Sue, T-Rex on Display in the Field Museum.

If we were far enough from earth, could we see the dinosaurs alive?

Short answer: In theory, yes. Light that left the earth 65 million years ago is now 65 million light years away, and an alien with a big enough telescope pointed directly at the earth might be able to see dinosaurs. Practically, such a telescope would be impossibly huge.

Longer answer: Light takes time to travel, so looking at anything is tantamount to looking back in time, and the further away it is, the further back in time you see. Assuming you’re about 2 feet or so from this page, then you’re reading this sentence as it was about 2 nanoseconds earlier. If you look at the moon, the light left it about a second ago. For sun, it’s closer to 8 minutes. If, for some crazy reason, the sun disappeared, there would be no effect on the earth for 8 minutes (but we’ll come back to that).

So in theory, the light that left the earth millions of years ago is now millions of light years away, and in that light is a glimpse of dinosaurs. Since the dinosaurs went extinct 65 million years ago, then the closest alien that could see dinosaurs, if he knew where to point his telescope, is 65 million light years away.

In practice, it’s much harder, because the earth is so small and 65 milllion light years is a long distance. For perspective, the vast majority of the visible stars in the night sky are within about 1,000 light years of us. The Milky Way is about 100,000 light years across. Andromeda is about 2.5 million light years away. At 65 million light years, our alien astronomers would be in the Virgo Cluster of galaxies.

So if these aliens want to see dinosaurs, they’re going to need a big telescope. Astronomers can just barely resolve planets in our own galaxy, let alone extragalactic planets. In fact, only two observations have been reported of extragalactic exoplanets. One is in Andromeda, and the other is cheating – it’s only 2,000 light years from earth because it was acquired by the Milky Way in a galactic merger.

Let me give you an analogy for how telescopes and light works. Imagine the sun is constantly casting a barrage of little tiny bouncy balls in all directions. I’m talking about fucktillions of balls here. These balls are photons, or particles of light. Some will bounce off of a T-Rex and then get caught in your eye, so your brain will be able to register, “Oh, that’s a T-Rex! I’m going to die!” The farther you are from the T-Rex, the fewer balls make it to your eye, and so the harder it is to see the T-Rex. Some other balls will hit the earth and bounce off, getting shot into space to travel the universe. Telescopes work by redirecting a bunch of these space-balls over a wide area to a small collector, so that we can get the benefit of a really big eye. Again, this is going to need to be a really big telescope.

But let’s not let reality get in the way of our adventure, after all, we’ve already assumed aliens exist. How big does their telescope need to be? Astronomy is hard, and gets harder the further away you look, because you need bigger and bigger telescopes to get the same resolution. If this alien wants to build a really big magnifying glass to see a T-Rex, then we can use the lens resolution equation to find an approximate size for his optics:

    Angular resolution = 1.22 × Wavelength / Lens Diameter

Let’s pick some numbers. We’ll use a wavelength of 500 nanometers since that’s in the middle of the visible spectrum and right at the transition between blue and green (and Rex from Toy Story is green), and a distance of 65 million light years. The angular resolution to resolve the earth would be:

    Earth Radius/Distance = 1.22 × Wavelength / Lens Diameter

Solving for the lens diameter gives about 5.8×1010 meters, which is about a third of the distance to the sun. This is big. This lens would fill up about half of Mercury’s orbit, and it only gets you a view of the earth as one pixel. That’s really big.

But these aliens wanted to see dinosaurs, not just the earth. If you want to resolve a dinosaur, even as just a dot, then we use the same equation again but with the size of a dinosaur instead of the earth radius. Since Triceratops was about 9 meters, and T-Rex was about 13 meters, we’ll use 10 meters for easy math. This lens needs to be 4.4 light years in diameter. I know human astronomers give their telescopes hilarious names, like the “Very Large Array,” and “The Overwhelmingly Large Telescope,” but somehow, these just feel inadequate.


Anyway, you’re going to run into a problem here because when you start putting a lot of mass in one spot space starts to curve a lot, and eventually it’s going to collapse into a black hole. For something with the density of glass, which is about 2.5 grams/cc, you’re going to hit this point fairly quickly. In fact, a ball of glass 14 light minutes in radius will have enough concentrated mass to collapse into a black hole.

Tough luck.



asked by /u/p0yo77 with special thanks to /u/Polyducks and /u/base736

cover image credit: Wikimedia Commons


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