According to theoretical physicist Carlo Rovelli, time is an illusion: our naive perception of its flow doesn’t correspond to physical reality. Indeed, as Rovelli argues in The Order of Time, much more is illusory, including Isaac Newton’s picture of a universally ticking clock.

The concept of time is simply an illusion made up of human memories, everything that has ever been and ever will be is happening RIGHT NOW. The laws of physics are symmetric ultimately meaning that time could have easily moved in a backward direction as it does forward.

Physicists define time as the progression of events from the past to the present into the future. Basically, if a system is unchanging, it is timeless. Time can be considered to be the fourth dimension of reality, used to describe events in three-dimensional space.

“While is it mathematically feasible, it is not yet possible to build a space-time machine because we need materials–which we call exotic matter–to bend space-time in these impossible ways, but they have yet to be discovered.”

For outside observers, a black hole is one solidary element, and there is no proper time inside the black hole, but there is only the observed coordinate time according to our spacetime coordinates.

The Short Answer: Although humans can’t hop into a time machine and go back in time, we do know that clocks on airplanes and satellites travel at a different speed than those on Earth. However, when we think of the phrase “time travel,” we are usually thinking of traveling faster than 1 second per second.

In fact, according to Albert Einstein’s famous equation, E = mc² , time travel is possible, at least in one direction.

Einstein’s theory of special relativity says that time slows down or speeds up depending on how fast you move relative to something else. Approaching the speed of light, a person inside a spaceship would age much slower than his twin at home. Also, under Einstein’s theory of general relativity, gravity can bend time.

The short answer is this: A day is the length of time between two noons or sunsets. That’s 24 hours on Earth, 708.7 hours (29.53 Earth days) on the Moon.

In the early 21st century, crew on the ISS were said to average about six hours of sleep per day.

Space is a vacuum and humans cannot exist in a vacuum. This means that we have to create crafts and suits which provide an environment where humans can survive. micrometeorites – danger from impact damage (to spacecraft and to astronauts during spacewalks) solar flares and radiation – danger from ionising radiations.