Speed of Light Relative?

Re: Speed of Light Relative?
No. The speed of light is a constant

benevolenthell wrote:

"Since time is relative, and the speed of light is the amount of distance light travels in a certain amount of time, isn't the speed of light relative?"

This is a very intelligent question.

The answer is that it is the *distance* that the light travels (not its speed) that is relative.

When person 'A' measures less time than person 'B', she will also measure less distance. This is called "Lorentz Contraction".

Incidently the phonominon that "objects moving very quickly are shorter" was known before the theory of relativity was discovered. The fact that relativity explained it was one of the reasons that scientists of the day knew the theory was a good one so quickly.

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To be really explicit about this, if any observer in a non-accelerated reference frame measures any object (such as a beam of light) as travelling at the speed of light, all observers will measure the same speed for that object. Think about it. It sounds wrong, but it's actually true. For example, if a space craft is heading towards you shining a beam of light out the front, they will measure the light as propagating away from them at the speed of light, and you will measure it coming towards you at the speed of light. You will NOT measure it as coming towards you at the speed of light plus the speed of the space craft.

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my two cents worth,

you would see the light coming toward you at the speed of light and the spacecraft inhabitants would see it going away from them at the speed of light, but you would see it as slightly bluer than they would....is that not right?

c.g.

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doesn't' a prism split up light because it forces it to move slower at certain places?
And light is effected by gravity just as time is effected by gravity. Time is also effected by speed, so I would assume that light is also. If the speed of light was constant, then you could get to a certain gravity and speed in which light would conceivably be travelling slower than sound.

Or am I way out of my league?

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Light is affected by gravity, so it would be variable in that way. We can slow light, or at least, it can be slowed. But what about speeding it up? Or light not having any constant at all?

I have also heard about a theory where a laser flashed and then hit a "mirror," where half the light was reflected, and half continued on. The mirrored light continued to be mirrored several times. Both beams of light would end up the same distance from the laser. When they tried the experiment, both beams hit the receiver at exactly the same time. Hopefully that explains it well enough, but this, too, would mean that the speed of light is variable, and can speed up.

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No, C is constant. Light is not slowed by gravity, it is distorted by gravity. It's speed remains constant.

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What about when light falls into a black hole. Doesn't it fall "forever," or, essentially, stop?

I also recall reading a Scientific American about "Freezing Light." I don't, however, know the details of the article. And I have heard that light does slow in the atmosphere, although not significantly.

The article is at http://www.sciam.com/article.cfm?articleID=000BAEB1-B2AA-1C6F-84A9809EC588EF21 And here is a segment of it:

Quote:

Our goal was to drastically slow down light, which travels through empty space at the universe's ultimate speed limit of nearly 300,000 kilometers a second. We saw the first sign of light pulses slowing down in March 1998. As happens so often in experimental physics--because it can take so many hours to get all the components working together for the first time--this occurred in the wee hours of the morning, at 4 A.M. By July we were down to airplane speed. At that time I had to go to the Niels Bohr Institute in Copenhagen to teach a class. I remember sitting in the plane marveling that I was traveling "faster than light";--that I could beat one of our slow pulses to Denmark by a full hour.

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For another 2 cents worth.

C is a constant only if the speed of time varies.

But if you declare time constant then C will vary.

Since mass varies the speed of time which in turn varies the accelerations of gravity the notion of space-time became necessary to define a location.

And to mess it up further "light" having mass (when it acts like light) is subject to inertia so consequently a light ray between two massive objects (stationary relative to each other) Will always be red shifted (wave length increased or energies per time unit reduced)

Stay with it Individual, Yes a prism does slow light. Light travels at different speeds on different mediums (just good old wave theory) The prismatic effect seems to imply that different wave lengths travel at different speeds also if there is any medium. Space time seems to be a medium. Still confused, good you are definitely not alone. Me too

Now do you still wonder why Albert E. tore at his hair Question

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Aka,

You can't just "declare" time constant (at least not in Physics). Light does not have "mass" per se and light between two objects (stationary relative to each other) is not red shifted. (Red shifting happens when two objects are moving apart).

Please stop making up physics you don't understand. You frustrate those of us who have taken the time to study it, and confuse those who are interested but haven't taken the time.

Sorry, but people who pretend to know something about science when they clearly haven't taken the time to really learn it is one of my pet peeves.

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Individual,

You are absolutely correct about a prism. Light slows down when it passes through matter. However when we say "the speed of light is a constant" we are talking about the speed, not the light. This is often more correctly said "the speed of light in a vacuum" is constant.

Any particle that moves the speed of light would have the same properties. Only the relative speed matters.

Benevolent,

McGentrix is right about gravity distorting light. We have seen gravity bend light.

However with relativity, gravity is even a bit more strange. Gravity has many of the same affects that relative speed has. If you are in a strong gravity, time goes slower and distances are a bit shorter. This is one of the consequences of General Relativity.

I believe what you are talking about with the "half-silvered" mirrors is a method used in a range of experiments to measure the distance that light travels very accurately.

The first experiment I am aware of that use this technique is the seminal "Michelson-Morely" experiment that is a very interesting story in itself. The experiment was trying to detect a substance called aether. In its goal, it failed.

Einstein realized it failed because aether doesn't exist. This discovery turned out to be much more important and this experiment is now famous because of its failure.

Similar techniques have been used to verify many of the predications of Relativity.

Here is a somewhat technical, but readable explanation of this experiment and the idea behind it.

http://galileoandeinstein.physics.virginia.edu/lectures/michelson.html

Finally the article about "freezing light" is a bit misleading. What really happens in these experiments can only be explained mathematically and in truth what is being "frozen" is not the same as freezing a photon in a vacuum.

It is often difficult to take technical mathematical ideas and express them in everyday language. Of course people try, but the results are often misleading.

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curious George, wrote:

my two cents worth,

you would see the light coming toward you at the speed of light and the spacecraft inhabitants would see it going away from them at the speed of light, but you would see it as slightly bluer than they would....is that not right?

c.g.

Your are correct. This is the Doppler shift. And, as ebrown_p points out, we are talking about a certain speed, not light.

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E.Browne,
I hate to annoy you but if light does not have mass then there would be no reason for for the various phenomena observed such as Einstein crosses, Gravitational lenses,displacement of starlight, reactions of gold leaf "sun vanes" in a vaccum, black holes and various other peremutations of "observed light".

So for those reasons and others I think it fair to assume that light has mass ie. weight, ie. subject to the influences of gravity.

Unfortunetly the "speed of time" varies with some relation to masses.

The Harvard Tower Experiment was one.(it's on the web, I'd link it but we had a catastrophic event in cyberspace Smile

Variation in the speed of a couple of clocks in the Empire State Building was another. Outlined by Paul Davies in a couple of his books.

I don't blame you for being annoyed. I have a strange propensity for sticking to observed phenomena. Our conclusions may be different. Ambiguous perhaps, but far from certain Exclamation

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Brandon9000 wrote:

curious George, wrote:
my two cents worth,

you would see the light coming toward you at the speed of light and the spacecraft inhabitants would see it going away from them at the speed of light, but you would see it as slightly bluer than they would....is that not right?

c.g.

Your are correct. This is the Doppler shift. And, as ebrown_p points out, we are talking about a certain speed, not light.

no it's not the Doppler Shift, look it up, the doppler has to do with sound waves not light...akin to doppler? I think I know what you mean, but then again, I may be mistaken, I thought you were talking about the speed of light for some strange reason...???

so, what was the certain speed then, if not light?

C is constant, time is relative, that's what the experiments showed, but maybe that's irelevant too?

but if you, like Frank, need absolute proof, alas I have none on me. Embarrassed

c.g.

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akaMechsmith wrote:

E.Browne,
I hate to annoy you but if light does not have mass then there would be no reason for for the various phenomena observed such as Einstein crosses, Gravitational lenses,displacement of starlight, reactions of gold leaf "sun vanes" in a vaccum, black holes and various other peremutations of "observed light".

So for those reasons and others I think it fair to assume that light has mass ie. weight, ie. subject to the influences of gravity.

Unfortunetly the "speed of time" varies with some relation to masses.

The Harvard Tower Experiment was one.(it's on the web, I'd link it but we had a catastrophic event in cyberspace Variation in the speed of a couple of clocks in the Empire State Building was another. Outlined by Paul Davies in a couple of his books.

I don't blame you for being annoyed. I have a strange propensity for sticking to observed phenomena. Our conclusions may be different. Ambiguous perhaps, but far from certain

Aka,

What annoys me is that you clearly don't know what you are talking about.

You are saying things that any freshman physics student (as well as many of my high school students) would readily see is pure nonsense.

Physics is a field of study (with the emphasis on study). It is based on the work of scientists over thousands of years. Each generation of scientists has built on the work of those who preceeded them.

You, of course can learn about physics. I would suggest a class at a local college.

In this class you will learn why speaking about the "mass" of light is problematic. You will learn that even though the "rest mass" of light is zero it does have a relativitistic momentum. You will learn about why physicists don't normally speak about the mass of light. If you take more advanced class (later) you will learn about why gravity "attracts" light (and why the word "attract" doesn't really explain what happens).

But my beef with you is that you pretend to understand these things, without any understanding.

When you throw around big sounding physics words out of context, it has the same effect as what would happen if I tried to preform in public with a jazz band. I have seen people play jazz clarinet and I could certainly move the buttons like they do. But I have never taken the time to study the clarinet with any seriousness. I am confident that with a lot of work I could reach some mastery of this instrument. But I would never pretend I have the current skills to play in public.

This is not a matter of different conclusions. Your "conclusions" fly in the face of physics.

There are people on this thread who clearly have taken the time to learn about physics. We have taken classes. We have read books and we understand what Einstein et al. actually said and *why* he said it. Brandon, for example, clearly knows what he is talking about.

There are other people in this thread who are curious. They are making conjectures and asking for help to understand. But, they are not pretending to have any knowledge they don't have. This is one way to get knowledge.

But on these thread you try to come accross as an "expert" when you clearly havent spent very much time at all to learn.

This hurts these discussions. You create controversy where, among people who study science, there is none. And, you add to confusion about science where there should be no confusion.

Please stop.

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I have two more quick questions: do different colors go different speeds? And if not, since infra-red and x-rays all fit on the same spectrum, wouldn't they also move the speed of light?

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Interesting question Individual.

In a vacuum, all light all goes the same speed regardless of color.

And yes, light is just one part of a group of things called the Electromagnetic spectrum. Radio waves, UV radiaton, X-rays and microwaves are all part of this spectrum. They all go "the speed of light" in a vacuum.

However when light passes through matter, the different frequencies (i.e. colors) do go different speed. If a substance has this property it is called a "non-linear medium".

A prism splits lighit into its component colors precisely because of this property. The glass of a prism and water are both non-linear media.

Here is a good explaination.

http://science.howstuffworks.com/rainbow1.htm

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E.Browne,

I will probably stop at the same time you stop presenting "theories" as facts. I am referring to your comments regarding "C" in this thread.

I know that normally physicists don't refer to the "mass of light" but mechanically speaking for practical purposes light does have mass when "it's acting like light".

Re your "this hurts these discussions"

Sorry, I can't allow that Sad

.

Currently on my desk is a book entitled "Cosmology and Controversy"; sub titled "the historical developement of two theories of the universe" by Helga Kragh, Princeton University Press, 1996.
Apparently controversy is not unknown in the world of physics, or religion for that matter.

It's one of my pet peeves. A confusing of theories with facts.

You could have said that the speed of light is a constant in relativity theory. That IMO would be fair.

I don't wish to hijack the thread. But would it be too much to ask to differentiate a bit when you are attempting to educate us Question

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curious George, wrote:

Brandon9000 wrote:
curious George, wrote:
my two cents worth,

you would see the light coming toward you at the speed of light and the spacecraft inhabitants would see it going away from them at the speed of light, but you would see it as slightly bluer than they would....is that not right?

c.g.

Your are correct. This is the Doppler shift. And, as ebrown_p points out, we are talking about a certain speed, not light.

no it's not the Doppler Shift, look it up, the doppler has to do with sound waves not light...akin to doppler? I think I know what you mean, but then again, I may be mistaken, I thought you were talking about the speed of light for some strange reason...???

so, what was the certain speed then, if not light?

C is constant, time is relative, that's what the experiments showed, but maybe that's irelevant too?

but if you, like Frank, need absolute proof, alas I have none on me.

c.g.

I have two comments to make here, George:

One: Brandon was correct, and you were incorrect here.

Doppler Shift applies to electromagnetic waves in all portions of the spectrum.

http://zebu.uoregon.edu/~soper/Light/doppler.html

Two: I damn near never ask for absolute proof of anything -- unless the individual with whom I am discussing a matter suggests he/she CAN PROVE the item in question.

I KNOW I have never asked you for proof of anything. (Like the man says: Ya can look it up.) In fact, I doubt you can find a single instance of me asking anyone for proof of anything in this forum.

Interesting, isn't it? :wink:

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Not so fast Frank. This argument is simply a matter of the definition of the Doppler effect.

The Doppler effect was first used to descibe a particular behavior of sound waves. It happens when either the source of the sound, or the person hearing the sound is moving relative to the medium (i.e. the air).

Red and Blue shift happen bacase the source of the light is moving relative to the observer. This is a fundamental difference. These two phenomina are similar, but also profoundly different.

Nowadays the term "Doppler Effect" is used to explain both phenomina. However the science and math behind them are quite different.

So, in this case, you're both right.

Incidently, Red Shift is a pretty convincing proof that the speed of light is a constant.

Brandon,

Let me try to clarify the "speed of light" and the speed that light goes. The "speed of light" is the name we give to a very special speed. For one thing, this is the Universal speed limit from *any* reference frame. Simply put, this is the fastest speed that any thing can go relative to anything else.

We call this the speed of light becuase this is the speed light goes in a vacuum. Don't get confused by the name. There are other things (e.g. x-rays and gamma-radiation) that also go this speed.

Light goes slower than this speed when it passes through matter. This speed of light through a certain type of substance does not have the special properties of the speed of light.

So, "the speed of light" as a special constant and a basic property of the Universe doesn't change.

The speed that light travels as it goes through matter does change.

I hope this is clear.

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