What is the SI unit of spacetime
Always forward, never backward What is time
What is time actually? Ironically, the founder of the theory of relativity, Albert Einstein (1879-1955), described the physical quantity of time with an amazingly simple sentence that could also come from an elementary school child: "Time is what you read on the clock."
The English natural scientist Isaac Newton (1643-1727), who at the beginning of the 18th century coined the concept of "absolute time", later refuted by the theory of relativity, could probably have lived well with Einstein's theorem.
Time and space cannot be separated
Yet Einstein's and Newton's definitions of time are two completely different things. While Newton claimed that time is absolute and runs the same everywhere in the universe and without any reference to an external observer, time is inseparable from space according to the relativity theory founded by Einstein. Since the Big Bang, the following has been true: without space, no time and without time, no space.
Instead, both terms form the four-dimensional space-time, also called the space-time continuum. Time is one of the four dimensions, explains nuclear physicist Steffen Turkat from the TU Dresden: "Time is exactly like space, which consists of X, Y and Z components. And time is a fourth component of so-called space-time. "
Gravitation bends space
In other words, the height, width and depth of the room form an inseparable unit with the dimension of time. From this, in turn, it follows that time - contrary to what Newton claims - can never be absolute. Instead, time is just as malleable as space. The gravity of the heavenly bodies bends space and the speed compresses and expands it. And time plays along with this game.
This in turn means that time runs differently in every place and for everyone. "Wherever someone is, they can define their own time. Time is something local," explains nuclear physicist Turkat. At the same time, however, the Dresden scientist also emphasizes: "Our arrow of time is moving forward. For all particles and antiparticles."
Time arrow always runs forward
Time also describes the sequence of events, so it has a clear, irreversible direction. But how does that relate to the established individuality of time? Even seasoned experts do not find it easy: "It is very difficult to speak of a continuous progression of time when this concept of time is in itself a very relative concept that has to be defined separately for each thing, for each particle," admits the director of the Max Planck Institute for Gravitational Physics in Potsdam, Prof. Hermann Nicolai.
Not noticeable but measurable
And yet both go together. On earth, however, we do not notice anything of the influence of space on time. The snail's pace of our rockets and jet planes does not noticeably affect time. Even for the astronauts who were on the moon, the clocks ticked like they did on earth. And yet the effects of gravity and speed on time are measurable. The lower the force of gravity, the faster time goes by. Even the 33 centimeters of a step make a difference. The resident on the top floor of a skyscraper ages 90 millionths of a second faster than those on the ground floor in 80 years.
No navigation devices without room components
But because such periods of time do not play a role in our tiny earthly reality, time appears to us the same everywhere - although it is not. Only this knowledge makes navigation devices possible. They only work because they take into account the time differences caused by the components altitude, speed and distance, explains astrophysicist Harald Lesch: "So every time this voice in the car tells you: 'You have reached your destination', then you have too the general and special relativity theory achieved their goal. "
Extreme speeds and forces
The changes in space and time only become huge at extreme speeds and gravitational forces, as the example of the speed of light shows. "There are particles for which time does not move at all. Light particles, i.e. photons, move at the speed of light. For them, time stands still," explains gravitational physicist Nicolai.
So this would mean: If you were to travel at the speed of light for a short time, you could visit your great-grandson in the old people's home after your return. Such mind games are just as much theory as antimatter, which runs backwards in time, or time loops, which exist in the horizon of the black hole.
Time as a qualitative dimension
For astrophysicist Lesch, one thing is certain: In a world in which the arrow of time does not always point towards the future, we humans could not even exist: "There is an incredible difference between space and time. We can go to the same place twice, but never the same time. That means: time is a qualitative dimension and space is a quantitative dimension. "
Einstein's simple sentence does not challenge this: "Time is what you read on the clock." In fact, a perfectly set clock would show the exact time that applies to that one place. The vibrations of the atoms, which serve as the basis of our modern time measurement, reflect the influence of gravity and speed on time. We too live in space-time, even if we do not notice it.
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