The absolute value of the frequency in quantum mechanics

[mccoy1]

I was reading Bransden's Quantum Mechanics 2nd edition, chapter 2 page 61. There,it says "It should be noted that since E =hv (v for nu), the absolute value of the frequency has no physical significance in Quantum mechanics..."
Why is that? Isn't this a contradiction?

 

[PeterDonis]

Why is that?

Because the absolute value of the energy has no physical significance in QM (more precisely, in non-relativistic QM or in QFT in flat spacetime, where gravity is absent). Only differences in energy have physical significance.

 

[mccoy1]

Only differences in energy have physical significance.

I see. I got it now. Thank you PeterDonis.