The apparent progression of events from the past to the future is known as time. While it's impossible to fully explain the essence of time, we all share a number of common time-related experiences: Causes naturally lead to effects, we recall the past but not the future, and time appears to evolve in a continuous and irreversible manner. Is Time Relative? The experience of the flow of time is relative to the observer and their situation, according to Einstein's special relativity theory. Isaac Newton's work had previously assumed the presence of a "master clock" that maintained time synchronised throughout the universe. Although this clock was not supposed to exist, the concept permitted Newton's equations to function. According to the Internet Encyclopedia of Philosophy, the central premise was that all observers might agree on the exact same moment in time (opens in new tab). However, based on previous research, Einstein determined that time is relative. Moving clocks run slowly in special relativity; the quicker you move in space, the slower you advance through time. The bigger this effect increases as you approach closer to the speed of light. Physicists have done several measurements that show this effect in the decades after Einstein first postulated this hypothesis. A jet airplane's atomic clock will tick at a slower rate than one on the ground. A muon is a subatomic particle that can't move from the atmosphere to the earth since it's created when cosmic rays collide with air molecules. Muons, on the other hand, appear to exist longer from our perspective because they travel near to the speed of light, allowing them to finish their journey. When Albert Einstein established his theory of general relativity, he extended the concept of "time dilation" to gravity-related conditions. According to physicist Christopher S. Baird, the presence of strong gravity slows the passage of time, hence a clock in a strong gravitational well (for example, on the surface of Earth or near a black hole) will tick at a slower rate than a clock in the middle of space. Is it possible to travel through time? Time travel into the future is not only allowed, but required. Indeed, we are all moving forward into our own futures with each passing second. The future is unavoidable, and there is no way to avoid it. However, due to the reality of relativity, "jumping" forward in time is completely allowed. When a twin takes out on a rocket ship and travels near the speed of light for a few years, they will have aged less than their Earthbound counterpart when they return. According to Cosmos magazine, although only a few years may have gone on the spaceship, decades or even centuries may have passed on Earth, depending on how swiftly the rocket flew. According to report, NASA astronaut Scott Kelly has spent a few milliseconds less time in space than his twin Mark (who is also six minutes younger). This is due to spending more time in space and travelling at speeds of roughly 17,500 mph (28,100 km/h). However, time travel into the past appears to be forbidden - at least in all previous tests and observations. For starters, the prospect creates a slew of troubling questions, such as the famous grandfather paradox, which wonders what would happen if you went back in time and murdered your own grandfather: You wouldn't exist, thus you wouldn't be able to conduct the act. Second, there is no known physics mechanism that allows one to travel backward in time. While general relativity allows for the creation of some time-traveling scenarios, those scenarios necessitate entities that do not appear to exist in our reality (like matter with negative mass, or infinitely long cylinders). However, physicists still don't fully comprehend why time travel into the past is prohibited. Is it possible to go back in time? Physicists employ almost all rules and equations to understand the natural world that are symmetrical in time. That is, they can be reversed without affecting the outcome. For example, without any additional background, you wouldn't be able to discern if a video of a ball rising into the air and falling again was being played forward or backward. However, one part of physics appears to follow the passage of time: the idea of entropy, which is a measure of a system's disorder. Entropy always rises in a closed system, according to the second law of thermodynamics, and this evolution cannot be reversed. According to the Stanford Encyclopedia of Philosophy, physicists aren't sure if the growth of entropy causes the "arrow" of time or if it's merely a coincidence. Is Time a Continuous or a Discrete? Time is treated as a continuous in almost all physical theories, which is how humans sense the passage of time. There is no such thing as a smallest "unit" of time. All of the activities flow seamlessly into one another, with no interruptions or hitches. One quantum gravity theory, known as loop quantum gravity, proposes the existence of the lowest feasible unit of space-time. This unit would represent the tiniest extension of space and time possible. What we think of as smooth, continuous time is actually a stuttering, stop-motion passage from past to future, according to this idea. According to physicist Carlo Rovelli's 1998 essay in the journal Living Reviews of Relativity, because this happens for such a short time, it looks to be continuous, like the frames of a movie blending together. Is Time Real? The nature of time has been debated by scientists, philosophers, and others. And, despite what humans have discovered about time, such as the existence of time dilation and the potential link between time and entropy, still haven't been able to come up with a comprehensive definition of what time is. Some philosophers and physicists suggest that what we think of as time is only an illusion, a product of our awareness. The passage of time isn't real in this view; the past and future already exist in their fullness, just as the entirety of space already exists. According to physicist Sean Carroll, what we perceive as time is a result of the way our brains interpret sensory information from our surroundings.