What Determines the Constant Speed of Light?

[Integral]

You are so close. I am saying you cannot ask a question about speed until you have defined units of time and distance. And our current definition of meter is defined in such a way that the speed of light is constant by definition.

The conclusion is then that it is not a constant in mph which used to be a different standard from the meter?

Do you still fail to see the circularity of your reasoning? You say the speed of light is constant because the meter is defined in terms of wavelengths of light.

But the meter was defined in terms of the wavelength of light because the speed of light is constant.

And again the speed of light is constant independent of our ability to measure it.

 

[Integral]

I believe that there is no evidence against this statement, or that our little region of the universe has strongly suggested to us that it is so. In other words, I accept that c is a constant of the universe. I appologize if you thought that I was arguing to the contrary.

I agree with every word of this.

No apologies necessary :)

 

[Jack Martinelli]

The experimental methods used to measure a value for \epsilon_0 are significantly different from those used to measure c.
The fact is with modern technology I believe it is easier to get a precise measurement of c then \epsilon_0 so it may well be that it is now defined it terms of c then the other way round.

That does not change the fact that \epsilon_0 is a basic property of space time which is a factor in the propagation of EM waves.
Which is the "more" fundamental constant. I personally do not know.

1. I don't know that \epsilon_0 universal (e.g., bigbang)
2. I don't know that EM is universal. Particularly near charged particles (r<< 10E-15m)

 

[jdavel]

turin asked: What do you mean, "you'd never do that" (calibrate lengths between each measurement)? That is exactly what would be done. How else would you calibrate length?


There's no need to calibrate length if all you're trying to do is test whether c is constant. Just measure the time t that it takes for light to go from one end of a stick to the other. If you always use the same stick, and you always get the same t, then you can say c is constant.

You don't even have to know the length of the stick! Michelson & Morley didn't know the relative length of their interferometer arms to anywhere near the precision that they got for the relative difference in light speed along the arms.

 

[Integral]

Jack,
The same qualifers apply to \epsilon_0 that turin stated with c up thread.

Why do you introduce QM to the question. Are you simply looking for arguments.

It's a yes or no question.

Is c constant?

 

[jdavel]

DrMatirx said: "you cannot ask a question about speed until you have defined units of time and distance."

You can ask (and answer) this question without defining units of distance: If two light beams start at the same time and place, and arrive simultaneously at some other place, then was their speed the same?

 

[turin]

turin asked: What do you mean, "you'd never do that" (calibrate lengths between each measurement)? That is exactly what would be done. How else would you calibrate length?

I'm sorry. You have misunderstood me. I meant that, if you were to calibrate, you would use light to do so.

 

[turin]

If two light beams start at the same time and place, and arrive simultaneously at some other place, then was their speed the same?

There is not enough information to answer this question. I can start a trip from NY to Miami in my car, and stop at a gas station every hour, going 80 mph between stops to make up for lost time. My friend can get in his car with a really big gas tank and and start the trip at the same time, but going nonstop at 70 mph. If we both get to Miami at the same time, was our speed the same?

Or a more relevant example:
If I first drive to San Diego, and then to Miami, but we still both leave NY and arrive at Miami at the same time, was our speed the same?

 

[Jack Martinelli]

Jack,
It is not clear to me, again as with Turin, I fail to see the significance of your words.

Its not my words, its what my words refer to.

Is or is not c a constant of the universe?

Yes it is, but the reason it is is the question under discussion.

It seems to me that you may well simply be agreeing with me in an argumentative fashion, this goes for Turin also. Which is why I suspect that this is all about semantics. You seem to be telling me I am wrong but then in the end I really can't pin down how you disagree.

You are not "wrong". Your explanations are just not very good. Just my opinion. I find an explanation about the physical more enlightening when the language used refers other physical phenomena -- not other constants. If your language just refers to the physical (line-like intervals) there is no argument. That they exist is self evident. Maxwell's equation, for example, are not self evident. Each term I use (ruler or clock) is a word that has a semantic anchor (an observable space-like interval defined by two distinguishable objects). When you use language this way there is no semantic argument. You can't refute the way the universe works. You can't really refute that, in our universe there are line like observables & that they may be used to measure other line-like observables. That the maximum possible speed of an object in the universe is a constant is simply a consequence of our measurement proceedure. It has nothing to do with EMR. Bosons are just test particles that let us find this speed and express it in terms of meters and seconds.

 

[DrMatrix]

The conclusion is then that it is not a constant in mph which used to be a different standard from the meter?

I don't understand what you mean here.

Do you still fail to see the circularity of your reasoning? You say the speed of light is constant because the meter is defined in terms of wavelengths of light.

But the meter was defined in terms of the wavelength of light because the speed of light is constant.

And again the speed of light is constant independent of our ability to measure it.

The fact that the speed of light is constant when the distance is measured by rigid rods allows us to use the speed of light (not the wavelength) to establish an equivalent definition based upon the speed of light.

Mach one is defined as the speed of sound. Is the speed of sound constant? It depends upon what units you are talking about. In mach units, yes -- the speed of sound is mach one by defintion. In meters per second, no -- the speed depends upon the temperature and the pressure.

There's no need to calibrate length if all you're trying to do is test whether c is constant. Just measure the time t that it takes for light to go from one end of a stick to the other. If you always use the same stick, and you always get the same t, then you can say c is constant.

You don't even have to know the length of the stick! Michelson & Morley didn't know the relative length of their interferometer arms to anywhere near the precision that they got for the relative difference in light speed along the arms.

You are assuming that your stick has a constant length. The stick has become your unit of length. And that's fine, so long as you realize the stick is your unit of length.

DrMatirx said: "you cannot ask a question about speed until you have defined units of time and distance."

You can ask (and answer) this question without defining units of distance: If two light beams start at the same time and place, and arrive simultaneously at some other place, then was their speed the same?

OK. So long as the beam of light traveled the same path, then yes. Are two beams emitted at the same point that travel along the same path and arrive simultaneously at some other point two beams or one beam?

 

[Integral]

The conclusion is then that it is not a constant in mph which used to be a different standard from the meter?

If it is constant by the definition of the meter then perhaps it is not constant in different units which are not defined in terms of the speed of light.

If it is constant in units not defined in the terms of light then what do you mean by constant by definition of the meter?

 

[Jack Martinelli]

Jack,
The same qualifers apply to \epsilon_0 that turin stated with c up thread.

Why do you introduce QM to the question. Are you simply looking for arguments.

It's a yes or no question.

Is c constant?

Sorry. This thread is beginning to unravel & I may have misread something you (or someone else wrote).

BTW, I think I've said just about all I can about the speed of light. You're right, c is a constant. You'll get no more arguments from me.

 

[DrMatrix]

If it is constant by the definition of the meter then perhaps it is not constant in different units which are not defined in terms of the speed of light.

If it is constant in units not defined in the terms of light then what do you mean by constant by definition of the meter?

Does this explain? The speed of light is constant in meters per second because the meter is defined in terms of the speed of light. The speed of light is constant when the distance is measured by rigid rods because that's just the way the universe works. The speed of light may or may not be constant in units not defined in terms of the speed of light. The speed of light is not constant in General Relativity. So if our unit of length is based upon the coordinates of a non-inertial reference frame, the speed of light would not be constant.

 

[Loren Booda]

Are \epsilon_0 and \mu_0 invarient regardless of reference frame as c is?

 

[turin]

Are \epsilon_0 and \mu_0 invarient regardless of reference frame as c is?

It is believed to be so, in empty flat space.

 

[Integral]

The speed of light is not constant in General Relativity

This is incorrect. c is constant in the General Theory.

Looks like you have some pretty fundamental misunderstandings. It is to bad that you cannot use this site to clear up those misunderstanding rather then attempt to present your incorrect opinions as physical fact.

One more time.

c is constant in all coordinate systems. These coordinate systems can even have a constant velocity. All will agree (after unit conversions) on the speed of light.

Jack,
For all of your words, and even considering the meaning of your words you have not and cannot prove that the speed of light is constant using only sticks and clocks. At least here and in words. To accomplish this would require actual measurements, even then you have not proven the constancy of c. You have only a single value at a single location. It is impossible to generalize that single measurement to apply to all locations and times. Historically, the constancy of c was not established until the work of Maxwell. It was his work that laid the theoretical basis of the constancy of c. His work forced the world of physics to come to grips with an ugly fact. This fact, that the speed of light was determined by well known and understood fundamental constants and only those constants, had very deep implications which shook the world of physics to its very foundations. To attempt to prove the constancy of c without reference to E&M is, in my opinion, impossible.

 

[DrMatrix]

This is incorrect. c is constant in the General Theory.

Looks like you have some pretty fundamental misunderstandings. It is to bad that you cannot use this site to clear up those misunderstanding rather then attempt to present your incorrect opinions as physical fact.

One more time.

c is constant in all coordinate systems. These coordinate systems can even have a constant velocity. All will agree (after unit conversions) on the speed of light.

A beam of light trying to escape a black hole will hover at the event horizon. There you have light that is motionless. If I attach a coordinate system to the rotating earth, pluto is traveling faster than c. The speed of light is not constant in a general coordinate system.

Inertial coordinate systems can hae a constant velocity, sure. And the speed of light is constant in inertial frames, but not in general reference frames.

 

[cookiemonster]

I don't think anybody was asserting otherwise.

cookiemonster

 

[Integral]

A beam of light trying to escape a black hole will hover at the event horizon. There you have light that is motionless.

No. light follows geodesics. Inside the event horizon of a black hole geodesics form closed paths. When light enters a black hole it essentially orbits the singularity. One must be very carefully when doing physics in the vicinity of the singularity. Let us stay in our everyday universe. If you must resort to black holes to support your argument you are lost.

The speed of light is constant in GR.

 

[Janitor]

Thought experiment.

A rocket is floating out in interstellar space. A laser is floating next to the rocket. On the outside of the rocket there is some device that measures the speed of light. If the laser is pointing at this device on the rocket, the astronaut in the rocket gets a reading of precisely c on his LCD readout.

The astronaut hits the button to fire the engine, and away he goes. The laser remains behind, in free fall. The laser continues to shine at the rocket. The astronaut takes readings of the speed of the laser light as calculated by the apparatus on the side of his rocket.

The rocket runs out of fuel, and coasts at some high speed. The astronaut sees that the light from the laser is redshifted, but (I think everyone in this thread will agree) he sees precisely c on his LCD readout.

So my question is, what does the readout show for the powered phase of the rocket? I thought I had read at least once somewhere that it would read less than c. Does anyone agree? Does anyone disagree?

Or is this little thought experiment not even relevant to what you are discussing? Certainly the rocket itself can be thought of as a reference frame, one that is inertial for a time, then non-inertial for a time, then inertial once again, and we agree to ignore gravitational fields from heavenly bodies, which by assumption are all very far away.

 

[DrMatrix]

The speed of light is constant in GR.

Wrong!
From A Few Inferences from the General Theory of Relativityhttp://www.bartleby.com/173/22.html by Albert Einstein:

In the second place our result shows that, according to the general theory of relativity, the law of the constancy of the velocity of light in vacuo, which constitutes one of the two fundamental assumptions in the special theory of relativity and to which we have already frequently referred, cannot claim any unlimited validity. A curvature of rays of light can only take place when the velocity of propagation of light varies with position.

Janitor, so long as your reference frame is inertial, you can apply Special Relativity where the speed of light is constant. In a non-inertial reference frame, the velocity of light is not constant.

 

[Integral]

In the second place our result shows that, according to the general theory of relativity, the law of the constancy of the velocity of light in vacuo, which constitutes one of the two fundamental assumptions in the special theory of relativity and to which we have already frequently referred, cannot claim any unlimited validity. A curvature of rays of light can only take place when the velocity of propagation of light varies with position.

Velocity is a very critical word in this paragraph. Without appreciation for the precise meaning of this word, one might think that he is saying that the speed of light is changing. But he did not say speed, he said velocity, he is telling us that light does not travel in straight lines (or what we consider to be straight lines) but changes direction in a gravitational field, therefore the velocity of light must change.

This says nothing of the speed of light. The speed of light remains constant but its velocity {direction} changes in a gravitational field.

 

[jdavel]

Integral said: "The speed of light remains constant but its velocity {direction} changes in a gravitational field."

I'm going to have to be careful here, because I know virtually nothing about GR. If the g field and the direction the light travels are perpendicular, then what you said seems right, direction changes but speed stays the same. But if they're parallel, wouldn't it be the other way around?

The SR postulate of constant c is limited to measurements in inertial frames. I thought that in GR the postulate is limited to both inertial frames and zero gravity locations.

 

[turin]

quoting Integral
Quote:

The speed of light is not constant in General Relativity

This is incorrect. c is constant in the General Theory.

The speed of light in a literal sense of meaning is different than c. I can say that c is a constant everywhere in the universe, and you have shown that we agree what I would fundamentally mean by that: the constant that shows up in the wave equation that has units of speed. I can also say that the literal speed of a beam of light is different than c, both in the context of refraction, as well as in a coordinate system that considers a beam of light to hover at a horizon.

quoting DrMatrix
A beam of light trying to escape a black hole will hover at the event horizon. There you have light that is motionless. If I attach a coordinate system to the rotating earth, pluto is traveling faster than c. The speed of light is not constant in a general coordinate system.

I agree. c is a constant in that coordinate system, but it only replaces the literally speed of light locally in general, where an experiment can be done to examine Maxwell's equations.

quoting Integral
When light enters a black hole it essentially orbits the singularity.

I don't know what you mean by "enters a BH," but once anything gets "closer" to the "singularity" than a certain critical radius (I believe it is the Schwarzschild radius), orbits do not exist. The path of a photon inside a BH is a spiral that terminates at the singularity within a finite time interval in a coordinate system outside the BH, if I'm not mistaken. I guess you could mean "orbit" in the loose context of "free particle trajectory," so, I don't want to disagree with you. Is that what you mean?

quoting Integral
Let us stay in our everyday universe. If you must resort to black holes to support your argument you are lost. The speed of light is constant in GR.

Say what? Why has this arbitrary rule of not resorting to a 90 year old well established theory been introduced? And the speed of light is not necessarily constant in GR, but c is.

quoting Janitor
Thought experiment.
... there is some device that measures the speed of light.
...
So my question is, what does the readout show for the powered phase of the rocket? I thought I had read at least once somewhere that it would read less than c. Does anyone agree? Does anyone disagree?

I will have to defer this to some investigation, but I would ask how the device measures the value. I believe that the thought experiment critically depends on this. I will assume for my investigation that it has two points of detection (or possible four in the shape of a pyramid) separated by a known distance, and that it compares the detection times on clocks at each detection point by translating the clocks under negligible acceleration to a midpoint, and then adjusting for any time difference that appear among the clocks.

quoting Janitor
... is this little thought experiment not even relevant to what you are discussing? Certainly the rocket itself can be thought of as a reference frame, one that ... non-inertial for a time, ...

I think it's relevant. I'm with you.

 

[russ_watters]

Good luck, Integral - I'm out.

 

[DrMatrix]

When light enters a black hole it essentially orbits the singularity.

I don't know what you mean by "enters a BH," but once anything gets "closer" to the "singularity" than a certain critical radius (I believe it is the Schwarzschild radius), orbits do not exist. The path of a photon inside a BH is a spiral that terminates at the singularity within a finite time interval in a coordinate system outside the BH, if I'm not mistaken. I guess you could mean "orbit" in the loose context of "free particle trajectory," so, I don't want to disagree with you. Is that what you mean?

I was wondering what Integral meant too. Perhaps he'll explain.

In The Universe in a Nutshell by Stephen Hawking on page 114 Figure 4.12 we see a diagram of a star with rays of light leaving the surface. The axes are chosen so that a velocity of c will have slope equal to 1 or -1. The caption reads:

Spacetime around a non-collapsing star. Light rays can escape from the surface of the star (the red vertical lines). Far from the star, the light rays are at 45 degrees to the vertical, but near the star the warping of spacetime by the mass of the star causes the light rays to be at a smaller angle to the vertical.

Is the speed of light constant here? I'd say no. What say you?

 

[Integral]

Is the speed of light constant here? I'd say no. What say you?

Sure, what evidence have you provided to show otherwise?

Going to work, I drive 60Mph west, on the way home I drive 60mph East. Same speed different velocity.

What! No response to my reply to your Einstein quote? going try and simply ignore it, perhaps?? Just trot out another quote from another book that you thought you understood. Nice tactics, how to keep the argument going and convince yourself you are ahead. Ignore your failures and move on to a new and different misunderstanding.

It appears to me your understanding of basic physics is not even at the Halliday and Resnick level. (ie not able to appreciate the difference between velocity and speed, consider a square root to be a "fancy equation" )It would do you well to move beyond the coffee table physics books and attempt to learn the real deal. Start with a calculus text. Move on to a college level Physics text. Open your mind a bit, currently you are blinded by preconceived notions and crippled by your lack of formal physics knowledge.

The event horizon separates the universe from the black hole, what is different inside the event horizon? According to your man Hawking, inside the event horizon geodesics form closed paths. Thus light that enters a black hole tangent to a interior geodesic follows that path endlessly.


Hey I thought these were Hawking's ideas. You did read Brief History of time ? Perhaps I got it out of the collection of lectures by Hawking and Penrose.

If this is going to turn into a discussion of GR I will move the thread to the appropriate forum where mentors, with better knowledge of the details then I, can answer your questions.

BTW it would be nice if you could learn to ask questions to clarify your understanding.

Edit: Grammar and punctuation.

 

[Integral]

Janitor,
Consider this. In reality the Earth is a constantly accelerating reference frame, we measure the speed of light to be constant. We live deep in the suns "gravity well". this means that, according to GR, Space time is not flat in our vicinity. We still measure a constant speed of light. If you conduct your experiment deep in space away from the influence of stars and planets would not space/time be flatter there then here? We measure a constant speed of light, why would you expect anything different on your space ship?

This is not meant to be an end all answer to your question, just something to think about.

 

[DrMatrix]

Sure, what evidence have you provided to show otherwise?

The caption I quoted. It's easier to understand with the figure in front of you, but I'll try to explain. The x-axis is space and the y-axis is time. A vertical line represents stationary and a line at a 45 degree angle represents c. Closer to the vertical than 45 degrees represents a speed slower than c. With that in mind, please consider: "near the star the warping of spacetime by the mass of the star causes the light rays to be at a smaller angle to the vertical."

Going to work, I drive 60Mph west, on the way home I drive 60mph East. Same speed different velocity.

What! No response to my reply to your Einstein quote? going try and simply ignore it, perhaps?? Just trot out another quote from another book that you thought you understood. Nice tactics, how to keep the argument going and convince yourself you are ahead. Ignore your failures and move on to a new and different misunderstanding.

OK. I'll grant you that constant speed does not mean constant velocity. My Einstein quote does not contradict what you said. Happy? Good.

It appears to me your understanding of basic physics is not even at the Halliday and Resnick level. (ie not able to appreciate the difference between velocity and speed, consider a square root to be a "fancy equation" )It would do you well to move beyond the coffee table physics books and attempt to learn the real deal. Start with a calculus text. Move on to a college level Physics text. Open your mind a bit, currently you are blinded by preconceived notions and crippled by your lack of formal physics knowledge.

In college, I studied calculus, advanced calculus, topology, and physics (not advanced physics). I graded the homework for elementary differential equations. I always try to keep an open mind, and I look forward to learning new things. Everyone has their preconceived notions, I suppose.

When I said "fancy equations" that was a failed attempt at humor. I now wish I had not said that. I do not consider square root fancy.

The event horizon separates the universe from the black hole, what is different inside the event horizon? According to your man Hawking, inside the event horizon geodesics form closed paths. Thus light that enters a black hole tangent to a interior geodesic follows that path endlessly.


Hey I thought these were Hawking's ideas. You did read Brief History of time ? Perhaps I got it out of the collection of lectures by Hawking and Penrose.

I don't know enought to say that you are definitely wrong, but that does not sound right to me. My copy of A Brief History of Time is packed away somewhere.

If this is going to turn into a discussion of GR I will move the thread to the appropriate forum where mentors, with better knowledge of the details then I, can answer your questions.

Well, the question of whether the speed of light is constant in a general reference frame does fall under relativity. If you feel the mentors in another forum would be better suited, that is your decision.

 

[Integral]

If you wish to pursue the implications of a variable speed of light, read Faster then the Speed of Light by Joao Maguelijo. The first half of the book discusses the current understanding of GR. I found this to be an excellent presentation. He spends a significant amount of text complaining about the rigidity and short sightedness of the current technical journal system. For him to seriously address, or even discuss, the idea of a variable speed of light was careerer threating. He and a colleague (who was the senior researcher, chickened out at the last minute, refusing to present their initial work or attach his name to it.) developed the theory behind locked doors. The implications of a variable speed of light are far reaching and fascinating. Find a copy and give it a read. But meanwhile this is still very controversial physics and not an appropriate topic for this forum.

But as I said earlier any real discussion of VSP must take place in the Theory Development Forum.