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In common interpretations of quantum mechanics, it can only be said that objects exist once I observe them - it is not legitimate to ask where an object was before I observed it.

Does this point of view extend to measurements of physical constants? Do physical constants exist or have definite values before we measure them? Do they have definite values when we aren't measuring them? For example, which of the following statements is correct?

  1. A Measurement determines the speed of light
  2. The speed of light determines the outcome of a measurement
  3. Nobody knows, but somebody will know
  4. Nobody will ever know

I think this is a legitimate question, since how could I know that the speed of light is constant, if I am not measuring permanently. Might it only be constant/meaningful to speak of the speed of light the moments I look at it?

innisfree
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NoMorePen
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    are you interested in the speed of light in particular? for technical reasons, it probably isn't the best choice for your question (because it really just converts between our choices of arbitrary units for time and length). – innisfree Mar 11 '15 at 11:45
  • No, it is just an example. I am interested in all physical constants. I see your point though. – NoMorePen Mar 11 '15 at 11:47
  • I've edited your question somewhat, because I think it could be a very good question. but if you think the changes are unwarranted/don't reflect your question, i encourage you to change it back – innisfree Mar 11 '15 at 11:54
  • What QM theory says that objects only exist once they're observed? – Kyle Kanos Mar 11 '15 at 12:44
  • @kyle i'm responsible for that. i mean can only be said to exist... This is the case in any operationalist interpretation of QM (such as Copenhagen), or science in general, isn't it? This is related to Einstein's criticisms of QM, e.g. "I like to think that the moon is there even if I am not looking at it". This is well-known, isn't it? – innisfree Mar 11 '15 at 13:19
  • @innisfree: Due to the uncertainty principle, a particles properties (i.e., $p$ or $x$) aren't known until observation. Extending that to "objects" (where QM rules are diminished) and further to an objects existence seems to be quite beyond the HUP. – Kyle Kanos Mar 11 '15 at 13:40
  • I have not mentioned HUP. Copenhagen is essentially an operationalist interpretation of QM: concepts are meaningful if and only if they correspond to a measurement. That applies to a particle's position and momentum, but also to a particle's existence. e.g. has an atom spontaneously radiated a photon? From when did that photon exist? Did it exist before I measured it? I don't think anything I'm saying is unusual. – innisfree Mar 11 '15 at 13:55
  • I think the answer to the question is that physical constants are not part of a quantum state that is evolving in time; they are constants in the Hamiltonian that we determine from the dynamics. If I can make that more intelligible/intuitive, I will write it up. – innisfree Mar 11 '15 at 14:00
  • Might it only be constant the moments I look at it? Explain how they could be dynamical, yet every time someone measures it they get the same results as everyone else. – Kyle Kanos Mar 11 '15 at 14:02
  • See also http://physics.stackexchange.com/questions/21721, http://physics.stackexchange.com/questions/1586, http://physics.stackexchange.com/questions/11584 – Kyle Kanos Mar 11 '15 at 14:05
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    @kyle it's a subtle point that I've tried to clarify. If we say a physical parameter is constant, we are saying it always has a particular value. In the context of QM/operationalism, that isn't meaningful, because we can't speak of values that the physical parameter might have had when we weren't measuring it. – innisfree Mar 11 '15 at 14:50

2 Answers2

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Let us divide a calculation in physics into two pieces: the dynamics, which describes how a state changes with time, and the initial conditions, which describes the state at a particular time.

In quantum mechanics, the dynamics are described by a Hamiltonian, $H$. The initial conditions are described by a quantum state, $\psi(t_0)$. We evolve the state at time $t_0$ to a later time $t_1$ with the Hamilton $$ \psi(t_1) = \exp(-iH(t_1-t_0)) \psi(t_0) $$ Of course, in QM there is also measurement, a non-unitary process that isn't described by the Hamiltonian.

Physical constants are not part of a quantum state that is evolving in time; they are constants in the Hamiltonian. We determine physical constants by observing the dynamics. For this reason, the usual philosophical conundrums in QM concerning the existence of something prior to measurement (for example, of the properties of an electron) don't apply to physical parameters in the Hamiltonian.

innisfree
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Some physical constants, such as the speed of light $c$, are not measured but are defined to be a certain value. In the case of $c$, it's defined by way of the current definition of the meter. Other physical constants that are measured are always measured against a like-dimensioned standard. All physical measurements are fundamentally of dimensionless values.

robert bristow-johnson
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