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Hrithik mudaliar
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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/Hrithik mudaliar said:but if doesn't 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
thanksNugatory said:Here 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.Hrithik mudaliar said:but if doesn't change anything how would we know that the speed of light was constant over the time ?
In case you didn't know...Hrithik mudaliar said: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.Hrithik mudaliar said:if speed of light were not 3*10 ^8 m/s and something else would it affect the reality ?
Hi @Dale:Dale said: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.
Buzz Bloom said: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
Buzz Bloom said:and the values of e and h are unchanged
Hi Peter:PeterDonis said: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.
Buzz Bloom said: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.
Buzz Bloom said:I am assuming that it is not a problem to measure the frequency f of a photon without knowing the speed of light.
Buzz Bloom said:If both h and f can be measured without knowing c
Buzz Bloom said:which seems to be measuring e without requiring knowledge of c
Buzz Bloom said:I was not able to find on the Internet what the method is for establishing the standard for the volt unit
Hi Peter:PeterDonis said: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.PeterDonis said: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.
Buzz Bloom 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.
Buzz Bloom said:If that is correct
Buzz Bloom said:what is the meaning of: "So there is no physical meaning to asking which one of c, e, and h 'actually' changed,"?
Hi Peter:PeterDonis said:The usual way of "measuring" the frequency of a photon is to measure its energy and divide by Planck's constant.
Hi Peter:PeterDonis said: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.Buzz Bloom 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
Buzz Bloom said:Obviously I misunderstood the meaning of:
PeterDonis said: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:PeterDonis said: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.Buzz Bloom said:If h, e, and c, can be measured independently of knowing the value of α,
Buzz Bloom said:I was able to satisfy myself that a voltage value could be determined without any knowledge of values for α, e, h, and c.
Buzz Bloom said: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:PeterDonis said:How?
Buzz Bloom said:My understanding from what I perused (as best I could) is that NIST has produced (1991) a stable 1 volt standard apparatus.
Hi Peter:PeterDonis said: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:PeterDonis said:Ok, but does the behavior of this apparatus depend on the value of α\alpha? (Hint: the answer is yes.)
Buzz Bloom said:it must be the case that efforts to measure a value for any of the four constants has hidden dependencies of the value of one or more other constants
Yes, right here:Ibix said:I seem to recall @Dale worked this out in more detail recently, but I can't find the post at the moment.
This one is very easy in SI units. In SI units the energy of a photon is measured in joules. 1 J is a derived unit which is equal to 1 kg m^2/s^2. So the meter is part of any measurement of energy in SI units. In SI units the meter is defined in terms of c. So to measure the energy of a photon in SI units does require knowing c in SI units also.Buzz Bloom said: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.
It is not an issue of the experiment, it is an issue of the units.Buzz Bloom said:Millikan’s experiment ... which seems to be measuring e without requiring knowledge of c.
PeterDonis said:There is no way to isolate the changes to just e, c, or h, because there is no way to tell that something changed by just looking at one device.
Dale said:It is not an issue of the experiment, it is an issue of the units.
Buzz Bloom said:This means that by definition the value of c (using any units) cannot change.
Buzz Bloom said:In effect c is defined as 1 light-second per second.
Buzz Bloom said:If it should (hypothetically) happen that the number of seconds (or picoseconds) it takes light to travel (in vacuum) over an actual specific physical distance is measured to have changed
Buzz Bloom said:I wanted to work out what would be the experimental result if the speed of light doubled but the fine structure constant was unchanged.
Buzz Bloom said:Assuming α changes due to the change in c
Hi Peter:PeterDonis said:if the fine structure constant changes, it is not "due to" a change in c, e, or h. Which of c, e, and h change if the fine structure constant changes is a matter of choice of units, not physics. The physics is all in the fine structure constant.
I was assuming that any alternate system of defining units would relate to SI units by constants. For example, in cgs units, 1 centimeter = 1/100 of a meter, and 1 gram = 1/1000 of a kilogram. Also in British Imperial units, 1 foot = 0.3048 meters, and 1 pound = 0.45359237 kilograms. If this is a generally correct assumption, then using any unit from a system of units with this kind of definition relative to SI units would leave the value of c unchanged.PeterDonis said:No, it just means the value of c in SI units cannot change, because it's not measured any more, it's defined to be a specific number.
I assume a suitable device which measures the time difference in picoseconds (ps) between two signals, where these ps units are carefully controlled to be in terms of the cesium definition of a second. I assume a straight underground tunnel which contains as close to a vacuum as practically possible. The length of the tunnel is assumed to be unchanging over time. There is an apparatus at one end which emits a laser pulse when a button is pushed by an experimenter outside of the tunnel. The button push also starts the counting of picoseconds by a suitable digital counter. The apparatus also includes a radar pulse detector which stops the counter when it detects a pulse. At the other end of the tunnel is a mirror that reflects the radar pulse from the emitter back to the detector. The use of this apparatus is repeated from time to time to measure the time (in picoseconds) it takes for a pulse to move from the emitter to the mirror to the detector. There may also be auxiliary devices to detect possible disturbances that would effect the time measurement, for example, a device for detecting seismological disturbances due to earthquakes.PeterDonis said:How would you measure this?
The quote I posted in #29 was an answer by Dale to a similar but different question (in which α did not change) posted in a different thread which I cited. I quoted it because @Dale's answer suggested to me that he was likely to be able to answer the different question (where α did change) which I asked in that context.PeterDonis said:But you just said the fine structure constant was unchanged. That's inconsistent with this sentence.
Buzz Bloom said:I was assuming that any alternate system of defining units would relate to SI units by constants.
Buzz Bloom said:If this is a generally correct assumption, then using any unit from a system of units with this kind of definition relative to SI units would leave the value of c unchanged.
Buzz Bloom said:The length of the tunnel is assumed to be unchanging over time
Buzz Bloom said:There may also be auxiliary devices to detect possible disturbances that would effect the time measurement, for example, a device for detecting seismological disturbances due to earthquakes.
Buzz Bloom said:The quote I posted in #29 was an answer by Dale to a similar but different question (in which α did not change) posted in a different thread which I cited. I quoted it because @Dale's answer suggested to me that he was likely to be able to answer the different question (where α did change) which I asked in that context.
Hi Peter:PeterDonis said:If those statements appear to contradict your "philosophical view of reality", then perhaps you should consider discarding your philosophical view of reality.
Buzz Bloom said:In particular I do not grasp the meaning of "physics" in this context.
PeterDonis said:The only way for experimental results to change is if the fine structure constant changes.
Buzz Bloom said:The examples you have given in response to my posts suggest another possible interpretation to this quoted statement.