Particle Trajectory & Velocity in Bubble Chamber vs HUP

[eoghan]

Hi! Let's say I have a bubble chamber and a particle traveling in it. I know the trajectory of the particle, because I can see the bubbles. But if I measure the time elapsed between the appearance of the first bubble and of the last one, I know how much time the particle took to follow the trajectory. So I can know exactly particle's trajectory and particle velocity, but this is in contradiction with HUP... so, where am I wrong?

 

[Bob_for_short]

You are wrong because first you do not have a bubble chamber.

 

[DrChinese]

Hi! Let's say I have a bubble chamber and a particle traveling in it. I know the trajectory of the particle, because I can see the bubbles. But if I measure the time elapsed between the appearance of the first bubble and of the last one, I know how much time the particle took to follow the trajectory. So I can know exactly particle's trajectory and particle velocity, but this is in contradiction with HUP... so, where am I wrong?

Well, this one is way off because you are measuring things at 2 different points in time. The HUP relates to knowing non-commuting observables simultaneously. The information you gained from measuring at T1 is no longer relevant after the measurement at T2.

 

[Bob_for_short]

Well, this one is way off because you are measuring things at 2 different points in time. The HUP relates to knowing non-commuting observables simultaneously. The information you gained from measuring at T1 is no longer relevant after the measurement at T2.

It is a common misunderstanding. The uncertainty remains even if you take the delta_x and delta_p at different times.

 

[Bob S]

A fast particle travels about 30 cm per nanosecond, so will (did, actually) travel across the largest bubble chamber in about 10 or 15 nanoseconds. Visible bubble formation takes several milliseconds after the piston moves and the pressure on the hydrogen (or other liquid) is reduced to initiate bubble formation.
Bob S

 

[DrChinese]

It is a common misunderstanding. The uncertainty remains even if you take the delta_x and delta_p at different times.

Not sure I follow your thinking here. You can measure delta_x to any degree of accuracy at T1, and then measure delta_q to any degree of accuracy at T2. Maybe not in a bubble chamber though.

 

[Cthugha]

I think you are talking past each other.

As Feynman pointed out in his lectures the HUP is about what we are able to predict. If we have a Gaussian wavepacket there is of course some uncertainty about predicting \Delta x and \Delta p at different times. However nothing will keeping us from backtracking for example by putting a position detector behind a long slit (assuring that the vertical momentum equals 0). You know the past pretty exactly, but you were not able to predict it.

 

[Bob_for_short]

You know the past pretty exactly, but you were not able to predict it.

Knowing the past is even better than predicting.