If simultaneity is not an absolute concept, does that mean that we must discard the concept of causality?

Answer 1

No, but you do have to discard the notion that the cause must come before the effect. On a quantum mechanical scale there are numerous events that can cause an effect a VERY short time in the past. By "VERY" I reffer to the time it take light to travel a few millimeters and that sort of thing. Search for "photon two slit experiment" for an in depth example of one such setup.

Answer 2

The concept of absolute simultaneity survives in relativity - but just not under the same words. Any pair of events in spacetime is separated by an invariant (i.e. absolute) interval, which is either spacelike, timelike, null.

Spacelike separation is the analog of simultaneity and in fact the word "simultaneity" could have been technically defined to mean a spacelike separation. Historically that didn't happen and the word "simultaneous" is still reserved to describe the less useful and physically meaningless concept of two events happening to have the same time coordinate in some particular frame of reference and we have to use the cumbersome phrase "separated by a spacelike distance" for the more meaningful concept.

For any two events which are *not* separated by a spacelike distance, one of them must be unambiguously in the past of the other, that is, in the past in every possible frame of reference.

The principle of causality in relativity says more or less the same simple thing it did in prerelativistic physics but it uses the interval definition: No event A can have an effect on another event B unless A and B are separated by a non-spacelike distance (i.e. either timelike or null), and A is in the past of B.

Answer 3

As said above, simultaneity is a "relative" concept. Events that may appear simultaneous to one observer may appear at different time for another (see A.E. Relativity).
However, even applying relativity, cause always preceed effect...
Until recently.
Experiments in "passing messages between photons" (in high gravitational fields), have shown that MAYBE effect could preceed the cause. Ended up by admitting that due to gravitational fields, the "light cones" were tilted inwards to each other, giving an IMPRESSION of non-causality.
A.E. relativity still holds!

Answer 4

No.

While it is not true that observers moving relative to one another will agree whether events are simultaneous or even which came first (contrary to another answer - draw a space time diagram and you can easily construct a case in which event order changes between frames), this does not mean we have to abandon causality.

It does mean we have to factor in relativity though. While relativity is often talked about in terms of "observers", it applies to all physical processes equally so you will not observe a different outcome than would be expected by combining relativity and causality.

Answer 5

Perhaps I misunderstand your question, but if two events are not simultaneous, can they not be both caused by or results of external events or each other? I fail to see how simultaneity has anything to do with causality. Spontaneity, on the other hand, maybe...

Answer 6

Not sure what you mean by your question. What does simultaneity have to do with causality?
An interesting note however... entangled quantum particles will exhibit instantaneous change if one of the particles exhibits a change. BUT, the very fact that this change was observed would obliterate the entanglement.

Answer 7

No it doesn't. If I'm moving fast relative to you, events that one of us judges to be simultaneous, the other will judge to have happened at different times. But if our relative speed is less than the speed of light, we'll agree on which happened first. So if faster-than-light travel is impossible, as seems likely, effect can never precede the cause.