 Hello everyone and welcome to our universe. Today we'll be talking about light is a time machine. So light seems to be everywhere in the universe. It pretty much illuminates everything that we can see and it tells us a lot about the age and the evolution of the universe. But the universe tells its incredible story through the medium of light and of course other wavelengths in electromagnetic radiation. We of course learn about planets, stars and galaxies by their light, mainly their visible light. We can also know a lot more about stars, planets and galaxies through shorter wavelengths like ultraviolet and longer wavelengths like infrared light. These are of course invisible to the naked eye, but they are detectable to certain telescopes on Earth and in space. But these waves do not arrive at Earth instantaneously, they take time. And because the universe is very big place, this can take a very long time. These waves seem to travel across the universe at the speed of light. This is the fastest known speed possible. So let's put that in comparison. How does light get from certain objects in the universe to Earth? And how long does it take? So let's start with the solar system. First stop, the moon. Its average distance is 240,000 miles. So this means that it takes approximately 1.3 seconds to get from the moon to the Earth if you travel at the speed of light. So when you are watching the astronauts of Apollo 11 land on the moon live, you are actually watching them 2.6 seconds behind the live event. That's because it takes light 1.3 seconds to go from the Earth to the moon and then back again. The sun is 93 million miles away, so this means that it takes 8.3 minutes to get to Earth. This means that when we look up at the sun, we see it as it was 8 minutes ago. So if the sun would suddenly disappear from the solar system, the people on Earth wouldn't quite know it for another 8 minutes. So let's go to the largest planet in the solar system, Jupiter. Jupiter is more than 5 times further out from the sun than the Earth is. This means that it takes 43 minutes for the sun's light to reach Jupiter. The return trip to Earth can take 35 to 52 minutes. This simply depends on which side we are relative to Jupiter's orbit. Okay, let's go really far out now and go to Pluto. Yes, Pluto. So Pluto is very small and quite remote. In fact, it wasn't discovered until 1930. Pluto orbits 40 times farther out from the sun than the Earth does. This means that it takes 5.5 hours for the light to reach Pluto and roughly the same amount of time to reach Earth. This means that once the light reaches the Earth from Pluto, it is very very dim. This means that you require a very good telescope to spot this very dim object. But we still love you, Pluto. So let's move beyond the solar system and because we're going out to such a grand scale, our scale of distance and travel needs to change a little bit. Instead of measuring light in seconds and minutes, we now measure them in years. This basically keeps the numbers down to a minimum and keeps it relatively easy to understand. The star system that is closer to the Sun consists of three stars and the brightest of these stars is Alpha Centauri. Light from Alpha Centauri takes more than four years to reach the Sun. So this means that Alpha Centauri is just over four light years away from our star. The brightest star in our sky is known as Sirius. This is the primary star in the constellation of Canis Major. It is roughly nine light years away. So this means that we are seeing this star as it was nine years ago. If we wanted to travel there at the speed of light and travel back again to Earth, it would take a round trip of 18 years. So let's go to a star that we know and love. This star is known as an unpronounceable name. People might say Betelgeuse, people might say Betelgeuse. Frankly, we just know it as the big red star that's in Orion. Betelgeuse, or Betelgeuse, is about 430 light years away. So this means that the light that you see from this star left in the late 1500s. In the same constellation of Orion, there is also the Orion Nebula. This is a place where new stars are forming. This is at a distance of 1500 light years, meaning that the light from this object left a thousand years before the invention of the telescope. The further away the object is in space, the longer it takes light to get to Earth, and therefore we can see more back in time. As we look deeper and deeper into the Milky Way Galaxy, we are looking deeper into the past. Light can take thousands upon thousands of years to reach us on Earth from distant parts of the galaxy, but our galaxy is only 100,000 light years across. So this means that we can only see 100,000 years into the past. Once we move outside of our galaxy, we encounter even larger scales of time travel. This is the domain of the epic time traveler, stretching for millions and billions of years. And in this domain are billions upon billions of galaxies just like our Milky Way. The nearest large galaxy to the Milky Way is known as the Andromeda Galaxy. The Andromeda Galaxy is two and a half million light years from Earth. This means that the light left this object more than two million years ago. This was at a time when our species was just beginning to start its evolutionary process and gaining foothold on our planet. Astronomy is a lot like ancient history. Wherever you look in the night sky, you are looking into the history of the universe. This is a very important thing for astronomers. Looking at more distant objects, we learn more about the ancient times and phenomena in the early universe. The large telescopes allow us to look billions upon billions of years into the past and reconstruct the story of the cosmos. So I hope that's given you a great insight into light is a time machine. It's quite remarkable once you think about what you're really looking at in the night sky. And of course, if you want to know any more about this, I'll put some links in the description below. And as always, if you've enjoyed the video, click the like button. And if you want to support the channel, click subscribe and support our Patreon page. Thank you for watching and thank you for your support.