Doubts in special theory of relativity

[nitsuj]

There is also a rule about off-topic discussions such as the one both of us are currently having. If you want, use the 'report' button on my post instead.

tl;dr is the dumbest acronym imo.

What's the semicolon for? To make it more clear? Choose your acronyms wisely. popular ones here are

SR
GR
FoR


the list is long. But never have I seen tl;dr. It doesn't even make grammatical sense the way you used it. It looks like you mashed the keyboard by accident, not actually trying to communicate something.

As you can see and to your point about "off-topic" posts, using unpopular acronyms is...

 

[ghwellsjr]

Please follow the rules.

There is also a rule about off-topic discussions such as the one both of us are currently having. If you want, use the 'report' button on my post instead.

How was I supposed to know you weren't following the rules? I thought, like justin, "you mashed the keyboard by accident" and I was simply bringing it to your attention so you could correct it. Instead, you put the burden of understanding your keyboard shortcuts on all your readers. You didn't even explain it simply. You made me go to a link which ended by saying "Was that so hard?". The question is: Is it so hard for you to type normal English? Instead, you prefer to save a couple seconds of your own time and waste a lot of time for everyone else. Please follow the rules and spend a tiny little bit more of your time writing out normal plain English so that we can all read it clearly instead of taking our time to go search on Google or elsewhere for what we assume is something that should make sense.

 

[Fredrik]

tl;dr is used fairly often in forums. It's not as standard as LOL, or "the OP", but it's certainly not the first time I've seen it. The semicolon would be correct if you type it all out: "Too long; didn't read." It simply indicates that the sentences are related.

Of course, that doesn't mean that it's a good way to respond to a post. It's kind of rude. The fact that many people will be forced to google it just to get the insult makes it worse.

The worst acronym I've seen used at this forum is in my opinion ATM, for "against the mainstream". I know people in the IT business, and some of them seem to be perverts. They explained to me a few years ago that it means "*** to mouth", so that's what I'm thinking when I see someone say "ATM" here.

 

[JohnWisp]

Again, we cannot measure the speed of light in any direction. We assign the speed of light to be c in all directions in any Inertial Reference Frame (IRF). Therefore, the time it takes for light to propagate from one spatial point to a second spatial point is only the same as it takes for the reflection to propagate back to the first point in an IRF in which the first spatial point is at rest because our definition of an IRF makes it so. In other IRFs moving along the direction between those two spatial points, the time it takes for the light to propagate along the same two paths in opposite directions are not equal.

There is no measurement, independent of previously assigning or assuming the two propagation paths to yield the same time, that will determine that they take the same time.

I don't know why this is so hard for you to accept. If you just think about Relativity of Simultaneity, you can easily see that the time it takes for light to go in two directions between two spatially separated points takes different times in different frames. Look at this post I just made for you in another thread:

https://www.physicsforums.com/showpost.php?p=4341630&postcount=30

In the first IRF diagram where the blue twin is not at rest, the time it takes for his signal to propagate to the red twin is not the same as the time it takes for the light to reflect back but in the second IRF diagram where the blue twin is at rest, the two times are equal.

Your statement above contradicts GPS. We measure c for triangulation (all directions), that's why it works.

Further, this is a quote from Einstein.

With the help of this result we easily determine the quantities $$\xi, \eta, \zeta$$ by expressing in equations that light (as required by the principle of the constancy of the velocity of light, in combination with the principle of relativity) is also propagated with velocity c when measured in the moving system.

http://www.fourmilab.ch/etexts/einstein/specrel/www/

 

[Vanadium 50]

Your statement above contradicts GPS. We measure c for triangulation (all directions), that's why it works.

You keep saying this. It is not true. (And the reference you claimed supported this said nothing of the sort, which is rather annoying).

The speed of light is assumed (actually, defined) to be the same in all directions, not measured to be the same. Indeed, in a world with length contraction and time dilation, it is impossible to measure the one-way speed of light.

 

[JohnWisp]

You keep saying this. It is not true. (And the reference you claimed supported this said nothing of the sort, which is rather annoying).

The speed of light is assumed (actually, defined) to be the same in all directions, not measured to be the same. Indeed, in a world with length contraction and time dilation, it is impossible to measure the one-way speed of light.

With all due respect, light measures c in the stationary frame, so length contraction and time dilation make absolutely no difference to this calculation.

GPS shows light measures c or it would not work. That is why the link is valid.
But,


Allan et al., IEEE Trans. Inst. and Meas., IM-32 no. 2 (1985), pg 118.

They discuss in detail how time and frequency comparisons among the various standards organizations of the world can be performed with an accuracy of about 1 part in 10^14, using GPS satellites.

http://math.ucr.edu/home/baez/physics/Relativity/SR/experiments.html#GPS

Obviously, like MMX, frequency deviations would indicate a measured speed of light that is not c.

 

[pervect]

While the speed of light _is_ currently defined to be equal to "c", it wasn't ALWAYS this way.

I'll trim the history a bit, since we have people who are so impatient on this thread that they can't even write out "too long" longhand...

http://physics.nist.gov/cuu/Units/meter.html

In 1889, a new international prototype was made of an alloy of platinum with 10 percent iridium, to within 0.0001, that was to be measured at the melting point of ice. In 1927, the meter was more precisely defined as the distance, at 0°, between the axes of the two central lines marked on the bar of platinum-iridium kept at the BIPM, and declared Prototype of the meter by the 1st CGPM, this bar being subject to standard atmospheric pressure and supported on two cylinders of at least one centimeter diameter, symmetrically placed in the same horizontal plane at a distance of 571 mm from each other.

The 1889 definition of the meter, based upon the artifact international prototype of platinum-iridium, was replaced by the CGPM in 1960 using a definition based upon a wavelength of krypton-86 radiation. This definition was adopted in order to reduce the uncertainty with which the meter may be realized. In turn, to further reduce the uncertainty, in 1983 the CGPM replaced this latter definition by the following definition:

The meter is the length of the path traveled by light in vacuum during a time interval of 1/299 792 458 of a second.

Note that the effect of this definition is to fix the speed of light in vacuum at exactly 299 792 458 m·s-1. The original international prototype of the meter, which was sanctioned by the 1st CGPM in 1889, is still kept at the BIPM under the conditions specified in 1889.

Historically, people have measured the speed of light. It would be confusing to argue that they didn't do this - history records that they did. But when they did, the were using a different definition of the meter, one based on a "meter prototype". Of which there were a couple of different variants, the particular variant used can be inferred from the time at which the measurement was made, and a detailed reading of the experimental protocol. Well-run experiments are expected to have callbrations traceable back to the bureau of standards.

On one hand, that the fact that we base our modern definition of the meter on the speed of light inspires, I hope, some confidence in the average reader that it is, in fact, constant.

On the other hand, I think that because people have historically measured the speed of light, it is a sensible concept to talk about. When I hear someone talk about "measuring the speed of light", I simply assume that a non-modern defintiion of the meter is being used, one based on a prototype meter.

It's not really all that important if one is using the 1889 prototype, or the 1927 prototype, for most purposes, I think.

 

[ghwellsjr]

While the speed of light _is_ currently defined to be equal to "c", it wasn't ALWAYS this way.

When Einstein formulated his second postulate, it had nothing to do with the value of c or the fact that it is measured to be a constant. This measurement is always a two-way or a round-trip measurement. Rather, his second postulate has to do with the propagation of light in any direction or to state it another way, whenever anyone in an inertial state measures the two-way speed of light, we can define the times it takes for each "half" of the trip to be exactly half of the total round-trip time. Unless we make an arbitrary definition like this, we have no way of knowing how to partition the two times.

Remember, virtually all scientists prior to Einstein did not believe that this could be the case. They believed instead that light propagated at c only in one inertial rest state, that of the ether. Einstein made the brilliant observation that even if that were true, his second postulate would still be consistent with all the facts of nature and provides for a simpler theory than previous ether-based theories.