Photon flight times and the uncertainty principle

In summary, the conversation discusses the concept of causality in quantum field theory and how it relates to the speed of light. The individual is confused about the possibility of a single particle being detected at a different location in less than the required time period, and questions if this can be experimentally investigated. The uncertainty principle is brought up as a potential factor in measuring the flight time of a photon, and the conversation ends with the conclusion that the measured value of the speed of light is extremely accurate.
  • #1
kurros
453
15
Ok I feel a bit silly asking about this, but I have gotten myself confused and need some help sorting something out. In QFT the operators of observable quantities commute over space-like intervals, so we cannot propagate effects faster than light, causality is safe, cool.
But I have somewhat convinced myself that it is perfectly ok for a single particle emitted at one place to be detected elsewhere after less than the required L/c time period. Right? Because Feynman propagators are non-zero outside the lightcone? Not to mention that the wavepacket of a photon is spread over space, which means that the flight times of a series of identical photons should follow some -probably gaussian-like- distrubution with a mean of L/c? (even in the absence of all equipment-related uncertainties)

Or have I just misled myself here?

Perhaps there is also some interesting interpretation of such an effect, something like you aren't really measuring the 'same' photon, it is just the there is some non-zero probability that the photon field will fluctuate in just the right way so that a photon pops out of the vacuum at just the right time to make it look like your original one went a little faster than c (or slower an equal amount of the time of course)?
 
Physics news on Phys.org
  • #2
kurros said:
But I have somewhat convinced myself that it is perfectly ok for a single particle emitted at one place to be detected elsewhere after less than the required L/c time period. Right?
No.
And you have no ways to put this question experimentally.
Experimentally, you are tight by delays in the emitting and detecting apparatus, which ruin your ambitions.
Just beaches on the mountains...
 
  • #3
I am not sure I understand your claim that this cannot be experimentally investigated. People do lots of photon flight time experiments, one just needs to get long enough flight path in order to measure small variations. If you are talking about fundamental restrictions due to uncertainty principle difficulties in measuring exactly when a photon is emitted or absorbed then I am interested to hear more, because it is this kind of argument that let me to imagine that because we probably can't help but know something about a photon's wavelength, we then cannot ever measure it's position perfectly, and thus there is always an uncertainty in it's flight time, which fundamentally -or so it seems to me- means that flight times cannot perfectly be c always.

Perhaps it is just that one cannot draw from this argument the conclusion that photons "really do" travel faster than c sometimes, it is just that we can't tell their flight time to better than a given accuracy.
 
  • #4
Due to the uncertainty principle, you wouldn't be able to tell if the delay was because the photon's speed was less than c, because it was emitted slightly after you thought it was, or because of some random interaction with a stray molecule. The measured value of c is extremely tight, and not just through experiments. I believe it is possible to derive c using math as well. If it wasn't VERY VERY VERY close to what we say it is we would have known by now.
 

Related to Photon flight times and the uncertainty principle

What is the uncertainty principle?

The uncertainty principle, also known as Heisenberg's uncertainty principle, is a fundamental concept in quantum mechanics that states that there is a fundamental limit to the precision with which certain pairs of physical properties of a particle, such as position and momentum, can be known simultaneously.

What is photon flight time?

Photon flight time refers to the amount of time it takes for a photon, which is a particle of light, to travel a certain distance. This can vary depending on the medium through which the photon is traveling and any external factors that may affect its speed.

How does the uncertainty principle relate to photon flight times?

The uncertainty principle affects photon flight times because it states that it is impossible to know both the exact position and momentum of a particle at the same time. This means that the more precisely we know the position of a photon, the less precisely we can know its momentum, and vice versa. Therefore, the uncertainty principle introduces a level of uncertainty into photon flight times.

What factors can affect the flight time of a photon?

The flight time of a photon can be affected by a variety of factors, including the medium through which it is traveling (such as air or water), the temperature and pressure of the medium, and any obstacles or objects that may alter its path. Additionally, the uncertainty principle introduces a level of inherent uncertainty into the flight time of a photon.

How is the uncertainty principle tested in relation to photon flight times?

One way to test the uncertainty principle in relation to photon flight times is through experiments that involve measuring the position and momentum of a photon. By varying the precision of the measurements, scientists can observe the trade-off between position and momentum and confirm the principles of uncertainty. This can also be tested through theoretical calculations and simulations.

Similar threads

Replies
2
Views
538
Replies
1
Views
923
  • Quantum Physics
Replies
12
Views
2K
  • Quantum Physics
Replies
3
Views
593
  • Quantum Physics
Replies
3
Views
2K
  • Quantum Physics
Replies
21
Views
1K
  • Quantum Physics
Replies
5
Views
792
  • Quantum Physics
Replies
6
Views
3K
Replies
1
Views
580
  • Quantum Physics
Replies
29
Views
2K
Back
Top