This gives you way more obfuscated information than you probably want: https://en.wikipedia.org/wiki/Bell_test_experiments
Basically, if you set up an experiment where two entangled particles are send far apart and you are not very careful with the derivations, it would look like you can communicate faster than light between the two entities holding the two particles. Turns out that it does not work, and while it does give fascinating insights about the weirdness of quantum mechanics it is not useful for FTL communication.
However, it is definitely useful for encrypting messages that can not be decrypted no matter how much computational power you have (even if you have fancy computational oracles).
We cannot determine which occurred first. This is a limitation due to the information communication limit. So the theoretical basis (for consistency purposes) is that faster than light motion would violate causality.
If on the other hand, there is an absolute frame of reference, then all events can be related (in time) to this absolute frame of reference. In this case, no matter how much faster we can move above the speed of light, no violation of causality would be possible, we could always determine the relative positions in time of any and every event, no matter how remote from each other.
Since we do not know (as in, we cannot determine if such a frame of reference actually exists) of any such absolute reference frame, we are unable to determine in reality what would happen in the event of faster than light motion. The mathematical basis of the prevalent theories give causality violations in such events.
The universe is much stranger and more unknown than what is purported to be understood. Always keep in mind that no matter what theories are currently in vogue, they are a very pale and simplistic understanding of the reality that we live in. Every theory we currently use is flawed and fails in some way. That is not to say that they don't have utility. They do, but they are also limited in that utility.
The problem that is faced today is that people (quite intelligent people for that matter) forget that the mathematical descriptions they use in formulating theories and models of the universe around are always simplified models. There are always simplifying assumptions made. Too often, these assumptions are so hidden that those promulgating the theories get so caught up in their theories success that they forget that it is a limited and faulty description of reality. This then leads to some of the fantastical beliefs that they will then propagate.
This effects things all areas of science and the upshot is that when problems are highlighted with specific parts of the theories in question, there is a great amount of effort undertaken to defend the theory instead of applying critical thinking to the problem and looking for better alternative descriptions.
An example is the Standard Model of Subatomic Particles. One of the basic assumptions is that the Big Bang occurred. This underpins the entire model. Yet how often does this particular assumption actually get stated as being a fundamental assumption for the theory/model.
In conflict with this is the Theory of General Relativity. One of the "results" of the GR Theory is these strange entities called "black holes". Singularities that are quite problematic in any finite universe. Simply put, a universe in which "black holes" exist cannot be started by a "big bang". The singularity that is the "big bang" won't go bang since the gravity is way above anything escaping.
So we have impasse and hence, we have illogical assumptions underpinning some of our quite workable theories/models at the macro and micro levels. We must always remember that, though our theories are useful, they are a very flawed and incomplete model of reality.
The other thing to remember is that it should be fun to try and discover more about our reality. Too often, many who support one view or another (in terms of what theory or theories are applicable - witness the conflict between string theory and non-string theory defenders), come across as being afraid of their opponents.
Having a robust defence of your view is fine, but when it descends to ad hominem attacks, then all I see is fearful little people and not intelligent critical thinkers.
Everyone does this in some form or fashion, but I see it happening with some aplomb in some of the sciences. These days, I treat any announcements of ground breaking levels put out by professional scientists with a large truck load of salt until they can provide very strong and reproducible evidence to support their conjectures. This is a pity since the scientific method has worked well over a number of centuries to increase our understanding of the universe around us.
I have seen a reaction increasingly arise against science as the years have gone by. Most of this due to the non-science philosophies and beliefs being touted by those scientists as being "proven" by their "science".
I used to believe in evolution but due to the experimental evidence being presented during the 70's and 80's, particularly in the various genetic experiments, I came to a quite different opinion. As the years and decades have gone by, the evidence is mounting that the evolutionary model as propagated today is nothing more than a religious dogma. Religious wars are an anathema to learning about the universe around us. YMMV on that one.
https://www.youtube.com/watch?v=dEaecUuEqfc
Or this paper:
Sending a message faster than light violates causality (sidenote, it would mean the end of free will). Fortunately , information cannot be sent faster than light via quantum entangled particles, the formal theorem is called the "no communication theorem".
Since we do not know if there is one or not, we make the simplifying assumption that there is none. Whether or not this assumption is correct is not determinable at this time.
If we are able to send messages at some speed faster than the speed of light (by any factor you wish), we will still be in the same situation as we are now, not being able to determine when two events have occurred relative to one another.
If we find that we are able to send instantaneous message between any two points, we will then have found the absolute frame of reference to measure when two events have take place. Causality will not be violated in either scenario.