[Camlost]

You guys seem like a smart lot and have likely dug through more than your fair share of science fiction, so I thought I'd query you. I'm having a bit of trouble wrapping my head around Relativity and light speed travel. I've done a bit of research online and I've found that those sites which strip the complexity down to a level that it's openly accessible don't give enough information and those that don't shy away from the mathematics of it are far too confusing. There are all sorts of relativity calculators but without a basic theoretical understanding I'm really just punching numbers. One fact I did find interesting, mainly because it had never really occurred to me, is that the effects of relativity don't really begin to become significant until you are closely approaching the speed of light (ie. >0.8c).

The reason I bring it up is that I'm having an issue with a short story I'm considering in which the transfer of information over vast distances is a significant element. I'd like to avoid the "ansible" route if possible because the instantaneousness would undermine other elements of the story.

Any of you guys have a working understanding of complex physics that might be willing to break this down to something more manageable? Or, how about any examples in books you've ready that you feel handle this issue well?

[Wilshire]

Summon: SilentRoamer.

You are probably going to want FTL information communication of some kind. It doesn't have to be instant, but if you are talking about distances outside of a solar system it would be next to impossible to have meaningful communication between stars at light speed. Taking years between messages and responses would make it very difficult to have any kind of dialogue, or even giving direction to distant recipients.

What distances are you considering?

[Camlost]

I suppose some narrative context would be helpful. Humanity has gone beyond the realm of scientific discovery and into economic viability with their relativistic space travel. They have begun colonizing other star systems. For a handful of centuries they systems were forced to essentially operate autonomously due to the time delay that occurs with any correspondence. They've been working on informational exchanges that would transcend the light speed limit. During their continued expansions they encounter another intelligent species and after a few mishaps find themselves involved in a war (a bit cliche I know, but I have some thoughts to freshen it up a bit). The necessity for FTL communication becomes paramount.

This is where I need help. Is FTL possible in regards to transmissions? Can I have FTL exchanges of information whilst still imposing the light speed limit on travel? I know I can take some narrative liberties, but I'd prefer it be as near to the facts as I can make it.

[Wilshire]

Quantum entangled particles could lead to something like FTL communication.

Basically what you do to one particle instantly affects the other particle, irregardless of space or distance. Spin states could be converted into some kind of binary and from there... anything you want. The idea being that you generate some entangled particles, drive half of each pair across the galaxy, and then you can still communicate like you are in the same room.

http://en.wikipedia.org/wiki/Quantum_entanglement

So you can move information in this kind of quasi 'FTL' travel without breaking any physics... Or at least a futuristic civilization who has figured out how to use the phenomenon to communicate would be able to. Kind of like an ansible, but with less magic and more science

And with this, matter still has to move through regular time/space and therefore maintain a speed under lightspeed.

[SilentRoamer]

oooh I missed this one Camlost! Thought the title was Light, Time and Gravity.

The problem with ANY FTL communication, (Entanglement, zero width wormhole, Ansible, etc etc) is they all break causality.

The sort of thing you are talking about is a well known thought experiment called a Tachyonic Telephone  - essentially if you can send information FTL then you can essentially send messages into the past due to the mathematics and informational worldlines (regardless of whether FTL is ever achieved or just the illusion of FTL due to some other exotic technology (such as space folding)). So essentially FTL comms if done "scientifically" still has to take into consideration plausible causality paradoxes.

Most often if you are using FTL as a mechanism limiting it to zero mass matter (as in radio waves which have no mass but do have momentum.) then you remove physical FTL transfer which does help (at least in terms of mass requiring infinite energy to attain C.) It is a common misconception that mass cannot FTL (well it can't) but the misconception is that it is only speeds exceeding C which are unattainable - in actuality it is C itself that is unachievable (at least with Baryonic matter (matter with mass))

Personally I would go two routes - handwave it away completely or base it very loosely on little understood physics (such as the effects of Quantum Entanglement) or removing electromagnetic waves from the Higgs Field.

If you need anything specific then let me know. When taking into consideration distances for communications bear in mind Proxima Centauri is our closest neighbour and a return communication would take approx. 9 years. You might want to look at artificial satellite governing worlds or something like that.

Anyway happy to help if you feed me some more info

EDIT: The mathematics behind the Tachyonic Telephone can be a bit daunting for some. The Novikov Self Consistency Principle might be good reading for you if the maths turns out to be a pain.

[Camlost]

Most often if you are using FTL as a mechanism limiting it to zero mass matter (as in radio waves which have no mass but do have momentum.) then you remove physical FTL transfer which does help (at least in terms of mass requiring infinite energy to attain C.)

I think I'm having trouble wrapping my head around anything with a m=0 having momentum. I stopped studying hard mathematics shortly after highschool when I discovered the humanities (much to my parents chagrin), but isn't momentum as expression of mass x velocity?

When taking into consideration distances for communications bear in mind Proxima Centauri is our closest neighbour and a return communication would take approx. 9 years. You might want to look at artificial satellite governing worlds or something like that.

For a portion of the story, the limitations on interstellar communication is an important feature which is eventually overcome (both yourself and Wilshire mentioned quantum entanglement and I think I can make that fit as far as the narrative goes), so without breaking physics or causality it looks like C is my limit here? Unless I'm not understanding a point you may have been trying to make?

Also, what do you mean by "artificial satellite governing worlds"?

[Wilshire]

Yup C is pretty much your limit.

As an example, at the advent of "hey look, we built the first quantum communication machine" you are still limited to C for quite some time. The entangled particles need to be generate in the same place, so at best you could do whatever the "entangling" consists of at, say, Earth, then take half of them on a spaceship traveling in regular C space, build the communicator around the entangled particle, and then when you get to where you want to go, drop of the device.

This process must be done with each 'entangled communicator': Make entangled particles, build machine (or build in transit), travel in C space to destination. You could further limit this by making a specific planet/starbase/outpost as the only place with the necessary machines to build/make not only the machine but also the quantum entangled particles.

FTL communication would slowly propagate as you built networks of the entangled machines, but you are limited by distance/'C space' until you physically fly each new device to a new planet. Its not something that each place could build a machine that will talk to another one built on another planet, so that suddenly everyone has FTL communication. You could even have this tech exist at the start of your story, even have it developed, but simply make it so that whoever is involved doesn't have access to it (maybe theirs was destroyed and they have to wait for a new delivery, or maybe its a new tech and hasn't been disseminated to everyone yet, etc. etc.)

No idea what SR is referring to regarding satellites.

[SilentRoamer]

Yeah I mean you could move at regular C and then Quantum Talk using some form of entanglement technology. Any form of communication which exceeds C (not in a local frame but from the viewpoint of an external observer) causes Causality issues. This is due to there being no preferential reference frames. The main thing to bear in mind is that the concept of "now" does not hold any universal temporal truth.

Anyhow if you are interested in exploring this (or even just for understanding sake) I would recommend becoming familiar with the "twin paradox", "grandfather paradox" and "Novikovs Self Consistency Theory". All should help you get a good understanding of causality. You will probably get familiar with the term World Lines lol.

What I meant by "artificial satellite governing worlds" was that it may be an idea to have all communications handled by an arbitrary AI or something which removes the human element from things - just a thought I didn't explain very well.

Well light does not have any invariant rest mass - which is what most physicists prefer to use (they love their invariance) but it does have relativistic mass and the associated mass-energy equivalence. This is fairly difficult to explain without resorting to mathematics but basically because light has relativistic mass this relativistic mass is proportional to its momentum or expressed another way the "mass" is inversely proportional to wavelength.