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How do you prove that the speed of light is absolute 
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#1
May1909, 08:48 PM

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I'm in grade 8, but this website looked really helpful
Im doing my science fair project on time dilation, and I am wondering how to prove that the speed of light is absolute. Can anyone help me? 


#2
May1909, 09:24 PM

P: 261

One thing that comes to mind is to consider the halflives of particles. Some particles in the laboratory decay radioactively at a certain rate. But I think I read somewhere that particles of the same kind that strike the Earth from outer space (and are traveling really fast) decay at a slower rate. This is evidence that very very speedy objects age more slowly, which is a consequence of the the speed of light being constant.



#3
May1909, 09:34 PM

P: 15,319

I'm not sure you can prove it. It's a postulate.
So, the logic is thus: Let's suppose the speed of light is constant. Then ... Thus etc., etc. etc... And we arrive at Einstein's theory of relativity. So, since Einsteins' ToR explains what we see exquisitely well, we think it is very likely right, thus our original postulate is very likely true. 


#4
May1909, 09:45 PM

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How do you prove that the speed of light is absolute
The speedoflight postulate has also been tested experimentally, many times. See the FAQ Experimental Basis of Special Relativity, in particular section 3.



#5
May1909, 10:33 PM

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Purely conjecturally (I am not espousing a crank theory!), suppose someone experimentally finds a very slight frequencybased dependency in the speed of light and others confirm the results. While Einstein's postulate will remain an interesting mathematical postulate, it will be in contradiction with how the universe truly works. Einstein's postulate will be deemed as approximately correct. One can prove things in mathematics. Science doesn't work that way. After seeing thousands of black crows, and never seeing a crow other than black, you might postulate that all crows are black. Scientists worldwide confirm your hypothesis. This does not prove your hypothesis is correct. It will in fact be provably incorrect the minute someone documents finding an albino crow. 


#6
May2009, 12:04 AM

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A more modern example in terms of special relativity that I would suggest is particle physics. Like Cantab stated, there are none decay times and halflife of particles which we can compare against in particle accelerators. In an accelerator, the particles produced will travel at relativistic speeds, allowing us to observe time dilation by comparing how long the particles lasted at high speeds with respect to how long they last at rest. I once had a workbook in high school of bubble chamber photos. Using overlays, you could measure the curvature of a path to find the energy and type of particle. You could measure the length of the path and using the energy try to deduce the lifetime. A fun and surprisingly accurate exercise but I do not know what the name of the text was. 


#7
May2009, 10:29 AM

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Instead, scientific theories are tested by trying to disprove them. If you have proved that a prediction made by the theory is false, the theory is said to have been "falsified". What you should be looking for are experiments with results that are consistent with the predictions of special relativity, but contradict some of the predictions made by Newtonian mechanics (the best theory we had before special relativity). Those experiments don't prove special relavity, but they prove that SR is a better theory than the alternative. 


#8
May2009, 10:34 PM

P: 4

Thanks everyone,
If you know of anything else I could put in my science fair j(on time dilation), it would be very helpful. I have heard of the Dopler effect, and how that is used in police radar, the red shift/ blue shift. Would this have any consequence with this question? 


#9
May2009, 10:45 PM

P: 4

Fredrik "What you should be looking for are experiments with results that are consistent with the predictions of special relativity, but contradict some of the predictions made by Newtonian mechanics"
What are some experiments that you would recommend? 


#10
May2009, 11:01 PM

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...in math, however, a theorem is either true or false: 100% proven or 0% proven. ...and in the wiki for theory, they quote the National Acadmy of Sciences, and though they don't use the word "proof", they do say "well established". And that's exactly synonomous with "well proven". 


#11
May2109, 07:16 AM

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Another thing you might want to include is the following very easy* proof that invariance of the speed of light implies time dilation: Imagine a train moving at speed v with a laser attached to the ceiling, aimed straight down. The laser is switched on, and hits the floor a time t_{0} later according to a clock on the train, and a time t later according to a clock on the ground. These times can't be the same, because to an observer on the train, the distance the light moved is ct_{0}, and to an observer on the ground, the distance the light moved is ct. The times can't be the same because these distances can't be the same. The observer on the ground will agree that the light moved a distance ct_{0} straight down, but he will also say that it also moved a distance vt to the side. This means that you can draw a right triangle such that the lengths of its sides are "the distance the light moved in the train's rest frame", "the distance the light moved in the ground's rest frame" and "the distance that the train moved". If you express those distances in the form "velocity*time" (as I did above), you can use the pythagorean theorem to find the relationship between t and t_{0}. *) Very easy for those of us who spent years studying physics at the university, but probably not so easy but still doable for an 8th grader. 


#12
May2109, 07:55 AM

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#13
May2109, 08:39 AM

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In physics, yes. In mathematics, no. Because we are talking about physics the "proof" or "evidence" is "experimental evidence".



#14
May2109, 01:41 PM

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How about verifying Ampere's law? I think that can be done with high school apparatus. Ampere's law can be related to Coulomb's law via Lorentz transformations, which are derived by postulating the Principle of Relativity and the constancy of he speed of light for all inertial frames.
http://physics.weber.edu/schroeder/mrr/MRR.html 


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