Understanding Quantum Theory: A Beginner's Guide to Particle Physics

[GreatBigBore]

I have a particle physics question. I know I'm in the wrong forum, but when I posted this to the quantum forum, it got lost because of all the students asking for help with their physics homework. Can anyone point me to a book or article or youtube video or some resource that could help me with this? I don't have a formal physics background, so I need a "For Dummies" version.

I'm watching a youtube video (watch?v=uNOghidK2TY) of an interview with Feynman. He says, "...there are a number of paradoxes with this quark picture...we've done some experiments at very high energy, hitting a proton with an electron...that can only be interpreted by supposing that the number of particles inside is really infinite. If there are particles inside it can't be done with just three." He goes on to say, "...the idea that there would be just three particles [quarks] is contradictory to the laws of relativity."

I thought that I knew a bit about physics, but obviously I'm in the dark. Could someone please point me to some books (that don't require a physics degree) that could shed some light on these statements by Feynman? Thanks in advance.

 

[Chalnoth]

Well, I'm not sure of any really good references of this, but the answer to the conundrum lies in the fact that protons aren't just 3 quarks: they're three quarks in a bound state connected by extremely high-energy gluons (gluons are the carriers of the strong force). These high-energy gluons are so high in energy that they produce quark-antiquark pairs. These quark-antiquark pairs pop out of existence as rapidly as they appear, but at any given time there can be a tremendous number of them within the proton. So when you strike the proton with an electron, you may be hitting a particle with just 3 quarks or 3 trillion. It all depends upon how many quark-antiquark pairs are present when the electron passes through.

 

[GreatBigBore]

...the answer to the conundrum lies in the fact that protons aren't just 3 quarks...

So are you saying that the basic problem is that we know more now than Feynman did, and his paradoxes are now solved?

 

[Chalnoth]

So are you saying that the basic problem is that we know more now than Feynman did, and his paradoxes are now solved?

Not in this case. He was basically one of the main people figuring this whole thing out. As far as the infinities are concerned, he was probably talking about the fact that quantum field theory has a number of infinities that have to be "swept under the rug", so to speak.

We generally deal with them by cutting off our integrals at some high energy, placing the value of the rest of the integral into some small set of parameters that must be measured experimentally, and then showing that the end result is independent of which particular energy cutoff we choose. This process is known as "renormalization". Put another way, it's a process of not making calculations in the regime that we don't know (high energies), and just experimentally measuring those little bits of the computations that we don't yet know how to make.

In the end it means that there's a part of the behavior of quantum systems that we simply don't understand, because we don't know what goes on at these high energies. Obviously one of the things that would be really really nice for a new theory of high-energy physics would be to predict from first principles these parameters that we now measure experimentally.

 

[marcus]

...
I thought that I knew a bit about physics, but obviously I'm in the dark. Could someone please point me to some books (that don't require a physics degree) that could shed some light on these statements by Feynman? Thanks in advance.

The Nobelist whose books or videos you want is Frank Wilczek of MIT. Feynman got the prize for QED. Wilczek got it in 2004 for QCD.
He has several very good online video lectures (about one hour each). You can find the links at his MIT faculty website.
A good talk is called the origin of mass. He also has a wide-audience essay online with a similar title that parallels the online video to some extent.

He also came out with a book last year called The Lightness of Being, which is intuitive and non-mathematical, for wide audience. It tries to give an understanding of QCD, and the origin of mass, and his vision of the future of particle physics depending on various results from the LHC, as it starts to produce data.

Quarks interact by chromo dynamics. The color charge rather than the electric charge of electrodynamics. How a proton holds together and what goes on inside the proton---the sea of pairs, (anti)screening, condensates, what Chalnoth was talking about---is described by QCD, wantum chromodynamics.

The key insight was in 1976 when Wilczek was a grad student at Princeton. Interestingly, there were FOUR people involved and the rules of the Nobel only allow the prize to be split THREE ways. So this technicality delayed the award (which everybody knew was due) until one of the four (a dutchman named Gerard 't Hooft) happened to be nobelled for some other work he'd done on something else, and it seemed fair that the other three should get it (in 2004) for QCD.

The key experimental signature, from collider rings, was JETS. A certain shape of *splat* that happens when a collision tries to knock quarks apart. The binding force between two quarks becomes stronger the farther apart. and blasting them apart creates an angry swarm of quark/antiquark which spurts out from the point of collision in jets of decay product particles. One never sees the actual swarm of devils, only what they immediately convert into. So there is a dramatic jet of debris, which is captured by giant banks of detectors, and which allows calculations which confirm QCD.

Wilczek is an extremely good speaker in these video lectures. See if you can google and find his online stuff. Or get your local public or college library to get the book in.

 

[DevilsAvocado]

Hopefully this works as a good start:

http://en.wikipedia.org/wiki/Quantum_chromodynamics"
http://en.wikipedia.org/wiki/Quantum_field_theory"
http://en.wikipedia.org/wiki/Standard_Model"

 

[Chronos]

Quarks are very shy creatures. They tend not to persist in nakedness.

 

[DevilsAvocado]

Since we are discussing QM – Can anyone explain the difference between the Graviton and the Higgs boson (waiting at LHC for 'attention')?

 

[Chalnoth]

Since we are discussing QM – Can anyone explain the difference between the Graviton and the Higgs boson (waiting at LHC for 'attention')?

The graviton is the mediator of the gravitational force. It is a spin-2 massless boson which couples (very weakly) to stress-energy. Any gravitational interaction can be thought of as being mediated by an exchange of a typically very large number of gravitons. The graviton is to the gravitational force as the photon is to the electromagnetic force, in other words.

The Higgs boson is a quantum of the Higgs field. The Higgs field is proposed as a field which takes a particular value at all points in space, and interacts with matter in such a way that it appears to give various particles different amounts of mass (depending upon how they interact with this Higgs field). Basically, in quantum field theory, if you attempt to give particles intrinsic mass, you arrive at a contradiction. So the masses of particles must arise from interactions. The Higgs field provides an interaction that could give this exact effect.

 

[DrChinese]

The Nobelist whose books or videos you want is Frank Wilczek of MIT. Feynman got the prize for QED. Wilczek got it in 2004 for QCD.
He has several very good online video lectures (about one hour each). You can find the links at his MIT faculty website.
A good talk is called the origin of mass. He also has a wide-audience essay online with a similar title that parallels the online video to some extent.

He also came out with a book last year called The Lightness of Being, which is intuitive and non-mathematical, for wide audience. It tries to give an understanding of QCD, and the origin of mass, and his vision of the future of particle physics depending on various results from the LHC, as it starts to produce data.

Quarks interact by chromo dynamics. The color charge rather than the electric charge of electrodynamics. How a proton holds together and what goes on inside the proton---the sea of pairs, (anti)screening, condensates, what Chalnoth was talking about---is described by QCD, wantum chromodynamics.

The key insight was in 1976 when Wilczek was a grad student at Princeton. Interestingly, there were FOUR people involved and the rules of the Nobel only allow the prize to be split THREE ways. So this technicality delayed the award (which everybody knew was due) until one of the four (a dutchman named Gerard 't Hooft) happened to be nobelled for some other work he'd done on something else, and it seemed fair that the other three should get it (in 2004) for QCD.

The key experimental signature, from collider rings, was JETS. A certain shape of *splat* that happens when a collision tries to knock quarks apart. The binding force between two quarks becomes stronger the farther apart. and blasting them apart creates an angry swarm of quark/antiquark which spurts out from the point of collision in jets of decay product particles. One never sees the actual swarm of devils, only what they immediately convert into. So there is a dramatic jet of debris, which is captured by giant banks of detectors, and which allows calculations which confirm QCD.

Wilczek is an extremely good speaker in these video lectures. See if you can google and find his online stuff. Or get your local public or college library to get the book in.

Marcus, I found these:

Origin of Mass:

The LHC and Unified Field Theory - Frank Wilczek (1 of 8):

 

[DevilsAvocado]

The graviton is the mediator of the gravitational force.
...
The Higgs boson is a quantum of the Higgs field.
...

Thanks a lot for the explanation Chalnoth.

So the Higgs field 'provides' the mass for particles, and the graviton then mediates the gravity force, corresponding to the mass ('given' by Higgs), to other particles, like: "Hey guys look at me! I got mass from Higgs!"

Correct?

But how is QCD compatible to the Higgs field? I looked at http://www.youtube.com/watch?v=ECkG_JdodMA" (thanks DrChinese!) and Frank Wilczek is showing that mass is the result of quantum fluctuations inside the nucleons?

(QCD seems very cool, baryons is only 3 'RGB quarks'... I know what that is, I have RGB on my computer! )

And how is all this compatible to GR? Gravity Probe B has shown (2007) that gravity is spacetime curvature??

AND what happens with all this, if the Higgs boson is discovered at LHC this year...!?

 

[Chalnoth]

So the Higgs field 'provides' the mass for particles, and the graviton then mediates the gravity force, corresponding to the mass ('given' by Higgs), to other particles, like: "Hey guys look at me! I got mass from Higgs!"

Correct?

Well, sort of, but gravity doesn't just couple to mass. It couples to any energy, as well as pressure, stress (by the way, if you twist an object, you're inducing stress), and momentum.

And the mass of an object can be understood as the total energy in the internal degrees of freedom. So if you have a box, and you raise its temperature, you're increasing its mass (which means it becomes both harder to accelerate and have a larger gravitational field).

The Higgs is just one sort of interaction that raises the energy of a particle in such a way that it acts as sort of an "internal" degree of freedom.

But how is QCD compatible to the Higgs field? I looked at http://www.youtube.com/watch?v=ECkG_JdodMA" (thanks DrChinese!) and Frank Wilczek is showing that mass is the result of quantum fluctuations inside the nucleons?

Hopefully the above answered your question, but the point here is that the mass of an object is the energy in all of the internal degrees of freedom. The Higgs interaction is but one part of that. For a nucleon, for instance, the individual quarks have masses of just a few MeV, while the nucleon itself has a total mass of around 930MeV. So as you can see, most of the mass of a nucleon stems from the strong force interactions, and not from the Higgs interactions with the quarks.

And how is all this compatible to GR? Gravity Probe B has shown (2007) that gravity is spacetime curvature??

This we don't completely know, because we don't yet have a complete theory of quantum gravity. But we expect that the collective action of gravitons is what is interpreted as the curvature of space-time in General Relativity.

AND what happens with all this, if the Higgs boson is discovered at LHC this year...!?

Then we'll learn a lot more about the properties of quantum mechanics, and hopefully get some idea as to where to go next (by the way, this will be the case whether or not we find the Higgs).

 

[DevilsAvocado]

Well, sort of, but gravity doesn't just couple to mass. It couples to any energy, as well as pressure, stress (by the way, if you twist an object, you're inducing stress), and momentum.

And the mass of an object can be understood as the total energy in the internal degrees of freedom. So if you have a box, and you raise its temperature, you're increasing its mass (which means it becomes both harder to accelerate and have a larger gravitational field).

Very interesting, this is completely new to me. Acceleration yes, but the rest is news. So, in a body 'on the way' to a singularity, some of the gravitation comes from the concentrated mass, and some from the concentrated heat?

So as you can see, most of the mass of a nucleon stems from the strong force interactions, and not from the Higgs interactions with the quarks.

So why do we need the Higgs field? I heard that 90% of the mass in nucleons comes from quantum fluctuation (virtual particles popping in and out all the time), and nucleons makes all the mass of the ordinary matter (fermions).

This we don't completely know, because we don't yet have a complete theory of quantum gravity. But we expect that the collective action of gravitons is what is interpreted as the curvature of space-time in General Relativity.

This is exciting times we live in! And I am so happy that I just found PF to have someone to discuss this 'matter' with! GR + LHC + Higgs boson + QCD + QG + DM + DE ≈ TOE, Cool!

When I think (= pure speculations) about the difference between gravitons and curvature of space-time, the gravitons seems to work like a 'magnet' – if you are 'nonmagnetic' you can walk thru this field without even notice it’s there, but if you are 'magnetic' the gravitons will start drag you.

And the curvature of space-time doesn’t care about 'magnetism'; it drags everything, even light, since there is no 'other way' to go...

Is this even close...? Or not even GNORW...?

 

[Chalnoth]

Very interesting, this is completely new to me. Acceleration yes, but the rest is news. So, in a body 'on the way' to a singularity, some of the gravitation comes from the concentrated mass, and some from the concentrated heat?

Well, you'd need relativistic levels of heat, which isn't really reasonable (relativistic temperatures mean that the typical energy of a particle is greater than its rest mass, which means that all matter behaves like radiation when it achieves such temperatures). But for a neutron star, for instance, the pressure is so large that an appreciable part of the gravity of a neutron star is generated by the pressure.

So why do we need the Higgs field? I heard that 90% of the mass in nucleons comes from quantum fluctuation (virtual particles popping in and out all the time), and nucleons makes all the mass of the ordinary matter (fermions).

Well, first, nucleons don't constitute all of the matter out there. Second, 90% is not all.

When I think (= pure speculations) about the difference between gravitons and curvature of space-time, the gravitons seems to work like a 'magnet' – if you are 'nonmagnetic' you can walk thru this field without even notice it’s there, but if you are 'magnetic' the gravitons will start drag you.

And the curvature of space-time doesn’t care about 'magnetism'; it drags everything, even light, since there is no 'other way' to go...

Is this even close...? Or not even GNORW...?

I honestly have no idea what you're talking about.

 

[DevilsAvocado]

... I honestly have no idea what you're talking about.

Well, neither do I ...

I we look at electromagnetism, the electromagnetic force operates via the exchange of (virtual) photons. If you put a wooden stick inside an electromagnetic field, not much happens. If you put an iron stick inside an electromagnetic field, the stick gets influenced by the field.

My pure personal speculation is that – the gravitons works in a similar way as the (virtual) photons in electromagnetism, as the carrier of the force.

Then, if science already knows how the gravitons bend the curvature of space-time, there’s nothing to discuss.

If not, there seems to be at least one question to solve: If we put a laser beam near a massive object, using gravitons to 'produce' the gravity – the laser beam would continue perfectly straight.

In GR (curvature of space-time), the laser beam would bend slightly towards the massive object, right?

Well, as I said... I’m not sure what the h**l I’m talking about...

Edit: (I.e. wooden stick = laser beam, hard to influence via the exchange of particles, easy to influence via curvature of space-time...)

 

[Chalnoth]

My pure personal speculation is that – the gravitons works in a similar way as the (virtual) photons in electromagnetism, as the carrier of the force.

Well, yes, this is correct.

Then, if science already knows how the gravitons bend the curvature of space-time, there’s nothing to discuss.

Basically, a spin-2 massless field produces field equations that look like GR in the classical limit.

If not, there seems to be at least one question to solve: If we put a laser beam near a massive object, using gravitons to 'produce' the gravity – the laser beam would continue perfectly straight.

In GR (curvature of space-time), the laser beam would bend slightly towards the massive object, right?

It bends slightly. This has been tested (not with lasers, but with starlight passing near massive objects like our Sun).

 

[DevilsAvocado]

Basically, a spin-2 massless field produces field equations that look like GR in the classical limit.

Thanks Chalnoth.

So would you say that basically, QM/QG is compatible to GR, except for the 'chaotic mess' appearing at extreme energies, like the Planck period at BB?

Edit: Stupid question. QM ≈ GR and QG is not completed, yet. Sorry...

 

[jtbell]

In GR (curvature of space-time), the laser beam would bend slightly towards the massive object, right?

Yes. The deflection of light by the sun was observed as far back as 1919.

 

[Chalnoth]

Thanks Chalnoth.

So would you say that basically, QM/QG is compatible to GR, except for the 'chaotic mess' appearing at extreme energies, like the Planck period at BB?

Well, no, there are some quite fundamental problems reconciling the two. The basic issue is that when you attempt the simplest sort of quantum gravity, you end up with a theory that isn't renormalizable. That is, the process of sweeping the infinities under the rug that works well in QCD and QED doesn't work: when you cutoff your integrals at some point, the result depends critically upon what energy you choose to cut them off at.

On the other side, if you just try to interpret quantum mechanics in terms of GR, there is no unique way to determine how to write down the gravitational field for something that is in a superposition of different energy states.

Currently there are two proposals for an actual theory of quantum gravity. On the one side, we have String Theory, which is particularly compelling because it predicts quantum gravity. String Theory might be thought of as a theory of quantum mechanics beyond the standard model which necessarily produces gravity as well. On the other side, we have Loop Quantum Gravity, which is an attempt to approach the problem more from the General Relativity side in order to ask the question as to how GR can be quantized so as to give a stable theory of gravity.

 

[DevilsAvocado]

Yes. The deflection of light by the sun was observed as far back as 1919.

Yes, I was aware of that. The exiting thing was – has anybody 'seen' gravitons producing this kind of 'space bending'? And Chalnoth gave the explanation.

Thanks anyway.

 

[DevilsAvocado]

Well, no, there are some quite fundamental problems reconciling the two. ...

Thanks again.
Maybe it’s luck that I don’t work with these things (as it probably would drive me crazy ).

This is so extremely weird... almost as if there are two separate worlds, the macroscopic and the microscopic...

Hasn’t anyone tried to build a 'bridge' between the two, to see what happens? Like extremely large fullerenes, or something else? How about running the EPR paradox with objects on the 'border' to the QM world...

Or is it just impossible...

 

[ZapperZ]

Thanks again.
Maybe it’s luck that I don’t work with these things (as it probably would drive me crazy ).

This is so extremely weird... almost as if there are two separate worlds, the macroscopic and the microscopic...

Hasn’t anyone tried to build a 'bridge' between the two, to see what happens? Like extremely large fullerenes, or something else? How about running the EPR paradox with objects on the 'border' to the QM world...

Or is it just impossible...

Read Adrian Cho's review in this week's Science (Jan 29, 2010) on "quantum machines" that make use of macroscopic quantum oscillators.

Zz.

 

[Chalnoth]

Thanks again.
Maybe it’s luck that I don’t work with these things (as it probably would drive me crazy ).

This is so extremely weird... almost as if there are two separate worlds, the macroscopic and the microscopic...

Hasn’t anyone tried to build a 'bridge' between the two, to see what happens? Like extremely large fullerenes, or something else? How about running the EPR paradox with objects on the 'border' to the QM world...

Or is it just impossible...

Well, yes. The fundamental problem, I think, is the difference in strength between gravity and the other forces. Gravity is some 10^40 times weaker than electricity and magnetism. This tremendous difference means that it is nigh impossible to measure how gravity behaves at short distances.

Similarly, the other forces have two (or more) charges, and thus tend to be masked a large distances (e.g. electrical charges come in positive and negative values, and the strength of the electromagnetic force ensures that nothing becomes strongly charged). Since gravity has no such cancellation (more mass just adds to the gravitational field), it becomes the dominant force at large distances.

Because of this large disparity in the areas where the different forces apply, it's just incredibly difficult to measure the region of overlap. And it doesn't help that the mathematical principles used in one area tend to break down when used in the other. We naturally expect that gravity and the other forces are just different aspects of the same underlying fundamental theory, but it has proven to be very difficult to discover what that underlying theory is.

 

[DevilsAvocado]

Read Adrian Cho's review in this week's Science (Jan 29, 2010) on "quantum machines" that make use of macroscopic quantum oscillators.

Zz.

Thanks Zz, very interesting. This made me check out some more along this path. See next post.

 

[DevilsAvocado]

... We naturally expect that gravity and the other forces are just different aspects of the same underlying fundamental theory, but it has proven to be very difficult to discover what that underlying theory is.

The huge difference in force seems like a serious trouble. Could we explain gravity as 'something else', not as an aspect of a fundamental force? (Please don’t laugh ) I once had this crazy idea that gravity was some kind of 'overpressure' in the universe... and the reason massive objects have gravity, is because the 'overpressure' gets 'neutralized' by mass... the more mass, the more 'neutralization'. Crazy, right? But then DE popped up, so it’s maybe not totally crazy, just plain madness!?

I’m curious where the 'breaking zone' between the GR-world and QM-world is? And ZapperZ gave me a hint that 'Macroscopic Quantum Phenomena' isn’t just my personal wild idea... I Google "Macroscopic EPR" and got 96 results. And here are some examples:

http://www.swinburne.edu.au/feis/caous/theory/projects.html#epr"
An important and topical question arises with the issue, originally raised by Schrodinger in his famous cat paradox, of how to reconcile quantum with classical realities at the macroscopic level. This requires an understanding of how to extend the EPR and Bell analyses to many-particle and massive systems. Tests of macroscopic inequalities may be able to verify or rule out alternatives to quantum mechanics in which macroscopic and gravitationally induced wave-packet collapse is proposed.

Topics for 2009-2012:

A. Macroscopic EPR paradoxes
Development of measurable signatures for macroscopic EPR paradoxes and macroscopic entanglement: how does one experimentally signify a macroscopic EPR paradox?

B. Macroscopic coherent superpositions
Development of signatures of macroscopic coherence and superpositions . The issue here is to develop criteria for the macroscopic separation of superposition states in quantum mechanics.

C. Macroscopic Bell inequalities
Development of macroscopic Bell inequalities is an issue that raises the question of whether the classical-quantum correspondence principle always holds.

D. Experimental tests of macroscopic EPR and Bell paradoxes
Investigate experimental tests of macroscopic superpositions, EPR and Bell inequalities relating to photonic and cold atom systems that may be implemented in the laboratory.
​

And:

http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6TVM-4VKDMS8-H&_user=10&_coverDate=03%2F30%2F2009&_rdoc=1&_fmt=high&_orig=search&_sort=d&_docanchor=&view=c&_searchStrId=1186908646&_rerunOrigin=google&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=c0ac95ad17420824d8a3e7a814dfa15c"
Abstract
A scheme is proposed to deterministically create maximal entanglement between hybrid artificial atoms: superconducting charge and flux qubits. By tuning the circuit, the two qubits are dynamically decoupled and entanglement can be long-lived. This provides a new version of the Einstein–Podolsdy–Rosen (EPR) situation where the components of a macroscopic EPR pair are in opposite regimes.​

Then I searched arXiv.org for "Macroscopic EPR" and got zero (0) matches... maybe it’s a little to 'early'...
But arXiv search on "Macroscopic Quantum Superposition" gave 27 results and "Macroscopic Quantum Coherence" gave 41 results. And here are some examples:

http://arxiv.org/abs/0907.0176"
Authors: Jin-Shi Xu, Chuan-Feng Li, Xu-Bo Zou, Guang-Can Guo
(Submitted on 1 Jul 2009)
Abstract: By implementing an optical controlled-Not gate, we quantitatively identify the transition from quantum to classical with Leggett-Garg inequalities in a dephase environment. The experimental results show clear signature of the difference between them, which will play important roles in the understanding of some basic physical problems and the development of quantum technologies. The method used in our demonstration is also crucial on the realization of macroscopic quantum coherence due to the violation of Leggett-Garg inequalities.​

http://arxiv.org/abs/0903.1992"
Francesco De Martini
(Submitted on 11 Mar 2009)
Abstract: Two quantum Macro-states and their Macroscopic Quantum Superpositions (MQS) localized in two far apart, space - like separated sites can be non-locally correlated by any entangled couple of single-particles having interacted in the past. This novel Macro - Macro paradigm is investigated on the basis of a recent study on an entangled Micro-Macro system involving N 10^5 particles. Crucial experimental issues as the violation of Bell's inequalities by the Macro - Macro system are considered.​

http://arxiv.org/abs/0912.4565"
Shiro Kawabata
(Submitted on 23 Dec 2009)
Abstract: We have theoretically investigated macroscopic quantum tunneling (MQT) and the influence of nodal quasiparticles and zero energy bound states (ZES) on MQT in s-wave/d-wave hybrid Josephson junctions. In contrast to d-wave/d-wave junctions, the low-energy quasiparticle dissipation resulting from nodal quasiparticles and ZES is suppressed due to a quasiparticle-tunneling blockade effect in an isotropic s-wave superconductor. Therefore, the inherent dissipation in these junctions is found to be weak. This result suggests high potential of s-wave/d-wave hybrid junctions for applications in quantum information devices.​

 

[Chalnoth]

The huge difference in force seems like a serious trouble. Could we explain gravity as 'something else', not as an aspect of a fundamental force?

Well, that's a perfectly good question to ask, but then we have to ask what we mean by a force in the first place. If by "force" we mean something that governs the movement and interactions between particles, then gravity certainly qualifies.

But the question remains: is gravity precisely analogous to the quantum mechanical forces (that is, governed by a mediating particle), or is it something completely different? This is a question worth pursuing. It might well be the case that gravity is something extremely different from the other forces. But this alone doesn't solve the problem: you still have to present a theory of gravity that is both consistent with quantum mechanics and reduces to General Relativity in appropriate limits.

I once had this crazy idea that gravity was some kind of 'overpressure' in the universe... and the reason massive objects have gravity, is because the 'overpressure' gets 'neutralized' by mass... the more mass, the more 'neutralization'. Crazy, right? But then DE popped up, so it’s maybe not totally crazy, just plain madness!?

Fair enough, but without describing this idea mathematically, it doesn't go anywhere: you can't show that it reduces to General Relativity in the appropriate limits if you can't even write down specifically what it is.

If you're really interested in this stuff, keep hold of that idea. Work at it. Flesh it out. But bear in mind that in order to get anywhere, you're going to need to learn about General Relativity in detail just to show that this idea becomes GR in the appropriate limit, and you're also going to have to learn about quantum mechanics in detail just to show that it's consistent with quantum theory. And the unfortunate thing is, this is not easy: if it were easy, physicists wouldn't still be searching for a quantum theory of gravity after some 95 years.

I’m curious where the 'breaking zone' between the GR-world and QM-world is? And ZapperZ gave me a hint that 'Macroscopic Quantum Phenomena' isn’t just my personal wild idea... I Google "Macroscopic EPR" and got 96 results. And here are some examples:

http://www.swinburne.edu.au/feis/caous/theory/projects.html#epr"
An important and topical question arises with the issue, originally raised by Schrodinger in his famous cat paradox, of how to reconcile quantum with classical realities at the macroscopic level. This requires an understanding of how to extend the EPR and Bell analyses to many-particle and massive systems. Tests of macroscopic inequalities may be able to verify or rule out alternatives to quantum mechanics in which macroscopic and gravitationally induced wave-packet collapse is proposed.​

This might well be a reasonable avenue of research for quantum gravity. Just bear in mind that you'd still be looking for an insanely tiny signal, just due to how weak gravity is. Consider the difficulty: basically, in order to get a measurement of the quantum nature of gravity, you'd need to prepare a system in a superposition of two (or more) states with measurably-different gravitational fields. This could mean a superposition of different energy states, or perhaps a superposition of different position states.

So in order to get quantum gravity measurements, you not only have to produce a system that is massive enough to produce a measurable gravitational field, but you're also going to have to have it in a superposition of two states that are different enough that the difference in gravitational field between the states is measurable. And given that the quantum behavior of systems drops off exceedingly rapidly with size, this is not an easy thing to do (note that one of your examples used a whopping 10^5 particles, which is not even remotely close to enough to produce a measurable gravitational field).​

 

[DevilsAvocado]

Well, that's a perfectly good question to ask, but then we have to ask what we mean by a force in the first place. If by "force" we mean something that governs the movement and interactions between particles, then gravity certainly qualifies.

That’s true, but what if it isn’t a force/interaction between the particles (that we know)? Instead it’s something pushing from 'outside'...? Like, if you were an alien that landed on the bottom of the ocean, and you don’t know what water is, and you see the windshield start crackle, and you are sure that the gravity on Earth is so massive that your spaceship is about to implode... or something like that.

But this alone doesn't solve the problem: you still have to present a theory of gravity that is both consistent with quantum mechanics and reduces to General Relativity in appropriate limits.

That’s very true. My idea with this was to make a 'shortcut' to GR = QM by 'lifting out' gravity from the mess that happens at Planck. But this is not working, I think, because at Planck everything is inside everything... though if it did work, it would not make GR break down, I think (guess)...

Fair enough, but without describing this idea mathematically, it doesn't go anywhere: you can't show that it reduces to General Relativity in the appropriate limits if you can't even write down specifically what it is.

If you're really interested in this stuff, keep hold of that idea. Work at it. Flesh it out. But bear in mind that in order to get anywhere, you're going to need to learn about General Relativity in detail just to show that this idea becomes GR in the appropriate limit, and you're also going to have to learn about quantum mechanics in detail just to show that it's consistent with quantum theory. And the unfortunate thing is, this is not easy: if it were easy, physicists wouldn't still be searching for a quantum theory of gravity after some 95 years.

Yes, that’s the Achilles’ heel. Basically you must 'invest' an entire life to get anywhere in this complex field, and you have to settle on your career early. And the biggest problem is time & money – you got to make a living.

And given that the quantum behavior of systems drops off exceedingly rapidly with size, this is not an easy thing to do (note that one of your examples used a whopping 10^5 particles, which is not even remotely close to enough to produce a measurable gravitational field).

This will save me some time & money. As I first said – this is a crazy mystery that could drive you nuts... The most basic fundamental physics – Quantum Mechanics – that was 'processing' in Albert Einstein’s brain when he discovered General Relativity; is incompatible with Einstein’s own discovery... Einstein’s brain was in fact incompatible with the brilliant theory he produced...?? Can it be any weirder...!?

We are living in a world that’s incompatible with the stuff we are made off, uhh!? I give up...

Something clearly must be wrong – either GR is wrong, or QM is wrong, or both are wrong...

 

[ZapperZ]

The huge difference in force seems like a serious trouble. Could we explain gravity as 'something else', not as an aspect of a fundamental force? (Please don’t laugh ) I once had this crazy idea that gravity was some kind of 'overpressure' in the universe... and the reason massive objects have gravity, is because the 'overpressure' gets 'neutralized' by mass... the more mass, the more 'neutralization'. Crazy, right? But then DE popped up, so it’s maybe not totally crazy, just plain madness!?

I’m curious where the 'breaking zone' between the GR-world and QM-world is? And ZapperZ gave me a hint that 'Macroscopic Quantum Phenomena' isn’t just my personal wild idea... I Google "Macroscopic EPR" and got 96 results. And here are some examples:

You really shouldn't be posting all these "references" when you don't have a clue what they are saying. Most of them have no relevance to your point, other than the word "macroscopic" in them. You need to pay attention to what they really mean by "microscopic" in these preprints. You should also pay attention to how difficult it is to get some macroscopic object to exhibit a quantum behavior, and all the gymnastic we have to put it through to make such observations.

BTW, we recommend that people do not simply cite arXiv preprints IF they haven't been published. We still use the criteria of peer-reviewed publications as the acceptable references.

Zz.

 

[Chalnoth]

That’s true, but what if it isn’t a force/interaction between the particles (that we know)? Instead it’s something pushing from 'outside'...?

That would be flatly contradicted by reality, because it is absolutely clear that gravity responds to matter.

Yes, that’s the Achilles’ heel. Basically you must 'invest' an entire life to get anywhere in this complex field, and you have to settle on your career early. And the biggest problem is time & money – you got to make a living.

Well, one of the nice things is that once you start graduate school, they start paying you (typically). The money isn't great, but it's livable.

Something clearly must be wrong – either GR is wrong, or QM is wrong, or both are wrong...

Yes, that's the expectation. Basically, we don't expect that any of our current experimentally-verified theories are completely correct.

 

[SpectraCat]

I’m curious where the 'breaking zone' between the GR-world and QM-world is? And ZapperZ gave me a hint that 'Macroscopic Quantum Phenomena' isn’t just my personal wild idea...

Actually, there are plenty of examples of "macroscopic quantum phenomena", depending on just how tightly you want to define that. The definition I prefer is, "any macroscopically observable phenomenon where purely classical explanations fail to reproduce experimental measurements". Taking that as the definition, one need look no further than the heat capacity of molecular hydrogen to find an example. If one fails to take the nuclear spin symmetry into account for that system, it is simply impossible to recover the experimental measurement from statistical mechanics calculations.

I think another example would be the superfluidity of liquid helium-4 ... that can be explained classically (as Landau did) if one assumes a two-fluid model, but that is a purely empirical treatment. The full explanation of the behavior comes again from symmetry arguments, since the 4-helium atoms are spin-0 bosons. (I think this explanation is due to Feynman, at least he discussed it in his "Statistical Mechanics" book).

Then depending on whether or not you consider objects in the 10-100 nm size range to be "macroscopic", there are plenty of other candidates, such as the size-dependent fluorescence of CdSe nanospheres (which is roughly consistent with a particle in a 3-D spherical box model). And so on ...

 

[DevilsAvocado]

You really shouldn't be posting all these "references" when you don't have a clue what they are saying.
...

Sorry Zz, you’re absolutely right – I have absolutely no idea what I’m talking about. And even worse – I dragged your name into the 'mess'. I apologize for that.

I have to explain what happened here:
This thread was first posted under PF section "Astronomy & Cosmology", with the name "Quantum theory question -- I know, wrong forum", and I did my first post #6 from there as a 'basic help' to OP. And from now on, I clicked my Reply mail to get here, not realizing that the thread had been moved to the correct section "Quantum Physics"...

Then I thought of QM question that interested me, and started the question "Since we are discussing QM ..." in post #8, still believing that we were in section "Astronomy & Cosmology" (stupid, I know ).

And then Chalnoth popped in (with whom I have long and very interesting discussions with in "Astronomy & Cosmology"), and then things got 'carried away' so to speak... (because of me, not Chalnoth).

It wasn’t until on page 2 somewhere that I realized I was posting in the 'wrong' section...

I will now wrap this 'adventure' up and return to home base "Astronomy & Cosmology", where at least I have some clues on what I’m talking about. Sorry for the 'inconvenience'...

(I will not do 'bulk' references to arXiv again unless I’m 100% sure it’s relevant and peer-reviewed.)

Just a last reply to SpectraCat, hope that’s okay...

 

[DevilsAvocado]

Actually, there are plenty of examples of "macroscopic quantum phenomena", depending on just how tightly you want to define that.
...

Thanks for the interesting reply SpectraCat.

I’m a complete novice in the QM field, but interested. As I understand this, there are things in QM that are on the border to 'magic' from a macroscopic view, as the nonlocality in the EPR paradox. The 'mechanism' in EPR, is quantum entanglement of two particles (electron, photon, etc).

Do you know if there’s any possibility to produce this entanglement on larger objects, like molecules? If it is – could it work on even larger objects like spherical fullerenes (buckyballs) or carbon nanotubes (buckytubes)? At these sizes, objects get visible under the microscope:

http://upload.wikimedia.org/wikipedia/en/2/22/CntHAADF.jpg
Electron micrograph showing a single-walled nanotube

I guess, once and for all, a visual test experiment of the EPR paradox would settle the question of nonlocality GR <> QM.

Or is this 'visual setup' theoretical impossible?

(Sorry if someone is offended by this purely speculative question.)

 

[DevilsAvocado]

That would be flatly contradicted by reality, because it is absolutely clear that gravity responds to matter.
...

Thanks for your answers Chalnoth, good as always.

 

[Chalnoth]

I guess, once and for all, a visual test experiment of the EPR paradox would settle the question of nonlocality GR <> QM.

The EPR paradox is basically solved by the many worlds interpretation of quantum mechanics, but in any case I don't know why you think it would help with the GR/QM incompatibility.

 

[DevilsAvocado]

The EPR paradox is basically solved by the many worlds interpretation of quantum mechanics, but in any case I don't know why you think it would help with the GR/QM incompatibility.

(I don’t know if I 'dare' continue to 'mumble' here in the QM section... Zz just say the word and I stop this...)

This is confusing...

Wikipedia - Nonlocality
As action at a distance, nonlocality is incompatible with relativity. However, with quantum physics nonlocality re-appeared in the form of entanglement, where its physical reality has been demonstrated experimentally together with the absence of local hidden variables. While entanglement is compatible with relativity, it prompts some of the more philosophically oriented discussions concerning quantum theory.

A more general nonlocality beyond quantum entanglement — retaining compatibility with relativity — is an active field of theoretical investigation and has yet to be observed.​

My thought was to run EPR at macroscopic level to actually see if nonlocality can be 'reproduced'...