Matter Waves

[BenSon]

We have finished learning about this in school and I have a problem with it.

The equation we get taught to use for this is

Wavelength = h/mv

Why can we not slow somethings velocity down sufficiently to cause an observable wavelength? Like 10^-40m/s.

Also while ive got a thread open what is mass? Why do some particles have it while others dont?

 

~Scott

[insane_alien]

we can slow things down to 10^-40 ms-1 its just that it's really hard to measure at such a slow speed. the errors get very large. but we can use something very light. say an electron and we can measure the wavelegnth of that even when its going at relativistic speeds.

 

what is mass? good question. no-one knows. some think its the higgs field(not quite sure what this is) and others think its strings etc. etc.

[BenSon]

Oh yeah, I understand that we can observe the particle as a wave (such as an electron or neutron) but I ment macroscopic objects. How about this we have a model car something small just wheels on a board on the slightest incline you can imagine in a controlled environment woulden't it be moving so slow that we could see it as a wave? What would it look like?.

 

Bummer about that mass thing, explains why my teacher coulden't tell me if no-one knows. Could someone elaborate on the higgs field a bit?

 

`Scott

[swansont]

I think you can answer your own question if you calculate the temperature of an object that had that average speed and was in thermal equilibrium with its surroundings.

[insane_alien]

it would have to be very light and moving slow then which would mean experiments to confirm it could take years. the experiment to prove it is diffraction but in order for this to happen with a macroscopic particle the necessary velocity is tremendously slow. why not use a molecule or an atom. they are after all real things.

[BenSon]

I think you can answer your own question if you calculate the temperature of an object that had that average speed and was in thermal equilibrium with its surroundings.

I know what thermal equilibrium is but I'm sorry I don't know how to calculate temperature from velocity:embarass:

 

it would have to be very light and moving slow then which would mean experiments to confirm it could take years. the experiment to prove it is diffraction but in order for this to happen with a macroscopic particle the necessary velocity is tremendously slow. why not use a molecule or an atom. they are after all real things.

I think it would just be wild to see a visable thing as a wave.

 

`Scott

[swansont]

I know what thermal equilibrium is but I'm sorry I don't know how to calculate temperature from velocity:embarass:

 

 

The average kinetic energy would be 3/2 kT' date=' k= 1.381 x 10[sup']-23[/sup] J/K. That means for a 1kg object to have its center-of-mass velocity be in thermal equilibrium at 10^-40 m/s, it would have a temperature of about 2.5 x 10^-58K

 

At room temperature (~300K), that same object has a COM speed of about 10^-20 m/s, but the problem is that T goes as v². At that temperature, the wavelength is 10^-13 m. At the slower 10^-40 m/s speed, the wavelength is 10^-3m, so it's still small - Planck's constant is a very small number. Once you get much above atomic size, it beguns to get very difficult to see wave-type behavior.

[insane_alien]

technically every time you walk through a door you get diffracted a tiny bit.

[BenSon]

The average kinetic energy would be 3/2 kT, k= 1.381 x 10-23 J/K. That means for a 1kg object to have its center-of-mass velocity be in thermal equilibrium at 10-40 m/s, it would have a temperature of about 2.5 x 10-58K

 

At room temperature (~300K), that same object has a COM speed of about 10-20 m/s, but the problem is that T goes as v2. At that temperature, the wavelength is 10-13 m. At the slower 10-40 m/s speed, the wavelength is 10-3m, so it's still small - Planck's constant is a very small number. Once you get much above atomic size, it beguns to get very difficult to see wave-type behavior.

Thanks that makes sense i am a little lost on how you first worked out the temperature though.

technically every time you walk through a door you get diffracted a tiny bit.

I don't think so we were always taught tht diffraction occurs when the wave passes through a gap that is similar is sive to the wavelength. I think the door is alot larger then your wavelength.

 

`Scott

[swansont]

Thanks that makes sense i am a little lost on how you first worked out the temperature though.

 

 

1/2 mv² = 3/2 kT

 

T = mv²/3k

[BenSon]

1/2 mv2 = 3/2 kT

 

T = mv2/3k

Oh right kinetic energy of an ideal gas (right?). I see your equating the kinetic energy of a solid mass against the kinetic energy of a ideal gas. So for the car to be in thermal equilibrium with its surroundings they must be equal, I think I get it...Thanks:-)

[swansont]

Oh right kinetic energy of an ideal gas (right?). I see your equating the kinetic energy of a solid mass against the kinetic energy of a ideal gas. So for the car to be in thermal equilibrium with its surroundings they must be equal, I think I get it...Thanks:-)

 

If your car is just sitting there, yes. All of the gas molecules impinging on it give it a small speed, assuming it has three degrees of freedom. (This concept was one of the questions on my acceptance oral in grad school)

[lxxvii24]

We have finished learning about this in school and I have a problem with it.

The equation we get taught to use for this is

Wavelength = h/mv

Why can we not slow somethings velocity down sufficiently to cause an observable wavelength? Like 10^-40m/s.

Also while ive got a thread open what is mass? Why do some particles have it while others dont?

 

~Scott

Good question, but i think i was thought in elementary skool that mass is the quantity of matter in a a body. BUt , i know that most of us know this definition, so then what is matter. i think that is the new question?

[BenSon]

If your car is just sitting there, yes. All of the gas molecules impinging on it give it a small speed, assuming it has three degrees of freedom. (This concept was one of the questions on my acceptance oral in grad school)

Thanks for the help:-) Can I ask you how much oppertunity there is for research in physics? I don't mean is there anythig left to research but how many positions are available in research. Thanks.

Good question, but i think i was thought in elementary skool that mass is the quantity of matter in a a body. BUt , i know that most of us know this definition, so then what is matter. i think that is the new question?

I think matter is fermions like quarks and electrons. But what gives matter the propety of mass?

 

`Scott

[swansont]

Thanks for the help:-) Can I ask you how much oppertunity there is for research in physics? I don't mean is there anythig left to research but how many positions are available in research. Thanks.

 

Depends on the kind of research. Generally the answer is there are many research opportunities, but as I'm not looking for a new job, I don't really have a good feel for that information.

[Klaynos]

Thanks for the help:-) Can I ask you how much oppertunity there is for research in physics? I don't mean is there anythig left to research but how many positions are available in research. Thanks.

 

Have a look at your countries professional physics body (Institute of Physics for the UK etc...), and science publications such as new scientist for jobs...

[BenSon]

Have a look at your countries professional physics body (Institute of Physics for the UK etc...), and science publications such as new scientist for jobs...

I have been looking at some job websites but unfortunately there is not many of them. Also the kind of research I thinking of is theoretical like string theory and other up and coming theories.

 

`Scott