How they see us from a planet located 90 light years away

How they see us from a planet located 90 light years away

«Stardate 100669.9. This is the scouting party from the planet TOI-836b, reporting from the third planet in the Sun star system. We have just witnessed the birth of a human being, a member of what is, at least in appearance, the most intelligent species of the observed world. We request confirmation of the registration of the event from our home planet, through the distance of 89.7 light years between the two systems. We guarantee that the Prime Directive has been respected at all times.”

The Prime Directive

Although we know that no extraterrestrial outposts have visited Earth, nothing prevents us from fantasizing that it has happened at some point. After all, the Star Fleets Prime Directive requires you to maintain discretion at all costs, not to reveal your presence or interfere in the course of history of the worlds visited. Suppose they are here but have managed not to notice.

Interstellar crew from TOI-836b claim to have witnessed on site the birth of a human being on a stellar date corresponding to January 2023. But what have they seen from that distant planet if they have been observing at that very moment, from so far away, the Earth with a telescope?

Human beings do not have the necessary technology to observe any world in the cosmos, not even the Moon, in sufficient detail to distinguish individual human beings. But we assume that the technology on TOI-836b is very advanced, so let’s admit that they have some prodigious telescope capable of such an optical feat.

They would be watching their grandmother being born

If in the planetary homeland of the expeditionary group they have been looking here carefully, it is possible that on stardate 100669.9 they have also witnessed a birth… but not the same one reported by the scouting party, but that of this baby’s grandmother . Indeed, from TOI-836b they see the Earth of the past, that of 89.7 years ago. They see a happy day in May of the year 1933, the date on which the grandmother of the baby was born about which the interstellar expedition reports today.

We perceive the cosmos through “messengers” that move with a finite speed, always equal to or less than that of light in a vacuum. One of those messengers, sound, a daily source of information for human beings, propagates so slowly that it sometimes surprises us with effects due to that slowness, like echoes, or like the delay between lightning and thunder. .

In astronomy we have three main messengers. The first and most traditional is electromagnetic radiation, which includes light, and which travels with the maximum speed allowed by physics, of almost 300,000 kilometers per second. Secondly we find gravitational radiation, which is just as fast. And finally, we have neutrinos, which move a bit slower because they have some small but not zero rest mass. We could add a fourth messenger, cosmic rays, although of lesser importance than the main three.

The light delay

No messenger can transport information faster than the speed with which it travels. In the case of light, this implies a delay of the same nature as the one mentioned above for the sound of thunder. But since electromagnetic radiation is so much faster than sound, much greater distances are needed for any delay effect to begin to be felt.

At a distance of more than 300,000 kilometers from Earth, it takes more than a second for light from the Moon to bridge the gap between our world and its natural satellite. The American astronauts who visited the Moon received the radio signals from Houston with that delay, which, although it may seem small, was a certain inconvenience in communications. Soviet pilots remotely driving the Lunokhod robotic vehicles also faced this problem when driving those off-road vehicles by remote control.

From the Sun they would see us eight minutes late

Someone on the Sun would see what is happening on Earth eight minutes late. And the network of terrestrial robots that populate Mars “see” what happens on our planet many minutes later, up to twenty in extreme cases: such is the interval that must be expected between sending an instruction from the command center and receiving it. the martian surface. The situation is symmetrical in the other sense: when applause breaks out in the control room because a robot has successfully landed on Mars, in reality that event happened in the past that can last several minutes.

The Voyager 1 probe obtained the famous image known as “a pale blue dot” on February 14, 1990, at 4:48 a.m. (UTC). But more than 6,000 million kilometers from Earth, what that probe captured was not the reality of the moment of capture, but another corresponding to more than five and a half hours before, when it was still the 13th. That in the case from Earth because, strictly speaking, each one of the planets captured in that historic photo was recorded at a different moment in time, corresponding to the distance that then lay between each world and the space probe.

From a planet like TOI-836b, which is 89.66 light-years from Earth, in January 2023 they could only have seen the birth of the baby’s grandmother, and that was provided they had the necessary technology.

the cage of light

Astronomy teaches us that we live in the cosmos within a cage of light. We capture the universe with an inevitable delay, linked to the finite speed of sidereal messengers. With the naked eye we see the stars in the sky as they were decades, sometimes centuries or millennia ago. From the closest large galaxy to our own, Andromeda, they can now see an Earth where what is born is not a human baby, but an entire biological genus, the genus Homo, whose first representatives learned to walk around these parts some two million two hundred thousand years ago. The abyss of cosmic distances shows us, at the same time, the abyss of time towards the past, as we consider more and more distant galaxies.

Sometimes this fact is received with some uneasiness, with the perplexity and sadness caused by knowing that we will never know the present, “real” state of the most distant regions of the cosmos. But we must take our cage of light as a blessing, rather than a condemnation. Just because looking far is seeing the past, we have access to studying regions of the universe as they were in abysmally ancient times. We even caught a glimpse of the afterglow left behind by the Big Bang, just a few hundred thousand years after it all started.

If light propagated with infinite speed, then the instantaneous communications that flood science fiction movies and series would be viable. The exchange of messages in the style Star Trek between the star fleet and their planet of origin could take place as described at the beginning of this article, and from TOI-836b they would have witnessed the birth of the baby in real time.

It would also be possible that, in Star Wars, Obi-Wan Kenobi perceived “a great disturbance in the force” at the very instant of the destruction of the planet Alderaan, despite being many light-years away (actually it should have taken him years, if not centuries, to find out). ). But, apart from the implications that this could have for the physics of the cosmos, an infinitely fast light would prevent us from seeing the past of the universe and contrasting through observations the theories about its origin and evolution. Wouldn’t the inability to know our origins mean blindness much worse than the one we suffer now, in this cage of light that turns the landscape into a photo from the past?

David Galadí Enríquez, resident astronomer at the Calar Alto Observatory, Institute of Astrophysics of Andalusia (IAA-CSIC)

This article was originally published on The Conversation. Read the original.