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"...I Don't Understand It. Nobody Does." R. P.Feynman

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Posted by Alan on August 16, 2002 07:58:40 UTC

Hi folks,

quote from Richard P Feynman, page 9, "QED. The Strange Theory Of Light And Matter":

"It is my task to convince you NOT to turn away because you don't understand it. You see, my physics students don't understand it either. That is because I don't understand it. Nobody does."

copyright 1985 Richard Feynman. (Penguin Books).

I strongly urge people to read this book. It is short. It is direct. It is astonishing.

Here is an idea what it is about:

Remember learning at school that light travels in straight lines; that a mirror can reflect light, that the angle of incidence equals the angle of reflection, light can be separated into colours,
light bends from air to water, etc.?

These ideas were once explained using waves. But it was found that very dim light makes a detector respond with the same intensity, but less often, no matter how dim the light is.

So light had to be particles.

Quantum electrodynamics allegedly explains all the things light does that you learnt at school.

R. Feynman notes that it is easy to explain something like "subtracting numbers" by saying you put so-many beans in a jar, take so-many out, and what's left is the answer. But it is more complicated to explain the system of symbols and rules to do subtractions more efficiently without having to count all the beans.

He notes that it is easy to tell us what physicists are really doing; whereas the tricky rules to do it efficiently take years of graduate study.

In the book he tells us what they are really doing. He says that everything that is known about light can be explained by quantum electrodynamics.

Light partially reflects from a sheet of glass depending on its thickness. The amount of partial reflection is given as a probability of a photon being reflected.

The answer repeats in a cycle from zero photons to 16% of photons. The cycle repeats itself millions of times over and over as you use ever thicker glass. This is a deep mystery.

How physicists calculate the probability for partial reflection from a sheet of glass of given thickness is "analogous to EVERY OTHER problem explained by quantum electrodynamics." (see p. 24)

R.P.Feynman, page 24 of QED:

"You will have to brace yourselves for this - not because it is difficult to understand, but because it is absolutely ridiculous:
All we do is draw little arrows on a piece of paper - that's all!"

Bean-counting physics style:

The rules include: probability of an event is the square of the length of the little arrow you draw.

Arrows that represent each way an event can happen are drawn and combined by adding the arrows head to tail without changing their direction; to get a 'final arrow' from the tail of the first arrow to the head of the last one.

The direction of the arrow: imagine a stopwatch attached to the photon, that spins 36,000 times a second. Start spinning when the photon leaves its source, stop it when it hits your detector. The direction of the clock-hand is the direction used for the arrow you draw.

Reflection off the back of a glass sheet is drawn in opposite direction to that off front of sheet.

Now, you learned in school that light travels in straight lines and reflects off a mirror with angle of incidence = angle of reflection. Apparently not so.

Light goes all over the show, wiggling here and there, taking every possible path! But add the arrows (the direction of an arrow corresponds to the time light takes using a particular path).

Add all the little arrows and lo, the final arrow gives reflection off the middle of the mirror as the most paths of similar time made the final arrow bias in that way. (See p. 43)

So it is roughly right to say light goes where the time is least.

When you scrape regular scratches in the mirror, the places where you thought light wasn't reflecting (places away from the center), suddenly can be seen to reflect. By subdividing the mirror as a diffraction grating, scraping away at regular intervals all paths of a particular bias; there is a similar time for paths all across the mirror- and strong reflection is seen all over.

There is an amplitude, an arrow, for every way an event can happen. An arrow for every way an event can happen must be added to get the final arrow (square that; and have the probability).

Compound events: break into steps. Draw arrows (amplitudes) for each step. Start with a unit arrow. Say the first arrow is 0.2 at 2 o'clock: shrink the unit arrow to 0.2 and turn it from midday to 2 o'clock.
Say the second arrow is 0.3 and 5 o'clock; then further shrink the 0.2 arrow to 3 tenths of 0.2, and turn it from 2'o'clock to 7 o'clock.
Result is 0.06 arrow at 7 o'clock for compound event. 0.06 squared gives probability of compound event of 0.0036.

Mathematicians call the arrows "complex numbers".
Page 63: "When an event can happen in alternate ways, you add the complex numbers; when it can only happen as a succession of steps, you multiply the complex numbers." (Same thing as: alternate ways for event: use adding arrows rules; succession of steps to event: use the shrink and turn arrow rules.)

Feynman goes on to explain in ever greater depth all sorts of stuff. By p. 85 he says all the phenomena of light and electrons arise from:
1. a photon goes from place to place; 2. An electron goes from place to place.
3. An electron emits or absorbs a photon.

"Each of these actions has an amplitude - an arrow - that can be calculated according to certain rules."

He tells us the rules "Out of which we can make the whole world (aside from nuclei, and gravitation, as always!)"

Page 89: "You found out that in the last lecture that light doesn't go only in straight lines; now you find out that it doesn't go only at the speed of light!"

He draws space-time graphs. Page 95: "Not that the rules are so difficult - it's like playing checkers over and over. So our difficulty in calculating comes from having to pile so many arrows together." P. 95: "That makes billions of tiny arrows that have to be multiplied and then added together!"

Things are simplified by a number j (which is the junction or coupling of electron and photon). Short cut is to work out the possibilities involving j.

Later:
Page 98: "Every particle in Nature has an amplitude to move backwards in time, and therefore has an anti-particle."

Eventually we learn about the Pauli exclusion principle, about weak and strong interactions, W and Z particles, gluons, quarks, colours, flavours, muons, spin, etc.

I haven't read the whole thing yet! But have looked ahead at it.

Hope I haven't upset copyright re: the amount of quotes I used.

So I'm trying to make sense of this highly accurate and successful physics theory that has baffled physicists for decades.

The whole thing looks, to quote David Bohm, like there is an "implicate order". My guess is something basic is going on. Dr. Dick's idea that it may be all "true by definition" is intriguing.

Given that to discuss anything, physicists have to agree on definitions; it appears that any definition laws might undrerpin their best theories.

When I found that the process of defining a word, as given in a year one philosophy textbook, might map physics laws very nicely; I figured I'd try that idea.

Since the whole thing is built up with operations with complex numbers, thus 2-D numbers (ordered pairs can represent them): why not the whole thing be the comparing and matching of patterns?

A way of looking at a way of looking even; three interdependent musical chairs games, of which one joins kids without chairs in one game to chairs without kids in the other?

The whole thing self-referential, structures built out of consciousness? Three consciousnesses in one?

Fair enough to consider?

-dolphin

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