Should the speed of light be slightly uncertain?

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Discussion Overview

The discussion centers on the question of whether the speed of light should be considered slightly uncertain, particularly in the context of quantum mechanics and the Heisenberg Uncertainty Principle. Participants explore the implications of defining the speed of light as a constant and its relationship to both quantum behavior and relativistic principles.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • Some participants argue that since the position and momentum of a photon are uncertain, the speed of light should also be considered uncertain.
  • Others assert that the speed of light is defined as a constant, thus implying no uncertainty exists regarding its value.
  • A participant questions the relevance of defining the speed of light as an integer and challenges the understanding of its implications.
  • Another viewpoint emphasizes that the speed of light is a consequence of relativity and not quantum mechanics, suggesting that quantum uncertainties do not apply to it.
  • Some participants discuss the definition of the meter and how it relates to the speed of light, indicating that the speed is fixed by definition.
  • There are claims that the uncertainty principle does not necessarily imply uncertainty in the speed of light, as it can be measured independently of a single photon’s position.
  • A later reply highlights that Lorentz invariance implies a maximum speed but does not dictate that light must travel at that speed without further context from Quantum Field Theory.
  • Participants express frustration over perceived semantic arguments and a lack of engagement with the original question regarding the nature of speed and uncertainty.

Areas of Agreement / Disagreement

Participants do not reach a consensus; multiple competing views remain regarding the relationship between the speed of light, quantum uncertainty, and the implications of relativity.

Contextual Notes

Some discussions reference the Heisenberg Uncertainty Principle and its implications, but there is no agreement on how it relates to the speed of light. The conversation also touches on the definitions and measurements of physical constants, which may depend on specific contexts and interpretations.

  • #31
Naty1 said:
velocity as the derivative of position wrp to time is a low speed approximation.
I didn't know this. What is the general relativistic relation then? Or the not approximated expression?

If it were accurate at high speeds, we could accelerate things to light speed.
Why? I really don't see the implication.
In other words, Vtotal= V1 plus V2 is an approximation...
I know this but I don't see how it relates to "v=dx/dt implies v=v_1+v_2".

About the quotes you posted on the HUP, in fact I think the problem is that we aren't making any measurement. Consider any photon, you know it's moving at a speed "c" without any uncertainty. You don't even need to make a measurement for that, else special relativity is violated.

So let's say I measure the position of a photon with a screen that reacts to photon by darkening or something like that. I'd have a finite uncertainty in the position and also in the momentum (the HUP is not violated), but apparently none in the velocity. I'm still puzzled on how to get no uncertainty in the speed (that ok, I did not measure since I know it no matter what) but I have an uncertainty in the position.

Edit: I think I'm starting to understand something. The HUP is not related -directly at least- to the speed of a photon but on its momentum and position. Unlike a massive particle, the momentum of the photon is not related to its speed so an uncertainty in its momentum does not imply an uncertainty in its speed. Instead, an uncertainty in position should raise an uncertainty in velocity, not necessarily speed.
Am I wrong if I think that it's possible to have an uncertainty in position and in velocity and no uncertainty in the speed at the same time?
This would solve the problem...

Basically you know how fast the photon is moving no matter what. You don't know "perfectly" its momentum nor its position, all this at the moment it hit your screen. However there's an uncertainty in the direction the photon when it hit the screen.
So to answer the OP, the HUP principle applied to a photon does not imply an uncertainty in speed. Instead, in the velocity due to the uncertainty associated to its position.
Does this make sense? This does to me.
 
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