Comment by bloopernova
Comment by bloopernova 10 hours ago
I am unable to grasp why FTL communication would break causality, it's like my brain just refuses to accept it. Seriously, I've had it explained several times over the years.
Comment by bloopernova 10 hours ago
I am unable to grasp why FTL communication would break causality, it's like my brain just refuses to accept it. Seriously, I've had it explained several times over the years.
When you communicate, you're sending energy - whether it's sound waves or radio waves or whatever. Energy can't travel faster than c through spacetime. Now if you manipulate spacetime, such as a wormhole or whatever, then the end result can effectively appear as if it's FTL but its still going at c, its just traveling through less/compressed space.
One intuitive explanation is that anything moving at the speed of light is experiencing no time at all (from the point of view of an observer) and if something is moving faster than light that means it's going backwards in time. (only massless virtual particles can)
If a clock stops or runs backwards that totally messes up "causality" which is about events interacting relative to a time order, and so time must exist for causality to make sense.
If you did manage FTL communication, it seems like it might let you detect the notional absolute rest frame, so you'd be breaking special relativity anyway.
You can't grasp this because there is no FTL communication. Quantum entanglement does not enable FTL communication, and wormholes etc. are entirely theoretical.
It doesn't even enable STL communication, other than eg superdense coding and similar. But that's not what people mean when they think entanglement can be used for communication.
There was a young lady named Bright,
Whose speed was much faster than light.
She went out one day
In a relative way
And returned on the previous night.
It's all a big ball of wibbly-wobbly, timey-wimey stuff.
Its probably mostly because you have an intuitive idea that there is some concept of "now" which is independent of the observer.
In special relativity this global "now" isn't a thing. It doesn't exist. There is no global now. Different observers who are in different places and/or moving at different speeds will describe different events as simultaneous.
In particular say we have an observer who sees an event A happening at time 0, and a second event (call it B) at time t and the distance between them is greater than c t. Then you can find observers who see A happening first, B happening first or the two happening at the same time. However all observers will agree that the distance between the events was greater than c times the time between them.
This seems like it would cause problems with causality, but it doesn't because we need the distance to be greater than c times the time, which means no lightspeed signal could get from A to B. If you allow ftl communication then this "escape" doesn't work anymore, and causality can be explicitly broken.