Extra dimensions

In physics, extra dimensions are proposed additional space or time dimensions beyond the (3 + 1) typical of observed spacetime, such as the first attempts based on the Kaluza–Klein theory. Among theories proposing extra dimensions are:^[1]

• Planck scale.^[2]
• Warped extra dimensions, such as those proposed by the
  brane or branes.^[3]
• Universal extra dimension
  , proposed and first studied in 2000, assume, at variance with the ADD and RS approaches, that all fields propagate universally in extra dimensions.
• Multiple time dimensions, i.e. the possibility that there might be more than one dimension of time, has occasionally been discussed in physics and philosophy, although those models have to deal with the problem of causality.

References