Exploring Relativity: Light Propagation Opposite to Movement

[Alle_]

I have a question about the propagation of light, I am not that technically versed in all the matters involved so it is a philosophical question.

The principle of relativity, deducible from Newton's laws, has long been considered verified experimentally, most famously by Michelson-Morley's interference experiment and consequential hypotheses. It's consideration as a fundamental principle of nature, and not only for material bodies, is a core part of contemporary mechanics. So that in uniform motion, if carried along in this motion there is no direct means of discerning it.

Further, mass can move no faster than the velocity of light. This on account of the electromagnetic origin of kinetic energy, and that electromagnetic waves have inertial mass.

This imposes on us the understanding that the velocity of light emitted in any uniformly moving body will as viewed with regard from that body as well as from any containing body be the same. Or at the very least the same in all bodies and never exceeding the a maximum viewed from any. It is this which prompts my question. I can grasp that a ray of light emitted in the same direction as the translation of the body will fulfill this as a result of contraction and slower time, similarly my mind can be satisfied with an explanation based on time for the same results for rays emitted perpendicular to the translation. What has me wondering is how the two preceding paragraphs can be fulfilled when light is emitted in the direction opposite to translation. This primarily stems from my view that it should have the same velocity viewing from the body with translation as from containing bodies, clearly contraction and slower time will not enable this but rather do the opposite. One possible explanation is that it simply will appear slower from "upper" bodies. I shall not be too much at peace with that, but is there empirical verification of this? For on second thought it would not directly violate the paragraphs above. But I am inclined to think it would violate something else which I can not put my finger on, if its velocity was different from different perspectives.

In summary my question is how an emitted ray of light in direction opposite to the movement of a translating source will appear from an external body.

I realize that this can spur a discussion, please try to stay on topic and be as direct as possible with answers, even if they are in the negative or correcting anything in the question.

 

[Doc Al]

In summary my question is how an emitted ray of light in direction opposite to the movement of a translating source will appear from an external body.

The light moves with the same speed in any frame regardless of the motion of the source or the direction in which the light is emitted.

 

[granpa]

Time dilation:
[URL]http://home.earthlink.net/~parvey/sitebuildercontent/sitebuilderpictures/time_dil.gif[/URL]

no length contraction:
[URL]http://home.earthlink.net/~parvey/sitebuildercontent/sitebuilderpictures/length_con1.gif[/URL]

with length contraction:
[URL]http://home.earthlink.net/~parvey/sitebuildercontent/sitebuilderpictures/length_con2.gif[/URL]
notice the need for Relativity of Simultaneity.the above was originally posted at
http://www.sciforums.com/showpost.php?p=2616523&postcount=3

length contraction and time dilation are easy. what confuses all beginners is 'relativity of simultaneity'.

if you are a beginner and you are confused then there is a very good chance that it is 'relativity of simultaneity' that is responsible.

http://en.wikipedia.org/wiki/Relativity_of_simultaneityremember:
the length of an object in a certain frame is the distance between the front and the back of the object at one simultaneous moment.

hope this helps

 

[Alle_]

^

I am assuming this. I suppose you answered that summarised question, but it may have been a bit flawed in formulation. The wondering is how this happens without contradictions. Contraction in direction of movement and slower time will not be enough to explain this without contradiction, as it can for the other directions I brought up.

Also, is there empirical verification of this? Has a ray of light been emitted from a body at very high speed and its velocity within that body measured as viewed from an external one? It certainly would be in a way satisfactory if there is, and if there is an explanation of it. In case of if it does not I would need to consider if it can be allowed for an alternate explanation which also agrees with observations so far.

hope this helps

I will take a look at that. Thank you.

 

[Doc Al]

I am assuming this. I suppose you answered that summarised question, but it may have been a bit flawed in formulation. The wondering is how this happens without contradictions. Contraction in direction of movement and slower time will not be enough to explain this without contradiction, as it can for the other directions I brought up.

If you want to understand how light travels at the same speed in all frames in terms of length contraction and time dilation, you must also consider the relativity of simultaneity (as granpa's post points out). Length contraction and time dilation alone are not enough. (Of course, these effects of relativity are deduced from the fact that light speed is invariant.)

 

[Alle_]

hope this helps

I do not think there is something new for me there, also the visualisations depend on light traveling in both directions, not in just one without reflections. I am considering a case without reflection back and forth.

As importantly can someone refer me to accounts of empirical verification of the velocity of light being the same viewed from different frames.

 

[Mentz114]

In summary my question is how an emitted ray of light in direction opposite to the movement of a translating source will appear from an external body.

This is not well posed. What does 'appear' mean ? Where is the external body ? If the light is received by a receeding observer it will appear red-shifted, it received by an approaching body it will be blue shifted. We can't see light in the same way that we see objects, i.e. by capturing light reflected off the object, so it's important to be operationally precise in formulating the question.

As importantly can someone refer me to accounts of empirical verification of the velocity of light being the same viewed from different frames.

No, but check out the FAQ if you haven't done so.

From Maxwell's equations we know that c (in vacuo) is a function of the permittivity and permeability of empty space, so there's no reason to expect any observer to measure a different value.

 

[Ich]

I do not think there is something new for me there

Sorry, Alle_, you're wrong. Read this part again:

if you are a beginner and you are confused then there is a very good chance that it is 'relativity of simultaneity' that is responsible.

This chance is somewhere around 100%.

Contraction in direction of movement and slower time will not be enough to explain this without contradiction, as it can for the other directions I brought up.

So you realize that there's something missing, granpa points you exactly to this something, and then you claim there's nothing new for you here.

Further, don't think about "alternate explanations" unless you have at least a vague idea how the standard explanation is supposed to work.

 

[Dale]

Hi Alle_, as mentioned by granpa and Ich and Doc Al what you are missing is the relativity of simultaneity. It is the single most difficult concept for students of relativity to learn.

 

[Alle_]

This chance is somewhere around 100%.

So you realize that there's something missing, granpa points you exactly to this something, and then you claim there's nothing new for you here.

Further, don't think about "alternate explanations" unless you have at least a vague idea how the standard explanation is supposed to work.

I appreciate interest being taken in answering this question. For some time it has been on my mind, the question for a solution/answer to the velocity of light emitted in this way being the same viewed from the emitting body as observed from an external body. To be more specific per the above request by a different poster; a body passing by another one in a given axis of choice with a very large difference of velocity in that axis between the two bodies, where one of the bodies is in translation with regards to the coordinates and this body emits a ray of light in the direction of the axis opposite to that in which it moves, let's say, at the moment of passing the with regards to the coordinates stationary body.

I have postponed asking it whilst reading up more on the issue, but have not found a solution contained in those works. So I am resorting naturally to questioning for evidence and considering alternate explanations which would not contradict the principle of relativity and the maximum velocity of light.

I shall not be discouraged from ever giving due consideration to alternate explanations since questioning and considering alike is very important. Science cannot be a dogma.

If I have not understood, I would appreciate if someone clearly explained it in clear words to me. That is explain the solution to this apparent contradiction. That is in fact the reason I asked the question to begin with. Or you could maybe give me a clue, if you know the answer, I would not be here if I did not consider I need that.

Empirical observation is the only definite way to ascertain something in science, why reference to equations is not verification. This has to do with the fundamentals of how science is cultivated and i.e. what it relies on, which would be off topicto discuss here. Further Maxwell's work does not at all depend on empty space, he considered a medium for electromagnetic perturbations and this implies that it should be the same under such conditions, meaning with the principle of relativity taken into account it should be the same for all uniformly moving bodies with respect to the concerned variables, such as time. There are several conceivable solutions that would satisfy these conditions, other than seeing it the way you do. It doesn’t matter if such a medium exists and if it does that it corresponds to our individual images of it or not, it is enough that something with those expected resulting properties can be outlined.

Someone also noted difficulties of measuring velocity of light, that is certainly an important point and I have considered its implications. That is if there can be any means of us discerning this anyhow until it has left the emitting body with no means of again interacting with the translating body. For example, how can we possibly measure from the external body how fast it moves in the emitting body? All such measurement needs to be connected to the emitting body through transmission of our measurement and will be affected by this. Even if the answer to this would be in the negative, it does not make it any less important for explaining without contradiction how it does occur, knowledge that is important for further work. From the above it seems the times measured could correspond to the same velocity in all frames, but qualitatively it would still be contradictory and it would also allow for other solutions that are just as compatible with all observations and not having these contradictions. Physics is not Mathematics, however unseparable they may be, it is not only about relations of quantities but also about the procedures and circumstances of phenomena. A mathematical explanation may describe the relations between quantities just well but not give a physical explanation. It is important to distinguish between description and explanation in physics.

Someone said that it “should” according to Maxwell’s equations. The question is, does it? Further the question is even can it be concordant with the equations but still be under certain circumstances different from different frames (but not within them), always slower than the velocity of light from all frames, but without necessarily a means for us discerning it. There are such possibilities. There is no reason why it should not be considered. Even though I have for some time been very reluctant to this, not being able to explain it otherwise is what drives me towards this perspective.

If there is still some simple solution I have overlooked, please do enlighten me, I am not here to "argue" (not that this is "arguing"), I am in a search for a solution, and I am so for I have not been able to come about it on my own.

You can say “look here and look there” but it is likely I already have, and that alone is no additional clue.

Be that as it may, based on what has been posted, for you who do have full clarity about this, I think that if nothing else this post should pretty much have isolated where any relevant lack of understanding lies, and you ought to be able to alleviate this.

Also thanks for the point of red shifted and blue shifted light, since I also had a question of the wave frequency as it relates to this, but was not at a stage where I would post a question about that.

 

[Dale]

If there is still some simple solution I have overlooked, please do enlighten me

The solution is:
Relativity of simultaneity
which has already been mentioned at least 5 times. Hopefully the large font helps you avoid overlooking it a 6th time.

 

[Alle_]

The large fonts are rude. The question then is how is this a solution. Care to give a clue as you apparently know why the question arises?

 

[granpa]

the length of an object in a certain frame is the distance between the front and the back of the object at one simultaneous moment.

velocity = distance / time

 

[Mentz114]

Alle, as I see it, you can accept that an approaching oberver will measure the same velocity for the light ( as predicted by SR) but you can't accept that a receding observer will do the same. You say you are using 'time dilation' and 'length contraction', but as you've been told, you also need to take into account the change in simultaneity when you change frames.

The three diagrams show a light beam passing through an approaching and receding observer from the three points of view. It's obvious that in both the measuring frames, ( 2nd and third pic) they will get c=1. But notice that the times the observers see between events is different for different frames. This has to be taken into account ( as it has been by the Korentz transformation).

The diagrams are accurate Lorentz transformations of each other.

 

[Alle_]

Alle, as I see it, you can accept that an approaching oberver will measure the same velocity for the light ( as predicted by SR) but you can't accept that a receding observer will do the same. You say you are using 'time dilation' and 'length contraction', but as you've been told, you also need to take into account the change in simultaneity when you change frames.

The three diagrams show a light beam passing through an approaching and receding observer from the three points of view. It's obvious that in both the measuring frames, ( 2nd and third pic) they will get c=1. But notice that the times the observers see between events is different for different frames. This has to be taken into account ( as it has been by the Korentz transformation).

The diagrams are accurate Lorentz transformations of each other.

Ok. I understand the diagrams, I need to give it some consideration. I may have driven the question bit far, so before posting more (such as thoughts on granpa's reply) I will first give this my thorough consideration.

 

[ghwellsjr]

Whenever you want to measure the speed of light, you must set up an experiment that measures a round trip. That means putting a mirror a fixed, known distance in front of you, for example. Then you wait for a flash of light coming from a source behind you. As soon as the light reaches you, you start your timer. Now you wait for the light to travel to the mirror and reflect back to you, at which point you stop the timer. You calculate the speed of light as twice the measured distance from you to the mirror divided by the time you measured on your timer.

It turns out that it won't matter which direction you do this experiment in or whether the light source is moving with repect to you or how fast you are moving with respect to the ground or anything else, as long as you are not accelerating.

You first expressed concern about how you could get the same answer for the measured speed of light if you were moving in one direction as opposed to the opposite direction (or if the light was moving in the opposite direction). This is easy to understand, once you realize that whenever you measure the speed of light, you must measure a round trip, which means the light has traveled in both directions, that is, in one direction and then in the opposite direction. It is the average "speed" that you are measuring. It is impossible to measure the speed of light in only one direction.

Futhermore, if you and I both want to measure the speed of the same flash of light and I am moving with respect to you, we both set up the exact same experiment but you have to realize that I need to set up a mirror that is moving along with me at a fixed, measured distance from me and I have my own timer. Let's say that we both detect the flash when we happen to be located at the same place and we start our timers together. But then I (and my mirror) continue to move away from you (and your mirror). When the light reflects off your mirror and when the light reflects off my mirror may be at different times (we can't know) and it doesn't matter. What is important is that some of the light will reflect off your mirror and come back to you and another portion of the light reflects off my mirror and comes back to me. When I stop my timer, I will be in a different location from when you stop your timer and we may stop our timers at different times (we can't know) but when we both calculate the speed of light, we will get the same answer.

All the above is what actually happens when the experiments are performed and has nothing to do with the Theory of Special Relativity. Prior to Einstein, scientists were trying to explain these results in terms of an absolute stationary medium in which light traveled and when the experiment was moving with respect to that medium, the measured distance from the timer to the mirror would get closer together and the timer would run slower. Einstein said, in effect, everyone (who is not accelerating) is stationary with respect to the medium. You will note that this is ridiculously impossible and cannot possibly be true which is why no one else suggested it. But it turned out to be a very useful concept and from it flows the entire Theory of Special Relativity.

 

[Dale]

The large fonts are rude.

So is ignoring 4 different experts all telling you the same thing. If you will read the small font a little more carefully then I won't use the large font.

The question then is how is this a solution. Care to give a clue as you apparently know why the question arises?

Certainly. Remember that the speed of light is equal to the change in coordinate position divided by the change in coordinate time. The time dilation formula only applies in the special case when the two events (emission and detection) are co-located in one of the reference frames. In other cases, where the two events are at different locations in each reference frame, the full Lorentz transform equation must be used, which includes the relativity of simultaneity. This effect systematically offsets clocks at different locations by different amounts such that the change in coordinate time is different from what you would expect by the time dilation formula alone. This causes the difference in coordinate time to match what is expected for light to travel at c.

If you would like a more mathematical discussion I certainly can provide that, and it may be much clearer.

 

[Alle_]

Of the thoughts I have had I will restrict myself to posting some of the conclusive ones. Algebraic demonstrations left out.

The geometric diagrams offer a different view which I have taken care to adopt to see if considering this in a different perspective may lead to understanding a solution. In a way it does solve the contradiction posed, but only by introducing a different contradiction. Meaning that either the same fundamental contradiction remains or there is still the same issue being overlooked.

Let’s fix our coordinates to a third body which is parallel to one of the other two bodies and in the same velocity in the only axis of movement. We may idealistically assume for our purposes that we have a complete overview of everything at every instant from this body of observation. From this third body we get our measurement and transform what is happening to the other two bodies. My earlier formulations of the question corresponds to having this third body parallel to the receiver, so the emitter is moving away in front of it.

If we reposition our coordinates parallel to the moving, emitting body, and view this such that a receiving body is receding from the emitting body, it may appear at a glance to solve it. But it does not; the contradiction here becomes that from the one body the other will be larger stretched with regards to itself, whereas from the other one it will be vice versa. Thus we have not solved this without introducing a different contradiction. They cannot both be longer than the other; or rather they cannot both contract with regard to the other.

This however, I now understand, is the “standard explanation” which entails to there being a contraction of both bodies with regards to the other. The contradiction I started this thread about is associated with and in practice, physically, the cause of the indeterminate factor which was used by Lorentz in the different revisions of his hypothesis. The solution, arrived at by using Poincaré's Lorentz group, offers no explanation and whether or not it is truly correct, that is corresponds to the truth, would in reality matter little to the results we hitherto use from Lorentz hypothesis.

My understanding of a contradiction is then fundamentally correct, and it is long known by those who have considered it by varying means. What does this imply? To some it implies “acceptance”, for they are content with prioritising learning an accumulation of facts itself with little worry about the cultivation of science. To me it implies examining the basis upon which this apparent contradiction arises (for surely there can’t be a real contradiction), and clearing that up. It prompts a question which has no certain solution. In other words and for a more leisurely way of looking at that; such cases are presentations of “career” opportunities.

If there are two hypotheses for the process by which this occurs, neither being proven nor disproven, one having contradictions and the other being free of them. It says itself which one should be considered first. Avoiding contradictions is important to take into consideration when forming hypotheses, but hypotheses are improved over many revisions and it is not possible to explain everything at once. Mechanics is far from complete, and it is a great thing of science that it is constantly imposing on us the fact that we do not know everything yet.

This contradiction can be resolved.

It is naturally still possible that there is something more to learn which will solve the contradiction, at any rate this thread has fulfilled its purpose as I have arrived at an answer to my question. Thanks for contributions.

"Mathematics are sometimes a nuisance, and even a danger, when they induce us to affirm more than we know" - Poincaré, H.

 

[ghwellsjr]

Let’s fix our coordinates to a third body which is parallel to one of the other two bodies and in the same velocity in the only axis of movement. We may idealistically assume for our purposes that we have a complete overview of everything at every instant from this body of observation. From this third body we get our measurement and transform what is happening to the other two bodies.[/I]

You can assume any other frame of reference (it doesn't matter if there is a body there or not) and analyze your scenario from that point of view but you cannot make any new kinds of measurements that help explain what is really happening to the other two bodies. If you see any contradiction between the first two bodies when analyzing from either of their frames of reference, you will just get what will appear to you as another contradiction when going to a third frame of reference.

What you need to do is focus on just one frame of reference at a time. Do all your analysis in that one frame of reference and don't concern yourself with other frames of reference. Then you can start over if you want and analyze the same situation from the point of view of another frame of reference and analyze everything from that point of view. Don't try to compare the result of the first observer in his frame of reference with the result of the second observer in his frame of reference, unless you are talking about events which are things that happen at a single point in space at a single point in time.

 

[ghwellsjr]

If there are two hypotheses for the process by which this occurs, neither being proven nor disproven, one having contradictions and the other being free of them. It says itself which one should be considered first. Avoiding contradictions is important to take into consideration when forming hypotheses, but hypotheses are improved over many revisions and it is not possible to explain everything at once. Mechanics is far from complete, and it is a great thing of science that it is constantly imposing on us the fact that we do not know everything yet.

This contradiction can be avoided.

I presume that one of the two hypotheses that you are talking about is Special Relativity and that you believe it has contradictions but what is the other hypothesis that is free of contradictions?