# Why is $y(t)=x(t/2)$ a non-causal system?

I was going through my signal and system notes.they say $y(t)=x(t/2)$ is a non causal system? As non causal system depend on future inputs. how $t=t/2$ is future value of time? i could not understand how? could somebody please explain it to me?

## 2 Answers

Because for negative values of $t$ you have, for example, $y(-2) = x(-1)$ which depends on a future value of $x(t)$ at $t=-1$ for the current value of $y(t)$ at $t=-2$.

Note that $t=-1$ represents a future time for $t=-2$.

• You beat me to it ... – Matt L. Jan 17 '18 at 10:56
• we never miss these questions :-) – Fat32 Jan 17 '18 at 10:57

HINT:

Use a negative time $t$ to see that $y(t)$ depends on future input values.