Hacker News Reader

Comment by superposeur | Hacker News Reader

I seem to remember an astronomer mentioning that it is possible to interfere the light from gravitationally lensed double images. Lacking appropriate intuition, this would be amazing for me as I’d think the photons would lose coherence over the course of their multi billion light year journeys (but maybe not?). Does anyone know about this?

View on HN

Interferometry is common in radio telescopes, and was the method used in the recent imaging of a black hole by the Event Horizon Telescope, which used interferometry between multiple telescopes on different sides of the earth. It's also used a bit in optical telescopes, but harder to do. Basically yes, interference can still be used even over these extreme distances. The speed of light is pretty exact I suppose!

https://en.m.wikipedia.org/wiki/Event_Horizon_Telescope

https://en.m.wikipedia.org/wiki/Astronomical_interferometer

Interesting, thanks for the links. Looking at them, though, it would appear that the two "paths" being interfered differ in length on the order of the separation between the two telescopes -- i.e. meters or kilometers -- rather than astronomical distances. Using the speed of light, this corresponds to a difference in flight time of the light on the order of nanoseconds or seconds. I can imagine how the light traversing the two paths could remain unmolested by stray random, environmental phase shifts during such short times (necessary if you want them to interfere). On the other hand, the double images in the photo would have a difference in flight times of years? But, again, I don't really understand precisely what the astronomer was referring to, or have proper intuition for the conditions of space through which astronomical photons travel...

Photons travel at c, so don’t they experience no time? Wouldn’t they be zero old?

That has always been my take. It explains how they always perform to spec even across unimaginable distance and time.

MTTF at infinity due to relativistic effects is a pretty funny idea.

True that, but from the point of view of an outside observer, their wavefunction does have a phase that regularly oscillates (with a frequency corresponding to the color of the photon)

Reply on news.ycombinator.com