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if speed of light were not 3*10 ^8 m/s and something else would it affect the reality ?
Here is a recent thread on the subject: https://www.physicsforums.com/threads/why-is-the-speed-of-light-what-it-is.948620/but if doesnt change anything how would we know that the speed of light was constant over the time ? and it was changing over the time as the universe was expanding
thanksHere is a recent thread on the subject: https://www.physicsforums.com/threads/why-is-the-speed-of-light-what-it-is.948620/
We can just define the speed of light to be constant over time.but if doesnt change anything how would we know that the speed of light was constant over the time ?
In case you didn't know....if speed of light were not 3*10 ^8 m/s and something else would it affect the reality ?
As others have mentioned, experimental tests to see of the speed of light changes with time usually revolve around looking for changes in the fine structure constant.if speed of light were not 3*10 ^8 m/s and something else would it affect the reality ?
Hi @Dale:We can just define the speed of light to be constant over time.
What physicists are actually interested in is whether the fine structure constant is constant over time. That is a matter of real physics, not just unit choices.
This is a meaningful question, yes.Suppose the fine structure constant, α = 2π e2 / h c, actually had a different value some time in the past, t, than it does now
But this is not. If the fine structure constant changes, it is entirely up to your choice of units which of c, e, and h you want to change; you could choose for any one of them to change, or any two, or all three. So there is no physical meaning to asking which one of c, e, and h "actually" changed. The only physically meaningful question is whether the fine structure constant changed.and the values of e and h are unchanged
Hi Peter:So there is no physical meaning to asking which one of c, e, and h "actually" changed. The only physically meaningful question is whether the fine structure constant changed.
Who said it did?I would much appreciate your explanation about how the energy E of a photon is measured, and why this measurement depends on knowing the speed of light.
How would you measure the frequency of a photon?I am assuming that it is not a problem to measure the frequency f of a photon without knowing the speed of light.
The usual way of "measuring" the frequency of a photon is to measure its energy and divide by Planck's constant.If both h and f can be measured without knowing c
Yes. So what?which seems to be measuring e without requiring knowledge of c
Did you try Google? I did and found the Wikipedia page on "Volt" pretty easily.I was not able to find on the Internet what the method is for establishing the standard for the volt unit
Hi Peter:Who said it did?
My interpretation was that given α = 2π e2 / h c, if α changes, and e does not change and h does not change, then the change is α must be due a change in c. If e can be measured without specifying a value for c, and e can be measured without specifying a value for c, then the changed value of c can be calculated: c = 2π e2 / h α.So there is no physical meaning to asking which one of c, e, and h "actually" changed. The only physically meaningful question is whether the fine structure constant changed.
Please go back and read my post again, carefully. This is not what I said.My interpretation was that given α = 2π e2 / h c, if α changes, and e does not change and h does not change, then the change is α must be due a change in c.
It isn't. Go back and read my post again, carefully.If that is correct
You are confusing yourself by thinking that you measure h, e, and c. You don't. You measure ##\alpha##. (More precisely, you observe and record physical events whose relationships depend on ##\alpha##.) The formula ##\alpha = 2 \pi e^2 / h c## does not tell you how to calculate ##\alpha## once you've measured h, e, and c. It tells you how your measurement of ##\alpha## is related to other measurements. Summarizing the results of lots of different measurements in constants like h, e, and c, which have units, is a matter of convenience (and historical practice), not physics; all of the actual physics is in dimensionless numbers like ##\alpha##.what is the meaning of: "So there is no physical meaning to asking which one of c, e, and h 'actually' changed,"?
Hi Peter:The usual way of "measuring" the frequency of a photon is to measure its energy and divide by Planck's constant.
Hi Peter:Please go back and read my post again, carefully. This is not what I said.
is what you said. I said it was my interpretation of what you said, and I also indicated that I realized my interpretation was incorrect.My interpretation was that given α = 2π e2 / h c, if α changes, and e does not change and h does not change, then the change is α must be due a change in c
I confess that I am having difficulty understanding the meaning of you posts, and reading them again will not help me. You last paragraph did provide some help.Obviously I misunderstood the meaning of:
What is still a mystery to me is that I do not understand the reason that,You are confusing yourself by thinking that you measure h, e, and c. You don't. You measure α\alpha. (More precisely, you observe and record physical events whose relationships depend on α\alpha.) The formula α=2πe2/hc\alpha = 2 \pi e^2 / h c does not tell you how to calculate α\alpha once you've measured h, e, and c. It tells you how your measurement of α\alpha is related to other measurements. Summarizing the results of lots of different measurements in constants like h, e, and c, which have units, is a matter of convenience (and historical practice), not physics; all of the actual physics is in dimensionless numbers like α\alpha.
Hi Peter:Did you try Google? I did and found the Wikipedia page on "Volt" pretty easily.
They can't. Or, more precisely, there's a hidden assumption somewhere in any experiment that purports to measure them. For example, if you try to measure the speed of light you'll need a meter rule somewhere and its length depends on the strength of the interaction between atoms, which depends on e. I seem to recall @Dale worked this out in more detail recently, but I can't find the post at the moment.If h, e, and c, can be measured independently of knowing the value of α,
How?I was able to satisfy myself that a voltage value could be determined without any knowledge of values for α, e, h, and c.
The fact that you can make a particular measurement without knowing the value of ##\alpha## does not mean that the process you are measuring does not depend on the value of ##\alpha##. This confusion seems to be a crucial one for you in this discussion.I was able to satisfy myself that a voltage value could be determined without any knowledge of values for α, e, h, and c.
Hi Peter:How?
Ok, but does the behavior of this apparatus depend on the value of ##\alpha##? (Hint: the answer is yes.)My understanding from what I perused (as best I could) is that NIST has produced (1991) a stable 1 volt standard apparatus.
Hi Peter:The fact that you can make a particular measurement without knowing the value of α\alpha does not mean that the process you are measuring does not depend on the value of α\alpha. This confusion seems to be a crucial one for you in this discussion.
Hi Peter:Ok, but does the behavior of this apparatus depend on the value of α\alpha? (Hint: the answer is yes.)