|
| Li
miao:
|
|
 |
|
 |
Li miao: I'd like to say a few words about causality in picyhss. In SR, two events with time-like separation maintain a time order for every observer. If event A happens before event B and is a necessary condition of event B, we say A is a cause of B, noted as P(B|A) bigger than 0. Let P(A,B) be joint probability such that both A and B happen, then we have P(A,B)=P(A)P(B|A). since A happens before B, no causality problems here. Bayesian statistics works as follows: given B happens later, can we infer P(A|B) earlier? Bayesians say yes, they have: P(A,B)=P(A)P(B|A)=P(B)P(A|B), thus P(A|B)=P(A)P(B|A)/P(B). the problem above is: does P(A|B) mean anything? can we infer cause from result? you also has to assume a P(B) independent of P(A). if A and B are indepent, you get P(A)=P(A) on both sides. if A is a cause of B, you get something violate causality on both sides. at time B happens, P(B)=1, why should B has a prior probability? Take A=cosmological constant, B=existence of human, and this is the case for anthropic principle. some big shots like susskind advocate it, thus they don't think causality as a untouchable principle. when you have GR, no one can prove causality must holds, time-like closed worldline is possible. but if causality fails, a l ots of stuffs like S-matrix and CPT theorem are in danger. Same thing with 2nd law of thermodynamics. No one can prove it for a real physical system except toy models like 2d sinai hard pinballs.if poincare recurrence happens anyway, then we have A,B,A,B . in time order. you take different time intervals, you get different causalities. when we say we can't go back in time because that violate both causality and 2nd law. but causality seem to hold for on large and small scale, while 2nd law applies on large scale. are these two principles conneted? if you think 2nd law has an cosmological origin, so does causality, then can you violate causality to infer the cosmological origin? I'd like to know your opinion.
|
 |
|
 |
|
|
|
|
|
|
(VISITOR) AUTHOR'S NAME
Saegedhalgedhoeltjagh
MESSAGE TIMESTAMP
17 december 2014, 10:51:17
AUTHOR'S IP LOGGED
116.193.76.111
|
|
|
|
