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Showing 501-525 of 1000 posts in this discussion
In reply to an earlier post on Jun 29, 2012 10:49:57 AM PDT
So you have no answer to my question: If a physical theory gives exactly the same calculated results as observed results for all phenomena in its domain, how can you tell whether its "just" a model versus a perfect representation of actual physical reality?

In reply to an earlier post on Jun 29, 2012 10:50:50 AM PDT
Why wouldn't QED be able to generate a perfectly accurate answer for all phenomena involving electrons and photons at energies below the cutoff?

In reply to an earlier post on Jun 29, 2012 11:04:55 AM PDT
Last edited by the author on Jun 29, 2012 11:07:06 AM PDT
The cutoff is a practical necessity, not theoretical. But all the phenomena that "actually exist" don't implement the cutoff, in "reality". That is, there ARE NO phenomena like [you are supposing]. Nature takes its own 'calculations' out to infinity, it would seem. If you want to model this, then you need to do the same.

It's like the classical "three body problem". The three bodies have no trouble figuring out where they are going, for themselves; It's only the physicists and their theories who can't nail it down, in detail. And for much the same reason: All the known methods are iterative; they work by successive approximation.

No such method can produce an exact answer (a perfect answer) in finite time, and by t[inf.], the bodies in question have moved on.

In reply to an earlier post on Jun 29, 2012 11:16:49 AM PDT
Humans perceive the 4th dimension "time" as fundamentally different than the first three because we are bound within the fourth, not because the 4th ("time") is a "unique" dimension unlike the "spatial" dimensions. All the dimensions are spatial. A being above the 4th dimension (say, 6th) would be able to interact with or perceive the past/future as we do the items on our desk. Just remember that time implies movement, and within what does something move? Space. Time is not fundamentally different than the other dimensions, and there are at least 10 dimensions, probably 11.

In reply to an earlier post on Jun 29, 2012 12:02:09 PM PDT
Agree the cutoff is a practical, ad hoc solution to the problem of infinities in QED. Agree that in reality when a physical system involves energies above the cutoff, it's still real, but QED no longer gives accurate results.

I still don't know why one couldn't argue that if QED gives calculated results which agree to measured results within the limits of accuracy of the measurements for all phenomena involving electrons and photons at energies below the cutoff, why you couldn't argue that QED is a perfect representation of physical reality in this domain. I.e., the distinction between "actual" reality and a "model" of reality would break down completely.

In reply to an earlier post on Jun 29, 2012 12:12:26 PM PDT
Jack Shandy says:
-"you are confusing your models with what is actually real "

The issue, which I think you are missing, is how do you -decide- what's actually real or not? How can you decide what's real if you don't even have a model of reality assisting you in your decision? The model will determine what you decide to be real. This model may just be an amalgam of common sense misgivings, but it can also be a collection of accurate physical theories.

For me, it's the latter. You, decide for yourself. But you will use a model in one way or another.

In reply to an earlier post on Jun 29, 2012 12:51:18 PM PDT
How about if we consider a question like:

Does "reality" even have an infinite decimal expansion? Or is that idea simply an idealization?

As noted, QED offers no specific reason to assign a cutoff of any particular value. Could we use it to generate more decimal places than Nature actually knows it has? In which case, QED would be like the example I gave of over-specifying the time of day; It would be giving us a false accuracy.

Likewise, relativity is currently formulated with real number coordinates that can, in principle, be specified out to any degree of precision imaginable, like those nuts who calculate pi out to 5 trillion places. Worse, the full specification of each event contains/requires Aleph 1 bits of data--an uncountable infinity. Surely, Nature could get by with somewhat less than this absurd information overload, and still do all its wonderful tricks. But relativity, as currently conceived, cannot! Quantizing all those events just creates another bunch of infinities in a different area of the carpet, and you get results that boggle the mind in equally unpleasant ways, if not worse ones.

In reply to an earlier post on Jun 29, 2012 12:58:11 PM PDT
af,

In case you haven't seen it. can I recommend:

http://www.brainpickings.org/index.php/2012/04/09/how-long-is-a-piece-of-string-bbc/

It is quite an entertaining look at some of the issues concerning "realism", and what measurement actually means.

In reply to an earlier post on Jun 29, 2012 12:59:31 PM PDT
Can't we just define the limit of a decimal expansion, and get around the problem?

In reply to an earlier post on Jun 29, 2012 9:55:17 PM PDT
Donohue said: I love to see people who have no background in physics critique some of the most advanced and accurate physics theories. Having said that, I have been plowing through quantum field theory with some texts by Weinberg and Kane and there is an ad hoc quality about QFT that I don't see in SR or GR. The equations don't help my understanding of quantum behavior -- they just predict it.

Yes. Quantum theory (in general) makes good predictions (based on probabilities) and has consistent results but no one knows what is really behind it or really understands any mechanisms behind it. Also no one knows how to explain things like gravity with it via QFT. Then again how accurate is a prediction that is really a set of possibilities and the solution is only known....when you actually make a measurement? Also, when you know more about one property (velocity) by measurement you must then know less about another (position).

In reply to an earlier post on Jun 29, 2012 10:25:09 PM PDT
Last edited by the author on Jun 30, 2012 8:33:16 AM PDT
How can a theory that no one really understands be perfect or complete?

No one knows why the quantum world (as opposed to just the theory) is the way it is, how it "knows" to operate that way, or what possible hidden "mechanisms" or interconnections could underlie it. I think we can assume that there should be an attempt to also understand things like this. We can assume that it should all be understandable at some point and realize that might not be possible. I doubt the latter though....it's certainly not a good assumption to make at the beginning.

How can a theory that can only predict certain things, within a certain domain, and not others be "perfect", exact, and the same thing as reality?
A theory like that is incomplete and not "perfect".
How can a theory be anything but a model and not reality, no matter how precise it's predictions?
How can you truly define reality? I think it objectively exists, but we can only perceive it imperfectly and incompletely.
Our theories are also partial, incomplete, and separate.

Newton's theories may have seemed perfect and complete at the time but we now know they were incomplete, imprecise when entering conditions of extreme velocity or with high gravitational fields. He also quantified gravity but he had no clue as to what it was (and is) or why it exists or how it interacts with other aspects of nature (beyond what he did quantify in his theories). We don't understand what it really is to this day.

No matter how remarkable and useful a theory is, it is just a model and not the same thing as reality.
A theory can model or correspond to reality but it is not the same thing.

We don't understand reality. We cannot currently define exactly what it is. We each also have our own mental (probably mostly visual) conception of what reality is but this is just a fragmentary model we use to construct a provisional, personal model of "reality" so we can perceive the world and live within it.

I suspect that physics and science are still far from complete and much more will be known in the future. Also, there are things that we will never know or understand.

Posted on Jun 29, 2012 10:31:16 PM PDT
Last edited by the author on Jun 29, 2012 10:32:26 PM PDT
When two particles are in an entangled, non-local state and they then travel far apart. What happens to each particle when there is a change to one particle (say it interacts with another particle or decays) or one particle changes it's quantum state? Do they then become un-entangled?

In reply to an earlier post on Jun 30, 2012 6:06:11 AM PDT
Last edited by the author on Jun 30, 2012 6:20:13 AM PDT
It is not clear to me that reality (if there is such a thing) is under any obligation to be either:
1. comprehensible
2. predictable
3. complete

That is, a "perfect" theory of reality could be incomprehensible by us, consistent with more than one future state, and [not] be the only possible reality. It could be all three, and still be isomorphic with reality. We hope the final theory won't be like this--and maybe we'll keep looking until we find one that isn't (or die trying)--but Nature doesn't really pay much attention to our wishful thinking.

In reply to an earlier post on Jun 30, 2012 6:55:26 AM PDT
Last edited by the author on Jun 30, 2012 9:47:46 AM PDT
D. Colasante says:
I would add that we should remember that models belong to US, not mother nature. Thus, if Many Worlds is incomprehensible, we can ignore it, even if Many Worlds is "reality". We are only going to experience one world, so our model can ignore even the possibility of others, for the sake of simplicity.

In reply to an earlier post on Jun 30, 2012 8:22:02 AM PDT
Last edited by the author on Jun 30, 2012 8:40:09 AM PDT
Randall:

I like these points. I would think however that so far things have been fairly comprehensible and predictable (to a point). I don't know if our theories will ever be complete, but think that we should continue to try for that. It is important to keep these things in mind though, otherwise you risk assuming your successful theory or model equals reality.

I think that we are able to comprehend "reality" in part because we and our minds evolved within it and so have to have some correlations with it. mathematics is also not "reality" itself but it is a theoretical study of relationships, quantities, rates of change, etc. All of these are also things that nature does. That's why mathematics works as a tool to understand how the universe works. It isn't the same thing as reality or the universe or the theory of something but it has a correspondence to these things.

In reply to an earlier post on Jun 30, 2012 9:13:14 AM PDT
It may be that the comprehensibility we perceive so far is a selection bias: We like to dwell in the small corner of the world that we understand, and so the world appears comprehensible, in general, just as it seems to me that everybody speaks English. Yes, everyone I *know* speaks english, but that's because if they don't, then I can't know them. This would be true, even if my only tongue was choctaw, but my circle of friends might likely be smaller.

In reply to an earlier post on Jun 30, 2012 9:13:40 AM PDT
I assume you mean by "comprehensible" "conforming to common-sense notions of how things are" as opposed to "able to be formulated in such a way that someone else can also learn it and learn to use it".

In reply to an earlier post on Jun 30, 2012 9:16:44 AM PDT
I believe the answer is "yes". Either you do a measurement on one of the particles at which point its wave function "collapses" to that of a pure state, i.e., an eigenvector of the operator for that observable with the eigenvalue equal to the measured result, whereupon the wave function of other particle automatically collapses also to another such pure state, or by interacting with the environment one gets decoherence.

In reply to an earlier post on Jun 30, 2012 9:17:14 AM PDT
Our minds have a correlation with a very special piece of reality's 'real estate'. For instance, try to imagine what it must be like to live at the average temperature of the universe. Do you have any concept of how cold 2.7 degrees Kelvin is? (I don't.)

In reply to an earlier post on Jun 30, 2012 9:22:31 AM PDT
I don't think I mean to construe 'comprehensible' that narrowly. I would settle for the kind of comprehension we might have of modular functions, Heterotic 26/10 space, or our wives.

In reply to an earlier post on Jun 30, 2012 9:36:54 AM PDT
You mean theories only teachable to and usable by extremely smart individuals? OK. But there have to be at least one such individuals in existence in the Universe (not counting the one who came up with the theory in the first place).

In reply to an earlier post on Jun 30, 2012 9:49:21 AM PDT
Jeff Marzano says:
Randall R Young says:

[pi (part of the expansion of h(bar)) is only known out to 5 trillion decimal places or so]

Would pi still create an infinite number of decimal values if we used 12 as the base for our number system instead of 10 ?

Jeff Marzano

In reply to an earlier post on Jun 30, 2012 1:31:34 PM PDT
Last edited by the author on Jun 30, 2012 1:37:01 PM PDT
Jack Shandy says:
Yes, because pi is irrational. But they are not -decimal- values anymore for the obvious reason.

In reply to an earlier post on Jun 30, 2012 3:23:08 PM PDT
nonsense

change is not time is not change are not equivalent

lock someone up in a hole (like the old alcatraz did)
no light no sound no nothing

time moves on
nothing changes
unless you mean the universal master clock that keeps ticking no matter what

In reply to an earlier post on Jun 30, 2012 3:24:14 PM PDT
it only works in one domain
so it is just a model
not reality which exists over many domains
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Discussion in:  Science forum
Participants:  54
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Initial post:  Aug 27, 2011
Latest post:  Aug 2, 2012

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