> Wait a second there. There are different attitudes about falsifiability?

Yes.

My understanding is that Karl Popper first proposed falsifiability as a criterion for a scientific theory in part because he found that some supposedly scientific ideas (Freudian psychoanalysis was a particular target of his) failed to make predictions that could be definitively tested: it was a feature of these 'theories' that they could be interpreted in such a way as to accommodate any conceivable observation.

I think you are right that there is a strong consensus among scientists that for a theory to be considered scientific, it must make predictions which can be tested empirically, so that those predictions can be either verified or falsified. The difficulty some scientists have with a strict attitude towards falsifiability though is that many theories that make falsifiable predictions in regimes where the theory can be tested, also make consequential predictions about phenomena that are not readily observable, and might not even be observable in principle.

For instance, observations suggest that the universe is expanding at an accelerating rate; that the pace of expansion increases with distance; and that the expansion is unconstrained by the speed of light. This means that there are vast numbers of galaxies which were once receding from the Earth slowly enough that signals from them (e.g. light that can be imaged by telescopes) could be detected on Earth, but which are now receding from Earth faster than light. Without instruments capable of probing the universe with signals that can exceed the speed of light (something that is currently thought to be impossible), the future of these distant galaxies that have crossed the 'particle horizon' [1] will be forever unknowable to observers on Earth. Predictions about the future evolution, or even existence, of those galaxies are therefore not strictly falsifiable, because it would be impossible to make the necessary observations. Despite this, astronomers have no reason to believe that those galaxies cease to exist, or that the known laws of physics cease to apply to them, once they can no longer be observed by scientific instruments on Earth.

In other words, apparently credible scientific predictions could be made about a large and ever-increasing volume of the universe, but those predictions could never be tested observationally: they would be unfalsifiable. Currently accepted models suggest that, in the distant future, the entire universe except our local group of galaxies will recede into this unobservable domain, and will therefore be inaccessible to any astronomers around at the time.

String theory similarly makes predictions that might never in principle be testable (and thus falsifiable) because doing so is thought to require experiments that would necessarily collapse into black holes were they ever to be attempted.

Proponents of the Many Worlds interpretation of Quantum Mechanics arguably have a similar problem in that it is typically thought to be impossible to interact with the hypothesised other worlds in a way that could confirm or refute their existence. The cosmologist Sean Carroll (who is a proponent of the Many Worlds interpretation) has written a paper on this subject titled 'Beyond Falsifiability' [2], which is discussed on his website [3].

The physicist David Deutsch, who has played an important role in establishing the theoretical basis of quantum computation, has argued that a quantum computer could effectively demonstrate the existence of the other worlds of the Many Worlds interpretation [4], so falsifiability is perhaps less of a problem from that perspective.

[1] https://en.wikipedia.org/wiki/Particle_horizon

[2] https://arxiv.org/abs/1801.05016

[3] https://www.preposterousuniverse.com/blog/2018/01/17/beyond-...

[4] https://thereader.mitpress.mit.edu/the-many-worlds-theory/