# Quantum theory question

1. Jan 27, 2010

### 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.

2. Jan 28, 2010

### Chalnoth

Re: Quantum theory question--I know, wrong forum

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.

3. Jan 28, 2010

### GreatBigBore

Re: Quantum theory question--I know, wrong forum

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

4. Jan 28, 2010

### Chalnoth

Re: Quantum theory question--I know, wrong forum

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.

5. Jan 28, 2010

### marcus

Re: Quantum theory question--I know, wrong forum

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.

Last edited: Jan 28, 2010
6. Jan 28, 2010

Re: Quantum theory question--I know, wrong forum

Hopefully this works as a good start:

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

Last edited by a moderator: May 4, 2017
7. Jan 29, 2010

### Chronos

Re: Quantum theory question--I know, wrong forum

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

8. Jan 29, 2010

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

9. Jan 29, 2010

### Chalnoth

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.

10. Jan 29, 2010

### DrChinese

Re: Quantum theory question--I know, wrong forum

Marcus, I found these:

Origin of Mass:

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

Last edited by a moderator: Sep 25, 2014
11. Jan 29, 2010

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...!?

Last edited by a moderator: Apr 24, 2017
12. Jan 29, 2010

### Chalnoth

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.

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.

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.

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).

Last edited by a moderator: Apr 24, 2017
13. Jan 30, 2010

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 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 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...?

14. Jan 30, 2010

### Chalnoth

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.

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

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

15. Jan 30, 2010

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...)

Last edited: Jan 30, 2010
16. Jan 30, 2010

### Chalnoth

Well, yes, this is correct.

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

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

17. Jan 30, 2010

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...

Last edited: Jan 30, 2010
18. Jan 30, 2010

### Staff: Mentor

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

19. Jan 30, 2010

### Chalnoth

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.

20. Jan 30, 2010