Light Speed question

[mooeypoo]

I don't think it can because you'd have to have someone on the planet or star with a (somehow) synchronised stop watch sending out a signal...

 

I understand that relativity states that c is constant to all observers, but this has generally been used to show how clocks are altered by movement and the intensity of a gravitational field.

 

I want to see results of an experiment of light being shone on a moving target... can it be done?

 

Sanandra, you're asking a question, you're given an answer, you're being asked to check out the background of the theory because it's quite clear you don't quite understand it, you ignore that request and post another question as if no one can answer your question to begin with.

 

That's an unfair misrepresentation of what people are telling you.

 

Special relativity is REPEATEDLY proven by experimental evidence and valid mathematics that produce *VALID* predictions.

 

Here's a tidbit:

"Special Relativity": The strongest direct evidence comes probably from particle accelerators, in which subatomic particles such as electrons and positrons are accelerated to within a few inches per second of the speed of light. We can observe very clearly and accurately the changes in, for instance, the apparent masses of the particles. They are observed to increase dramatically, and in fact new and much heavier particles can be created by making counter-rotating beams of, say, electrons and positrons, collide head-on with each other. Special relativity has played a key role in the design and operation of particle accelerators for many decades.

Source: http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/980327b.html

 

and this:

Because an observer on the ground sees the satellites in motion relative to them, Special Relativity predicts that we should see their clocks ticking more slowly (see the Special Relativity lecture). Special Relativity predicts that the on-board atomic clocks on the satellites should fall behind clocks on the ground by about 7 microseconds per day because of the slower ticking rate due to the time dilation effect of their relative motion.

 

urther, the satellites are in orbits high above the Earth, where the curvature of spacetime due to the Earth's mass is less than it is at the Earth's surface. A prediction of General Relativity is that clocks closer to a massive object will seem to tick more slowly than those located further away (see the Black Holes lecture). As such, when viewed from the surface of the Earth, the clocks on the satellites appear to be ticking faster than identical clocks on the ground. A calculation using General Relativity predicts that the clocks in each GPS satellite should get ahead of ground-based clocks by 45 microseconds per day.

 

The combination of these two relativitic effects means that the clocks on-board each satellite should tick faster than identical clocks on the ground by about 38 microseconds per day (45-7=38)! This sounds small, but the high-precision required of the GPS system requires nanosecond accuracy, and 38 microseconds is 38,000 nanoseconds. If these effects were not properly taken into account, a navigational fix based on the GPS constellation would be false after only 2 minutes, and errors in global positions would continue to accumulate at a rate of about 10 kilometers each day! The whole system would be utterly worthless for navigation in a very short time. This kind of accumulated error is akin to measuring my location while standing on my front porch in Columbus, Ohio one day, and then making the same measurement a week later and having my GPS receiver tell me that my porch and I are currently about 5000 meters in the air somewhere over Detroit.

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Relativity is not just some abstract mathematical theory: understanding it is absolutely essential for our global navigation system to work properly!

(emphases not in original)

Source: http://www.astronomy.ohio-state.edu/~pogge/Ast162/Unit5/gps.html

 

 

The above issue with the GPS explains how this is a valid experiment using atomic clocks. This shows you that not only was an experiment conducted with clocks, but that it is CONSTANTLY being "conducted" by the GPS satellites. The mere fact we predicted precisely how much delay the clock would have relative to Earth clock -- and successfully! -- is your requested experimental evidence using clocks.

 

You should read a bit about Special Relativity. It's very interesting, and it will likely solve some of the misconceptions you seem to have about the theory.

 

~moo

[swansont]

 

I want to see results of an experiment of light being shone on a moving target... can it be done?

 

Got a radio? In your car?

 

Maxwell's equations yield a wave equation for electromagnetic waves, as long as c is constant. So if you still have a radio wave, i.e. your radio works, c is constant. I have personally verified this thousands of times.

[munion]

The are not relatives with the thread and my questions

[mooeypoo]

The are not relatives with the thread and my questions

munion your question was answered in the first few posts, and the discussion moved onwards to related question. Is there anything else you didn't get an answer on?

[munion]

Answered ?? where? tell me the post number... I don't think that from post 12 and above i got something..

[mooeypoo]

Let me make it easier on all, then. I thought it was answered, but it seems the discussion strayed a bit. So, for the sake of consistency, here's munion's latest question (I think):

As the SR says (and is proved with many experiments) that is the speed of everything in this world is same and it has the same value with the SOL. So far that we know...

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Let put this a little bit different: Now we have a SOL as we know it and then a cosmic acceleration is happening; the world would be the same before and after that acceleration? in this case something has changed an acceleration happened (even is theoretical and forbidden by the SR).

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Ps: with cosmic acceleration i mean SOL acceleration sorry any misleading s

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I think that my questions are boring.... (and my English bad)

 

Was that the one you didn't get an answer for, munion?

[munion]

Yes from this point and then i lost it!!!

[kleinwolf]

Maybe you refere to atomic units, where c=hbar=e=1 if I remember a few. We redefine the context.

 

then formally the relativistic speed addition gives 1+1=1, (representing c+c=c) which can be disturbing.

[munion]

????????????

[D H]

????????????

You are right to be confused by that post. The poster was confused.

 

I think that you might be trying to bite off a bit too much at once, munion. You started with a reasonable question, but then you delved into more esoteric stuff before (inflation, expansion of the universe) before getting a good understanding of special relativity. One step at a time!

 

Let's start with the first word in special relativity. Special here means that the theory is of limited domain. It does not apply to objects undergoing gravitational acceleration, and it does not apply to the expansion of the universe. For that, you need general relativity.

 

Your opening post regarded what would transpire if the speed of light was very small. Nobody really delved into this question:

 

What would we see if the speed of was 5 km/hr? For grins, how about making that just a bit faster? (There's no need for wheeled vehicles if the top speed is a leisurely walk.) Somewhere out on the 'net is a site that helps with visualizing special relativity by asking what if the speed of light was 20 km/hr, or something like that.

 

The answer is that things would like quite bizarre. Round bicycle wheels would appear to go out of round as a bicyclist accelerated toward the speed of light. The bicycle will appear to get longer and longer to someone on the ground. Getting the bike up to speed would take a lot of energy. Getting the bike up to racing speed (and racing speed in this universe is low) would take immense amounts of energy. If you are the one on the bike, the landscape will change in bizarre ways as you get up to speed. Things will get compressed from the perspective of the rider.

[munion]

Dear D H

Thank you for your answer it was my mistake to involve cosmic accelerations but i hadn't any better example in order to answer to the previous post. This which is not clear in my mind is : how we can measure something which is absolute (or invariant if you prefer). If you kind enough and read my first posts (until the post #13 i think) you will see what it is my confusion.

Edited September 6, 2009 by munion

[swansont]

Dear D H

Thank you for your answer it was my mistake to involve cosmic accelerations but i hadn't any better example in order to answer to the previous post. This which is not clear in my mind is : how we can measure something which is absolute (or invariant if you prefer). If you kind enough and read my first posts (until the post #13 i think) you will see what it is my confusion.

 

If it's invariant it makes it easier, because there are fewer conditions on has to meet.

 

But I think you might be asking a different question. If it's invariant, no matter what you do, you are going to get the same answer, so in that sense it's like a self-fulfilling prophecy. Is that what you mean?

 

The speed of light has units, so it's going to depend on the unit system you choose: meters and seconds. And because time and length change in similar fashions, they could change (as they do in a moving frame of reference) to give the same answer for c.

 

However, there are other fundamental constants and invariant quantities. The fine structure constant, for example, is unitless — so it doesn't depend on your unit system. It depends on the electric charge, as well as c. So if c changes independently, then the fine structure constant has to change, and this has ramifications as to the wavelength of emitted light in atomic/molecular transitions and whether certain bonds can even form. So there are other measurements one can make to ensure that c is really invariant.

[munion]

No i m asking that: from the fact that the speed of all in 4D time space is the same C how i can measure it?. An example is the following: you have a device which measuring the speed of light. Suddenly the 4D speed is changing with the same rate; your device will show something different?

 

Regarding with fine structure constant really i don't know what would happened int this case.... the value would be the same but i can't say anything else... i have a wiki in fine structure http://en.wikipedia.org/wiki/Fine-structure_constant and it has a topic about the constancy of fine structure.

Edited September 6, 2009 by munion

[swansont]

I'm not understanding what you mean by "how can I measure it?" You can measure a distance and an elapsed time. The ratio gives you the speed.

 

If, hypothetically, all of the constants were scaling at the same rate, such that these measurements of any invariant didn't change, we wouldn't notice. But why would it matter? And I'm not familiar enough with all of the dimensionless constants — it may be that this cannot happen.

Edited September 6, 2009 by swansont

[munion]

Ok forget the phrase how i can measure it. Focus in the previous experiment which i restate it in that: You have a device which measuring the speed of light. Suddenly the 4D speed is changing for all with the same rate; your device will show something different? You say "If, hypothetically, all of the constants were scaling at the same rate, such that these measurements of any invariant didn't change, we wouldn't notice." And of course you will notice that because of your inertia (your mass).The SOL would be "unchangeable" and you would feel a mysterious acceleration. (Hypothetically speaking IF the SOL changing).

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One last thing for the fine structure constant from Wikipedia we have:

 

Arnold Sommerfeld introduced the fine-structure constant in 1916, as part of his theory of the relativistic deviations of atomic spectral lines from the predictions of the Bohr model. The first physical interpretation of the fine-structure constant α was as the ratio of the velocity of the electron in the first circular orbit of the relativistic Bohr atom to the speed of light in the vacuum.[4] Equivalently, it was the quotient between the maximum angular momentum allowed by relativity for a closed orbit, and the minimum angular momentum allowed for it by quantum mechanics. It appears naturally in Sommerfeld's analysis, and determines the size of the splitting or fine-structure of the hydrogenic spectral lines.

 

So this ratio (the ratio of the velocity of the electron in the first circular orbit of the relativistic Bohr atom to the speed of light in the vacuum) would be unchangeable to the SOL variations. I m not expert in that so this could be incorrect.

Edited September 6, 2009 by munion
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[D H]

Suddenly the 4D speed is changing for all with the same rate; your device will show something different?

What do you mean by this?

[munion]

4D (SOL if you prefer) speed acceleration.... help me to understand what is your question here.

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for all i mean for all the matter which included in time space

Edited September 6, 2009 by munion

[insane_alien]

you mean the 4-vector for velocity? its always constant in magnitude, it can only change direction.

 

handled nicely by general relativity. input the data into its equations and you'll get your answer.

[munion]

"you mean the 4-vector for velocity? its always constant in magnitude, it can only change direction.

handled nicely by general relativity. input the data into its equations and you'll get your answer."

 

 

Nope i mean 4D accelaration an hypothetical SOL change ok? read the previous posts you are completely out of scope.

Edited September 7, 2009 by munion

[mooeypoo]

I recommend all members be civil to one another and go over our rules. Impolite behavior will not be tolerated. Also, if you don't have anything consructive to add to the debate, don't post at all. That goes to everyone.

[munion]

I recommend all members be civil to one another and go over our rules. Impolite behavior will not be tolerated. Also, if you don't have anything consructive to add to the debate, don't post at all. That goes to everyone.

 

thank you mooeypoo

[D H]

munion, insane_alien did the best he could to interpret what you asked in terms of 4D speed. "Speed" means magnitude of a velocity vector, so what else could 4D speed mean other than the magnitude of the 4-velocity? The magnitude of the 4-velocity is indeed always constant.

 

I think you meant something else, but I do not know what that something else is. Your question doesn't quite make sense, munion. That is why I asked what you meant by "Suddenly the 4D speed is changing for all with the same rate; your device will show something different?"

 

Try asking without using the term "4D speed".

[munion]

munion, insane_alien did the best he could to interpret what you asked in terms of 4D speed. "Speed" means magnitude of a velocity vector, so what else could 4D speed mean other than the magnitude of the 4-velocity? The magnitude of the 4-velocity is indeed always constant.

 

I think you meant something else, but I do not know what that something else is. Your question doesn't quite make sense, munion. That is why I asked what you meant by "Suddenly the 4D speed is changing for all with the same rate; your device will show something different?"

 

Try asking without using the term "4D speed".

 

Yes DH i m sorry for the misunderstanding you have right. But i don't know how to express it different. My question is about a hypothetical speed of light change...

 

Lets restate it and i hope to be clear in this time:

You have a device which measuring the speed of light. The SOL is changing ; your device will show something different?

Edited September 8, 2009 by munion

[swansont]

Yes DH i m sorry for the misunderstanding you have right. But i don't know how to express it different. My question is about a hypothetical speed of light change...

 

Lets restate it and i hope to be clear in this time:

You have a device which measuring the speed of light. The SOL is changing ; your device will show something different?

 

We went over this already — there are many processes that depend on the speed of light. If c changed, without a scaled change in all other physical constants (if that's even possible), something would be different, such as the strength of atomic or nuclear forces.

[D H]

Let's back up a bit. I think what you are really asking is "are the physical constants really constant with respect to time?" Physicists assume that the key physical constants are indeed constant in the theories they develop. Our current understanding of the universe cannot answer your question because it is a nonsense question in terms of that understanding: The constancy of the key physical constants is built-in to that understanding. One obvious consequence, however: Energy would not be conserved if the physical constants varied with time. Energy is conserved because the laws of physics are invariant with respect to time. Get rid of that invariance and you get rid of conservation of energy.

 

Because physicists do not like unwarranted assumptions, they test the heck out of these assumptions of the physical constants being constant. There are ways of looking back in time, a long ways back in time. For example, telescopes essentially see back in time. Another example are the Oklo deposits in Africa. Two billion years ago these formed a natural nuclear reactor. The decay products we see now from these two billion year old reactions demonstrate that the laws of physics have not changed over the last two billion years.

 

Here is one web site that tries to answer your question: http://www.phys.unsw.edu.au/einsteinlight/jw/module6_constant.htm. It starts with this personal preamble (emphasis mine):

As a child, I asked my parents "What if everything in the world had shrunk to half size overnight. How would we know? I'd be half as tall, but the ruler would be only half as long, so I couldn't tell." But this invited further questions: how long would it take to walk to school? If my half-length legs propelled me at half the speed, I wouldn't notice the change in the distance. Or perhaps shrinking all the molecules in my leg muscles would make the electrical forces stronger so I'd walk faster. Or maybe all the clocks would be sped up by a factor. Pendulums and planetary orbits would be shorter, and crystals would be stiffer (per unit area). Both gravitational and electric clocks would run fast. At the time, I didn't know about inverse square laws or quantum mechanics, but it seemed reasonable that the (electrical) muscular forces and gravitational forces should be changed, too.

Conclusion in retrospect? Starting from this point makes the question more complicated and confusing than it need be - one ends up going around in circles. It's easier to start logically and work up from the basics.

I think this same confusion is part of your problem. Read this article, see if it answers some questions.