Undergrad Why Is a Violation of Causality a Problem?

[Keln]

I've never understood the problem with a violation of causality.

Obviously we are talking about in the context of FTL travel, probably one of the most asked about topics there is. Everyone wants it to be a thing. While I see the benefits of it, I don't care about if it could be possible or not. I just care about understanding why it can or cannot be, with the assumption that it cannot be.

The normal response is "violation of causality!". Also, the many many reasons why you can't go faster than light (mass, infinite energies, etc). But I am not interested in them. This is merely a discussion for understanding a certain piece of the puzzle.

I've come to understand, for my own visualization, that the speed of light is the speed of time itself. Or at least the speed of the time component of space-time. It cannot be violated because it simply isn't possible. To do so would be to travel back in time.

From the reference frame of the one doing the speeding, on a regular old graph, less than c has a v=d/t slope of /

A photon as a frame of reference slope of |

FTL sees a slope of \

Backwards in time is what that plot suggests, unless I am missing something there in my understanding.

But it should be that backwards time travel would be within the reference frame of the one traveling, not an outside observer. And over any distance, their arrival time at any speed would be that at which light or other means could convey that information. It almost seems like one could make what is effectively an instantaneous trip, which is to any other reference frame FTL, yet not appear at that location literally earlier, relative to "universal time" than one left.

In other words, I point my super fast rocket ship at a star 5 light years away. I put it into ludicrous speed which is FTL, and I experience going back in time 5 years and arriving at the point where it looks exactly as it had when I left. Everything there is exactly as it looked to me when I left my planet 5 light years away. I personally have gone back in time 5 years (which I suspect would actually age me 5 years contrary to popular thinking), but the net change is nil, as far as time, I am just in a different location. I could never have made the trip without having seen the destination, approximately around the time just before I left for it. So the cause was distance related, not time related, with respect to the effect.

Effectively, I have just traveled at around light speed, 5 years later in my frame of reference, to arrive at a destination as it looked when I first observed it 5 ly away. The net effect I am 5 years older biologically, but in a different physical location at around the same local time as when I first observed it from Earth.

Why is this a problem?

 

[Herman Trivilino]

I've never understood the problem with a violation of causality.

Obviously we are talking about in the context of FTL travel, probably one of the most asked about topics there is. Everyone wants it to be a thing. While I see the benefits of it, I don't care about if it could be possible or not. I just care about understanding why it can or cannot be, with the assumption that it cannot be.

The normal response is "violation of causality!".

Are you referring to the notion that FTL travel implies a violation of causality?

It's not clear to me just what it is you're wanting to know.

I've come to understand, for my own visualization, that the speed of light is the speed of time itself. Or at least the speed of the time component of space-time.

While this is a popular notion, I've never been able to make sense of it. Speed is a ratio of distance to time, so speaking of the speed of time is nonsense. Objects have speed, time is not an object.

In other words, I point my super fast rocket ship at a star 5 light years away. I put it into ludicrous speed which is FTL, and I experience going back in time 5 years and arriving at the point where it looks exactly as it had when I left. Everything there is exactly as it looked to me when I left my planet 5 light years away. I personally have gone back in time 5 years (which I suspect would actually age me 5 years contrary to popular thinking), but the net change is nil, as far as time, I am just in a different location. I could never have made the trip without having seen the destination, approximately around the time just before I left for it. So the cause was distance related, not time related, with respect to the effect.

Effectively, I have just traveled at around light speed, 5 years later in my frame of reference, to arrive at a destination as it looked when I first observed it 5 ly away. The net effect I am 5 years older biologically, but in a different physical location at around the same local time as when I first observed it from Earth.

Why is this a problem?

It's not a description of the way Nature behaves.

 

[Keln]

Are you referring to the notion that FTL travel implies a violation of causality?

It's not clear to me just what it is you're wanting to know.

I am wanting to know, basically, if I arrive at a physical distance of whatever you like, at t=1 second, after I made the decision to go there at t=0 seconds, regardless of my speed, how that violates causality.

While this is a popular notion, I've never been able to make sense of it. Speed is a ratio of distance to time, so speaking of the speed of time is nonsense. Objects have speed, time is not an object.

Speed is a ratio of d/t, which I noted. However, the "speed limit" of light is related to time. Time, by its very nature has to have some sort of "rate". It's not distance over time, obviously, but it is a rate. And the ratio between c and the rate of time is 1:1, which is why anything traveling at light speed can go an infinite distance over time, from its own reference frame. Calling it the "speed of time" is merely an analogy. They are simply connected. Whether actually going back in time is possible by breaking that barrier is possible...I somehow doubt. I think you get into some impossible math at that point.

But the most common answer to anything about FTL is "breaks causality, stop asking questions". That's not an answer.

It's not a description of the way Nature behaves.

Unless someone has a full model of how Nature behaves already, that's not an answer either. I'm trying to understand as best I can why instantaneous travel in and of itself somehow violates causality, if there is a cause then an effect. I'm not talking about "warp travel" either, which some have conceded is possible, but still grumble about causality. I don't understand why, if there is an actual cause then effect. I'm not talking about arriving before you ever existed here. I'm attempting to understand causality outside of my layman's understanding of the term.

 

[Grinkle]

The normal response is "violation of causality!".

But the most common answer to anything about FTL is "breaks causality, stop asking questions".

Do you have a reference?

 

[PeterDonis]

I've come to understand, for my own visualization, that the speed of light is the speed of time itself.

I don't know where you're getting this understanding from, but it's not even wrong. "The speed of time itself" doesn't make sense.

From the reference frame of the one doing the speeding, on a regular old graph, less than c has a v=d/t slope of /

A photon as a frame of reference slope of |

FTL sees a slope of \

I don't know where you're getting this from either.

Why is this a problem?

There are too many misconceptions in your post to make a start at trying to answer this. I think you need to first spend some time understanding what relativity actually says.

Thread closed.

 

[Nugatory]

I am wanting to know, basically, if I arrive at a physical distance of whatever you like, at t=1 second, after I made the decision to go there at t=0 seconds, regardless of my speed, how that violates causality.

Google for "Tachyonic antitelephone"