Why does Electro-magnetic radiation go the speed limit? How come that particular energy source travels the fastest? Why cannot something exceed that limit?

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According to current theory; mass and energy are equivalent; associated through the equation E=Mc^2. A derivation from this equation leads to the relationship M=1/(1-v/c)^2. As the velocity of an object approached c, the mass approaches infinity. This prevents anything with mass from reaching the speed of light.

The speed of light likewise tops out at c, and it is due to the same relationship: Since mass and energy are equivalent, accelerating even a single photon beyond the speed of light is kinetically impossible.

There are better reasons and explanations, but I think this is about as simple as it gets.

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jwj

It's a big universe out there...is it really unwinding, really burning out?

The truth is we do not know if C is the universal speed limit. According to current theories there are particles that have yet to be detected, who's to say those particles don't travel faster than C?

Science is full of assumptions.... Do yourself a favor and keep asking why, and keep doubting the answers they give.

The same theories that say those particles exist in the first place.

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Science is like sex. Sometimes something useful comes out, but that is not the reason we are doing it.
-- Richard Feynman

The radiation you refer to is carried by particles that have "zero rest mass", which means they would not exist in any reference frame in which they were not moving. Since existence is not allowed to depend on reference frame, that means there is no reference frame in which those particles are not moving. That also means they must move faster than any allowable reference frame, i.e., there is a "speed limit" for observers. Other arguments indicate that all particles with zero rest mass must move at this same speed limit. Now, that speed could have been infinite, but that's a very strange universe where things can move infinitely fast! Ours doesn't have that property, so the speed of light is finite. Why that speed happens to be 300,000 km/s, no one has any idea.

I am curious about something, How can you tell how fast you are going? I mean if you are on a bus that is going 20 MPH and you walk forward 3 MPH are you going 3 MPH or 23 MPH. This can be applied to C. If you are on a ship going as close to the speed of light as possible and you walk forward are you going faster than the speed of light? Or is it not even possible to walk forward on a ship that is going that fast? Also if you held out your arm would your blood be able to pump to your fingers? Because if it did than your blood vessels are going faster than the speed of light.

Edit: another thing, if you are on a bus and you can see that is the light inside the bus moving faster than the light outside the bus?

Well light never overtakes light. But if you are on a ship travelling at 99% of light speed and turn on the ships front light, you would see that light move away from your ship at the speed of light relative to you, i.e. it would look like it was moving away from you at 300,000 km/s, from your point of view.

But would that mean it was travelling faster than light from somebody else's viewpoint? The answer is no. Light never overtakes light. So what would someone else see?

They would see your ship travelling at 99% of the speed of light, and that light would be moving at the speed of light, so they would see the light moving away from the front of your ship, moving 1% of the speed of light faster than your ship is! They see the light moving at c, and your ship travelling at 99% of c.

So what is happening here? How can anyone, whatever speed they are travelling at, see light move 300,000 km/s faster than themselves?

Well, if light is the constant, then it must be distance and time which are changing depending on the viewpoint. Your rulers and your clocks measure the universe differently, depending on your velocity relative to somebody else's rulers and clocks.

We have found that clocks tick at different rates when they travel at different relative speeds. This is unnoticeable in daily life, but we have tested this by putting very accurate atomic clocks on jet planes and flying them round the world and then comparing them to a clock that stayed on the ground. It seems that when you travel relative to another object, your clock ticks at a different rate relative to that objects clock.

Another effect of different relative speeds is a difference in the way you measure the universe around you. As speeds increase, objects seem to shrink in the direction of travel.

You asked how we can tell how fast we are going, but you answered the question in the same sentence - "I mean if you are on a bus that is going 20 MPH and you walk forward 3 MPH are you going 3 MPH or 23 MPH?". You can only judge your speed relative to another observer. You are walking forward at 3mph relative to a passenger sitting on the bus, but you are moving at 23mph relative to a person standing on the ground watching the bus drive past.

But who is to say who is moving relative to who? If two ships pass each other in deep space, who is to say which is moving and which is not? Well it turns out that either ship can think of themselves as being at rest and that the other ship is the one that is moving, and they would both observe the same amount of change in the length and the clock of the other and would both measure light as travelling 300,000 km/s faster than themselves!

One way to think of it is that the closer you approach the speed of light relative to another object, the bigger the difference there is in the rate that your clocks tick and the length of your rulers, which means you will always measure the speed the light as the same figure, and it is your perception of space and time that change for you both to make up the difference. And it works the same for everyone, whatever their relative speed.

Using the same equations, how could a photon be so small? If they are going the speed of light and they are mass, then by the standard models' equation; the photons should be almost infinite in size!!!!!!!

Seems to me that the equation itself says it's wrong, or there is no such thing as a photon in light.

I prefer the latter of the two.

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Just because it's complex doesn't mean it's true!

If you can't explain it, then you don't understand it!

(bold mine. extra exclamation points, all yours.)

Well, there's your problem. The part about mass. Photons have "no rest mass", remember? They don't exist unless they're moving at lightspeed. The only time they even "slow down" (sort of) is when they get absorbed into a particle with mass-- which means they're not photons any more. they've been converted into another form of energy. If they're re-radiated, they become photons again, and therefore must move at lightspeed. Their "mass" never changes, only their energy.

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"If this were play'd upon a stage now, I could condemn it as an improbable fiction."
Shakespeare, Twelfth Night
"The Mayan symbol for "book" looks a lot like a triple hamburger, but I've never seen them claiming it as proof the Mayans had Big Macs." - KaiYeves
"Distance doesn’t matter much in space, where if you just start a thing off with the right kind of shove, sooner or later it will get where you want it to go." -Frederik Pohl, Mining the Oort

I agree; characterizing a photon as a particle is a poor choice. But it is ok to say a photon has a mass-equivalance of energy. Photons have wave functions, and personally I think even the photo-electric effect can be properly characterized as a wave, rather than particle function.

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jwj

It's a big universe out there...is it really unwinding, really burning out?

Well, it may be a "poor choice" for you and rebel, who do not need to do actual calculations involving photons, but it's a good choice for all kinds of physicists who actually do ultra-accurate calculations involving photons. Still, I will give you the benefit of the doubt that what you really mean is that the classical concept of "particle" is of limited value.All particles have wave functions, that does not in any way compromise what we mean by "particle", it just requires the term be better understood than classical analogies allow.

Scientists have supposed to have been able to move certain particles (I think it may be light particles through a certain type of gas) faster than the speed of light.

You are talking about the speed of light in some material that slows the light. It's confusing, but what is normally meant by "the speed of light" is a constant called c, the speed of light in a vacuum. Nothing has ever been observed to carry information or particles faster than c.

Why the speed of light is c in the vacuum?

I´ve seen people proposing that light is contantly subject to a 'drag' by the vaccum energy. That would result in its observed speed. It would be akin the the slowing produced by interactions in a medium, like water. But I´m sure people more knowledgeable than me here can easily show why this idea is rubbish.

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What brings us together is stronger than what pulls us apart

A drag is not a very good way to think of it-- drag is a force that can achieve a slowed speed if there is always some force pushing it forward, like a motorboat engine. In fact, if the idea is to slow light down to c from some other faster speed, then what was that other speed? It doesn't answer anything to imagine it that way. I would just say, either the speed of light would be infinite, or it wouldn't. If the former, what a strange world that would be, and if the latter, then why not c? For the reasons mentioned above, particles with zero rest mass define a speed limit-- and it wouldn't make much sense for that speed to be infinite. Basically, the spirit of relativity requires there be a single speed for massless particles, and why that speed turns out to be c is totally unknown and probably always will be.

The field equations of all known forces are wave equations with propagation speed of the speed of light. When you think about "things" going faster than light, your thoughts are wrong because all "things" are really composed of waves. These waves are electromagnetic and gravitational and propagate at c.

The only way that something can go faster than light is by a conspiracy of converging waves known as the scissor effect - see http://math.ucr.edu/home/baez/physic.../scissors.html

For more information on particles travelling 'faster than light', please research Cerenkov radiation. It is a nice shade of blue and very interesting.

http://en.wikipedia.org/wiki/Cherenkov_radiation

Make note the particles never travel faster than c(theoretical tachyons exempted). It is the speed of light in a medium that slows down and allows the particles that are travelling very close to the speed of light to surpass their massless friends.

Well, we aren't the only ones that disagree with photons not agreeing with E=mc^2, the wikipedia doesn't like it either. Here is a clip from "Elementary Particles" from the wiki...
Main article: Standard Model
The Standard Model of particle physics contains 12 flavours of elementary fermions, plus their corresponding antiparticles, as well as elementary bosons that mediate the forces and the still undiscovered Higgs boson. However, the Standard Model is widely considered to be a provisional theory rather than a truly fundamental one, since it is fundamentally incompatible with Einstein's general relativity. There are likely to be hypothetical elementary particles not described by the Standard Model, such as the graviton, the particle that would carry the gravitational force or the sparticles, supersymmetric partners of the ordinary particles.

I highlighted the discrepancy for you!

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Just because it's complex doesn't mean it's true!

If you can't explain it, then you don't understand it!

I hate to tell you, this is not news to me. But if you think it makes sense to argue that "characterizing photons as particles is a poor choice" on the basis that "the standard model has not unified gravity and quantum mechanics", I'm afraid I've lost your train of thought. Again, I can only point to the accurate calculations made by practicing physicists that quite successfully treat photons as particles, within the context of what "particle" means in quantum mechanics, not the casual way many nonphysicists might use the term. I suspect the latter is what you are doing, but such casual usage of "particle" is neither precise nor scientifically useful. On another thread, it was pointed out that the word "space" is used very differently at different levels of abstraction, and the same is true with the word "particle"-- it is not a phrase that means the same thing to you as it does to a "particle physicist".

Yes, that's Wikipedia, "the online encyclopedia that anyone can edit." Which means anyone can put their unproven and unsupported opinions in there.

I highlighted the important parts for you.

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"If this were play'd upon a stage now, I could condemn it as an improbable fiction."
Shakespeare, Twelfth Night
"The Mayan symbol for "book" looks a lot like a triple hamburger, but I've never seen them claiming it as proof the Mayans had Big Macs." - KaiYeves
"Distance doesn’t matter much in space, where if you just start a thing off with the right kind of shove, sooner or later it will get where you want it to go." -Frederik Pohl, Mining the Oort

And actually, there's nothing wrong or even controversial with the Wiki entry-- and it provides no support for rebel's position, either.

So what exactly sets the speed of light? Why that particular number; not the photon itself?

Permittivity and permeability, what are these beastly words?

If memory serves me right, permittivity is related to changes in the electric field. That leaves permeability for the magnetic field. If I increase the permittivity of the vacuum, what becomes of the permeability?

I don't think ratios of permittivities and permiabilities brings any insights into why c is what it is, it just changes the language of the question. We don't know why it takes on that particular value-- and it's a deep question if science is even capable of knowing that.

I think this is the best answer to the OP. And I also think that anybody who can answer that question (if it can be answered at all) would certainly be invited to have dinner with the king of Sweden.

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As above, so below

is it possible to think of the speed of light as being some kind of base from which all else is derived, a kind of zero?

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It's possible, indeed often done in theoretical work, to set the speed of light to one, and express all other speeds as fractions of it. It is, if you will, the natural unit of velocity, and I guess you could think of it as the "base" velocity - but it's not zero in any meaningful sense.

(You could apply some one-to-one transformation to velocity so that lightspeed becomes zero in the derived quantity. But no such that I know of has any physical meaning - it'd just be a mathematical game.)


From another point of view, everything moves at the speed of light. This would be looking at the four-speed, which always has the magnitude of c - it's just that for ordinary objects the overwhelmingly largest components points in the direction of future time rather than in a spatial direction. In this view time dilatation is simply the fact that more of the constant total velocity points spatially so the particle goes slower in time, and photons (and other massless particles) travel in an exclusively spatial direction, so being in a sense timeless.

From this POV the question of the thread title is trivial - the spatial component of your four-velocity can obviously not be greater than your total four-velocity.

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Science is like sex. Sometimes something useful comes out, but that is not the reason we are doing it.
-- Richard Feynman

A note here is that there is a lot of curiosity regarding the speed of light.

The point is which ever frame you are in you will measure light as having the same speed. But what about the speed of space?

This is from the Hubble site glossary section

OK so it is cheating by taking space out of the reference frame of space. But space is big ... really big and by using those numbers it is clear that beyond the Hubble Limit (outside the frame) space is travelling faster than light.

This is an insightful way to look at the question, and it is often expressed this way. I would actually say it slightly differently-- I would say that any observer always sees the 'rate of flow of time' to be unity-- one second of action elapsed per second on his/her clock (which is really just a statement that all local processes proceed according to a local standard clock, no one knows why, but that is the core concept of time). However, they do not perceive other observers as having a unity rate of flow of time, when referencing their time to the observers' time (that's time dilation). That's bad, we've lost an invariant. The way to recover that invariant is to look at both space and time, as AndreasJ says, and include the "rate of flow" of both, together into a single rate (or "spacetime velocity", if you will). The problem is, space and time are different entities, so are measured in different units, and a conversion is required to be able to unite them into a single rate. The conversion that allows the rate to be invariant is that all distances get multiplied by 1/c, and then we combine the whole business into an effective rate-- an effective one second of action per one second of the observer's clock. So c is handed to us as part of the desire to convert space into a timelike format so that we can get an invariant rate of flow of something. Then massless particles must also move at that speed c, in a vacuum, so that no time can pass for them in "their own frame" (a frame in which they would have no mass so could not exist). What I'm saying is I don't really like the idea that "we are all moving at rate c", I like to say "we are all moving at rate unity", and we think everyone else is also moving through time at rate unity, where we just need to generalize our concept of 'movement through time' to account for an appropriately converted distance contribution.
True, but the question persists, why is that speed limit equal to c? Or as I've put it above, why is 1/c the conversion factor from distance to time that makes the rate invariant? We still have no idea. Could that number ever come from some other number, or would it always just have to be inserted as an observed parameter? Some people might use an anthropic argument, "it had to be c, or very close to it, or we couldn't be here". Although that may be true, I wouldn't call that a scientific explanation (as it relies on untestable assumptions), but it may be the best science can ever do.

Well, obviously the value of c can be derived from other constants (eg. the permittivity and permeability of vacuum), but then you have to measure those. The real question is rather how many free parameters a "theory of everything" would have. The number is given by nature, but exactly which those parameters are would be up to how we chose to formulate the theory.

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Science is like sex. Sometimes something useful comes out, but that is not the reason we are doing it.
-- Richard Feynman

That's a view that comes about from getting a bit too high on Maxwell, and ignoring the deeper foundation on which it is built. Years ago, I was of that mind.

We have this relation mu*epsilon = 1/c^2. You can see that as saying c is function of these two fundamental properties of the vacuum. However, the deep view shows that the two constants are really functions of 'c', plus, and this is very important, the electrical units we choose.

The way it's done in SI, is the electric force constant goes as
1/epsilon, and the magnetic force constant goes as mu directly. So a better way to write for purposes of these discussions is:

E/H = c^2, where E and H are the force constants.

Although, we could ramble on for ages about this, getting deep into the notion of "duality", basically we have these E and B things, and either can be viewed as fundamental, and the other is sort of a "relativistic effect". The latter can be a bit misleading, because in the general case, one can't transform either one away globally, so you can't really see B as a pure coordinate effect of a pure electric thing, or E as pure coordinate effect of B.

However, the two transform where the one transforms into the other, and 'c' is the governing factor of that transform.

So, by choosing a unit system, how much one unit of charge will be, we fix E, the electric force constant. That immediately determines H from 'c'. And that can be seen as the fact that once we choose a unit of charge, the unit of current is immediately determined as charge per time.

In practice, since it's so much easier to circulate charge, making current, than it is to separate charge, making static electric forces, we actually choose the unit of current, fixing H, fixing E. mu_0 is simply chosen to be 4pi*10^-7 H/m, making a certain current in two wires that produces a certain force be one unit of current, one amp as we call it, one coloumb of charge per second.

Dimensionful constant are sort of artifacts of arbitrary choices of units.

So the way to think about this is we have just one degree of freedom in choosing electrical units. The other constant is immediately determined by our choice of space and time units.

That's "all c is", our choice of space and time units.

Indeed, nature gives us so many degrees of freedom. We could set up some system where we choose both E and H independently. That would then *fix c*, forcing a certain relation between space and time.

In General Relativity, gravity itself plays a wave game, although a non-linear, tensor complicated mess of a fame, but the same E vs H relation can be seen there.

However one doesn't think of it that way, one just uses G and c, rather an E and H (G plays the role of E). One could, and Gaussian units do, formulate things entirely in terms of E and c (and choose E to be one).

All this stuff is about dimension and the units we tick mark them off with.

THe most elegant, high priestly way to do it is to let the universe itself choose its units (equivalent to setting all the degrees of freedom to one). That gives you Planck, or God's units. Or I should, gives you a lack of units, because you're ticking off all dimensions in fixed way.

-Richard