,

 

Speed of light - Wikipedia, the free encyclopedia

[freeztar]

See this link,

 

Speed of light - Wikipedia, the free encyclopedia

 

I've previously read all of that. I'm still unsure how this relates to your dot idea.

 

Just to make sure, this is how I envision what you are implying (sorry for the crappy drawing, but a pic is worth [math]10^3[/math] words):

[LaurieAG]

A point of light, like a laser dot, can be made to travel faster than light. If you projected a dot of light on a distant object you could move the projector at it's source slower than light but the dot it projects could move faster then light.

 

Hi Moontanman,

 

The link isn't exactly what is being discussed. Are you talking about something like setting up a series of pixels (projected on a distant object i.e. screen) that appears to be something moving FTL? i.e. apparent movement appears 90 degrees to the individual dots being sent.

[modest]

A point of light, like a laser dot, can be made to travel faster than light. If you projected a dot of light on a distant object you could move the projector at it's source slower than light but the dot it projects could move faster then light.

 

I agree Moontanman, a laser point like a shadow can move faster than light. They are not physical things and only by considering them a "thing" can we say they move faster than light. As you would no doubt agree, there is no way to communicate with this effect. This discusses it:

 

Faster Than Light

 

It also looks like a good site for the rest of the common FTL ideas.

 

-modest

[Moontanman]

I agree Moontanman, a laser point like a shadow can move faster than light. They are not physical things and only by considering them a "thing" can we say they move faster than light. As you would no doubt agree, there is no way to communicate with this effect. This discusses it:

 

Faster Than Light

 

It also looks like a good site for the rest of the common FTL ideas.

 

-modest

 

The site I was talking about says this:

 

Light spots and shadows

If a laser is swept across a distant object, the spot of light can easily be made to move at a speed greater than c.[14] Similarly, a shadow projected onto a distant object can be made to move faster than c. In neither case does any matter or information travel faster than light.

 

While it does say that information cannot travel faster than light like this I disagree. If you had a detector on the moon and you swept a laser beam across it you could send something like Morse code faster than light. As you flicked the beam back and forth across the detector it would move on the Moon at the instant it was moved on the earth. Using this to stimulate the detector would allow you to send a digital code instantaneous. OK why not?

[Moontanman]

Hi Moontanman,

 

The link isn't exactly what is being discussed. Are you talking about something like setting up a series of pixels (projected on a distant object i.e. screen) that appears to be something moving FTL? i.e. apparent movement appears 90 degrees to the individual dots being sent.

 

negatory on that

[modest]

While it does say that information cannot travel faster than light like this I disagree. If you had a detector on the moon and you swept a laser beam across it you could send something like Morse code faster than light. As you flicked the beam back and forth across the detector it would move on the Moon at the instant it was moved on the earth. Using this to stimulate the detector would allow you to send a digital code instantaneous. OK why not?

 

No, it still takes like the regular amount of time to get to the moon. What you outline is no different from blinking a laser on and off to construct a message. The speed you blink it on and off or the speed you wiggle it back and forth is independent of the amount of time it takes the signal to reach its destination.

 

-modest

[Moontanman]

No, it still takes like the regular amount of time to get to the moon. What you outline is no different from blinking a laser on and off to construct a message. The speed you blink it on and off or the speed you wiggle it back and forth is independent of the amount of time it takes the signal to reach its destination.

 

-modest

 

No you misunderstand, the beam is already at the moon. According to the article you can move a shadow or a spot of light faster than the speed of light. If you moved the spot back and forth across the detector you could signal faster than light since the spot moves at the speed of it's source being moved back and forth. At it's source the speed could be just a few feet per second but the spot would be moving faster than light. Before I go any further with this, lets get some basics settled out. While I'm sure I am wrong in this lets see if I have the idea correct. If you had a small rod with a laser coming out of both ends and you caused it to turn in a circle at a few hundred RPS what would the beams look like if you could look down on them from a great distance? (lets assume you can see a laser beam) Would the beams be straight and true as a laser or would they spiral like the arms of a hurricane? If they would be straight then you could move the laser back and forth and at some point the beam would be moving faster than light but in sync with the source. Then you could turn a detector off and on with no time lag from the Earth to the detector. In essence you would signaling faster than light. Yes? No? Why? Why not?

[Overdog]

No you misunderstand, the beam is already at the moon. According to the article you can move a shadow or a spot of light faster than the speed of light. If you moved the spot back and forth across the detector you could signal faster than light since the spot moves at the speed of it's source being moved back and forth. At it's source the speed could be just a few feet per second but the spot would be moving faster than light. Before I go any further with this, lets get some basics settled out. While I'm sure I am wrong in this lets see if I have the idea correct. If you had a small rod with a laser coming out of both ends and you caused it to turn in a circle at a few hundred RPS what would the beams look like if you could look down on them from a great distance? (lets assume you can see a laser beam) Would the beams be straight and true as a laser or would they spiral like the arms of a hurricane? If they would be straight then you could move the laser back and forth and at some point the beam would be moving faster than light but in sync with the source. Then you could turn a detector off and on with no time lag from the Earth to the detector. In essence you would signaling faster than light. Yes? No? Why? Why not?

 

The problem with that is if you move the laser beam while standing on earth, the laser light that is hitting the moon will not shift instantaneously, as the photons emitted from the laser since you moved it still have to travel all the way to the moon before any movement will be seen on the moon.