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In this paper written by physicist Max Tegmark (https://arxiv.org/pdf/0704.0646.pdf) it talks about "External Reality Hypothesis". Specifically, he says:

Although many physicists subscribe to the ERH and dedicate their careers to the search for a deeper understanding of this assumed external reality, the ERH is not universally accepted

and

We will see that, although it sounds innocuous, the ERH has sweeping implications for physics if taken seriously. Physics theories aim to describe how this assumed external reality works. So it seems that there are theories in physics which accept the existence of this so-called "External Reality".

Apparently in the paper no example of these types of physical theories appear. So, do you know of any theory which assumes this? What theories is Tegmark talking about? Do you know of any physics theory that would assume a Tegmark's External Reality where his Mathematical Universe Hypothesis and universes described by different matematical structures could be real?

minnafotter
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    The hypotheses of External Reality Existence ? Thus, e.g. the nuclear bomb on Hiroshima was only an "hypothesis" ? – Mauro ALLEGRANZA May 19 '19 at 17:16
  • This seems to be a rehash of the [realism vs. idealism debate](https://philosophy.stackexchange.com/q/9940/2014), where "External Reality" is another word for "[mind-independent (or physical) being](https://books.google.com/books?id=zAsjkHJ8aP8C&pg=PA935#v=onepage&q=%22mind-independent%20or%20physical%22&f=true)" (vs. ["mind-dependent being" or _ens rationis_](https://books.google.com/books?id=zAsjkHJ8aP8C&pg=PA934#v=onepage&q=%22mind-dependent%20being%22&f=false)). – Geremia May 20 '19 at 18:54

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Afaik the theories of physics have no need to decide ERH. It is a metaphysical hypothesis of no concern in physics.

I'm unable to think of a scientific theory that depends on ERH being true or false. This may be why no examples are given in the paper. ERH is not testable in physics so a theory dependent on it would not normally be considered scientific.

Right now I find it difficult to see why ERH matters in physics. Nobody is ever going to falsify solipsism so a scientific theory cannot depend on this hypothesis but must be independent of it.

Or so it seems to me.

  • but Tegmark says that there are physics theories that assume an ER. This suggest that there are theories in physics that are compatible with ERH @PeterJ – minnafotter May 19 '19 at 12:12
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    @minnafotter Compatible yes. But a scientific theory cannot depend on an assumption that is untestable so must be agnostic on the reality of the external world. Clearly it is real in some sense but to ask in what sense is to wander into metaphysics. –  May 21 '19 at 08:58
  • @minnafotter Does Tegmark also say that those theories that are compatible with ERH are NOT compatible with idealism? or, otherwise, are compatible with both, and how to know that? Because if there are such theories that are compatible with one but not the other, then hoooray ! we can say goodbye to many metaphysical questions. – SmootQ Jun 18 '19 at 22:19
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    @SmootQ - Well put. –  Jun 19 '19 at 10:23