# Theory of Relativity

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#### drwayne

##### Guest
mental_avenger":o7b6icnh said:
drwayne":o7b6icnh said:
An object in a constant velocity frame of reference is experiencing no net force.
Wayne
An object in orbit is experiencing constant acceleration. That is countered by the force of gravity between the object and the whatever it is orbiting. The fact that the two forces are in balance to keep the object in orbit does not mean that the object is not under constant acceleration. While gravity may counteract the affect acceleration has on orbiting, it may not counteract the affect that acceleration has on atomic interactions.

You will notice I said nothing about the word orbit, because it is a case where there *is* a net
force on the object, which leads to an acceleration.

An object does not remain in orbit due to countering forces. It is falling all the time.

By the way, accelerations do not counter forcees.

Wayne

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#### drwayne

##### Guest
OK, I hope I have a little more time this time.

The physics of objects in orbits has the problem that people try to teach it in
a weird sort of pseudo-statics mode. In that pursuit, they introduce pseudo-forces
like the "centrifugal" force, and start balancing things. I have even seen grad
students still doing this stuff.

Here are some of the essential points:

An object in orbit, even in a circular orbit, is in fact accelerating all the time.
Even though the magniitude of it's velocity does not change. Hoow is this?
Remember for a moment that velocity is a vector quantity, with both direction
and magnitude. The direction of the velocity is changing as it sweeps out
its circle.

Wayne

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#### ArcCentral

##### Guest
If every mechanism we use to measure time gives us a result that indicates time for them is slowed (and they all agree!)...what other conclusion can we draw?

The mechanism can slow? Thats the other option. In this option, time isn't anything at all, it doesn't do anything, it could be considered the non-event between events, essentially what changes here under gravity, acceleration, or just going real fast is that there is a greater non-event between events.

When you say that time slows, I'm not sure what you mean. What is the mechanism? Is there a mechanism, if not, then what do you mean when you say time slows?

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#### MeteorWayne

##### Guest
mental_avenger":1939q936 said:
The fact that they work as TOR predicts does not mean that TOR is correct. Remember, the theory was developed to explain observed phenomena, not the other way around. Since AFAIK, there would be no way to tell the difference between time dilation and the slowing down of atomic interactions, the observed results would be the same regardless of which is taking place.

.

AFAIK, that is not correct. In fact TOR was created first, then the effects it would have on time were figured out afterward.

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#### SpeedFreek

##### Guest
mental_avenger":m9dlzu45 said:
SpeedFreek":m9dlzu45 said:
A second always lasts a second, in any valid frame of reference. Time dilation shows that different frames of reference will measure different elapsed times for the same event, but the difference only becomes apparent if they can meaningfully compare their results. Nobody experiences time speeding up or slowing down, they only find that they have experienced different amounts of elapsed time, depending on what they were doing, relative to the other.
That statement contradicts the concept of time dilation.

Quite the opposite, actually. That statement is the essence of relativity, that the laws of physics are the same in any valid frame of reference. The difference in results when comparing the measurements of an event from two different frames of reference only shows up when you can meaninfully compare those different frames of reference. In the case of two observers who are coasting through space with relative speed to each other, each measures his own clock as ticking normally, but each will calculate that the others clock is running slower than his own, by the same amount! Time-dilation is symmetrical between inertial frames of reference.

http://en.wikipedia.org/wiki/Time_dilat ... _observers

mental_avenger":m9dlzu45 said:
SpeedFreek":m9dlzu45 said:
The energy that runs around the microchips on a GPS satellite seems to cycle at a different rate up there when compared to the rate it cycles down here. We had to set the chips to run slow by the amount predicted by relativity, for us down here to measure that clock up there as running at the same rate as our own.
And that agrees with what I said might be happening, that is, the atomic interactions may change as opposed to time actually changing. BTW, did you get the slowing down reversed in your example?

No, I did not reverse it. The clock on a GPS satellite should gain time (run "faster") when compared to a clock on the surface of the Earth, according to relativity. The predicted difference due to SR relative motion is that the clock should lose 7 microseconds (run "slower"), but the predicted difference due to GR gravitational time-dilation is a gain of 45 microseconds. The net result is that the GPS satellites would gain 38 microseconds per day, when compared to a clock on the ground. We set those GPS clocks to run slow by the predicted amount, so that when they are in orbit they run at the same speed as the clocks down here on the surface.

http://www.astronomy.ohio-state.edu/~po ... 5/gps.html

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#### mental_avenger

##### Guest
Notice that all the explanations revolve around “observations” between frames of reference. Observations require information transfer via some medium, usually light. That requirement automatically introduces restrictions into the ability to determine the actuality of the events. IOW, if A observes that B’s clock is running slower, and B observes that A’s clock is running slower, those errors are a direct result of the propagation delay of information caused by the limited speed of light. Imagine the discrepancy in the observed speed of the clocks if the observations were made by letters on shuttle craft. Personally, I couldn’t care less about what can be observed. IMO that is irrelevant. The only relevant thing here is what is actually happening.

It has been said that there can be no simultaneous events in widely spaced parts of the galaxy because there is no way to verify them with the transfer of information. If a star 100 ly away explodes, we won’t know it for 100 years. However, that does not change the fact that it did explode at the exact same time as a given event here on Earth. And that is regardless of the relative velocities of the separated objects.

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#### Saiph

##### Guest
direct result of the propagation delay of information caused by the limited speed of light

Except time dilation experiments aren't just an example of a signal delay. You take two clocks, you keep one and move the other. Then you sit them next to eachother again...they will not agree on how much time passed.

And as for simultenaity: It isn't said that events can't be simultaneous, and the reason certainly isn't due to a signal delay. It's a consequence of relativity saying you can trade span of time for a length of distance. What's simultaneous for one observer, having two events in different locations occur at the same time, isn't necessarily simultaneous for another observer. That observer can see the two events take place at different distances (length contraction) and slightly different times.

Now, I see if you reject time dilation and length contraction why you won't buy this. But don't chalk up relativity's simultaneity claims to a mere finite signal speed/delay.

A further note on the claim that relativity was created and designed to explain observations, patently untrue. Yes we knew about mercury's odd orbital oscillations before hand. But relativity was created to explain the michelson-morley experiment and what that means. GR then extended it by considering acceleration (and it's equality with gravity) and was applied to mercury's orbit.

But other tests were done, much later, to test predictions of the theories that could not be tested at their formulation. Clocks weren't accurate enough to test time dilation for instance, and they had to wait for a solar eclipse to test gravity bending light travel paths. And nobody KNEW about, let alone looked for, altered stellar positions during a solar eclipse.

Black holes are another consequence of GR, and it wasn't for 50 or so years that we actually started seeing any real evidence that they existed.

Those three 'observations' were made to check relativity. Relativity was not made to explain them.

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#### SpeedFreek

##### Guest
Yes, the relativity of simultaneity, time-dilation and length contraction are definitely not due to light-travel time. It is after you calculate out the light travel time from your observations that you come across these effects (which is why I always try to say that one observer will "calculate" the others clock to show a different elapsed time, these relativistic effects are apparent after you calculate out light-travel time, doppler effect and the aberration of light).

Consider two observers. One is sitting in the middle of a train carriage and the other is standing on a station platform. As the train passes through the station, the observer on the train turns on a light in the middle of the carriage. From that observers frame of reference, the light moves at c, so it would reach the opposite end walls of the carriage at the same time.

But from the point of view of the observer on the platform, the light moves at c too. As the train passes him on the platform, he sees the light turn on in the middle of the carriage, but due to the fact that the train is moving past him at speed, he will see the light hit the back (trailing) wall of the carriage before it hits the front. This is not about the time it takes the light to hit the retina of the observer, it is about the time it takes the light to move from the middle of the carriage to the ends of the carriage it was emitted within. It is about how the observer sees the light move relative to another frame of reference when compared to their own. It all boils down to the fact that every observer sees the light moving at 300,000 km/s, relative to themselves.

The observer on the platform can measure what the distance was to each end of the carriage when he saw the light hit the ends, even though he saw the light hit the ends at different times. He can then work out the light travel time and subtract it from his calculations, to see if the light actually hit the ends of the carriage at the same time or not, and he will find that it did not. He will find that the light was moving in opposite directions at 300,000 km/s relative to him, rather than the carriage. The observer on the carriage calculates the light moved at 300,000 km/s relative to the carriage, rather than the platform.

So, did the light actually hit each end of the carriage at the same time, or not?

http://en.wikipedia.org/wiki/Relativity ... experiment

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#### Saiph

##### Guest
Now, to elaborate a bit on why speedfreak's example is so odd, I'll outline how it should look, and work out, if the speed of light isn't constant, i.e if it works just like say, throwing a baseball.

The man in the carriage moving at .5c turns on the light, right in the middle. It spreads out at 300,000 km/s and strikes both ends at the same time.

The man on the ground watches the train go by, and sees the man turn on the light. As the speed of light (in this experiment) is entirely relative, he see's it move at 450,000 km/s in the direction the train is heading, but 150,000 km/s backwards. However the ends of the train are also moving, which results in the ground side observer seeing the the light strike both ends at the same time, as the moving carriage brings the back edge up to the slowly moving light, and takes the front edge away from the faster moving light.

So if the speed of light is relative, like everything else, we don't have a problem at all, everything works out fine and fits into our everyday understanding of how things work.

The problem, of course, is that the speed of light is observed to be constant regardless of who is measuring it..so the speed is ALWAYS 300,000 km/s...thus speedfreak's example where we are left wondering: Who is right?

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#### mental_avenger

##### Guest
Saiph":3hf75gyw said:
Except time dilation experiments aren't just an example of a signal delay. You take two clocks, you keep one and move the other. Then you sit them next to eachother again...they will not agree on how much time passed.
That is incorrect. They will not agree on the results of their measurement of elapsed time. If that disagreement is due solely to the effects of acceleration on the atoms of one of the clocks, then that is what is being measured, not time.

Saiph":3hf75gyw said:
And as for simultenaity: It isn't said that events can't be simultaneous, and the reason certainly isn't due to a signal delay. It's a consequence of relativity saying you can trade span of time for a length of distance. What's simultaneous for one observer, having two events in different locations occur at the same time, isn't necessarily simultaneous for another observer. That observer can see the two events take place at different distances (length contraction) and slightly different times.
Again with the observation of events, not the actuality of events. How a given observer observes an event has no bearing whatsoever on the actuality of the event itself.

Saiph":3hf75gyw said:
But other tests were done, much later, to test predictions of the theories that could not be tested at their formulation. Clocks weren't accurate enough to test time dilation for instance, and they had to wait for a solar eclipse to test gravity bending light travel paths. And nobody KNEW about, let alone looked for, altered stellar positions during a solar eclipse.
Which brings up another scientific unknown. Currently, we have no idea of the nature of light. Wave-particle duality is proof of that. It is likely that light is neither a wave nor a particle, but rather something different than anything else we are familiar with. And if we don’t know what light is, how can we possibly understand why it is bent by gravity?

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#### mental_avenger

##### Guest
SpeedFreek":37s4bihw said:
Consider two observers. One is sitting in the middle of a train carriage and the other is standing on a station platform. As the train passes through the station, the observer on the train turns on a light in the middle of the carriage. From that observers frame of reference, the light moves at c, so it would reach the opposite end walls of the carriage at the same time.

But from the point of view of the observer on the platform, the light moves at c too. As the train passes him on the platform, he sees the light turn on in the middle of the carriage, but due to the fact that the train is moving past him at speed, he will see the light hit the back (trailing) wall of the carriage before it hits the front. This is not about the time it takes the light to hit the retina of the observer, it is about the time it takes the light to move from the middle of the carriage to the ends of the carriage it was emitted within. It is about how the observer sees the light move relative to another frame of reference when compared to their own. It all boils down to the fact that every observer sees the light moving at 300,000 km/s, relative to themselves.
Ah, yes, Einstein’s classic train Gedanken experiment. Except that, as I proposed in my own Gedanken experiment, that has never been proven by eliminating the effects the measuring instruments have on measurements. In my opinion, the Speed of light = 1.802,265,898 MegaFurlongs / MicroFortnight relative to it’s starting point.

SpeedFreek":37s4bihw said:
The observer on the platform can measure what the distance was to each end of the carriage when he saw the light hit the ends, even though he saw the light hit the ends at different times. He can then work out the light travel time and subtract it from his calculations, to see if the light actually hit the ends of the carriage at the same time or not, and he will find that it did not. He will find that the light was moving in opposite directions at 300,000 km/s relative to him, rather than the carriage. The observer on the carriage calculates the light moved at 300,000 km/s relative to the carriage, rather than the platform.
The observer again. The real question is what did the light actually do.

SpeedFreek":37s4bihw said:
So, did the light actually hit each end of the carriage at the same time, or not?
If the source was in the exact center, then yes.

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#### Saiph

##### Guest
That is incorrect. They will not agree on the results of their measurement of elapsed time. If that disagreement is due solely to the effects of acceleration on the atoms of one of the clocks, then that is what is being measured, not time.
Not true, acceleration does not have to be present to measure these effects. In fact, acceleration adds additional time dilation effects (General relativity) that are seperate from and and addition to the special relativity effects. You can observe time dilation on clocks that are kept at a constant speed. And if you think phycisists can't take into account something as simple light travel time...you have very little faith in modern scientists.

Again with the observation of events, not the actuality of events. How a given observer observes an event has no bearing whatsoever on the actuality of the event itself.
Actually this has lots to do with the 'actuallity' of the event, as the observer is physically interacting with the system. If the light is a high energy bath of gamma rays, in one case he'll be fried by both at the same time. In another he'll be fried by one, THEN the other. The difference is a single intense blast, or a longer milder blast.... Observation IS 'actuality'. If we cannot trust what we cannot observe....then all of empirical science and experimentation don't hold up.

Currently, we have no idea of the nature of light
I beg to differ. While we do not understand it in it's entirety, it is actually one of the best characterized and understood phenomena in science today. And while we may have further breakthroughs in how light works in extreme circumstances, none of those results will completely invalidate the understanding we have today.

This feeds into my next point where you state:
In my opinion, the Speed of light = 1.802,265,898 MegaFurlongs / MicroFortnight relative to it’s starting point.
Okay, so now you're throwing out the constancy of the speed of light entirely? A basic observed fact since the 1900's (michelson-morley experiments), replicated in countless experiments with various designs, and the foundation of a lot of modern technology and put to use every day in things ranging from modern materials science (quantum physics uses this in understanding atomic structure and behavior), to computers and communications devices...

Okay MA, are you just pulling my leg?

As it is, if you refuse to accept this fact, the rest of this arguement is entirely pointless, as time dilation is predicated upon (and a consequence of) the speed of light being a universal constant completely independent of an observers motion and acceleration. I.e. if you cannot accept this, it's only logical that you don't accept time dilation, and I'm not going to argue the point further.

Also, I'm going to throw in another confirmation of time dilation for the curious (as I'm not going to argue the point with MA until we clear up the speed of light = constant issue): The lifespan of Muons created by high energy cosmic ray impacts in the upper atmosphere. These particles live a very, very short time. So short that in newtonian physics they do not live long enough to travel from the upper atmosphere where they are created, down to the ground observation stations at the speeds they are observed at. They should die before they arrive. However they DO arrive in very significant numbers, indicating that the half-life of the muons as we observe it is increased by the time dilation factor of their velocity. They still experience the short half-life, but since they move so rapidly compared to us they last longer, and are able to make the trip. Here is another concrete consequence of time dilation, that isn't just a reading on a clock.

And i'll anticipate a couple counter points: Yes, we do know they are formed in the upper atmosphere, not lower and Yes this is replicated in particle accelerators every time they do experiments in precisely monitored and controlled conditions...

M

#### mental_avenger

##### Guest
Saiph":2avaaog8 said:
Not true, acceleration does not have to be present to measure these effects. In fact, acceleration adds additional time dilation effects (General relativity) that are seperate from and and addition to the special relativity effects.
Ok, what compensation is used to nullify the effect of acceleration on an object in orbit, keeping in mind that an object in orbit is under constant acceleration?

Saiph":2avaaog8 said:
You can observe time dilation on clocks that are kept at a constant speed.
How? By using light transmitted by the moving clock and processed through instruments that necessarily insert their own errors into the equation?

Saiph":2avaaog8 said:
And if you think phycisists can't take into account something as simple light travel time...you have very little faith in modern scientists.
Bingo!

Saiph":2avaaog8 said:
Actually this has lots to do with the 'actuallity' of the event, as the observer is physically interacting with the system. If the light is a high energy bath of gamma rays, in one case he'll be fried by both at the same time. In another he'll be fried by one, THEN the other. The difference is a single intense blast, or a longer milder blast.... Observation IS 'actuality'. If we cannot trust what we cannot observe....then all of empirical science and experimentation don't hold up.
I will illustrate where you are incorrect about that. Lets use the train example, a really long train car. The light in the center is a high energy source. A man on the train at one end of the train car will have a certain amount of time to duck out of the way of the energy beam. It doesn’t matter how many people are observing from the sidelines, or how fast the train is going, or which direction it is going, the man will still have exactly the same amount of time to duck out of the way. IOW, observation by anyone else will have no effect whatsoever on the actuality of the event.

Saiph":2avaaog8 said:
I beg to differ. While we do not understand it in it's entirety, it is actually one of the best characterized and understood phenomena in science today. And while we may have further breakthroughs in how light works in extreme circumstances, none of those results will completely invalidate the understanding we have today.
Ok, what is light? Is it a wave? Is it a particle? What is the makeup or composition of light? We don’t know what light is, we only know what happens to other substances when it interacts with them. Even then, we only know the observed effects of light’s interaction with other substances, not necessarily what is actually taking place.

Saiph":2avaaog8 said:
Okay, so now you're throwing out the constancy of the speed of light entirely?
Not at all. I am only questioning the manner in which we observe light and characterize it. In a vacuum, light emitted from a source will leave at 299,792,458 m/s and will continue at that velocity, relative to its source, until it encounters something. If that light passes by someone who is traveling at .5c, it will continue at .5c relative to that person, and at c relative to its origin. Someone observing that light will neither make it speed up nor slow down. Here’s the catch. While the original light will proceed upon its way at c relative to its source, those parts of that light that are intercepted in an attempt to measure it will be changed by the instruments to appear to be moving at c relative to the instruments.

Saiph":2avaaog8 said:
Okay MA, are you just pulling my leg?
Nope. If you recall my Gedanken Experiment many years ago, I challenged anyone to provide a setup for an experiment for the measurement of the speed of light that would not be dependent upon any interactions of the light with the instruments or equipment that might change its velocity. There were no successful submissions.

Saiph":2avaaog8 said:
As it is, if you refuse to accept this fact, the rest of this arguement is entirely pointless, as time dilation is predicated upon (and a consequence of) the speed of light being a universal constant completely independent of an observers motion and acceleration.
And therein lies the problem. The whole theory is predicated upon that yet unproven item. IMO, the instruments used in the measurements necessarily interact in a manner that will cause them to measure the light they receive as c, regardless of its actual velocity.

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#### derekmcd

##### Guest
Hi, MA

I'm not sure what, exactly, your point of contention is. So, I'll start with the most basic of questions:

Do you agree or disagree that the speed of light is constant regardless of the reference frame?

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#### Saiph

##### Guest
And therein lies the problem. The whole theory is predicated upon that yet unproven item. IMO, the instruments used in the measurements necessarily interact in a manner that will cause them to measure the light they receive as c, regardless of its actual velocity.

Okay...so you're saying we can't know despite direct observation, because we can't trust the instruments, any instruments at all. Taking that to it's logical conclusion and applying it to all experiments (as you've failed to illustrate why it's just THIS experiment)... I can't know anything because I can't trust what any observation tells me! :roll:

Here's the problem with your 'instrumental error' idea. It's not completely wrong, you must look for systematic errors in any device you use, it's also why you calibrate any measuring device. The problem MA, is that after calibrating them, every mechanism using a variety of timing and detection techniques ALL give the same answer. If the issue was the instruments introducing their own bias then various instruments would introduce various biases. They would not all produce the same error. This is precisely why you use different methods and devices to measure the same thing.

Anyway, my point is if all matter perceives light to have a constant speed, it must have a constant speed as everything acts just like it does have a constant speed.

If I can't trust a simple set of instruments to tell me when something arrives at point B after leaving point A then we have a problem and you have no faith in empirical data. And since I doubt you're a core believer in completely theoretical physics we'll get no purchase there. And since you don't think a very well trained and educated group of experts can correctly account for light travel time...you have absolutely no faith in science.

And until we get this little bit hammered out I'm not going to bother with the rest.

M

#### mental_avenger

##### Guest
Saiph":lavlhhm9 said:
Okay...so you're saying we can't know despite direct observation, because we can't trust the instruments, any instruments at all. Taking that to it's logical conclusion and applying it to all experiments (as you've failed to illustrate why it's just THIS experiment)... I can't know anything because I can't trust what any observation tells me! :roll:
Not at all. That only applies to the measurement of light, and only using current test setups. Remember, that was my challenge, to design a test system that would eliminate possible instrument generated errors. I actually designed such a system, but it has to be used in space.

Saiph":lavlhhm9 said:
Here's the problem with your 'instrumental error' idea. It's not completely wrong, you must look for systematic errors in any device you use, it's also why you calibrate any measuring device. The problem MA, is that after calibrating them, every mechanism using a variety of timing and detection techniques ALL give the same answer. If the issue was the instruments introducing their own bias then various instruments would introduce various biases. They would not all produce the same error. This is precisely why you use different methods and devices to measure the same thing.
The errors introduced by the instruments are the result of the way light is propagated. Remember the BEC experiments? They found that apparently light does not “pass through” a medium. Rather, it is absorbed by the medium (gas, lens etc.) the pattern travels through the medium as information stored in the spin of the electrons of the medium, and it emitted out the other side. That also explains why and how lenses work as they do. As the light is emitted, it is emitted at c. That is regardless of the velocity it had when it entered the medium. Therefore, each time you use a lensed instrument to capture a ray of light, it automatically ensures that the exit velocity will be c relative to that instrument. Mirrors function in a similar manner, absorbing and re-emitting the light.

Saiph":lavlhhm9 said:
Anyway, my point is if all matter perceives light to have a constant speed, it must have a constant speed as everything acts just like it does have a constant speed.
Ok, lets examine that theory. Lets say we have two space stations far enough outside our solar system to not be measurably affected by gravity from our Sun. Station B is fixed exactly one light day past Station A. Now, station A sends a supply ship to station B, and that ship cruises at .1c. When that ship is about halfway to Station B, Station A sends a short pulse of light to Station B. That pulse passes the supply ship about 5.5 days out. That pulse of light is traveling at c relative to Station A and to Station B. What speed is that light pulse traveling relative to the supply ship?

Saiph":lavlhhm9 said:
If I can't trust a simple set of instruments to tell me when something arrives at point B after leaving point A then we have a problem and you have no faith in empirical data.
I would trust a properly set up set of instruments.

Saiph":lavlhhm9 said:
And since I doubt you're a core believer in completely theoretical physics we'll get no purchase there. And since you don't think a very well trained and educated group of experts can correctly account for light travel time...you have absolutely no faith in science.
I think they can. I don’t think they have attempted the correct setup of instruments in the vacuum of space.

Can you devise a test setup that would measure the velocity of light from a distant star in such a manner that the re-emission of light by the optical components would not alter the incoming velocity of the light?

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#### mental_avenger

##### Guest
derekmcd":3b8yop3q said:
Hi, MA

I'm not sure what, exactly, your point of contention is. So, I'll start with the most basic of questions:

Do you agree or disagree that the speed of light is constant regardless of the reference frame?
Unless acted upon by an external force or object, the speed of light in a vacuum is 1.802,265,898 MegaFurlongs / MicroFortnight relative to its source.

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#### Saiph

##### Guest
Why are you using such odd units MA?

Anyway, simple interferometer throws your entire claim out the window. You take a laser light and bounce it off a mirror, reflecting it back on itself to create interference fringes. Move the mirror towards the source at speed V. By watching how the fringes change you can determine if the light reflected off the mirror is C + V (i.e. light is emitted at C relative to the mirror moving at V), or just C. The measurement takes place BETWEEN and only via interaction with itself (which does not change propagation speed). Thus your concern is moot.

Sorry MA, doesn't work and it's been looked for.

As for my suggesting a device that meets your specifications to prove your point...ain't gonna happen. Especially as you say you've already designed one. Burden of proof lies on you.

Edited to fix a brain fart, thanks DrRocket!

M

#### mental_avenger

##### Guest
Saiph":3mw0p0h7 said:
Why are you using such odd units MA?
Variety.

Saiph":3mw0p0h7 said:
Anyway, simple interferometer throws your entire claim out the window. You take a laser light and bounce it off a mirror, reflecting it back on itself to create interference fringes. Move the mirror towards the source at speed V. By watching how the fringes change you can determine if the light reflected off the mirror is C + V (i.e. light is emitted at C relative to the mirror moving at V), or just C.
Using that setup, the light beam aimed at the mirror is absorbed and re-emitted at c from the mirror regardless of its entrance velocity. Since both the light from the source and light from the mirror are being emitted at c, there should be no difference when they are moved towards or away from each other. Therefore, that setup does not prove anything.

Saiph":3mw0p0h7 said:
The measurement takes place BETWEEN and only via interaction with itself (which does not change propagation speed).
No, the light that hits the mirror is not the same light that is “reflected back”. According to observations with Bose-Einstein Condensate, the light is absorbed by a medium and re-emitted. That would include a mirror.

Saiph":3mw0p0h7 said:
Of course you realize that with that answer, you have the exact same light traveling at c and also at .9 c at exactly the same instant at exactly the same point in space. Notice that I didn’t ask what an observer would observe, but rather how fast light was actually traveling.

Saiph":3mw0p0h7 said:
As for my suggesting a device that meets your specifications to prove your point...ain't gonna happen. Especially as you say you've already designed one. Burden of proof lies on you.
An experiment can be set up on the moon to measure the speed of light from a distant star without using optics or mirrors. We would use light from a distant star that would be moving radially relative to us. Two detectors are set up exactly one kilometer apart, both facing the same direction. Exactly halfway between them a timing device is placed, connected to the two sensors by precisely equal length cables. In line with the two sensors, a high speed slit shutter is placed.

At the precise instant that a specific star is directly in line with the shutter and the sensors, the shutter opens and closes quickly. The light from the star reaches one sensor, then the second sensor. The timer measures the time that it took the light to travel from one sensor to the other, and the speed of the light beam can be calculated. This eliminates any possible change in the speed of the light by the instruments.

I would love to set that up to test my hypothesis, but my current financial situation precludes purchase of that equipment and transport, setup, and operation on the Moon.

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#### Saiph

##### Guest
In the interferometer setup, according to you the light from the mirror is re-emitted at C' relative to that mirror. Right, thats the point of the experiment. The light from the original laser source is moving C, our baseline speed. The mirror, since it's moving, will emit, according to you, light at C plus Vmirror, so C'.

If there is a difference between C and C' we'll be able to see it via the interference fringes. Heck, I can take out the mirror entirely, and just use two laser lightsources, and move one relative to the other.

The experiment you describe is basically a straightforward one. The variation I've heard of is you have two highly accurate clocks, one hooked to a light source, the other to a photosensitive detector. Start both clocks at the same time, and the source is activate as well. The detector switches off it's clock when it detects light.

No big deal, and it's probably been done.

Why do you need it to be done on the moon?

I think I will throw in some time dilation aspects here. Time dilation is, according to relativity, a direct consequence of the constant speed of light. The derivation is actually pretty straightforward. One of the pretty clearcut cases of time dilation is the observed lifespan of various particles (such as the muon). At high speeds these live longer than those that are stationary: EXACTLY as predicted by SR time dilation equations. This applies to ALL particles and their half-lives. This alone is, IMO, very good proof of SR's validity and it's claim to a constant speed of light. And this is without having to measure it directly.

You can also throw in Maxwell's equations that do an incredible job describing light as propagating EM wave. But you'll throw that out as we 'don't really know what a photon is' despite how aptly these equations describe it and predict it's behavior.

B

#### bowman316

##### Guest
Its just like passing a car going past you in the opposite direction. Both cars are going 50 mph, but to you, it looks like the car passing you is going 100 mph. But it is still only going 50. It does not matter what the perceived speed is, that is not the actual speed.

M

#### mental_avenger

##### Guest
bowman316":17ykif63 said:
Its just like passing a car going past you in the opposite direction. Both cars are going 50 mph, but to you, it looks like the car passing you is going 100 mph. But it is still only going 50. It does not matter what the perceived speed is, that is not the actual speed.
Your example is not commensurate with the situation in this discussion. A more applicable example would be two cars going from town A to town B. The first is traveling at 10mph. When it is about halfway there, a second car leaves town A and heads towards town B at 100mph. When it passes the first car, it is going 100mph relative to town A and to town B, but it is going 90mph relative to the first car.

M

#### mental_avenger

##### Guest
Saiph":2hd2bxx6 said:
In the interferometer setup, according to you the light from the mirror is re-emitted at C' relative to that mirror. Right, thats the point of the experiment. The light from the original laser source is moving C, our baseline speed. The mirror, since it's moving, will emit, according to you, light at C plus Vmirror, so C'.
Hmmm………. I’ll have to think about that. In any case, if that experiment is performed using the air as a medium through which the light passes, it may be possible for the molecules in the air to successively absorb and re-emit the light in both directions, invalidating the intent of the experiment.

Saiph":2hd2bxx6 said:
The experiment you describe is basically a straightforward one. The variation I've heard of is you have two highly accurate clocks, one hooked to a light source, the other to a photosensitive detector. Start both clocks at the same time, and the source is activate as well. The detector switches off it's clock when it detects light.
The difference is that my setup removes all variables and the possibility of unknown variables. It would be the cleanest setup with available technology.

Saiph":2hd2bxx6 said:
No big deal, and it's probably been done.

Why do you need it to be done on the moon?
To eliminate the possibility of light absorption and re-emission by the molecules of the atmosphere, which is a gaseous medium.

Saiph":2hd2bxx6 said:
I think I will throw in some time dilation aspects here. Time dilation is, according to relativity, a direct consequence of the constant speed of light. The derivation is actually pretty straightforward. One of the pretty clearcut cases of time dilation is the observed lifespan of various particles (such as the muon). At high speeds these live longer than those that are stationary: EXACTLY as predicted by SR time dilation equations. This applies to ALL particles and their half-lives. This alone is, IMO, very good proof of SR's validity and it's claim to a constant speed of light. And this is without having to measure it directly.
And that same effect would be measured if acceleration and/or high velocity effects the all atoms equally and slows down their interactions proportionally to the degree of acceleration and/or velocity.

Saiph":2hd2bxx6 said:
You can also throw in Maxwell's equations that do an incredible job describing light as propagating EM wave. But you'll throw that out as we 'don't really know what a photon is' despite how aptly these equations describe it and predict it's behavior.
What about experiments such as double slit, which show wave/particle duality? Why is light sometimes observed to act like we would expect particles to act, and sometimes observed to act like we would expect waves to act? “Photons” are not waves, they are particles according to theory. So, which is it? We don’t know. I suggest that light is neither waves nor particles, but something we don’t have an analogy for. Also note that the concept of a “propagating EM wave” is also just an analogous approximation, not necessarily an actual physical manifestation.

(post recreated)

D

#### derekmcd

##### Guest
mental_avenger":36fp0mp2 said:
derekmcd":36fp0mp2 said:
Hi, MA

I'm not sure what, exactly, your point of contention is. So, I'll start with the most basic of questions:

Do you agree or disagree that the speed of light is constant regardless of the reference frame?
Unless acted upon by an external force or object, the speed of light in a vacuum is 1.802,265,898 MegaFurlongs / MicroFortnight relative to its source.

I've always appreciated your debating style, so I'm a bit surprised by this non-answer to a rather direct question. All you have done is offer what the speed is in one frame of reference.

Just so we are clear, in the context of Special Relativity, the speed of light doesn't concern itself with whatever medium you might conjure up that it is propagating through. It also doesn't concern itself with the various scattering theories.

I framed my question in a basic way... a basic answer will do.

D

#### derekmcd

##### Guest
mental_avenger":cqacgc4b said:
bowman316":cqacgc4b said:
Its just like passing a car going past you in the opposite direction. Both cars are going 50 mph, but to you, it looks like the car passing you is going 100 mph. But it is still only going 50. It does not matter what the perceived speed is, that is not the actual speed.
Your example is not commensurate with the situation in this discussion. A more applicable example would be two cars going from town A to town B. The first is traveling at 10mph. When it is about halfway there, a second car leaves town A and heads towards town B at 100mph. When it passes the first car, it is going 100mph relative to town A and to town B, but it is going 90mph relative to the first car.

Replace the second car and 100 mph with a photon and C respectively, do you get similar results? Does the reference frame of the first car see the photon as travelling at 10 mph less than C?

Again, a simple question. A simple answer will do. A direct answer here would answer my question in the previous post and would assist me in try to understand you point of view concerning time dilation.

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