A drop of water falls in an endless, still lake. The ripple spreads out, leaving a circular wave spreading out endlessly. Tiny disturbances create their own ripples; one side travelling with the main ripple, causing wonderful interactions in the wavefront; but the main ripple grows faster than these disturbances spread across it.
The beings of the ripple look across the main ripple, seeing the disturbances as their interactions propagate across the main ripple. Looking back far enough to the earliest disturbances, one thing becomes clear; the entire ripple comes from one drop, and most of the ripple is moving away faster than a disturbance can propagate.
An expanding universe where every point of the universe started from the middle is pretty easy to conceptualize with the right analogy.
It’s the weak point of the analogy, surface gravity waves like you’d get in a shallow lake do have nonlinear behaviour though.
Maybe a more accurate description would be to describe the wavespeed of the medium having tiny variations that cause extremely small, close range kinks where the wavefront crosses past itself, relating speed through time vs speed through space as the radial and tangential propagation of the wavefront. But that’s a less clean analogy, and the lake ripple is still good for describing how an entire universe can appear to be in the middle no matter where you look, despite originating from (suspectedly) a singular point.
A drop of water falls in an endless, still lake. The ripple spreads out, leaving a circular wave spreading out endlessly. Tiny disturbances create their own ripples; one side travelling with the main ripple, causing wonderful interactions in the wavefront; but the main ripple grows faster than these disturbances spread across it.
The beings of the ripple look across the main ripple, seeing the disturbances as their interactions propagate across the main ripple. Looking back far enough to the earliest disturbances, one thing becomes clear; the entire ripple comes from one drop, and most of the ripple is moving away faster than a disturbance can propagate.
An expanding universe where every point of the universe started from the middle is pretty easy to conceptualize with the right analogy.
The disturbances propagate at the same speed as the ripple, unless it’s some crazy nonlinear ripple.
It’s the weak point of the analogy, surface gravity waves like you’d get in a shallow lake do have nonlinear behaviour though.
Maybe a more accurate description would be to describe the wavespeed of the medium having tiny variations that cause extremely small, close range kinks where the wavefront crosses past itself, relating speed through time vs speed through space as the radial and tangential propagation of the wavefront. But that’s a less clean analogy, and the lake ripple is still good for describing how an entire universe can appear to be in the middle no matter where you look, despite originating from (suspectedly) a singular point.