The slowness of time. The speed of light, while fast, is not infinite. Light takes time to travel from one place to another. Einstein proved that nothing can travel faster than the speed of light. It is the speed limit built into the nature of space and time.
The Universe has a speed limit. This creates some weird effects. We cannot witness these weird effects in the everyday human world, because the speed of light is so much faster than anything in our daily experience. But the effects are very real: clocks in satellites measure time a little differently than clocks on the surface of the Earth, as Einstein’s theory predicts.
What are “simultaneous” events? Suppose there are a number of space colonies in different star systems that are more or less in a line. Call them A, B, C, D, and E. Each is about 10 light years apart.
Let’s make this an exciting story.
Stars A and E unexpectedly go supernova, and the humans have to evacuate into hyperspace. Colony B sees the light from star A’s explosion before colony C sees it. Colony C sees the explosion before colony D sees it. Colony D sees the light before colony E sees it.
The reverse is true of the light from the explosion of star E, which travels first to D, then C, B and finally A.
Light from star A’s explosion reaches colony C in their year 72, which is 72 years after their founding. That same year, the light from star E’s explosion also reaches them. They write in their history books, “Stars A and E both were observed to go supernova this year!”
The history books of colony B would read differently. The light from star A reached them 10 years before it reached colony C. Then again, the light from star E reached them 10 years *after* it reached colony C. Therefore, their history records would say, “Star A was observed to go supernova, and 20 years later star E was observed to go supernova.”
Neither history is incorrect. The colonies really did observe the events happen at those times.
When the citizens of colony E come out of hyperspace, they will consider the damage to their colony world and begin to file lawsuits against the insurance companies, which will probably need a bail-out. These lawsuits could still be in progress 40 years later, when the light from the explosion of star A reaches them.
Therefore, star E’s history books say, “40 years after our star went supernova, colony A’s star was observed to go supernova, too.”
Of course, star A’s history books would record the fact that their own star went supernova, and 40 years afterward star E was observed to also go supernova.
We have not yet mentioned colony F, which is 10 light years further away than colony E. Colony F’s history books would agree with colony E’s, observing star E going supernova 40 years earlier than star A.
On the other hand, colony Zero is 10 light years further away than colony A. It is 20 years from colony B, 30 from colony C, and so forth. Colony Zero’s history would agree with colony A’s, observing star A going supernova 40 years earlier than star E.
Which supernova happened first? Einstein said that this was a meaningless question. The sequence of events is relative to the observer’s frame of reference. It is relative to their location and their speed.
Bad news for Captain Kirk! To build Captain Kirk’s warp drive, Starfleet would have to figure out when the Enterprise, traveling from colony A to colony E, would pass through colony C. The idea that there is an absolute frame of reference — called “hyperspace” — that the Enterprise can zip around in, popping out into “normal space” at colony C to pick up more dilithium crystals, is impossible.
There is no absolute frame of reference that ties together the universe into a single NOW. There is only the current frame of reference, from which different events in the past can be said to be observed simultaneously — and any frame of reference is equally valid.
(Then again, consider the “slingshot manuver,” in which the Enterprise zooms very close to the Sun in order to travel into the past. When they want to return, they do it again. Why don’t they travel into the past again? Apparently the idea here is that it matters in which direction you travel while slingshotting!)
- We should build the Starship Enterprise. Seriously (blogs.telegraph.co.uk)