Can the James Webb Space Telescope really see the past?

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A combined optical/mid-infrared image featuring data from both the Hubble Space Telescope and the James Webb Space Telescope. It is in a spiral pattern. (Image credit: ESA/Webb, NASA & CSA, J. Lee and the PHANGS-JWST Team; ESA/Hubble & NASA, R. Chandar. Acknowledgement: J. Schmidt)

The James Webb Space Telescope (JWST) made history on July 12 when it unveiled its first image; a gem-filled image dubbed the "deepest snapshot of the cosmos ever taken."

The James Webb Space Telescope can look farther into the past than any other telescope, observing far-off stars and galaxies as they were 13.5 billion years ago, not long after the beginning of the universe as we know it. This also looks farther across space than any observatory before it.

NASA's James Webb Space Telescope has produced the deepest and sharpest infrared image of the distant universe to date. Known as Webb's First Deep Field, this image of galaxy cluster SMACS 0723 is overflowing with detail. (Image credit: NASA, ESA, CSA, and STScI)

How is this possible? How is it possible for a machine to travel "back in time"? It's not magic; it's just how light works.

"A telescope may travel through time. It may sound fantastic, but the process is actually rather straightforward: Light requires time to travel through the immense stretches of space and reach us. The universe is like a window into the past, "'s explanation from NASA scientists.

This landscape of "mountains" and "valleys" speckled with glittering stars is actually the edge of a nearby, young, star-forming region called NGC 3324 in the Carina Nebula. Captured in infrared light by NASA's new James Webb Space Telescope, this image reveals for the first time previously invisible areas of star birth. (Image credit: NASA, ESA, CSA, and STScI)

It takes time for whatever light you see to get to your eyes, whether it's the brightness from your desk lamp a few feet away or the twinkle of far-off stars. Fortunately, light travels at an incredible speed of around 670 million miles per hour (1 billion kilometers per hour), so you won't see it moving from the desk lamp to your eyes.

Most objects in the night sky are millions or billions of miles distant, so when you look at them, you are actually witnessing light that has traveled a very long distance to get to you.

As an illustration, consider the sun. The distance between Earth and its star, which is its home star, is typically 93 million miles (150 million kilometers). Accordingly, the time it takes for light to reach Earth from the sun is around 8 minutes and 20 seconds. In other words, when you stare at the sun, you are gazing 8 minutes into the past rather than at it as it is right now (although you should never gaze directly at it).

Astronomers prefer to measure vast distances in space using light-years rather than miles or kilometers because the speed of light is so crucial to their field of study. The length of a light-year, or 5.88 trillion miles or 9.46 trillion kilometers, is the maximum distance light can travel in a year. For instance, Polaris, the North Star, is located around 323 light-years from Earth. Every time you see this star, you take in light almost 300 years old.

Therefore, you can travel across time without using a fancy telescope; you only need your own unaided eyes. But scientists require telescopes like JWST to peer far into the past (like back to the beginning of the cosmos). JWST can detect light at wavelengths unseen to the human eye, such as infrared waves, in addition to focusing on distant galaxies to view visible light from millions of light-years away.

The most difficult to detect things in the cosmos can be found when infrared radiation is observed with the proper tools. Infrared energy, which is produced by many objects, including people, is heat. Without special equipment, this energy is invisible. According to NASA, infrared light has a significantly longer wavelength than visible light, allowing it to traverse through thick, dusty areas of space without being dispersed or absorbed. Infrared radiation is emitted by many stars and galaxies that are too far away, too faint, or too veiled to be seen with the naked eye.

JWST's most useful tactic is this one. The telescope can observe the light generated by the oldest stars and galaxies in the cosmos more than 13 billion years ago by peering across hazy portions of space using its infrared-sensing equipment.

JWST used this method to get its well-known deep field image, and it will use the same method to try to peer much further back in time, to the first few hundred million years following the Big Bang. Although the telescope will show us truly long dead stars as their old light travels over the vast cosmos, JWST gives our mortal eyes a one-of-a-kind time travel exhibition.

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