![superluminal define superluminal define](http://images.thebubble.org.uk/beam.jpg)
More importantly, by choosing Bob's speed fast enough, we can make B occur as long before A as we want. If Bob's space ship is traveling away from Alice at a significant portion of the speed of light, then in S' event B can happen before event A, so according to Bob, the signal is received before it's sent, just as we said could be the case. Let's call Alice's frame of reference S, and Bob's frame of reference S'.
![superluminal define superluminal define](https://static.cambridge.org/content/id/urn%3Acambridge.org%3Aid%3Aarticle%3AS0074180900078062/resource/name/firstPage-S0074180900078062a.jpg)
We said that Alice sends a signal to Bob, who is on a distant space ship. Now it makes sense to ask, "Could Alice use superluminal signals to communicate with her past self?" The answer turns out to be yes, using the postulates of relativity. So, basically, what we're arguing is that the sort of "back in time" communication we've described so far isn't very interesting, since it doesn't allow Alice to communicate with her past self or any of the stuff that leads to cool sci-fi stories and troubling paradoxes. Remember that for event A the past is defined as all the events that fall inside the light cone of A and happen at an earlier time. You might well object that the signal is being sent to "elsewhere", and, though it does appear to arrive at an earlier time in some frames of reference, it does not really go into the "past", in our relativistic sense. Thus, we've already found that it looks like the signal is being sent to an earlier time, and we have to begin to question our ideas of causality. That means that, although according to Alice event A clearly happens before event B (the signal is sent before it's received), there are some reference frames in which event B happens at an earlier time than event A, so the signal is received before it is sent according to such observers. As a result, they have a different order in time in different frames of reference, as is discussed in the light cone node. Because the signal is superluminal, events A and B have a spacelike separation, Δx > cΔt. Alice sends the signal to Bob, who is out on a space ship, and he receives it at event B. Let's call the event of sending the signal event A. Suppose Alice is sitting in her lab on her space station, and she sends out a superluminal signal. The reason that sending a superluminal signal enables you to send a signal back in time is that the events connected by a faster than light worldline have a spacelike separation. The Basic Conceptsįor simplicity, let's talk about sending a superluminal signal (meaning one that moves faster than light), but the same arguments apply to sending matter-energy. At the end we discuss whether there are ways to get around this result, if it might be possible to send things faster than light without the implications of time travel. First we will describe the general picture of what's going on, then look at some numbers to make it concrete, and finally write down the algebra. It may also be helpful to read about relative simultaneity. These topics are discussed in the light cone node. In order to understand why this is the case, one must first understand what we mean by past and future in relativity and the meaning of timelike and spacelike separation of events. According to special relativity, being able to send something ( information or matter-energy) faster than the speed of light (in vacuum) is equivalent to the ability to send it back in time.