All visual astronomy is based on light, of course; only, what is light?
It is an electromagnetic emission released through an energetic reaction. If material is hot enough it emits light; if excited electrons fall back to a state of reduced energy, they release light. Light’s speed in a vacuum, about 186,000 miles per second, somehow coincides with an absolute speed limit that applies to everything with no increase possible. Light emitted from a moving vehicle travels at the same velocity whether it is shining ahead of or behind the vehicle.
The nature of light has been one of the greatest conundrums in the history of physics. For centuries scientists thought about light and experimented to comprehend it. Only in the 20th century did physicists agree that light has properties of both particle and wave, and yet, they know that’s not the whole story either.
Nobelprize.org, the web site of the Nobel Foundation, records that the 1965 prize for physics was awarded to Sin-Itiro Tomonaga of Japan and the Americans Julian Schwinger and Richard P. Feynman “for their fundamental work in quantum electrodynamics, with deep-ploughing consequences for the physics of elementary particles.”
“Thanks to their work one now has one of the most beautiful and accurate theories mankind has achieved in this area,” the foundation says on another Internet page, “the QED or Quantum Electro-Dynamics.” QED is too complex for many practical applications and usually is offered only in advanced graduate courses, it adds.
“Every new physics student therefore has to struggle with the duality problem, allowing the simultaneous existence of both particle and wave concepts and holding that the two are mutually exclusive,” a mind-bending challenge.
If we didn’t have eyes we could still detect the heat that comes with the absorption of this type of radiation. With eyes, light allows us to comprehend the shape, size, texture and color of objects even at great distance. The lenses of our eyes focus the incoming light into recognizable images.
Telescopes lenses do that too; the bigger the aperture, the more light is captured and focused.
Often I think about the immense reaches that light travels through, starting from the objects that our telescopes search out. In the vastness of space, the M101 galaxy, 25 million light-years distant, appears to us as it was 25 million years ago. For that length of time, from far before the advent of humans, its light has been streaming through space, precisely aligned so that when focused at last, photons show the locations of the galaxy’s star clouds and spiral arms, its dust lanes and, occasionally, its supernovas — that is, where they were during Earth’s Miocene era.
Undoubtedly, some of the stars that the Hubble Space Telescope photographed in M101 were extinguished in cosmic explosions millions of years ago. Yet when we examine them through this grand instrument, their light still shines. The light continues through space even though the source of its luminosity went out. Astrophotographers like to say they are capturing “ancient photons.”
M101 is about 170,000 light-years across. If the disk of such a galaxy were tilted in our perspective, so that the far edge was 100,000 light-years beyond the near edge, we would be seeing light that was emitted at different stages: the closest part shows us the way it was X million years ago, and the other edge is from X million plus 100,000 years ago. I wonder if the galaxy’s rotation in the 100,000 years caused any changes in one section compared with another.
Sometimes I try to imagine light in space, the way it actually is without anything to focus it. (In a phenomenon called gravitational lensing, extremely dense objects like galaxy clusters focus light coming from farther away in space; that’s not what we’re talking about here.)
A galaxy emits light in all directions, not only toward Earth, but in a vast nimbus spreading through millions of light years above, below, and around the galaxy. Each star in the galaxy is doing that too. Other galaxies and stars, dust clouds, planets, comets, moons and nebulae are generating, reflecting or absorbing light, increasing the complexity of these light spheres. Light passes right through empty space and right through other light fields.
Imagine the swiftly growing spheres of electromagnetic radiation. They fly from every glowing object in space, yet they do not impart any information about color or shape, size or distance — until an eye or a telescope captures some of that light and focuses it.
And finally, our minds engage to interpret the views.