Register to reply 
I don't understand something...by The_Thinker
Tags: None 
Share this thread: 
#1
May1207, 09:21 AM

P: 148

An electron is a charged particle... A time varying electric field produces an electromagnetic wave perpendicular to it. So when an electron moves, it should also produce an electromagnetic wave perpendicular to it. Isn't this true?
Now if this is true... then should we not be able to detect its presence as it moves around, courtesy this emwave. Now if we can detect this.. then we should be able to find out its velocity through the shift in frequency and we should be able to detect its position by putting detectors at the right place. Is this wrong? 


#2
May1207, 09:25 AM

Emeritus
Sci Advisor
PF Gold
P: 9,772

Is this thread wondering toward Heisenberg's Uncertainty Principle by any chance?



#3
May1207, 11:01 AM

P: 1,235




#4
May1207, 01:20 PM

P: 1,295

I don't understand something...
Sure, you would get measurements of positions and velocities.
I don't know what you mean by "the position" and "the velocity". 


#5
May1207, 01:34 PM

P: 148

ah... yeah.. Hootenanny... thats what i was wondering about... if you can measure the position and velocity of the thing... then how does that not go against the uncertainty principle?
and lalbatros: 


#6
May1207, 01:41 PM

Emeritus
Sci Advisor
PF Gold
P: 9,772




#7
May1207, 01:51 PM

P: 148

so... hmmm... if it is just a mathematical principle... then how is it that people use it as a fundamental principle? I have read and heard of things where they say that the HUP also applies to energy and that is why QM states that a pair of protons and antiprotons can just appear and annihilate each other stating HUP as a reason...
Have I heard or read wrong?? 


#8
May1207, 01:56 PM

P: 688

the uncertainty principle is a mathematical one. It can be derived from Schrodinger's equation (the nonrelativistic one... I don't know anything about the relativistic one though) and theories from linear algebra.



#9
May1207, 02:00 PM

Emeritus
Sci Advisor
PF Gold
P: 9,772




#10
May1207, 02:03 PM

P: 148

cool k man... have fun...
(and do let me know how it is... ive been dying to see it myself...) K... well ill just read up on it... physics sure is weird... [=(] 


#11
May1207, 06:37 PM

P: 2,292

I posit that that core problem is that we do not fully understand the nature of momentum; not that I have explanation.



#12
May1207, 11:53 PM

P: 688

even if you have electromagnetic fields to measure, I doubt the wave would have an exact value of momentum, and/or "position" to give you an exact value of the position of the electron itself.
I may be wrong though, as photon obey the relativistic Schrödinger's equation and the commutator of the position and momentum operator might be different. (I'm pretty sure the derivation based on CauchySchwartz's inequality would still be valid though). quantum mechanics is indeed a very abstract subject... as momentum and position become operators rather than definite numbers.... and you can only talk about the average of all the position and the average of all the possible momentum... 


#13
May1307, 08:34 AM

Admin
P: 21,887

What is the magnitude of the electric field of one electron? Then think about what is the 'change' in the electric field of one electron as a function of its velocity.



#14
May1307, 10:02 AM

P: 148

but Astronuc 


#15
May1307, 02:39 PM

Admin
P: 21,887

I was getting at 



#16
May1307, 04:13 PM

P: 688




#17
May1307, 05:14 PM

P: 687

Astronuc: This is independant of any particular measuring apparatus, or how that equipment may or may not affect the electron. 


#18
May1407, 04:37 AM

P: 148

ah... k... will do that surely tim_lou..
And.. hmmmm... k, i see your point NeoDevin, but what does the position of the wave have to do with the velocity and the position of the particle? all we need to measure is which angle its coming from to find its velocity (this can be done by measuring the energy we get at a static detector, when the magnitude is highest, then thats the position isn't it?) and the frequency shift to find out the velocity.. Am I right? Oh... here's another question... What if in the double slit, if we put this kind of a detector at the 2nd slit, then without doing anything! to the experiment we can measure which slit the electron goes through right...? So... if we did that.. then what pattern do we observe? 


Register to reply 
Related Discussions  
HELP I do not understand!  Calculus & Beyond Homework  2  
Help me understand  Introductory Physics Homework  3  
Please help me understand this  General Math  16  
Help me understand!  Introductory Physics Homework  9  
Can you please help me understand..  Electrical Engineering  2 