Is spacetime physical or just a tool to describe change?

[tom.stoer]

You can seriously see the wavefunction - the absolute square of it.

Sorry to say that, but that's a huge conceptual difference.

Lets take the graviton instead. To distinguish a graviton signal from a neutrino signal would require shielding that would collapse into a black hole. Therefore the graviton is not observable in principle. http://arxiv.org/abs/gr-qc/0601043

I am sorry but as long as there is no reliable theory of quantum gravity which explains what the "mathematical entity graviton" means we cannot safely say whether a "graviton as physical entity" does exist. In addition it may very well be that the final theory of quantum gravity does not even contain a graviton as fundamental d.o.f., therefore it could be that it does neither exist mathematically nor physically and that there is no such problem at all.

How many things are simply not observable because their detection or creation has unphysical requirements or even physically impossible requirements?

A lot - but why should we care? OK, you may complain about the fact that a specific theory (which one?) contains too many unphysical entities and that (according to Ockhams razor) it should be replaced by something which is less wasteful ontologically. Of course you are invited to develop such a theory (what do you have in mind?).

 

[nitsuj]

chill_factor - What about a photon moving at c, it's not observable between events. What do you make of this stuff that separates events?

 

[Drakkith]

I believe it does. I'd say science is not a subset of philosophy that deals with empirical things; it is a subset of engineering that deals with fundamental limits instead of practical limits.

I think you should review what engineering and science are. Here are a couple of links.
http://en.wikipedia.org/wiki/Engineering
http://en.wikipedia.org/wiki/Science

As such, the physicality (observability) of the systems we deal with is important. At the end of the day science is not philosophy because science is about data.

Here's a link to philosophy. And on the philosophy of science.
http://en.wikipedia.org/wiki/Philosophy
http://en.wikipedia.org/wiki/Philosophy_of_science

Just read some arXiv high energy, cosmology, etc. papers and tell me with a straight face that if those disappeared tomorrow, the world would be a worse place.

It would be, for we would not know what paths have already been taken.