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Author Topic: Why should particles be pointlike if one dimensional strings are permitted?  (Read 5629 times)

Offline QuantumClue

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Ok, may seem a very layman question. It is true that physicists today believe that particles have no internal structure which leads to classical problems involving the nature of spin. In order to remove this problem, the particles are not allowed to have what is called a classical angular spin, but instead must be allowed to have an intrinsic property which acts like a spin, and this is known as angular momentum.

If you have a debate with a scientist over any possible structures to electrons for instance, there is a cold stern ''No'' - and the debate usually ends with the scientist telling you this cannot be true. However, we have been slowely comforted into a relatively new idea in physics, and this is string theory, which was first postulated in the 1980's - yes, this is relatively new, even to physicists.

So how come all of a sudden we are being told to appreciate that particles are 1 dimensional extended objects, when particles are left to always be dimensionless?


 

Offline peppercorn

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Sorry, who's returning to the idea that they're point-like?

"...1 dimensional extended objects" - an extended 1 dim. object is 2 dimensional, surely?

'scuse me if I'm missing the obvious...
 

Offline QuantumClue

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Not by my understanding. Anything which has dimensions is an extended object in space and time. A little search on the web shows that this is the type of terminology which is used.
 

Offline QuantumClue

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And I never stated any has returned anywhere with the idea, I am asking why it is not permitted in QM for electrons, but we are being led to believe in one-dimensional strings, seems like one is allowing one thing and not the other.
 

Offline QuantumClue

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http://www.wordiq.com/definition/String_theory

It states that strings are one-dimensional extended objects.
 

Offline Bill S

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Would it not be true to say that a string is considered as a one dimensional object simply because only one "co-ordinate" is needed to specify any position along its length; not because any reasonable person would actually believe that a one dimensional object could actually exist in our three spacial dimensions?
 

Offline QuantumClue

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I agree that no one could truely believe a string exists in three spatial dimensions at once, it could only occupy one of these dimensions at a time. Along the same line of thought, apply that to particles, which are allowed a four momentum.
 

Offline Bill S

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QC, Were you agreeing with what I had just said? If so, perhaps I didn't make my meaning clear.  My point was that even a string must exist in three spatial dimensions in order to be present in the three spatial dimensions which we experience.  Any object that does not exist in all three dimensions is not in fact an object, it is only an idea.  [8D]
 

Offline QuantumClue

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Yes I am agreeing. Even a particle needs to be exist in three spatial dimensions to be present in the three dimensional world we view. However, they are dimensionless, pointlike, and obviously don't have any dimensions about them. Causes some conceptual paradox, one might say.

But of course, any more of my own speculations and this will be sent to the appropriate subforum :)
 

Offline yor_on

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I'm not sure on that one, but I think the idea of allowing one dimensional strings, and 'multi dimensional' branes that to us in SpaceTime only shows one dimension, just as a possibility? is allowed. Remember that the whole idea of 'dimensions' in physics so far have built on three singular 'room property's' criss-crossing each other in all 'points' in SpaceTime, together with the 'dimension' of time.

I have my own views there though, and as far as I think? Well, I haven't seen any two-dimensional systems inside SpaceTime, yet?

==

One thing, when it comes to particles taking 'place' it seems better to define them from their 'confinement' to me, than to say that they are 'there' :) Quantum weirdness huh.
« Last Edit: 09/12/2010 16:59:56 by yor_on »
 

Offline JP

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And I never stated any has returned anywhere with the idea, I am asking why it is not permitted in QM for electrons, but we are being led to believe in one-dimensional strings, seems like one is allowing one thing and not the other.

I think electrons are considered point-like because our best model says they behave that way.  But since the standard model breaks down at the Planck scale, doesn't that mean they could have sub-Planck length structure?  Doesn't string theory deal with sub-Planck length structure?
 

Offline QuantumClue

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And I never stated any has returned anywhere with the idea, I am asking why it is not permitted in QM for electrons, but we are being led to believe in one-dimensional strings, seems like one is allowing one thing and not the other.


I think electrons are considered point-like because our best model says they behave that way.  But since the standard model breaks down at the Planck scale, doesn't that mean they could have sub-Planck length structure?  Doesn't string theory deal with sub-Planck length structure?

Excellent question, I will review this soon. If that be the case, electrons are certainly allowed some substructure.
 

Offline Bill S

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Quote from: JP
But since the standard model breaks down at the Planck scale, doesn't that mean they could have sub-Planck length structure?  Doesn't string theory deal with sub-Planck length structure?

Could it be that the Planck length is the quantum of space, in which case sub-Planck lengths would have no meaning?
Where would that leave string theory?
 

Offline QuantumClue

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Sorry, I've still not looked into this further, I've been really busy. Promise I will though.
 

Offline QuantumClue

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I can't find an answer to that at all. I don't know if sub-planck lengths are permissible or not.
 

Offline Bill S

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Quote
I don't know if sub-planck lengths are permissible or not.

Could that be because no-one really knows?
 

Offline JP

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I don't know if sub-planck lengths are permissible or not.

Could that be because no-one really knows?

No one really does know, but different theories have different descriptions of what happens on that scale.  The question I was asking was whether string theory says that sub-planck lengths were physically meaningful.
 

Offline Geezer

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Quote
I don't know if sub-planck lengths are permissible or not.

Could that be because no-one really knows?

No one really does know, but different theories have different descriptions of what happens on that scale.  The question I was asking was whether string theory says that sub-planck lengths were physically meaningful.

It's been a while, but I think that was the general idea. The additional dimensions are only apparent on sub-planck scales.
 

Offline JP

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That's what I vaguely recall from pop-sci accounts too, Geezer.  Quantum loop gravity, which is a different technique for getting gravity and QM to work together, quantizes space itself, so I think within that theory sub-planck length scales might be meaningless.

And of course, no one really knows if one of these theories is right.
 

Offline peppercorn

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And of course, no one really knows if one of these theories is right.

But I think it should be mentioned that some theories have more validity (are more likely to be right and offer predictions that if not testable today have some hope of becoming so) than others.  I feel we should add this before we hear "Well, if no one knows then my [ill-thought-out half-baked] 'theory' is as good as any other" ... Just saying ::)
 

Offline JP

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True, peppercorn.  String theory and quantum loop gravity are theories that match observations and have make quantitative predictions that are hopefully testable in the future.
 

Offline Bill S

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Quote from: Geezer
The additional dimensions are only apparent on sub-planck scales.

Does that mean that these theoretical quanta of space are sub-Planck in size, and that the additional dimensions are "rolled up" to sub-Planck sizes?
 

Offline yor_on

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JP, I think you may like this one?
Check up "Three roads to quantum gravity" by Lee Smolin.

In it he states that there is one simple way of defining a smallest 'meaningful' string length. It's by changing Heisenberg's uncertainty principle slightly. Normally you can make the uncertainty in position arbitrarily small by making the uncertainty of its 'motion' arbitrarily large. But by inserting a term regulating this you can't assume that any longer, as when you do the new term in the equation comes in and 'forces' the uncertainty in position to grow instead of diminish. The result is that you will have a 'smallest value' for the uncertainty of a position. I'm sorry, I had to translate this from the Swedish version so I can't give you the equation itself but you'll find in the book.

As for how true they will be?
That's a interesting question, and may change the way we look at mathematics.
 

Offline yor_on

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And no, not as I know? A point like particle may have a confinement, but no defined position? Ah? As I understands it. It's also a 'probability wave' wherein you have a highest probability of that electron 'existing' at some x,y point of it. We know it must have a 'confinement', otherwise we wouldn't have what we call matter as that needs confinements placed aside each other in 3-D geometry.

As for how a one-dimensional surface, or string would look? I don't know, as you say it is strange to call something 'stretching out' one-dimensional. But I like the idea of 'properties' though. The particles own 'peculiarities' and 'affirmations' telling us that they are 'there'. It would have been easier though if they really had the same properties we see macroscopically.
 

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