On July 12, the James Webb Space Telescope (JWST) made history by releasing its debut image: a jeweled image which has been touted as the deepest image of the universe ever taken.
As well as seeing further into space than any observatory before it, The James Webb Space Telescope has another trick in its mirrors: It can see further back in time than any other telescope, observing distant stars and galaxies as they appeared 13.5 billion years ago, not long after the beginning of universe as we know it.
How is this possible? How can a machine see “back in time”? It’s not magic; it’s just the nature of light.
“Telescopes can be time machines. Looking out into space is like looking back in time,” explained NASA scientists at WebbTelescope.org. “It sounds magical, but it’s actually very simple: Light needs time to travel great distances to reach us.”
All the light you see—from the twinkle of distant stars to the glow of the desk lamp a few feet away—takes time to reach your eyes. Fortunately, light travels dizzyingly fast – about 1 billion km/h – so you’ll never notice it moving from, say, your desk lamp to your eyes.
But when you look at objects that are millions or billions of miles away — as most objects in the night sky are — you see light that has traveled a long, long way to reach you.
Toe the sun, for example. Earth’s home star sits an average of 93 million miles (150 million kilometers) away. That means it takes light about 8 minutes, 20 seconds to travel from the Sun to Earth. So when you look at the sun (even though you should never look directly at the sun), you see it as it looked more than 8 minutes ago, not as it looks right now – in other words, you see 8 minutes back in the past.
The speed of light is so important to astronomy that scientists prefer to use light years, rather than miles or kilometers, to measure great distances in space. One light year is the distance that light can travel in one year: approximately 5.88 trillion miles, or 9.46 trillion km. For example, the North Star, Polaris, sits approximately 323 light years away Earth. Every time you see this star, you see light that is more than 300 years old.
So you don’t even need a fancy telescope to look back in time; you can do it with your own naked eyes. But to look really far into the past (say back to the beginning of the universe), astronomers need telescopes like JWST. Not only can JWST zoom in on distant galaxies to observe visible light coming from millions of light-years away, but it can also capture wavelengths of light invisible to the human eye, such as infrared waves.
Many things, including humans, emit heat as infrared energy. This energy cannot be seen with the naked eye. But when infrared waves are viewed with the right equipment, they can reveal some of the hardest-to-find objects in the universe. Because infrared radiation has a much longer wavelength than visible light does, it can pass through dense, dusty areas in space without being scattered or absorbed, according to NASA. Many stars and galaxies that are too far away, faint or hidden to see as visible light emit heat energy that can be detected as infrared radiation.
This is one of JWST’s best tricks. Using its infrared sensing instruments, the telescope can peer past dusty regions of space to study light emitted more than 13 billion years ago by the oldest stars and galaxies in the universe.
That’s how JWST took its famous deep-field image, and that’s how it will try to look even further back in time, to first couple of hundred million years after The big bang. The stars that the telescope will reveal may actually be long dead today, but as their ancient light makes the long journey across the universe, JWST treats our mortal eyes to a unique time-travel display.
Originally published on Live Science.