Dear Lorraine,
When I say "people should be at the centre of the entire scientific activity", I don't mean that we should reject any physical law or hypothesis which makes humans appear insignificant. The universe has its laws, and while we appeared by virtue of these laws, Nature doesn't seem to care about us. Nature hits us with epidemics, catastrophes, aging and death. It just doesn't seem to care. What I said it is that WE should care. And science appeared as a means to convert Nature on our side. Not by denying its laws, but by curiosity, by loving and trying to understand Nature. For this, scientists propose hypotheses, with the purpose of explaining the laws of Nature. Either by curiosity, or to learn how to improve human's lives.
You asked: "how could people be at the centre of the entire scientific activity if we live in a multiverse where just as many universes exist where people are not the centre of the entire scientific activity?"
May I ask in turn "how could people be at the centre of the entire scientific activity if we live in a 13.7 billion year old universe where humans exist only for 100 000 years, and as far as we know, only on a tiny pale blue dot, out of an infinitely vast number of planets?"
I think we should consider any hypothesis that has explanatory power, no matter how speculative may appear. Do you think that we should choose only the hypotheses that make us feel good about ourselves? This would sabotage our efforts to understand Nature, and therefore to improve humans' lives.
What if Newton decided to burn his theory of mechanics, because in a world where mechanics determines everything, people will not have free-will, will not be accountable or rewardable for what they do?
Should we execute guys like the Church did with Giordano Bruno, who dared to claim that human's planet is not at the center of the universe, and that many other worlds may exist?
Should we refute Maxwell's equations, theory of relativity, and any deterministic theory, because we don't like our deeds to be predetermined by imutable laws? Should we refute quantum mechanics, because we don't like that its intrinsic randomness implies that our deeds are random? Should we refute evolution because it says that we are what we are because of random chance?
You say "Tegmark's multiverse implies that every good work that I personally do is countered by a universe in which I do just as many bad works: this view of reality IS dehumanising.". I don't even think that from the MUH follows that YOU in other worlds are bad. It may be someone with your name and many similarity with you which is bad in other worlds, but that is not you. You are what you are in this worlds. The other "you"s are not you, even if they may share the same DNA, because they have different histories, different choices. I can understand your frustration that all your good nature and efforts to do good works will only apply to this version of you, and are not guaranteed to also apply to other versions. So what? You are different from them.
But I find this more humanizing than you think. Because it makes us think that in different situation, we may have made some choices for which we now so easily blame or despise other people. This gives us the opportunity to realize that we shouldn't blame others for what they are, if we are not in their shoes. And I think this is the most humanizing thing.
Just like you are so proud of what you did in this world and are afraid that if the multiverse is true, some other "you" may do some bad things in that world, there may be others in this world who regret their doings, and hope that in other world they would do better. Or people who may find comfort that in other worlds a tragedy that happened to someone they loved didn't happen.
"How can physicists disclaim responsibility for the effects that their views have on people's attitudes towards reality, and on people's attitudes towards themselves and their good works?"
If you are so responsible, then how can you not think that the multiverse may be a chance of redemption or of a completely better life than this one, how can you disclaim responsibility for the effects of your view?
I have two boys, and the youngest one is severely autistic. It was not the choice of me and my wife to be so. If there is a cause, the cause is Nature, but I don't blame Her. I don't blame the laws of physics and evolution for doing this to him, and to many others. If God exists, should I blame Him for not being as good and merciful as advertised? If there is someone to blame, maybe it is more practical to blame the corrupt government, which doesn't care about these problems, and because of which I need to have two jobs just to be able to provide him therapy (which leads to very small progress anyway). If the multiverse theory will be proven to be true somehow (which I doubt), should I just institutionalize him and be comfortable that in a parallel world he is OK? I don't think so, I just have to do my best to take care of him in this world, and give him the best chance I can, no matter how the universe is, how God is, and how the government is. I see no moral implications at all of the multiverse in this issue, in any possible world my duty is to do my best. And I see no moral implications in other aspects of our lives. We just do our best, that's all. I leave the game of blame to others.
So far, all scientific ideas and technological advances were found offensive and dehumanizing by some. Yet, I think that you can see statistics showing that in overall human behavior improved in time. I doubt you have any study that shows that the multiverse ideas led to dehumanization and to fascism or other bad things. It is just a worry you have.
Best wishes,
Cristi