The de Broglie wavelength of any massive particle is $\lambda = \frac{h}{p}$. We know that if a large object (say, a baseball) is at rest, it will have $p=0$ and hence $\lambda$ will be infinite. This should allow us to observe quantum interference of the baseball.
Online sources say something about the uncertainty principle: we can never be certain that $p=0$. There will always be some uncertainty in the value of $p$. If we want to decrease $\Delta p$, we have to increase $\Delta x$, the uncertainty in the position of the baseball. Why doesn't this allow us to observe quantum effects?
