If there are 4 (or more) physical dimensions, and physical objects can move through the 4th dimension in paths perpendicular to our 3 dimensions, the physical objects must pass through our 3-dimensional universe every now and then. To an observer, it would appear that a physical object appeared out of nothing, grew, shrank, and disappeared. Mass would appear to be created and destroyed.

So why haven't we ever witnessed that? Does the lack of that kind of observation mean one of the following?

  1. There are only 3 physical dimensions.
  2. Physical objects can't move perpendicular to our 3 dimensions.
  3. There may be 4 dimensions, but physical objects only ever move parallel to our 3 dimensions, so they don't intersect our 3-dimensional space.
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    $\begingroup$ Have you ever seen a soap bubble burst? $\endgroup$ – WillO Mar 10 '15 at 17:17
  • $\begingroup$ 1) Time is a physical dimension (actually imaginary time is). 2&3) Physical objects can only move up to 45 degrees out of parallel with the 3 space dimensions (that's the speed of light). $\endgroup$ – Mike Dunlavey Mar 10 '15 at 20:17
  • $\begingroup$ If you want to understand this stuff, I highly recommend Sam Lilley, "Discovering Relativity for yourself". $\endgroup$ – Mike Dunlavey Mar 10 '15 at 20:32
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    $\begingroup$ From ths OP's comment to Leon's answer, it's clear that they are asking about a spatial 4th dimension, not time. $\endgroup$ – PM 2Ring Feb 6 '19 at 3:14

Good question. The main difference is that we cannot freely move in time or in other words, we and everything else are moving together. Other than that, I think nobody can say for sure yet if the answer is 1, 2, 3 or something else. The important thing to realize is that time as a 4th dimension is used to make models or theories of reality. Compare that to a drawing of a house on a piece of paper. That is a model of a house but it is clearly not an actual house.

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  • $\begingroup$ Thanks. I know that some TOE's require multiple dimensions, which led me to this question. It's probably based on the apple example in "Flatland" (which I haven't read for over 30 years). $\endgroup$ – Brett Juilly Mar 11 '15 at 17:00

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