Why is Causality all-important?

[Rasori]

In many cases, the answer to "why can't information travel faster than light?" is causality. Taken from Wikipedia, there are three ways to get rid of the light-speed barrier: ignore special relativity, ignore absolute relativity, or ignore causality. Yet causality is the link between two events. Basically, from my layman's understanding, object A can't travel faster than light because light from object A's starting position has to be seen at object B before object A can reach it's destination. An object is considered to be in causal contact (this understanding coming from Wikipedia) if an event can effect both objects.

 

What, then, makes it impossible to send information faster than the speed of light? Perhaps you cannot tell that the information has been SENT from somewhere, but you have the information. I don't understand why time changes at the speed of light, and so if you're moving faster than the speed of light you're traveling through time. From the way I see it (which is really what I hope to be corrected on here), if an object travels a lightyear in two months, it still left it's origin two months ago. Time has past--though the light may not have come to an observer from the origin by the time the object reaches said observer, the light is in transit. If the light is being sent, then who cares if it isn't there yet? It's still going to get there eventually, because it happened in the past. It's not time travel, it's just superluminal travel. Still, an asteroid travelling perpendicular to object A's path can possibly hit by object A--it's not like object A doesn't exist, it just can't be accurately seen. The same asteroid, if it's near object A's destination, can be seen by the destination observer before object A gets there.

 

Just for a more clear example, let's say an object, let's call it Bob, is in orbit of Earth. At Bob's location, before Bob leaves, a clock (let's say it's atomic to say it's very accurate, and let's call it Clock A) reads 11:30 AM. A light-hour away, at Bob's destination, is another atomic clock (Clock B) reads 11:30 AM as well (in other words, said clocks are synchronized to a certain point on the Earth). So Bob leaves Clock A at 11:31 AM (Clock B reads 11:31 AM). Bob travels this light-hour in 30 minutes, and Clock B reads, when Bob reaches it, 12:01 PM. Clock A, at the point that Bob reaches Clock B, also reads 12:01 PM. Both clocks have noticed a change in time. This means, then, that time has passed. If time has passed, it isn't time travel, although Bob's destination will actually see Bob leave his origin at 12:31 PM. Even so, Bob definitely was there, and is at Clock B's location--he has traveled faster than light.

 

I don't get why causality prevents this (I do understand the concepts behind other things preventing it, however). Can someone explain this to me?

[swansont]

This might help.

[Rasori]

That is, indeed, helpful. Still, while I'm not going to say that it's wrong or anything, I'm curious about other sources too. It is believable (and understandable) so that's really all I need, but the more the merrier.

[Thomas Kirby]

There is nothing like using an unprovable theory to prove something. If the theorist were not so highly respected it might even be called pseudoscience.

[swansont]

There is nothing like using an unprovable theory to prove something. If the theorist were not so highly respected it might even be called pseudoscience.

 

No, it mightn't. The theory makes testable predictions, which have been confimed, many times over. All theories are unprovable, as science is inductive. However, they can be confirmed to the point that we have an extreme amount of confidence that they are correct, as with relativity. Did you have something to actually add to the discussion?

[Rasori]

Hate to bring this back from the dead, but I realized now how the above site didn't prove fully satisfactory to me.

 

It claims that the superluminal message is received instantaneously by the person it's being sent to. I don't understand why that has to be the case. If you're going to send a message at four times the speed of light to someone a light-second away, you're still waiting a quarter of a second for the receiver to get it. Again, if the receiver is looking at the sender, the sender will appear to send the message three-quarters of a second after the message is received, but that's not a problem. In fact, that just reminds me of watching a fireworks display and not getting the sound until after you know it was made--this is just not knowing it was made until after you get the sound.

[JohnB]

I must admit, this is an idea that I'm having trouble getting my head around too.

 

I've been thinking of say, a repeater station on Mars. (To keep things easy assume that Mars is 30 light minutes away and the control signal travels at 10c.)

 

So I send the signal and 3 minutes later the station recieves it, 3 minutes after that I get the signal from the station that it has recieved the message. This, as I understood Swansont's link would not be a causality violation. However, if we add a third person, say on Venus, then there would be a causality violation. I send the signal to Mars, it's repeated to Venus and then sent back to me before I sent the first signal.

 

But I send the signal, 3 minutes later it is recieved and retransmitted to Venus where it arrives after say 8 minutes where it is retransmitted to me arriving some 3 minutes later. All told the signal has taken some 15 minutes for the trip after I sent it.

 

Is the problem that I'm assuming a Universal frame of reference which all things are relative to (as well as each other) whereas Relativity has only relative frames of reference between the two bodies with no Universal frame?

 

I just can't seem to picture it properly in my mind.