[pdonis]

> Obviously the thought experiment requires energy that doesn't 'exist'

No, it requires energy to be added to the system from outside the system. Which is precisely what you cannot do with the universe as a whole. That's what makes such thought experiments meaningless for the universe as a whole.

[thx-2718]

> it requires energy to be added to the system from outside the system.

Yes that is exactly what energy that doesn't 'exist' means.

[pdonis]

But you can't do that for the universe as a whole. Asking what would be the case if you could is meaningless; it's like asking what would be the case if 2 + 2 were 5. No consistent model exists of such a situation, so the question is meaningless.

[thx-2718]

> you can't do that for the universe as a whole.

Yes correct, that's why that energy doesn't 'exist' because it's outside of the universe.

[pdonis]

There is no such thing as "outside of the universe".

If you disagree, please show me the consistent model on which your scenario is based.

[thx-2718]

> There is no such thing as "outside of the universe".

Yes that's correct in the general accepted sense for the term 'universe'.

Edit: question if the initial thought experiment was posed as 'observable universe' would it make a difference to you?

[pdonis]

> if the initial thought experiment was posed as 'observable universe' would it make a difference to you?

It would address my "can't operate on it from the outside" objection, yes. But it still wouldn't make the thought experiment meaningful, for the reasons I gave in response to wyager elsewhere in the thread.

[faangsticle]

Now take off your nitpick hat and ignore that detail. Its what makes it a thought experiment.

[pdonis]

You can't ignore it because it's not a "detail"--it's a crucial feature of the thought experiment that doesn't work for the universe as a whole. What you're suggesting is like saying, in my thought experiment I assume that 2 + 2 = 5, just ignore the fact that 2 + 2 is actually 4.

[faangsticle]

Yes, that's what a thought experiment is. A cat can't be alive and death at the same time, either.

[pdonis]

> that's what a thought experiment is.

No. A thought experiment cannot be based on a contradiction.

> A cat can't be alive and death at the same time, either.

And the Schrodinger's Cat thought experiment does not claim that it is, even though many pop science discussions try to claim otherwise. The Schrodinger's Cat thought experiment is based on the math of QM, i.e., on a consistent underlying model. It simply points out consequences of that model that might not be obvious to many people.

The "thought experiment" I have been objecting to in this thread, by contrast, is not based on any consistent mathematical model. The operation it is proposing to do on the universe as a whole is inconsistent with GR, which is the only consistent model we have of the universe as a whole. That means it's not a valid thought experiment. "Thought experiment" does not mean you can make up whatever you want.

[faangsticle]

> "Thought experiment" does not mean you can make up whatever you want.

The only thing we can agree on is that "thought agreement" means that you can't make up whatever you want :-)

[pdonis]

"Thought experiments" that allow you to make up whatever you want are pointless, because you can also make up whatever answer you want. So the "thought experiment" tells you nothing.

As actual physicists actually use thought experiments, they cannot make up whatever they want. Thought experiments involve taking a known consistent model and working out consequences of it that were not previously obvious or well known, and seeing where that leads. You can only do that usefully if you constrain the thought experiment by the model, i.e., if you do not allow yourself to make up whatever you want.

Since the discussion here is about an article on physics, it seems appropriate to treat proposed thought experiments the way actual physicists would treat them. That is what I have been doing.