Need some clarifications about light speed

[ram2048]

the other thread got the lockdown before i got a chance to gather all i could from it <sad>

so i guess i have to start a new one.

when people say "The speed of light is constant in a vacuum at C or 299,060 kps" (or whatever the freaking number is) what exactly do they mean?

from the last post we've determined that all things are relative and all realities are valid, so when they did this test for light speed what was it relative to? seems kind of unsafe to say that it's constant everywhere there's vaccuum. maybe it's just in our solar system or our galaxy that light behaves this way.

or even more so, light speed had to have been measured relative to some "stationary" position which isn't allowed because who's to say that position is actually stationary?

so it comes down to that light speed is constant in a vacuum RELATIVE to SOMETHING that is stationary.

and BECAUSE it is constant then stationary frame(s) MUST exist. these two things are mutually co-dependant.

to make it simpler. the speed of light can only be constant relative to something stationary. it CANNOT be constant relative to something that can move or does move.

so here comes a kicker. if we devise a space instrument that consists of 4 strobes on poles, some receptors in the center, and some thrusters, oh you and some computing power ;D. set this thing adrift in space with the right program to measure time from strobes to receptors, and use the thrusters to compensate for the deiscrepancies from all 4 strobes, then we can come up with an object that sit ABSOLUTELY STILL in space.

relative to the whole freaking universe :D (if our assumptions are true)

 

[jdavel]

In #1, ram2048 said, "...the speed of light can only be constant relative to something stationary. it CANNOT be constant relative to something that can move or does move."

Yes it can, and it is. The speed of light in a vacuum is constant relative to whoever measures it. That constant speed is c. It doesn't matter whether you're moving or what you're moving relative to (including the source of the light!). It doesn't matter. You'll always get c when you measure the speed of light.

That seems counterintuitive. But we aren't born with intuition; it comes purely from our experiences in the world. So intuition can fail if we try to apply it to situations with which we have no experience. Most people have no experience with speeds anywhere close to that of light. So we have no valid intuition about what things are like at that kind of speed.

A good analogy is how counterintuitive it must have seemed to people when they were first told the world was round. All their experiences were with travel over very short distances. And over short distances the world seemed flat. Why should that change for long distances? And if they kept going far enough around, wouldn't they eventually be upside down and maybe even fall off? Surely this round theory had to be wrong; it was so counterintuitive!

Lesson? Intuition is useful for getting us through our day to day lives. But when you set out to understand basic truths about the physical universe, it's best to leave your intuition behind, and base your conclusions on what has been objectively measured. Only a very few, very brilliant physicists seem able to use intuition without running amock!

 

[Integral]

There have been many threads covering this.

The constancy of c was first derived by Clerk Maxwell in about 1867 when he formulated the fundamental relationships of Electromagnetism (Now called Maxwell's Equations) in the form of the wave equation. In this equation the speed of the wave appears in a characteristic manner. The quantity which represented the speed of a electromagnetic wave (which had not been experimental observed at that time) was the quantity

$${\sqrt \frac 1 {\epsilon_0 \mu_0}}$$
Where the Permittivity of free space is
$$\epsilon_0 = 8.85 x 10^{-12} \frac F m$$

and the Permeability of free space is
$$\mu_0 = 1.26 x 10^{-6} \frac H m$$

If you work out the basic units and do the math you will find c. Maxwell observed this agreement with the experimental value of the speed of light with some amazement. To the best of my knowledge this was the first real evidence that light was electromagnetic in nature. The implications of this result are startling, simply because the speed of light is NOT DEPENDENT on the motion of the observer but on fundamental and well known constants of the universe. When this result was published it initiated a half century of turmoil in Physics community which finally ended in Einstein's publication of Special Relativity.

 

[jdavel]

Integral,

Well, I know we've had this argument before, but it wasn't Maxwell's discovery that caused the turmoil. Maxwell believed there was an ether, and he believed that the speed in his wave equation was wrt the ether. That was perfectly consistent with all other wave motion. It wasn't until the MM experiment failed to detect the ether, that all the turmoil started. A wave without a medium? Impossible!

In fact during the time between Maxwell's discovery and the MM experiment, many physicists believed that their work was pretty much complete, that there was no more physics to discover. You can check it out.

 

[Integral]

One would wonder then why they bothered with the MM experiment? The reason it was done was to COUNTER Maxwells result, thus it is known as the experiment that failed.

 

[NateTG]

One would wonder then why they bothered with the MM experiment? The reason it was done was to COUNTER Maxwells result, thus it is known as the experiment that failed.

Well, even if there were an ether, determining the 'ether wind' would be an interesting physical experiment, much like Cavendish weighing the earth.

 

[ram2048]

Yes it can, and it is. The speed of light in a vacuum is constant relative to whoever measures it. That constant speed is c. It doesn't matter whether you're moving or what you're moving relative to (including the source of the light!). It doesn't matter. You'll always get c when you measure the speed of light.

i think you must have it wrong...

if this were the case then shooting a photon at something 10 light seconds away, then running after it as 1/2 the speed if light, the photon would be moving at 1.5 c in order to maintain speed of C relative to me.

it can't be constant relative to something moving. i think you must be measuring it wrong :|

 

[pallidin]

i think you must have it wrong...

if this were the case then shooting a photon at something 10 light seconds away, then running after it as 1/2 the speed if light, the photon would be moving at 1.5 c in order to maintain speed of C relative to me.

it can't be constant relative to something moving. i think you must be measuring it wrong :|

I agree. How on God's green Earth can something alter it's relationship dependent on frame of reference and yet maintain a different relationship with another frame at the same time?
Something is seriously wrong here.

 

[jcsd]

i think you must have it wrong...

if this were the case then shooting a photon at something 10 light seconds away, then running after it as 1/2 the speed if light, the photon would be moving at 1.5 c in order to maintain speed of C relative to me.

it can't be constant relative to something moving. i think you must be measuring it wrong :|

Well let's have a look at what special relativity has to say then:


We know that length contraction and time dialtion occur governed by the equations:

x' = γ(x - vt)

t' = γ(t - vx/c²)

therefore we can say:

Δx'/Δt' = (Δx - vΔt)/(Δt - vΔx/c²)

So let's put in your figures which are v = c/2 and Δx/Δt = c (as the photons speed is constant we can say v_photon = Δx/Δt), using the substituion &Delta;x = c&Delta;t.

Δx'/Δt' = (c&Delta;t - c&Delta;t/2)/(&Delta;t - c²&Delta;t/2c²) = (c&Delta;t/2)/(&Delta;t/2) = c

Therfore the velocity of the phton in the primed frame is also c due to the effects of time dialtion and length contraction (of course should've expected this as time dialtion and length contraction are derived from the constancy of c in all inertial refrence frames).

 

[jdavel]

ram said, "it can't be constant relative to something moving. i think you must be measuring it wrong :|"

After all the nonsense you've posted in the last few days, my intuition told me not to respond to your post. This time I should have followed my intuition.

Jerk somebody else's chain; I've got plenty of time, but not enough for you. You'll never understand any physics.

 

[ram2048]

o.. k...

time dilation has nothing to do with the rate of covering distance.

velocity means it will cover X distance in X time. there is no variable for perception.

the photon is GOING to travel 299,792,458 meters in one second and 2,997,924,580 meters in 10 seconds.

this is a what's commonly referred to as being constant

now if you say this is RELATIVE to ANYTHING, moving or non, it means that going 20 miles an hour or 299,792,457 meters per second, that photon will still be ACTUALLY moving 299,792,458 MORE than THAT speed.

which makes it NOT constant.

 

[jdavel]

Integral said, "One would wonder then why they bothered with the MM experiment? The reason it was done was to COUNTER Maxwells result, thus it is known as the experiment that failed."

Sorry, but you've got this totally wrong. There's a very famous story about Michelson getting his idea for measuring the speed of the Earth through the ether after seeing a letter written by Maxwell suggesting such a measurement. Google maxwell and michelson; I'm sure you'll find it.

 

[jcsd]

o.. k...

time dilation has nothing to do with the rate of covering distance.

velocity means it will cover X distance in X time. there is no variable for perception.

the photon is GOING to travel 299,792,458 meters in one second and 2,997,924,580 meters in 10 seconds.

this is a what's commonly referred to as being constant

now if you say this is RELATIVE to ANYTHING, moving or non, it means that going 20 miles an hour or 299,792,457 meters per second, that photon will still be ACTUALLY moving 299,792,458 MORE than THAT speed.

which makes it NOT constant.

velocity does NOT mean "it will cover X distance in X time", but by making the simplifying assumptions of constant velocities along a single axis we are looking at the case where it does mean an object will cover "X distance in X time".

The problem is you are using the assumptions of Galliean relativity which we now know to be the limiting case only.

If you disagree with the results, go through the equations and their initial assumptions and pick the steps that you disagree with.

 

[jcsd]

Even in Galliean relativity (which is what your argument is based on) you can see that the measured velcoties are dependent on the motion of the observers.

 

[Integral]

Sorry, but you've got this totally wrong. There's a very famous story about Michelson getting his idea for measuring the speed of the Earth through the ether after seeing a letter written by Maxwell suggesting such a measurement. Google maxwell and michelson; I'm sure you'll find it.

Gee! I wonder why MAXWELL would have ever sugested such an experiment. Seems to me you cannot see the forest for the trees.

Please show me where an ether enters into

$$\sqrt {\frac 1 {\epsilon_0 \mu_0}$$

http://www.worldretailstore.com/item/BE-0967694418.html

 

[ram2048]

velocity does NOT mean "it will cover X distance in X time", but by making the simplifying assumptions of constant velocities along a single axis we are looking at the case where it does mean an object will cover "X distance in X time".

The problem is you are using the assumptions of Galliean relativity which we now know to be the limiting case only.

i know of no other method of describing velocity other than direction + distance over time. Since the direction is irrelevant because we have stipulated movement in one axis it can be ignored.

Even in Galliean relativity (which is what your argument is based on) you can see that the measured velcoties are dependent on the motion of the observers.

saying "the speed of light will not cover 299,792,458 meters in one second for a moving observer" is the same thing as saying it's NOT constant relative to the moving observer.

which is what I've been trying to verify. the equations that Integral brought up base the speed of light upon "Permittivity" and "Permeability", two scales for quantifying space (i guess) but has anyone actually verified it using REAL TOOLS?

We base SO much upon this constant light speed yet it seems to lack quite a bit of substansive proof

 

[ram2048]

doing some research I'm seeing that michelson's experiments in 1879 seem to find light speed at 186,350 miles per second with a likely error of around 30 miles per second

so what happens when we take his experiment into space and give it an inertial reference speed (from earth) of 1/2 c?

pointing the main vector of the light beam at the direction we're travelling, the splitter will shoot the beam off in two directions at right angles to one another.

relative to the Earth the light beam reflected "to the right" must travel with a "forward" motion at 1/2 C to keep up with the reference frame of the experiment.

but relative to the earth, the light is actually traveling at an angle with a total velocity greater than the acceptable constant value of the speed of light

 

[ram2048]

this is a damned good page

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

 

[jdavel]

Please show me where an ether enters into
$$\sqrt {\frac 1 {\epsilon_0 \mu_0}$$

That's easy. It's the velocity of a wave. And Maxwell believed it was the velocity of the wave with respect to the ether. Just like the velocity of all other waves is the speed at which they propagate through their mediums. The only issue anyone had with this was that the only other fundamental field that was known at the time (gravity) seemed to be independent of velocity. EM fields evidently weren't. But if there was really an ether, that made perfect sense. Then MM showed there was no ether, and they had to find something else for c to be relative to. Einstein postulated that c was the speed of light relative to whoever observed the light.

 

[ram2048]

IF lightspeed is in any way constant relative to something, it's going to conform to emitter theory (my opinion)

basically wherever light is dropped into the universe or created from exists as a TRUE fixed position, universally so. Light's speed is constant and relative to that absolute position.

what the universe does has an effect on how we measure it from there, be it bending space with gravity, or slowing the propagation medium with permeable mass (air water etc)

think of it as a photon being a rotating wheel with a certain velocity equal to the rotation speed and size of the tire. this wheel has infinite traction with the universe when in contact with it. when introduced into the universe it immediately takes off at full speed depending on the medium it's in.

well that's my take on it according to everything I've read about it so far. we'll see how that develops :D

 

[Integral]

I see no reference or need of ether in that expression. Many physicist of the day recognized that fact also. Indeed the idea of the ether died hard. The final end was the MM EXPERIMENTAL VERIFICATION of Maxwell's theoretical prediction. As usual, and as should be, it is experimental verification which finally gives credence to theory. But the theory was in place and understood, thou most were more convinced of an error by Maxwell. They were waiting for him to uncover the error. Thus the issue was know as Maxwell's Conundrum. They felt is was his problem, he should fix. Experimental verification was probably the last thing they expected.

 

[jdavel]

Integral said, "I see no reference or need of ether in that expression."

Of course there's no "need of ether". How could there be? There IS no ether. You and I know that, but Maxwell didn't. He believed in the ether! Do you think he didn't? Can you show me a reference (that I don't have to spend money on) that says he didn't believe in the ether? There are dozens of web sites where his belief in the ether is alluded to. You can google them at "maxwell ether" or "maxwell michelson" just to name a couple.

If anyone else wants to jump in and straighten whichever of us is wrong here, feel free. Otherwise, I guess we'll just have to disagree on this one. Maybe I'm not expaining what I mean very clearly, but I've done the best I can.

 

[jcsd]

i know of no other method of describing velocity other than direction + distance over time. Since the direction is irrelevant because we have stipulated movement in one axis it can be ignored.



saying "the speed of light will not cover 299,792,458 meters in one second for a moving observer" is the same thing as saying it's NOT constant relative to the moving observer.

which is what I've been trying to verify. the equations that Integral brought up base the speed of light upon "Permittivity" and "Permeability", two scales for quantifying space (i guess) but has anyone actually verified it using REAL TOOLS?

We base SO much upon this constant light speed yet it seems to lack quite a bit of substansive proof

Velocity is dx/dt where x is dispalcemnt and t is time. You should be able to see that if v is not constant in general velocity will NOT equal displacement/distance. In special relativity all inertial refrence frames are equal.

It's a postulate of special relitvity that the speed of light is the same in all inretial reference frames this comes from experimentation. tTime dialtion and length contraction allow this to be true.

 

[Alkatran]

Light doesn't go faster or slower depending on how you're moving. YOU move faster or slower (through time) depending on how you're moving. Now this doesn't work exactly the way I just described, and I think it be be best summed up by this site here: (which has pictures which give you a good idea what einstein was getting at) http://casa.colorado.edu/~ajsh/sr/paradox.html

 

[russ_watters]

doing some research I'm seeing that michelson's experiments in 1879 seem to find light speed at 186,350 miles per second with a likely error of around 30 miles per second

so what happens when we take his experiment into space and give it an inertial reference speed (from earth) of 1/2 c

It works exactly the same.

but relative to the earth, the light is actually traveling at an angle with a total velocity greater than the acceptable constant value of the speed of light

Nope. Any observer who is in a position to meausure the speed of a photon of light will always measure it at C. Why? While different observers will always agree on the speed of a photon, they will not necessarily agree on how far or for how long it has traveled.

Lets say I'm here on Earth and I fire a laser at Mars. You go chasing it in a spaceship at .5C (according to me). I would measure the speed of the beam front at C and you should (in Galelean relativity) measure it at .5C. But you won't. When you get to Mars, you and I compare notes about the trip and we'll find that we don't agree on how far you traveled or how long it took to get there. But the time dilation and length contraction both work out in such a way (from the Lorentz transformations) that we will still agree that the beam front of the laser was traveling at C.

From your first post:

so it comes down to that light speed is constant in a vacuum RELATIVE to SOMETHING that is stationary.

and BECAUSE it is constant then stationary frame(s) MUST exist. these two things are mutually co-dependant.

to make it simpler. the speed of light can only be constant relative to something stationary. it CANNOT be constant relative to something that can move or does move.

Well that's just it: "stationary" and "moving" is a matter of opinion. Is a train moving past the station or is the station moving past the train? Doesn't matter. Yeah, it makes more sense to consider the train to be moving, but what if you are out in space? Are you moving toward Mars or is Mars moving toward you? Mars orbits the sun, so depending on how you go, you might be letting Mars come to you (though you are still orbiting the sun). The point is, regardless of how you choose to define your position and velocity, if you measure the speed of light from it, it'll always be C.

the other thread got the lockdown before i got a chance to gather all i could from it <sad>

so i guess i have to start a new one.

Trust me, its better this way: You're asking good questions and there was too much trash in those other threads for you to get good answers.

 

[ram2048]

it's starting to come together, but as mentioned before it's very counter intuitive so it's tough to get a grasp on.

i'm still convinced there's something wrong with it, but now at least i see how it works (and how genius it is, in respect to it being the ultimate trump card "if everything is relative, then nothing can be disproven")

i appreciate the help of everyone here (and in the other thread) in getting me started :D

 

[quantumdude]

i'm still convinced there's something wrong with it,

You have only to look at the experimental evidence to be convinced. When the speed of light is measured in particle accelerators from sources that are moving at over 0.99c, the speed of the emitted light is still measured to be c.

the ultimate trump card "if everything is relative, then nothing can be disproven")

That is not the case at all. First, SR doesn't say "everthing is relative". All its predictions are derived from two things that are not relative, namely the laws of physics and the speed of light. And SR certainly does make some predictions that, if shown wrong, could disprove the theory.

 

[geistkiesel]

You have only to look at the experimental evidence to be convinced. When the speed of light is measured in particle accelerators from sources that are moving at over 0.99c, the speed of the emitted light is still measured to be c.

descrbe the experimental arrangement where the speed of light is measured with respect to the particle moving over .99c.

 

[quantumdude]

descrbe the experimental arrangement where the speed of light is measured with respect to the particle moving over .99c.

Since I don't believe you are really interested in learning, I am going to give you the reference so you can look it up yourself. That way, if you're just screwing around again, my time won't be wasted.

"Velocity Of Gamma Rays from a Moving Source," Physical Review 135, (1964), pp. B1071-75; T. Alvager, F.J.M. Farley, J. Kjeliman

 

[Hurkyl]

SR really began to make sense to me when I understood space-time diagrams; maybe a couple of them will help you too.


Suppose we consider a simple situation; a photon traveling through space. Let's make a plot of distance by time:

|       /
|      /
|     /
|    /
|   /
|  /
| /
|/
*--------

The vertical axis is time (the units are seconds) and the horizontal axis is distance (the units are light-seconds). At any time t, the photon is at x position x = ct, so the plot of the photon's position vs time looks like the drawing above.

Let's first analyze the situation classically. We want to consider how things look in another frame, so let's draw the new frame's coordinate axes on this diagram:

    |   /
   |   /
   |  /
  |  /
  | /
 | /
 |/
|/
*--------

Again, the nearly vertical is time and the horizontal axis is distance. I'm not sure if you've worked with oblique coordinate axes before, so I'll briefly describe how to read measurements. To determine the time an event occurs, you draw a line parallel to the distance-axis, and it will meet the time axis at the correct time. Similarly, to determine the position where an event occurs, you draw a line parallel to the time-axis and it meets the position axis at the correct place. For example:

    |   /
   *---X
   |  /|
  |  /|
  | / |
 | / |
 |/  |
|/  |
*--*-----

So, in this example, the event marked by X occurs at the x-coordinate of 3 light-seconds, and at the t-coordinate 7 seconds. We divide, and we see that in this frame that the photon is not traveling at the speed c.

If we like, we can make a transformation of the plane so that this moving coordinate frame is made perpendicular; in other words, we convert to the moving frame. The picture would look like:

|   /
|   /
|  /
|  /
| /
| /
|/
|/
*--------

where the photon is clearly moving at the "wrong" speed.


Now, let's do the same thing in Special Relativity. The main difference is the position axis changes as well as the time axis:

    |   /
   |   /
   |  /
  |  /
  | /   -
 | /  --
 |/ --
|/--
*-

(The scale on the axes change also, but that is somewhat more difficult to describe; the change in scale is the same for both of the axes, though)

Now, if we pick an event to measure in these new coordinates:

    |   /
   |  -*
   |--/|
  *- /|
  | / | -
 | / |--
 |/ -*
|/--
*-

In this case, we see that the event was recorded at the 5 unit position along the distance axis, and the 5 unit position along the time axis. So, the velocity of the photon is still measured to be c!

And, if we convert everything to this new frame, we get:

|       /
|      /
|     /
|    /
|   /
|  /
| /
|/
*--------

!

Going back to the diagram with oblique coordinates,

    |   /
   |   /
   |  /
  |  /
  | /   -
 | /  --
 |/ --
|/--
*-

The idea is that this slanted horizontal line is a "line of simultaneity"; just like the slanted vertical line traces out where the postion measurement is zero, the slanted horizontal line traces out where the time measurement is zero.


I've found these diagrams very helpful in showing, graphically, how the two frames relate in a way that both will measure the speed of light to be c.