Light Speed (c) is constant value

[JohnStu]

Has any experiments varified that light speed cannot increase or decrease that it is constant, no matter where you go?

 

 

 

I've Googled around this topic and only bumped into experiments that are about Einstein's relativity.

 

 

 

Experts please help

[Schrödinger's hat]

I guess it depends on what you mean by verification.

We haven't gone to other galaxies to measure how long light takes to travel from point a to point b, but the speed of light is so intricately tied up in so many phenomena that it would require a large amount of special pleading to set a different value for c in other places.

 

Things like absorbtion and emission spectra of stars and gas clouds, the energies at which nuclear reactions take place, etc etc depend on the constant c.

 

I suppose one could argue that it's just a coincidence that photons travel at c in this location and travel at other speeds elsewhere, but that would require even more contorted reasoning re. ideas about massive and massless particles.

[Sergeant Bilko]

I'm no expert, but I recall that I did read some experiments where they have slowed down the speed of light in certain circumstances.

 

 

 

http://news.harvard.edu/gazette/1999/02.18/light.html

[Schrödinger's hat]

I'm no expert, but I recall that I did read some experiments where they have slowed down the speed of light in certain circumstances.

 

Light propagation through a medium is a bit different. It can be viewed as the average speed of light which is being absorbed and re-emitted repeatedly. Although this doesn't describe the way phase velocity and group velocity can differ (and how phase velocity can exceed the non-medium-dependant constant c) without going into much more involved detail.

[swansont]

Has any experiments varified that light speed cannot increase or decrease that it is constant, no matter where you go?

The constancy of c is an intimate part of Maxwell's equations. Every time you listen to your radio and move (e.g. in a car) you are confirming that c is a constant.

[JohnStu]

Thank you, great answers, that's what I needed to know.

[elfmotat]

Light propagation through a medium is a bit different. It can be viewed as the average speed of light which is being absorbed and re-emitted repeatedly. Although this doesn't describe the way phase velocity and group velocity can differ (and how phase velocity can exceed the non-medium-dependant constant c) without going into much more involved detail.

 

Light isn't absorbed and re-emmited when it travels through a medium. This is a common misconception. If it were true, then absorption spectrum would be discrete because atoms only have discrete energy levels.

[swansont]

Light isn't absorbed and re-emmited when it travels through a medium. This is a common misconception. If it were true, then absorption spectrum would be discrete because atoms only have discrete energy levels.

 

The QED picture is absorption and re-emission from virtual states. The only process that works is one in which no net momentum is transferred to the atom, i.e. the photon is re-emitted in the same direction as it was originally going. But absorption spectra are discrete in media when there are real states present.

[scourge]

The constancy of c is an intimate part of Maxwell's equations. Every time you listen to your radio and move (e.g. in a car) you are confirming that c is a constant.

 

I know some physics but i am by far no expert. You mentioned "Maxwell's equations". I looked a little into that and found the lorentz force and found out this has largely to do with electromagnetism i would say it in fact has entirely to do with that. But anyway my question would be is that something i should look into? I know the operations and calculations behind acceleration and gravitation in the area of newtonian physics. Should i study into those equations? I have to ability to read the actual paper if i would like to. But anyway, It is something i should look into?

[The Observer]

If you want to study Maxwell's equations, you will want to familiarize yourself with a bit of vector calculus first. If you are interested in physics, I would highly recommend looking into it.

[scourge]

If you want to study Maxwell's equations, you will want to familiarize yourself with a bit of vector calculus first. If you are interested in physics, I would highly recommend looking into it.

 

Okay i am a decent way through regular algebra, I figured to truely understand what is beneath them i would need to learn complex mathematics, I will indeed start. I have a website that i can go to and get a thorough education in mathematics all the way to linear algebra. So as of now i will just take the concepts of these things in physics as i study mathematics to the calculus and perhaps maybe linear algebra if i find it nessicary. Guess there isn't a reason to take a free and well guided education when its layed out for you to take.

[The Observer]

Okay i am a decent way through regular algebra, I figured to truely understand what is beneath them i would need to learn complex mathematics, I will indeed start. I have a website that i can go to and get a thorough education in mathematics all the way to linear algebra. So as of now i will just take the concepts of these things in physics as i study mathematics to the calculus and perhaps maybe linear algebra if i find it nessicary. Guess there isn't a reason to take a free and well guided education when its layed out for you to take.

 

Well keep it up man. Mathematics gets beautiful when you start to really get into it. I am very sure you will find the study of linear algebra necessary, much of mathematics is an attempt to reduce all sorts of different looking problems to linear algebra problems!

[swansont]

I know some physics but i am by far no expert. You mentioned "Maxwell's equations". I looked a little into that and found the lorentz force and found out this has largely to do with electromagnetism i would say it in fact has entirely to do with that. But anyway my question would be is that something i should look into? I know the operations and calculations behind acceleration and gravitation in the area of newtonian physics. Should i study into those equations? I have to ability to read the actual paper if i would like to. But anyway, It is something i should look into?

 

Yes, absolutely. Maxwell's equations describe electricity and magnetism, which is a very important part of physics.

[morgsboi]

I'm not an expert but Einstein is.

http://www.physicsforums.com/showthread.php?t=482955

[alpha2cen]

Has any experiments varified that light speed cannot increase or decrease that it is constant, no matter where you go?

Is the speed of light outside of our Universe the same? If not, the light pass, i.c., vacuum will be an important basic component of the Universe. We have not known the difference of the vacuum component between in and out of the Universe. The something we only know is the world, Dark Energy.

Edited June 15, 2012 by alpha2cen

[swansont]

"outside of our universe" doesn't really have a meaning.

[alpha2cen]

Has any experiments varified that light speed cannot increase or decrease that it is constant, no matter where you go?

 

Light speed does not change at any circumstance. Is it a fundamental component in the Universe? We can change light frequency, but not change light speed. Whenever the energy is high or low, it only runs the speed, C. Even very tiny energy state, it only runs the speed, C. It does not have any tiny activation energy for travelling. Why such state is possible?

[juanrga]

Has any experiments varified that light speed cannot increase or decrease that it is constant, no matter where you go?

 

Not only any experiment made since a century ago verifies that c is a constant, but c is listed --in the tables of universal constants-- as one of the constants that characterize our universe

 

http://en.wikipedia....ersal_constants

Edited June 15, 2012 by juanrga

[alpha2cen]

Not only any experiment made since a century ago verifies that c is a constant, but c is listed --in the tables of universal constants-- as one of the constants that characterize our universe

 

http://en.wikipedia....ersal_constants

 

There are many factors on measuring the speed of light. One of them is vacuum quality. Are all vacuums equal?

In the case of measuring the speed of light in the space, are there any difference ?

The measured value form January to July has any difference? Or, 1800 years measured value has any difference from recently measured it?

If the value has any regular difference, we could consider the vacuum quality difference in the space.

That is, all vacuum quality in the space is not the same. The Earth is moving through the different quality vacuum in the Galaxy.

Edited June 16, 2012 by alpha2cen

[Volumes]

If are considering the light stays in the same medium, then the short answer is, c is in fact constant.

[D H]

There are many factors on measuring the speed of light. One of them is vacuum quality. Are all vacuums equal?

In the case of measuring the speed of light in the space, are there any difference ?

The measured value form January to July has any difference? Or, 1800 years measured value has any difference from recently measured it?

If the value has any regular difference, we could consider the vacuum quality difference in the space.

That is, all vacuum quality in the space is not the same. The Earth is moving through the different quality vacuum in the Galaxy.

In order, nonsense, nonsense, more or less, no, no, silly question, nonsense, nonsense, and nonsense.

[alpha2cen]

In order, nonsense, nonsense, more or less, no, no, silly question, nonsense, nonsense, and nonsense.

 

Recently some scientists say "Some physical constants are varying with time." There is true value of the speed of light at this time in the Universe. But, Does the measured value have not a position dependency? If we know the vacuum property exactly, and the light travelling interfering factors in the vacuum are the same, we could say the light speed does not have a position dependency. Probably, the measured values have small differences at the long significant digit?

Edited June 17, 2012 by alpha2cen

[ACG52]

Recently some scientists say "Some physical constants are varying with time." There is true value of the speed of light at this time in the Universe. But, Does the measured value have not a position dependency? If we know the vacuum property exactly, and the light travelling interfering factors in the vacuum are the same, we could say the light speed does not have a position dependency. Probably, the measured values have small differences at the long significant digit?

 

'Some scientists'? Who, where, and when.

 

The Laws of physics have translational symmetry, meaning they are the same no matter where you are, so the measured value of c does not have a position dependency.

[D H]

'Some scientists'? Who, where, and when.

There indeed are some. Phys. Rev. Lett. 82, 884–887 (1999), Phys. Rev. Lett. 87, 091301 (2001), MNRAS 327:4, 1208–1222 (2001), MNRAS 345:2, 609–638 (2003), ..., Phys. Rev. Lett. 107, 191101 (2011), MNRAS 422:4, 3370–3414 (2012) just to name a few. Fringy, definitely, but that those are definitely not crackpot journals. Aside: Never so fringy as to attribute the supposed variations to "vacuum quality difference in the space".

 

The 2011 version caused quite a stir, but even it has pretty much been refuted in the eyes of most astrophysicists (e.g., A&A 540, L9 (2012)).

 

 

The Laws of physics have translational symmetry, meaning they are the same no matter where you are, so the measured value of c does not have a position dependency.

That's quite the bold statement! There's a huge difference between physics and philosophy: Physicists need evidence of their bold statements. One job of experimental physicists is to come up with tests that either confirm or falsify the grandiose statements made by their theoretical brethren. So they test. And test. And test again. And again. It's important.

 

The speed of light isn't of near the interest to physicists as are the dimensionless constants such as the fine structure constant. There are 25 or so of these in the standard model of physics. Change your units and you change the speed of light. Pick a convenient set of units and the speed of light, Newton's universal gravitation constant, the Planck constant, etc. all become one. That trick doesn't work for those dimensionless constants. They truly are supposed to be constant. That "supposed to be" is central to all of physics, so physicists test and test and test for variations in these fundamental constants. The consensus is that all variations seen to date are statistically consistent with the null hypothesis, which is of course that these fundamental constants are constant.

[alpha2cen]

Thank you for good reference. Data is important. In the future we will more approach the truth of our physical world by using this data.