How can you measure a particle?

[ChrisDK]

Hi everyone,

 

It's been a while since I've been on here.

Last time I asked a questions about what generates gravity, and that turned out to be a very interesting discussion.

 

Now I have a new question about particles.

 

So, everyone knows about Heisenberg's uncertainty principle, which basically says that you can never know both the position and momentum of a particle at the same time.

So by my logic (which might be off ) that means you can never exactly know where a particle is at any given time, right?

 

So, with all the research into quantum mechanics and quantum computers, I just can't figure out how they are able to measure anything about a particle at all.

Let's say I am building a quantum computer, and I want to know if a particle is in spin up, or spin down, how can you do that, when you don't/can't know exactly where the particle is?

 

Now I realize that things in the quantum world, do not behave at all like things in the "macro world", but to my mind, that's kind of like trying to measure the speed of a car, without knowing where it is. That would more or less be impossible right?

 

Or is it possible to decrease the uncertainty in position enough, to actually measure the particle?

 

I hope it all makes sense

[Mordred]

The more you know a particles position, the more uncertain you are on its momentum. A few years back this was reduced by a method that reduces the amount of interference caused by measurements.

 

Essentially take multiple samples of multiple particles to increase the accuracy. Then look at its influences on other particles etc.

[studiot]

 

Let's say I am building a quantum computer, and I want to know if a particle is in spin up, or spin down, how can you do that, when you don't/can't know exactly where the particle is?

 

Heisenberg's Uncertainty Principle does not forbid us to know any and every one of the quantum numbers of a particle and the spin is one such.

 

Quantum numbers are just that, they only take on certain specific values, they do not take on any values between these.

 

The HUP is derivable from some mathematics of periodic functions known as The Bandwidth Theorem or from the Swartz Inequality, which limit the resolution of calculating one continuous function, given the value of a second continuous function, for certain pairs of functions.

 

In the case of the HUP the pairs are

 

Energy and time

 

Momentum and distance (position)

 

Note that this is not just a measurement issue where the measurement affects the result, as the HUP is often presented in elementary texts, it is fundamental.

[imatfaal]

in case anyone is following up Studiots excellent post - the inequality is the Schwarz or Cauchy-Schwarz; just a simple typo

[ChrisDK]

 

Heisenberg's Uncertainty Principle does not forbid us to know any and every one of the quantum numbers of a particle and the spin is one such.

 

Quantum numbers are just that, they only take on certain specific values, they do not take on any values between these.

 

The HUP is derivable from some mathematics of periodic functions known as The Bandwidth Theorem or from the Swartz Inequality, which limit the resolution of calculating one continuous function, given the value of a second continuous function, for certain pairs of functions.

 

In the case of the HUP the pairs are

 

Energy and time

 

Momentum and distance (position)

 

Note that this is not just a measurement issue where the measurement affects the result, as the HUP is often presented in elementary texts, it is fundamental.

 

So in terms of actual measurement, how is it possible to actually figure out the value of a quantum particle, that's the thing I really can't figure out.

 

I am somewhat familiar with the Cauchy Schwartz inequality, I believe it was in relation to vectors, but I am not familiar with the Bandwidth Theorem.

 

Now I might be getting at this from the wrong angle, but in order to physically measure a particle, disturbing it or not, then you must be able to know exactly where it is. But if you can never know the energy and/or position at any time, how can you actually measure anything at all? Let alone watch it interact with other particles, since they suffer from the same problem (I assume)?

 

I think one of the hardest things for me to understand as well - somewhat related - is when do these types of effects "blend" into the real world. In other words, when do things obey classical Newtonian mechanics, and when does an object obey quantum mechanics?

A molecule for instance, is not a quantum object to my knowledge, as in it doesn't have the particle/wave duality, but it's made of particles that do. So when does the switch occur so to speak, is there a certain mass or number of particles in a system, that determines how an object behaves? Why don't molecules and atoms all behave like particles and waves?

 

(Is it obvious I don't have a Ph.d in physics? lol)

[imatfaal]

With a photon you can just see if it goes through a polarizor - but things get much more complicated: Stern Gerlach was one of the first https://en.wikipedia.org/wiki/Stern%E2%80%93Gerlach_experiment

 

You can actually demonstrate the wave nature of a large macromolecules (C60 - buckyballs) by obtaining a double slit defraction pattern. In most circumstances other effects swamp out QM effects - but there is no firm switching from one regime to another

[studiot]

 

Imatfaal

 

in case anyone is following up Studiots excellent post - the inequality is the Schwarz or Cauchy-Schwarz; just a simple typo

 

 

Thank you, I like to keep the mods on their toes. +1 for vigilance.

 

 

 

ChrisDK

 

Now I might be getting at this from the wrong angle, but in order to physically measure a particle, disturbing it or not, then you must be able to know exactly where it is. But if you can never know the energy and/or position at any time, how can you actually measure anything at all? Let alone watch it interact with other particles, since they suffer from the same problem (I assume)?

 

 

You may not be a Phd physicist any more than I am a Lexicographer, but you would find it easier if you tried to be more precise in your thinking. (No offence meant)

 

I have underlined a couple of examples in the quote.

 

You don't measure a particle whatever that means. You measure one or more properties or characteristics.

 

To suggest you can never know the energy etc..... is over the top.

The UP places limits on the accuracy with which you can know certain properties in certain circumstances.

That is a far cry from never knowing anything about these properties.

 

We use the bandwidth theorem in another guise to determine the minimum sampling frequency to copy a waveform eg for making CDs

 

In this case we are using it the other way not to limit but to ensure we overcome the limit.

Edited August 29, 2015 by studiot

[swansont]

 

To suggest you can never know the energy etc..... is over the top.

The UP places limits on the accuracy with which you can know certain properties in certain circumstances.

That is a far cry form never knowing anything about these properties.

 

Quoting to emphasize this.

 

Planck's constant is really, really small. So the exactness we are limited to is a small value.

[Acme]

Hi everyone,

...

So, with all the research into quantum mechanics and quantum computers, I just can't figure out how they are able to measure anything about a particle at all. ...

Particle detectors measure many different attributes of particles.

Particle detectors @WIKI

In experimental and applied particle physics, nuclear physics, and nuclear engineering, a particle detector, also known as a radiation detector, is a device used to detect, track, and/or identify high-energy particles, such as those produced by nuclear decay, cosmic radiation, or reactions in a particle accelerator. Modern detectors are also used as calorimeters to measure the energy of the detected radiation. They may also be used to measure other attributes such as momentum, spin, charge etc. of the particles. ...