Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: jeffreyH on 24/04/2019 11:11:27
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Since motion is relative then in some frames of reference we can be considered to be moving close to the speed of light. Are we actually moving at close to the speed of light?
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Maybe "faster" as we are disappearing from view in some FOR's owing to the "pudding sponge" expansion effect.
There is also the hypothetical FOR of a tachyon....
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We can select an infinite number of reference frames between stationary and arbitrarily close to the speed of light with reference to some other object. It is not possible for us to be travelling at all those speeds simultaneously. This would mean we are experiencing an infinite number of time dilation effects all at once. With respect to everything else.
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But we are! We see red and blue shifts out there, which means that, simultaneously, other observers see us as red or blue shifted, and all shades between.
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We can select an infinite number of reference frames between stationary and arbitrarily close to the speed of light with reference to some other object. It is not possible for us to be travelling at all those speeds simultaneously. This would mean we are experiencing an infinite number of time dilation effects all at once. With respect to everything else.
That's like arguing that because different people would measure the Left-Right distance between the ends of the red line shown below as being different (as indicated by the distance between the vertical lines), depending on how the line is rotated with respect to them, that the line would have to point in an infinite numbers of direction at once.
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Time dilation is akin to measuring that Left-Right distance between the end points of the line as done from different orientations to the line..
It like the red and blues lines below comparing each other's respective "lengths" by sighting along lines perpendicular to themselves ( the thin lines in the diagram)
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The simple answer is. what do you compare it too?
The more difficult one is
can you still accelerate?
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Actually, what you ask is if there is a local speed limit
Or if it is globally defined.
Pick a sum. or measure your acceleration
I'm proud of you Jeffrey.
Took me some time to wonder about that one
And it's essential to relativity
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So. a 'golden standard'?`
Or purely local
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Jeffrey
I would call it 'local'
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What one has to remember when asking this kind of question is how we define it.
It's a local speed limit, but one that you can translate to other frames of reference.
There is a subtlety to that, purely cerebral.
the question following becomes
What does it mean?
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The point is that it either depends on your frame of reference.
Or if you still find a acceleration.
It's one of those things that made me define the universe as 'locally defined'.
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Also a question that made me take properties and conservation laws seriously :)
Weird as it might sound
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No, because light reaches us. And nothing, NOTHING is allowed to move faster than light. If we where travelling apart from our twin planet at over half the speed of light it would dissapear, but because there are two motions that does not break the nothing rule. Unless light is pre programmed somehow.
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Since motion is relative then in some frames of reference we can be considered to be moving close to the speed of light. Are we actually moving at close to the speed of light?
The question is incomplete. You need to ask "are we moving near the speed of light in as measured in frame S." where S is specified by the person asking the question. In our frame (sitting on earth) the answer is no.
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Take two objects meeting each other, both close to the speed of light. As they collide the combined 'potential energy' must become a sum surpassing the potential energy that each one of them have on their own, and that seems allowed? But that sum can't be able to surpass the sum of one of them at 'c' hitting something defined as being 'still' by the ship/object. So there must be a limit to the 'energy' released in a collision, mustn't there? Any ship defining a speed will need have to arbitrarily choose another frame of reference as its anchor of being at a 'null motion'. And as it accelerates it will be the acceleration (weight) that informs it of it still not having reached 'c'. Not saying that it can of course but just for the fun of it.
But it's also so that you can use the relative motion of Earth. Think of spinning rope with a stone resting in it to see how. Although in this case the spin is a 'acceleration'.
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another way of thinking of it is to define Earth as moving in one direction (with the solar system/galaxy/whatever) relative some other frame, then testing if that gives you a boost depending on where you place the ship, with the 'motion' or against it. As far as I get it it won't be any energy saved by that approach intrinsically. And that is the most important point of it to me. Because if you find the energy needed differing depending on choice of take off then that motion is not 'relative' anymore, at least not to me.
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Halc, it's a thought experiment. It's not meant to be 'practical', you can also consider it a exercise in logic, depending on if you accept the prerequisites. There seems to me there must be a limit for that potential energy I'm discussing due to relative motion. I call it potential because it's not locally measurable experimentally. What happens as they collide is definitely 'kinetic' though. As for when a acceleration ends if there now was such a possibility :) Nah, it doesn't, the energy needed is 'infinite' if you want to reach 'c'.
But that's not the point of the exercise.
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The last part is about what a 'relative motion' means(?) Three objects in different relative motion in a space is a proof of it being as 'real' a motion as a acceleration is. But when it comes to the idea I discuss in where you measure energy spent by your rocket, going 'against the motion' or with the 'relative motion' of Earth that you 'decided to choose', I don't expect there to be a difference. Could be wrong there but I don't think I am.
What the last part comes down to in my mind is that if there was a difference relative motion would no longer be relative. Thinking of it as 'displacements' you might reach a different conclusion, but it wouldn't be correct as I see it.
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Not when looked at from the standpoint of relativity Halc, for that you need three objects to prove it to yourself. With two objects in relative motion you're free to associate any speed you like to yourself from a total standstill to the other object being the one unmoving. But put in one more and it becomes different, then you can prove that different speeds exist even if looked at as relative. Which is weird in one way, in another perfectly fitting to the way we normally look at motion. We do find different speeds to exist every day in our life.
the first one is also about 'potential energy'. In the two body system I described the combined kinetic energy in a collision doesn't change but the 'potential energy' of one or the other becomes diffuse. It's a matter of choice, and not locally measurable.
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Actually that holds for the three body system too :)
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Not sure how you think there Halc. With two objects in relative motion you are free to associate this 'relative speed' to any of those objects. That's what changes when you introduce a third object. Even though the motion still is 'relative' for each one you now can prove that different speeds exist. May not seem important , but if you combine it with the fact that your potential energy is a fluid expression depending on frames of reference, and that you locally have no way of measure any of this possible energy it becomes interesting, at least to me :)
and it's not about accelerations at all Halc, it's about relative motion, and what it might mean. To it you can add that locally defined there is no way to differ Earths 'relative motion' from Earth being unmoving (black box scenario) the only thing measurable should be the spin. There is some subtlety to the proof in that you still will be unable to give yourself a definite 'speed', even though you now can prove different speeds to exist.
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spelling
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Maybe I should fill in some background to it. Accelerations becomes GR in relativity, 'relative motion' as in a uniform is SR. Accelerations are intrinsic, there will be a measurable force to it locally (weight by scale) also defined as 'gravity' when discussing uniform accelerations.. Relative uniform motion has no such coupling, and can't even be locally defined. And it is weird when you think of it in those terms, at least I find it so. Almost as if they were two disconnected phenomena
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I'll give you another thought experiment. Take those two rockets again, one going in the 'motion' of Earth, the other against that 'relative motion'. Then accelerate Earth at some meaningful number, to then to 'coast' again, uniformly moving. Let the rockets leave Earth. Will they measure any difference locally from the former 'lift of', before that Earthly acceleration?
Will it cost any of them more (or less:) energy?
I would say no, all uniform motions are (locally) equivalent. If they weren't we would have a way to measure it locally.
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If that is correct, how will you measure yourself gaining 'c' in your rocket?
Your relative motion (Earths) at lift of not meaning a thing?
this one included a acceleration, but for the relative (uniform) motion before and after it didn't change a thing.
Another thing worth noticing is that even though we can assume that Earths acceleration had a vector, it didn't change the way Earth in its relative motion still don't have any preferred direction, locally measured.
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A third example: Imagine a rocket constantly accelerating through a space. The idea is that as it gets closer to 'c', it will cost it more energy right, fuel spent etc? Now stop the acceleration, let it coast for a while, then start accelerating again. Will it now draw more fuel than when it once started its journey? (assume that to have been in space too)
Now, if the postulate I presented before where an acceleration didn't change a thing is correct. What do you think will happen in this circumstance?
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Now, what does this remind me of ?
'c'.
It becomes just as weird, thinking of it in those terms :) I call it 'potential energy', but then you could also include the stress energy tensor acting on you with different 'relative motions' as well as accelerations.
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So to summarize.
I can prove different uniform motions to exist.
I can not prove my own uniform motion though.
All uniform motions must be equivalent, or we would be able to measure a difference locally.
We can't
Equivalent, but still having different speeds, although not possible to define locally.
Now the question becomes, how get those to fit each other :)
Either they do, or they don't.
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Actually I don't see how to understand the stress energy tensor here? If the postulate is correct in where there is no possibility for me to measure locally, and if you simultaneously can prove different 'uniform speeds' then you either gain such or you don't. Reason seem to argue that you can, as with earths 'acceleration' in the above example, but if there is no way to define a 'cost' for those rockets? The last example is just naturally following, a extreme of it.
The other way to see it is that there will be a cost. If that is true then relative motion isn't relative anymore.
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The easiest way to prove that all uniform motions are the same (equivalent) is to use red and blueshifts. With Earth accelerating to a higher 'speed' you could expect a blue respectively redshift to show itself, and it will under the acceleration. But not before, and neither after it.
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there are some 'outlandish' possibilities to it. You might want to assume that accelerations are all there is for example. Just as some argue that a 'real' time dilation only can exist with accelerations. 'gravitational time dilation's' 'Earths spin' all fall under the category 'accelerations' in relativity.
Against that one you have the proof that different uniform motions can be proven to exist.
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Let me add one thing that I find just as weird. The ability to gravitationally accelerate to 'c'. That's how I think of something in falling towards a 'real event horizon'. That one can only be argued theoretically but presuming homogeneity and isotropy, conservation laws etc I expect it to be correct. Should have added 'symmetries' here, because that is how I look at it. It's a symmetry.
What that really say is very similar to the examples above, as you locally measured won't notice a thing (ignoring tidal forces). No acceleration involved, locally defined. No measurable 'cost' existing, locally defined. How to test that is to look for those blue respectively red shifts measuring inside your 'black box'. You will still reach 'c', or at least where 'speeds' breaks down for matter.
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This one is actually a perfect counter to anyone arguing that it is only accelerations that present 'real' time dilation's.
Why?
No 'gravity' existing either, locally defined.
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But we are! We see red and blue shifts out there, which means that, simultaneously, other observers see us as red or blue shifted, and all shades between.
I will come back to this shortly.
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A third example: Imagine a rocket constantly accelerating through a space. The idea is that as it gets closer to 'c', it will cost it more energy right, fuel spent etc? Now stop the acceleration, let it coast for a while, then start accelerating again. Will it now draw more fuel than when it once started its journey? (assume that to have been in space too)
Constantly accelerating ( by this I assume that you mean maintaining a constant value of acceleration), according to who?
If you means as measured by the ship, then the ship uses fuel at a constant rate, as measured aboard ship. For an observer measuring the ship motion from an inertial frame, the rate at which the ship burns fuel decreases as the ship gets closer to c relative to to this observer ( time dilation). This also means that the acceleration of the ship falls off as it approaches c. It never reaches c because the rate at which it approaches c decreases.
From the ship view, it is just constantly accelerating at a fixed rate. The rate at which it changes its own velocity ( relative to what it was moving just a moment ago) never changes. However, this does not mean that it can ever measure its relative velocity with respect any inertial frame as being c or greater.
Imagine it starts at Earth. it accelerates at a constant value ( as measured by itself) for a period of time until it is moving at 0.1c relative to the Earth. It drops a marker, which after it is dropped, maintains a constant 0.1c velocity with respect to the Earth. The ship continues its acceleration for a time period equal to the first. It will measure its velocity with respect to the marker as being 0.1c. However, it will measure its relative velocity with respect to the Earth as being 0.198c, not 0.2c, the velocity difference between Earth and marker will, as measured by the ship, decreased to 0.98c.
Drop another marker, accelerate to 0.1c relative to it, and the ship will measure its velocity as being 1.98c relative to the first marker and 0.292c relative to the Earth. The rocket can keep dropping markers and accelerating to 0.1c relative to the las marker it dropped for as long as it wants but will never measure its relative velocity with respect the Earth as ever reaching c.
If the Ship stops, coasts, and then start accelerating again, both frames ( ship and inertial) measure the acceleration/fuel rate) to pick up right where it was.
If on the other hand, you mean a constant acceleration relative to the inertial frame, then both ship will measure a need for the fuel rate to increase. For the inertial frame, in order to maintain the constant acceleration to c, for the ship, to constantly increase its rate of acceleration. Again, if the ship stops, coasts, and then accelerates again, the fuel rate picks up right where it left off. If the Acceleration has to be at the same value as it had before as measured in the inertial frame, it has to have the same value as before as measured by the ship.
Of course, the fuel usage of the ship doesn't automatically increase, the occupants would have to deliberately ramp up their engines. They would have to work out just how much they would have to adjust their fuel consumption in order to arrange it so that the inertial observer would measure a constant rate of acceleration( this also means that they would be experiencing constantly increasing g forces).
This is a bit of an artificial scenario which requires the cooperation of one participant in order to create certain measurements for the other. When the ship stops and then accelerates again, they would have to actually say to themselves; " Before we cut engines, we were accelerating at a rate of y as measured by ourselves, and x as measured by the inertial observer. To return to that state, we have to fire up our engines so that they are burning fuel at the same rate at they were then." Nothing magical about that.
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I didn't use any inertial frame for the 'space example', because it seemed quite meaningless to me Janus. What I did instead was assuming that as I can't define locally what (relative) speed anything has, calling it 'inertial' should hold for any frame of reference being in a relative motion. I just let the rocket 'coast' at first in its relative motion. And logically it seems to me that if I can define different uniform motions, there either exist a need for defining what sort of speed something has 'relativity moving' or all 'relative motions' are equivalent. That's the problem I see, although you might see it differently.
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If they (relative speeds) all are equivalent (although differing in speeds) then a acceleration won't help putting it right. What one will get is a arbitrarily defined figure, defined from a arbitrarily defined 'inertial frame' initially 'at rest' with the rocket. And that is in a way the whole point of my thoughts there.
Or, turned around. All relative motions being equivalent, then accelerations doesn't add a thing, once turned of? What I mean by that is this 'energy density / mass' or just 'gravity' perhaps? That adds up locally with the rockets speed building up, closing in on 'c'.
Thinking that way one might assume that with a added speed, aka another step of acceleration, the engines should have to work harder, this continuing each time it leaves a uniform motion to then accelerate again.. But that shouldn't hold if indeed all relative motions are equivalent as it seems to me.
( For those that find me slightly confusing, think of it as you being in a car, Each time you accelerate it up its 'relativistic mass' should have changed with the added speed you got from the last acceleration. Like accelerating a heavier and heavier car for each step, the engine will have to work more for it. )
And that's another thing that itches, calling the inhibiting effect 'energy density'. I'm not sure what it is, but I don't think calling it 'energy' is meaningful here. Calling it relativistic mass might be a better choice perhaps? Although, considering that you can reach infinitesimally close to 'c' staying at one gravity of acceleration locally measured, relative mass doesn't seem to describe it properly either, does it?
Welcome to 'potential energy' :)
anyway, ignoring the questions, all relative motion being equivalent should make all definitions of measuring 'c' purely local. And that it holds everywhere for everyone, locally defined, can't be the same as there being a 'absolute background' of speeds from where you measure. Then again, we do agree on relative speeds astronomically, but we do it from one shared frame of reference, earth. Although, a big pinch of salt might be needed here ...
Which makes a kind of sense, to me then :)
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So, presuming the ship to accelerate at a constant one gravity the fuel consumption will raise to infinity :)
And 'coasting in between' won't change a thing if I get you right Janus? The consumption will start at a the level it was when one stopped accelerating. Then that should question what one mean by a 'relative motion', because they are no longer relative, as it seems to me? From red and blue shifts they are, but from 'relative speeds' they're not.
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To Alan"s point. An observer is midway between two distant objects at an instant in time. He measures the light from each. One is red shifted but the other is neither red or blue shifted. The observer must conclude that the time dilation is different for each remote object. If we say the midpoint observer is in an inertial frame he can say no such thing. He could be moving away from the more red shifted object at the same speed and in the same direction as the other remote object. Since motion cannot be detected in an inertial frame nothing definitive can be said about any shift detected. The shift too is relative. What we detect is not absolute. So receding objects are not giving us absolute information, only relative information. This is complicated by the fact that there is no absolute scale for time dilation.
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Let's see
the conditions I use is a black box. That's why I refer to local changes only, as fuel consumption. No use using external as you can't define your initial motion. We have two ideas here, one is that you can 'know' your speed, aka fuel consumption as from some 'global parameter'.even though you can't know it initially. the other is that you can know it, but locally defined.
Did you agree to my resume of your thoughts btw Janus?
From my perspective it will be enough with proving that each acceleration for that 'black box' we can call the rocket uses more fuel relative ones local wristwatch inside it. For the same gravity that is, what might confuse is that one should have to adjust for the time dilation relative ones 'speed' here at each new iteration/acceleration, if I get it right.
( although that actually depends on what 'conventions' you use for defining it. The most used is to consider proper time (wristwatch), at the same time as people then also introduce gravitational and motion induced time dilation's :) And presuming that different accelerations 'slow' your proper time, then adjusting for it? Well, that becomes somewhat of a headache, doesn't it? From where would you define the acceleration? Doing it locally seems the best approach to me if so, and then using blue / red shifts. Then again, you're now presuming that there should be some standard of 'time' that you need to adjust too doing it. What we could call the 'initial' undefined 'speed' before the acceleration, depending on how we define that. And that is one of the questions I'm wondering about here.)
But it's also a statement saying that relative motion, although undefinable initially, isn't relative at all, isn't it? Especially if we although defining it locally thinks of it at some global truth.
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The problem I see with it is that it seems as if you from assuming this to be true, would be able to 'know' the initial parameters by accelerating from your 'inertial platform' at a defined gravity. It's no longer a relative motion? Using your idea with buoys, letting the platform become one, then measuring the acceleration relative your wristwatch?
presuming different 'real motions' for the platform, you should get to different answers locally measured inside that rocket, shouldn't you Janus? Although one in each case can define it (the platform) as a 'inertial frame of reference'.
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Let me express it this way. By moving the platform relative some fixed star(s), in different directions and using different relative motions, you actually should be able to define a 'null motion'? If we assume that the results inside the rocket, varying fuel consumption, is related to the relative motion of the platform.
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Clarifying my thoughts a little
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There is no problem with defining a speed inside that black box, as far as I think. You just need a light bulb to then measure the blue respectively red shift it show under a acceleration. That should tell you how close you're getting to 'c' locally defined. But the same measurement done under a relative motion, aka 'free falling' uniform motion won't differ one relative motion from another, again as far as I see.
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To Alan"s point. An observer is midway between two distant objects at an instant in time. He measures the light from each. One is red shifted but the other is neither red or blue shifted. The observer must conclude that the time dilation is different for each remote object.
Not true. Most red/blue shift is due to Doppler effect, no dilation.
What causes Doppler shift? Relative motion. What causes time dilation? Relative motion. If things were not moving apart we wouldn't see any shift. They would be relatively 'stationary' with respect to each other.
http://theconversation.com/explainer-the-doppler-effect-7475
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What causes Doppler shift? Relative motion.
What causes time dilation? Relative motion.
Time dilation, yes. Doppler shift no. Doppler shift is due to a changing distance between the two objects, not due to relative motion.
An object can be moving at some silly speed tangential to me and will exhibit only redshift due to time dilation, but none due to Doppler since its distance from me is not changing. The two can be balanced, where one cancels the other resulting in no shift and apparently synchronized clocks (at least from one observer).
If things were not moving apart we wouldn't see any shift. They would be relatively 'stationary' with respect to each other.
Again, no. A thing in orbit around me would not be relatively stationary with me and yet we would not be moving apart.
http://theconversation.com/explainer-the-doppler-effect-7475
The site is misleading when it implies that it always occurs when there is relative motion. It is assuming non-tangential motion, but fails to state that.
The site also assumes slow relative speeds and thus totally ignores relativistic effects.
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Explain what you mean by moving tangential to you. Do you mean orbiting?
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Another question. If we have a photon starting at point a and travelling to point b would all observers agree on the path the photon takes from a to b?
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Explain what you mean by moving tangential to you. Do you mean orbiting?
Draw a line between observer and the object. If the velocity vector of the object (relative to the observer) is oriented perpendicular to that line, its motion is tangential, meaning its distance from the observer is not changing. This includes orbital motion, but is not confined to it. Somebody pacing back and forth could be doing it.
Another question. If we have a photon starting at point a and travelling to point b would all observers agree on the path the photon takes from a to b?
A path is a series of events (points in spacetime) and such points are fixed and have frame independent relations to each other. So in that sense, yes, all observers would agree (be it a photon or a sparrow). They would not agree on the spatial length of the path nor the time taken to do so, but since the events are fixed, the separation of A and B in spacetime is a frame independent value (called the interval), and similarly, any subset of the path has that fixed interval. So again, all observers would agree on the path taken.
What has that question got to do with the subject of this thread?
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would all observers agree on the path the photon takes from a to b?
Halc gave a good answer for a macroscopic particle.
For photons, the classic experiment is the dual-slit experiment.
- There are two (groups of) paths between a and b.
- No observer knows which slit a particular photon passed through
Attempting to determine the path of a photon at an intermediate point between a and b causes the waveform to collapse and changes the path(s).
The best consensus you could hope to achieve for a photon is that all observers agree that they don't know which slit a particular photon passed through.
PS: For certain combinations of photon wavelength and slit spacing, you can effectively guess (with high confidence) which slit a particular photon passed through. But this is the limiting case where the wave nature of the photon can be ignored, and you don't get a diffraction pattern.
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would all observers agree on the path the photon takes from a to b?
Halc gave a good answer for a macroscopic particle.
True. The question was about a QM level particle and I didn't give a QM answer. The topic wasn't a QM topic, so I guess I'm not thinking along those lines.
Why aren't there more QM topics in this forum? The philosophy forums are full of them. Is it totally not a scientific topic then?
- No observer knows which slit a particular photon passed through.
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The best consensus you could hope to achieve for a photon is that all observers agree that they don't know which slit a particular photon passed through.
Most interpretations say the photon takes both paths. It isn't just an epistemological thing where the path taken isn't known.
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Why aren't there more QM topics in this forum? The philosophy forums are full of them. Is it totally not a scientific topic then?
I have had a go(posted a new thread) .Hope the question was relevant to this thread.
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If you read me without preconceptions you will see what I say.
In the end it's perfectly simple
'c' is a local definition :)
Prove me wrong
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And no Halc, definitely not.
Use your own mind, Janus might have a broader setting from where to reach his thoughts, but that does not exclude us others.
But you need to see what I mean to react.
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And actually Halc. Doppler is due to relative motion . That's one of the things I find difficult. 'Relative' in this case means that a distance is growing, at a 'sphere' from where you measure. Jeffrey is perfectly correct in that you're free to define whatever speed' you define as to be a relation between you and what you measure. That you find it to 'fit' at a 360 degrees doesn't prove it to not be relative.
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What it do prove is that there is a 'constant'. or if you like, a accelerating constant.
Heh :)
Think I just introduced a new definition of a 'constant' :)
Maybe?
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Relative is relative Halc
If you don't know your base you don't have a 'golden standard'
I think that's what I'm trying to point out.
Which doesn't exclude a local definition in where you use blue and red shifts locally to define a 'speed'.
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Maybe one just need to turn ones head?
I don't know. I have no interest in physics, and I love those that has.
Weird, isn't it?