# jaydnul

Members

17

1. ## Timer in C++

All I need is to run a certain section of my code (that loops forever) for a certain amount of time, about 10 seconds. The obvious problem is when it gets to that section, it loops forever and doesn't move on. Is there a function that will allow me to run a given chunk for a given amount of time? Thanks
2. ## Teaching at Community College... Career Suicide?

I remember hearing a fellow student talk about how it was a bad idea to teach community college physics. I'm just curious, would it really be that bad to teach at a CC while looking for a research position after obtaining a PhD in physics? Why would this hinder your opportunity to get a real research position when I would think the teaching experience would be a good thing?
3. ## Definition of observing

Yes, I see now how I was misenterpreting it. So in this post talking about the single slit, paragraphs 6 and 7 (labeled #1,#2), when the particle hits the detector, is the information about the instantaneous momentum lost? Couldn't you measure the momentum given to the detector plate? Sorry, should have been more clear but I'm not sure when I critisized you
4. ## Definition of observing

Ahh I see. The previous momentum has nothing to do with the current position. I think I understand now. Thanks swansont
5. ## Definition of observing

I thought I was going crazy so I dusted off my Feynman Lectures. This is from volume 3, pg. 2-3 (talking about the single slit experiment): "Sometimes people say quantum mechanics is all wrong. When the particle arrived from the left, its vertical momentum was zero. And now that it has gone through the slit, its position is known. Both position and momentum seem to be known with arbitrary accuracy. It is quite true that we can receive a particle, and on reception determine what its position is and what its momentum would have had to have been to have gotten there. That is true, but that is not what the uncertainty relation refers to. [HUP] refers to the predictability of a situation, not remarks about the past. It does no good to say "I knew what the momentum was before it went through the slit, and now i know the position," because now the momentum is lost. We are talking about a predictive theory, not just measurements after the fact. So we must talk about what we can predict." There's a few possibilities here. 1) There has been sufficient evidence after Feynman's death that refutes him. 2) I am misinterpreting the text and need clarification. 3) The great Richard Feynman was simply wrong. (Which I'm sure is possible, but I suspect someone would have caught the error by now given that the lectures were written in the 60's) Also read this: http://physicsandphysicists.blogspot.com/2006/11/misconception-of-heisenberg-uncertainty.html
6. ## Definition of observing

What is the evidence that the momentum and position don't exist together in precise states (genuine question, I'm really quite curious)? Sure the operators don't commute but that is math, which is only used to predict. Are you saying that once we detect a particle on the detector plate, we can calculate the momentum it must have had to get there, but we can't assume that is the correct value?
7. ## Definition of observing

No The best example is a simple single slit experiment. The slit width is the uncertainty in position. If we perform the experiment and see a hit on the detector plate, we know the particle went through the slit, and thus its position. Then we can calculate the momentum it must have had using the time it took to hit the detector and the distance between the slit and the detector. Now we know what the particle's momentum and position were. The HUP still hasn't come in to play yet. When we perform the experiment again and try to predict the momentum with the same slit width, that is when the HUP inequality must be satisfied. The rest of your post seems a little irrelevant to me, but that is probably my own fault in understanding .
8. ## Definition of observing

I was also trying to make this point in a different thread. It is a common misinterpretation of the uncertainty principle to assume we can never know what the momentum and position of a particle was. If the electron has been detected, then we can easily deduce its position and the momentum it needed to get there. The H.U.P. is if we ran the experiment again, there would be an uncertainty in our ability to predict the momentum or position, which has to satisfy the uncertainty principle.
9. ## Definition of observing

A "particle" is neither a wave or a particle, it is a quantum object. In some instances, it exhibits properties that are able to be modeled by the math of a standard macroscopic wave, but by no means is it actually a wave.
10. ## The Official "Introduce Yourself" Thread

Hi, I'm jaydnul and I'm an alcoholic. No, but I am an senior level undergrad majoring in physics. Also a big Star Wars fan. Emperor Palpatine is probably the closest thing to a father figure I will ever have... A friend of mine was telling me about the benefit of being a member of a forum like this, where you can ask questions, but also share knowledge that you might be qualified to give. When I was looking for a forum to join, science forums seemed the most pleasant and well informed of the lot. Excited to come aboard!
11. ## What happens to a particle after it stops being observed?

If we setup a single slit experiment, the slit width would be our uncertainty in x. If the particle hits the detector we know it went through the slit. We can then calculate the momentum with the position on the detector and the time it took to get there. What I'm saying is that those two uncertainties have nothing to do with HUP. HUP is about what would happen if we repeated the experiment and tried to predict the two values beforehand.
12. ## What happens to a particle after it stops being observed?

A particles position and momentum can be known to exact precision. Sure, once you confine it to a small space, the momentum for future measurements will be uncertain in accordance with the HUP, but the values for instantaneous position and momentum can be known exactly.
13. ## What happens to a particle after it stops being observed?

This is not necessarily true. The particle's position and momentum could both be known to an arbitrary accuracy given the quality of the measuring apparatus. The HUP is about what can be predicted if the experiment was repeated, not about what can be measured.
14. ## Just an idea...

Having more or less mass does not dictate whether or not you can have a stable orbit. The further you get from the center of the earth, the more mass you would need, or just a slower tangental velocity. Theoretically, the moon could orbit the earth from 10 lightyears away if it was moving slow enough and they were the only two masses in the universe. I'm not really sure what these gravitational 'layers' are supposed to be.

For some reason, I have always had an abysmal public speaking ability. My brain will randomly deny me certain memories that I know like the back of my hand when not in front of a crowd. I also forget how to simultaneously breath and talk, taking deep breaths and not being able to find a pause in my sentence to take another. I realize public speaking is a vital tool in a person's toolbox, so I want to perfect it. For those of you who were once like me, what were some of the steps you took to improve? Thanks!
16. ## I.Q. Is Rubbish

What does it matter? I've never been asked my IQ in a job interview. What is the point of such a system? To gain self-confidence?
17. ## The behaviour of electrons

You will meet a dead end when attempting to relate the quantum world to your everyday experience. The word particle is just that, a word. It doesn't have any physical comparison in the macroscopic universe. There is no way to properly picture it in your head, which is where the mathematics comes to the rescue. That being said, virtual particles are different from your everyday electron. In QED, the interaction between the nuclei of the to atoms and the shared electron is described as the exchange of virtual particles. Look up QED and Feynman diagrams.
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