I Synchronizing clocks at different locations to measure speed of light

[ESponge2000]

TL;DR Summary

I’m aware this is a challenge in physics with catch22, to measure c in one direction we need 2 clocks in sync, which we can’t do without assuming knowledge of the one-directional speeds of light, the whole point of why we need to sync in the first place …

just have some questions & ideas here and want to gather information on

When I tap my left hand and my right hand against the floor 3 feet apart, at the same time, I know that I’m not “really stretching 3 feet” at the same time, my brain is reading the nerves of each hand at a speed actually even slower than c but no matter how my circuits were designed it would be not faster than c.
The start point would be my brain sending the signal for me to touch equally on each side , and the result of what I touched would come also from my brain (a 2-way flow not a one-way flow)

I even tried to examine rare phenomenon in Siamese twins where there’s 2 conjoined brains but in rare cases some signaling is shared by one brain and not the other. The significance is if ANY conscious existence of 2 places on one clock could be ascertained there could be the physical infrastructure to synch 2 locations 2 brains 1 conscience but no , all cognition would flow from one brain to the other there’s no loophole around causality

But then another thought occurred to me. One brain, even a computer detector, is going to be larger than an infintessimal dot. That means the computer laptop brain spans a range of locations one clock…. Or does it span one clock … and for better precision is off by the assumption of containing only one clock ? And the lack of an infintessimal dot for a brain is a limitation on the meaning of any clock altogether to decipher one-way time discrepancies?

Thinking this through I THINK I see 2 possible areas of hope for testing this
1) Finding objects that occupy one state across more than an infintesimal volume and then examining variations in the workability when the object is rotated at set angles …. Would be due to directional effects Zzz The main thing that comes to mind are Quantum system states … possibly one system simultaneously covering a range of localities and comprised of a system-wide state

Or
2) Something that can be done with gravitational curving of spacetime : curvature of space time can maybe allow theoretically one direction path of light that starts and ends at the same
Location , and then a separate patch work to correct for gravitational time dilation interfering in the calculation ?

or for a final question is it determined in physics that we cannot synchronize 2 clocks at 2 distant locations ? Or is it something that we just haven’t been able to achieve at present?

 

[Nugatory]

we cannot synchronize 2 clocks at 2 distant locations

Only by making an assumption that is equivalent to assuming that we already have two synchronized clocks. So no.

The main thing that comes to mind are Quantum system states … possibly one system simultaneously covering a range of localities and comprised of a system-wide state

When you see these states in a QM text… You’re reading a text on non-relativistic QM, the only kind you’ll see in undergrad physics classes. Non-relativistic physics just assumes that there is one true time and all properly constructed clocks are synchronized to that time.

 

[Ibix]

is it determined in physics that we cannot synchronize 2 clocks at 2 distant locations ?

You cannot do it without assuming a one-way speed of light in a universe governed by relativity.

People do search for failures of Lorentz covariance (a broader collection of phenomena which would include an assumption-free synchronization convention), but nothing has been found.

 

[ESponge2000]

You cannot do it without assuming a one-way speed of light in a universe governed by relativity.

People do search for failures of Lorentz covariance (a broader collection of phenomena which would include an assumption-free synchronization convention), but nothing has been found.

That’s what my understanding has been. It does get weird to think that we are able use an assumption of choice, simplest being that c is the same in all directions for each IRF, for relativity of simutaneity and using those Lorentz transformations, getting all the same answers for explaining observations we are able to test. We can maybe guess the Einstein convention holds merit when we stipulate it is a fixed truth that all 2-way transmissions obey the same velocity constant …. Assuming then any path deviated from the constant it would have to be offset with paths deviating the other way going in reverse, And when we get how any angles one can travel a straight line path it becomes difficult to imagine such a lack of symmetry in directional speeds of light, or to where is that point in the universe where light beams move the fastest towards ?

It also I suppose can get very confusing when our galaxy or cluster of galaxies may be rotating just like earth rotates on its axis, and we wouldn’t know if our angles in space shift over time even when we use the constellations as our compass for space compass , what’s to say the entire observable universe isn’t in some type of axis rotation

 

[Ibix]

It does get weird to think that we are able use an assumption of choice

Ultimately, it's just a choice of whether the imaginary coordinate grid you draw over spacetime is made of imaginary orthogonal lines (isotropic one-way speed) or imaginary non-orthogonal ones (non-isotropic one-way speed). As long as you keep track of which set of lines you're using and the mathematical consequences of using a non-orthogonal grid, you're fine. Physics can't care about your personal choices.

 

[ESponge2000]

I think then the answer is it doesn’t matter. The observations come out the same regardless right?

If the assumptions is the south direction has speed of light infinity because the north direction it’s half of c, Then the same effect both north and south , for both directions an infinite energy results in crossing everything in 0 time , to the south because you would be approaching a truly infinite speed, And to the north because when traveling at near c/2, Which would appear to earth the instant light emitting back south ( it would appear as c) , the north c/2 traveler would be time dilated to experience the same exact effect as infinite speed limit the other way ….. and since light observing is a 2-way we would observe the same c value

 

[ersmith]

Your comments seem to suggest that there's a "true" one way speed of light that we just can't measure. That's not the way it works in relativity. Rather, the one way speed of light is defined by the synchronization convention chosen. Hence it is in a sense arbitrary (although if you want your coordinates to define an inertial reference frame you'll have to ensure that the two way speed of light comes out to c).

It is convenient to use the Einstein synchronization convention for many reasons, including that it makes the speed of light the same in all directions. So that's the convention physicists choose in practice.

 

[PeterDonis]

The observations come out the same regardless right?

Yes, but "the one way speed of light" is not an observation. It's a convention.

 

[ESponge2000]

Your comments seem to suggest that there's a "true" one way speed of light that we just can't measure. That's not the way it works in relativity. Rather, the one way speed of light is defined by the synchronization convention chosen. Hence it is in a sense arbitrary (although if you want your coordinates to define an inertial reference frame you'll have to ensure that the two way speed of light comes out to c).

It is convenient to use the Einstein synchronization convention for many reasons, including that it makes the speed of light the same in all directions. So that's the convention physicists choose in practice.

If I’m understanding this right, we don’t even know if there is a “true” one-way speed of light, we have that observation affirms the fundamental constant c time delay for each and every IRF using each single clock against itself. That means then that we may not even know how light moves altogether beyond what it needs to do to preserve that relationship, which is to present the observations we validate by what we observe directly. Science doesn’t care about what I think.

But thought exercises can be fun. Alpha proxima is said to be the light from 4.2 light years away because a round trip is 8.4 light years away. We just know a round trip is 8.4 years. Our knowledge of the distance is that the relative motion of the star apart from the motion due to the rotation of the earth, orbit around the sun, and the motion of the solar system, reflects distance from earth, other knowledge affirming its distance is what kind of star it is with a telescope vs its brightness in the sky , through science comparing stars we have numerous scientific methods to assert the distance

Based on objects closer and larger like the sun and an eclipse on Jupiter we have ability to know what the light source should be seeing and then how long it takes till we see what we should see , and determine the delay is the speed of causality
What we see to what happens far away + what happens far away traveling back to our eyes is a round trip

If we somehow could one day make the trip to alpha proxima and see the earth from what we won’t know is 0-8.4 years ago which by convention we call a distance of 4.2ly away, actually 4.2 (averaged light years) , we STILL wouldn’t know the answer as when we move from one location to another , even in the same IRF, We don’t know how it impacts our own clock

So let’s temporarily assume that alpha proxima light instantly travels to earth and earth’s light takes the whole 8.4 years to reach the star. And we get hung up by saying we aren’t really looking at 4.2 years ago after all! What does that mean? What does that change communication-wise, it still takes 8.4 years either way to communicate with the Star. By goin to the star then would we be living in a different time than we thought, the same as earth’s ?
This is confusing to think about but if this were the convention we use instead , we would infer yes and no…. The voyage to the star took us , relative to earth , ignoring our own clock synchronizations, 4.2 years deeper into the future for earth than we expected it would …:. But that doesn’t matter because we might be 4.2 years into the future but we have to wait 8.4 years to see the earth’s light so it’s as if we were not 4.2 years forward in time and saw the light of an earth 4.2 years into the past

Or maybe I am missing the whole point , MAYBE the Question of “what time is it on alpha proxima at the time exactly they they see it’s 2025 on planet earth” is a question that is not answerable at all… simply altogether ,
simultaneity is just a convenient way for us to make sense of pairing times of different locations which honestly don’t sync and don’t have to sync period.

Then for earth it’s “omg we can see what this star is doing almost in simultaneity with us , we can tell the star to let us know any info they see we will get an immediate alert ! But …. That info of excitement takes 8.4 years to reach them … communication is a 2-way process . Or maybe that’s the lesson here …. Moreover it wouldn’t even matter if the directions where light travels at c/2 vs instant shifts around in such a way that the 2-way pathways remain average c … even if Proxima Centauri is continuously moving or the directions are shifting because if this isn’t altering the 2-way speed of light then the synchronization of all clocks will be such that preserves the fundamental 2-way constant to the extent the one-way differences change, if the one-way speeds are different.

One-way communication renders time lapse irrelevant, is that a true statement?

 

[weirdoguy]

we don’t even know

We do know, it's a convention, and you've been told this numerous times in all of your threads.

 

[ESponge2000]

I think I’m finally getting it. Relativity of Simultaneity for different locations, regardless of IRF being assumed to be the same or not, has no confirmed absolutes. We make assumptions only for practicality to make it easier to work with the observations which are that observed distances in an IRF obey the 2-way signal fundamental maximum speed limit, nothing more nothing less. Outside that, there is no relevance to any absolute time at any location B that holds any absolute meaning to a time at location A, other than obeying the causality 2-way limit

What that means is if we see there is a clock right now with a telescope on alpha proxima that indicates its the year 2021, we can affirm that any relevant time paired to us now, if any, in any convention, will sensibly be one that’s at least on or after 2021 if it’s in the same IRF
But even This criteria assumes “light has a ‘true’ one-way velocity ! Which is not affirmed either !!! In other words the answer to what time is it “right now” on alpha proxima can be a question with no
Answer at all that doesn’t state the use of 2 conventions

Convention 1: light has a true one-way velocity
If convention 1 is assumed,
then
Convention 2 applies,
else
there’s just us people making up a framework of simultaneities for practicality purposes


Convention 2: the true one-way velocities for same IRF must average A and B in opposite directions for any certain distance X in a single IRF where (X/A + X/B ) = 2X / c (units of time).

And standard convention A=B=c

Almost like … you are given a same flight flies back and forth between New York and Paris and only takes 60 minutes between arriving and redeparture.. at each. Assume Paris changed its time to a secret other timezone … we just know the time lapse between each time this plane arrives in New York. Paris keeps a separate log of time lapse between each time the same plane arrives in Paris.
Do we have enough info to calculate the jet stream ? Nope

 

[Nugatory]

Do we have enough info to calculate the jet stream ? Nope

There is an important difference between this aircraft example and one-way light speed measurments: The aircraft is following a timelike worldline along which proper time is defined. Thus we can put a clock on the plane to directly measure the proper time (along that worldline) between takeoff and landing events, and calculate the jet stream from that.

 

[ESponge2000]

There is an important difference between this aircraft example and one-way light speed measurments: The aircraft is following a timelike worldline along which proper time is defined. Thus we can put a clock on the plane to directly measure the proper time (along that worldline) between takeoff and landing events, and calculate the jet stream from that.

Correct but that’s why I said the only info in the flight problem hypothetical is the time, interval times between the round-trip, and the Timelapse at Paris without the Paris clock making sense …. I did that on purpose .

In reality obviously we say airplane speeds are too low for SR to be taken seriously so it’s Newtonian style where one time is defined as The Clock

 

[PeterDonis]

If I’m understanding this right, we don’t even know if there is a “true” one-way speed of light

No, we know there isn't any such thing as a "true" one-way speed of light. The one-way speed of light is a convention, not a physical quantity.

we have that observation affirms the fundamental constant c time delay for each and every IRF using each single clock against itself.

In other words, the two-way speed of light is a physical quantity--although even there care is needed in how it's specified and measured.

That means then that we may not even know how light moves altogether beyond what it needs to do to preserve that relationship

I don't know what you mean by this.

But thought exercises can be fun.

Only if you properly constrain them using consistent premises and math. You're not doing that. You need to.

 

[ESponge2000]

No, we know there isn't any such thing as a "true" one-way speed of light. The one-way speed of light is a convention, not a physical quantity.


In other words, the two-way speed of light is a physical quantity--although even there care is needed in how it's specified and measured.


I don't know what you mean by this.


Only if you properly constrain them using consistent premises and math. You're not doing that. You need to.

We know there isn’t a true one-way speed of light ? Or we don’t know the answer to whether there is or isn’t one?

 

[PeterDonis]

We know there isn’t a true speed of light ? Or we don’t know the answer to whether there is or isn’t one?

Go read the first sentence of my post #14 again. It doesn't say either of these things. Read carefully, paying attention to all the words.

 

[Ibix]

We know there isn’t a true one-way speed of light ?

A uniquely defined one-way speed of light is incompatible with the theory of relativity. We do not know of any theory that is compatible with a uniquely defined one-way speed and also with observations of reality.

 

[ESponge2000]

Go read the first sentence of my post #14 again. It doesn't say either of these things. Read carefully, paying attention to all the words.

I’m overthinking it, I think you mean we don’t attempt to deal with if there is or isn’t a true speed of light in one direction because we know it’s irrelevant to how SR interacts with us … we know we can apply a convention and the math will work as it’s supposed to

It better said, it really doesn’t matter what convention you apply, among any number of valid ones, as far as calculating the results that we are able to scientifically test

 

[Ibix]

I think you mean we don’t attempt to deal with if there is or isn’t a true speed of light in one direction because we know it’s irrelevant to how SR interacts with us …

No, @PeterDonis is saying the same thing I did, that a unique definition of the one-way speed of light (or indeed, anything) is incompatible with relativity. We almost always assume isotropic speed because the maths is much simpler and easier to interpret, but we are free to assume otherwise and we do not become more right or wrong if we do so.

 

[ESponge2000]

No, @PeterDonis is saying the same thing I did, that a unique definition of the one-way speed of light (or indeed, anything) is incompatible with relativity. We almost always assume isotropic speed because the maths is much simpler and easier to interpret, but we are free to assume otherwise and we do not become more right or wrong if we do so.

Isn’t that the same thing i said ? You said Incompatible I said extraneous because incompatible implies something more than extraneous.

A cable TV remote is incompatible with an iPhone, it has no use in anything to do with an iPhone .

What paint you use for a Honda Civic would be extraneous to how well it drives but it’s not incompatible because it can go on the car it just doesn’t matter if it’s green or orange

I would think the latter is more the case for the speed of light convention

 

[PeterDonis]

I’m overthinking it

You're thinking instead of just reading what I actually said. What I actually said is quite specific and clear. Read it.

 

[Ibix]

You said Incompatible I said extraneous because incompatible implies something more than extraneous.

Yes. That's why I said incompatible. If there is a uniquely defined one way speed of light, then relativity is wrong.

 

[ESponge2000]

Yes. That's why I said incompatible. If there is a uniquely defined one way speed of light, then relativity is wrong.

Well relativity under a hypothetical universal construct where speed of light is made to be “absolutely isotropic” and c , preserves relativity of simutaneity between different IRFs but with an absolute simultaneity for objects at rest with each other

Relativity under a hypothetical universal construct with “uniquely defined one way speeds of light”
Also preserves relativity of simutaneity for different IRFs and also for objects at rest in some directions but possibly not others …. For some directions merely perpendicular to where the one-way speed of light velocities are in contrast would be neutral lines of motion closer to isotopic … you can also have speed of light in one direction isn’t a constant velocity but shaped identically in the reverse direction to preserve the 2-way speed of light concern… these I think Ibix and Peter are saying are beliefs that have no support for how relativity works … this is all anyone’s guess.

The framework of relativity in its entirety … what makes it relativiry, follows that simply … clocks at different locations do not have “absolute” simultaneities, when objects in motion or signals convene on a same location, only then is there an absolute simultaneity

 

[jbriggs444]

Well relativity under a hypothetical universal construct where speed of light is made to be “absolutely isotropic” and c , preserves relativity of simutaneity between different IRFs but with an absolute simultaneity for objects at rest with each other

You are suggesting that there could be one true absolute frame with absolute simultaneity and a bunch of false frames with relative simultaneity.

Yes, but if the theory of relativity is correct then no experiment can identify the one true frame. Any inertial frame will do. You've failed the uniqueness requirement.

 

[Nugatory]

but with an absolute simultaneity for objects at rest with each other

Simultaneity is a property of pairs of events not objects even in classical physics where there is no relativity of simultaneity.

 

[ESponge2000]

You are suggesting that there could be one true absolute frame with absolute simultaneity and a bunch of false frames with relative simultaneity.

Yes, but if the theory of relativity is correct then no experiment can identify the one true frame. Any inertial frame will do. You've failed the uniqueness requirement.

Ahhh I see now!!! The part of relativity that we have to look globally and say which IRF is The simultaneity of absolute and the theory of relativity is firm on there is not an absolute resting frame. With that theory there is no assertion of unique absolute one-way directional speed of light because that would violate the core principle that “there is no master resting frame over another!” Each Simultaneity in physics only exists independently for each one single point in space,
Other than that we just have a range of time values across locations, the range being such that time at other locations must satisfy that light to and from travels at the cosmic speed limit. Like we would say with certainty it’s not the year 1500 on the other side of my backyard because light to and from it being merely instantaneous, the time range is less than milliseconds of uncertainty…

So
Each object is an independent clock… always at rest with oneself, non-reconciliable to all remaining objects’ other clocks, and where time reconciling is done only among things that are convened in a common location.

I have a friend named Andy and a friend named Jonah. They meet for lunch without me. Then Andy calls to tell me he met with Jonah for lunch. With “absolute time precision” there is no reconciliation of what time they met for lunch, at the nanosecond level, to my clock. Because light round trip
Is merely instantaneous and atomic clocks are not precise to a nanosecond level, the range of discrepancy due to SR is so miniscule we can ignore the discrepancy and forge an absolute clock
But if we want to go to absolute precision we have none.

I also don’t know to the nanosecond level if the time on Andy’s clock when he called me is what it would have been if some synch was attempted earlier between my location and where he called me from . And all I do have simultaneity for is my clock …(im always at rest) , And the electromagnetic signal of Andy’s voice which reached my location at c from traveling through a cable from the entry point of either a router or a 5g tower … is absolute time comparable.
Later, I meet Andy in person , at only that moment , Andy’s clock and mine have an absolute reconciliation, if Andy and me had earlier met each other and synched our clocks and assuming our clocks have perfect atomic flawless precision, According to relativity , our reconvening tells us with absolute precision which one of us underwent more changes in inertia, the one who did will have a clock a few nanoseconds if that … slower

 

[jbriggs444]

unless there’s a place in theory of relativity that events at other locations are not real with regard to ANY time reconciling our local clock … and ONLY become real when the photons reach our point location, and then it is that which is formulated on distance and the speed of light constant…

Unless you can define it, the word "real" is just philosophical mumbo jumbo. Stick to experimental predictions instead.

If events make the transition from "unreal" to "real" as our past light cone sweeps past them, that fact has no experimental consequences.

and that when we change locations we never change locations , we only change inertial frames but with respect to ourselves we are always at rest

This does not make much sense. An inertial frame is basically a coordinate system. You can use a coordinate system without being at rest in that coordinate system. You are not obliged to use a (non-inertial) coordinate system that is anchored to your feet.

 

[PeterDonis]

with an absolute simultaneity for objects at rest with each other

No, there is no such thing in relativity.

the time on Andy’s clock when he called me

Is a convention, not a physical thing. The urge to keep thinking of it as a physical thing, as something "real" instead of just a convention, is something that needs to be unlearned if you are going to really understand relativity. You need to really internalize the fact that there is no such thing as "what time it is now" for you on a clock that is spatially separated from you. There just isn't. You can adopt a convention for such things for describing events and doing calculations, but such a convention has no physical meaning. Please think about that carefully. You have been going round and round in this thread trying to give a physical meaning to something that has none.

 

[ESponge2000]

No, there is no such thing in relativity.


Is a convention, not a physical thing. The urge to keep thinking of it as a physical thing, as something "real" instead of just a convention, is something that needs to be unlearned if you are going to really understand relativity. You need to really internalize the fact that there is no such thing as "what time it is now" for you on a clock that is spatially separated from you. There just isn't. You can adopt a convention for such things for describing events and doing calculations, but such a convention has no physical meaning. Please think about that carefully. You have been going round and round in this thread trying to give a physical meaning to something that has none.

I am getting that it’s a convention, you’re right, but I think I meant rather “the signal received by me at my location of a time showing on Andy’s clock when i answered Andy’s call”, would not be a convention. But also what would not be a convention is the time for Andy at Andy’s location when Andy called me … because it’s the same clock.

The convention is reconciling the time the signal was transmitted to the time it was received, For either location you have one and not the other…. For my location I can, however, choose a convention. The most common one being c velocity all directions. Then we take for me :
The time on my clock I receive the signal of a time on Andy’s clock after a tone we both hear over the phone the same signal, i then adjust to my time the value of (1-v/c)*time where time set equal to the light-timeunit which is actually a distance, and where if we are not in relative motion then v=0… what I mean is a distance of 1 foot would be referred to as 1 light-nanosecond …. For a signal 1 foot distance away by convention we adjust by a nanosecond to my clock to assume a simultaneity that doesn’t REALLY exist.

If we are about 5000 feet away (1.6km
Away) and light travels about 1 foot per nanosecond and we are measuring my clock in nanoseconds and we are at v=0 then this means by convention my clock time adjust 5000= By convention “the time at my location corresponding to the now shown on Andy’s clock at Andy’s location when Andy sent signal
because at that time I receive the signal we calculate that the signal is broadcasting a past event on my timeline that occurred by convention, 5000 nanoseconds ago…
But ALL of this is convention ….. as stated by Peter, there is NO WAY to reconcile what time it is NOW in 2 different locations

something to think about … if something is 4 light-years away from us …. It is a defined absolute distance relative to our perception of length …. We people created this terminology … we say a light-second means roughly 300,000km
But this terminology already uses a convention because we are implying light IS traveling this distance in this time in each direction . That assumes that convention or else it can’t be taken literally. In other words how many seconds does it take light in a vacuum to travel 1 light-second (aka 186,000 miles) is actually a trick question. The answer is not 1 second unless the question states to apply a convention. But that’s human terminology
a true literal light-second meaning a true speed of light in one second times t would imply a distance we can’t calculate

But that also means our world works by perception is reality. Next time I get pulled over for speeding on the highway and try telling the cop that he can’t be certain on the time stamp because he can’t identify the correct nanosecond I was speeding, the judge will not take my defense because it is an “unreasonable defense”.

Reasonable transparency : based on the way everyday communication works, it is clear regardless of relativity differences our clocks at a level of all times material to our daily actions forge with a Newtonian model of absolute time , for the level of precision that is reasonable, the proof being for instance a good driver doesn’t get into accidents because of special relativity. And perfectly reasonable for an officer to use language “the car was over here when the other car was over here, a witness on another car agrees they saw this happen at the same time as me”
In fact the prosecutor if they brought in a physicist as an expert witness , The physicist would testify to the court that the units of time that affect SR are below the threshold of altering the perception of an absolute time, over a distance threshold significantly farther than the ones concerning highway traffic. So would it be an accurate statement that SR is not a reasonable defense? Correct your honor.

 

[PeterDonis]

“the signal received by me at my location of a time showing on Andy’s clock when i answered Andy’s call”, would not be a convention.

Yes, it would, because "the time showing on Andy's clock when I answered Andy's call" is a convention.

What would not be a convention would be "the time showing on Andy's clock when he emitted the light signal that I receive at a certain time on my clock". But that is not the same thing as "the time showing on Andy's clock when I answered Andy's call". Think carefully.

also what would not be a convention is the time for Andy at Andy’s location when Andy called me … because it’s the same clock.

That's true, this is not a convention, because it's a time at Andy's location of an event at Andy's location. But it's not the same thing as "the time showing on Andy's clock when I answered Andy's call"--the latter is a convention, because it's assigning a time on Andy's clock to an event that is at a different location. Any such time is a convention.

if something is 4 light-years away from us …. It is a defined absolute distance relative to our perception of length

No, it's not an "absolute distance". There is no such thing in relativity.

that also means our world works by perception is reality

Not as far as physics goes, no. The physics of how the cop's radar detector works is the same whether the judge accepts your argument or not.

 

[ESponge2000]

Yes, it would, because "the time showing on Andy's clock when I answered Andy's call" is a convention.

What would not be a convention would be "the time showing on Andy's clock when he emitted the light signal that I receive at a certain time on my clock". But that is not the same thing as "the time showing on Andy's clock when I answered Andy's call". Think carefully.


That's true, this is not a convention, because it's a time at Andy's location of an event at Andy's location. But it's not the same thing as "the time showing on Andy's clock when I answered Andy's call"--the latter is a convention, because it's assigning a time on Andy's clock to an event that is at a different location. Any such time is a convention.


No, it's not an "absolute distance". There is no such thing in relativity.


Not as far as physics goes, no. The physics of how the cop's radar detector works is the same whether the judge accepts your argument or not.

here’s what’s still tricky . Distances are not absolute, time is not absolute, velocities are not absolute , even a directional speed of light is not absolute, even the roundtrip apparent speed of light isn’t absolute unless it travels through no medium. But all reference frames can use SI or can they? What we sent out on the Golden record on Voyager One Space probe is still universally accurate , correct ? Because if there is intelligent life they would be able to model earth and earth at rest and then use our own space-time diagram for calculating lengths and time, right?

 

[PeterDonis]

all reference frames can use SI or can they?

If you mean SI units, the choice of units is part of the choice of frame. So not, not "all" frames use SI, because some frames choose different units. SI units are a convention.

What we sent out on the Golden record on Voyager One Space probe is still universally accurate , correct ?

I have no idea what you think this has to do with relativity or the issue under discussion in this thread.

 

[Nugatory]

In fact the prosecutor if they brought in a physicist as an expert witness , The physicist would testify to the court that...

Or the physicist could testify that the car's coordinate velocity using a frame in which the roadway is at rest (which is what the statute means by "speed") is unaffected by all this stuff so no relativity/frame dependence hanky-panky can be relevant to the facts of the case... and they're pretty sure that they were only brought into the case because someone still doesn't understand what relativity does and doesn't do.

 

[weirdoguy]

here’s what’s still tricky

What's tricky is that it seems that you don't belive (maybe subconsciously) that relativity is correct. What's the point of this long dissertations? As a teacher, I can assure you, they are not helping, even though you may think otherwise. Go get a textbook on relativity and use your time usefully.

 

[ESponge2000]

Am I correct to say nothing in relativity is not compatible with QE because we already have that 2 locations don’t have a simultaneity and QE does not attempt to assign a time at more than one location of a measurement, which is already consistent with relativity there’s no way of knowing a time in another location correlated to a choice of action at a different location

But maybe that’s not even the main reason both QE and SR compatible the main reason is SR concerns casually connected events in spacetime but does not impede on events that are correlated but not causally connected , correlations that don’t have meaningful cause-effect are an exception to the communication velocity limit since communication isn’t transmitted. To establish simultaneity in a quantum collapse you must have FTL communication since that contradicts relativity we also have no simultaneity framework for quantum collapses either, knowing it or a true simultaneity here is incompatible with both frameworks.

The frameworks don’t contradict each other but both are incomplete when applying GR to quantum theory

 

[PeterDonis]

QE

What is QE?

 

[Nugatory]

Am I correct to say nothing in relativity is not compatible with QE….

I think that by “QE” you mean quantum entanglement? If so what you’ve written is kinda OK as a B-level answer to how we reconcile relativity with the apparent action at a distance behavior of entanglement.

But be aware that there are some subtleties here…

If you aren’t working from a graduate level textbook, it is a near certainty that you’ve been reading about non-relativistic quantum mechanics. Absolute time is baked into this non-relativistic formulation - the $t$ that appears in Schrodinger’s equation is the reading of a mythical giant clock in the sky to which everyone’s clock is synchronized, inconsistent with relativity.

 

[ESponge2000]

I think that by “QE” you mean quantum entanglement? If so what you’ve written is kinda OK as a B-level answer to how we reconcile relativity with the apparent action at a distance behavior of entanglement.

But be aware that there are some subtleties here…

If you aren’t working from a graduate level textbook, it is a near certainty that you’ve been reading about non-relativistic quantum mechanics. Absolute time is baked into this non-relativistic formulation - the $t$ that appears in Schrodinger’s equation is the reading of a mythical giant clock in the sky to which everyone’s clock is synchronized, inconsistent with relativity.

But that’s a fault of quantum mechanics using a “t” in a way that isn’t the same usage as in SR .
The t in quantum should be outside of any local clock and property of no point in space , a quantum field … The way this was explained to me is you have stochastic probability wave yes like schrodingers cat, at measurement nothing is created nor altered , a past present future is revealed that wasn’t revealable otherwise …
In quantum mechanics there’s no relevance to the question “who measured first”, It just is what the correlation is at measurement and can’t be before so. In this framework there’s also no simultaneity
But the “t” just like in SR a convention is used for relativity of simultaneity … You can perhaps say the quantum mechanics conventions are the quantum system’s inertial frame defines the clock, as if it’s one object , and also the same Einstein convention we use in relativity of simultaneity …. Those assumptions are would create a framework where a single t value holds

Where terminology is arbitrary :

Didn’t Einstein also create a confusion by we say objects with mass increases with kinetic energy. But in E=MC^2 Einstein means Only “rest mass” not total mass , Which makes perfect sense because kinetic energy is “relative” ! The total mass of an object if altered by its velocity is relative to the object it is in relative motion with , so the only meaningful value of an object’s mass is its mass at rest . But then we have .::: photons have energy , blue light photons in fact have more energy than red light photons …. Light has energy
But … photons travel at c because they are massless … energy and mass are interchangeable… contradiction?

Einstein redefined mass to mean rest mass only . That means we have non-zero mass and require infinite energy and would be total mass Infinity to accelerate to the speed of light, hence impossible. Einstein M though is our rest mass which doesn’t change when we accelerate

Photons travel only at c , they have zero rest mass, but in motion at c the effect is a convergence on a finite , not infinite , total energy. Am I understanding this right ?,

 

[PeterDonis]

that’s a fault of quantum mechanics using a “t” in a way that isn’t the same usage as in SR .

It's a fault of non-relativistic QM using "t" that way. But non-relativistic QM is just an approximation anyway.

Quantum field theory does not have any such issue; it is perfectly consistent with SR.

The t in quantum should be outside of any local clock and property of no point in space , a quantum field

No, it isn't. Please do not try to speculate. QFT is a complex subject, and if you want to understand how QM is reconciled with SR, you need to take the time to learn it.

 

[PeterDonis]

The way this was explained to me

By whom? From what source?

 

[PeterDonis]

you have stochastic probability wave yes like schrodingers cat, at measurement nothing is created nor altered , a past present future is revealed that wasn’t revealable otherwise …

Some QM interpretations might work this way, but not all of them do.

In quantum mechanics there’s no relevance to the question “who measured first”

There is if the measurement events are timelike or null separated. Only if the measurements are spacelike separated is there no invariant time ordering.

Discussion of QM belongs in the QM forum, not this one. And discussion of QM interpretations, which is what you're getting into here, belongs in the QM interpretations subforum. And any such discussion would need to be based on a valid reference, not just a vague "someone told me".

 

[ESponge2000]

I agree and I’ll stop here. You would know more about quantum and SR than me.

 

[cianfa72]

Ultimately, it's just a choice of whether the imaginary coordinate grid you draw over spacetime is made of imaginary orthogonal lines (isotropic one-way speed) or imaginary non-orthogonal ones (non-isotropic one-way speed).

Just to take it simple, consider a 2D spacetime in the realm of SR (no gravity at all). We want to draw coordinate lines over it. As first set of coordinate lines, take a flock of identical, free-falling clocks filling the entire space such that:

• their (timelike) worldlines do not intersect
• round-trip time of light signals traveling from any clock to any other and back doesn't change (as measured from the clock sending and receiving back the light signal)

That the above construction results to be feasible, follows from SR flat Minkowski geometry.

Next, we need to draw over the other set of (spacelike) coordinate lines. To do that, we can use for instance Einstein's synchronization procedure/convention starting from a "master clock". This way we label each event with a time coordinate. Now draw over the lines connecting all events with the same "assigned time label": they define the coordinate lines in the other set.

From the above procedure, we get a coordinate grid (i.e. global chart) drawn over the 2D flat spacetime made of Minkowski-orthogonal coordinate lines. In this chart/coordinates the one-way speed of light is, by very construction/definition, isotropic with the same speed.

P.s. note that free-falling is a physical/invariant definition (i.e. zero proper acceleration/zero reading of accelerometers attached to each of those clocks).

 

[cianfa72]

From the above procedure, we get a coordinate grid (i.e. global chart) drawn over the 2D flat spacetime made of (Minkowski) orthogonal coordinate lines. In this chart/coordinates the one-way speed of light is, by very construction/definition, isotropic with the same speed.

Conversely, we can pick a synchronization procedure different from Einstein's one giving us a different synchronization convention in which the one-way speed of light turns out to be no longer isotropic.