All of us know that when we see an object in space through a telescope, we see it how it was many years ago. Light is what enables us to see. For nearby objects like your laptop screen and the walls of your room, distance doesn’t matter. But when we are talking about celestial bodies, the distance is too large to be measured in miles and km. Hence we use the “light year”, a measure that signifies the distance light travels in a year. Given how fast light is, it takes less than a nanosecond to travel from your eye to the laptop screen. Or the wall, or even a tree some 400-500 meters away.
But talking about a star some 30 quadrillion km away, it will take around 3 years. So you will see your laptop screen as it was less than a nanosecond ago which really doesn’t matter. But when you see the star through a telescope, you will see it how it looked 3 years ago, because it’s being illuminated by light that reached it now, having traveled from where you are 3 years ago. For all you know, that star may not even be there at present. But you will see it.
Coming to the topic of the day
Dinosaurs existed on our planet some 60-65 million years ago. So if light and sight behave as I explained to you above, then theoretically, an alien 65 million light-years away should be able to see dinosaurs on Earth. The light that bounced off the dinosaurs 65 million years ago will reach this far-off planet right now, enabling aliens to see them. So, theoretically, the answer is yes. But practically, the truth is a lot more complicated.
The verdict of Science
Think about it, were dinosaurs the only object light bounced off of 65 million years ago? Definitely not. And considering the enormous size of the universe, the Earth is smaller than the smallest existing microbe compared to it. And dinosaurs are even smaller.
Light is everywhere. The photons that constitute light bounce off every opaque object, and scatter in all directions. And when these photons are captured by the eye, they enable us to see the object. When the distance between our eye and the object is less, the photons aren’t able to scatter much before they reach the eye. But the greater the distance, the more these photons will scatter. And the more they scatter, the fewer photons make it to our eye. Hence, it becomes more difficult for us to see the object.
Now consider a celestial object, like a star several light years away. This star is so far away that we can only see a dot in place of the star unless we see it through a telescope. A telescope basically collects more photons that have bounced off the star, and concentrates this light so that we can see the star more clearly. The farther the star, the bigger and more powerful your telescope should be.
The alien telescope prognosis
So again, theoretically speaking, our alien needs a huge, powerful telescope that will enable it to see dinosaurs on planet Earth. And considering that this alien lives 65 million light years away, you can only imagine how big and powerful the telescope needs to be to collect all the scattered photons that will enable it to see dinosaurs.
For an estimate, consider the world’s most powerful telescope, the James Webb Space Telescope. Its main lens has a diameter of around 6 meters and a resolution of 0.1 arc seconds, which means that this telescope can easily spot a penny more than 40 km away.
The JWST shall enable humans to see galaxies even a couple of billion light years away, but as small clusters of luminous dust, and not actually galaxies. If you want to be able to see a planet or star clearly using this telescope, the star should not be more than half a million light years away, at max. At this maximum limit, the star will only appear as a big dot in the sky.
This, my friends, is the largest and most powerful telescope built by humans. So assuming that these aliens use human technology, then the telescope has to be bigger than a city. By human standards, this telescope should have a lens more than 60kms in diameter. Not to mention a crazy resolution I can’t even give you an estimate of. And even with this huge telescope, planet earth will only appear as a dot to our alien friend. To see dinosaurs on Earth, as a dot, the diameter of this lens should be more than 4.5 light years, bigger than Earth itself!
The impossibilities don’t end here
Considering that somehow, our aliens have the capability to build a telescope this huge. But according to nuclear physics, a body this huge cannot exist in space. There are some mass density rules our universe follows. Every object has a limit to how big it can be, depending on its density. This is because after an object acquires a certain mass if the mass is increased, a black hole is formed at the spot. It happens because every object that has mass bends the space-time fabric a little. More the mass, more the bend/curvature. When the mass is too high, this fabric curves so much that a black hole is formed.
So considering that the lens of this telescope is made of glass, the diameter of this lens should not exceed 28 light minutes. If it does, a black hole will be formed. So how will the alien construct a telescope 4.5 light YEARS wide without creating a black hole?
Theoretically, an alien 65 million light years can see dinosaurs on Earth. But practically, the laws of physics make it impossible, unless the aliens have some technology humans cannot comprehend. And why would they want to see dinosaurs, anyway?