What is space-time?
Answered by Science Channel
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    Science Channel

  1. While the idea of space-time is closely linked with Albert Einstein's 1905 theory of special relativity, mathematician Hermann Minkowski actually coined the term three years later in 1908 in response to Einstein's theory.

    Time is a measurement of change that takes place in what we call space. The series of changes that makes up your life happens over time and in space. The word "space-time" is our merging of the two concepts into a single continuum: three spatial dimensions plus a fourth dimension of time. While we have the ability to control our experience of the first three dimensions (height, width and depth), we do not seem to have the ability to navigate, manipulate or control our experience of time, even though physics tells us that it is merely a dimension like all the others. For human beings, time seems to be a one-way street with a pretty strict speed limit.

    Despite our inability to significantly manipulate our experience of time, we can observe the existence and unity of space-time by using experiments. If you've ever swung a bucket of water around in a circle, you know that with sufficient speed you can turn the entire bucket sideways without any of the water coming out. This is due to the equivalence principle, a key concept in Albert Einstein's theory of general relativity, which states that gravity working in one direction is equivalent to acceleration in the other. That's also why an ascending elevator provides a feeling of increased gravity and a feeling of decreased gravity during descent. The equivalence principle means gravity affects measurements of time and space, warping space-time itself.

    The concept of an object of great mass warping space is familiar to us -- a planet or a star warps the topography of an area of space, causing nearby objects to be pulled into the depression it creates. But scientists have also been able to observe with empirical data that objects of great mass can warp time as well. For example, if you synchronize two clocks and take one of them into space (away from Earth's center of gravity), they will lose their synchronization. This proves that time is part of the same continuum as space, and that space-time is a real and useful concept.

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