Can spacetime have an outside? I sometimes read stuff where it is mentioned. Is it true?
Please provide a specific reference. "Stuff where it is mentioned" is not enough to go on.
You may wish to read about embedding theorems and pseudo-Riemannian geometry.
Where did you read such "stuff"?
What physical experiment would you do to measure an "outside" of space-time? Do any of the things you "sometimes read" describe such an experiment that could be carried out to answer the question - i.e. an experimental result that gives one answer if there is an 'outside' and a different result if there is not? Lastly, are such discussions that you "sometimes read" in the context of General Relativity, or in some other theory (for example, some sort of Brane theory).
In the context of General Relativity, there's no reason to assume that there is anything "outside" space-time, in fact the geometry is carefully set up to be an intrinsic geometry. Many popularizations use an embedding diagram to illustrate the concept of curvature, but this is more or less a visual aid, it's not part of the theory itself.
I don't necessarily have a good answer outside the context of GR - the question though is rather broad, if you think you have some specific theory that requires an "outside of space-time", it would probably be a good idea to identify that particular theory more closely, and then find the appropriate forum to talk about that question. It's always worth asking the question if you're thinking about something that is testable by physical experiment - or if you are asking about something that can never be tested. Sometimes the tests may not be obvious, but if you realize you're asking about something that can never be experimentally decided, you're asking yourself a philosophical question and not a scientific one.
I forgot.. but maybe something about entanglement using traffic outside spacetime..
With that this thread is closed and will remain closed.
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