Comparative speed of moving vs unmoving

[gonegahgah]

I've done a little diagram to hopefully represent my question.

I've represented three different sources on three moving vehicles:
Source 1: cannon (emitting cannon ball)
Source 2: horn (emitting sound)
Source 3: light (emitting light)

For the cannon the cannon ball will reach the target sooner than a cannon ball fired from a stationary cannon.
For the horn the sound will reach the target at the same time as sound from a stationary horn.
For the torch the light will reach the target (at the same time)/(sooner than) light from a stationary torch?

Could someone with enough credentials just clear up for me once and for all what the answer is for the last one? Will the light from the moving torch reach the target at the same time as the light from a stationary torch or would it reach the target sooner than the light from the stationary torch?

 

[ghwellsjr]

Same time.

 

[gonegahgah]

Cool, thx. What about if the target is moving vs not moving instead of the source - say towards the emitter?

For the cannon ball the situation remains the same.
For the horn the sound will reach the target moving towards it sooner than the target that stays still. (This is different and is due to the non-movement of the air).
For the torch the light will reach the target moving towards it (sooner)/(at the same time) as for the target that stays at the line?

Which of these is the correct one for the torch and light in this example?

 

[ghwellsjr]

Two light beams, starting at the same place but without regard to the relative motion of whatever is emitting them, will travel through space together and arrive at any location coincidentally.

 

[gonegahgah]

Thx. I'll try to understand what you are saying then?

For the second example the moving target will be at a different position than the stay put target when the light reaches the closer one so that would mean the light will reach the moving target sooner?

Is that what you mean?

 

[ghwellsjr]

The motion of the sources and targets have nothing to do with the propagation of light. But the light is traveling in a straight line so any targets, moving or not, that are closer to the source of light, moving or not, will receive the light before targets that are farther away.

 

[gonegahgah]

Thx. The reason I am asking is I am trying to work out if I have looking at it all wrong all this time and if maybe things can fall into place in a way that I can be comfortable in accepting. I'm not totally sure of this but I'm hoping.

But I'm thinking about it and I'm not sure now...
I thought maybe it had something to do with the observer - which is what is stated but it doesn't fix things the way I was hoping...

I have trouble seeing any difference between the source moving towards the target or the target moving towards the source yet they produce different transmission time results for the same initial separations.

The cannon ball example is easy and is just added velocities.
The sound example is easy and relates purely to the movement of air and the speed of sound in air. If you produce a sound in a wind it will carry faster in the direction of the wind and slower against the direction of the wind. Speed of sound is relative to the body of air.

But, light speed through vacuum is not affected by any medium or the movement of that medium; only the observer; yet the results more closely parallel the sound example and not the cannon ball example? Why? I'm confused.

If you move towards the Sun then you expect to meet younger light (time elapsed since it was emitted) then a ship that stays behind. But if you have two Sun's, one moving towards you and one staying put, then if they pass through the same point light from both will reach you at exactly the same time.

Or if we drop it to a single ship and sun example: If you move towards the Sun then you expect to meet younger light than if you had stayed put. But if instead the Sun moves towards you or not then the 'age' (time since emission) will be the same.

To me these two situations sound exactly the same but for whatever reason:
- if you are moving towards the Sun you will reach 'younger' light
- if the Sun is moving towards you then you will receive light that is the same 'age' as light from a Sun that weren't moving...

It sure has me confused? If I'm mixing something up badly can you help me?

 

[Doc Al]

I have trouble seeing any difference between the source moving towards the target or the target moving towards the source yet they produce different transmission time results for the same initial separations.

Different transmission times according to whom? Who is measuring the transmission time?

(And realize that this is different than asking which light hits the target first.)

 

[ghwellsjr]

But, light speed through vacuum is not affected by any medium or the movement of that medium; only the observer; yet the results more closely parallel the sound example and not the cannon ball example? Why? I'm confused.

The movement of light through space is not affected by an observer. We don't know how light moves through space, only that two parallel light beams or photons travel together. We have no way of tracking the progress of light. We don't know when the light reaches a given location. We don't know where the light is at any given moment in time. And the reason is that we have nothing that can travel faster than light to give us information about its whereabouts like we do for cannon balls or sound where we use lightwaves to track their progress.

So to solve this otherwise intractable problem, Einstein came up with the novel idea of rearranging our concepts of time and space so that we instead define a Frame of Reference in which the speed at which light propagates through space is c. It's a Frame of Reference in which the movement of light is now knowable, whether or not there are any observers at rest in that FoR.

 

[gonegahgah]

You're right Doc. This is the part I'm trying to think into it; how the observer can make the difference.

I'm thinking that the observer is outside the emitter and the target rather than being in either. So,

- If the outside observer is moving in step with the emitter (so that the emitter appears stationary) then the result is supposed to be the same amount of time that the light reaches the target (whether the emitter moves or not),
- or if the outside observer is moving in step with the target then the result is supposed to be 'younger' light for the target that is moving towards the emitter as opposed to if the target doesn't move.
(*I've just reread these and I may have them back to front. I'll have a rethink about them tomorrow. Maybe there is something for me understand here? Not sure yet?)

But why? I was hoping the observer's movement somehow affected the result mathematically but I'm failing to register how?

Using the observer as a mathematical foundation I would have expected similar results but in the opposite way than they appear. So I'm still confused. I'm a bit sleepy but I'll try to understand it a bit better tomorrow...
(*As mentioned I'll recheck my thinking tomorrow; maybe there is hope?)

The tricky bit may be (?) that we use light to measure light which certainly pertains to the observer and maybe that is what is confusing me... Thanks for assisting me; I'll look again tomorrow.

 

[ghwellsjr]

The tricky bit may be (?) that we use light to measure light

As I said before, we use light to measure cannon balls and sound waves but we cannot use light to measure light, we need something faster if we are going to measure the propagation of light in space but there is nothing faster and that's how Einstein comes to our rescue, by redefining what time and space are so that we can talk meaningfully about the propagation of light through space.

 

[HallsofIvy]

I have trouble seeing any difference between the source moving towards the target or the target moving towards the source yet they produce different transmission time results for the same initial separations.

The cannon ball example is easy and is just added velocities.
'
But, light speed through vacuum is not affected by any medium or the movement of that medium; only the observer; yet the results more closely parallel the sound example and not the cannon ball example? Why? I'm confused.

The "cannon ball" and "light" examples are not all that different but you are using the wrong formula.

If a vehicle is moving, relative to the ground, at speed u and you fire a cannonball forward at speed, relative to the vehicle, v, then the cannonball's speed, relative to the ground is NOT simply "u+ v", it is, according to special relativity,
$$\frac{u+ v}{1+ \frac{uv}{c^2}}$$
Of course, for normal "vehicle" and "cannonball" speeds u and v are so small compared with c that $1+ uv/c^2$ is essentially 1.

If you shine light forward, at speed c relative to the truck, by the same formula, its speed relative to the road is still
$$\frac{u+ c}{1+ \frac{uc}{c^2}}= \frac{u+ c}{1+ \frac{u}{c}}= (u+ c)\frac{c}{u+ c}= c$$

 

[gonegahgah]

Thanks Ivy. That's a cool formula. Thanks for showing it for both to clarify the similarity.

I am still confused...
There still seems to be the matter that if you, the observer, are moving with the target you will be at a closer point to the light when the light reaches you traveling at c so the final distance is shorter; whereas if you, the observer, are moving with the emitter then distance to the target remains the original distance and so will take the full amount of time to travel to the target.
For the cannon ball this doesn't matter and the final distance covered is always shorter than the original distance and so the time is shorter whether you, the observer, are moving in step with the emitter or the target.

This is why I'm trying to think how the observer fits into the light example.
If the outside observer moves in step with the target as it closes on the emitter then they will get a report of the light sooner - than if the target and emitters weren't moving - because they will move towards the light as the light travels towards them; so they meet part way.
But if the outside observer moves in step with the emitter then the light travels to the target for the full amount of time dictated by the distance but the report back of the target being hit would be affected by the movement of the observer. The report back to the outside observer now takes less time to get back because the observer is moving towards the target.

For this later case will they measure expiration of the full time for the original distance plus the time for the report to get back to the observer where they are now closer to the target because they are moving in step with the emitter. Using those relative formulas is that correct?

 

[ghwellsjr]

Up until now, you've been asking about the one-way propagation of light but now you're introducing an outside observer without making it clear where in relation to the emitter and the target he is located so it's not possible to provide you with a simple answer. Why don't you stick with the one-way propagation issues until they make perfect sense to you?

For purposes of understanding the one-way propagation of light, you can consider it to be exactly like your analogy with the propagation of sound in air. So let's work out some examples. In these examples, you and I will be the targets listening for the sounds from three horns, each with a different pitch so we can distinguish them.

First, let's take a situation where you are stationary with respect to the air and I am going to be moving but I will explain how later. The first horn is stationary. The second one is moving toward you and the last one is moving away from you but they are all far away from you and in line with each other and with you. It just so happens that all three horns arrive at the same location at the same time and they each emit their respective sounds. Isn't it obvious that all three sounds will travel together through the air and arrive at you simultaneously? Now suppose we repeat the experiment but this time, I'm traveling toward you from farther away than the horns and I happen to arrive at your location at just the moment that you hear the three horns. Isn't it obvious that I also will hear all three horns at the same time too? And if I were approaching you from a position closer to the horns than you are but arrived at your location at the moment you heard the horns then I would hear all three horns at the same time as you did too?

OK, got all that? Good.

Now let's repeat the whole thing but this time the wind is blowing in the direction from the horns toward you. Will that change the order in which the sounds of the horns reach either you or me? No, they will still all arrive simultaneously both for you and me, won't they? Same thing if the wind is blowing in the opposite direction or any other direction. It also won't matter how fast the wind is blowing, will it?

Now, instead of having the wind blow in different ways, let's just have you, me and the horns all moving with respect to each other, thus creating our own wind. This, again, won't make any difference, it terms of the simultaneity of the arrival of the sounds of the horns, will it? (I'm assuming that we don't go so fast as to break the sound barrier.)

This is exactly like our situation with light. When two or more light sources traveling with respect to each other emit flashes of light when they are co-located, those flashes propagate through space together and arrive simultaneously at co-located targets, no matter how the sources or targets are moving.

Please note that we are not comparing the time it takes for the sound to travel from the horns to you and me under different conditions of the wind because that will definitely affect the propagation time and we're not measuring the propagation time. We're only demonstrating that it isn't affected by motions of the emitters or targets. In the case of sound in air, we can know the actual propagation time for the sounds from the emitters to the targets because we can use light to let us know when the horns were tooted. But we can't do that with light for the same reason we couldn't use the sound waves all by themselves to measure the propagation of other sound waves.

 

[gonegahgah]

Thanks that is good. You've no-noed it but I like that idea of using sound to measure the arrival time of sound at the target as this is kind of like what I am wondering for light. The communication of arrival time has to be done by some means and generally that is emr (including current in a wire). But as you say, I should take little steps to see if I can understand this.

What you are saying is making sense; though again light doesn't have any media in a vacuum to affect its speed whereas sound does as you say. I'll give this some more thought tomorrow. Thanks ghwellsjr.

 

[ghwellsjr]

I hope you aren't thinking that we are measuring the arrival time of light. We aren't doing that. All we are doing is illustrating that the propagation of light through space is independent of the motion of the source or the destination. I haven't said anything about measuring anything. And I have specifically said that we cannot measure the propagation of light because we don't have anything faster than light with which to measure it.

 

[harrylin]

[..] I was hoping the observer's movement somehow affected the result mathematically but I'm failing to register how? [..]

When you talk about "the observer's movement", you imply "as seen by a different observer". And indeed, according to that other observer, the motion of the moving observer affects the result mathematically, because the moving measurement system is affected by motion.
However, from the perspective of the "moving" observer who assumes to be in rest, it's just the other way round.

PS: And as mentioned before, SR treats light just the same as sound in air that is in rest.

 

[gonegahgah]

That is sounding like how it works harrylin with the air/space-time being at rest relative to the observer or something like that? I think? I'm guessing?

Another question...

When you head towards a light wave you meet the light wave part way but how much time does it take to reach you. Does it take a major fraction of the time that it would have taken if you hadn't moved from your starting point; or does it take the same amount of time to reach you?

Like the cannon ball. If you move towards the cannon you will meet the cannon ball part way and the time elapsed to being hit is less than if you stayed put instead of moving towards it.

And the sound wave. If you move towards the horn you will meet the sound part way and the time elapsed to hearing the sound is less than if you stayed put instead of moving towards it.

What is the situation for a light wave? If you move towards a torch you meet the light part way but does the light take less time to reach you than if you stayed put? Or does it take the full amount of time of your original separation?

 

[harrylin]

If you still use the standard measurement system in which you were in rest also when you are moving, then light will behave just like sound in air that is in rest in that system.

However, if you immediately after you start to move towards the torch, adapt your measurement system to one in which you are then in rest, you will recalibrate and resynchronize your measurement system in such a way that you'll measure the light to move at c towards you; however the distance that the light has to travel will appear less in that system.

So, with either of these choices you will measure that the light will take less time to reach you.

 

[ghwellsjr]

When you talk about "the observer's movement", you imply "as seen by a different observer". And indeed, according to that other observer, the motion of the moving observer affects the result mathematically, because the moving measurement system is affected by motion.
However, from the perspective of the "moving" observer who assumes to be in rest, it's just the other way round.

PS: And as mentioned before, SR treats light just the same as sound in air that is in rest.

If you still use the standard measurement system in which you were in rest also when you are moving, then light will behave just like sound in air that is in rest in that system.

However, if you immediately after you start to move towards the torch, adapt your measurement system to one in which you are then in rest, you will recalibrate and resynchronize your measurement system in such a way that you'll measure the light to move at c towards you; however the distance that the light has to travel will appear less in that system.

So, with either of these choices you will measure that the light will take less time to reach you.

When you say "measurement system", do you mean anything other than what everyone else means when they say "Frame of Reference" as defined by Einstein's Theory of Special Relativity?

 

[harrylin]

When you say "measurement system", do you mean anything other than what everyone else means when they say "Frame of Reference" as defined by Einstein's Theory of Special Relativity?

No, just trying to keep it understandable for laymen :tongue2:

 

[ghwellsjr]

When you say "measurement system", do you mean anything other than what everyone else means when they say "Frame of Reference" as defined by Einstein's Theory of Special Relativity?

No, just trying to keep it understandable for laymen :tongue2:

"Measurement System" already has a standard, accepted meaning and "Frame of Reference" is not one of them. How does your non-standard use of an accepted term contribute to understanding if a laymen looks it up and believes that you mean what the reference says instead of your secret meaning known only to you?

On the other hand, if you would use "Frame of Reference" when that is what you mean, the layman can look that up and increase his understanding of what you are trying to tell him. Please try to educate laymen instead of confusing them (and the rest of us, for that matter).

 

[harrylin]

"Measurement System" already has a standard, accepted meaning and "Frame of Reference" is not one of them. [..]

I'm sure that most laymen don't know what "Frame of Reference" means - you could just as well write "cage" or "bubledidok". I also considered "coordinate system", but that's still overly technical.
I find throwing jargon at people in an explanation bad ("evil": very impolite!), except if it's absolutely necessary.

Anyway, there's no reason for second guessing, we can easily and quickly verify this and then get on with the topic!

Gonegahgah, supposing that you are not versed in these topics:
- what do you think of when you read "frame of reference" (if anything)?
- what do you think of when you read "coordinate system" (if anything)?
- what do you think of when you read "measurement system" (if anything)?

PS: I tend to think that "measurement system" means a system for making measurements... If someone thinks that that's wrong, please don't interfere with the discussion here but start a topic on it.

PS.PS: I see here below that you ignored my request not to interfere with this thread as you started to discuss here your assertion that "measurement system" cannot refer to an assemblage or set of instruments for making measurements. I will similarly ignore that interruption.

Harald

 

[ghwellsjr]

The Metric System (also known as MSI or Measurement Systems International) is a Measurement System as is the English (Imperial) System.

It doesn't really matter what you or I or gonegahgah think terms mean if we make up our own definitions. Anyone who is on this forum to learn Special Relativity needs to learn the proper terms, even if it's a challenge. It doesn't help to make it more of a challenge by using your own invented terms, especially when they already have accepted definitions.

 

[gonegahgah]

I do find using common phrases an understanding trap as I find I read them and other laypeople read them differently even though there maybe be a textbook definition/explanation of what they are used for by professional to encompass. There probably are precise terms that are used to mean a whole thing but I am only guessing what they mean? True I should probably look them up too but I don't know if they will just complicate or direct me to the answer quickly. That's why I try to look at the question in the simplest way I can imagine it so that I can try to gain some insight into something that is puzzling me.

I guess the "frame of reference" is the stationary frame of the observer?
I guess co-ordinate system again refers to x,y,z points in a frame that is stationary to the observer.
I would probably have guessed at what you explain for "measurement system".
I'm probably wrong in all cases?

My question is just to understand how light behaves in comparison to other systems.
Like for instance I'm just trying to basically understand the following:

If you are 8 light minutes from the Sun then the light should take 8 minutes to reach you if you stay at that distance shouldn't it?
If you move towards the Sun then I'm guessing that you and the next light wave will meet part of the way?
I know that this throws in acceleration and changing frames or something but the last statement would remain true wouldn't it? We can still expect to meet the next photons to emit part way can't we? We should even meet the photons that were already traveling part way when they reach us?
I'm guessing that is correct?

But I'm just asking about the time taken for the next photons to reach us.
In eight minutes (about) we can walk about a kilometre.
So we would probably meet the light somewhere at approximately (8 light minutes - 1 kilometre) distance. Is that right?

Of course the light travels much faster than we do; much faster.
At a slow walk I'm guessing there wouldn't be too much SR & GR effect? Or is that wrong?
So my question then is would we meet the next photons, to emit from the Sun, part way and still at 8 minutes time or would it be more like 7:59.99999999...9999321 minutes (so just under 8 minutes)?

We can't measure that of course but I'm just trying to understand what the understanding of this is so I can know as well?
1st part: Do we meet light part way if we move towards it? I guess yes?
2nd part: Do we meet it in the same time we would have if we had stayed still?
or, Do we meet it in less time than we would have had we stayed still?

 

[harrylin]

I do find using common phrases an understanding trap as I find I read them and other laypeople read them differently even though there maybe be a textbook definition/explanation of what they are used for by professional to encompass. There probably are precise terms that are used to mean a whole thing but I am only guessing what they mean? True I should probably look them up too but I don't know if they will just complicate or direct me to the answer quickly. That's why I try to look at the question in the simplest way I can imagine it so that I can try to gain some insight into something that is puzzling me.

I guess the "frame of reference" is the stationary frame of the observer?
I guess co-ordinate system again refers to x,y,z points in a frame that is stationary to the observer.
I would probably have guessed at what you explain for "measurement system".
I'm probably wrong in all cases?

Not too bad - of course the above discussion may have helped a little.

- "Frame of reference" can also apply to "moving" ones; probably you mean that observers who choose it take it as being "in rest". But do you know what "frame" means in this context?

- "Co-ordinate system" means indeed nearly the same as "frame of reference": a system of measurement instruments that allow to map either position coordinates or position and time coordinates.

- And as you indicated, you would probably have understood correctly that with "measurement system" I meant a system of measurement instruments for determining position and time coordinates.

My question is just to understand how light behaves in comparison to other systems.
Like for instance I'm just trying to basically understand the following:

If you are 8 light minutes from the Sun then the light should take 8 minutes to reach you if you stay at that distance shouldn't it?
If you move towards the Sun then I'm guessing that you and the next light wave will meet part of the way?
I know that this throws in acceleration and changing frames or something but the last statement would remain true wouldn't it?

As I tried to clarify in my earlier posts (please check), it does not necessarily throw in a "changing of frames". But yes, you meet the light ray at a certain point in the "stationary" coordinate system; and obviously that point is closer to the Sun if you are moving towards the Sun than if you stay at the original distance.

We can still expect to meet the next photons to emit part way can't we? We should even meet the photons that were already traveling part way when they reach us?
I'm guessing that is correct?

Sorry, what does "to emit part way" mean?

But I'm just asking about the time taken for the next photons to reach us.
In eight minutes (about) we can walk about a kilometre.
So we would probably meet the light somewhere at approximately (8 light minutes - 1 kilometre) distance. Is that right?

Roughly, yes.

Of course the light travels much faster than we do; much faster.
At a slow walk I'm guessing there wouldn't be too much SR & GR effect? Or is that wrong?
So my question then is would we meet the next photons, to emit from the Sun, part way and still at 8 minutes time or would it be more like 7:59.99999999...9999321 minutes (so just under 8 minutes)? [snip repetition]

Obviously if it would take 8.00000000..0 minutes or so to reach you at a certain point, then it will take 7:59.99999999..9 minutes or so to reach you at a point closer by the Sun. The calculations are straightforward if you stick to the same coordinate system.

Does that help?

 

[ghwellsjr]

You are wrong about all three. You should look them up instead of just assuming you understand them.

A Frame of Reference does not have to be related to any stationary observer. All observers and objects exist in all Frames of Reference.

In Newtonian physics, a co-ordinate system could refer to just the x, y and z components of a point in space but in a Frame of Reference, t (for time) is also included so there are four parameters or co-ordinates and instead of using the term "point", we use the term "event". But again, if you are talking about a frame where an observer is at rest, the x, y and z parameters are always 0 but not the t parameter. Why shouldn't it also be 0 for consistency's sake? Please don't make the mistake of thinking that a Frame of Reference is linked to an observer stationary in that frame.

When you ask questions about the one-way speed of light coming from the sun to the earth, it is because there is the assumption that a "clock" on the sun has been previously synchronized to a clock on the Earth so that the reading on the sun-clock is eight minutes earlier than the reading on the Earth clock. That's why we can say that it takes eight minutes for the light to make the trip.

If you then start moving toward the sun and you want to discuss how much earlier you encounter the photons, you can do this from the common rest frame of the sun and the earth, but according to your clock and your experience of time, you will encounter the photons in less time. If you go fast enough, you can encounter them all within any arbitrarily short period of time but of course you will encounter them in the same order in which they were emitted.

EDIT: I forgot to mention the most salient point about the difference between a Newtonian spatial co-ordinate system and an Einsteinian Frame of Reference which includes time but not just any arbitrary time, it is time in which remote clocks have been synchronized in a particular way. This is the reason why Co-ordinate System and Measurement System in no way correlate to Frame of Reference. Those do not address the issue of defining time for remote clocks which is what SR is all about.

 

[gonegahgah]

I assume that I probably don't understand them precisely.
Thanks harrylin for helping me to understand the answer I was after.
I do also appreciate what you are saying ghwellsjr. I have read the efforts that they went through to co-ordinate time.

Whether we synchronise clocks or not, I just wanted to know if physically when we move towards a light source if the time the light takes to reach us is less than if we stayed put - due to us meeting the light part way. I'm sure it has probably been experimentally confirmed by synchronizing clocks together, moving them apart, calculating for drift/shift, and then calculating back the results; but I just wanted to know what the reality was.

It was what the reality was that I was finding myself confused about. I was just trying determine what the answer was. I was starting to think that the answer was that the light would take 8 minutes (in the example) based upon the original separation no matter how you moved towards or away from the light. Which was certainly different to the cannon and the horn.

But, that is not the case, and the meeting time is less due to the meeting part way aspect. Of course things like the provided formula have to be thrown in too but I was only interested in the actual reality of the situation; not in measuring it; just to help clear up my confusion.

So now I understand that if you move towards a galaxy that you will meet the light of the galaxy part way which will reduce the time the light takes to reach you. And vice versa, if you move away from another galaxy then you will be moving away from its light as the light travels so that your meeting point will be a little further away and in correspondence the light will take a little longer to reach you.

That's all I wanted to know on this question. I think harrylin most saw that that was all I was simply after. Thanks for your clarity harrylin.