Quantum P and GR

[xelasnave]

As a casual reader it is hard to understand the. "conflict" between QPhysics and General Relativity so I read what I can find.
One thing I came across gave me the impression Quantum Physics seems to suggest (at least in the case of the hypothised graviton that thijg can happen instantly which for me raises the question "isn't that wrong nothing can exceedC."..
Moreover why can't the two be reconcilled.
Does each camp accept the premises of the other and they simply can't satisfactorily link the two..?

[bojan]

They can't be "linked" just like that because completely different math is used to describe each one of them..
It's a bit like digital and analogue electronics... Digital deals with discrete levels (one's and zero's) while analogue electronics deals with any voltage in between and out...
In this crude example the link of course exists (currents and voltages)... but I can tell you from my experience that not many "digital" guys will understand analogue electronics at all .. it is "black magic" for them .
Which is very good for us, "analogue" people

[CraigS]

Hmm .. there's a lot more to this than meets the eye.

The fundamental difference between the two is 'background independence' or 'universality' which in turn, stems from a philosophical difference. There has been heaps written on this and the philosophical debate still rages.

Quantum Gravity has been attempting to unite it all for years .. unsuccessfully. String theory comes closest.

Don't ask me to distinguish the philosophical differences though … deep stuff.

Cheers

[xelasnave]

Thanks for that I was starting to think it was just me.At one level they seem to agree but of course the material I cover is like a documentary where a complex topifvgets 45 minutes to cover the lifetime work of many scientists.
Actually some of the discussion you guys have becomes so informative and helpful as you somehow give more sides to matters unapparent.
The complexity of it all is stagering...I gave up on advanced math support after going to a math site and finding the index was book lenght let alone the material...
I find the prospect of instant comunication difficult to accept one would think time must elapse between particle interaction. .even if they eant faster than C surely it can't be instant...mmm. only one entity can do that...oh no Better revise my atheiest stand. .
Craig why don't you throw some more light on the background irrespective of its speed.
alex

[sjastro]

Alex,

The inability of combining QM and GR is a scale related issue and has nothing to do with instantaneous interactions.

QFT (Quantum Field Theory) is a relativistic QM theory.
In the development of QFT the main issues encountered by theorists was that by combining QM and SR (not GR) produced a couple of very strange calculations.

Firstly since QM is a theory based on probabilities, the probability P of a particle being in a particular state is in the range 0<= P<=1.
In QFT however probabilities could have negative values which is of course an absurdity.
Secondly in QFT particles could exist in negative energy levels which is difficult to interpret in QM.

Unfortunately this where a knowledge of mathematics comes into play on how theorists overcame these hurdles by incorporating concepts such as second quantization where a field could be quantized like the characteristics of particles describing energy, angular momentum etc.

Instantaneous travel in QFT is meaningless. It's like trying to explain the double slit experiment in terms of a photon or particle being at both slits simultaneously due to instantaneous travel rather than the Heisenberg uncertainty principle at work, or the photon or particle wavefunction collapsing when an observer makes a measurement. A graviton should it exist will operate on the same principle.

Regards

Steven

[CraigS]

Hmm .. Steven .. just been doing some reading up … it seems that a basic difference between GR and QM/QFT is that GR theory is not dependent on any preferred type of space-time (ie: no frame of reference is 'special' over another), hence it is regarded as a 'background independent' theory.

In QM/QFT, however, the properties of the elementary particles have to do with their relationships to the state of the 'background' vacuum. Ie: the vacuum has different states which ultimately gives rise to the different properties of the elementary particles: ie: mass, position, momentum, spin, and charge.
Hence QFT is called a 'background dependent theory'.

If so, I think I can get this ..

Cheers

[xelasnave]

Thank Steven that was excelent. Again I focused on something that confused me.
But thanks to you all of you I am trying to understand it all without trying to rewrite anything...but its not easy for me. I looked into getting a math tutor but gave up on the idea so it is a frustration to have these interests and no tools so to speak. You would have noticed the thread re the trcking in the double slit..that was very interesting.
alex

[sjastro]

Originally Posted by CraigS

Hmm .. Steven .. just been doing some reading up … it seems that a basic difference between GR and QM/QFT is that GR theory is not dependent on any preferred type of space-time (ie: no frame of reference is 'special' over another), hence it is regarded as a 'background independent' theory.

In QM/QFT, however, the properties of the elementary particles have to do with their relationships to the state of the 'background' vacuum. Ie: the vacuum has different states which ultimately gives rise to the different properties of the elementary particles: ie: mass, position, momentum, spin, and charge.
Hence QFT is called a 'background dependent theory'.

If so, I think I can get this ..

Cheers

Craig,

The vacuum is a field in the lowest or ground state. There are different types of vacuums, such as the quantum electrodynamic (QED) vacuum or the quantum chromodynamic (QED) vacuum.

The issue with gravity as a Quantum Field theory is that it is non- renormalizable. A gravitational interaction between 2 particles changes the particles from a state A to a state B. If you sum all the momenta going from state A to B you end up with infinite values. This is prevalent at the Planck scale. There is no way you can renormalize or "cut out" the infinite terms as you can with QED or QCD.

This is where string theory is an advantage. By making your strings larger than the Planck scale, the infinities are averted.

Regards

Steven

[sjastro]

Originally Posted by xelasnave

Thank Steven that was excelent. Again I focused on something that confused me.
But thanks to you all of you I am trying to understand it all without trying to rewrite anything...but its not easy for me. I looked into getting a math tutor but gave up on the idea so it is a frustration to have these interests and no tools so to speak. You would have noticed the thread re the trcking in the double slit..that was very interesting.
alex

No problems Alex.

The maths behind QFT is very difficult.
Maths for GR on the other hand is simpler and much more intuitive.

Regards

Steven

PS Here is Richard Feynman who won the Nobel prize for his work on QED.
It's encouraging that QED is also difficult to a genius.

http://www.youtube.com/watch?v=iMDTcMD6pOw

[CraigS]

Originally Posted by sjastro

Craig,

The vacuum is a field in the lowest or ground state. There are different types of vacuums, such as the quantum electrodynamic (QED) vacuum or the quantum chromodynamic (QED) vacuum.

The issue with gravity as a Quantum Field theory is that it is non- renormalizable. A gravitational interaction between 2 particles changes the particles from a state A to a state B. If you sum all the momenta going from state A to B you end up with infinite values. This is prevalent at the Planck scale. There is no way you can renormalize or "cut out" the infinite terms as you can with QED or QCD.

This is where string theory is an advantage. By making your strings larger than the Planck scale, the infinities are averted.

Regards

Steven

Thanks Steven …
More reading to do on all this I'm afraid ..

Cheers

[Zaps]

Originally Posted by xelasnave

One thing I came across gave me the impression Quantum Physics seems to suggest (at least in the case of the hypothised graviton that thijg can happen instantly which for me raises the question "isn't that wrong nothing can exceedC.".

That's "action at a distance". There is no conflict or contradiction.

[xelasnave]

E

Originally Posted by Zaps

That's "action at a distance". There is no conflict or contradiction.

Thanks very much for this link. I am only half way thru but the math was totally understandable because it showed what each input symbol represents.
I am at chapter 7 which covers specificaly what concerned me.
I have not read any of the references but I will select some on my second reading.
alex

[xelasnave]

I did find..."many believe relativity per se prohibits transport of matter-energy and or action at a distance"...there is another view presented after that but that is what
I was interested in mostly.

I find this just like reding a High Court judgement...it seems to labour upon things one would never think twice about...and many references to associated law and "dicta" of other judges. And do we ever get to know exactly what a particle is doing or is it all probability.
And that poor cat in the box. ..is it probably dead...and. if so is it dead on the bottom of the box...it could be stuck on the ceiling I guess....my attempt at humour.
I will read it again and some of the references ..thanks something I can get my teeth into.
Alex

[CraigS]

Originally Posted by xelasnave

And do we ever get to know exactly what a particle is doing or is it all probability.

Y'know Alex .. that's a very interesting question. I don't know myself.

A few articles have appeared lately that have thrown me when it comes to this question … here's a thread about 'The Shape of an Electron' .. one of the articles in the thread says ..

the electron is a perfect sphere, give or take barely one part in a million billion.
...
The result comes from the latest in a long line of experiments to probe the shape of the fundamental particle that carries electrical charge.

… so how is it that they can measure accurately the shape of an electron, at this degree of precision, and yet the Heisenberg Uncertainty Principle is frequently cited as a reason why such precision cannot be achieved ? Clearly, it can be achieved ! What purpose does the Uncertain Principle serve in this case ? Is it as relevant as it once was ?

And I've been pondering another one … Startling thermal energy behavior revealed by neutron scattering.
.. in this one, they have found that the vibrations of atoms in a crystal lattice in thermal equilibrium, (the vibrations are indicative of thermal energy), self-organise into discrete packets, called intrinsic localized modes (ILMs) that break the symmetry of the crystal. They found that the ILMs self-organised according to a regular pattern. Whilst this is indicative of a chaotic phenomenon forming fractal patterns of both regularity and irregularity, (which is no great surprise to yours truly), my question is: How do they measure this kind of stuff when QM is telling us that its all a matter of probability ? This particular test made use of neutron scattering to collect the vibration data of atoms trapped within the lattice structures.

Another recent one is: Quantum physics first: Researchers observe single photons in two-slit interferometer experiment.
In this, they have developed a way to make momentum and distance measurements on photons which may ultimately lead to tracing their paths backwards to the specific slit they went through.

So I'm starting to wonder just how much use are these QM Principles going forward in such research ? Just about every day, I find that some scientists have put these 'guidelines' (ie: QM 'guiding principles') aside, and measured very small particles, such that very useful insights can be obtained.

What physical applicability do these principles have, if they are simply guiding researchers by challenging them to find ways around them ?

Are we not picking up on the message that these QM Principles are not quite the deterrent they once used to be and hence, are they applicable guidelines for researchers, going forwards ?

Interesting questions …

Cheers

[xelasnave]

I am glad you asked Craig as I can answer all your questions very simply...I have no idea...
From my view the uncertainty principle pointed out an exception to the rule but in effect the. exception became the rule. The rcent double slit experiment suggests to me others think similar..they look where they expect is my simple view.
I am still trying to understand how some of this counter intuitive stuff gets a run but I guess the way things work is state a premise and prove it with math if that works the idea will be observed in experiment.
I find it difficult to accept because if a premise appears counter intuitive any subsequent proof won't satisfy me.
I amnot sure how the things you ask about are worked out..maybe a computer model and it linked to an experiment that suggests the model can be relied upon and the model is where measurement is made.As I said I don't know....unless we introduce an eather that is...
alex

[xelasnave]

Thanks for the links a I will read them with interest

[sjastro]

Originally Posted by CraigS

Y'know Alex .. that's a very interesting question. I don't know myself.

A few articles have appeared lately that have thrown me when it comes to this question … here's a thread about 'The Shape of an Electron' .. one of the articles in the thread says ..


… so how is it that they can measure accurately the shape of an electron, at this degree of precision, and yet the Heisenberg Uncertainty Principle is frequently cited as a reason why such precision cannot be achieved ? Clearly, it can be achieved ! What purpose does the Uncertain Principle serve in this case ? Is it as relevant as it once was ?

They not measuring a shape. What they are trying to do is measure an electron's dipole moment. The more distorted the shape, the greater the dipole moment. If an electron is perfectly spherical the electron dipole moment is zero.

And I've been pondering another one … Startling thermal energy behavior revealed by neutron scattering.
.. in this one, they have found that the vibrations of atoms in a crystal lattice in thermal equilibrium, (the vibrations are indicative of thermal energy), self-organise into discrete packets, called intrinsic localized modes (ILMs) that break the symmetry of the crystal. They found that the ILMs self-organised according to a regular pattern. Whilst this is indicative of a chaotic phenomenon forming fractal patterns of both regularity and irregularity, (which is no great surprise to yours truly), my question is: How do they measure this kind of stuff when QM is telling us that its all a matter of probability ? This particular test made use of neutron scattering to collect the vibration data of atoms trapped within the lattice structures.[

Hmmm if the size of the crystal is greater than it's Compton wavelength, then the measurements are entirely predictable and not quantum mechanical.

Another recent one is: Quantum physics first: Researchers observe single photons in two-slit interferometer experiment.
In this, they have developed a way to make momentum and distance measurements on photons which may ultimately lead to tracing their paths backwards to the specific slit they went through.

Interestingly enough the reasercher's themselves are not claiming to violate the uncertainty principle.

Steinberg stresses that his group's work does not challenge the uncertainty principle, pointing out that the results could, in principle, be predicted with standard quantum mechanics. But, he says, "it is not necessary to interpret the uncertainty principle as rigidly as we are often taught to do", arguing that other interpretations of quantum mechanics, such as the pilot-wave theory, might "help us to think in new ways".

What the experiment does challenge is the Copenhagen interpretation of QM, where a wavefunction collapses when a measurement is performed.

Regards

Steven

[renormalised (Carl)]

I might have a look at your replies, Steven, tomorrow. Getting a bit bleary eyed here

The last one has quite a few consequence for QM and many of its offshoots...one in particular, the many worlds interpretation. I think you'll know where I'm heading with this one

[CraigS]

Thanks Steven;
I don't necessarily see that any of these 'violate' the Uncertainty principle. I don't think any of them do, as they all seem to make use of indirect measurements.
The observation I'm querying here is that the Uncertainty Principle was for many years, an absolute in QM. These researches don't seem to accept it as such, and in fact are using it as a challenge to push measurement technologies beyond any perceived limits.
If this is an accurate observation, then I'm left wondering what purpose does the Uncertainty principle serve in doing this research ?

Originally Posted by sjastro

They not measuring a shape. What they are trying to do is measure an electron's dipole moment. The more distorted the shape, the greater the dipole moment. If an electron is perfectly spherical the electron dipole moment is zero.

Yep .. fair enough .. the Scientific American article says that an electron actually drags a cloud of virtual particles around. As the electron whizzes around, it drags this particle cloud, which ultimately results in an imbalance of the dipole magnetic moment. (It seems the cloud is considered to be part of the 'electron'). If they find this cloud is deformed, then they say …

The discovery would show that time is fundamentally asymmetrical, and could prompt an overhaul of the 'standard model' of particle physics.

… so, there are a few caveats in there, and until they can uncover the asymmetry, all is good, but my point is that the absoluteness of the Uncertainty principle is clearly not serving as a deterrent factor for these researchers. So what role is it serving in this case ?

Originally Posted by sjastro

Hmmm if the size of the crystal is greater than it's Compton wavelength, then the measurements are entirely predictable and not quantum mechanical.

They seem to be careful in pointing out that the vibrations they're measuring are at the atomic level and are independent of the crystal lattice (??)

Originally Posted by sjastro

Interestingly enough the reasercher's themselves are not claiming to violate the uncertainty principle.

Yep and that point should be made very clear, also.

Originally Posted by sjastro

What the experiment does challenge is the Copenhagen interpretation of QM, where a wavefunction collapses when a measurement is performed.

Hmmm … so there's 'interpretation' of QM coming into this ? Does this mean that the Uncertainty principle is also an interpretation of something ? I'm not challenging the Uncertainty principle here .. I'm really just challenging my understanding of it and other such principles in QM, as well as the role it plays in guiding research.

There's nothing 'wrong' with QM … there's only (as always) something wrong with my understanding.

Cheers

[xelasnave]

The uncertainty principle is opperational in my quantum mechanics and QFT as I am uncertain about all of it...not its validity but my comprehension of what I read.
And by the way I have no problemwith an answer being the result of probability if it came across that way.
I can't wait to read views re. "other worlds".
alex