# Causal signal is defined in electrical eng.

1. Apr 15, 2009

### krindik

Hi,

As I read, A Causal signal is defined in electrical eng. context as

$f(t) = \{^{f(t), t\geq{0}}_{0, t<0}$

However what does it mean to have f(t) when t<0 ? ie. non-causal signal?
Is there any meaning when t<0 for any signal? In that sense all signals are causal. Isn't it?

Could u pls expain and point me correct direction.

Thanks

2. Apr 15, 2009

### atyy

3. Apr 15, 2009

### krindik

Re: Causality

In fact, I'm refering to Causal Signals

Pls see below for there definitions,
http://cnx.org/content/m11495/latest/

However, my question is regarding the actual physical interpretation of these signals.

what is the meaning of an anti-causal or non-causal signal which has values for negative time?

what's the meaning of negative time? is it with respect to the point in time which we start to observe the system?

can u give some examples of such signals?

4. Apr 15, 2009

### Andy Resnick

Re: Causality

Maybe it would help to keep in mind that the point t = 0 corresponds to 'now'. In terms of constitutive relations (e.g. Kramers-Kronig), it means that the present state of the material can only depend on *past* states of the material and incident fields, not future values.

The same type of formulation was used by Wheeler and Feynman (wheeler-feynman abosrber theory)

http://www.npl.washington.edu/npl/int_rep/dtime/node2.html [Broken]

Last edited by a moderator: May 4, 2017
5. Apr 15, 2009

### krindik

Re: Causality

So when we refer to non-causal it means that the signal has existed before now until future and an anti-causal signal means it has existed in the past.

So when we interpret Kramers-kronig relatioship we say that $\chi^{'} \, ,\chi^{''}$ depend on each others present and future values but not on any of past values?
http://en.wikipedia.org/wiki/Kramers-Kronig_relations" [Broken]

Last edited by a moderator: May 4, 2017
6. Apr 16, 2009

### Andy Resnick

Re: Causality

I'm not familiar with the way you are using non-causal as opposed to anti-causal. My understanding is that causal signals can influence present effects, non-causal signals cannot.

As for the Kramers-Kronig relation, I think you have a sign error: the response functions only depend on past (or present) stimuli.