What's so special about C?

[Dustin Kesselberg]

When?



His genius.



When?



He didn't.

1.When he said it was constant.

2.That's not an answer.

3.When he said it was always constant.

5.No?Who did?

 

[Darat]

1.When he said it was constant.

But he said it at different times of his life, and at during his lifetime many measurements were made, do you mean when he first came up with his theories?

2.That's not an answer.

Sorry not only is it an answer it's also the correct answer! His genius was that he thought about a new way to perhaps describe, via mathematics, the world, subsequently his theories became testable and it was found out that his maths does seem to pretty much describe how the world "works". However we also now know (from other observations of the world) that his theories are not a complete description of the world.

3.When he said it was always constant.

He didn't know.

5.No?Who did?

No-one. Remember these are theories, not a form of religious truth so its all about remembering that when someone says they “know” they are probably meaning something more like “all the observations we have confirm this part of his theory so we can say we know it to be true” but that doesn’t mean it is true.

 

[Dustin Kesselberg]

1.When Einstein First came up with his theories he based them on the speed of light.

2.Assuming something is always constant when it has never even been measured makes no sense.

3.So what you are telling me is that Einstein did not know the speed of light was constant but assumed it was constant?This also makes no sense.

5.In experaments,it has been observed that when speeding a particle to the speeds of light..It does not pass the speed of light. It's mass increases while it's speed does not.

 

[Darat]

1.When Einstein First came up with his theories he based them on the speed of light.

No, that's not quite correct the idea that the speed of light is a constant is part of his theory. I don't know the history of his development of his various theories well enough to provide you with when he first started using that idea.

2.Assuming something is always constant when it has never even been measured makes no sense.

Quite right, that's why he was a genius! Remember many people considered the idea "crazy" at the time, even ones who could speak the same language! If those people had been able to show his maths was wrong or the theories had been found not to (accurately) describe the observable world Einstein would just be an obscure footnote in the long history of scientists who came up with theories that didn't "work".

3.So what you are telling me is that Einstein did not know the speed of light was constant but assumed it was constant?This also makes no sense.

And I daresay that is why he is today considered a genius and you and I aren't!

5.In experaments,it has been observed that when speeding a particle to the speeds of light..It does not pass the speed of light. It's mass increases while it's speed does not.

?

 

[Dustin Kesselberg]

2.Being "genius" does not mean you are illogical,Or Psychic. Einstein must of had a way of knowing the things he claimed otherwise he was just good at guessing.

3.See No.2

5.Nevermind.

 

[Darat]

2.Being "genius" does not mean you are illogical,Or Psychic. Einstein must of had a way of knowing the things he claimed otherwise he was just good at guessing.

...snip...

He did have a way in that he created a theory that was consistent (mathematically speaking), however he made assumptions in that theory, probably the now most famous is his "cosmological constant" that he described as his greatest "blunder". In other words he "fiddled" his equations because at the time he thought without the constant they didn’t actually describe the universe as he understood it.

I think you are missing out why Einstein is today still considered such a genius, it was the fact that he thought (at the time) like no one else had ever done before and had the sheer intelligence and mathematical ability to turn that new way of thinking into a coherent theory.

 

[Roboramma]

I have a few questions which do not seem to have been answered even though this thread pertains to them...


1.How did einstein know the speed of light?

It was experimentally tested.

2.Why did he assume that it was always constant?

The Michelson/Morely experiment showed that there was no mesureable differance between the speed of light from the sun as the rotation of the earth carried the experimenter toward the source of the light, and when moving away from it.
At least, that's how I remember the experiment... someone else might correct me if I've got the details wrong, this is only high school physics I'm recalling.
Anyway, from that experiment it seemed that the speed of light was constant regardless of the observer moving toward or away from it.

As for your other questions, I don't know enough to comment intelligently.

 

[Dustin Kesselberg]

It was experimentally tested.


The Michelson/Morely experiment showed that there was no mesureable differance between the speed of light from the sun as the rotation of the earth carried the experimenter toward the source of the light, and when moving away from it.
At least, that's how I remember the experiment... someone else might correct me if I've got the details wrong, this is only high school physics I'm recalling.
Anyway, from that experiment it seemed that the speed of light was constant regardless of the observer moving toward or away from it.

As for your other questions, I don't know enough to comment intelligently.

It was tested 80 years ago?

 

[Roboramma]

5.In experaments,it has been observed that when speeding a particle to the speeds of light..It does not pass the speed of light. It's mass increases while it's speed does not.

Almost.. as the speed of the particle increases, it's mass increases exponentially(or something like it). It's a curve that goes up and up as the speed increases, but will only reach the speed of light at infinity.
The fact that mass increases as velocity increases is easy to see from:
1. E=m c squared (how do I use superscript?)
2. E increases as v increases.
if E increases and c is constant, m must increase as well.

Well, that doesn't tell you how much m increases, but E = mc(c) is only one simple equation in the theory, and there are other parts of the math that would show the exact relationship.
Again, I don't really know enough about it to comment, though...

 

[Darat]

It was tested 80 years ago?

I think the first person to give a value for the speed of light was Roemer back in the 17th century.

(Edited to add.)

There is a very good website (IMO) for the The Michelson-Morley Experiment see http://galileoandeinstein.physics.virginia.edu/lectures/michelson.html

 

[Roboramma]

It was tested 80 years ago?

I think it was more than that. Actually, I thought it was at the end of the nineteenth century, but again, I could be wrong about the exact date.
I'm not exactly sure how it was done, but they used rotating mirrors, and based on how far a mirror moved before light from another mirror struck it, they could determine the speed.
Well, something like that anyway. I'm in an internet cafe in bangkok right now, and they connection isn't the greatest for looking these things up.

 

[drkitten]

It was tested 80 years ago?

More like 120. The results of the famous Michaelson-Morley experiment were published in 1887, and showed that the speed of light did not vary with the earth's direction of travel through the hypothesized ether -- or in other words, that the speed of light appeared to be constant irrespective of the motion of the observer.

By 1903, when Einstein published the Theory of Special Relativity, this experiment and its findings were well-known.

There's a brief discusson on Answers.com if you want more information.

 

[NarrMaster]

I believe (I think I read this in "Einstein's Universe") that Einstein attempted to concieve of a wave of light travelling at less than light speed, and came to the conclusion that it was a contradiction. Part of the "thought experiment" included travelling at "C" with the beam of light, and measuring it. A beam of light that is stationary relative to an object is meaningless, apparently.

Just thought I'd throw that out there.

 

[Pragmatist]

I have a few questions which do not seem to have been answered even though this thread pertains to them...


1.How did einstein know the speed of light?


2.Why did he assume that it was always constant?


3.How did he know it was always constant?


4.Why is mass infinite at the speed of light?


5.How did einstein know mass is infinite at the speed of light?

1. Einstein knew the speed of light because it had been measured many times. I have a paper of Maxwell's that predates Einstein by about 50 years in which he mentions various different measurements of the speed of light.

2. Because no matter how they did measurements of it, regardless of whether the apparatus was standing still or moving, the measurement always seemed to come out the same (within the limits of experimental error). In particular, interference type measurements, would have shown a noticeable phase shift when the apparatus was in motion if the speed of light was not a constant - they didn't.

3. Already answered in 2. He didn't know it absolutely, but all the experimental evidence available at the time seemed to indicate that it was.

4. Because once you accept that the speed of light is constant for all observers it changes the fundamental geometry of the universe itself. Space is no longer Euclidean at high speeds but rather Minkowskian. In order to maintain observed physical laws and in particular to maintain conservation laws, when one thing changes, other things have to change. The 3 fundamental properties recognised in physics are mass, length and time. Setting the speed of light as a constant causes changes in time. Therefore there have to be corresponding changes in length and mass as well, otherwise all our physical laws and conservation goes out the window. The result of adjusting time to suit the observed effects results in length apparently contracting at high speeds and mass increasing. The limit of all such effects is the speed of light itself. Time goes to zero, length goes to zero and mass becomes infinite.

 

[IIRichard]

I believe Gallileo made a rough determination of the speed of light by timing the re-appearance of the moons of Jupiter. When Jupiter was farther away from the earth, the eclipse and reappearance took longer. He correctly believed it was due to the fact that light had a finite speed.

Over the years, the value was calculated more precisely.
James Clerk Maxwell formulated equations of electromagnetics that predicted the known value.

Michaelson measured the speed of light to great precision. He also wanted to show that the assumed medium for the propagation of light, the aether, existed. He compared the speed of light in the direction of the earth's motion with the speed at right angles to it. Presumably, if the aether existed and the earth were moving through it, the speed of light plus the speed of the earth's motion through it would add and the two would not agree.

However, the two did agree and Michaelson proposed the the measuring apparatus shrank in length due to the speed of the earth.

So by the time Einstein was working on the theory of relativity, the speed of light was well known and Michaelson's experiments showed that it was constant irrespective of any movement through the aether.

Einstein took these two results and others and ran the ball past the goal posts. He said forget the aether, we don't need it and c, the speed of light, is constant for all observers. He then showed all sorts of remarkable things followed. These effects, gravity bending spacetime, increased mass at high speed, time dilation have been experimentally verified to very precise degrees.

 

[SkepticalScience]

Thanks for your answers, but this is still a bit odd to me.

I share many of the questions that Pragmatist brought up. Why would your mass go to INFINITE when you get closer to light? I can understand that your mass could increase as you accelerate but can't understand how your mass would become infinite.

Also, does light accelerate? When I turn on my flashlight, does light actually accelerate from 0 to 186,000mps or does it just INSTANTLY arrive at 186,000mps.

I can't imagine it would work instantly - but there are many supposed truths about light that boggle my imagination.

 

[IIRichard]

Thanks for your answers, but this is still a bit odd to me.

I share many of the questions that Pragmatist brought up. Why would your mass go to INFINITE when you get closer to light? I can understand that your mass could increase as you accelerate but can't understand how your mass would become infinite.

Also, does light accelerate? When I turn on my flashlight, does light actually accelerate from 0 to 186,000mps or does it just INSTANTLY arrive at 186,000mps.

I can't imagine it would work instantly - but there are many supposed truths about light that boggle my imagination.

The quick answer is that you can't get to the speed of light and therefore the mass approaches infinite but never quite gets there. The reason is e=mc^2. As more energy is put into the particle, spaceship, whatever, to push it closer and closer to the speed of light, that energy increases the mass.

As far as I know, a photon, a "particle" of light, is always at the speed of light from the instant of its creation except in certain laboratory environments.

 

[Dustin Kesselberg]

The quick answer is that you can't get to the speed of light and therefore the mass approaches infinite but never quite gets there. The reason is e=mc^2. As more energy is put into the particle, spaceship, whatever, to push it closer and closer to the speed of light, that energy increases the mass.

As far as I know, a photon, a "particle" of light, is always at the speed of light from the instant of its creation except in certain laboratory environments.

He is asking WHY it approaches infinite mass at the speed of light.

 

[CurtC]

Since spacetime is Galilean, nothing prevents one observer from moving at "c" relative to another. Then what? One observer sees the other keeping pace with a wave, while the other sees the wave passing himself at "c"?

I don't get it at all. [/B]

Ah, yes you do! Your example is the perfect vehicle to understand why, in a universe where c is constant for all observers, it's impossible to ever attain c. Your example can't happen, because of the reason you describe - nothing with mass can ever go exactly c. Yours is a reductio ad absurdum explanation of why.

Dustin, Einstein already knew the speed of light to a high degree of accuracy, and that it seemed to be constant no matter what speed the obervers were going in. Further, Maxwell's equations, which seemed to do a really good job of describing electromagnetism, implied that c is a fixed constant no matter who is doing the measuring - those equations have c, and they didn't seem to specify which reference frame it was for.

It should have been obvious to everyone that the speed of light is constant for all observers. It's just that when you make that assumption, you get strange effects (such as time occuring at different paces for different observers), which everyone thought were ridiculous. Einstein had the genius to stick with it, and to grind through the math of what the world would be like if that assumption were true. His genius is that he followed where the science led him, and didn't automatically stick with his preconceptions. When he did grind through all that math, the equation e=mc^2 fell out into his lap. Something else that fell out was that apparent mass increases as an object approaches the speed of light, and as it gets closer, the mass increases without bound.

Understandably, this made a lot of physicists uncomfortable. But it sure resolved the issue of the speed of light and reference frames, and further, detailed predictions of the idea's time dilation effects were confirmed by measurements.

 

[rppa]

The quick answer is that you can't get to the speed of light and therefore the mass approaches infinite but never quite gets there. The reason is e=mc^2. As more energy is put into the particle, spaceship, whatever, to push it closer and closer to the speed of light, that energy increases the mass.

As far as I know, a photon, a "particle" of light, is always at the speed of light from the instant of its creation except in certain laboratory environments.

Not to throw too much confusion in here, but physicists no longer use the term "relativistic mass", nor do they talk about "mass increasing", nor do they use E = mc^2. The term "mass" is reserved for the invariant mass, or rest mass. So it is true that it takes infinite energy for an object with mass to achieve the speed of light, but not that the "mass" becomes infinite. For particles with nonzero mass, the modern version of Einstein's famous equation is E = gamma*mc^2.

Even Einstein himself was trying to get away from the whole "relativistic mass" usage, since he said it created more confusion than it resolved.