2 ms·

Maybe their math is correct, but the presentation here doesn't seem right. "Not so — there is no focal ‘point’ here but a focal line. As we move through and pas

Maybe their math is correct, but the presentation here doesn't seem right. "Not so — there is no focal ‘point’ here but a focal line. As we move through and past 550 AU, we take advantage of the fact that the focal line extends to infinity." Light is reversible, even if the space is curved. If all light from the "focal line" gathers at the focal point on the other side of the star, where does it go in reverse?

dandelany 10y ago
> If all light from the "focal line" gathers at the focal point on the other side of the star... It doesn't - the light on the focal line may be from any source that falls on the same line on the opposite side of the star. But since the chances of more than one light source (distant star) falling on this exact same line are pretty minuscule, it's still a (potentially) useful lens
- credit_guy 10y ago
  I don't understand what you are trying to say. There is no such thing as a focal line. More generally, it's not possible for light from several points to focalize at the same point because you can't revert those rays. I guess Randal Monroe explains it better than me here: https://what-if.xkcd.com/145/ https://what-if.xkcd.com/145/ However, I can see one way to read what they mean. If you take the Sun's "focal point", the point where parallel lines from one side intersect after being bent by its gravity, and move a tiny away from the Sun, then you form the image of a point that resides on the line going from you through the Sun all the way to infinity on the other side. You need to move very little for the opposite to move a lot, so for measurement purposes you are still at 590 AU from the Sun. This way with very tiny moves you could survey the planets in a different solar system, and in particular you could resolve them to multi-pixels.

2 ms·

Maybe their math is correct, but the presentation here doesn't seem right. "Not so — there is no focal ‘point’ here but a focal line. As we move through and pas

by credit_guy 10y ago

dandelany 10y ago
> If all light from the "focal line" gathers at the focal point on the other side of the star... It doesn't - the light on the focal line may be from any source that falls on the same line on the opposite side of the star. But since the chances of more than one light source (distant star) falling on this exact same line are pretty minuscule, it's still a (potentially) useful lens
- credit_guy 10y ago
  I don't understand what you are trying to say. There is no such thing as a focal line. More generally, it's not possible for light from several points to focalize at the same point because you can't revert those rays. I guess Randal Monroe explains it better than me here: https://what-if.xkcd.com/145/ https://what-if.xkcd.com/145/ However, I can see one way to read what they mean. If you take the Sun's "focal point", the point where parallel lines from one side intersect after being bent by its gravity, and move a tiny away from the Sun, then you form the image of a point that resides on the line going from you through the Sun all the way to infinity on the other side. You need to move very little for the opposite to move a lot, so for measurement purposes you are still at 590 AU from the Sun. This way with very tiny moves you could survey the planets in a different solar system, and in particular you could resolve them to multi-pixels.