Could You See the Dinosaurs from 65 Million Light Years Away?

Dinosaur with Telescope by Eli Helman

This is common fun fact to introduce people to the idea the light travels at a finite speed. That when we look out into the cosmos, staring out at a star a 1000 light years away, it takes that light 1000 years to reach us and we are then seeing that star as it was 1000 years ago. This is then often followed by the thought experiment of what would happen if you could teleport to a galaxy 65 million light years away, could you use a telescope to  ‘see the dinosaurs? Let’s take a look at this thought experiment to see how feasible this could be.

The Telescope

Well first of all, we are going to need a telescope, and not surprisingly, quite a large telescope. The resolution of a telescope can be given by what is known as the Rayleigh criterion which states angular resolution can be given by the 1.22 times the wavelength of light observed divided by the telescope diameter. We can say this angular resolution needs to be the same as the angle a dinosaur would take up in our field of view from 65 million light years away. Taking the size of a dinosaur to be around 10m long, giving us an angle of ~1×10^-23 degrees. Then by having our telescope operate within visible light (~500nm) we can find the necessary telescope diameter to be ~3×10¹⁶m or around 3 light years. This is huge! For reference the earth is around 0.000000001 light years across and the closest star to the sun is 4 light years away. This is already quite far out of the realm of possibility but lets assume that this telescope could be built by an advanced alien civilization.

Technical Feasibility

Lets say this telescope could be build out of an ultra-strong, ultra-thin reflective material, only 1 micro meter thick. This incredibly thin telescope would be greater than the mass of Jupiter. You could attempt to negate some of this gravitational effect by rotating the telescope. However, if the telescope were to only rotate 1 degrees every 800,000 years then the end of the telescope would be orbiting at faster than the speed of light (ignoring special relativity) which in reality would end in the telescope being torn apart by centrifugal forces.

Telescopes must be rigid in some way to avoid any distortions and keep the shape of the mirror. This would be extremely hard in this case due to the fact that in any object, motion is transferred at the speed of sound in that material. In this case if you were to give the mirror a big push at one end it would take 10,000 years for the other end of the mirror to feel that effect.

Optical Limitations

If we were to have the distance from the telescope at which it focuses the light (its focal length) be also 3 light years, we would get the size of the focused Earth be just 60cm. You would then need each sensor to be around 5×10^-7m which is the same size as the wavelength of light we assumed we were viewing! In astronomy a telescope with its focal length the same size as its diameter would be considered an extremely fast telescope which is somewhat ironic in this case as the light would go three years between reaching the telescope surface and then being detected. Additionally due to the expansion of the universe the light from Earth would undergo a slight redshift to 502nm.

Earths atmosphere would cause some slight aberrations but that would be the least of our worries as the accumulated light absorption from interstellar and intergalactic dust would make this task essentially impossible, along with the fact that it would be extremely difficult to view the Earth with glare coming from the Sun along with the entire galaxy.

Conclusion

So even if we could travel 65 million light years away and take a look back at Earth, we would be limited by practical challenges, resolution limits, the construction scale, cosmic dust and light interference. So as fun as this idea may be, I don’t think any aliens will be getting a good look at the dinosaurs.

By Ross Gaynor.

#AP