Involuntary
Time is often described as a 4th dimension with the others being length, width and height. Unlike the other dimensions, moving through time appears to be involuntary. In theory, you can stop at a point in space and stand there for a while although the planet Earth and Milk Way galaxy are actually moving. It does not appear to be possible to do the same with time as you move through it without an ability to stop.Irreversible
Time occurs as an irreversible sequence of events such that once something happens it can't unhappen. There is a relationship between events known as cause and effect whereby the past influences the future but the future can't influence the past.Required
Time appears to be a requirement for anything to happen. In other words, nothing occurs in an instant without the progression of time.Measurable
Time can be measured as regular intervals known as seconds. This is a fundamental measurement that is important to practically every human pursuit including science, art, business and daily life.Absolute Time
Absolute time is the idea that time is constant across the entire universe for all observers. This is a basis for the system of physics proposed by Sir Isaac Newton known as Newtonian physics.Time Dilation
Relative time is the idea proposed by Albert Einstein's theory of special relativity that suggests that time slows down if you are moving fast or if you are in a gravitational field. This has interesting implications as it provides a basis for forward time travel. For example, cosmonaut Sergei Krikalev is believed to have gained 22.68 milliseconds of time by spending a total of 803 days, 9 hours, and 38 minutes on the international space station because it is moving at about 7.7 kilometers a second or 17224 mph. Gravitational fields can also slow time. It has been calculated that the core of the Earth is 2.5 years younger than the crust due to stronger gravity at the core.Subjective Time
People commonly report the sensation that time feels slow or fast depending on the situation. This is commonly viewed as variations in human perception as opposed to an actual ability to slow time itself. However, philosophers such as Immanuel Kant have proposed that time is a human concept that is necessary for a rational mind to organize events and that the universe doesn't necessarily have any such constraints.Arrow of Time
The arrow of time is the theory that time moves in a single direction from past to future. This can be demonstrated by watching a movie backwards such that the laws of physics don't make any sense in reverse. For example, if a character jumps off a cliff in a movie it makes sense run from past to future. In reverse, the character appears to defy the laws of gravity.Entropy
Entropy is the amount of disorder in a system. According to the Second Law of Thermodynamics, the entropy of an isolated system can only increase with time. This explains aging that appears to effect all things over time including the universe itself.| Overview: Time | ||
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Definition | The progression of existence and events from past to future. | |
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