# B Uncertainty Principle -- Fundamental limit, or a limitation of our measurement capabilities?

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1. Jan 14, 2019

### Kevin Chieppo

I'm a hobbyist physicist and I just started studying QM through watching Leonard Susskind's lectures on the Stanford Youtube channel. I get the idea of it being impossible to precisely know both a subatomic particle's position and momentum, but is this actually a physical limitation? Or is it just a limitation by how we know how to measure subatomic particles today?

I don't have any idea how, but I feel like there will be a day when we understand matter on a small scale well enough that we can abandon this whole probabilistic model. Thanks for reading.

2. Jan 14, 2019

### Orodruin

Staff Emeritus
It is a physical limitation in quantum mechanics. It has absolutely nothing to do with the accuracy of your instruments.

Your feeling is common, but there are experiments that have ruled out hidden variables.

3. Jan 14, 2019

### haushofer

Local (!) hidden variables.

4. Jan 14, 2019

### Orodruin

Staff Emeritus
Valid point.

5. Jan 14, 2019

### Anubhav Srivastava

Well Its not a physical limitation rather its only a limitation of how we measure and see things using the most precise instruments we have.

So lets take an example where we want to know an electrons position. But we cannot directly see an electron, the only thing you can do is shine a light o an electron and it reflects it back to us so we see it through the interaction of photon with the electron.

Now you might be think how ?
Let me give you an elaboration of the above experiment.
We shot a photon at the electron to know its position and then the electron hit the photon and came back to us in a time of t.

Now we know that the Tie taken would be t = 2x/c where c = speed of light and x is distance b/w electron and the photon source.
So now we can calculate x easily.

But wait !!!
As soon as the electron hit the photon, it must have recoiled back because linear momentum must have conserved during the collision. And the Electrons velocity must have changed.
So now you see that you know the position well but since you changed the velocity of the electron, you no longer know the momentum.

So its not that uncertainty is physical law rather its just our inability to determine a systems states without disturbing it.

And Yes maybe in the future we will be able to remove this inability.

Last edited: Jan 14, 2019
6. Jan 14, 2019

### stevendaryl

Staff Emeritus
But the EPR experiment seems to show that this interpretation of the uncertainty principle is not correct.

7. Jan 14, 2019

### Staff: Mentor

This explanation of the uncertainty principle is not right, even though Heisenberg himself explained it that way at first and it's been repeated ever since. Not long after Heisenberg came up with this explanation, our understanding of the math behind quantum mechanics had advanced and it became clear that the uncertainty is fundamental to the theory - you cannot have a system with definite position and momentum even if you never measure either. No improvements in measurement technology can remove the limitation if QM is correct.

You will find the correct explanation of the uncertainty principle in some older threads here.

8. Jan 14, 2019

### stevendaryl

Staff Emeritus
I agree. The only caveat being that the Bohmian interpretation of quantum mechanics actually does say that uncertainty is due to lack of precise knowledge about the state (and location) of a particle.

On the other hand, certain observables, such as the spin of an electron along an axis other than the one that you actually measure, just don't have values in the Bohmian interpretation. Whether an electron goes to the left or to the right in a Stern Gerlach device is a complicated (but deterministic) function of the state of the measuring device. So there is no unique answer to the question: "What would have happened if I had measured the spin along a different axis?"

9. Jan 14, 2019

### DrChinese

An EPR pair (a system of 2 entangled particles) can be measured independently. They act as "clones" of each other, in the sense that the entangled properties obey certain conservation rules. Yet, you cannot exceed the Heisenberg limit when you measure non-commuting properties: one on one, another property on the other.

So no, as already pointed out by stevendaryl and Nugatory, you are quite incorrect. Yours is a common misconception.

10. Jan 25, 2019

### Kevin Chieppo

Just wanted to say thank you for the thoughtful replies.