(no subject)

[beccaelizabeth]

If you think you have invented an ansible
instant, faster than light, communication
and you only have one planet to test it on

how would you test?

if magic is ftl, how would they figure that out?

Comments

[brithistorian]

Depending on the technology level, I think probably some variation on the Michelson-Morley experiment (https://simple.wikipedia.org/wiki/Michelson%E2%80%93Morley_experiment) would work for this.

[beccaelizabeth]

which led to me trying to learn more physics and mostly not getting it... thanks :-)

[brithistorian]

Hope it was at least a little bit helpful - you've got some really good ideas below. If all you need is a hand-wavy approach for a book or something, a variant on Michelson-Morley would work, I think: Regular light in here, magical light in here, an intricate arrangement of mirrors to combine the two light beams and spit them out over there. If magic is no faster than light, the projected beam will look normal, but if magic is faster, it will look different somehow (dark interference lines, splitting into colors, sparkling, whaever).

[petra]

Place one at one location and the other at its antipode. Make a planet ansible sandwich. That's the maximal delay.

[beccaelizabeth]

is good plan
and getting there to try it is good adventure

[melannen]

It would depend on your tech otherwise - if you had really good clocks and/or orbital capability you could directly measure it pretty easily (microsecond-level speed of light issues come up in current earthbound tech all the time now that we've got that far.) If you were using pre-18th century tech, I think it would depend a lot on the exact properties of what you were measuring - stuff like mikkelson-morley, fr ex, relied on optical interference for the measurements, but w your classic ansible that seems unlikely to work.

So you'd have to figure out what else you can measure first.

[melannen]

If you were using pre- 19th century tech and no extra magic (other than your ftl device) I think the best you could do would be have people at opposite edges of night observing the same distant astronomical event (conjunction of jupiter and a star, maybe). That wouldn't give you any comparison to speed of light but it would confirm "instantaneous to the extent of our ability to measure."

[melannen]

Okay here's a mikkelson-morley-ish idea using the quantum entanglement mechanism but not needing atomic clocks: you have an ansible with two separate outgoing lines. One of them connects to a unit in the same room, the other to a unit at the antipode. You rig it so that as soon as one of the particles gets flipped, it immediately flips back. You let it run, recording the number of flips, until one of the sets has more flips recorded than the other. If there's a speed-of-light delay, it would only take less than fifty flips for the antipodal one to be a second behind, and that's measurable even with mechanical clocks. if it's truly simultaneous they will stay in sync forever. (Of course you would have to be very very careful that nothing else could be causing a difference in rate of flips.) I'm not sure you could be precise enough to do that experiment with, say, telepathy, though.

[beccaelizabeth]

bouncing and synch
very neat
a good clear experiment to try.

[beccaelizabeth]

Tech is pretty sketchy, but GURPS Magic has a Tell Time spell that does say *exact* time, though presumably limited to units caster would think to ask for...