# Casuality [Hitting]

1. Sep 6, 2014

### ChrisVer

In fact I don't know if I'm supposed to say it here, but why do physicists care so much about casuality? It seems as if we are trying to impose something "logical" to us over nature...In fact mathematics don't need casuality in the first place to work, right?
So what is so strongly suggesting to us that nature should be casual? For example, from what I understand so far, the Big Bang was not casual -it just happened-. Also a probabilistic view of nature is going against determinism and thus casuality as well [cause→result]. That's because a 'cause' event $A$ can give you the $B_i$ 'result' events...as an example of this I'm looking into path integral formalism, which actually takes the integral of every path, casually connected or not, to give the final result.

I'm not really trying to hit casuality [apart from the misleading title which I can't change anymore], but see how it can be on par with what I mentioned above.

Last edited: Sep 6, 2014
2. Sep 7, 2014

### Staff: Mentor

In general, the kind of people that are interested in understanding Nature and have the determination to learn the mathematics necessary and put in all the time to study the physics properly to become physicists put less importance in social interactions or, more properly, social norms. Therefore, they care less about the perception others have about them and will often go for what is more confortable or takes less effort. For instance, socks and Birkenstock are fine, who cares what people think!

Or did you mean causal instead of casual?

3. Sep 7, 2014

### microsansfil

Hello,

Somehow, causality rather refers to models, theories.

Causal relationships are what allows to predicts future events.

Patrick

4. Sep 7, 2014

### ChrisVer

Well i cried laughing... but yes I meant causality/causal etc...It was a post right before I went to sleep...

5. Sep 7, 2014

### strangerep

I would have said the opposite. i.e., that causality must be an essential ingredient to our theories to model physical situation successfully. E.g., energy needs to be bounded below, even though Poincare invariance by itself makes no such imposition.

Well, mathematicians need very little from the real world in order to work (except maybe a pencil and paper).

Past and future are manifestly not interchangeable.

Using the word "happened" like that implicitly suggests there was a time "before" the Big Bang, which is not what modern cosmology theory says.

A non-deterministic quantum framework can also respect causality -- since the quantum framework is just a way to represent a dynamical group (or semigroup!) unitarily, with a statistical interpretation. Indeed, QFT is only as successful as it is because it's constructed in terms of causal field representations of the Poincare group (i.e., not just any representations). Moreover, in scattering theory, the spaces of "in" and "out" states are distinct. The scattering operator is then a mapping from the former to the latter.

Another manifestation of the importance of causality in nature is the applicability of dispersion relations (aka Kramers-Kronig relations). Take a look at the early chapters of Nussenzweig's book on causality and dispersion relations to see the important link between causality and energy-analyticity.

The path integral still relies deep down on an assumption that energy is bounded below, and that we go "from" an initial state "to" a final state.

6. Sep 8, 2014

Currently accepted understanding in science is probably the best way to look at it. If time travel were proven possible tomorrow, causality would be violated (and vice versa). What is interesting is that current understanding of science (with experimental verification!) would allow your son to be older than you, but still would not allow him to be your father! Chew on that.

7. Sep 8, 2014

### nitsuj

Can you elaborate on that?

8. Sep 8, 2014

You would probably know the often used example: If you went back in time and killed your grandmother before your father was born, then you couldn't have been born. But you are there just the same! So causality is violated.

Since causality is assumed to hold under all circumstances, the inference is that time travel is impossible.

9. Sep 8, 2014