cybersharp

Consider the sun, It gives off light, about every second or ...maybe it was millisecond that light travels it moves more or less a foot.
Now, considering everything has to have force applied to move, what propells light that fast..from any source of which light is rendered light goes faster than a human can move. So why can light as small as it is move so fast over so much distance without any seeming propellsion where as a human can only move slowly?

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John Henry

It doesn't move.

At the speed of light, time doesn't pass, so it never moves.

To answer the force question, light has no mass, only momentum, so it needs zero force to propel with infinite acceleration.

Or does it?

How's that for a headfuck?

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stingc

You're a few centuries out of date . Things need a force to accelerate them; not to move at a constant speed.

An heuristic explanation would say that light is composed of particles with zero mass. Since a=F/m, any force whatsoever will cause light to move at the fastest speed possible. Relativity tells us that there is a finite maximum speed, and that it is a universal constant. So light moves at that speed (in a vacuum).

To be honest about it, that explanation is quite wrong. It is a useful mental picture, though. A more detailed explanation would get technical pretty quickly.

Zeraph

It really is, and it feels good

cybersharp

So then movement is confined to how much the mass is then...the more the mass the longer and more accelloration you must have to move?

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stingc

I have no idea what this means. I think you've made up your own definitions for half the words you used.

tecoyah

Light is radiation....it is propelled by its very creation.

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Suave

Angels. Duh.

cybersharp

Thank you! Thats the answer i was looking for.

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Willravel

So it wasn't angels then. Oh well.

shaggs_manoa

I like the "FM" story best...i.e. It's Frigging Magic!

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Suave

I heartily disagree. If angels don't move it, then they make it with their angel powers.

Ustwo

Any science or technology which is sufficiently advanced is indistinguishable from magic.
- Arthur C. Clarke

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flstf

186,282.4 miles per second : Not just a good idea, it's the law.

JJRousseau

Visible electromagnetic radiation, to be exact. But radiation is just movement (to or from a point - or in this case, a source) so in one sense, light IS movement.

John Henry

roflmfao. Best answer yet.

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cybersharp

So then in a sense...If one were to travel the speed of light then one would "almost" cease to exist?

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stingc

The word 'radiation' has more than one meaning. It can indicate the act of moving outward from a source, or it refers to a class of physical objects which includes light. It doesn't mean anything to say that light IS movement.

Saying light is radiation is correct, but it's is a label, not an explanation. Saying that light is specifically electromagnetic radiation gets you a lot more, but only if you know electromagnetism. Otherwise, it's just more jargon.

Furry

Specifically, any electromagnetic beam (whatever the frequency) is made up of electric and magnetic components, which propagate each other. It can be modelled as a vibration on a membrane.

Is it just me, or does anyone else consider a universe with a speed limit particularly sadistic? Ok, C's important, it can tell us lots of good things, but quite frankly:

The C's the limit. *Bwaha*

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ObieX

What's cool is scientists have been able to slow light down to like 35mph. It would be more acurate to say the maximum speed of light is 186,282.4 miles per second. I've always been interested in the actual definition of what light IS. Visible electromagnetic radiation is the best explaination anyone has ever given to me and it was from this thread - thank you TFP

(I still have little idea what that means, though. My head would probably explode violently but feel free to give a more detailed, technical, explaination someone )

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Suave

Light is made up of electromagnetic waves that oscillate quickly and move in packets to form particles, or that oscillate more slowly and act as waves.

/\_/\_/\_/\_/\_/\_/\_/

or

/\/\/\/ /\/\/\/ /\/\/\/

Or maybe I'm wrong. That's how I remember it from my grade 11 physics course back in the day though.

JustDisGuy

So anyone care to enlighten me on the physics of solar pressure (aka solar wind) based on the above explanations? If light has no mass, how can it propel something, even at a relatively insignifcant rate simply by reflecting from its surface?

I know I could Google for it, but it seems like a reasonable question to pose in this thread...

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GMontag

Even though a photon has no mass, it still has momentum and kinetic energy.

The equation for the total energy of an object is:

E^2 = p^2*c^2 + m^2*c^4

where p is the momentum, m is the mass, and c is the speed of light.

For massive particles with no velocity (and therefore no momentum), this reduces to the familiar E=mc^2. For massless particles, it reduces to E=pc, or rewritten, p=E/c. The energy of a photon is given by E=hw, where h is Planck's constant and w is the frequency of the photon.

ObieX

Solar winds are made up of more than light i think. Solar matter is ejected constantly with different forms of radiation, the combination of all the crap that flies out is solar wind. Or atleast was my understanding, tho im kinda talkin outa my ass :P

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fckm

Moment and Energy are the two of the most fundamental concepts in all of physics. For classical objects, the momentum of the object is equal to it's mass times it's velocity:

p=m*v

During the early 1900's, a bunch of physicists came up with some wacky ideas, now refered to as quantum mechanics. One of these guys named deBroglie had a wacky idea that the momentum of an object is inversly poportional to the object's "wavelength":

p=h_bar * k
h_bar = angular planck's constant
k=2*pi/lambda = wavevector

Thus, even though light doesn't have any mass, by deBroglie's theory, it still has momentum. (This has been verified experimentally).

For a classical particle, the Force acting on that particle has the effect of changing that particle's momentum.:

F=dP/dt , Force is the derivative of momentum with respect to time.

Thus, if you have a particle who's mass is constant with time, the force acting on the particle will change it's velocity:

F= m*dv/dt = m*a.

Since a photon carries momentum, any classical particle which absorbs the photon will have it's momentum changed by an amount equal to the photon momentum. If there's a stream of N photons, of wavevector K, in an amount of time T, then the momentum imparted per unit time is:

N*h_bar*K/T

then this acts as a Force on the classical particle, such that the particle is accelerated:

m*a=N*h_bar*K/T

The problem is a simple momentum conservation problem, as long as you realize that photons do indeed have momentum, despite not having any mass. Because of these two things, 1) photons have no mass but do have moment 2) momentum is conserved, physicists usually say that momentum is a fundamental quantity, like energy. There are ways of measuring or calculating momentum and energy, but no answer to the question "what _is_ momemtum/energy?".

Now, one of the issues with modern society is that everyone is so quick to point out all the "cool" predictions of quantum mechanics without giving a good explanation of the classical background to it all. When talking about light, a lot of people are quick to point out the particle nature of light, and completely forget about the fact that light is nothing more than changes in the electromagnetic field.

Looking at Maxwell's equations (the equations which describe the behavior of electric and magnetic fields), we can see two fundamental constants. Epsilon0 and Mu0. These two constants can be measured experimentally and are properties of the free space of the universe. (I'm sure there are more fundamental explanations, but I don't know them).
Maxwell's equations are a set of Differential Equations, and when combined in a certain way, form Wave Equations. Solutions to the wave equation are usually sines and cosines. That's why you usually see "waves" graphed as sinusiods. Since the wave equations govern the evolution of waves, they also govern how fast waves can propogate through a medium. The so call "speed of light" is determined by the two constants, epsilon0 and mu0 in the wave equation. The "speed of light in a vaccum" is simply calculated using the above constants. Since light is a wave in the electromagnetic field, it travels at the speed governed by the constants epsilon0 and mu0.

When light is traveling inside a different medium, such as water, the constants epsilon and mu are no longer equal to the free-space constants epsilon0 and mu0. Consequently, light traveling in water actually travels slower than light traveling in a vaccum. Thus, when people talk about The Speed Of Light, they're talking about the c (speed) as defined by the free-space epsilon0 and mu0 (free-space permiability and permiability).

Before Einstein came on the scene, people had already measured the two constants, and had determined The Speed Of Light. However, they didn't understand it's full importance, or it's fundamental nature until the theory of special relativity came out. The Speed Of Light isn't just the speed at which EM fields propogate, but coincidentally, it was also some sort of cosmic speed limit.

In almost all cases, if a person has never taken quantum mechanics, there is NO need to think of light as a particle. It is too confusing and not very illuminating. I suggest that in the future, think of light as ocsillations of the EM field, just like waves on a string.

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JJRousseau

And they are called "radiation" because..... ..... they radiate! See the term 'metonymy' But seriously, light can be defined in many ways. "Physical objects" is not one I'd ever use.

Maybe not to you, but to many of us dummies it conveys better what light really is. One can say it is photons or EM waves and one can say wind is molecules of air filling an area of low pressure. But both are essentailly "movement". And in my mind it conveys a meaning without the use of "jargon".

Yes...

And since most of us don't understand EM as well as you, let's just call it "movement" Well. ObieX seemed to like it .

fckm

Oh, here's a cool thing. although I can't draw it.

Newton's 3rd law is a law of reciprocity. I push on an object, the object pushes back. It's also in a sense a formulation of Mometum Conservation.

Going into EM theory, one can set up a thought experiment where you have two electrons traveling perpendicular to each other. They produce radial electric fields, which obey Newton's 3rd law. However, since they are moving charges, they also generate magnetic fields. The force on electron A due to the mag. field of electron B is _NOT_ equal and opposite to the force on electron B due to the mag. field of electron A. Newton's third law is violated. uh oh.
Since Newton's third Law is pretty much a verson of Momentum Conservation, this situation breaks Momentum Conservation. This is bad. So how do we resolve this? Well, some genius comes up with the idea that the missing momentum is in the Field! Thus, we now see where this idea that light has momemtum comes from. The field itself can carry energy and momentum. This was an odd concept at the time. the Idea of a field was pretty much a mathematical abstraction to allow for "action at a distance" and there wasn't any "physical" significance.

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JustDisGuy

I envy your understanding, and thank you for your responses.

Unfortunately, I cannot discourse on the subject on a par with the rest of you, so I'll resume lurker mode here. It is indeed a most enjoyable thread though.

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fckm

Light is not movement. Nothing is moving.
(this gets hard to describe)

The Electric Field (and the mag. field for that matter) are Force Fields. So, a force is a vector that points in a direction and has a magnitude. The magnitude of the force tells us how much the momentum is changing, and the direction tells us in what direction it's changing.

A field, is a mathematical concept, where every single point in space is assigned some value. For example, think of Temperature. Every single point in your room has some temperature. The temperature can vary. The temperature by your computer is probably larger than the temperature at your window. This is called a scalar field.

Now, let's think about wind. You go outside on a summer day, and it's windy out. But its not windy everywhere, some places are more windy than others. The wind is a vector field. A every point in space, the wind has a direction (the direction can be different depending on where you are) and a certain magnitude (the windyness can be different depending on where you are).

The E-field is a vector Force Field. By itself, the E-field doesn't do much. But, if you take an electron and hold it in your hand, and move it around in the E-field, you can feel a force on the electron. The force will vary in direction and magnitude depending on where that electron is. Now imagine that the E-field changes in time too. So if you hold your electron at a certain point in space, you will feel the force on that electron change in time. At one instant, the electron is pushed to the left. A second later, the electron is pushed to the right.

Now, let's take away the electron. Even though the electron is gone, the FIELD is still there. This is where our analogy to wind breaks down. Take away the air, and there's no more wind.

Light, is "waves" in the E-Field. Think of a wave on a string. Think of one point on that string. A one moment, that point is very high. A second later, that point is now in the rest postion of the string. In the next moment, the point is really low.

Transfer this over to the E-Field. Take our test charge and put it at a point. At one moment, the test charge feels a force to the left. As time progresses, the pull of that force decrease, untill it stops feeling a force at all. Then, there is a small force to the right, and the pull of that force increase with time, and starts decreasing again. This is an oscilation in the E-Field. Now imagine this happening at every point in space. Only, each point is slightly "out of synch" with it's neighbor. This is a wave in the E-field.

Now, lets take that electron away again. Even though the electron isn't there, the E-field still exists and that oscillation in the E-field still exists. This is light. Light is not a movement of anything, it is a change in the Electric Force Field. This change has certain characteristics in space and time.

How do we see/measure light? Well, our eyeballs are made of atoms, and atoms have charged particles. When that "wave" hits our eyeball, it gives atoms and electrons energy, which causes chemical reactions, which causes nerve impluses, etc.

How do we make light? Light is oscillations of the EM field. Where do EM fields come from? They come from charged particles (protons and electrons, etc). Each charged particle gives off a constant field. If we wiggle the particles, we can imagine that this wiggle propogates throughout space. This wiggle is the oscillation of the field. Thus, wiggling/accelerating charged particles creates light.

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fckm

Well, the whole point with special relativity is that if you have mass, you can't travel at the speed of light. The amount of energy it takes to make you go faster increases the faster you are traveling. Eventually at some point when v<c, it will take more energy than is contained in the universe to make you go any faster.

Similarly, if you are a particle with no mass (neutrino, for example), it must travel at The Speed Of Light. If the particle didn't travel at the speed of light, then according to relativity, it would have no momentum and no energy. Thus, it's not a particle anymore and can't be detected so we can ignore it.

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GMontag

Perhaps an easier explanation of the whole light's momentum thing to understand would be thus:

Light actually does have mass, it just doesn't have rest mass. Particles with 0 rest mass can still have a relativistic mass if, and only if, they are travelling at c. Having this relativistic mass allows it to have an impart (relativisitic) momentum upon the solar sails.

This explanation is somewhat discouraged though as the concept of relativistic mass has been obsoleted because it isn't an invariant property under frame changes.

MageB420666

Ok, here's what I don't understand.

According to the theory of relativity, the closer an object comes to travelling at the speed of light, the slower it moves through time, until at the speed of light time stops. So wouldn't light have an infinite velocity? velocity is distance\time and if time equals zero the velocity becomes infinite.

Since light obviously has a measurable velocity and does not travel instantaneously, I'm going to assume that the flow of time just becomes extremely close to zero at the speed of light, now this is where it gets a little tricky: Since we are not travelling at near light speeds, we observe time at close to what we assume is it's normal passage, now light(travelling at light speed) comes whizzing by us, So my question is, Is it really travelling as fast as it appears to be? We observe light as travelling at 186,282.4 miles per second, but that is a second as we observe it, what about time as it affects the photon? Wouldn't that second of observation to us be a couple of hours(just picking a random time frame) to the photon due to the relative slowing of time? Wouldn't the photon actually be travelling at 186,282.4 miles per couple of hours? Due to the relative time rates, it could travel for a couple of hours, but to our perspective have only travelled for one second. So the closer you come to the speed of light, less velocity you need because you gain more time to travel.

I hope that wasn't to messy, if it doesn't make any sense let me know and I'll try to rephrase it.

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stingc

You are mixing different definitions of time. Light travels a finite distance in finite time. distance/time is therefore well-defined, and will in fact always be the same for any (non-accelerating) observer looking at a beam of light (i.e. the speed of light is universal).

Time dilation refers to the fact that time intervals are measured differently by different people. So you were trying to mix measurements from two different frames of reference. If you stay consistently in one frame, then there are no problems. It is also worth noting that no particular frame is more special than the others. Somebody in a spaceship moving at 0.99c relative to the earth will not feel like he is moving slowly or particularly heavy or anything else. Instead, he'd think that everything on earth looked really strange. But everyone on earth would think the opposite is true .

ObieX

I know im going off on a slight tangent here, but I'm still not sure how objects cant keep accellerating. I know there's the "the faster you get to the speed of light the more massive you become" arguement, but since speed is relative this makes little sense to me. For example, say i was traveling at 99% the speed of light relative to a small rock floating in space. Now lets say here comes the earth speeding toward me, for simpliscity i'll just say that earth in this case is moving at 2% the speed of light. Now relative to the earth im traveling faster than the speed of light (99% + 2% = 101%). Another example, say im in a rocket, i accellerate to 50% the speed of light relative to a rock floating in space (lets assume that space in this instance is a vaccume except for the rock, my ship, and my fuel), and i turn off my rockets. Now everything in the space ship, fuel included, has "caught up" and we're all moving at the same speed. Now if i were to reignite my engines how would i not gain a new momentum and accelleration ability? Sure i can only go as fast as my thrusters can push out of the back of my ship, but relatively speaking i gained a new starting point for starting accelleration. Another example, we're all traveling around the sun, on earth, at lets say 30000mph (not the real # im just using it as an example) Relative to the eart we arent moving yet, we havent left the launch pad. Now lets say we blast off in the direction the earth is moving, increasing our speed relative to the sun. Lets assume our rocket can accellerate to 156283.4mph. This would put us 1mph over the speed of light relative to the sun, right? Why cant we use methods similar to this to travel faster than light? Say we build a space station that is constantly moving at 50% the speed of light, and use that as a launch pad to gain speed over the speed of light relative to earth. Is everything just limited to some relative speed to the very fabric of space-time? Does space-time have a speed? Can we measure and use this speed of space-time in some way to over-come this drawback? Like finding a way to move whol sections of space-time? (actually now that i think about this i think this is how warp drive in startrek is supposed to work, warping space-time around the ship to make a bubble of spacetime, the enterprise wouldnt actually be moving in "real" space-time they create their own little bubble of space-time that moves in the greater space-time.. i think, anyway)

WeEEeEe! My brain hurts!

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cybersharp

For a short answer i would think it would have to do with friction..

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GMontag

First of all, in SR you measure all velocities relative to you. You just can't say a space station is moving at 50% of the speed of light, it has to be moving that speed relative to something else. Second, and here's the non-intuitive part, you can't just add velocities like you are trying to do. Lets take your first example:

You have three different frames of reference in the example, the one where the rocket is stationary, the one where the rock is stationary and the one where the earth is stationary. All the speeds you gave were from the rock's frame of reference.

So, in the rock's frame of reference, the rocket is moving at .99c, and the earth is moving at .02c. We can then ask what the earth's velocity will be in the rocket's frame of reference. The rock's velocity will, obviously, be .99c, but the earth's velocity will not be 1.1c, because the formula for adding velocities is actually w = (u+v)/(1+(uv/c^2)). This comes out to approximately .9904c. So the rocket is travelling at 99.04% of the speed of light relative to the earth, not the 101% one would naively assume.

EDIT: added more digits to make it more clear that the relative velocity between the rocket and the earth is larger than between the rocket and the rock.

John Henry

Cybersharp: It's not to do with friction, because it would still happen in a vacuum.

ObieX: Shut up! You're not supposed to mention that. Now you've gone and made physics look dumb and I'm going to get the sack. I would hazard that nothing is able to move faster than c relative to anything else, so you just consider the object moving away from it fastest. Not sure if that's right, though.

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aKula

Gmontag is right. Anybody wanting to read more about the subject I would recommend this book. http://www.amazon.com/exec/obidos/tg...glance&s=books
It's really good at explaining the concepts behind relativity and then goes on to look at relativity itself and scenarios like you mentioned.
Also light is self propogating because a moving electric field creates a magnetic field and a moving magnetic field creates an electric field.

Stug

This song always helps me to remember the basics, whenever things seem hard or tough...

The Monty Python - Galaxy Song

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tiberry

I hope someone hasn't already asked or clarified this - but I can't seem to read thoroughly through all the posts without my head exploding...

Why would the speed of light (particle, wave, waves of particles, or whatever) or any other "thing" that has no mass be affected by the medium it travels through? If the "thing" has no mass to interact with the mass of the medium, what's slowing it down?

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