We haven't visited a black hole – no one will ever return alive from the black hole interior. We haven't directly and/or clearly observed the Hawking radiation, the Unruh radiation, a singularity in general relativity, gravitational waves let alone gravitons, excited strings, additional compactified spatial dimensions, superpartners, quanta of the inflaton field, regions with a different value of the inflaton field, regions with differently compactified extra dimensions than our way, and many other things.This is a fair comment. There are atheist-evolutionists who get very upset at the notion that scientists have beliefs, but they clearly do. The beliefs preferably have some evidentiary support, but that is almost nonexistent in his examples.
Nevertheless, theoretical physicists generally assume that all those things – or at least most of them – exist. They do so most of the time. They say that they "believe" that those things are needed. Is the word "believe" another proof that their activity has evolved into a religion?
Not at all. The words "I believe X" simply means "my opinion is that X is probably right". Both religious and irreligious people have the right to "believe". The churches have no monopoly over the word. And religious and irreligious people may become convinced about something equally staunchly and feel the same psychological certainty about something. Where the churches differ from the scientists is in the methods to arrive to a "belief".
He skips some other common physicist beliefs: many-worlds, other forms of the multiverse, proton decay, holographic universe, quantum nonlocality, super-Turing quantum computing, and Barack Obama Hope and Change. Okay, I am just joking about the last one.
A person who is viscerally hostile towards physics may call the opinion that gravitational waves exist "religion". A physicist knows that the person from the previous sentence is an ignoramus. After all, a Nobel prize in physics has already been given primarily for the indirect detection of gravitational waves. ...Just about everything shows wave behavior at some scale, so I can believe in gravity waves.
OK, I obviously consider those who disagree with the existence of gravitational waves to be close to the "deniers of high school science" who also reject the existence of ice ages or heliocentrism. If you're one of them, please try to adjust your comments to the fact that I basically consider you a wild animal, a skunk of a sort. This is not meant to be insulting. It's meant to be as accurate an appraisal of the intellectual depth of two mammals as I am capable of producing.
I have beliefs against many of the above things. I am skeptical about superpartners (SUSY particles), proton decay, and quantum computing partially because these are farfetched and dubious theories, but mostly because experimental attempts have been such failures.
My skepticism in extra compactified dimensions and extra universes is a little different. These are ideas that are not testable and do not really explain anything either. I am not even what there is to believe in, because their ideas do not have much to do with observational science, as it has always been understood. I certainly do not believe in that nonsense, but I am not sure it makes any sense to disbelieve it either.
If you tell me that you have a spiritual connection with your grandmother, and then ask me my opinion, I would not have any opinion. I do not know how I would ever be able to come to a conclusion that your belief is true or false. It does not have any objective meaning that can be analyzed by the tools that I have.
Likewise, you are free to believe in other universes with different inflaton fields, if you wish. Motl is right that irreligious people have the right to have their own beliefs, even if there is no way to test or demonstrate them. I only raise objections when you claim to have some sort of scientific backing for them.