FTL Communication

[sanctus]

Originally Posted by Overdog

Ok, I read the thread. Great thread...I may revive it later. Thanks.

So the answer to my question is no, the part of the universe we can see appears to be very close to flat.

Well the almost flat or flat models are the ones that fit best the data atm if i remeber right, that doesn't mean that other models that are not flat aare necessarily wrong.

[Overdog]

Are there any theoretical ways to communicate faster than light in our universe?

A highly theoretical idea I heard once was this:

Imagine a pair of scissors that is light years in length. You close the scissors, and cut a piece of string that is 4 light years away from where you are standing.

Hmmmm...you could send morse code with it if you wanted...

No laws are violated. Unfortunately it requires a physical, mechanical connection across the entire distance...

[modest]

Originally Posted by Overdog

Are there any theoretical ways to communicate faster than light in our universe?

A highly theoretical idea I heard once was this:

Imagine a pair of scissors that is light years in length. You close the scissors, and cut a piece of string that is 4 light years away from where you are standing.

Hmmmm...you could send morse code with it if you wanted...

No laws are violated. Unfortunately it requires a physical, mechanical connection across the entire distance...

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

[sanctus]

That wouldn't work because, assuming you have enough strength to close the scissors, the speed of propagation of the force you applied to close the scissors also propagates no faster than the speed of light...that means the end of the scissors at 4 lightyears away would at the earliest start to move 4 years after you applied the force...


EDIT:lol modest you beat me by one minute

[Overdog]

Originally Posted by modest

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]

Originally Posted by Overdog

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

Originally Posted by sanctus

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]

Originally Posted by 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...

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