A telescope with a 400-meter diameter mirror would be an unprecedented achievement in astronomical observation, far surpassing the size of any current optical telescope. Such a telescope could revolutionize our understanding of the universe in several ways:
Resolution and Detail:
The telescope would provide incredibly high-resolution images, allowing astronomers to observe finer details of celestial objects than ever before. This could include surface features of planets and moons in our solar system, detailed views of star surfaces, and structures within distant galaxies.
Deep Space Observation:
With its massive light-gathering capacity, the telescope would be able to detect fainter and more distant objects. This might lead to the discovery of new galaxies, quasars, and potentially, the detection of the faintest objects in the universe like rogue planets or isolated neutron stars.
Exoplanet Studies:
The increased resolution could allow direct imaging of exoplanets around nearby stars, potentially enabling the study of their atmospheres. Such detailed analysis could detect chemical compositions, including signatures of water, oxygen, or other molecules indicative of life.
Understanding Cosmology:
It could probe the structure of the universe on large scales, helping to refine our models of dark matter and dark energy. By observing the most distant galaxies, we can gain insights into the early universe and the processes that shaped its evolution.
Astrobiology and Life Potential:
If equipped with the right instruments, the telescope could potentially analyze the atmospheres of distant worlds in significant detail, looking for signs of bio-signatures or other markers of life.

In summary, a telescope of this magnitude would not only expand our current boundaries of observable universe but likely lead to an era of new discoveries, offering insights into the fundamental laws of the cosmos, the potential for extraterrestrial life, and a clearer understanding of the universe’s past and future.