When we look out into the cosmos, in a sense, everything we see is an echo. The light we see bounces off surfaces, through the air, refracted, and reflected, all at the fastest local speed physically possible. This speed takes the "echo" of light as it interacts with the world we see and is translated into sight. In our everyday experience, a light echo isn't something we could ever perceive.
Likewise, if you took two laser beams and shot one of them in a straight line so that one hit a far wall; and you angled the second one so that it bounced once off the floor and then hit the same point as the first laser, then in theory turning on both laser beams at precisely the same time wouldn't do you much good in seeing which gets to the far wall faster. The laser's light moves so quickly that the infinitesimally slight delay in the angled laser reaching the wall is indistinguishable from the instantaneous transmission to human eyes. Even some of the most sensitive light-measuring sensors would be unable to register a difference between the travel times for the two light beams.
But what if, rather than using light to look around our everyday world here on Earth, we instead used it to look out into the deepest reaches of space? The Sun's light takes about eight minutes to reach us here on Earth, and the farther out you go, the longer it takes light to reach us. Get far enough away, and now the extra distance that angled light must travel starts to matter. This is how a light echo results.
What is a light echo?
Light, as a wave, propagates in all directions equally and at a constant speed. However, light can also be reflected. Because light moves at a steady rate, it is possible to measure the difference between when light emitted from a source is expected to arrive at a particular location and when light actually arrives. A delay from the expected time could mean the light was reflected off some object. Because it moves so quickly, a light echo is only really observable over astronomical distances.
In practice, a light echo can occur from light waves originating in a nova, supernova, or some other bright cosmic event and interacting with the interstellar or even intergalactic medium between the source of light and an observer. The resulting light echo that an observer can measure is the visual analog of the sound echo they'd hear if they shouted in an empty symphony hall, with rippling patterns of light reaching the observer at different times.
How does a light echo work?
In most cases, this appears in the form of a "bubble" or "ripple" of light that can spread outward as it works its way through clouds of space dust, gas, and more.
"As light from the outburst continues to reflect off the dust surrounding the star, we view continuously changing cross-sections of the dust envelope," said astronomer Howard Bond of the Space Telescope Science Institute in Baltimore.
And just as sound echoes can become distorted, strengthened, or otherwise change as a consequence of the intervening material it interacts with, the space dust and gas that light interacts with can absorb parts of the color spectrum so that the light that reaches the observer in the form of a light echo might shift colors as it propagates through the interstellar medium.
A very famous case of this is Tycho Brache's supernova, which was observed by the Danish astronomer on November 11, 1572, and inspired him to dedicate his life to serious astronomy. The light from the original supernova reached Brache in 1572. Still, light echoes from that supernova were recently observed by astronomers after traveling away from the direction of Earth and being reflected by interstellar dust and gas back towards us—offering astronomers the rare opportunity to observe light from the same event that was witnessed by one of the most storied names of Europe's scientific revolution.
"I think it is cool that I can look in the sky and still see the same light that Tycho did at the time of his truly revolutionary discovery," Texas A&M University astronomer Nicholas Suntzeff said. "This supernova proved [Aristole's unchanging geocentric model of the universe] wrong and quickly led to a freedom of thought in science — that we can question any theory with observations — which is central to the way science operates today."
How can we use light echoes in astronomy?
Like how sonar and radar help scientists map ocean floors and measure the distance to the Moon, light echoes have proven to be a useful astronomical tool. In the case of Tycho Brache's supernova, seeing the light echo of the event is the next best thing to seeing it happen firsthand.
"It's like finding a color photo of Napoleon," said Armin Rest, an astronomer on sabbatical from Harvard University who led the study into Brache's supernova light echo. "We suddenly get a chance to take a snapshot of an event very influential in the history of astronomy."
In the case of supernovae and other event-centric phenomena, a light echo can fill in critical information gaps in events that occurred a long time ago - gaps that otherwise would not be possible to fill in.
“We can see the ‘before and after’ simultaneously by studying the light echo and supernova remnant, respectively,” Rest said. "Normally, in astronomy, the time scale for events is so long that you can’t watch a single object evolve. You can see a light pulse from a distant supernova, or you can study a nearby supernova remnant, but you can't study both the supernova explosion and the remnant for the same event. With light echoes, though, you can do both for the same event."
Having the ability to access what is essentially historical observation brought into the modern-day also has the ability to fill in holes in the timeline of astronomical events. A recent example used light echoes to determine that the Milky Way galaxy's supermassive black hole, Sagittarius A* (Sag A*), had a burst of activity about 300 years ago (in contrast to its primarily quiet posture today).
"We have wondered why the Milky Way’s black hole appears to be a slumbering giant," says Tatsuya Inui of Kyoto University in Japan, who lead the team using light echoes to probe Sag A*. "But now we realize that the black hole was far more active in the past. Perhaps it’s just resting after a major outburst."
In the case of Sag A*, researchers examined records from 1994 to 2005 for gas clouds known as Sagittarius B2, which is located about 300 light-years from Sag A*. The historical data showed these gas clouds brightening in the X-ray part of the light spectrum in response to activity around Sag A*.
X-ray emissions are a significant feature of active black holes that are accreting material since material in the accretion disk brushes up against other in-falling material at substantial fractions of the speed of light. These interactions produce a bright X-ray source that can outshine the entire galaxy that's hosting them, so these are excellent candidates for light echo observation and give us a more detailed history of an active galactic nucleus like Sag A*.
"By observing how this cloud lit up and faded over 10 years, we could trace back the black hole’s activity 300 years ago," says Katsuji Koyama of Kyoto University, an astronomer who worked on the study. "The black hole was a million times brighter three centuries ago. It must have unleashed an incredibly powerful flare."
Rest certainly agrees with that assessment. After studying a light echo in the Large Magellanic Cloud from a powerful supernova in 2008, Rest and his colleagues were able to pinpoint the explosion to an event about 400 years ago, demonstrating the light echo's utility in studying astronomical events.
"People didn't have advanced telescopes to study supernovas when they went off hundreds of years ago," he said. "But we've done the next best thing by looking around the site of the explosion and constructing an action replay of it."
"This is the first case where the conclusions that are drawn from the supernova remnant about the original explosion can be directly tested by looking at the original event itself. We'll be able to learn a lot about supernovas in our own galaxy by using this technique."