Position of a particle and photons

[Maslova]

Once I have read that we can’t know a actual position of a particle because to see the particle we need to send photons and when we send photons it colides with the particle and change it’s position. Is this true?

 

[PeterDonis]

I have read

Can you give a specific reference?

 

[Maslova]

Can you give a specific reference?

but it’s in Portuguese

 

[PeterDonis]

it’s in Portuguese

What is the title of the book, or article, or whatever it is? Is there an English translation?

 

[Maslova]

What is the title of the book, or article, or whatever it is? Is there an English translation?

This is the book. Just in Portuguese

 

[PeterDonis]

This is the book.

What kind of book is it? Is it a textbook? A popular science book?

 

[Maslova]

What kind of book is it? Is it a textbook? A popular science book?

Scientific divulgation

 

[PeterDonis]

Scientific divulgation

I don't know what "divulgation" means.

 

[Maslova]

I don't know what "divulgation" means.

Science popularization, science communication

 

[PeterDonis]

Science popularization, science communication

Ok, that was the impression I got from looking the book up on Amazon. Popularizations are generally not good sources for learning the actual science.

In this particular case, the book appears to be describing the uncertainty principle in quantum mechanics. However, the description you give, which I assume is based on what the book says, is not quite correct. The uncertainty principle does not place any limit on how accurately you can measure a particle's position. What it does place a limit on is this: the more accurately you measure the particle's position, the greater the uncertainty in its momentum will be after the measurement.

For example, if you are measuring the position with a photon, the uncertainty in the position measurement is approximately the wavelength of the photon, so to make the position measurement very accurate you will have to use a photon of very short wavelength. But such a photon will have a very large momentum, so when it collides with the particle it will change the particle's momentum by a very large, unknown amount.

 

[Maslova]

Ok, that was the impression I got from looking the book up on Amazon. Popularizations are generally not good sources for learning the actual science.

In this particular case, the book appears to be describing the uncertainty principle in quantum mechanics. However, the description you give, which I assume is based on what the book says, is not quite correct. The uncertainty principle does not place any limit on how accurately you can measure a particle's position. What it does place a limit on is this: the more accurately you measure the particle's position, the greater the uncertainty in its momentum will be after the measurement.

For example, if you are measuring the position with a photon, the uncertainty in the position measurement is approximately the wavelength of the photon, so to make the position measurement very accurate you will have to use a photon of very short wavelength. But such a photon will have a very large momentum, so when it collides with the particle it will change the particle's momentum by a very large, unknown amount.

Thanks for the explanation!

 

[PeterDonis]

You're welcome!

 

[Demystifier]

But such a photon will have a very large momentum, so when it collides with the particle it will change the particle's momentum by a very large, unknown amount.

Why unknown?

 

[PeterDonis]

Why unknown?

First, because prior to the measurement, you don't know where the particle is, so you can't shoot a tightly collimated photon (one whose momentum you know in advance) at it.

Second, because you can't control how much of the photon's momentum gets transferred to the particle.

 

[cosmik debris]

Ok, that was the impression I got from looking the book up on Amazon. Popularizations are generally not good sources for learning the actual science.

In this particular case, the book appears to be describing the uncertainty principle in quantum mechanics. However, the description you give, which I assume is based on what the book says, is not quite correct. The uncertainty principle does not place any limit on how accurately you can measure a particle's position. What it does place a limit on is this: the more accurately you measure the particle's position, the greater the uncertainty in its momentum will be after the measurement.

For example, if you are measuring the position with a photon, the uncertainty in the position measurement is approximately the wavelength of the photon, so to make the position measurement very accurate you will have to use a photon of very short wavelength. But such a photon will have a very large momentum, so when it collides with the particle it will change the particle's momentum by a very large, unknown amount.

Now I'm confused. I thought that the uncertainty principle wasn't about measurement but about preparation, i.e. that the principle described the combined standard deviations of say momentum and position in the preparation.

Cheers

 

[PeterDonis]

I thought that the uncertainty principle wasn't about measurement but about preparation

A measurement is a preparation in the sense you are using the term here. The more general way of looking at it is that the uncertainty principle is a restriction on states: any physically realizable state of the particle must have combined standard deviations of momentum and position that satisfy the uncertainty principle.

 

[Ibix]

To be fair to the book, it does appear to be talking about the uncertainties in pairs of measurements (duas variáveis - two variables) having a product not less than $h/2\pi$. But (at least in the excerpt above) it doesn't seem to explain that these are specific pairs of measurements (e.g. position and momentum). And it does talk about bouncing photons off your object of interest but doesn't explain (at least on that page) about the high energy photons needed to get more precise position measurements and why that might lead to greater uncertainty in momentum.