[Digital Today reporter Jinju Hong (홍진주)] An explanation said that even if alien civilizations exist, there are clear physical constraints that could keep them from visiting Earth.

On June 19, local time, online media outlet Gigazine reported that Professor Carol Oliver, who studies science communication and astrobiology at the University of New South Wales’ Sydney campus, presented three reasons aliens do not seek out Earth: distances in space, the energy required for travel and Earth’s environment.

The first reason cited was that the universe is too large. Proxima Centauri, the star closest to the sun, is about 40 trillion km from Earth. By astronomical standards, that is 4.3 light years. Even traveling at the speed of light, it would take more than 4 years.

With current technology, the gap feels even larger. The outlet cited the Parker Solar Probe as the fastest space probe. Its top speed is 191 km per second, about 0.064 percent of the speed of light. At that speed, it would take about 6,650 years to reach Proxima Centauri. That means if an alien civilization began space development around a similar time as Earth, it could take thousands of years to arrive.

The explanation also connects to the Fermi paradox. The Fermi paradox raises the question of why no traces or messages have been found even though the possibility of alien civilizations appears high. Still, the point at which humanity began sending radio waves into space was presented as 1901. Those signals have reached only about 120 light years so far, or about 0.000002 percent of the galaxy. Oliver said that considering that range, it is hard to see the lack of any response so far as strange.

The second reason is an energy demand so large it is difficult to imagine. Spacecraft require more energy as speed increases. Oliver explained that even if a civilization far more advanced than Earth developed a means of travel near the speed of light, a spacecraft moving at the speed of light would require "infinite energy," making it impossible.

Even if there is a civilization for which such travel is technically possible, the motive to visit remains a separate issue. Oliver also raised the question of why a highly developed civilization would spend energy and time to come to Earth, which lags in technological level. If the cost of travel itself is too high, the need to visit could be even lower.

The third reason presented was the danger of Earth’s environment. Life on Earth evolved with Earth’s environment over hundreds of millions of years. Even oxygen, essential to humans and many living things, is a highly reactive and strongly corrosive gas. That means this environment could be harmful to alien life not adapted to Earth. The point was that the perception of Earth as a planet familiar to living things cannot be assumed to apply equally to alien beings.

The search for extraterrestrial intelligent life continues. Since 1960, researchers have built exploration capabilities using radio astronomy. Projects mentioned included the SETI Institute in California and the Oxford University-based Breakthrough Listen project. Still, the period in which humanity has actually continued observations amounts to about 100 years, compared with a cosmic history spanning 13.8 billion years. Oliver judged that the likelihood of discovering intelligent life within such a short time is very low.

Even so, she said the need for exploration remains. Quoting wording from a 1959 paper, Oliver said, "It is difficult to estimate the probability of success, but if we do not search, that probability becomes 0." The remark reflects an awareness that rather than concluding alien civilizations are absent, we should first examine current technological limits and the range of observations.