why does light have a finite speed

@Zarathustra,

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but it is, in no way of common usage of the term, a philosophy

I suppose, Zara, it depends on what you mean by the term

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@cheeser,

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So what defines velocity, …...mass, energy, space…... Is there an answer?

My feeling too for what it's worth, evidently not much around here

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@tomr,

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…...that it takes time for an electric field to generate a magnetic field in an electromagnetic wave. I have never heard that.

Me neither, thought I was missing out somehow

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…...I always took this to be instantaneous…..

Me too but who am I after all

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But I am with you on the speed of light. It cannot be infinite.

Yes, no, Tomr, I'd backed off on that assertion since it seemed to pose conundrum, contradiction, paradox

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…...There would be no explanation for the delay of a signal over long distances.

I give you my concentric circles, eg, at the instant it's 12:00 here it's 12:0500000000000001 on Mars, thus the wavefront makes the trip almost instantaneously

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@dalehileman,

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I'm not sure, Tom, that's been conclusively decided. Don't some thinkers still maintain a strong case for light as an oscillation of space (See Tomr #…565)

If you are thinking of light as an oscillation of space you are giving space properties of a carrier medium. Since space does not initiate the electromagnetic wave. An accelerated charged particle or a changing current initiates an electromagnetic wave, and I think of that light as being propagated by those charge's fields which is apart of the charges themselves. So in your case, I think space would act like an "inbetween" or medium for the propagation of the wave. This is alot like aether theories.

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@Thomas,

Light has momentum doesn't it? If so does that not mean that it must have mass ?

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@cheeser,

Don't let the word "mass" confuse you, because in physics it can be used in two different ways. Look at Einstein's equation for example: E=mc^2, where m=E/c^2, and so mass is really just the same thing as energy (E) but measured in different units. Light has this "m" because it has energy. Therefore, light is indeed affected by gravity (and not only by black holes but in other less extreme situations). In 1919, general relativity was confirmed when it was discovered that light from stars bends as it goes by the Sun.

A better explanation is given here: http://physicsforums.com/showthread.php?t=511175

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@tomr,

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…….you are giving space properties of a carrier medium.

Yes I guess so

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Since space does not initiate the electromagnetic wave.

It's initiated by a energy source of some sort such as a lamp or radio transmitter

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An accelerated charged particle or a changing current initiates an electromagnetic wave,

Yeds tht's how I understood it

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and I think of that light as being propagated by those charge's fields which is apart of the charges themselves.

Dunno, that's somewhat beyond me but isn't a gravity wave somewhat analogous

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So in your case, I think space would act like an "inbetween" or medium for the propagation of the wave. This is alot like aether theories.

I had understood that theory maintained light as an aether wave

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@cheeser,

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does that not mean that it must have mass ?

It's tempting to guess that yes it does and that's what determines its velocity. Maybe then the neutron's is ever-so-slightly less so it goes a tad faster

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@Enzo,

Does that mean that all energy has momentum associated to it, even in the case of energy which has no carrier medium?

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@cheeser,

A body can posses energy without momentum, look at potential energy for example.

In special relativity, the energy of an object of rest mass m is given by:
E = mc^2/ (sqrt(1-(v/c)^2))
When v=0, you get E=mc2, and so E = mc^2 is the rest energy of some object or mass. And so you have a relationship between mass and energy.

The kinetic energy of an object is the energy in excess of the rest energy, so it's given by:
K=E-mc^2

Conclusively something can not have momentum without energy, but something can posses energy without momentum.

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@dalehileman,

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Dunno, that's somewhat beyond me but isn't a gravity wave somewhat analogous

Yeah I suppose there are theorists out there that do think this about gravitational waves. But waves like these have never even been detected. Its a different realm of scientific theory though. Einsteins theory of gravity proposes alot of weird stuff might be possible but is hard to reconcile with some of the more concrete and experimentally detectable science. Be careful when assuming ideas of this kind in your own theory because there are an infinite number of ways we can go wrong and only one theory will be right in the end. But then again there is nothing wrong with exploring this analogy with light as long as it is not contradicted by some more readily verifiable theory.

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@tomr,

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…...gravitational waves…..never even been detected.

Very surprised Tomr to learn that. One would suppose, eg, that if we're near a large object that suddenly moves that our grav attraction to it would also change, presumably having reached us at c. Grav wave

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Be careful …...in your own theory because there are an infinite number of ways we can go wrong…...

My own isn't exactly a theory, it's just a different way of looking at time at a distance

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But then again there is nothing wrong with exploring this analogy with light….

That was my immediate reaction

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@dalehileman,

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Very surprised Tomr to learn that. One would suppose, eg, that if we're near a large object that suddenly moves that our grav attraction to it would also change, presumably having reached us at c. Grav wave

Yes gravitational attraction occurs. But the form that gravity is assumed to take in this kind of case has never been dectected. The form being the rippling of space-time creating somekind of actual distortion. I got this from wikipedia:

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As a gravitational wave passes a distant observer, that observer will find spacetime distorted by the effects of strain. Distances between free objects will increase and decrease rhythmically as the wave passes. The magnitude of this effect will decrease the farther the observer is from the source.

It is true that einstein predicts this kind of distortion. I just find the contractions of space and time to be odd, and I suppose have a personal bias against it to start with. I also have a bias against theories that assume more dimensions than I can count on one hand-what ever happened to string theory anyway?

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@tomr,

tomr wrote:

Where do we come across a delay in the creation of either a magnetic or electric field?

It's implicit in equations 34.5, 34.6, and 34.7. The book doesn't explain it at this point, but there's a footnote pointing to section 16.9 of the same book, which presumably explains the point you're asking about for waves in general. Do you have a link to chapter 16 of the book? (As an aside: what's the title of the book? It seems rather well written.)

Either way: on reflection, you're probably right with your objection. The actual theoretical argument may not be as easy as easy as solving the equation. You probably have to start with a Hertzian dipole in a field-free vacuum, then wiggle the electrons in the dipole to create a pulse in E(t, x=0), then look at what happens to B(t, x=X) and E(t, x=X) for some small, nonzero value of X, by integrating equation 34.5 over t. You will find that B(x=X) will change with a time delay proportional to X. That's all I was trying to say in my earlier post, but you're right, it's more complicated to show than I thought.

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@dalehileman,

dalehileman wrote:

Don't some thinkers still maintain a strong case for light as an oscillation of space

Not if the thinkers are competent at thinking and informed about the relevant experiments. (For details, search the web for "Michelson experiment")

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@cheeser,

cheeser wrote:

Light has momentum doesn't it? If so does that not mean that it must have mass ?

No, not necessarily, although it happens to be the case in Newton's original formulation of mechanics. But in Hamiltonian and Lagrangian mechanics, which generalize Newtonian mechanics to work for arbitrary coordinates, the concept of momentum is generalized to the point of defining momentum in terms of energy, space, and time. You no longer need any explicit reference to mass. And because electromagnetic fields have energy as a function of time and space, you can ascribe a momentum to them even though light has no mass.

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@Thomas,

interesting, if i do physics at university how much of this am i likely to do?

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@tomr,

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…...But the form that gravity is assumed…... has never been dectected.

I assume what you're explaining is that the wave caused by jolting a heavy object is simply too weak to detect. Yet it just has to exist doesn't it

And isn't that exactly what the Wiki quote says

Thus we ought to be able to devise an experiment. Wouldn't it be crackers if we found it to move faster than light--or the neutrino

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It is true that Einstein predicts this kind of distortion.

I'd think it would have been predicted long before Einstein

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I just find the contractions of space and time to be odd,

…….Yea Tomr just as the mysterious changes taking place in moving object and the limitation of its velocity (which incidentally my weird concentric-time perspective does explain)

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and I suppose have a personal bias against it to start with.

Why though Tomar, it seems perfectly logical to me

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I also have a bias against theories that assume more dimensions than I can count on one hand

I fully understand

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-what ever happened to string theory anyway?

Yes what exactly

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@Thomas,

Don't some thinkers still maintain a strong case for light as an oscillation of space

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Not if the thinkers are competent at thinking

Guess that leaves me out

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@cheeser,

cheeser wrote:

interesting, if i do physics at university how much of this am i likely to do?

That depends on what you mean by "do physics". If you choose physics as your major, you will learn all of it. If you choose some other major major --- call it "X" --- and take a "physics for X" course, you won't learn the relativistic and quantum-mechanical aspects of light, but will learn most of the rest.

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why does light have a finite speed

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