The concept of "outer space" refers to the vast expanse beyond Earth's atmosphere and encompasses everything beyond our planet, including other celestial bodies like stars, galaxies, and even other universes if they exist. So, in a sense, we can say that outer space begins immediately beyond Earth's atmosphere.
As for the question of whether we can reach other universes or stars by continuing to travel in outer space, it's important to understand the current scientific understanding and limitations.
Stars: Stars are scattered throughout galaxies, and galaxies are spread across the universe. We can reach individual stars by traveling through space, as we have done with robotic missions to neighboring celestial bodies like the Moon, or with crewed missions to planets within our solar system. However, the vast distances between stars pose a significant challenge for human space travel. The nearest star system to our solar system, Alpha Centauri, is about 4.37 light-years away, which is roughly 25 trillion miles (40 trillion kilometers). Given our current technology, it would take many thousands of years to reach even the nearest stars.
Other universes: The concept of multiple universes is a topic of speculation in theoretical physics and cosmology, particularly in the context of theories like the multiverse. However, the existence and nature of other universes, if they indeed exist, are still theoretical and not yet scientifically confirmed. It is currently unknown how one could potentially access or travel to other universes, as our understanding of the universe is limited to what we can observe and study within our own observable universe.
Regarding the notion of hitting a wall at infinity, it's important to note that the universe is not considered to be infinite in size according to our current understanding. The observable universe, which is the part of the universe we can see, is estimated to have a diameter of about 93 billion light-years. However, the universe itself may extend beyond our observable universe, and its true size and boundaries are still a subject of ongoing scientific research and investigation.
In summary, while outer space begins immediately beyond Earth's atmosphere, the vast distances to reach other stars make human interstellar travel extremely challenging with our current technology. The concept of reaching other universes or encountering a wall at infinity goes beyond our current scientific understanding and remains speculative.