How are space-like intervals important?

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Discussion Overview

The discussion centers on the concept of space-like intervals in the context of relativity, exploring their implications for causality and the nature of information transfer between events. Participants examine the distinctions between space-like, time-like, and null-separated events, as well as the philosophical and technical aspects of these separations.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants question how "information" can travel faster than light, given that space-like intervals imply that no causal relationship exists between events.
  • Others argue that space-like intervals indicate that different observers will disagree on the order of events, thus preventing any definitive causal relationship.
  • A participant notes that space-like intervals can be seen as a relativistic generalization of simultaneous events in Newtonian physics.
  • Clarifications are made regarding null-separated events, which are described as having a spacetime interval of zero and are associated with light signals.
  • Some participants emphasize that information cannot travel faster than light, reinforcing the idea that causally related events must be timelike-separated.
  • Discussion includes the concept of light cones, with space-like separated events lying outside each other's light cones, while time-like events lie within them.
  • There is mention of constructing local axes in the Fermi-Walker way, with a focus on the nature of spatial and time directions in relation to space-like intervals.

Areas of Agreement / Disagreement

Participants express a range of views on the implications of space-like intervals, particularly concerning the nature of information transfer and causality. There is no consensus on the question of how information can be perceived to travel faster than light, and the discussion remains unresolved regarding the interpretation of these concepts.

Contextual Notes

Limitations include the potential for misunderstandings about the nature of information and causality in relation to space-like intervals, as well as the dependence on definitions of terms like "null-separated" and "light-like." The discussion does not resolve these ambiguities.

Cluemore
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Time-like and light like separations between events make sense, because a particle or a light flash can travel between them. However, how can you have a space-like interval when even light cannot travel such an interval (because its velocity is not greater than c). Obviously the first event cannot cause the second event for a space-like interval.

I suppose it boils down to what I understand about WHAT is actually traveling between two events. "Information" is said to travel between these events. So what is this "information" that can travel above the speed of light? Or have I got it all wrong?
 
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Spacelike intervals are important because if two events are separated by a spacelike interval then different observers will differ about which one happened first. Thus, there is no observer-independent way of saying which happened first, and thus no possibility that one of them caused the other (because the the cause must precede the effect for all observers).

In contrast, with timelike-separated events all observers will agree about which happened first and therefore it is possible that the first event was the cause of the second.
 
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To add to what Nugatory said, events with spacellike separation are the relativistic generalization of Newtonian simultaneous events. Instead the Newtonian case, where you can definitely say simultaneous events occurred at the same time, in relativity you can say that any pair of events with spacelike separation may plausibly be considered to be simultaneous.
 
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Nugatory said:
with timelike-separated events all observers will agree about which happened first

Just a clarifying note, the same is true of null-separated events.
 
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Thanks for the replies. I suppose I now have anroader understanding of the purpose and necessity of such an interval. However (I may very well be mistaken), this does not answer my question about how "information" can travel above the speed of light.

BTW what are null separated events?. Is it when some time interval is zero or...?

PeterDonis said:
Just a clarifying note, the same is true of null-separated events.
 
Clueless said:
Thanks for the replies. I suppose I now have anroader understanding of the purpose and necessity of such an interval. However (I may very well be mistaken), this does not answer my question about how "information" can travel above the speed of light.
Information cannot travel faster than the speed of light.
If it were (hypothetically) possible, the events "message transmitted" and "message received" would be separated by a spacelike interval, which means that the two could not be causally related - but we can't have reception without transmission, so they must be causally related so must be timelike-separated.

BTW what are null separated events?. Is it when some time interval is zero or...?
Events are null-separated when the spacetime interval between them is zero. You'll often hear this called "lightlike" because the emission of a flash of light at one place and its arrival at another are always null-separated eventS.
 
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If you are familiar with the concept of a light cone, then events that are space-like separated are outside each other's light cones, events that are time-like separated are inside each other's light cone (one in the other's future, the other in the one's past), and events that are null- or light-like separated are in the surface of each other's cones.

That's kind of the diagrammatic version of what Nugatory said.
 
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Clueless said:
Thanks for the replies. I suppose I now have anroader understanding of the purpose and necessity of such an interval. However (I may very well be mistaken), this does not answer my question about how "information" can travel above the speed of light.

BTW what are null separated events?. Is it when some time interval is zero or...?

When constructing local axes in the Fermi-Walker way, the spatial axes are the 3-spacelike lines orthogonal to each other and the time direction.

All axes may be space-like, I think.
 
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Mentz114 said:
All axes may be space-like, I think.

All three of the spatial axes are, yes. If you are constructing an orthonormal basis (which is how Fermi normal coordinates are constructed), it must have three spacelike axes and one timelike axis. It is possible to construct coordinate charts with all four basis vectors spacelike (for example, Painleve coordinates inside the horizon of a black hole), but the coordinate basis for such a chart cannot be orthonormal.
 
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