| HN Mirror

We do not usually consider a frame duration to measure a length, because for most of the durations we use the frame's size is absurdly large in practical terms, ranging from thousands of miles to larger than the diameter of the earth. So we can take a shortcut of identifying a frame as some area from the camera's point of view identifying an infinite, instantaneous volume in front of us, and don't consider the fact that light isn't instantaneous and in fact has a transmission time.

Frames this short mean that there is a sort of volumetric aspect to what is being catured. From the point of the camera, when light collection begins an area sweeps out in front of the camera and back in time at the speed of light, and when the light collection stops, the end of the area sweeps out from the camera lens in the same direction. The result of this is a spherical shell volume travelling backwards in time that represents the spacetime locations that can emit light and appear in the frame [1]. The resulting picture is an integration of the photons emitted in this 4D spacetime volume, in the direction of the camera, projected onto a 2D image.

(That's assuming a point-like camera and instantaneously turning the camera on and off; in reality it'll be fuzzier but the principle holds.)

The difference is that at these incredible fast speeds, the resulting 4D spacetime volume is of human size in most directions in the places we care about [2], measured in human-sized measuring units like "centimeters" rather than "light-seconds". Normally we can neglect thinking about this volume because we simply spray so "much" out that we don't have to think much about capture exactly what we want, in much the same way that in normal day-to-day life we tend to act as if lightspeed is simply infinite. In this case, we actually have to think about it to get what we want.

A similar effect can be seen in network equipment, for what it's worth; at the highest network speeds, we've now significantly passed the point where a given bit being transmitted is now a human-sized fraction of a wire. If you could "snapshot" a 100Gb network cable, you could see bits on the wire. If I'm getting my math right, each bit is on the order of 2-3 centimeters, give or take the medium not being fully light-speed.

[1]: It may be more intuitive for a moment to instead imagine that the camera is starting to emit the shell at the time that it is turned on, and stops when turned off, forward in time, as that creates a more intuitive initial image of what-seems-to-be-cause preceding what-seems-to-be-effect. Imagine the camera emitting light instead of collecting it. It may be easier to then imagine this shell going out farther and farther, getting larger as it goes (and the light inside dimming as it has to fill that volume with the same amount of energy it started with), indefinitely out into the universe. But since the camera is receiving instead af transmitting photons, the reality is the same image, just with time reversed; as the time goes farther back, the shell is farther away from the camera and larger.

[2]: Technically it extends all the way in the direction the camera is all the way out in space back to the Big Bang or the CMB or something, but unless you're deliberately photographing space, and why you'd do that with this camera I have no idea, you don't have to worry about that.