[coldtea]

>If alternate realities exist, and we can get to them from this reality, then they are no longer an alternate reality.

All that means is that they are somehow connected.

They are still 2 different environments, which different rules -- which is what people actually mean when they say "alternate reality", and not "complete isolated reality space".

So the counter-proof is more like saying "If there's Canada, and we get to there from here, the they are no longer Canada".

[chongli]

Canada has a well-defined border which you can point to not only on maps but on physical landmarks as well.

What are the borders of reality?

[coldtea]

It the hypothetical example described it could be anything -- in fact it could be exactly like the "borders of Canada", some stretch of land with guards and fences. Or it could be some "portal", a mirror you can go through, a black hole, whatever.

That place would just have to comply with a different set of rules compared to anywhere else to be justified to call it an "alternate reality" (e.g. no gravity, fire is cold, etc).

Heck, we even call experiences like living with the Amish an "alternative reality", and those are on our very same universe/world, and with all the known laws of physics and basics intact.

[astrobe_]

If you are levitating, then you know that you're dreaming. That's a border.