Need a physicist to help with this one...

SkyHog

SkyHog

Touchdown! Greaser!
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Everything Offends Me
My brain is hurting trying to understand this one. I'm sure there's an explanation in the Theory of Relativity that I am missing, as this is mostly a Relativity issue.

Assumptions:
1. There are 4 viewpoints: A, B, C, D
2. Light always travels at the speed of light
3. Nothing travels faster than the speed of light within a system

Note - if any of the assumptions above are incorrect, then the entire premise is incorrect, so I need to make sure I understand that part first.

So here's the setup. We have a giant onlong cement box on a flat bed truck (or any other movable surface). Inside the cement box, we have a directional light source. This light source is positioned at viewpoint A, on one end of the box, and it is pointed to the other side of the box at the furthest distance (viewpoint B). It is safe to say that the light travels from viewpoint A to viewpoint B at the constant speed of light.

Outside the box, there is a sensor that is tripped when the light is sent from viewpoint A, which displays on viewpoint C. There is another sensor that is tripped when the light is received at viewpoint B, which displays on viewpoint D. Essentially, this serves as a form of measurement of the speed of light. The delay caused by the transmission from A-C and B-D is irrelevant because it is constant between the two (i.e., it always takes 5ms to get from A-C and 5ms to get from B-D).

So - that said, the box is on a stationary truck, light leaves A and hits B, and is measured outside the box as "the speed of light."

Now, the truck starts moving at a very high speed, and the light is measured again.

My hypothesis: it measures at the speed of light, still, because to viewpoint A, and to viewpoint B, the light is traveling at the speed of light. However, because the truck is moving, from outside the box, the light has traveled a further distance than it had originally, and therefore, the light traveled at a speed faster than the speed of light, right?

Its a matter of relativity, that within the box, the light traveled the same distance in the same time, but outside the box, it traveled a longer distance in the same time....

What am I missing?

Edit: A diagram to follow:

Code: 
    C                D
*-----------------------*
*|  A                B |*
*-----------------------*
       O            O
 
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Light in a medium travels slower than light in a vacuum.

I don't think assumption 3 is correct. Light propagated from a moving source will travel faster than light propagated from a stationary source, relative to the two source points.
 
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Time slows down for the flatbed truck.
Light travels at the speed of light, period. As you accelerate to near the speed of light, the light must still travel at the speed of light. Therefore, to make everything work out, the time for the people on the fast moving truck slows down. In other words, they still believe that they are moving through time at the same rate, but compared with people not moving fast, they actually age very slowly.

In the 70's a bunch of scientists actually proved this by sending an atomic clock either into orbit or onto a concord (I can't remember which), after some time of moving really fast, the atomic clock on the fast moving platform was some small amount slower than on on the earth.

EDIT: I also don't really understand your hypothetical. If the truck is moving really fast, how do sensors A and C or B and D line up?
 
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I can't answer your question but I ran across this on a site while looking for landing lights for my Mooney. The answers are relative to your question. It may depend on what type of light you are using.
FAQs

What is the meaning of life? You're a pilot and fly a Mooney, and you still need to ask that?
How much more speed can I expect? This light should give you an extra 5 kts because of reduced optical friction from the light. If you install one of these on a Piper, Bonanza or Cessna, you should get an additional 20 kts because the air molecules see the Fresnel lens and figure it's a Mooney coming at them.

http://www.scn.org/~bk269/eclipse.html
 
From outside the truck, the length from A->B appears shorter than it does on the truck.

Crack your textbook and look up "Lorentz" (or "length") contraction.

FYI, assumption #3 is not necessary. You can derive it from #2.
 
SkyHog said:
Assumptions:
1. There are 4 viewpoints: A, B, C, D
2. Light always travels at the speed of light
3. Nothing travels faster than the speed of light within a system

Note - if any of the assumptions above are incorrect, then the entire premise is incorrect, so I need to make sure I understand that part first.
Click to expand...

Point 2 should say that the speed of light in an inertial frame (i.e. unaccelerated) will always be measured the same speed. So if you measure the speed of a photon emitted from a passing truck, you get a speed c relative to your reference frame. A person inside the truck also gets c in their reference frame.

Point 3 is just a consequence of point 2 when you go work the math to make point 2 possible in all cases. Things like relative time and distance get affected.

So - that said, the box is on a stationary truck, light leaves A and hits B, and is measured outside the box as "the speed of light."

Now, the truck starts moving at a very high speed, and the light is measured again.

My hypothesis: it measures at the speed of light, still, because to viewpoint A, and to viewpoint B, the light is traveling at the speed of light. However, because the truck is moving, from outside the box, the light has traveled a further distance than it had originally, and therefore, the light traveled at a speed faster than the speed of light, right?
Click to expand...
It isn't clear to me which way the truck is moving in this frame of reference. If in the direction of A to B, the same direction as the light, then at the instant that the light is emitted from point A, the length of the truck in our reference frame is, according to Lorentz contraction, L = L0*(1 - V*V/C*C)^(1/2) where V is the speed of the truck, C is the speed of light, and L0 is the original length of the truck.

Suppose L0 = 1, C = 1, and V = 0.5. Then L = (1 - 0.25)^(1/2) = 0.866. (Using arbitrary units for simplicity, where C = 1.)

The truck appears to us to be 86.6% of its original length. Point B no longer can line up with point D when point A lines up with point C. With point B located 0.866 units to the right at the instance A and C are lined up, and B is traveling rightward at a speed of 0.5 while light is also traveling that same direction at 1.0, how long before the light reaches B?
 
Jim Logajan said:
Point 2 should say that the speed of light in an inertial frame (i.e. unaccelerated) will always be measured the same speed. So if you measure the speed of a photon emitted from a passing truck, you get a speed c relative to your reference frame. A person inside the truck also gets c in their reference frame.

Point 3 is just a consequence of point 2 when you go work the math to make point 2 possible in all cases. Things like relative time and distance get affected.

It isn't clear to me which way the truck is moving in this frame of reference. If in the direction of A to B, the same direction as the light, then at the instant that the light is emitted from point A, the length of the truck in our reference frame is, according to Lorentz contraction, L = L0*(1 - V*V/C*C)^(1/2) where V is the speed of the truck, C is the speed of light, and L0 is the original length of the truck.

Suppose L0 = 1, C = 1, and V = 0.5. Then L = (1 - 0.25)^(1/2) = 0.866. (Using arbitrary units for simplicity, where C = 1.)

The truck appears to us to be 86.6% of its original length. Point B no longer can line up with point D when point A lines up with point C. With point B located 0.866 units to the right at the instance A and C are lined up, and B is traveling rightward at a speed of 0.5 while light is also traveling that same direction at 1.0, how long before the light reaches B?
Click to expand...

Yes, the truck is moving the same direction as the light. Are you saying that the distance from A to B gets shorter when the truck is in motion?
 
SkyHog said:
Yes, the truck is moving the same direction as the light. Are you saying that the distance from A to B gets shorter when the truck is in motion?
Click to expand...

Yes. Such a shortening is known as Lorentz contraction. First devised by Dutch physicist Lorentz (and independently by Irishman FitzGerald, so also known as Lorentz-FitzGerald contraction) in 1892 to explain the result of the Michelson-Morley experiment. It was later shown that the same equation could be derived from Einstein's postulates.

To a person in the truck, the distance between A and B appears unchanged at 1. But to the truck person the distance between C and D appears to be shorter because those points are traveling at high speed to the left!
 
SkyHog said:
Yes, the truck is moving the same direction as the light. Are you saying that the distance from A to B gets shorter when the truck is in motion?
Click to expand...

Yeah but ..? Was the truck on a treadmill:dunno:;):rofl::lol:
 
Just like sound in air or surface waves on the water the propagation speed of light is independent of the source speed but on the fabric of space. But unlike the timing clock in air or water the timing clock is affected by its speed over the fabric of space when moved at speeds close to the speed of light. The atomic internal structure velocities (orbiting electrons) cannot be faster than what the fabric of space allows (speed of light). Otherwise the internal atomic velocities will exceed the speed of light when moved very fast. This why the clock slow downs as it moves faster. The speed of light is a function of the granularity (density) of the space medium. It is believed by some that this granularity may be different in some regions of the universe like the intergalactic space vs the vicinity of a black hole.

José
 
Piloto said:
Just like sound in air or surface waves on the water the propagation speed of light is independent of the source speed but on the fabric of space. But unlike the timing clock in air or water the timing clock is affected by its speed over the fabric of space when moved at speeds close to the speed of light. The atomic internal structure velocities (orbiting electrons) cannot be faster than what the fabric of space allows (speed of light). Otherwise the internal atomic velocities will exceed the speed of light when moved very fast. This why the clock slow downs as it moves faster. The speed of light is a function of the granularity (density) of the space medium. It is believed by some that this granularity may be different in some regions of the universe like the intergalactic space vs the vicinity of a black hole.

José
Click to expand...

Huh?

The propagation speed of light is the same in ALL frames, not just in the "fabric of space" (i.e., medium). There is no luminiferous ether -- no medium in which light waves travel. The speed of light is the same in the source frame as in any possible detection frame. This is dramatically different from acoustic waves in air or water.

That "fabric of space" [sic -- it's spaceTIME] analogy is a big red herring for special relativity, and it's only marginally useful for the curved spacetime in general relativity it was intended to describe.

Time doesn't slow down because of "internal structures." Time itself slows down, regardless of any clock. It's a consequence only of the constancy of the speed of light.
 
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The light in a medium will not travel faster when in motion - it will shift frequency - becoming bluer toward a stationary source and redder as you travel away it will not change speed . . . .

You will never be able to measure this phenom in a truck. . . unless its a space truck . . . even guys who spend 6 months on the ISS are only something like a heartbeat younger than the rest of us.

The relativistic effects only become truly apparent within 3% of the speed of light to those outside the system.
 
infotango said:
In the 70's a bunch of scientists actually proved this by sending an atomic clock either into orbit or onto a concord (I can't remember which), after some time of moving really fast, the atomic clock on the fast moving platform was some small amount slower than on on the earth.
Click to expand...

I was sure I knew the answer, but .... everything is spinning so fast... I need to sit down...Oh, I already am seated


Will you stop touching me!

:rofl::rofl::rofl::rofl:
 
infotango said:
In the 70's a bunch of scientists actually proved this by sending an atomic clock either into orbit or onto a concord (I can't remember which), after some time of moving really fast, the atomic clock on the fast moving platform was some small amount slower than on on the earth.
Click to expand...

You are probably referring to the Hafele-Keating experiement.

The GPS satellites have been validating the special and general relativity for the last 20+ years. The designers have to take into account special and general relativity - if they did not, the difference in clock rates would cause positional accuracy to drift by up to 10 km per day!
 
comanchepilot said:
The relativistic effects only become truly apparent within 3% of the speed of light to those outside the system.
Click to expand...

Depends on what you mean by "apparent."

It's measurable (and corrected for) in GPS satellites, which are essentially orbiting atomic clocks. The effect is of the order of tens of microseconds per day, corresponding to several miles in line of sight distance (the speed of light is pretty close to a foot per nanosecond). Positioning error were that not corrected for would be comparable, as half of the sky is less than 30 deg from the horizon.

GPS satellites are pretty fast, but they are far, far below "within 3% of the speed of light."
 
I've always had difficulty believing point #2 - I understand it's significance in the theory, but nobody has ever sufficiently explained to me why light behaves this way. I've also never heard a sufficient explanation of an experiement that actually proves it. A constant speed of light is an assumption of the special theory of relativity, but the assumption of the theory is not a proof of it. Our knowledge of how objects behave at very high speeds is very incomplete and our knowledge of light behaves at very high speeds is even more incomplete.

Second - assumption #3 comes from e=mc^2. However, keep in mind that Einstein did not say that c was the speed of light, he said it was a very large constant, such as the speed of light. However, for the sake of this discussion, that distinction is not important.

As far as your diagram goes, I'm not following what you're attempting propose. You seem to have a fixed object and a means of measuring the time of the propagation of light, but what are you actually proposing?
 
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bflynn said:
I've always had difficulty believing point #2 - I understand it's significance in the theory, but nobody has ever sufficiently explained to me why light behaves this way. I've also never heard a sufficient explanation of an experiement that actually proves it. A constant speed of light is an assumption of the special theory of relativity, but the assumption of the theory is not a proof of it. Our knowledge of how objects behave at very high speeds is very incomplete and our knowledge of light behaves at very high speeds is even more incomplete.
Click to expand...
Consider for a moment that light does not travel through space during a period of time; rather, consider the possibility that light is an excitation in space at points in time and when you are measuring the speed of light, you are actually measuring time's granularity in terms of measuring things within a spacial frame of reference.

Then consider that this may be a huge pile of steaming dung and put on some Doors records.
 
ChrisK said:
Consider for a moment that light does not travel through space during a period of time; rather, consider the possibility that light is an excitation in space at points in time and when you are measuring the speed of light, you are actually measuring time's granularity in terms of measuring things within a spacial frame of reference.

Then consider that this may be a huge pile of steaming dung and put on some Doors records.
Click to expand...

Or rather, a huge bong of smoking weed to go along with those Doors records.
 
Brian, look up a description of the Michelson-Morley experiment. That is the experimental proof of "#2."

They measured the speed of light 6 months apart, so the Earth would be moving in the opposite direction in its orbit for the second measurement as it was during the first. They designed an apparatus that would be capable of measuring it (a Michelson interferometer -- still used today for accurate velocity measurements), and got a null result. It's one of the most important experiments in physics.

#2 was used as the starting point for special relativity, not because it's some kind of pretty theory that has to be "just because" (that's not physics -- it's philosophy). Rather, it's what experiment said reality did. Einstein's leap was to take that at face value, and see what it did to Newton's equations.
 
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Sac Arrow said:
Light propagated from a moving source will travel faster than light propagated from a stationary source, relative to the two source points.
Click to expand...

That is pretty much the idea of what is now called the old "Galilean relativity" - it was simple and intuitive - but it was rejected over a hundred years ago because it doesn't correctly explain experiments.

When people talk about the theory of relativity, they usually mean the theory that replaced "Galilean relativity" and that is "Special Relativity." The main difference between the two is what they say about light. Galilean says "velocities are relative for everything" while Special says "velocities are relative for all objects, but not for light."
 
NoHeat said:
That is pretty much the idea of what is now called the old "Galilean relativity" - it was simple and intuitive - but it was rejected over a hundred years ago because it doesn't correctly explain experiments.

When people talk about the theory of relativity, they usually mean the theory that replaced "Galilean relativity" and that is "Special Relativity." The main difference between the two is what they say about light. Galilean says "velocities are relative for everything" while Special says "velocities are relative for all objects, but not for light."
Click to expand...

If Special is right, it would be impossible for a light bulb to emit light in all directions.
 
SkyHog said:
My brain is hurting trying to understand this one. I'm sure there's an explanation in the Theory of Relativity that I am missing, as this is mostly a Relativity issue.

Assumptions:
1. There are 4 viewpoints: A, B, C, D
2. Light always travels at the speed of light
3. Nothing travels faster than the speed of light within a system

Note - if any of the assumptions above are incorrect, then the entire premise is incorrect, so I need to make sure I understand that part first.

So here's the setup. We have a giant onlong cement box on a flat bed truck (or any other movable surface). Inside the cement box, we have a directional light source. This light source is positioned at viewpoint A, on one end of the box, and it is pointed to the other side of the box at the furthest distance (viewpoint B). It is safe to say that the light travels from viewpoint A to viewpoint B at the constant speed of light.

Outside the box, there is a sensor that is tripped when the light is sent from viewpoint A, which displays on viewpoint C. There is another sensor that is tripped when the light is received at viewpoint B, which displays on viewpoint D. Essentially, this serves as a form of measurement of the speed of light. The delay caused by the transmission from A-C and B-D is irrelevant because it is constant between the two (i.e., it always takes 5ms to get from A-C and 5ms to get from B-D).

So - that said, the box is on a stationary truck, light leaves A and hits B, and is measured outside the box as "the speed of light."

Now, the truck starts moving at a very high speed, and the light is measured again.

My hypothesis: it measures at the speed of light, still, because to viewpoint A, and to viewpoint B, the light is traveling at the speed of light. However, because the truck is moving, from outside the box, the light has traveled a further distance than it had originally, and therefore, the light traveled at a speed faster than the speed of light, right?

Its a matter of relativity, that within the box, the light traveled the same distance in the same time, but outside the box, it traveled a longer distance in the same time....

What am I missing?

Edit: A diagram to follow:

Code: 
    C                D
*-----------------------*
*|  A                B |*
*-----------------------*
       O            O
Click to expand...

This whole question is an exercise in futility.... If a cement box is small enough to fit on a flat bed truck then the distances and timing you want to quantify are impossible to measure... :dunno:
 
Sac Arrow said:
If Special is right, it would be impossible for a light bulb to emit light in all directions.
Click to expand...

The light itself is not moving at 2c, you can have independent systems traveling at the speed of light in opposite directions because there can be no inertial reference frame from which each particle is moving at >1c. This is where General Relativity and space-time comes in to play. If you were a photon traveling south at 1c, and there were a photon traveling north directly towards you at 1c for its inertial reference frame, the photons would see each other moving at different speeds over space-time, and that sum would never exceed 1c.

This is one of the reasons that warp-bubble technology would be far different from actually obtaining near-light speed in a ship. Let's say theoretically that we obtained eight 9s towards light speed travel, you fire up those engines and have them cancelled almost immediately, to the ship and everyone in it, it will seem as though you've teleported to alpha centauri, but to the stationary observer (e.g. the humans on earth and the vogons in the planning room on alpha centauri) just over 4 years will have passed.
The warp-bubble principal relies on keeping the reference observers within the "bubble" effectively stationary in space-time, and the bubble itself is the only thing which experiences time dilation by compressing space in front, and expanding space behind moreso than standard with massive particles. At "maximum warp" (9.3 Warpfactor, which is about 1100c) it would take "about a day" to get to alpha centauri, but it will also be about a day for the crew, the humans on earth, and the depressed cyborgs on alpha centauri.

I could go into the math, but I'm at work, so I'll throw some links out instead:
http://www.thebigview.com/spacetime/timedilation.html
http://www.fourmilab.ch/documents/OhMyGodParticle/

This is an internet argument in a web forum, so assume everything I said was either wrong or a lie.
 
Sac Arrow said:
If Special is right, it would be impossible for a light bulb to emit light in all directions.
Click to expand...

Nope. The frequency will shift, but SR doesn't prohibit emission in all directions. It's always at the same speed, regardless of how you look at it. You're still thinking like a Galilean. You don't add emission velocity to source velocity for light. It's just c. Period. That's what the Michelson-Morley Experiment says, and it's the derivation of SR.

If it were that easy to disprove SR, do you think anyone would have given it a second look in late 1905, let alone 107 years later? Experiment is everything in physics.

SR does make a significant change to Newtonian dynamics when speeds get close to c. Velocities do not add linearly. Constant acceleration follows a hyperbolic trajectory in the x-t plane, and never crosses the x=ct line. And so on.
 
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N801BH said:
This whole question is an exercise in futility.... If a cement box is small enough to fit on a flat bed truck then the distances and timing you want to quantify are impossible to measure... :dunno:
Click to expand...

Huh? It's not hard to measure distances to a few inches with a differential GPS, and close to the wavelength of light (0.7 microns for red) with an interferometer, and nanosecond timing isn't all that difficult either. The speed of light is about a foot per nanosecond. Many of us have torn our hair out over 40 nanosecond spurious signals on a 20 foot coax cable.

Besides, you missed the point. It's a thought experiment to learn about how time and space behave.
 
MAKG1 said:
Huh? It's not hard to measure distances to a few inches with a differential GPS, and close to the wavelength of light (0.7 microns for red) with an interferometer, and nanosecond timing isn't all that difficult either. The speed of light is about a foot per nanosecond. Many of us have torn our hair out over 40 nanosecond spurious signals on a 20 foot coax cable.

Besides, you missed the point. It's a thought experiment to learn about how time and space behave.
Click to expand...
..

The margin for error in this experiment is open to debate.
 
MAKG1 said:
What margin for error?

The measurements are possible in principle (though making a truck go that fast isn't), and you're severely missing the point.
Click to expand...

I must be missing the point.... Something in "priciple" is entirely different then something that is verifiable...
 
N801BH said:
I must be missing the point.... Something in "priciple" is entirely different then something that is verifiable...
Click to expand...

That is not a description of a test of SR. It's a teaching tool to understand its implications. Yes, you've missed that point. No test that proposes taking anything bigger than an insect up to relativistic speeds is close to practical. Not because measurements can't be done (they can). Because there is no mechanism to set up preconditions.

There are several better methods for verification. The atomic clock in orbit (GPS satellite) has been mentioned. Another is penetration depth of unstable accelerated charged particles.

If you're going to argue that SR is unverified, you're going to have to go cast your vote for Woodrow Wilson's successor and collect your pension for the Great War.
 
To put this simply light travels in effect without a medium unlike sound which needs gas, liquid or a solid. So a reference for the speed of light doesn't matter.

If you have the truck with point A and B and the truck is moving very fast, say half the speed of light going left to right then you switch the light on at point A and detect it at point B the light outside the truck will have passed point D and meet up with point B at the same time.

However inside the truck due to the conservation of time it will seem like it took the same amount of time for light to travel from point C to D as it took for point A to B. This is because time will have slowed down in the truck.
 
The speed of light is constant within a moving system, and the system's movement doesn't accelerate or decelerate it. The laser gyros used in many glass-panel systems use the principle.

inn-gyroscope.gif


A pulse of light leaves the generator and travels in both directions around the mirrored track. If the gyro does not rotate around its axis, the pulses of light arrive at the detector at the same time, and computers determine that no motion has occurred. Rotating the gyro shifts the arrival times of the pulses, the computer detects that and calculates a rate and direction, and applies that to the the pilot's display.

Three of them are required to get roll, pitch and yaw.

The principle was invented around 1900, using a glass fiber as a track. The light beamed into the end of the fiber exited the other end and intersected the ingoing beam, and shifting the whole affair around its axis caused an interference so that the exiting light on the target would dim or extinguish.

Dan
 
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Dan, FOGs are interesting devices, essentially round interferometers (no mirrors). But the principle they work on is the Doppler shift, not time dilation. When the ring is rotating, the clockwise beam gets Doppler shifted one way, and the counterclockwise beam gets shifted the other way. The difference is proportional to rate of rotation.

The Doppler shift for light is somewhat different than it is for sound, but to first order in v/c, they are identical. Since v/c is really, really small for an aircraft, you'll have a hard time finding Relativistic effects with an FOG.

Because they sense rate rather than attitude, they drift like mad. They also have natural quantization (it's hard to count half an interference fringe).

The simplest, most direct measurement of SR effects is the atomic clock in orbit (GPS), which directly measures time dilation.
 
Brian Greene, "The Elegant Universe". It will explain everything.
 
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