Is the speed of light truly constant in all directions?

[seb7]

Something that's been bugging me, but unable to find an answer for..

The speed of light (in a vacuum) is absolute, well not really, its relative to the space/time its in. ie. The distant galaxies are moving away from us, but their local light speed is relative to their space/time.

So since the Earth is moving through space (around the Sun, around the galaxy etc), does light on Earth appear to travel faster in one direction than the other? Has anyone made a device for measuring the speed of light in different directions, as to show how much faster, and which direction? and how would it read as the device is taken across and out of solar system? Wouldnt it be useful for measuring spacetime.

 

[Bandersnatch]

You're basically talking about Michelson-Morley eperiment from the early 1900s. It found out that light speed doesn't depend on the choice of reference frame, and is indeed always the same for all observers, which later lead to the development of Relativity.

 

[seb7]

Thanks Bandersnatch, Reading up his experiment, yes, but his apparatus would not have been accurate enough as to be able to detect it. Today's atomic clocks shouldn't have too much trouble in getting fast/accurate measurements.

 

[Bandersnatch]

his apparatus would not have been accurate enough as to be able to detect it.

It only had to be accurate to 1 part in 5000 to detect the anisotropy due to Earth's orbital motion.

Here's a recent measurement accurate to 1 part in 10^17:
http://arxiv.org/abs/1002.1284

 

[seb7]

I'm not totally sure I understand that experiment, but if its like the original comparisons of resonance frequencies, I wouldn't expect it ever to show anything abnormal, as the thesis assumes c can't deviate upwards.

If c is truly absolute, surely you can use that fact to measure speed of the apparatus through spacetime?

 

[Dale]

If c is truly absolute, surely you can use that fact to measure speed of the apparatus through spacetime?

That makes no sense whatsoever. How could you possibly use a frame invariant speed to measure the "speed through spacetime"? Let alone "surely".

 

[seb7]

might be hard to explain. here's an example:

I'm in the centre of a carriage of a moving train, say its traveling at 100kph. I'm stood shining a touch in front and behind me. Lightspeed isn't relative, thus shining it forward doesn't create a lightspeed of c+100kph, its still travels at c. So then it should take longer to reach the front of the carriage then it takes when shining it to the back of the carriage. Thus, you should be able to calculate the speed of the train, by measuring these time differences. (though only a slight difference, approx.1/11000000)

 

[Nugatory]

might be hard to explain. here's an example:

I'm in the centre of a carriage of a moving train, say its traveling at 100kph. I'm stood shining a touch in front and behind me. Lightspeed isn't relative, thus shining it forward doesn't create a lightspeed of c+100kph, its still travels at c. So then it should take longer to reach the front of the carriage then it takes when shining it to the back of the carriage. Thus, you should be able to calculate the speed of the train, by measuring these time differences. (though only a slight difference, approx.1/11000000)

Google for Einstein's thought experiment on relativity of simultaneity; "einstein train simultaneity" are good keywords for this search.

To perform this experiment you have to have synchronized clocks at both ends of the train. The measurement you're describing won't give us the speed of the train through spacetime, it will give us the speed of the train relative to an observer for whom the two clocks are synchronized.

 

[ghwellsjr]

Something that's been bugging me, but unable to find an answer for..

The speed of light (in a vacuum) is absolute, well not really, its relative to the space/time its in. ie. The distant galaxies are moving away from us, but their local light speed is relative to their space/time.

So since the Earth is moving through space (around the Sun, around the galaxy etc), does light on Earth appear to travel faster in one direction than the other? Has anyone made a device for measuring the speed of light in different directions, as to show how much faster, and which direction?

No, no one has made such a device. If some one could, Special Relativity would be proved invalid.

There are two types of devices that can be made:

1) A device that measures the total time for light to traverse a distance to a reflector and back again. Note that this device cannot determine how much time it took for the light to get to the reflector compared to how much time it took for the light to get back. This device has only one clock and one ruler. When we divide twice the distance by the time interval, we always get the same answer, no matter which direction we point the device in.

2) A device that compares the speed of the light from two distant relatively moving sources to see if different light traverses the same distance in the same amount of time. Note that this device is not measuring the speed of light, only determining if one light is faster than the other. This device always determines that the light from two different sources takes the same amount of time to traverse a given distance.

and how would it read as the device is taken across and out of solar system?

Since your device cannot be made, then your question doesn't have an answer.

Wouldnt it be useful for measuring spacetime.

Same answer.

The fact that we cannot measure light the way you want us to is the reason that Einstein could postulate that the time it takes for light to traverse to the reflector is the same as the time it takes for the light to traverse back from the reflector. He uses this to define remote time. You should read the first couple sections of his 1905 paper.

 

[seb7]

Nugatory,

it will give us the speed of the train relative to an observer for whom the two clocks are synchronized

If the clocks were previously sync'd to each other while together, then separated to each end of the carriage? How would different observers result in different readings? Sorry, if I sound stupid.

ghwellsjr, on your second point, so a tube of fixed distance, c would always be constant? No matter if the tube was moving? I can understand it would when measured A-B-A, but not A to B.I will do some more reading.. thanks for the replys.

 

[jbriggs444]

If the clocks were previously sync'd to each other while together, then separated to each end of the carriage? How would different observers result in different readings?

That's "slow clock transport". The difficulty is that one clock is moving "upstream" (from the point of view of the train station) and that one clock is moving "downstream". Accordingly, they are time-dilated to different extents. When they reach the ends of the train, this will have caused them to go out of synchronization (from the point of view of the train station).

The clock at the front of the train will read slow. The one at the back of the train will read fast.

 

[PAllen]

Nugatory,

If the clocks were previously sync'd to each other while together, then separated to each end of the carriage? How would different observers result in different readings? Sorry, if I sound stupid.

ghwellsjr, on your second point, so a tube of fixed distance, c would always be constant? No matter if the tube was moving? I can understand it would when measured A-B-A, but not A to B.


I will do some more reading.. thanks for the replys.

Imagine someone in a train moves two synchronized clocks from the center to the ends of a train. Imagine someone relative to which the train is moving, moves two synchronized clocks a distance apart equal to the rest length of the train. Each such observer finds that their moved clocks are still in synch (per the Einstein synchronization convention). However, each finds that the other's clocks have become out of synch in the process of being moved.

Of additional interest, if a third clock remains in the center for each observer, then each observer finds the moved clocks to be NOT in synch with the clock that never moved.

These affects are all small for ordinary speeds, but all of these effects have now been observed directly with ultra-high precision atomic clocks at ordinary laboratory speeds.

 

[seb7]

Thinking about it, the clocks don't even have to be sync'd. Measurements could be done, then put the train in reverse as to take comparison readings.

 

[Nugatory]

Thinking about it, the clocks don't even have to be sync'd. Measurements could be done, then put the train in reverse as to take comparison readings.

Then the clocks at the ends have to be synchronized with the clock at the middle; otherwise we have no way of calculating the start and end times of the light flight.

You're posting quickly enough that I'm not sure that you have googled for, read, and understood that relative of simultaneity thought experiment yet. That is a necessary prerequisite for understanding your scenario.

 

[seb7]

(..still reading..!). Though everything I've read so far is what I already understood.
Yep, you wouldn't be able to know the start and end times, but you could still measure the relative changes in length of time (caused by the training moving forward or backward)

 

[ghwellsjr]

ghwellsjr, on your second point, so a tube of fixed distance, c would always be constant? No matter if the tube was moving? I can understand it would when measured A-B-A, but not A to B.

On my second point, I didn't say that the speeds of the two lights were being measured. I said they were being compared.

 

[seb7]

Hi George, yes, though comparing two A to B, or comparing two A to B to A?Im halfway through Einstein 1905 ON THE ELECTRODYNAMICS OF MOVING BODIES. I agree with his assertions about time syncing at different coordinates in different motion states, but I disagree as to his reasons why. c can play tricks with observable time, and in his correct observations of this, he's seemed to made wrong conclusions and now confused c with time itself, then goes on stacking up other examples all based on his flawed conclusion. Think I need to read it from the beginning again!

 

[dauto]

Hi George, yes, though comparing two A to B, or comparing two A to B to A?


Im halfway through Einstein 1905 ON THE ELECTRODYNAMICS OF MOVING BODIES. I agree with his assertions about time syncing at different coordinates in different motion states, but I disagree as to his reasons why. c can play tricks with observable time, and in his correct observations of this, he's seemed to made wrong conclusions and now confused c with time itself, then goes on stacking up other examples all based on his flawed conclusion. Think I need to read it from the beginning again!

So you think Einsteins conclusions were flawed. You're not alone. Many people did. But he was proved right by experiment. You don't seem to grasp the meaning of the principle f relativity. It basically states that there is no such a thing as an object's absolute speed through spacetime. You're not moving through space time.

 

[ghwellsjr]

Hi George, yes, though comparing two A to B, or comparing two A to B to A?

Comparing two A to B.

Im halfway through Einstein 1905 ON THE ELECTRODYNAMICS OF MOVING BODIES. I agree with his assertions about time syncing at different coordinates in different motion states, but I disagree as to his reasons why. c can play tricks with observable time, and in his correct observations of this, he's seemed to made wrong conclusions and now confused c with time itself, then goes on stacking up other examples all based on his flawed conclusion. Think I need to read it from the beginning again!

You should also read the wiki article on the one-way speed of light.

 

[Dale]

might be hard to explain. here's an example:

I'm in the centre of a carriage of a moving train, say its traveling at 100kph. I'm stood shining a touch in front and behind me. Lightspeed isn't relative, thus shining it forward doesn't create a lightspeed of c+100kph, its still travels at c. So then it should take longer to reach the front of the carriage then it takes when shining it to the back of the carriage. Thus, you should be able to calculate the speed of the train, by measuring these time differences. (though only a slight difference, approx.1/11000000)

Sure, you can do this. You don't even need light, just anything that propagates with an isotropic speed in a given reference frame. This has nothing whatsoever to do with measuring speed wrt spacetime, only speed wrt the given reference frame in which the train is moving.

We should clear up some terminology which I am afraid may be getting in the way. The speed of light is called "invariant", which means that its value is the same in all reference frames. When the value of something depends on the reference frame then it is called either "frame variant" or "relative". The word "absolute" is rarely used scientifically in this context, and its usual connotation is that it is the value in the "one true reference frame" which is also commonly known as the "aether frame". Since there is no experimental evidence for the "aether frame", despite a lot of effort to find it, it is a discarded concept except by novices and crackpots.

 

[seb7]

spacetime detector..

Has this device been made before? Tube of fixed length with light source and light receive, each has its own clock. These are accurate clocks, but not required to be synchronize. We measure the time light left, time light received. Then comparing the differences when the device is in different states of motion. eg. take measurements on a train, then rotate the device 180 degrees and take second measurements.

I've found similar experiments, but always with synchronized clocks, but I'm proposing not using synchronized clocks, as we are comparing time differences.

Would we still see no time variations from different states of motion?

 

[seb7]

Sorry, I've just noticed the page 2 replies! thank you.

yes, I'm trying to measure the aether frame. I can see flaws in every experiment I've read that supposed to disprove the existence of an aether frame.

 

[Dale]

I can see flaws in every experiment I've read that supposed to disprove the existence of an aether frame.

You are kidding yourself. Even if you do NOT assume the postulates of relativity you can deduce the Lorentz transform to within about 1% with just three experiments.

http://authors.library.caltech.edu/11476/1/ROBrmp49.pdf

 

[Dale]

I've found similar experiments, but always with synchronized clocks, but I'm proposing not using synchronized clocks, as we are comparing time differences

If you are only interested in time differences then synchronization is not relevant. If having them not synchronized is fine then having them synchronized wouldn't be a problem either.

 

[ghwellsjr]

Has this device been made before? Tube of fixed length with light source and light receive, each has its own clock. These are accurate clocks, but not required to be synchronize. We measure the time light left, time light received. Then comparing the differences when the device is in different states of motion. eg. take measurements on a train, then rotate the device 180 degrees and take second measurements.

I've found similar experiments, but always with synchronized clocks, but I'm proposing not using synchronized clocks, as we are comparing time differences.

Would we still see no time variations from different states of motion?

It doesn't matter whether or not we see time variations from different states of motion because when you accelerate two clocks, you change their relative timings so your experiment is flawed.

 

[ghwellsjr]

Sorry, I've just noticed the page 2 replies! thank you.

yes, I'm trying to measure the aether frame. I can see flaws in every experiment I've read that supposed to disprove the existence of an aether frame.

There is no experiment that can disprove the existence of an aether frame, assuming that it is just like any inertial frame defined according to Special Relativity. The problem is that any inertial reference frame could be the presumed aether frame, so how would you know which one it was?

 

[seb7]

George, yes, been reading about one-way speed of light.

DaleSpam, Robertson doesn't really add anymore substance to the same ideas/conclusions of what Einstein and Michelson-Morley wrote.

I've read quite a lot of material over the last 24 hours, and it seems the existence of aether seems to have been disproved based on a handful of bad experiments. I don't believe you can disprove it based on any two-way speed of light experiment, or one-way but using one motion, or without a clock at both light source and light sensor, or by any doppler effects.

Sorry if I'm being a pain in the ***! Its not intentional, honest.

Wish I had access to some equipment to run my own experiments, or read of experiment that I could say yes, that would prove I'm wrong!

 

[Nugatory]

I've read quite a lot of material over the last 24 hours, and it seems the existence of aether seems to have been disproved based on a handful of bad experiments. I don't believe you can disprove it based on any two-way speed of light experiment, or one-way but using one motion, or without a clock at both light source and light sensor, or by any doppler effects.

It's impossible to prove that something doesn't exist.

What we have demonstrated is that there is no observed phenomenon that it is consistent with the existence of an ether but not special relativity; or that is more easily explained if we assume that there is an ether than if we assume there is not.

This discussion is somewhat sterile except when you or someone else proposes an experiment that will have a different result if there is or is not an ether. So far, all such experiments have produced results that are consistent with either of two possibilities:
1) There is no ether.
2) There is an ether, but it has properties that are different than we hypothesized in the experimental design; in particular, it does not have whatever property we hypothesized would be observed in our experiment.

 

[russ_watters]

yes, I'm trying to measure the aether frame. I can see flaws in every experiment I've read that supposed to disprove the existence of an aether frame.

It would be of help to you if you accepted the fact that scientists are not idiots and you can't possibly have found, in 24 hours, a flaw in the work of millions of hours.

 

[Dale]

DaleSpam, Robertson doesn't really add anymore substance to the same ideas/conclusions of what Einstein and Michelson-Morley wrote.

Sure he does. You are reading from a very biased perspective if you don't recognize that.

I've read quite a lot of material over the last 24 hours, and it seems the existence of aether seems to have been disproved based on a handful of bad experiments.

The problem isn't the disproof of the aether, the problem is the lack of any proof of the aether. Despite many scientists performing many experiments designed to detect it they have all failed. If it cannot be detected by any experiment then you will never make a wrong prediction about any experiment by assuming it doesn't exist.

If you wish to believe in it you are free to, provided you believe in an aether which behaves exactly the same as though there were no aether.

Sorry if I'm being a pain in the ***! Its not intentional, honest.

Being a pain isn't a problem, but on this forum experimental evidence and professional scientific publication are the rule. You cannot simply dismiss the preponderance of evidence, such as the Robertson paper, simply because it doesn't fit with your agenda.

Here is another good review: http://www.edu-observatory.org/physics-faq/Relativity/SR/experiments.html

The evidence in favor of SR is overwhelming. Not just a "handful of bad experiments" but more than a century of the most precise and careful experiments ever devised.