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Quantum On Feynman's QED

Charles Link

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I started reading Feynman's "QED the Strange Theory of Light and Matter" (a paperback) , and an item that some of you might find of interest comes up in the first chapter. He basically says it is a mystery why Venus goes through five cycles in eight earth years, but I can offer a simple explanation: We can write for the orbital period ## T ## of earth relative to Venus as ## \frac{1}{T}=\frac{1}{225}−\frac{1}{365} ##, (the "rates=frequencies" subtract, since Venus and the earth orbit in the same direction around the sun, to get the net rate), where Venus has an orbital period of 225 days. This makes ## T=\frac{(225)(365)}{365−225}=(1.61)(365) ## earth days ##=1.61 ## earth years. Meanwhile 5 cycles in 8 earth years makes it to be one cycle in 1.60 earth years. ## \\ ## I think it is likely Feynman knew this, (I'll let you read this part of the book, and you can reach your own conclusion), but he was trying to make a point that Nature can be somewhat mysterious. We do the calculations, but don't always know precisely why they work. ## \\ ## The Mayans, who he was discussing in the book, may not have known why, but we know why.
 
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dRic2

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And it is a mystery that Venus completes a cycle every 1.60 earth years :smile:


I think what he wanted to say is that we can perform very accurate calculations (tricks as he calls them), but still know nothing about the underling principles of Nature.
 

Charles Link

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Thank you @dRic2 . It's interesting seeing the lectures from which the book was made. The book is the text from a series of what are called the Mautner lectures that were presented at UCLA, but in the book, Feynman says he also tried out the material in the lectures in New Zealand first. Perhaps the video above is from New Zealand=it says "University of Auckland" at one point (3:30) in the video. (Edit: A google shows that University of Auckland is, in fact, in New Zealand). In any case, thank you very much.:smile::smile:
 
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"Why?" is hardly ever a good question in the physical sciences. We are pretty good at what and how, but "why?" is better left to poets.
 

Charles Link

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"Why?" is hardly ever a good question in the physical sciences. We are pretty good at what and how, but "why?" is better left to poets.
In this case though, knowing the orbital period of Venus to be 225 days, (which the Mayans probably did not know), and knowing earth's to be 365 days, the 5 Venus cycles in 8 earth years does have a simple answer to the question "why?", with just a little bit of computing, which I did in post 1. (Basically, the time for Venus and earth to pair up in their orbit relative to the sun is 1.61 earth years, i.e. Venus orbits the sun faster than the earth and laps the earth every 1.61 years).
 
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In this case though, knowing the orbital period of Venus to be 225 days, (which the Mayans probably did not know), and knowing earth's to be 365 days, the 5 Venus cycles in 8 earth years does have a simple answer to the question "why?", with just a little bit of computing, which I did in post 1. (Basically, the time for Venus and earth to pair up in their orbit relative to the sun is 1.61 earth years, i.e. Venus orbits the sun faster than the earth and laps the earth every 1.61 years).
That sounds more like "what" than "why" to me. These are observations, not reasons. I think you are missing Feynman's point.

BTW, Venus' year is 224.7 days which means that in 8 earth years it will have 13 orbits, I'm not sure how that relates to "5 cycles" of morning/evening Venus observations.
 

Charles Link

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That sounds more like "what" than "why" to me. These are observations, not reasons. I think you are missing Feynman's point.

BTW, Venus' year is 224.7 days which means that in 8 earth years it will have 13 orbits, I'm not sure how that relates to "5 cycles" of morning/evening Venus observations.
It is quite clear how Feynman plans on showing us the QED next, and he is making the point that QED doesn't seem to have a good reason why. ## \\ ## For this one, I urge you to compute how long ## T ## it takes for Venus to lap the earth , considering them as running on a circular (or oval) track, where Venus goes around in 225 days, and the earth takes 365 days to circle the track. If you compute it, you will likely get the same equation that I did in post 1 to compute it. The result is ## T= (1.61)(365) ## days. Note that ## 5T= (5)(1.61)(365) \approx (8)(365) ##, thereby the 5 to 8 number of the Mayans. ## \\ ## From this, you can readily compute what the arithmetic shows: In 8 years, Venus laps the earth 5 times, so that it orbits the sun 13 times, just as you computed. One cycle of Venus occurs simply as one lapping of the earth by Venus.
 
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Charles Link

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Note that in the above analysis, basically the earth is standing still relative to the sun, while Venus takes 1.61 years to go around. Meanwhile the earth is rotating once every 24 hours. Depending on where Venus is in its "effective" 1.61 year orbit (and it orbits at 67 million miles from the sun, compared to the 93 million earth-sun distance), it will appear either as the morning star or the evening star.
 
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Charles Link

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I'm going to give this one a "bump", so others may see it. It already received some good feedback, but I could enjoy a little more. ## \\ ## Edit: And here is a "link" that gives the equation that I have in post 1 to derive what is called the synodic period. https://in-the-sky.org/article.php?term=synodic_period Undoubtedly, Feynman knew how to do this calculation, even though he pretends that no one seems to know why it is such that 5 cycles of Venus gives 8(365)=2920. This makes one cycle of Venus to be 584 days=the synodic period, which can essentially be computed from the 225 and 365 orbital periods. (Using 224.7 makes it more accurate). @DaveE You may find this "link" of interest.
 
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Spinnor

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"Why?" is hardly ever a good question in the physical sciences. We are pretty good at what and how, but "why?" is better left to poets.
But in this case for the orbital periods of the planets the "why" question has a good answer, the planets orbit the sun via Newtonian physics which a clever Mayan priest might have figured out. Likewise for Q.M., calculations yield predictions of probabilities but one might ask why does Nature work in this way, I am confident there is an answer to such a question.
 

dRic2

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@Spinnor if you think that Newton's laws can answer that question, then "why" do you you think that ? Why are Newton's laws true? Because that's the way it is, you might say, but why does Nature work this way ? And why not an other way ? You can't answer that. You can't know why Nature is the way it is, you can only take it as a fact. And from that you can describe how nature works, not why.
 

Spinnor

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And from that you can describe how nature works, not why.
Clearly our understanding now is greater than that of the Mayans in regards to the orbital periods of the planets. They could predict the orbits but they had no idea why, now we have a better answer to that question. Maybe that just leads to a different why question that can't be answered.

If you think we have reached the end of the road in regards to Q.M. then our opinions differ.
 

dRic2

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I agree. But we are talking about different things.

For example, imagine that someday physics will be good enough to predict every observed phenomenon with en extraordinary precision. Everything will be explained. Gravity, dark matter, time, symmetry violations, turbulence (o0))... To name a few. There will be nothing that out model can't describe. Do you think you would have answered the "why" question ? Because I don't.
 

Spinnor

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Do you think you would have answered the "why" question ?
I am thinking of a particular why question in regards to Q.M. and I think we can answer that why question but maybe we will have to replace it with a deeper one. Feynman's lecture of the Mayans about how they could predict the motions of the heavens even though that did not know the reason why is an analogy about how we can predict say where electrons will be likely detected on a screen but we have no picture about how such a thing can occur, all we have is the mathematical apparatus to predict such things. Like the Mayans who were blind to the idea of Newtonian celestial mechanics I feel we are currently blind to a deeper representation of Quantum Mechanics, the math of quantum mechanics won't change but we will in time understand why Nature uses it. Feynman tells us that it will be hard to find some deeper representation but I don't think he rules out the possibility. He does though say

"...Do not keep saying to yourself, if you can possible avoid it, "But how can it be like that?" because you will get 'down the drain', into a blind alley from which nobody has escaped. Nobody knows how it can be like that. "

But that was just to encourage those who do the opposite of what they are told to do and try and find a deeper representation?
 

Charles Link

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Feynman seems to take a couple of different approaches in his various books. In "the Pleasure of Finding Things Out", he talks about the excitement of new discoveries. Meanwhile, towards the end of his book, "the Character of Physical Law", he takes what seems to be a somewhat pessimistic approach in that he thinks the age of discovery may be coming to an end. ## \\ ## The Quantum Electrodynamics was a very important advancement, but I do think there will be quite a number of others still to come. With each advancement, the question of "why?" never gets entirely answered, but that's what keeps it interesting. The joy of learning new things is always there.
 
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dRic2

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I think you are not getting my point. I think it's mainly my fault as my english might sound confusing at times.

I'm struggling to put the right words together because it not a simple argument. I suggest to read the work of the great philosopher Immanuel Kant, which is basically what Feynman is trying to say. I'll try to summarize it quickly.

The world around us is based on what we feel: it is just a representation inside our mind. It is what Kant called the Phenomenon. Physics comes up with theories, "fundamental laws" to explain what we see and what we feel, so it is merely a tool to describe the Phenomenon. Then there is the Noumenon which is "the thing-in-itself". This is the true nature of reality that is beyond out perception. We can never reach that. That would be meta-phyisics. You can come up with all the nice theories you want but you'll never get to the "thing-in-itself" and that's why you can't answer any "why" question. Mayans math "tricks", Newton's law, General Relativity... They are all attempts to look at the Phenomenon from a different point of view, that's all. There is nothing fundamental about those things.

So now I repeat my question:

If the times comes when we will understand everything, (even the question about QM that you referred in your post), do you think you can answer the question "why is nature the way it is?" ? Well no! Because you would still be confined within the "wall" of the Phenomenon!

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I have nothing more to say (and we are going off topic). It's a matter of opinion, but I strongly suggest studying the work of Immanuel Kant.

Cheers
Ric

PS: I'm not pessimistic, and I think that people should study physics and get new ideas, and view things from others point of view. But I think people should do that only because it is a fun thing to do, like a puzzle, or game. Nothing more. 😄
 
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Spinnor

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The world around us is based on what we feel: it is just a representation inside our mind.
I also have trouble conveying my thoughts as well, and English is my native tongue! Consider mathematicians, they can create ideas, math, geometrical spaces that have nothing to do with the world around us, they are not limited by their everyday experiences, neither are physicists.


If the times comes when we will understand everything, (even the question about QM that you referred in your post), do you think you can answer the question "why is nature the way it is?" ? Well no! Because you would still be confined within the "wall" of the Phenomenon!
That wall is still very far away, our understanding grows with time even if there is some limit plane.

But I think people should do that only because it is a fun thing to do, like a puzzle, or game. Nothing more. 😄
I agree!
 

dRic2

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Consider mathematicians, they can create ideas, math, geometrical spaces that have nothing to do with the world around us, they are not limited by their everyday experiences, neither are physicists.
Yes but mathematics is not physics. I think is just sheer luck that we can use it to express physical law.

BTW Kant also pondered about mathematics and geometry. He used this metaphor:

Imagine we see the world through colored glasses. For example blue glasses. Things will look blue to us, but not because they are blue, but because we have this special glasses. Mathematics and geometry are those glasses, so they are a "filter" between us and the Noumenon (the thing-in-itself).

Anyway, if you are interested, read something about Kant. He was a smart guy ;)
 

vanhees71

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But in this case for the orbital periods of the planets the "why" question has a good answer, the planets orbit the sun via Newtonian physics which a clever Mayan priest might have figured out. Likewise for Q.M., calculations yield predictions of probabilities but one might ask why does Nature work in this way, I am confident there is an answer to such a question.
Why do you consider the answer to the question about the periodical periods of the planets: "Because of Newtonian physics." a better answer than that of quantum theory about probabilities of measurement outcomes? Both are just a mathematical description what has been figured out by more and more accurate and quantitative reproducible observations of phenomena.

One profound general discovery is that according to quantum theory even if one has complete knowledge about a system (i.e., its preparation in a pure quantum state) most of the observables of that system have no determined values but we can have only the information about the probabilities for the outcome of a measurement of each observable.

This is just umfamiliar for us since we live in the illusion of having predetermined values of all observables, but that's of course just because we don't look too accurate on not too detailed information of macroscopic objects. According to quantum theory this "determinism" is just an emergent phenomenon for sufficiently coarse-grained perspectives on the phenomena concerning macroscopic bodies surrounding us.

Of course, also in previous times the findings of the natural scientists was quite unfamliar if not even scaring for some contemporaries. E.g., Newton's idea that actually the gravity pulling an apple from the famous tree in his garden is the very same force obeying the very same natural laws as the force pulling on the Moon. He could even show that he was right, being able to derive Kepler's famous empirical laws about the motion of the planets around the Sun (as well as the Moon around the Earth) from his general laws of mechanics plus the general law of the gravitational force.

Such thoughts were even quite dangerous some time before. Giordano Bruno was burnt at the stake just for thinking that planets may rather run around the Sun than vice versa. Galileo already was "only" arrested at home for such ideas, and Newton got famous for his work. Fortunately men get used for discoveries of natural scientists. I think one day also the general public will get used to what QT has to tell us. Maybe it will take longer than in the "ancient times", because we have the internet an mass communication which keeps the esoteric hype at life for "more clicks", because it sells better than the true theory itself :-((.
 

Spinnor

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Why do you consider the answer to the question about the periodical periods of the planets: "Because of Newtonian physics." a better answer than that of quantum theory about probabilities of measurement outcomes? Both are just a mathematical description what has been figured out by more and more accurate and quantitative reproducible observations of phenomena.
The Mayans made observations and came up with formulas that allowed them to predict future events in the heavens, for example a total solar eclipse. One of the things that they observed and I am sure they could predict was the retrograde motion of the planets. Their formulas could predict retrograde motion but they did not know why such motion occurred. Once you assume that the planets orbit the sun via Newtonian mechanics such motion becomes obvious. The math does not change but now we have a deeper understanding of why the math works. We are still left with more why questions but our understanding has increased.

I think the situation is similar in regards to Quantum Mechanics, there is a deeper representation of Nature that implies the mathematics of quantum mechanics.

Being one of the more clever members here I would appreciate your suggestions on how to discover such a deeper representation, play the Devil's advocate if you would.
 

vanhees71

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It may always be true that there is a "deeper representation", i.e., a more comprehensive theory describing the phenomena, but as the history of science shows, you have nearly no chance to find such a "better theory" from pure thought alone. Your arguments are a good example: You have some feeling (or prejudice) about how nature should look like. For some reason you are simply unsatisfied by the best theory we have, which is QT, but you don't have any concrete idea what's wrong with it. It's just not following "good old Newtonian physics", but that's progress of science: We learn that some theory is not valid for all phenomena, but is only an approximation of a more comprehensive theory.

Newtonian mechanics had to be revised twice in a relatively quick time in the 20th century: First it was discovered that the very foundation of Newtonian mechanics, namely the Galilei spacetime model, is only approximately valid, and special (and later even general) relativity is a much better spacetime model. The success of Newtonian mechanics was then explained by the fact that it dealt only with bodies moving much slower than the speed of light and that only tiny gravitational fields of the solar system have been considered. In this realm it was a very good approximation, but as was shown by accelerating particles through electromagnetic interactions to higher speeds (I think most probably that the experiments by Kaufmann et al with electrons was among the first) indeed Newtonian mechanics breaks down, and the better relativistic description was needed.

The 2nd need for revision was also evident at least from 1911 on: Rutherford had discovered that matter indeed consisted of atoms and that these are mostly "empty space", i.e., they consist of a pointlike (or at least quite small) positively charged "nucleus" surrounded by negatively charged electrons. The problem was that then the obvious fact that atoms and matter formed by them is stable. This couldn't be true within classical physics, because charged electrons running around the nucleus on ellipses (the bound states of the classical mechanics problem well known from planetary motion around the sun, but here with electromagnetic forces). That was one clear motivation to find a better theory, and it turned out (after some predecessors like Bohr's and Sommerfeld's ad-hoc "solution" of the problem) to be quantum theory.

Within 20-30 years QT was also extended to the relativistic case, and today everything (except gravity) is described by relativistic QFT, and all the earlier theories (classical newtonian or relativistic mechanics, classical electrodynamics) stay valid as approximations for well-defined circumstances.

This rough summary of the historical development up to our best theories today shows that new theories are not found by pure thought but only if you have clear scientific hints at where the contemporary theories fail.

Today we have only one really fundamental problem, namely a consistent quantum theory including gravitation, but there seems to be quite little progress in finding a satisfactory answer. I think the problem is that we don't have clear empirical evidence for quantum effects on gravity (or spacetime?).

So it is very likely that there's a need for a more comprehensive theory, and for sure both the classical spacetime model to describe gravity of general relativity as well as quantum (field) theory to describe all known particles and interactions except gravity need some revision or extension. Maybe a solution of these problems will lead to some theory that's more satisfactory for your worldview than QT is today, but it's hard to say since we so far don't have any such satisfactory theory.
 

Spinnor

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It may always be true that there is a "deeper representation", i.e., a more comprehensive theory describing the phenomena, but as the history of science shows, you have nearly no chance to find such a "better theory" from pure thought alone. Your arguments are a good example: You have some feeling (or prejudice) about how nature should look like. For some reason you are simply unsatisfied by the best theory we have, which is QT, but you don't have any concrete idea what's wrong with it. It's just not following "good old Newtonian physics", but that's progress of science: We learn that some theory is not valid for all phenomena, but is only an approximation of a more comprehensive theory.
You don't quite understand me. I am quite impressed with Quantum Theory, who could not be. Every quantum like phenomenon we discover is eventually modeled with QT, the situation is quite remarkable. Quantum Theory as a mathematical framework is here to stay, it is so accurate in its predictions how could it be replaced. Just as the Mayans could predict retrograde motion of the planets but did not know why such motions occur we can predict where electrons and photons are likely to be found in a double slit experiment but we have no clue why such a pattern occurs other than electrons and photons are excitations of the underlying quantum fields and that is just the way these excitations interact with a double slit screen, there is no deeper representation, and maybe there is none and maybe there is.

As for pure thought coming up with a deeper representation it can only come to people who understand current physics, like you or any of the other very clever people on this forum, and also search for a deeper representation.

Thanks.
 
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vanhees71

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Well, I think that all our theories are preliminary, and with better understanding maybe one day we'll need a revision of QT. The revised theory must of course describe everything that QT has described well until today. So it can well be that QT is an approximation to an even more comprehensive theory, which we don't know yet. It's the same as with the astronomy of the Mayans: They had a remarkably well working description, but it was superseded by Kepler's more accurate descriptions, then by Newton's much more comprehensive theory of mechanics and gravitation and so on. The Mayan's results were simply refined by the later descriptions and then even with Newton's theory which is much more general and reduced to a handful of fundamental laws. Nevertheless also Newton's theory was found to be not the complete picture yet. It needs to be refined by relativity, and with respect to gravity even by the general theory of relativity. Nevertheless Newton's theory stays valid as an approximation of general relativity, which leads for weak fields and small velocities to tiny refinements of Newton's results. At Einstein's time this was known from the mercury perihelion shift (after all the "Newtonian causes", i.e., the influence of other planets have been taken into account). It was a great triumph for Einstein to get from his general theory of relativity the corresponding missing perhelion shift of mercury out right.
 

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