# B DeBroglie wavelengh

1. Oct 4, 2016

### fanieh

deBroglie wavelength says that wave-like properties of object or particle is inversely proportional to mass. Since dark matter has so very few tiny micro iota of mass.. does it mean the wave-like properties of dark matter (should this really exist) are more enhance and closer to quantum objects?

2. Oct 4, 2016

### Simon Bridge

No it doesn't.
To get to grips with this part of modern physics you have to start being very careful about your language.
The deBroglie wavelength is just a length - it does not say anything at all. You want to try again?

Getting mixed up can lead to questions like this:

... consider: if I understand you right, and I am just guessing what you mean;
* an electron is a "quantum object", in that it's behaviour is better described by quantum mechanics than by Newtonian mechanics;
* a baseball is not a quantum object in this way because it's behaviour is very well described by Newtonian mechanics.
* The deBroglie wavelength associated with a baseball is very much smaller than the deBroglie wavelength associated with an electron.
Now what does that do to your question?
(Or maybe you have a different definition for "quantum object"?)

However - this notion of wave-particle duality that you are getting used to: don't get used to it. It is a stepping stone to help you understand quantum physics that comes later and it contains a number of traps for the unwary. Just be aware that you are being introduced to something that is not correct to pave the way for a deeper understanding that we know is hard to get to.

3. Oct 4, 2016

### fanieh

So what is the effect or consequence of having bigger deBroglie wavelength? an electron and dark matter the size of the moon has let's say similar deBroglie wavelength.. so the consequence (I think) is the dark matter the size of moon is quantum object and can be in pure state that is non decohered. They say macroscopic object can never be in superposition. But can't we say the dark matter object the size of moon is a macroscopic object and at the same time in superposition?

4. Oct 4, 2016

### Simon Bridge

There is not enough information about the structure of dark matter to comment.
At the moment, afaik, the nature of dark matter particles as quantum objects in unknown - they exist as a proposed manifestation of a solution to the Einstein equations. This makes them gravitational objects. To get to grips with this part of modern physics you have to start being very careful about your language.
You need to define your terms.

What do you mean by "macroscopic object", for example?
The passage seems to suggest you think electrons cannot exist in a "pure state that is non-decohered" ... is that the case?
What do you understand by these terms? What do you mean by "size of the moon" for example?
Can you give a careful definition of the terms you are suing - because you seem to be using them in a different way to, and I could be mistaken here, everyone else.

Some notes:
Latest results tentatively suggest dark matter particles to have mass on the order of nanograms .... which is 19 orders of magnitude more massive than an electron. The largest object to be successfully used in interference experiments is a molecule of buckminsterfullerine and the dark-matter particles are anticipated to be 9 orders of magnitude more massive than those.
http://www.space.com/32295-super-heavy-dark-matter-particle-proposed.html

The deBroglie wavelength of a single material object the diameter on the order of kms or bigger does not mean much if anything - if you somehow threw it at a wall (what sort of object would qualify as a "wall" in this context?) with a slit in it that is roughly an angstrom wide, would it fit, do you think?

Have you seen:

5. Oct 4, 2016

### fanieh

Thanks for the dark matter link.
I know electron can exist as pure state and not decohered. I was asking what if for example.. just for sake of discussion.. what if a baseball has the same deBroglie wavelength as an electron. How would the baseball behave? would it interfere with itself if passed thru a corresponding slit?

6. Oct 5, 2016

### Simon Bridge

All right - a baseball with the debroglie wavelength of an electron would have very different properties to what you'd usually label "a baseball".
For instance - it would have a very low density, so would not be solid, more like an extremely diffuse gas and certainly not visible to the naked eye.

But lets pretend we have a quantum baseball without worrying too much about what it is.
If a pitcher threw it right at a solid wall of quartz, then it would pass right through ... you wouldn't be able to see it do this, it would just leave the pitcher's hand and that's all anyone knows unless a catcher on the other side of the wall announced "hey! something hit me!" thus detecting it.

As it passes through the wall the ball no more "interferes with itself" than an electron does ... that is a common misconception.
Quantum mechanics does not say anything about the details of what happens as the ball is between the pitcher and the catcher.
The pitcher throws one baseball, and the catcher catches one baseball ... if he is in the right place to catch it that is.

What we can do is line up catchers on the other side of the wall and repeat the experiment, recording which catcher got the ball on each trial.
In this way we build up a picture of how often the ball gets caught in a particular place.
It is this picture that shows the interference pattern

7. Oct 5, 2016

### fanieh

Thanks. Supposed the dark matter sector has many kinds of density and particles... in fact I read in the Scientific American how the dark matter can form dark chemistry. Then supposed the mass of this special dark matter is not the same as the one you depicted in the web you gave where it is nanograms but much lighter than electron. Then this dark matter baseball with deBroglie wavelength greater than say electron is a quantum object... a macroscopic quantum object. Shouldn't it be in pure state theoretically assuming it has no interaction with any of the known forces preventing decoherence except the force of gravity? Can the quantum baseball be decohered by gravity.. but it looks like it can't.. why can't it?

8. Oct 5, 2016

Good luck.

9. Oct 5, 2016

### fanieh

What? Double slit experiments on photons and electrons can be performed without it be decohered by gravity. So I guess gravity can't decohere quantum systems. Right guys?

10. Oct 5, 2016

### fanieh

This is key. For very small particles like electrons. We can't know what happen between emission and detection. But for quantum baseball size object with similar deBroglie wavelength as electron. Still we can't know what happens in between? but it's so big.. so I am wondering if it can be known at all?

11. Oct 5, 2016

### Simon Bridge

Please provide a citation so I know what you are talking about.

So what? Lets suppose that dark matter comes in different particles with different masses... we are now in the realms of pure speculation unless you can provide a citation. Mine was really recent. I can find stuff on dark Matter that says pretty much anything I want if I don't care about being up to date or accurate.

What dark matter baseball? Why is it important that the baseball be made of dark matter? Why is it important that it is a baseball?
What do you mean by "quantum object"? What do you mean by "macroscopic object"? How do you determine size?
I have repeatedly asked you to speak clearly and you continue to refuse to do so.
Until you do you will get nowhere and nobody can help you.

You are not listening:
The quantum baseball in my example was not a dark matter baseball, I never said it was. ... it was normal matter, so that it could interact as an electron does with other normal matter. Let me repeat this as it is not sinking in: the quantum baseball in my example was purely imaginary with no correspondence to anything in Nature. It was a fiction, it was not real, and this is important: it has nothing to do with dark matter.

You seem to want to imagine a material object, roughly baseball shaped, but with the mass of an electron (or less - since you want the debroglie wavelength to be longer), which does not interact electromagnetically (dark matter). So lets say a sphere the volume of a baseball containing nothing but an electron's mass amount of dark matter. Fine... there is no reason to believe that such a thing is even physically possible, and indications that it isn't. So we are probably positing a situation that violates known laws of physics ... just so you realise that. Do not expect the conclusions to make sense.

Do you realise that such an object has a density somewhat less than that of a vacuum ... it is literally less than nothing. But lets just say ... it can still interact by the weak nuclear interaction, and by gravity. (There are three forces besides the electromagnetic interaction, but current models of dark matter tend to exclude the strong nuclear force as well.)

Now you want to know if such an object, already doubly impossible, can exhibit interference at slits ... well, at this stage, while you are making stuff up why not make that up as well? But OK - the slit experiment involves somehow restricting the possible paths between some dark-matter-baseball emitter, and a detector for same. Whatever is able to restrict the path of such an object must do so by interacting with it somehow.

Does gravity interact in the same way as other quantum mechanical forces?
The short answer is: "we don't know". We do not currently have a working theory of quantum gravity and we are wayy off being able to manipulate dark matter.
It may be that dark matter will provide clues to quantum gravity - ie we can try to do interference experiments with it where the possible paths are restricted by gravitational fields. But: and here is where I want you to hear me: none of this needs a baseball sized lump of dark matter!!
I don't understand your obsession with this.

Maybe you are thinking that you want to be able to watch the particle as it travels between source and detector?
This is a totally different type of question.
Please understand, we cannot see what happens in between because that would spoil the effect we are trying to watch - it has nothing at all to do with the physical size of the object in question.
This bit does not need dark matter either - it has nothing to do with dark matter.

We could do the electron interference experiment in something like a cloud chamber where little bubbles trace out the trajectory of the electron.
If the important property of a "macroscopic object" is that you can watch it move and know where it went ... then the electron is now "macroscopic". Better in fact because it leaves a record of where it's been as if the baseball could draw a line in the air as it flew.
But if we do this - then the interference effect vanishes. Guess why?
It is the same for the hypothetical quantum baseball.

There are some other issues with the quantum baseball ... ie. with a normal baseball you know with good precision when it was pitched and when it was caught.
But if you do that with the quantum baseball you lose the interference effect as well... because the energy of the baseball (which affects it's debroglie wavelength) will be very uncertain.

The bottom line is that big objects do not map well to quantum processes ... this is not surprising: if they did, we would not need quantum mechanics in the first place.

12. Oct 5, 2016

### vanhees71

Sigh, when will the textbookwriters and professors stop to use "wave-particle duality" to introduce quantum theory? Was Planck really wrong when he said that old-fashioned outdated ideas die out, because the young generation is taught the modern thing right away, because it's a better description of nature than the old stuff?

There is no wave-particle duality, and modern quantum theory doesn't need it. The "wave functions" are just probability amplitudes to find the described particle at a position, i.e., $|\psi(t,\vec{x})|^2$ is the probability distribution to find the particle around the position $\vec{x}$ when looking at time $t$. That's it. There's not more meaning in the wave function than this Born rule.

Further QT is valid for objects of any size. The classical behavior of macroscopic objects is due to a coarse-graining procedure describing effectively collective degrees of freedom.

13. Oct 5, 2016

### Simon Bridge

I think it's an artifact of the historical approach to teaching science... but I agree, it is possible to introduce quantum theory without mentioning deBroglie wavelengths and a lot of confusion would be avoided if people didn't do this.

I'd like to address the problems here without having to refer to dB, but OP seems stuck on it. Instead I keep getting caught in a level of silliness that I'm having trouble pulling the thread away from. We don't even need to invoke dark matter, which does not do what OP seems to want it to do. I think OP is trying to find a metal picture of QM in terms of intuitively understood physics.

14. Oct 5, 2016

### fanieh

I'll give the reasons why the wave aspect is embedded deep in our psyche. First deBroglie won Nobel Prize for the wave-particle duality. Second G.P Thomson proved the wave property of electrons and got a Nobel Prize too. Third. deBroglie said "When the electron moves around in an atom, its associated wave is stationary, i.e. in a standing wave pattern, like a wave moving along a violin string fixed at its ends. In this situation, only certain discrete frequencies are produced - the fundamental and its overtones, as any good music student knows. This is just what Bohr need in 1913 for his hydrogen atom postulate (Remember the unexplained 2pi factor?) By just fitting a whole number of electron waves along the circumsference of the atom, and using de Broglie's relations, Bohr could have given a complete theoretical justifications for the orbital quantization" (from Zarate "Introducing Quantum Theory).

Then Schroedinger came into the scene. He made the Bohr and deBroglie wave 3D. So the wave is really 3D. Or as the book described

"Visualizing Schrodinger's Atom

What Schrodinger did was reduce the problem of the energy states in an atom to a problem of finding the natural overtones of its vibrating system using Fourier analysis. The natural frequencies and the number of nodes of one-dimensional standing waves (e.g. a violin string) are easy to visualize. This picture can be extended to a two-dimensional system, such as the vibrations of a struck drum head..... Though it is very difficult to visualize three-dimensional vibrating systems in something like the hydrogen atom........ The integers called quantum numbers by bohr, Sommerfeld and Heisenberg were now related in a natural way to the number of nodes in a vibrating system."

So even in Schrodinger Equation, the wave thing is retained.. so how could you guys say there is no wave (even 3D wave)??