Another Hairbrained FTL Scheme

Main Question or Discussion Point

Arguments against FTL communication are usually based on causuality arguments, which I tend to agree with. But it seems to me that the 'backwards in time' problem seems to make an assumption that might not always apply, ie that FTL velocity would be relative to C for all observers. Here's another (most likely incorrect) idea for sending FTL signals ...

I have a powerful laser, and a fancy optical system. I can use this to create small traveling pulses containing negative energy relative to vacuum, (ie 'squeezed light'). This in itself has already been done (to a small extent), so is not totally unrealistic. Assuming that I can create these pulses as the result of interference in the beam, I could possibly make these 'bubbles' travel at very high speed, (ie >C phase velocity - the group velocity of the beam is of course just C). Assume that I can 'inject' an extra photon into some of these bubbles, and since there is 'less space' that the photon experiences relative to free space, the photon can 'ride' the bubble and move FTL.

The above might be *totally* impossible in practice, but is meant mostly as thought experiment. To me it demonstrates an interesting twist on the paradox problem, because as the transmitter/receiver speed up, the velocity that they can transmit becomes slower, ie to another observer, the 'FTL' transmission can be much slower than C, since the speed of the interference pattern is related to the transmitter's clock, not C itself. Could such an arrangement be used to send a message 'back in time'? It seems at first glance to me that the answer is no, which would at least not rule out this method of FTL transmission based on causuality problems. I would much appreciate a comment from someone in the know.

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If one produces this set-up inside a craft that is travelling to some future destination(ie in the forward motion) then one produces an interesting result with respect to Time reference.

The MM experiment using lasers, set up onboard a spacecraft?

What you need:

Two Lasers, one beam horizontal across the craft, the other pointing in the direction of crafts motion.

Horizontal Light(light tranversing across the craft from side to side) will be bent downwards, as Einstein's thought experiment detailing Acceleration and Gravity Equivilence shows. The Vertical light in the direction of motion however, will this light be "bent-backwards" ? in the opposite direction to motion? this has its workings contained within Einstein's thought experiment where he ride's at the front of a beam of light, holding a mirror.

If it was possible you could now have two similtaineous experiments detailing a number of aspects of light and FTL communications.

Some questions: Does the forward Laser beam follow the standard predicted experimental consistant MM results? for example..will the Laser-Light remain a constant?..will it remain a Light of L.A.S.E.R?..will its beam (speed) be pulled backwards by Acceleration Direction?..simplified into Einsteins Light beam thought experiment:If you were travelling at the speed of light, and shone a Laser light in the direction of travel, would a Laser work? No it would not, whilst ordinary light remains constant, and always registers the S.O.L to be constant, a L.A.S.E.R would actually have to turn into "ordinary" light to remain constant, the laser would fail due to the emission process needed to create the beam.

Now to an observer onboard the craft, the Laser Beam has been "reversed" into ordinary light!..it's light energy has been converted by the forward 'equivilence' principle, the tightly packed beam is rindler-shifted into ordinary light, it is specifically "un-tuned".

The FTL experiment you state cannot occur, because of 'interference', mixing ordinary light and Laser beams, you may have a phase velocity injected into the experiment, and produce "novel" results.

But place the apparatus into the Realms of Relativity, General and Special, and it falls back to earth with an almighty crash!

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Spin_Network said:
If you were travelling at the speed of light, and shone a Laser light in the direction of travel, would a Laser work? No it would not, whilst ordinary light remains constant, and always registers the S.O.L to be constant, a L.A.S.E.R would actually have to turn into "ordinary" light to remain constant, the laser would fail due to the emission process needed to create the beam.
I don't think this is correct. First, you can't travel 'at' the speed of light. If you are travelling at *nearly* the speed of light, you would not measure any different behavior from the laser. Another observer travelling at a different velocity would detect a different frequency coming from the laser, but the light would still be coherent, and everyone would agree that it's velocity is C. If you are travelling rapidly towards an observer and fire a red laser beam, the receiver could detect coherent UV/x-rays/gamma rays (based on the relative velocity). If you send the same beam backwards to an observer you are receding from, that observer might detect microwave/rf energy (but it would still be coherent).

pervect
Staff Emeritus
Iridium said:
Arguments against FTL communication are usually based on causuality arguments, which I tend to agree with. But it seems to me that the 'backwards in time' problem seems to make an assumption that might not always apply, ie that FTL velocity would be relative to C for all observers. Here's another (most likely incorrect) idea for sending FTL signals ...
The Lorentz interval of your FTL signal will be space-like for the observer sending the signal, by defintion.

Relativity says the Lorentz interval is invariant for all observers. This implies that the Lorentz interval of your FTL signal will be space-like for ALL observers as long as the Lorentz transform applies.

This rules out a FTL signal that appears to move slower than 'c' relative to some subset of observers.

ie to another observer, the 'FTL' transmission can be much slower than C,

pervect said:
Relativity says the Lorentz interval is invariant for all observers. This implies that the Lorentz interval of your FTL signal will be space-like for ALL observers as long as the Lorentz transform applies. This rules out a FTL signal that appears to move slower than 'c' relative to some subset of observers.
Agreed. I should have been clearer. The 'true' signal velocity can never be less than C relative to any observer, although it can approach C as the relative velocity increases.

However, the effective transmission speed from transmitter to receiver can be less than C. Assume a transmitter/receiver that could send at 1.5C (as they measure it). It takes a certain number of laser oscillations to send a 'bubble' a certain distance. Due to time dilation, these oscillations can be slowed for another observer. I would think an observer in a different frame who measures the tx/rx pair's clock as running at 1/2 speed would measure an effective speed (from tx to rx) of ~.75C.

pervect
Staff Emeritus
I really don't understand the details of your proposed mechanism. The "bubble" part is especially unclear unfortunately.

However, as long as you can assign a definite arrival time to your "bubble", (i.e. if we can treat it as a classical object with a well determined position at a well determined time), we can use the argument above to say that the bubble must appear to be traversing a space-like interval in any inertial frame - ASSUMING that the Lorentz transforms work. So if you're getting a time-like interval for your bubble, you are doing more than postulating an unkown physical mechansim, you are re-writing relativity.

Basically I think you've made an error (that you didn't intentionally re-write relativity) - so that if you do your calculations correctly you'll never find the "bubble" (whatever that turns out to be) travelling slower than light.

The "usual" (fictional!) means of avoiding causality problems with FTL travel is to use a preferred frame which defines how the FTL signals travel. (This has made it into a FAQ, BTW:)

http://www.physicsguy.com/ftl/

It's obvious that anyone living in the preferred frame will alwyas see signals return after they are sent, so there is no causality problem within the preferred frame itself. I think that things work out reasonably well for other observers who are moving relative to the preferred frame too, but it's been too long since I've thought seriously about the issue for me to be positive about this point.

pervect said:
I really don't understand the details of your proposed mechanism. The "bubble" part is especially unclear unfortunately.
Here is a bit more info about what I was thinking. By 'Squeezing' coherent light, one can apparently create 'bubbles' whose energy content is below that of normal vacuum. There are fairly severe restrictions on doing this, for example any region of negative energy must be surrounded by regions of positive energy such that the average is well above the vacuum level. Even if (by some extreme miracle), my proposed ftl transmission method could actually work for photons, it would be very unlikely that it could ever be used to transport atoms (or starships!). For some simple background info on 'squeezed light', try:http://www.physics.hku.hk/~tboyce/sf/topics/wormhole/wormhole.html [Broken]

I tried to diagram this using 'ascii art', but I couldn't make it display properly here. Imagine an 'array' of lasers+optics spaced in a straight line. Each can be made to emit a pulse of 'squeezed vacuum' at an angle relative to the 'line', (say 45 degrees), which travels at C. There are two such 'lines' placed parallel to each other with a separation between them, (say 1mm). A pair of emitters on opposite sides can emit these 'mini bubbles' at the same time, such that the two beams will meet in the middle at a certain time, and create a temporary region of space with (double) negative vacuum, due to constructive interference (a 'bubble'). Successive pairs of lasers are fired, and timed such that the intersection point for successive pairs of 'mini bubbles' moves forward at >C speed. The intersection points are constructed such that they overlap (relative to the 'bubble' width). I can imagine a 'fishing line' type of arrangement such that I can 'unspool' a transmission line as I travel.

I 'inject' a photon into the first intersection point ... since the 'sides' of the two mini bubbles have positive energy density, the photon is slowed when it first encounters the sides of the bubbles, but then encounters overlapping negative regions and (fictionally) begins moving at ftl velocity, since it is effectively travelling through 'less space', (successive regions of negative vacuum).

I suppose I will have this settled for myself by creating the clearest 'paradox' argument I can. Rather than a basic argument using Minkowski diagrams, I want to try and create a simple sequence of events that causes a paradox, using only lorentz transformations, and with the restriction that all observers begin at the same coordinates with the same frame of reference, (but are then free to accelerate relative to each other, send signals, etc.)

Example: A, B, and C are next to each other and are stationary relative to each other. They syncronize their clocks. C moves 1 light minute away from A and B, (at velocity .5C relative to A and B) and then stops moving. While travelling, C has been 'unspooling' a transmission line attached to A. A and B measure the time on C's clock to be '?' ... C measures the time on A and B's clocks to be '?'. (Etc)

The purpose would be to prove for myself that all the silliness above can be used to send a signal backwards in time relative to some observer, as agreed upon by comparing to the original frame (a 'semi-preferred' frame). Assuming that it is easy to demonstrate a paradox, then I would expect 'squeezed' light can never be used to send ftl signals. (I already assume that it *is* impossible, but you can't blame a guy for looking for loopholes).

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Phobos
Staff Emeritus