I’ve been reading up a bit on time dilation and near-light-speed travel for a personal project, and I’m having trouble wrapping my head around some of the numbers involved. Any help would be appreciated!

Alice and Bob are located on interstellar objects exactly one light-year apart. Alice is going to visit Bob, and right before she leaves, she sends a message to Bob that she’s on her way. As soon as Bob receives the message, he sends one back.

With her spaceship, Alice travels at 0.98c. The messages are sent at 0.9999c.
a) If Alice leaves home on January 1st, 3019, when does Bob receive her message?
b) From Alice’s point of view, when does she receive Bob’s return message? How long does she have from when the message arrives until she lands at Bob’s place, in her subjective time?

For the first one, my understanding is that Alice’s message would arrive shortly after January 1st 3020, and she herself would land about a week later, around the 8th. After that, however, the variables start getting a bit jumbled in my head, so I figured I’d ask here. Thanks in advance!


I'm not sure from your question whether you are interested in this from a science fiction point of view or from a real-world point of view. Poor old Alice! She would never receive Bob's reply, because she would be dead. Whatever force boosted her to 0.98 percent of c would have killed her and destroyed her spaceship. You can't accelerate humans as though they were subatomic particles, which are accelerated by the electromagnetic force, trillions of times more powerful than gravity. If by some miracle she could avoid being killed by the acceleration, she wouldn't survive the storm of deadly radiation generated as protons moving at 0.98 percent of c smashed into her spaceship. Sadly, Bob would never be able to meet up with Alice again, but at least he is alive. It was a shrewd move to get Alice to make the journey instead of trying it himself. And if he knew the secret of the wonder-fuel that Alice used, he would probably end up as a billionaire. No known fuel could accelerate a spaceship to 0.98 percent of c, so whatever she used, scientists would dearly like to know about it.

  • $\begingroup$ You can assume that Alice and Bob are both photons, in this case. Consider it from a science-fiction perspective, if that helps you. $\endgroup$ – DealerUmbra Jul 14 at 10:41

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.