Take this non-FTL scenario, involving a phone call and the postal service.
I send a postcard to my friend in Paris, asking whether they would like to visit me. Since it will take some time to arrive, I decide to telephone instead and ask the same question. I mention there is a postcard on the way, and my friend says yes, they would like to visit me.
After we finish speaking, my friend realises we didn't agree on a date for the visit, so my friend sends me a postcard to ask which date suits me best. After realising that it will take some time to arrive, I get a telephone call from my friend asking the same question. We agree on a date for the visit and hang up.
Some time later a postal worker in Paris is very confused, as they first see a postcard thanking the recipient for the invitation to visit, and then a few days later they see a second postcard inviting the same person to visit. Somehow they have received the reply before the invitation. They decide to telephone both my friend and I as our phone numbers are listed on the postcard, but we explain to the postal worker that we used the telephone to convey the information some time ago, and the postcards are arriving out of order due to the speed of the postal service.
Shortly afterwards, my friend receives my letter, and a day later I receive my friend's letter.
None of this violates causality, despite the postal worker seeing the events out of order, by getting the response before seeing the invitation.
Now if my friend was located in a distant star system, the postcard and postal service was replaced by a message sent at the speed of light, and the telephone was replaced by an instant FTL communication device, what exactly would change?
Apparently FTL communication allows things like the reply to arrive before the message was sent, but I cannot see how this would happen any more than telephoning before a postcard arrives results in time travel.
Even if my friend was travelling close to the speed of light (relative to me), experiencing a slower ticking clock, I cannot see how this would result in causality violation. Sure, when I call my friend on the FTL-phone they'd hear my voice sound like a chipmunk because I am experiencing time faster than they are, but I can't see how any of this could result in anything going backwards in time.
Sorry to ask yet another of these questions but all the other answers I can find seem to involve a certain element of hand waving that avoids the key points of the issue. In the hopes of avoiding this question being flagged as a duplicate, here are similar questions and my reasons why they don't answer this question:
- How does faster than light travel violate causality? States you will arrive before you leave but doesn't explain why. Says the speed of light is constant no matter your reference frame, but seems to assume the speed of a spaceship travelling at c is also constant regardless of the reference frame. That doesn't seem right to me, I can't see how a spaceship would "speed up", just because you view it from a different vantage point that happens to be moving in the same direction as the ship. Photons may not be able to go slower than c under any circumstances, but a space ship sure can.
- What are some scenarios where FTL information transfer would violate causality? Seems to consider two frames with different clock speeds as synchronised, so a message sent at T1 = 10 will arrive at T2 = 10 even though T2 might be running at half the speed.
- How does light travel create time travel violating causality? Explains that there is no universal timeline. None is needed if you can calculate the speed of your friend's clock relative to your own.
- Can FTL-Communication between two points in the same frame of reference break causality? Sadly I lack the physics degree required in order to understand the answer here.
I would really appreciate a detailed example showing exactly how FTL could produce a paradox and more specifically, why the paradox cannot be avoided. Thanks in advance for any help you might be able to provide to help me wrap my head around this!