Bridges and Special Relativity

I wrote earlier about how Bridges can cause problems when measuring time.

My brother-in-law wrote back, saying that Bridges can cause problems in actually allowing travel backwards in history, beyond their effects of messing up accounting. His argument was not quite thorough, but it show me that my earlier example is deeply flawed.

In that example, I had treated Bridges as teleportation devices, similar to "transporters" from Star Trek. Transporters would cause problems when measuring time, as I described. It would also allow very blatant conflicts with causality.

(This link explains how nicely, if you understand spacetime diagrams. I'm not sure how readable spacetime diagrams are to most people. I did my undergraduate physics thesis in Relativity. I was glad to see my professor's book, at the time a draft photocopied and bound by Kinko's, is now in print and getting rave reviews.)

But Bridges are more than teleportation devices. They are actual physical connections, like the "wormholes" modern physics believes are possible but without any messy tidal forces or event horizons. So my other example was deeply flawed because the pilot could have spent most of trip standing halfway through the Bridge, with one side of his/her body on Earth and the other side on the spaceship.

Phrased most simply: Bridges can cause two locations, moving at velocities and/or accelerations, to become one frame of reference.

(In the language of special relativity: a setting with Bridges has no inertial reference frames.)

Duh. I should have seen this sooner.

Anyway, all science-fiction settings are faced with the problem that all three the following cannot co-exist:

• the Principle of Relativity
• a constant speed of light
• faster-than-light travel or communication
  

(The Principle of Relativity states that the laws of physics behave identically in all smoothly moving inertial reference frames. In other words, if you were alseep on a train that always moved completely smoothly, upon waking you would not be able to tell if the train was currently moving or stationary until you uncovered the window.)

Many science-fiction settings discard the constant speed of light. For example, lasers create slow-moving "bolts" of photons in Star Wars and Star Trek. While this might not seem a detail of the setting related to a spaceship "drive" that moves the ship faster than light, it still renders futile most attempts to apply special relativity to the setting.

Other science-fiction settings disallow faster-than-light travel and communication. Firefly is probably an example.

For my setting it does not make sense to try to explain how special relativity shows causality paradoxes or allows traveling back in history just because the Principle of Relativity has nowhere to be applied. There are no smoothly moving inertial reference frames to work with: even to be locations for an experiment whose results differ in violation of the Principle of Relativity.

Moreover, my setting also lacks a constant speed of light! Here is a brief narrative to explain:

Imagine two very long spaceships with lots of windows pass each other in opposite directions. These spaceships are so long that a laser beam turned on at the center of a ship, traveling to one of end, takes a measurable amount of time.

One spaceship has two lasers at its exact center. These simultaneously shoot laser beams towards targets at the front and back of the ship. Within this ship, observes see the targets are hit simultaneously.

You are sitting in the other ship, in a room that has two Bridges that lead to the front and back of the other ship to give you a simultaneous view of both targets.

During the duration of the laser beam progress, you see the front of the other ship has moved slightly farther from the laser firing location and the back of the ship has moved slightly closer. Nevertheless, through the Bridges you see both targets hit by the laser beams simultaneously.

Thus the two laser beams, from the point of view of your ship, must have moved at different speeds.

In real life, the two target impacts would not be simultaneous in both locations. This has been experimentally observed with how muon travel relates to muon half-life decay: scroll down in this article for an example.

But in my setting, it would not make sense for you to observe the two target impacts as simultaneous when looking through the Bridges, but not simultaneous when looking through the windows of both spaceships. That would contradict my description of how Bridges work.

So I finally have the answer to my question. In what ways do Bridges make the setting unrealistic?

First, a beam of light must behave like any other projectile in moving faster when cast from a fast-moving source. Second, there are no frames of reference to which special relativity can apply.

(At this point a physicist might wonder how the setting would be affected since Maxwell's Equations now involve a non-constant speed of light. But since my setting is for adventuring, not laboratory work, I will ignore this issue.)

There may be more effects, but I do not now have time or brainpower left to consider them.