# How to Demonstrate that FTL Travel Violates Causality? [closed]

I'm trying to develop a story where a vast and sinister Alien Intelligence is bent on testing humanity's primitive knowledge of physics via the Q & A answer sites on their world wide communication system to make sure that they are not a threat to him (not entirely dissimilar to what Emperor Ming does in the Flash Gordon movie):

The Emperor Ming: Every thousand years, I test each life system in the Universe. I visit it with mysteries, earthquakes, unpredicted eclipses, strange craters in the wilderness... If these are taken as natural, I judge that system ignorant and harmless - I spare it. But if the Hand of Ming is recognized in these events, I judge that system dangerous to us.

What question should the AI ask the humans in order to test them and what matching answer would fully demonstrate that FTL travel violates causality? Something like this perhaps?

If I have a spaceship that can instantaneously and infallibly travel to any point within a few hundred light years show me how I can travel back in time and prevent myself from being conceived?

Obviously in order to adapt something like this to a story the answer needs to be without complicated maths (especially as I can't follow it).

The background that prompted this idea is that I've occasionally seen the phrase/assumption 'FTL travel violates causality' (or variations thereon) used by sci fi fans and authors such as Charless Stross but I have never seen a good explanation of why.

For seconds: faster than light travel would appear to be a necessary precondition to writing wide-screen space opera. But if you permit violations of special relativity, you're also implicitly permitting global causality violation — time travel. (Go read a physics textbook if you're not sure why.) Permitting violations in the first place suggests that there'll be more than one way of doing FTL travel (just as there's more than one way of doing heavier than air flight — compare a helicopter to a jet airliner and a bee). And then you've got to ask, what are the implications of time travel?

The best explanations I have seen usually involve two frames of reference moving in opposite directions at the speed of light where one observer sees an event (e.g. shooting a giant space weapon at a harmless planet) before the other observer has started the event and is thus assumed to be able to (near) instantaneously travel to the other frame of reference (thus effectively back in time) and prevent the event.

The trouble with those explanations is that surely, once an observer in one frame of reference tries to become an actor in the other frame of reference, the frames of reference must somehow merge or expand and therefore the initial conditions are no longer valid thus invalidating the violation of causality?

## closed as off-topic by overactor, Tim B♦Dec 10 '14 at 11:42

This question appears to be off-topic. The users who voted to close gave this specific reason:

• "This question does not appear to be about worldbuilding, within the scope defined in the help center." – overactor, Tim B
If this question can be reworded to fit the rules in the help center, please edit the question.

This is a question that's been repeatedly asked on Physics.SE and you're likely to get a better answer there than here.

Rather than migrating this question I'm going to close it. Hopefully these linked answers will give you what you need but if not feel free to ask a follow on question. Note that a question purely about the Physics would be best asked on Physics.SE, questions about changing the rules of physics or consequences to the world/society/etc would be best asked here.

Asking in those places will make it more likely that a relevant expert will see the question and be able to respond.

https://physics.stackexchange.com/questions/52249/how-does-faster-than-light-travel-violate-causality

https://physics.stackexchange.com/questions/196/what-are-some-scenarios-where-ftl-information-transfer-would-violate-causality

https://physics.stackexchange.com/questions/128075/how-would-wormhole-based-ftl-violate-causality

https://physics.stackexchange.com/questions/113690/superluminal-travel-without-time-travel

It might also be worth you dropping by the Physics genera chat room first though: http://chat.stackexchange.com/rooms/71/the-h-bar

They'll be able to help fill in some blanks and point you in the right direction to get the most from your question.

• @ClaraOnager just in case there is any confusion: the people who voted to close this question are saying it's off topic for this site. Period. Because of that, it's not going to be reopened/taken off hold here. As a courtesy, Tim B is pointing you toward some resources that might help you understand FTL and causality, but whether those resources actually do help you or not, that doesn't change the fact that the question is off topic for Worldbuilding. – David Z Dec 10 '14 at 12:33
• @DavidZ Well, closed questions can be reopened by either a moderator or by vote, if I'm not mistaken. This is still a young site and its scope is still being defined. I personally feel that this question is off-topic and it seems TimB feels the same strongly enough to insta-close it. But this doesn't necessarily mean it can never be opened again because of a community consensus. – overactor Dec 10 '14 at 12:44
• @overactor yep, sorry, you're right. I should have worded that differently. Of course my point was really that the existence or lack of helpful resources elsewhere has little to no bearing on whether the question is on topic here. – David Z Dec 10 '14 at 12:58
• Anyone got a suggestion as to how to reword it? – user3042 Dec 10 '14 at 15:16
• Come join us in chat chat.stackexchange.com/rooms/17213/worldbuilders-general-chat and we can discuss it. – Tim B Dec 10 '14 at 15:33