# Few questions on uncertainty principle

1. Nov 17, 2006

### AlbertEinstein

Can we mix SR and uncertainty principle? for ex. from uncertainty principle we have $$\Delta x \Delta p \geq h/4\pi$$.
Books then write $$m \Delta x \Delta v \geq h/4\pi$$ with an assumption that m can be measured accurately.
However form SR we know that mass depends on velocity; with mass increasing with velocity. Now if we can't measure v exactly how can we measure m exactly?

please explain.
Thanks

Last edited: Nov 17, 2006
2. Nov 18, 2006

### AlbertEinstein

Hey guys. please help. Have I written something silly.? pls explain. I hope this is not homework type question.

3. Nov 18, 2006

### DaveC426913

You have just tripped over what is arguably the greatest problem in 20th century physics.

SR and QM are both theories that have been tested with exquisite precision, two of the best tested theories in the history of science, and they passed with flying colours. SR beautifully describes the macroscopic world - gravity, stars, galaxies and the universe. QM beautifully describes the subatomic world - atoms, electrons, quarks.

Yet they cannot be combined. Putting the two together generates infinities. They are incompatible as we currently understand them. Our understanding of the universe is broken.

Last edited: Nov 18, 2006
4. Nov 18, 2006

### AlbertEinstein

Then what should I assume:write the eq quantum way or non-quantum way? If both are incompatible then how masses at high velocties are measured? btw I heard that only GR is not compatible .

5. Nov 18, 2006

### selfAdjoint

Staff Emeritus

Albert, the relativity problem DaveC was talking about was the general covariance of General Relativity. For Special Relativity there are good unifications with Quantum. Dirac's equation for the electron is one, and all the accepted Quantum Field Thoeries from QED to the Standard Model are "manifestly covariant" in the SR sense of the term. If you just want to do electrons, look up Dirac, it's usually in the first or second chapter of any QFT textbook.

6. Nov 18, 2006

### AlbertEinstein

oh, can I get dirac eq in QM by resnick

7. Nov 18, 2006

### DaveC426913

My bad. I was talking about GR and QM, not SR and QM.

8. Nov 19, 2006

### Staff: Mentor

If you mean, "can I find the Dirac equation in Resnick's QM book", try looking in the index.

9. Nov 19, 2006

### Los Bobos

It is actually more consistent to say, that from SR we know that mass does not increase with velocity. Here you would have to think what is momentum in SR. And of course the biggest problem of elementary QM, the difference in the handling of spatial and temporal coordinates.

10. Nov 19, 2006

### Staff: Mentor

The postings discussing mass in relativity, which used to be located here, are now in the following thread in the relativity forum:

https://www.physicsforums.com/showthread.php?t=144545

Please make further postings along this line in that thread. Discussion about the uncertainty principle can continue in this thread here. Thanks in advance for your cooperation.

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