The first: Force required to accelerate the object increases (some people say mass increases). It has been more or less proven that, as an object approaches the speed of light, two things happen. The very idea of reverse time travel is pretty ludicrous in the scientific world, but, if you can accept that time is a dimension, then you might have some luck with time "slowing." This example uses ansibles, which transmit instantaneously, because I wanted to use as few numbers as possible, but in principle they could just be using radios that send a signal so fast it takes only seconds to pass the one lightyear divide. 2 lightyears away from me, she'll be her soon." But form Dave and Carol's perspective, Alice sends the message and Bob says "Carol's here right now, I'll give it to her" while Alice thinks that Dave is still. If Alice sent the message the moment Dave hands it to her, Bob would say "Carol is still. ![]() But Carol's flyby happens after Dave's, so Alice hears her message from Dave before she sends it, before she's even written it. But Alice sends the message, which arrives instantaneously at Bob, who immediately hands it to Carol. So Alice and Bob think that Dave flies past Alice, and hands her a message. They can't pass simultaneously, Alice and Bob are 1 lightyear apart while Dave and Carol are. 8 lightyears apart, meaning that Dave passes Alice first, then later Carol passes Bob. Alice and Bob think that Dave and Carol are only. The problem is, contraction works both ways. So Alice's message gets back to Alice as Dave flies past her. Alice decides to send a message, Bob receives it, Carol gets the message from Bob as she passes, sends it instantaneously to Dave, who will later hand it off to Alice. This makes sense, since Alice and Bob are closer together (.8 lightyears) than Carol and Dave, so Dave is still. Now, Carol flies past Bob's planet (still moving very fast) while Dave is approaching Alice's planet. Notice that Alice and Bob think they are one lightyear apart, but due to length contraction, Dave and Carol think that Alice and Bob are only. Also, Alice and Bob on their planets can talk to each other instantaneously because they have an ansible, and Carol and Dave can speak to each other using an ansible.ĭ-> C-> A B ![]() The spaceships are moving very fast, some fraction of the speed of light, but they are not moving relative to each other. Dave and Carol are also at rest relative to each other, but they are on spaceships one lightyear apart. Okay, Alice and Bob are at rest relative to each other, on planets one lightyear apart.
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