An alien looks up at their sky and gazes into their advanced telescopes. They look at a distant yellow star with many planets orbiting it. An asteroid belt encircles the four planets within it and four larger planets outside it. They look at the third planet, its surface is green and blue and they can see feather and scaled animals of large size battling each other and their environment.
They are seeing dinosaurs
While the numbers don't currently truly match up for this exact scenario, look-back time is a very real phenomenon. When looking at distant objects across the galaxy or in other galaxies, we are looking at objects that are very different to what they might be now. This is because it takes time for the light to travel… anywhere. Light is not instantaneous.
For a teaching resource on this topic, then I suggest this science article.
Light has a limited speed which is 3 x 10^8 m/s (3 times 10 with 8 zeros after it); light takes time to travel, it takes time to cover distance and in doing so, it will eventually reach the person - or telescope - looking at the light. This must mean that if light is traveling to us from distant planets, stars and galaxies, that it must have taken some time to reach us. This simple statement has deeply wonderful consequences because it reveals the fact that every-time we look at the night sky we are looking back at the past.
For example, when you feel the heat on your face from the energy and light from the Sun, that light has traveled for approximately 8 minutes until it has reached Earth. So if the Sun were to blow-up or suddenly stop shining then it would take 8 minutes for us to feel any of the effects of this event. Our closest star (Alpha Centauri) is about 4 light years away and this means
that if it were to die or change in any way it would take 4 years for us to have any knowledge of this change.
If we look further away, the closest galaxy that we can see to the Milky Way is about 2.5 million light-years away - the Andromeda galaxy. This means that the light and the image that you are looking at from the galaxy will actually be what the galaxy looked like 2.5 million years ago. If it suddenly vanished or exploded somehow at this moment in time, it would take 2.5 million years before we knew that anything was wrong!
I just love this concept and the unbelievable perception that when you walk
outside at night you are walking beneath the past, millions of years of it. Taking this amazing view further, Andromeda is actually the closest galaxy - outside of the Milky Way galaxy group - and this has a ‘look-back time’ of only 2.5 million years. So depending on what galaxies you are looking at, then we can look back billion of years in time. The Hubble Space Telescope has conducted many deep field surveys over the years looking back deep into the past. So in theory we can look back to the point when the first galaxies and star were formed and alive almost 13 billion years ago. Yet, there is a problem, unfortunately in order to this, we need to be looking at objects that are very bright in order to shine bright enough for us to observe from deep field images. Of course that means that before the very first stars and galaxies we couldn’t look back since no light would be being emitted in the early universe.
The image below shows how we can look-back into the past by probing high redshift, deep field observations. The top left image shows a ground based telescope beginning to look back several billion years, while the main progress in recent years has been thanks to the space-based Hubble telescope looking back all the way to the first stars (before galaxies were even formed). Finally on the bottom left we can see that a new generation of space based telescopes could let us peer back even closer to the beginnings of the universe such as the recently launched James Webb space telescope.
James Webb has already done incredible things, showing the earliest galaxies in the universe and how their existence changed the entire structure of gas in the universe. James Webb has managed to look back over 13 billion years ago. To some of the earliest galaxies ever to have existed. This is such a mind-boggling fact, it is just truly incredible to learn and this is happening now, in our time that we can see these amazing images.
Image credit: Image credit: NASA / ESA / CSA / Simon Lilly, ETH Zurich / Daichi Kashino, Nagoya University / Jorryt Matthee, ETH Zurich / Christina Eilers, MIT / Rongmon Bordoloi, NCSU / Ruari Mackenzie, ETH Zurich / Alyssa Pagan, STScI / Ruari Mackenzie, ETH Zurich.
This is worth learning - I believe - because it puts into perspective how far we have come in our understanding of the universe. How vast the universe is and what incredible things we can learn even if we just spend some time looking for long enough and far enough away.
I hope this has helped you perhaps learn a little more of the universe or reminds you of a wondrous fact of the place that we live.
If you are interested to learn more about the universe, then perhaps consider my ebook which is sold on all major retailers - Link here.
Thanks for reading.
Cheers and stay curious
Oliver - The Teaching Astrophysicist
(Note: This blog post was NOT generated by AI and is conceived, typed and uploaded by a real person.)
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