Zeraph

Something I never got was how this really works. If things are relative then how am I ever going a real speed at all? To the ground I may be moving 5mph but to a galaxy moving away from me I could somehow be moving faster than the speed of light. After all, if two bodies in space are both moving near the speed of light, away from each other... then relative to each other, how are they not moving faster than the speed of light?

Speed seems completely arbitrary under relativity.

JamesB

All that matters is the frame of reference - no need to concern yourself with how fast you are moving in relation to another galaxy as you describe.

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Martian

For the second part, you're missing a crucial part: speed of light is constant relative to the observer. This is where realtivistic effects come into play -- speed being an expression of distance over time, if the speed and distance stay the same the time has to be variable. In other words, a second isn't always a second -- your second may be more of a second than mine. If I'm going fast enough, I may experience hours while you do not, so that when we meet up again significantly more time has passed for me.

You seem to be hung up on the idea of an absolute frame of reference, when in reality there is no such thing. It's all relative, my man.

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I get through cryin' and I'm sadder than before I wept
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The_Jazz

Martian, you've got it backwards: if you're going suitably faster than Zeraph, he would experience hours where you would experience minutes or seconds, depending on the differential.

But otherwise, yep, that's it.

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DaniGirl

A real thought provoking thread. But its not something I really think about. I mean I do think about how small our galaxy is and how insignificant everything is compared to the big picture. I don't think there is faster then the speed of light.

Zeraph

Score one for Jazz...I'm shocked
Yep, if youre going near the speed of light you come back "younger" than the people you left behind.

OK let me rephrase my example.

Imagine three bodies in space stationary relative to each other. One person is on each body, call em A B and C. A and C take off in opposite directions from B. Both go near the speed of light. Looking from B using A as a reference point, how is C not moving faster than the speed of light?

shakran

Now I'll throw a monkey wrench into the whole mess.

The speed of light isn't constant. It can be sped up and slowed down. It can even be stopped.

Putting the Brakes on Light Speed - washingtonpost.com

there's one article that covers it.

The_Jazz

But C isn't moving at C (see what I did there?).

OK, smart-assness over, I don't think you've constructed your question quite right. A and C depart point B at the speed of light in opposite directions. From B, both A and C will always be seen as moving at the speed of light since B is stationary. B is moving at the speed of light as seen from both A and C. That's an important distinction because when A or C looks at each other in reference to themselves, even though they're moving at the speed of light, the other is still moving at the speed of light - which is the same as B. If light departed from one of them back towards the other, it would depart at the speed of light in that direction, which is not zero to compensate for the direction of travel in the opposite direction but the actual speed of light. If the light was shone from C towards A and A wasn't 100% efficient, eventually the light from C would eventually catch A.

At least as I understand it.

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"There ought to be limits to freedom." - George W. Bush
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Martian

Thanks for the catch, Jazz, you're indeed correct. Dashing off quick replies about complex subjects? Bad.

And yes, to your analysis. Sums up my understanding on the matter as well.

__________________
I wake up in the morning more tired than before I slept
I get through cryin' and I'm sadder than before I wept
I get through thinkin' now, and the thoughts have left my head
I get through speakin' and I can't remember, not a word that I said

- Ben Harper, Show Me A Little Shame

The_Jazz

Well, sort of kind of. Most of the techiniques that they're using simply trap the light and give the illusion that it's slowing down. It's still moving at the same speed but the medium causes it to appear to move slower. The Harvard experiments were something a little different, but at the end of the day the light basically just has a much more inefficient route to take than normal.

__________________
"They that can give up essential liberty to obtain a little temporary safety deserve neither liberty nor safety." - B. Franklin
"There ought to be limits to freedom." - George W. Bush
"We have met the enemy and he is us." - Pogo

n0nsensical

The speed of light is constant in a vacuum.
The speed of light is constant in all inertial reference frames.

Zeraph, in your example each of A B and C is an inertial reference frame. Time and space in one frame as viewed from any other frame is relative to the velocity of the other frame. Even if, relative to B, the sum of the velocities of A and C is greater than the speed of light, viewed from either of those frames, the velocities of the other frames WILL be less than the speed of light. This seems like a paradox but the way you can look at it is that time and space warp to prevent anything from moving faster than light, in special relativity this is known as time dilation (special relativity does not really get into the case of space warping but it can happen). Basically time slows down for the moving observer relative to the observed frame which causes the relative velocity to be less than the speed of light in any case.

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Zeraph

Good stuff. Ok, let me re-rephrase my question. something is moving at the speed of light. I'm riding a photon that is shot in the opposite direction of the obj moving at the speed of light. From my perspective on the photon moving "backwards" at the speed of light, wouldn't the object I was shot from seem to be moving faster than the speed of light? If we can only move away from each other at the speed of light then kinetic energy must disappear somewhere in there since it doesn't cancel momentum when I'm shot out backwards...which obviously isn't right so I must not be getting something.

nice answer n0nsensical...ok so time is taking the hit in energy I guess...but then, well, why? why time and not speed like it would normally work at our level...insanity, i dont know how he ever figured this stuff out.

Daniel_

Right.

Thought experiment. You are in a vacuum. You are hanging in an empty part of space.

Designate the long axis of your body as UP. Designate the direction you are facing as NORTH.

Shine a YELLOW laser EAST, and another WEST.

From your POV, there are two yellow photons going away from you at C

Therefore from your perspective, the photons relative speed seems to be 2C.

If you were sitting on the yellow photon, looking back past the person shining the light what would you see?

You would see not a yellow photon going away at 2C, you would see a RED photon going away at 1C.

Look up Redshift, and it will help explain.

__________________

╔═════════════════════════════════════════╗
Overhead, the Albatross hangs motionless upon the air,
And deep beneath the rolling waves,
In labyrinths of Coral Caves,
The Echo of a distant time
Comes willowing across the sand;
And everthing is Green and Submarine
╚═════════════════════════════════════════╝

shakran

Unless I grossly misunderstand it, which is indeed possible, they actually slowed down the light itself. It took a straight path, but at a slower speed. And the wavelength compressed when it went into the "brake section" for want of a better term.

The_Jazz

I'm not a physicist, but when I read this:

it implies - to me - that the light is still moving at the same but through a medium that inhibits its progress. So the speed of light is still constant but these particular photons are spending more time moving throught the cesium. They haven't changed the speed of light, just impeded these particular light particles in a convenient way.

Not trying to split hairs or anything, but when you say

it's not entirely true, only for the photons involved in this experiment. C (the speed of light) can never exceed 300,000 km/second. You cannot vary C in any way known to man since the photons in these experiments instananeously "accelerate" back to C when they exit the medium - that's because they were never truly slowed down, just taking a much longer route than normal but still at C.

Make sense?

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"They that can give up essential liberty to obtain a little temporary safety deserve neither liberty nor safety." - B. Franklin
"There ought to be limits to freedom." - George W. Bush
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shakran

No, but then I'm a physics layman. If something's moving in a straight line at speed X, and it goes through a medium still in a straight line, and the time it took to traverse that distance indicates that it had to be moving at X-10, and on the other side of that medium it's back to speed X, I don't see how it didn't slow down. I've read other articles on this, that didn't indicate the medium caused the light to take a circuitous path through the braking medium, so I've been assuming that the light travels a straight line through it. Is that where I'm hung up?

The_Jazz

I think that's the issue. Again, I'm no physicist, but as I understand it, the light is still moving at it's constant speed but the medium is basically making it move in a more circuitous route. So instead of exiting the medium at the shortest point, it's moving in an arc, or - in this case I think - a series of arcs. Maybe I don't understand the process well enough, but unless they're going to change the nature of a photon, it can't decelerate and then instantly accelerate once it reaches the exit. The fact that they're talking about it still traveling at C once it reaches the other end makes me think that it never slowed down, just took a very long route to get to the end.

Then again, I could be the one who has it wrong. At the end of the day, I suspect we'd need someone intimately familiar with the research to give us the right answer.

__________________
"They that can give up essential liberty to obtain a little temporary safety deserve neither liberty nor safety." - B. Franklin
"There ought to be limits to freedom." - George W. Bush
"We have met the enemy and he is us." - Pogo

n0nsensical

You're hitting on another aspect of relativity, which is relative mass and energy. Indeed, mass and kinetic energy are also relative and subject to the same rules. You've heard the equation E=mc^2. This is the rest energy of matter. The total energy is the rest energy plus the relativistic kinetic energy. This energy is different depending on the velocity of the two frames of reference. As the velocity approaches the speed of light, the relativistic mass/energy approaches infinity. Another way to say that nothing with mass can move at the speed of light is that it would need to have infinite energy.

If you're mathematically inclined, look into the Lorentz Transformation for the mathematical explanation of special relativity. Some people can visualize equations better than words. From that and other equations, such as Maxwell's equations, and a whole lot of experimental data, is how physicists figured this out. Some of the consequences of relativity were observed before anyone had any idea what was going on. Other parts were beyond the accuracy of instruments of the time. (It's pretty hard to measure time dilation without an atomic clock) Most prominently Einstein, but he stood on the shoulders of other giants. Other physicists really made the important discoveries separately, Einstein just put all the pieces together into a few neat papers. Then he moved on to General Relativity which is a whole other can of worms that even fewer people understand.



Light really does slow down when it travels through matter, and it travels in a straight line (actually the precise motion of individual photons is a statistical distribution, but the whole distribution travels straight on average), but there isn't much of a why about it, it just does. I'm not sure it can be explained in macrophysical terms. It can be modelled in different ways by some complex quantum mechanics that I wouldn't have the slightest clue about. The simplest way I could explain it is that different media are differently susceptible to the application of electric and magnetic fields, and light is a moving electromagnetic field. (Electricity and magnetism are equivalent through relativity) It's similar to the way sound travels faster in water, because the speed of sound waves is dependent on the density of the medium. It's similar for EM waves, but somewhat more abstract, because they don't necessarily need a medium to travel at all whereas sound waves do.

So the speed depends on the properties of the medium in which it travels, sometimes expressed as electric permittivity and magnetic permeability. (They are really just one number but if you're not into tensors and things it's usually more convenient to express them separately) In a vacuum, these are known as the permittivity and permeability of free space and are simply some magical universal constants the square root of the inverse product of which is equal to c. The permittivity and permeability are different in various media, and even within a single medium can vary with the field strength and frequency.

The reason that visible light does not travel through usual matter is that the permittivity is approximately 0 at the appropriate frequencies; the energy is either reflected or absorbed. The permittivity of air is very close to that of free space and I think the permeability is equal. (There are some wave equations that can tell you what will happen to light at the boundary of media but I didn't do too well in that part of electromagnetics and ended up with a C-. Hehehe. I don't necessarily know what I'm talking about. =P) But glass has an unusually high permittivity. In (some?) nonmagnetic media, the permeability is exactly equal to the permeability of freespace and thus the speed of light is only dependent on the permittivity.

__________________
"Prohibition will work great injury to the cause of temperance. It is a species of intemperance within itself, for it goes beyond the bounds of reason in that it attempts to control a man's appetite by legislation, and makes a crime out of things that are not crimes. A Prohibition law strikes a blow at the very principles upon which our government was founded." --Abraham Lincoln

Daniel_

Think of someone walking through a crowd.

Before you get to the crowd, you take a 1m step each second, meaning you travel at 1m/s.

When you hit the crowd, you take a 1m step each second, but instead of straight line, you now have to jink and bob through the people, so your speed is still 1m/s, but your progress through the crowd has dropped to >1m/s.

On the micro scale, your SPEED is unchanged, but on a macro scale your VELOCITY has dropped.

Latices that trap photons are like this - the photons move at C, but because they bounce back and forth past atoms in the lattice, their path length is massively increased, and so they take longer to get through the lattice than L/C, where L is the external size of the lattice.

__________________

╔═════════════════════════════════════════╗
Overhead, the Albatross hangs motionless upon the air,
And deep beneath the rolling waves,
In labyrinths of Coral Caves,
The Echo of a distant time
Comes willowing across the sand;
And everthing is Green and Submarine
╚═════════════════════════════════════════╝

n0nsensical

Ahh, is this a property of all matter or only certain types? If the latter, what is different about it to cause this kind of behavior? Is this the entire reason for varying permittivity and permeability of different media? Is there any sort of delay associated with bouncing off an atom or is it simply because of the additional distance? This is where my knowledge starts to get really hazy, I am horrible at microelectronics and microphysics, why I ended up as a computer scientist than a EE, EE is practically turning into material physics and quantum mechanics these days as circuits shrink and shrink.

__________________
"Prohibition will work great injury to the cause of temperance. It is a species of intemperance within itself, for it goes beyond the bounds of reason in that it attempts to control a man's appetite by legislation, and makes a crime out of things that are not crimes. A Prohibition law strikes a blow at the very principles upon which our government was founded." --Abraham Lincoln

Daniel_

Don't know, I'm a chemist.

__________________

╔═════════════════════════════════════════╗
Overhead, the Albatross hangs motionless upon the air,
And deep beneath the rolling waves,
In labyrinths of Coral Caves,
The Echo of a distant time
Comes willowing across the sand;
And everthing is Green and Submarine
╚═════════════════════════════════════════╝

Zeraph

hah, awesome thread.

SSJTWIZTA

read it, thank me later.

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Then they came for the communists and I did not speak out because I was not a communist.
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Then they came for me And there was no one left to speak out for me.
-Pastor Martin Niemoller

TUDelftstudent

Isn't the speed of light in relation to the direct surroundings?

Soshi Aroso

First, picture a two coordinate grid. (The kind you used to plot equations in school). A length of exactly one whatever, call it a foot, lying horizontal takes up one foot horizontal and no space vertical. The same length vertical takes up on vertical foot, but no horizontal space.

Take that length, and lay it at an angle. Now, it takes up space both vertically and horizontally. You are familiar with perception in two dimensions, and so you can easily see this. If the length tilts more towards the horizontal, then it takes up less vertical space and more horizontal space. However, at all times the length is exactly the same.

Now, imagine that acceleration can be plotted onto a coordinate grid instead of a vertical spatial measure. After all, time is a measurable dimension, and acceleration and speed are simply functions of time compared to a physical distance. So call this new coordinate (y*t). You are unfamiliar with perception into this new dimension, and so your brain attempts to explain them in terms of your familiar three coordinate system. However, when it does this, things don't sync up perfectly.

A length at rest (that is, horizontal) appears exactly the same in this new grid as it did in the old grid. When you start applying acceleration to that length (tilting it up), It starts to take up less space on the x coordinate and more on the (y*t) coordinate. Your brain will continue to think of this coordinate in spatial terms, and so will be unable to intuit why is doesn't act the same as plain old y.

This mental grid helps to explain temporal dilation as well. An object exactly at rest (perfectly horizontal) would be experiencing no time. This is effectively impossible (or performed only by things like photons; take your pick). Objects subject to a greater acceleration experience more personal time than those subjected to less acceleration, or, alternatively, experience less distance.

This isn't an exactly correct way of describing relativity, but perhaps it's a helpful mental exercise nonetheless? Hopefully at some point your brain will just click, and then you'll understand it fine.

Tesla

And what about the mass affecting space-time fiber? If an object with mass accelerates, it's mass increases accordingly. And when it approaches near the speed of light, it gains almost infinite mass, that means no matter what force is applied at the object, it can't gain the speed of light, because of the mass increment. And I have heard about an interesting theory about the Higgs boson, the particle "accused" for giving mass to things. It's more like ether(oh yeah, ether enters the physics once again) that prevents objects with that burden from moving without force applied(like photons).

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MSD

Let v be your velocity and w be the object you were fired from. let u be the observed velocities by a neutral observer. Let c be the speed of light in a vacuum.

U=[(v+w)/(1+vw/c^2)]

To convert your frame of reference to their frame of reference, you need to apply the Lorentz transformation. You're thinking in relativistic velocities in terms of standard mechanical velocities.