Aliens on the other side of the Universe see the Earth through a "time machine": astrophysicist explains the paradox
Representatives of alien civilisations, who are likely to be observing our planet from their distant worlds, see the Earth through a kind of "time machine", and the further away the observer is, the further back in time the Earth itself will be. Thus, some extraterrestrial civilisations may not even be aware that there is a war in Ukraine, while others may not see any signs that intelligent life exists on Earth.
The paradox of space observations was discussed in an article for Big Think by Ethan Siegel, Doctor of Astrophysics. "The time machine" paradox is related to the speed of light.
Why we look into space and see the past
Siegel explains that when you see something in the Universe, you don't see it exactly as it is at the moment of observation, but rather a "past" version of it. The fact is that the speed of light, although the highest at which any signal can propagate through the Universe, is finite.
When you observe a celestial object, you see it in the state it was in when it reflected the light that came to an observer on Earth. You have to guess what the real situation is on the observed object based on the available information.
For example, some of the stars that people can see in the starry sky now are actually long gone, but they are located so far away that their light still reaches the earth.
Given the knowledge that mankind has, Siegel speculated on how observers from different points in the universe might see planet Earth.
Observations from Voyager 1
Voyager 1 is currently located at a distance of 157.8 AU: that is 23.5 billion kilometres away. It was launched in 1977, and it took 45 years to reach its current location, which is outside our solar system. "Voyager 1 is the most distant of the 5 spacecraft currently moving away from our solar system.
But even being so far away by human standards, Voyager 1 is able to "see" the Earth as it was less than a day ago. More precisely, the way it was 21 hours and 46 minutes ago.
For example, an observer on the Moon sees the Earth "in the past" for a little more than 1 second, an observer from Jupiter - for 33 minutes, and from Pluto - for 4 hours and 44 minutes.
Observations from Sirius
Sirius is a star within the Milky Way that is the brightest in the Earth's sky. It is located 8.6 light years away, which means that conventional Sirians see the Earth as it was in early February 2014. Yanukovych had just fled Ukraine, and the country had no idea what tragic events were about to take place. Barack Obama is still the president of the United States and few believe that Donald Trump will be next.
Bilateral communication, if the Sirians decide to respond to our presence, can be established in just 17 years.
Observations from the TOI 700 star system
TOI 700, which is known to be home to at least 3 (and possibly 4) exoplanets, is located 101.6 light-years from Earth. From there, the planet looks like it did just after the end of 1920. 85% of the Earth's surface is still wilderness; only 15% has been altered, mostly for food production. The Earth is definitely inhabited, and there are the first signs of a technologically advanced species living on it.
The round trip would take more than 2 centuries.
Observations from the Large Magellanic Cloud
The Large Magellanic Cloud has about 10 billion stars and is located at a distance of 160,000 light years.
An observer looking at Earth would see our planet as it was 160,000 years ago. Homo sapiens had already evolved, but were not the only species on the planet, as our direct ancestors were joined by Neanderthals, Denisovans and possibly the last surviving members of Homo erectus.
The Earth would have been rich in signs of life, including complex and differentiated life, but the planet was completely in a pre-technological state. The Earth had been in a long ice age for almost 80,000 years. It is not known how an observer could determine the presence of intelligence on Earth.
Observations from the Andromeda galaxy
Andromeda is located about 2.5 million light-years away: it is the only galaxy in the Local Group that is more massive and has more stars than our own Milky Way. Someone who is now in Andromeda would see our planet as it was 2.5 million years ago: long before modern humans appeared.
In fact, an Andromedan would be able to see the dawn of the Paleolithic era, when human ancestors first began using Stone Age tools. In addition to fires caused by heat, lightning strikes and volcanic eruptions, there were fires at night, which were lit by our hominid ancestors: the first members of the genus Homo, but before the appearance of Homo habilis.
For more than a million years, our ancestors lived side by side with representatives of the Australopithecus genus exclusively on the African continent: only there did these unique "night fires" appear. At this time, the Earth was experiencing giant, long-lasting eruptions, including those at Mount Kenya, the Lesser Barrier Island, Norfolk Island, and the Boring lava field.
Observations from Messier 87, the most massive galaxy at the centre of the Virgo cluster
Approximately 55-60 million light-years away from us is the centre of the Virgo galaxy cluster. With a mass of about 1000 times that of the Milky Way, it is the closest huge galaxy cluster in the Universe to us. The light we see from it originates from ~55-60 million years ago, and thus an observer there would have seen the Earth as it was so many years ago.
The planet is in a period of recovery from a mass extinction following the giant Chicxulub impact, which led to the Cretaceous-Paleogene extinction that occurred 5-10 million years ago. Almost all of the largest species of plants and animals on Earth went extinct, including all non-African dinosaurs and all flying reptiles.
Mammals, which had previously been small creatures, began to grow rapidly, and at this time the eohippus, the ancestor of the horse, first appeared. The land was dominated by small carnivores such as the ankalagon and the tree-climbing chryacus: the earliest examples of hoofed mammals. Marsupials and placental mammals evolved in parallel to each other when the split supercontinent Pangaea began to split the great ancient ocean into two separate bodies of water, separated by the Americas.
The Earth clearly showed itself to be a living, inhabited world, albeit very different in both land and atmospheric composition from the one we know.
Observations of 3C 273, the brightest quasar in our night sky
3C 273 is the first quasar ever discovered, which later turned out to be an active supermassive black hole in the centre of a distant galaxy. The object is located at a distance of more than 2 billion light years from us.
Anyone observing the Milky Way from such a great distance, if they could somehow see our planet, would see the Earth as it was almost 2 billion years ago. After recovering from the Great Huronian glaciation, which lasted 300 million years but ended 2.2 billion years ago, the oxygen level on Earth began to rise to several percent for the first time.
Eukaryotes began to appear on Earth, whose cells now contain closed, divided organelles that perform separate functions within the cell.
Photosynthesising organisms such as cyanobacteria and blue-green algae persist, and carbon dioxide levels drop to a few percent. The Earth's early atmosphere, rich in methane and ammonia, has completely disappeared.
The planet Earth is definitely alive, as the analysis of the atmosphere would show, but there are no signs of complex, differentiated or intelligent life. Earth is just another world with successful but ultimately simple life forms.
Observations from the galaxy 3C 295
In 1960, the galaxy 3C 295 was discovered, which is located at a distance of about 5.6 billion light years.
In a young star cluster in the Milky Way galaxy, at a distance of about 27,000 light-years from the galactic centre, a new star system has emerged over the past 60 million years.
The young, cold star orbits four inner planets, one of which has just formed a large moon as a result of a giant collision, followed by an asteroid belt, four gas giants, the Kuiper belt and the Oort cloud. Pluto is only the second largest member of the Kuiper belt, dwarfed by Triton, a world that will one day be taken over by Neptune.
Planet Earth at such an early stage probably has a volatile early atmosphere dominated by hydrogen, helium, water vapour, ammonia and methane. The same is likely to have been the case on Mars and Venus.
Life may already be emerging on any or all of these young worlds, but an outside observer would have no identifiable signs to look for. Until biological activity begins to visibly alter the atmosphere, surface or waters of this world, it will look like any other rocky planet in the universe: unremarkable and uninhabited.
Anyone observing the Earth from this vantage point would not be able to predict that intelligent life would ever emerge on this water-rich world.
The end of observations
Siegel notes that any observer studying the Milky Way from a greater distance would not be able to see the planet Earth because our planet, its parent star, and the entire solar system have not even formed yet.
The astrophysicist concludes that humans, the only intelligent and advanced civilisation we know, have existed for only a few hundred thousand years. Only a few thousand nearby star systems could know that we have become technologically advanced; only those within our home galaxy and a little beyond could even know of our existence.