Human travel is currently limited by the technology available to us and the constraints of physics. Some of the main limitations of human travel include:
Speed Limit: According to Einstein's theory of special relativity, the speed of light in a vacuum is the ultimate speed limit in the universe. As objects with mass approach the speed of light, their energy requirements increase significantly, and it becomes practically impossible to reach or exceed this speed.
Long Distances in Space: Even with the most advanced technology currently available, human spacecraft can only travel at a fraction of the speed of light. This makes traveling to even the nearest stars an endeavor that would take tens of thousands of years or more, considering the vast cosmic distances involved.
Harsh Space Environment: Space is an unforgiving environment with high levels of radiation, microgravity, and other hazards that can be harmful to human health. Protecting astronauts during long interstellar journeys would pose significant challenges.
Life Support and Resources: Extended space travel requires sustainable life support systems and sufficient resources for food, water, and other essentials. Current technology struggles to provide these for long-duration missions beyond Earth.
Now, if we could somehow overcome these limitations and travel at a speed very close to the speed of light, some fascinating effects would come into play due to special relativity:
Time Dilation: Time would pass differently for the travelers and the observers back on Earth. As the travelers approach the speed of light, time would slow down for them relative to Earth. A journey that may take several years from the travelers' perspective could correspond to much longer periods on Earth, potentially hundreds or thousands of years.
Length Contraction: As an object approaches the speed of light, its length along the direction of motion appears to contract significantly relative to an observer at rest. This means that space would appear compressed in the direction of travel.
Relativistic Doppler Effect: The light from stars and galaxies would be significantly blue-shifted (compressed to higher frequencies) in the direction of motion and red-shifted (stretched to lower frequencies) behind the spacecraft.
Cosmic Background Radiation: As the spacecraft approaches the speed of light, it would encounter an intense blue-shifted radiation, primarily in the microwave range, which is the cosmic background radiation present throughout the universe.
Visual Effects: The stars in the direction of travel would appear shifted toward the blue end of the spectrum due to the relativistic Doppler effect, while stars behind the spacecraft would appear red-shifted.
Brightening: The light from stars and other cosmic objects would become more intense and brighter due to relativistic effects.
Forward-focused Vision: Due to length contraction, objects in the direction of travel would appear compressed and more focused, while objects behind the spacecraft would appear elongated.
It is important to note that achieving such high speeds is currently far beyond our technological capabilities, and we have only begun to explore the possibilities of interstellar travel in theory and with unmanned missions. The challenges of relativistic speeds and the preservation of human health and well-being during such journeys remain significant hurdles to overcome.