Dictionary

Wormhole

Einstein’s general theory of relativity can be used to construct model universes in which shortcuts in space exist. For certain regions of space one has two possibilities to travel from one to the other – one conventionally through space, the other by entering a spherical region, a “mouth” of the wormhole, to come out near the target in a similar region, the other mouth – without having to travel a long distance on the way from one mouth to the other. If the two wormhole mouths could be set in motion relative to each other, it would even be possible to create a time shift: anyone who dives into one mouth of the wormhole and comes out at the other end would have travelled a little into the future or the past.

According to all we know, wormholes are certainly very unrealistic and speculative model universes. In order to stabilize a wormhole to such an extent that it would be traversable, one would need matter with highly exotic properties – it is quite possible that for this reason alone, there are no wormhole shortcuts in the real universes.

Definitioner

time

It is a fact of life that not all events in our universe happen concurrently - instead, there is a certain order. Defining a time coordinate or defining time, the way physicists do it, is to define a prescription to associate with each event a number so as to reflect that order - if event B happens after event A, then the number associated with B should be larger than that associated with A. The first step of this definition is to construct a clock: Choose a simple process that repeats regularly. (What is "regular"? Luckily, in our universe, all elementary processes such as a swinging pendulum, the oscillations of atoms or of electronic circuits lead to the same concept of regularity.) As a second step, install a counter: A mechanism that, with every repetition of the chosen process, raises the count by one. With this definition, one can at least assign a time (the numerical value of the counter) to events happening at location of the clock. For events at different locations, an additional definition is necessary: One needs to define simultaneity. After all, the statement that some far-away event A happens at 12 o'clock is the same as saying that event A and "our clock counter shows 12:00:00" are simultaneous. The how and why of defining simultaneity - a centre-piece of Einstein's special theory of relativity - are described in the spotlight topic Defining "now". With all these preparations, physicists can, in principle, assign a time coordinate value ("a time") to any possible events, and describes how fast or how slow processes happen, compared to that time coordinate.

space

In a strict sense: Space as we know it from everyday life: the totality of all locations in which objects can sit, with three dimensions. In a more general sense used by mathematicians, all kinds of sets of points are spaces - a line for instance, which has but a single dimension, or a two-dimensional surface, but also higher-dimensional spaces. Also, in such more general spaces, geometry can be different from the standard Euclidean geometry taught in high schools - such spaces can be curved.

matter

In general relativity: All contents of spacetime that contribute to its curvature: particles, dust, gases, fluids, electromagnetic and other fields. In particle physics: All elementary particles with half-integer spin, such as electrons and quarks, as well as their composites such as protons and neutrons, in contrast with force particles.

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Wormhole

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