The speed of light is so rapid that it would take months to get there safely! When we change speeds or directions quickly, we experience higher or lower g forces. Normal people can only sustain 9 g's for a few seconds at a time. A person could die from g-force trauma even if they were in space alone.
At the moment, astronauts inside the International Space Station (ISS) can survive for several hours with their blood pressure kept constant by drugs and fluids, but they would eventually suffer damage to their organs due to prolonged exposure to such high levels of stress. However, it may be possible in the future to protect ourselves from the effects of high g-forces by using equipment such as anti-g suits or inertial confinement devices. Humans have already survived much greater levels of stress than this, for example during space walks when astronauts are exposed to up to 40 g's for an hour at a time.
It is currently impossible for humans to survive at absolute zero temperature. The lack of energy required to move atoms around at such low temperatures would actually cause them to collapse into a dense mass at which point they would no longer be able to react with other substances.
Humans cannot survive in deep water for more than a few minutes because of the pressure difference between the surface of the water and its interior.
It would take 353,7 days of steady 1G (9,81 m/s2) acceleration to reach the speed of light. You'd travel 4,58 billion kilometers in that period. However, the human body can withstand greater than 1G. I'm not sure what the limit is or how long it will last. Probably no more than a few seconds.
The closest any human has ever come to reaching light speed was on October 5, 1959, when American astronaut Alan B. Shepard Jr. became the first person to go into space. He was traveling at 3 miles (4.8 km) per second when he launched into orbit around Earth on a Soviet spacecraft named "Korolev".
Shepard returned to Earth eight hours and 49 minutes later. The average speed over that time was about 24,000 miles per hour (39,300 km/hr). This rate of speed exceeds that of sound (which is approximately 740 mph or 1,200 km/hr), but not by much! Astronauts experience very strong G forces during takeoff and landing which can cause them to be injured or even killed. For this reason, they are not allowed to go faster than sound or else they might get blown away from their vehicles.
So, although it's possible for humans to reach speeds close to light speed, there are many factors that could prevent this from happening. Also, light speed isn't really fast enough to do anything interesting!
"Other than the speed of light, there is no actual practical limit to how fast humans may move," adds Bray. Light moves at a speed of one billion kilometers per hour. As a result, people should be able to move at a velocity that falls just short of the "universe's speed limit": the speed of light.
The theoretical limit to human movement comes from Einstein's theory of relativity. According to this theory, mass and energy are related to each other; you can't increase one without decreasing the other. For example, if you try to accelerate a car beyond the speed of sound, it will disintegrate into flames before it reaches high speeds. In order for humans to travel at or near the speed of light, we would have to build machines that have equal or less mass than ordinary matter, such as energy particles.
In reality, humans are limited by several factors. The most important of these is gravity. If you were to try to escape Earth's gravitational pull, you would need to be moving faster than about 36,000 km/hour (22,500 miles/hour) in order to stay in orbit around the planet. This is more than enough to cover the distance between Earth and the moon in a single second! But even this speed is not high enough to avoid crashing into Earth again and again.
The reason for this is simple: Energy equals mass times the speed of light squared.
Light travels at approximately one billion kilometers per hour. That's about 300,000 miles per second, or about 186,000 kilometers per second.
However, even if we were able to travel at half the speed of light, it would take us only about 20 years to reach the nearest star, Alpha Centauri. Even with fuel economy improvements, such a trip would use up most of our available energy supply in less than 10 years. And the journey back home might as well be made forever because we'd be dead by then anyway.
The closest stellar system outside of our galaxy is the Andromeda Galaxy, which lies 2 million light-years away from Earth. If humanity ever wants to explore beyond our solar system, we'll need to develop faster-than-light travel technology.