Does Feynman's Multiple Histories mean what I think it does?

Discussion in 'Physics & Math' started by Tailspin, Dec 16, 2023.

  1. Tailspin Registered Senior Member

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    I was Just reading The Universe in a Nutshell for the first time.
    In it, Steven Hawking says that Feynman's Multiple Histories certifies that theory that everything that can exist, does or will exist. That out there in the cosmos there is an exact replica of Erath, you, me ect...

    However, when I entered Feynman's Multiple Histories into a search engine the results don't seem to match what Hawking said in his book.

    So who is right and what does Multiple Histories mean?
     
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  3. Seattle Valued Senior Member

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    I think that Hawking and Feynman understood the same thing by Feynman's use of Multiple Histories. I think Hawking also extended that in his belief system to include the interpretation you first mentioned. There is no proof for that interpretation.
     
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  5. geordief Valued Senior Member

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    For an exact replica of "Earth, you, me etc", those relicas would, imo have to have the exact same pasts as the originals.

    In other words each replica would subtend an entire,evolving replica universe

    If that was the case it would be a meaningless assumption and the possibility of it happening/having happened can be dismissed as of no interest .

    Any body that can be said to exist(Earth, you, me etc) does not exist as static definable separate entitiy but as part of the changing ,dynamic environment it is part of.
     
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  7. Tailspin Registered Senior Member

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    Recently I started reading The Universe in a Nutshell by Steven Hawking and I came across his chapter on Feynman's Multiple Histories. Hawking said it confirmed the theory that, somewhere out in the cosmos, there's exact duplicate of Earth, you and me doing what we're doing right now, purely by coincidence.

    I looked up Feynman's Multiple Histories online to see if that's what it meant. Not only did I find that's not what it means but I found a conversation on Physics Stack where someone had voiced a similar issue. The other person said that Universe in a Nutshell was a bad book and they should stop reading it at once. A few other said Hawking was arrogant and his writings had no real value.

    I was shocked by all of this. Ever since I first heard about him back in the 90s, he's been portrayed as this icon of genius, the greatest mind of a generation. When TV, movies and videogames want to create a genius character, they create him in Hawking's image. And I know that's not a scientific way to rate intellect but it wouldn't be this way if most people didn't think he was a genius and I can't help but think they thought that because he is. Apert from some references in Futurama accusing him of plagiarism, I've never heard a bad word against him until now.

    So I want to ask you, was Steven Hawking a good scientist? Is his work considered fact or theory? Is it done scientifically? Should his book be considered to be full of immutable proven facts or current models and theories? Because I can't tell if someone says everything like it's true. And should I keep reading The Universe in a Nutshell? Because I'm only half way through.
     
  8. James R Just this guy, you know? Staff Member

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    Tailspin:

    If that's what he said, then all I can say is that I personally disagree with him that Feynman's "multiple histories" implies any such thing.

    Essentially, the "multiple histories" idea uses a much older idea from classical physics, involving a mathematical quantity called the "action". For example, suppose we want to work out the trajectory that a thrown ball will take through the air. Newton's way to do that would be to work out the various forces acting on the ball and then to compute the acceleration, velocity and, finally, the position of the ball as functions of time. But there are other ways to solve the problem. The one involving the action says that we consider all possibly trajectories that the ball might take to get from point A to point B. We calculate, for each trajectory, a certain mathematical quantity. Then, it turns out that the trajectory with the smallest value of that quantity (the action) will be the trajectory that the ball actually follows.

    Feynman extended this idea from classical to quantum mechanics.

    One interpretation of quantum mechanics suggests that, perhaps, all of the possible trajectories actually exist, only in different universes. But this is just one among several possible interpretations of what the maths is saying (and not, I think, one that Feynman himself was fond of).

    I don't know whether Hawking subscribed to the "many worlds" interpretation of quantum mechanics. It is possible that, in the book you read, he was merely describing the idea, rather than endorsing it. But I haven't read the book, so I can't be sure.
    It is important to distinguish between what appears in peer-reviewed scientific papers and popular science descriptions of scientific ideas. Scientific papers in Physics tend to be full of mathematical proofs and justifications. But popular science books are written with the layperson in mind. A tongue-in-cheek rule of thumb for popular science writing is to remember that every equation you include in the book will probably halve its sales, since most of the target audience will lack the mathematical training necessary to understand anything beyond high-school mathematics.

    With this in mind, I will try to answer your questions.

    Was Hawking a good scientist? Yes. He published many professional scientific papers. His work is highly regarded among his scientific peers.

    Is his work considered fact or theory? Answer: both. Every valuable statement in science is "theoretical". Scientific claims are all provisional, and are only accepted by the scientific community to the extent that they are (a) plausible; (b) consistent with everything else we know and (c) supported by appropriate evidence.

    There are many unproven hypotheses in science. Some of Hawking's hypotheses ("theories") are unproven, in that they have not yet been supported by observational or other evidence. However, a hypothesis is not disproven until it is. At the frontiers of science, it is quite normal - indeed desirable - to keep multiple competing hypotheses in mind when planning experiments and observations.

    Some of Hawking's work is well verified. Other claims of his are more speculative. This is normal in physics (and, more generally, in science).

    Should Hawking's pop-science books be considered to be full of immutable proven facts? No. Some of the content is well supported by evidence and widely accepted in the scientific community. Other parts of the content are more speculative. Some of the content could be right, but is doubted by many other physicists. This, too, is not unusual in science.

    Personally, I think it is very important when communicating science in a popular way to be clear about what is considered to be very well established in science and what is more speculative.

    Only a few physicists ever achieve the level of public recognition that Hawking achieved. Hawking became a household name. He had biographical movies made about his life. He became famous, in a similar way to Einstein. None of that means that he was an infallible human being, or that everything he wrote must be consider incontestable fact.

    Should you keep reading A universe in a nutshell? If you find it engaging and interesting - why not? But watch out for the clues that are in there about what is widely accepted as fact and what is more of a speculative idea that is more personal to Hawking. (I hope the relevant clues are in there!)

    More generally: be wary about putting scientists on pedestals. Scientists are just people who have attained expertise in science, just as lawyers are people who are experts in the law. You probably don't put lawyers on a pedestal or assume that their every statement is a hard fact.
     
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  9. exchemist Valued Senior Member

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    "Fact or theory" is a test you need to apply correctly in science. The only facts in science are suitably confirmed observations of nature. These observations are carried out by experimental scientists, of whom Stephen Hawking was not one. He was a theoretician so naturally what his research produced was theories, not facts. No doubt his book includes a fair amount of speculation, some of it metaphysical. This will go beyond his scientific theories and will doubtless be very much open to question. It should be noted that the various "interpretations" of quantum theory, including Copenhagen, Many Worlds, Relational etc are really metaphysics rather than science. They are the subject of much debate but do not affect the theory.

    Hawking is regarded as a very good scientist. That does not make him infallible. Even Einstein made what we now think of as mistakes. But Hawking's public reputation was due not just to his science, which was good (though apparently not good enough for a Nobel Prize), but also to his impressive personal achievement of managing to do so much considering his terrible physical disability. One should not overemphasise his scientific contributions just because of that, which I think has been a problem in some popular portrayals of the man.
     
  10. Pinball1970 Valued Senior Member

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    Based on his published papers and H index yes.
     
  11. QuarkHead Remedial Math Student Valued Senior Member

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    I think there may be some misunderstanding here.

    Recall that when a quantum system is known to be in certain state, say A, but is measured to be in either state B or in state C, it is said to be in a superposition of states. Recall also that in this circumstance, there exists no measurement corresponding to state A, as it is the sum of the states B and C. Hence states are said to be descried by a state vector.

    Feynman's idea as I understand it is that, since the history of a system is nothing other than its trajectory through configuration space, the present state of a system is the sum of these trajectories, that is, the superposition of multiple histories.

    I am not aware that he intended it to apply outside the quantum realm, but I could be wrong.
     
  12. C C Consular Corps - "the backbone of diplomacy" Valued Senior Member

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    If going by Dyson, then privately Feynman might have been reifying Sum Over Histories or attaching physical correlates and significance to its abstract description. Or (more safely), maybe it was purely "pictures" he was dabbling in rather than projecting such upon objective reality as a literal situation.

    Freeman Dyson: Dick [Feynman] fought back against my skepticism, arguing that Einstein had failed because he stopped thinking in concrete physical images and became a manipulator of equations. I had to admit that was true. The great discoveries of Einstein’s earlier years were all based on direct physical intuition. Einstein’s later unified theories failed because they were only sets of equations without physical meaning. Dick’s sum-over-histories theory was in the spirit of the young Einstein, not of the old Einstein. It was solidly rooted in physical reality. --Disturbing the Universe (1979)

    Dyson as quoted by Nick Herbert: Thirty-one years ago [1949], Dick Feynman told me about his "sum over histories" version of quantum mechanics. "The electron does anything it likes," he said. "It just goes in any direction at any speed, forward or backward in time, however it likes, and then you add up the amplitudes and it gives you the wave function." I said to him, "You're crazy." But he wasn't. --Quantum Reality: Beyond the New Physics (1987)​

    I can't risk typing the "magnet" of his name directly in this post for obvious reasons, but coincidentally it also sounds like what a certain metaphysically-prone someone (below) advocates sans referencing and utilizing that particular item (as Hawking did).

    Parallel Universes (2003)
    https://space.mit.edu/home/tegmark/multiverse.pdf

    EXCERPTS: If space is infinite and the distribution of matter is sufficiently uniform on large scales, then even the most unlikely events must take place somewhere. In particular, there are infinitely many other inhabited planets, including not just one but infinitely many with people with the same appearance, name and memories as you. Indeed, there are infinitely many other regions the size of our observable universe, There every possible cosmic history is played out. This is the Level I multiverse.

    [...] As reviewed in this article, it is becoming increasingly clear that multiverse models grounded in modern physics can in fact be empirically testable, predictive and falsifiable. Indeed, as many as four distinct types of parallel universes have been discussed in the recent scientific literature, so that the key question is not whether there is a multiverse (since Level I is rather uncontroversial), but rather how many levels it has.
    _
     
    Last edited: Dec 31, 2023
  13. Pinball1970 Valued Senior Member

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    You can get misconceptions from popsci
     
  14. James R Just this guy, you know? Staff Member

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    Moderator note: Two threads about Hawking's comments on the multiple histories thing have been merged into one.
     
  15. phyti Registered Senior Member

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    The graphic shows multiple possible paths, leading to 5 outcomes.

    The choice at each branch determines which path becomes a reality (red).

    You can't return to the origin (0) and do it over. Time accumulates.

    Floyd driving to the airport runs out of gas on (3-6) and misses his flight. The plane crashes (6) and no one survives. Had he not run out of gas he would have made his flight and been a fatality (3-7).

    Floyd says "running out of gas saved my life". No so Floyd. Running out of gas and the plane crashing are independent events. There is no cause and effect.

    His other choice at (3-8) would have resulted in severe injuries in an auto accident (8).

    This is an example showing the future is not known until it happens. Theories like

    many worlds are extreme attempts to predict the future. No better than astrology or tea leaves.

    There seems to be a trend toward fantasy replacing objective science.

    All things thinkable are not realizable.

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  16. QuarkHead Remedial Math Student Valued Senior Member

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    Unless your friend Floyd inhabits the quantum world, you are missingthe point
    On the contrary, they are an admission that, in the quantum world, the future is by definition, unkmowable.

    Let me repeat my earlier post:
    If a quantum state is the sum of at least 2 other supparposed states, and if, for every such state this is true for each of its antecendent states, and, referring to the set of all such antecendents as a state's history, then the theory says that the present state of a quantum system is the sum over all its history.
     
  17. James R Just this guy, you know? Staff Member

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    You guys aren't quite getting the "sum of histories" idea correct. It's a specific thing in quantum physics. Did you read post #5?
     
  18. phyti Registered Senior Member

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    QuarkHead;

    We have a particle P that has 2 decay states, A and b with probability rates of .4 and .6 respectively. The probabilities are values based on the history of P. Which result happens now?
    The future is unknowable period, since it hasn't happened. The images from distant events haven't arrived yet. We always observe events after the fact. All observations are historical. The so called successful 'predictions' are only the result of consistent laws regulating the universe.

    My post was only contrasting many possibilities vs. one actuality. Many worlds or the block universe attempts to explain events that happen and even the ones that don't happen. In the manner of a popular comedian, 'it just don't look right to me'.

    If the block universe contains all events that have and can happen, where do we put new events or new objects that are formed?
     
  19. phyti Registered Senior Member

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    James;

    It might help to refer to 'QED' by R. Feynman.

    Feynman uses the 'least action' principle of Hamilton in his QED light interference experiments. Light as particles move wherever they can*. Since light has only one speed in vacuum, it's analogous to a race. The winner is the first to reach the finish line.

    In the graphic, light from source S reflects from mirror M. Feynman envisions millions or billions of photons reflecting from the entire M surface. If the simple (for clarity) photon trajectories continue through M to the virtual image of D (D'), the comparison is easy to visualize. The shortest and least time is the line through location 1 on the M. Analysis of the angles a, reveals the equal angle of incidence to angle of reflection rule. Since the variation to either side of path 1 is minimal, its also the area of greatest intensity.


    * I don't agree with any particle moving backward in time. Clocks moving at speed v>0 lose time. A clock cannot move slower than v=0, thus there is no speed by which a clock gains time. (a clock moved to a higher altitude runs faster only due to less gravitational influence)

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  20. Tailspin Registered Senior Member

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    You said to be wary about putting scientists on pedestals. But the thing is I do, I always do, I don't think I can help it. I do it every time I read about science in books and online and every time I watch a science documentary.
    You said "You probably don't put lawyers on a pedestal or assume that their every statement is a hard fact." Well I would if I were seeking legal advice or asking them about laws and trials. And are you saying I shouldn't assume their every statement is a hard fact. Are you saying they're not? Why wouldn't they be?
    I also do the same thing with makers of history documentaries, doctors I visit and the teachers I've had.
    Because all these people have knowledge, education, experience and access to resources I don't. And are probably a lot smarter then me.
    Who am I to question them? How am supposed to learn anything if I doubt everything they say? You know who do question experts? Anti-vaxers, flat-Earthers and people who want evolution banned from all schools.
    I do put scientists on pedestals. Because, as far as I know, they are the ones who know, the ones who figure out what is real and false and write the books about it, the book teachers teach from. As well as being the smartest people in the world. You don't think of politicians or artists being smarter then cosmologists and quantum physicists do you?

    You say scientific claims are all provisional? Not according to every science documentary I've ever seen. And they don't say what's established and speculative. If anything they say it's all established.
    On an episode of the show Nova, the presenter once said "Since there is a finite number of ways matter can be arranged in a finite space, there exists out there exact duplicates of you, me and everyone."
    He didn't say "If there is a finite number of ways..." or "Let's suppose there is a finite number of ways..."
    He said SINCE. So I can't help but assume that this thing about matter in a finite space is 100% true, and has been proven beyond a shadow of a doubt by scientists and is part of the things we know to be absolute truths. Because he wouldn't have said it like that otherwise.
    Not long ago I saw another show presented by physicist Brian Cox. He was talking about how entropy will end the universe in a dark and lifeless void forever. With every sentence he said that this was what was going to happen. He never once said that any of this was supposition or there was any chance of this not happening. In fact I've never heard any scientist or virtually anyone at all say the universe won't end in my entire 37 year life.
    Incidentally I've brought up these 2 issues here and on other sites and in each case the response has been quote "Of course it will" "It's as sure as 2+2=4" and some made fun of me for even questioning these things.
    And now your telling me it's all provisional? You said I should look for the clues but what clues are there when they say everything as if it's a fact? How am I supposed to even begin to figure out what parts of science are factual and suppositional when everyone, fans and presenters, say it's all factual? And when people say it like that I can't help but think it's because it's the truth.

    You've sent me into a bit of a panic here because you're asking me to think for myself. And while I know that's noble and vital, I don't know if I can do it.
    For one thing, to do it I'd need a lot of scientific knowledge to challenge pop-science and if I had that I wouldn't need pop-science. How am I supposed to think for myself on a subject I know nothing about?
    And for another there's my neurology. I have Asperger's syndrome, which means I tend to take statements literally and I think in absolutes, black and white, right and wrong with nothing in between. I know of grey areas and partial truths but at the same time my brain can't process them.
    I'm not gullible or a mindless sheep but when someone, anyone makes a statement I can help but think it's true and if it conflicts with what I think I must be wrong.
    It even happens with my own opinions, I like the show Fresh Prince of Belair, once I hard my dad say what a stupid show it was and I thought "Oh, my opinion of this show's quality must be wrong." And for a while I stopped enjoying it.
    Another time I had seen this American-made documentary about the American revolution and I was awed at how it changed the course of human history. Then I saw this British-made about the revolution and the historian presenting it said it was started by the rich who thought they could get richer without British tax law. He even implied these people fired the first shot at Lexington to get it going.
    It took some time but I came to the thought that this man's documentary fell more along the lines of conspiracy theories then anything else. But even now I feel I'm not allowed to think that. Really I can't help but think I'm not allowed to disagree with experts and the overwhelming masses who agree with them.
    You said I should look for clues to what is suppositional? The only clues I can pick up on are when they say something like "All statements made in this programme are provisional, theories and hypotheticals based on current scientific knowledge which is subject to change." Which they never do.
    Unless someone says something reeeeealy outrageous or obviously wrong I can't pick up on that at all.

    In conclusion how am I supposed to think for myself and spot what is provisional on subjects I know little about, when my intellectual superiors state over and over it's true, when everyone else believes them and call me a fool for questioning them, when fact and opinion sound exactly the same and I'm fighting my own neurology?
     
  21. exchemist Valued Senior Member

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    What James says about science is basic philosophy of science and not in the least controversial. No theory can be proven true as all theories in science could in principle be overturned, if some new and unforeseen observation of nature came along that showed a theory to be wrong. One can never logically exclude the possibility of that.

    Also you already know of several theories in science that are in a sense "wrong". What about Newtonian mechanics or his theory of gravitation? Einstein's special relativity showed Newtonian mechanics breaks down when objects are in relative motion that is a significant fraction of the speed of light. His general relativity also showed that Newtonian gravitation breaks down under certain circumstances, too. Nevertheless engineers and physicists continue to use Newton's theories because they are good enough for most purposes and far simpler to work with. They just keep in mind that their scope of application is limited.

    Secondly, if you know anything of the history of science you will be aware of many more theories that served a purpose for a time and then were discarded after being shown to be poor models: the Ptolemaic (geocentric) system of astronomy, phlogiston, the aether, the caloric theory of heat.......etc etc. We cannot assume that none of the theories we have today will not one day be superseded.

    So no need to panic. Just get used to the idea that theories are models of nature that may not be exact. Some work well all the time, so far as we know. Others work in certain circumstances, or are approximations. Some are contested by rival groups of theorists. And some are tentative because we do not yet have a full wealth of observations to know how good they are. That last limitation applies to a fair amount of cosmology, since a number of the relevant observations are difficult to make.

    I'm sure you are right that TV programmes about science tend not to dwell on the philosophical limitations of scientific theories. They will be simplified for the purpose of teaching some specific theory and generally won't want to get side-tracked into a debate about the possibility, however faint, that the theory in question might get overturned one day.

    But, Asperger's or not, you have to face the fact that scientific theories are not absolute and eternal. They are the best models we have for the time being. That's all they can ever be. Scientific research is working on improvements all the time.
     
    Last edited: Jan 18, 2024
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  22. Pinball1970 Valued Senior Member

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    Don't put yourself down.
    Pop science can give you a flavour, I used to read lots of it.
    I will say that you can get a decent view of the players, what they did and when. A history/chronology.
    The technical details however, the real meat gets lost or brushed over.

    Science is hard, you can't read a couple of books and understand it, it takes years of studying and practice.
    Pop sci gives the impression you can take it in via YouTube videos or a Brian Cox book.

    In terms of where to start, review what you learned school 14-16, school books. If you did any A level type courses or above I would still do that.
    Reason? They usually have questions at the end of the chapters, have a go at them.
    14-16 covered? Move up a notch. 16-18 A level.
    You may not get this if you are not from UK.
     
  23. James R Just this guy, you know? Staff Member

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    Tailspin:
    Experts aren't always right about everything - not even about everything that you might imagine might be encompassed in their field of expertise.

    Lawyers who give legal advice aren't always right. They might make a (hopefully, educated) guess at what a court will do, for example, but they might be wrong when push comes to shove. A lawyer might honestly be trying to give you good legal advice, and yet the advice he gives you still might not be the best advice in all the circumstances.

    Experts are human beings, and human beings make mistakes and get things wrong. Experts are usually considered as such because they have a good track record for getting things right, at least when it comes to their specialised fields of expertise. But they aren't always right.

    When it comes to scientists, part of the job description is to speculate on how things might be in nature. Scientists put forward hypotheses, which sooner or later are tested to see whether they correctly describe the phenomena they are attempting to explain. A lot of hypotheses turn out to be wrong. Some turn out to be right - or at least not provably wrong given the current state of the available data. But, like lawyers, scientists are human beings. They aren't right all the time. And an individual scientist or lawyer might be right about 95 out of 100 things he says and wrong about the 5 other things, for instance.

    It's also worth bearing in mind that that "hit rate" is likely to be lower as the expert strays further away from his own speciality, to comment on other matters. An expert in criminal law is by no means guaranteed to be great at Family Law. An expert biologist is by no means guaranteed to be great at physics. These things are true even if the criminal lawyer is more likely to know something about Family Law than the average non-lawyer on the street, and when the biologist is more likely to know something about physics than your average chef on the street, for example.

    You're naive if you expect that once somebody is an expert in something, his every statement must be taken to be "hard fact". You'd be better off recalibrating your unrealistic expectations of experts.
    While it is a good bet, in most cases, that your family doctor is going to give you reasonably good medical advice - and almost certainly better medical advice than you're going to get from somebody who isn't a trained doctor - you should still not take everything your doctor says as hard infallible fact. The doctor offers you his expert opinion. He draws on his knowledge, but his knowledge has limits. That's why there are medical specialists that family general practitioners refer you to when they reach the limits of their own expertise. But those specialists have their own limits, too. And there are things that no doctor - specialist or otherwise - actually knows or can guarantee to be the case.
    I would also be wary of assuming that educated automatically equals smart, or reliable. How many shonky lawyers have you heard about? How many shonky doctors?

    Now, on average, medical doctors are smarter than the average person on the street. We can take that as a given, because it's hard to get into medical school and become a doctor. It's an academically competitive process. But, nevertheless, some people make it into medical school, blunder their way through with bare passes and get admitted to practice. They aren't necessarily great doctors. There are good doctors and bad doctors. There's all sorts of reasons and ways for why a doctor can be bad doctor. So why would you assume that every time a doctor gives you advice, he must be right, or that you should trust him? You've heard of asking for a second opinion, haven't you? Why would you need one? Think about it.
    It is more accurate to describe those people as deniers than honest questioners, because those people can, in a very short time, discover that there are expert consensuses among relevant experts on the subjects of vaccines, the shape of the Earth and evolution. In fact, the deniers are, in most cases, aware of the prevailing consensus positions of the experts, and yet they continually raise the same faulty arguments against the expert view, again and again, ignoring the fact that their own (non-expert) positions have already been proven to be untenable.
    It's a question of having the right kind of expert for the right kind of job. An artist might be clueless about politics but, equally, a politician might be clueless about art. Why would you assume that being an expert in one thing makes you an expert in everything? That's a silly notion.

    And how do you propose to compare the "smarts" of a painter like Rembrandt, say, with the "smarts" of a scientist like Galileo? It's like comparing apples to oranges, isn't it?
    Yes.
    I don't think you're listening carefully enough. If you do watch carefully, you'll find many statements in documentaries about "scientists believe that" or "our best theories suggest that" or "Scientist X suggests that phenomenon Y might be caused by Z. If so, then..."

    However, you ought to bear in mind that scientists don't usually have the final say in what goes to air and what does not when a science documentary is made. Rather, the producers and editors of the documentary decide what to include and what to leave out. And those producers and editors are not usually experts in the science. Some of them, also, aren't as careful to distinguish fact from speculation as they should be.
    And what did he say before all of that? I assume there was some lead-in to the claim about the number of ways that matter can be arranged in a finite space, etc., and some prior talk about exact duplicates or such. You might need to look earlier for the relevant disclaimer, if there was one.
    What do you mean you can't help but make that assumption?

    You're free to make your own assumptions. Nobody is holding a gun to your head.

    You always have the choice to think critically about what you're viewing, or not to. If you choose not to, that's on you. And that doesn't just apply to science documentaries. It applies to everything you watch on TV or in the movies.
    What makes you think that? People can be motivated for all sorts of reasons to speculate about things they don't know for sure. People can be overconfident about the conclusions they have reached about things. People can overestimate their own expertise. People can make mistakes.
     
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