In simple terms, a wormhole is a shortcut through space and time. To understand this, spacetime (which is a mathematical model that combines space and time into a single continuum) can be imagined as a 2D surface that, when folded over allows a bridge between the two to be formed. The bridge, or wormhole, has at least two openings. These two mouths are connected by a tunnel, which is the shortcut between space and time. In theory, these wormholes have to be held open by exotic matter, which is matter that violates one or several energy conditions or is made of unknown particles. Exotic matter could have a negative mass or be repelled by gravity rather than attracted to it.
Theoretically a wormhole could be used for time-travel or travel from one universe to another. It is thought that a traveller would not be able to pass through the center of the wormhole although possibly be able to see lights from the other universe. The traveller would have to wait for the two white holes to merge and would most likely die from the collapse of the wormhole.
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Also, click here for a lovely diagram of spacetime as a folded 2D surface with a wormhole forming a connection between two locations in spacetime.
wow. that is an amazing thought. i think we should find something to use to let us travel through space to see other universes. i think that it would be amazing if we could go to the future or to a different galaxy.