# Down the relativistic rabbit-hole (Split from official news)

#### EricB

##### Member
I badly phrased it. I meant the speed relative to the train
I my example rear and front observer are equally distant to the light source.
Since we established light has constant speed regardless of momentum of the source, The rear observer would see the flash first as the light would travel faster relative to the train.
A central observer would always see both wall receive light at same time because the relative speed towards the back is faster. On the return trip, relative speed is faster from the front.
To observe light it HAS to travel back but observers at each end of the train don't have a return trip they observe the light directly hence seeing it at different times.

That's the way i understand it anyway. I could totally be wrong. Not a physicist but it make sense to me that way.

Edit: just saw Grench post.
Of course if train is not moving everybody see light at same time except central observer would have a slight delay compare to both end observers.

#### Grench

I wonder how complicated the math would be to determine how accurate of a clock would be required to measure the arrival differences.

We know that we're moving 390,000 m/s (872,000 mph) relative to the CBR:
https://www.scientificamerican.com/article/how-fast-is-the-earth-mov/
I would call that the minimum threshold of detection. The thing to keep in mind, is that measuring this is in relation to the speed of light. c = ‎299792458 m/s

So...
390000 / ‎299792458
Interestingly enough, Google won't perform that calculation. Good thing I still have an HP 48s handy.
1.30089997E-3
So, the minimum variance needed to be detected is going to be 0.001301 of a second.

However, to be able to measure any variance worth noting, we would need to have a more local reference point. We're buzzing around the Sun at 30,000 m/s. Ideally, the experiment should be able to detect and classify the Earth's progression around the sun as being 'forward' or 'backward' relative to the Sun's motion through space.
1.0007E-4

Without using laser cooling on the atomic clocks, the accuracy gets down to around 1ns per day.
1 ns 1 day
1 day 86400 s
Cancel days off ...
Atomic clocks used in GPS sattelites are accurate to +/- 0.00001157407 ~ 1.16E-5 seconds.
The clocks are just past the threshold of being able to detect a difference if it is large enough.

Hmm... the experiment could be done using 3 synchronized atomic clocks within line of sight of each other in close enough to a straight line, two mirrors, two lasers and four detectors. Mirrors are used during setup to determine the exact distance from center to the two outlying clocks. (Laser range finding.) Experiment should last ~ 10 years.

Does anyone have a few million dollars in research grants lying around?

Of note, the GPS atomic clocks in orbit have to be corrected for the relativistic effects from how fast they're moving at orbital distance compared to how fast we're moving on the Earth's surface. To state that another way, they are moving at a net velocity fractionally closer to the speed of light than denizens on the surface are. We can measure that difference and correct for it.

The idea here is to be able to use light's consistency (constant velocity) to determine via experiment, what proportion of the speed of light are we moving relative to 'full stop', which is admittedly a widely rejected concept.

On the subject of, 'full stop'. When we measure the light from distant galaxies, it is common for us to refer to that light as having been X billions of years old. The idea that the light was from a single point of time and from exactly where that star/object was at that single point in time is well accepted.

Light, at the point in time & space from which it was emitted travels at a constant velocity regardless of the momentum or direction of travel of the emanating source. A point of light, at the moment it is generated, effectively defines 'full stop'.

#### levi

##### Still fresh, damnit!
Your full stop seems to be defined in terms of zero time, not zero velocity. Of course, velocity is defined as δd/δt

If δd/δt=v, and δt=0
δd=v*δt=v*0=0

In other words, velocity becomes irrelevant if there's no time. You can't say nothing's moving, because you can't determine how fast anything would go if time were to restart. Sure, a system out of time would remain in statis forevermore, but you wouldn't be able to do any experiments in that space, so I'm not sure what value it has.

#### Grench

Your full stop seems to be defined in terms of zero time, not zero velocity. Of course, velocity is defined as δd/δt

If δd/δt=v, and δt=0
δd=v*δt=v*0=0

In other words, velocity becomes irrelevant if there's no time. You can't say nothing's moving, because you can't determine how fast anything would go if time were to restart. Sure, a system out of time would remain in statis forevermore, but you wouldn't be able to do any experiments in that space, so I'm not sure what value it has.
For zero time, you're assuming a division by zero. It isn't δt=0 that I'm wanting to measure, but δd instead. As δd approaches 0, so does v. If anything, δt would appear to approach infinity by ratio/proportion to δd as δd approaches zero.

Light has a constant v = c. Over a given time δt it will go a prescribed distance δd. Ignoring gravitational lens effects, and allowing for measurement tools that may be beyond our current capabilities, an experiment could be conducted to determine how fast a system is moving by making δd constant for two test points opposing the light source (3 equidistant points on a line) and highly accurate, highly synchronized clocks on opposing ends.

If the system/line is in motion through space along the length of the line described by the system, the arrival time of light at the end detectors should be different even though any reflected light back to the source would arrive at the same instant. Through reorienting the system/line, the direction of travel should become apparent. How well the system's velocity can be calculated is entirely dependent on how accurate and how well intertwined the two opposing clocks are.

The reference point in all of this is the speed of light. Accepting that as a known known constant allows for measurement of how far the system/line has traveled (linear for orientation) during the variable δt required for the light to travel a constant δd distance in opposing directions. Measured at the ends. The speed of light in the system is constant. There is no change or effect to the net speed of light from the system that creates it.

The position of the system is changing over the measurement period. The point in space where the light was initiated is the reference point for zero motion. The light marks the spot, the entire measurement system moves relative to that point in space. The δt detected at the ends for a constant system δd means that the system is moving relative to the position of the light at the moment it was created.

#### ible

##### professional vim user
Grench, there's two perspectives you have to worry about, the person on the train and the person off the train. you like being the observer off the train (where the light hits both sides of the train at different times), but there is an equally valid point of view for people on the train. they see the light hit both ends at the same time. trust me, i'm a physicist. everything you know is wrong. there are two pictures of what's happening, and both are equally valid (they are connected via a Lorentz transformation).

light doesn't hit the ends at different times for the people on the moving train, because to them, the train isn't moving. there is no measurement you can make to determine if you're moving or not. otherwise you get all sorts of nonsensical things, like i'm moving "1000000 mph West" -- that wouldn't make sense. every velocity is measured with respect to something else. in the moving train, if you're on it, you can't tell that you're moving. to you, the earth is moving backwards, and you're standing still. and light hits both sides of the train at the same time, because the train walls aren't moving (relative to you).

that's what relativity is all about, baby.

#### levi

##### Still fresh, damnit!
For zero time, you're assuming a division by zero. It isn't δt=0 that I'm wanting to measure, but δd instead. As δd approaches 0, so does v. If anything, δt would appear to approach infinity by ratio/proportion to δd as δd approaches zero.
No, my GCSE-level maths there shows that as δt approaches 0, δd approached zero, and v approaches irrelevance.

I did think I might be landing on a division by zero there, but using basic calculus, I multiplied both sized by δt, which happens to be 0. Multiplication by 0 is permissible and well defined as resulting in 0 all and any time you do it.

Light has a constant v = c. Over a given time δt it will go a prescribed distance δd. Ignoring gravitational lens effects, and allowing for measurement tools that may be beyond our current capabilities, an experiment could be conducted to determine how fast a system is moving by making δd constant for two test points opposing the light source (3 equidistant points on a line) and highly accurate, highly synchronized clocks on opposing ends.

If the system/line is in motion through space along the length of the line described by the system, the arrival time of light at the end detectors should be different even though any reflected light back to the source would arrive at the same instant. Through reorienting the system/line, the direction of travel should become apparent. How well the system's velocity can be calculated is entirely dependent on how accurate and how well intertwined the two opposing clocks are.
I'm afraid not. As ible says, the light will arrive at equally spaced detectors at exactly the same instant, for any train or spaceship that's not accelerating. There is no magnetic ether that we're moving thorough, and light runs at the same δd/δt no matter how slowly your t clock is, or how distorted your d is.

I think that on earth, if you were to build one of those L shaped detectors used to disprove the magnetic ether theory, and point one end up in the air, the interference pattern should change, because the rotation of the earth means we are receiving a slight vertical acceleration due to centripetal forces.

#### Djoga'Ro

##### moonstruck
What I don't get is, when I'm the observer off the whatever-vehicle and I see the back receiving its photon prior to the front meaning time runs slow in the back, how does the future look. Assume the vehicle stops later (come to what would be a halt relative to me), will the back be younger than the front from my perspective and both be the same age to their perspective?

#### Grench

Grench, there's two perspectives you have to worry about, the person on the train and the person off the train. you like being the observer off the train (where the light hits both sides of the train at different times), but there is an equally valid point of view for people on the train. they see the light hit both ends at the same time. trust me, i'm a physicist. everything you know is wrong. there are two pictures of what's happening, and both are equally valid (they are connected via a Lorentz transformation).

light doesn't hit the ends at different times for the people on the moving train, because to them, the train isn't moving. there is no measurement you can make to determine if you're moving or not. otherwise you get all sorts of nonsensical things, like i'm moving "1000000 mph West" -- that wouldn't make sense. every velocity is measured with respect to something else. in the moving train, if you're on it, you can't tell that you're moving. to you, the earth is moving backwards, and you're standing still. and light hits both sides of the train at the same time, because the train walls aren't moving (relative to you).

that's what relativity is all about, baby.
No, my GCSE-level maths there shows that as δt approaches 0, δd approached zero, and v approaches irrelevance.

I did think I might be landing on a division by zero there, but using basic calculus, I multiplied both sized by δt, which happens to be 0. Multiplication by 0 is permissible and well defined as resulting in 0 all and any time you do it.

I'm afraid not. As ible says, the light will arrive at equally spaced detectors at exactly the same instant, for any train or spaceship that's not accelerating. There is no magnetic ether that we're moving thorough, and light runs at the same δd/δt no matter how slowly your t clock is, or how distorted your d is.

I think that on earth, if you were to build one of those L shaped detectors used to disprove the magnetic ether theory, and point one end up in the air, the interference pattern should change, because the rotation of the earth means we are receiving a slight vertical acceleration due to centripetal forces.
For the above statements to be correct:
Light must have infinite velocity and reach all points in a system simultaneously. We know this to be incorrect.
A system behaves differently with "walls" than without. This is incorrect. These rules must apply regardless of the system design and definitions. A train car or a point in the galaxy, both must behave the same.

This ignores Einstein's postulates:
1. The laws of physics are invariant (i.e., identical) in all inertial systems (i.e., non-accelerating frames of reference).
2. The speed of light in a vacuum is the same for all observers, regardless of the motion of the light source.
What Einstein is stating in #1 is that what applies in a shoebox applies in a train car applies to planets applies to galaxies.

What Einstein is stating in #2 is that light does not inherit momentum from the motion of it's origin. It's speed is a constant c, regardless of the speed of it's source OR the speed of the observers.

Time for crude ASCII art.
Code:
``````[FONT=Courier New]@ = observer (atomic clock & sensor)
0 = light source origination point, 3rd observer
* = light propogation
Static system reference with reflection:
T01 @----------------------------0-----------------------------@
T02 @----------------------------*-----------------------------@
T03 @-----------------------*----0----*------------------------@
T04 @------------------*---------0---------*-------------------@
T05 @-------------*--------------0--------------*--------------@
T06 @--------*-------------------0-------------------*---------@
T07 @---*------------------------0------------------------*----@
T08 *----------------------------0-----------------------------*
T09 @---*------------------------0------------------------*----@
T10 @--------*-------------------0-------------------*---------@
T11 @-------------*--------------0--------------*--------------@
T12 @------------------*---------0---------*-------------------@
T13 @-----------------------*----0----*------------------------@
T14 @----------------------------*-----------------------------@
T15 @-----------------------*----0----*------------------------@[/FONT]``````
In the above, static example:
The observers on the ends detect the light simultaneously.
The observer at the point of origin sees the reflection from both ends simultaneously.

Code:
``````[FONT=Courier New]@ = observer (atomic clock & sensor)
0 = light source origination point, 3rd observer
* = light propogation
Moving system reference with reflection:
T01 @------------------------------0-----------------------------@
T02  @------------------------------*-----------------------------@
T03   @------------------------*-----0---*-------------------------@
T04    @------------------*-----------0-------*---------------------@
T05     @------------*-----------------0-----------*-----------------@
T06      @------*-----------------------0---------------*-------------@
T07       @*-----------------------------0-------------------*---------@
T08        @----*-------------------------0-----------------------*-----@
T09         @--------*---------------------0---------------------------*-@
T10          @------------*-----------------0------------------------*----@
T11           @----------------*-------------0------------------*----------@
T12            @--------------------*---------0------------*----------------@
T13             @------------------------*-----0------*----------------------@
T14              @----------------------------*-0*----------------------------@
T15               @-------------------------*----0-*---------------------------@[/FONT]
(Note: I did not get my ratio of dashes to right shifting correct for both of the examples to land where they should.  This is a rounding error for the crude instrument above.  In both cases the net time for light to go from origin back to center observer is the same.  Blame the artist.)``````
In the above, moving system example:
The observers on the ends detect the light at differing points in time.
The observer at the point of origin sees the reflection from both ends simultaneously at T14.
The light is instantly decoupled from the motion of the system at inception.

To an observer in the center of both systems, the light appears to hit both walls at the same time. The observer at the center/origin cannot tell if the system is in motion in either example

However, to a pair of truly synchronized 'perfect' clocks at either end (both of which consider the other to be wrong), they will record differing times for the arrival of the light. It is this difference that I am speaking to. The observers on the ends could be the walls of a train car or distant planets moving in the same direction with one moving towards and one moving away from the light source.

This does not violate relativity, but rather enforces it. The speed of light is constant. Light will take less time to cover a shorter distance than a long distance (just like a car). The above end observers with one moving toward and one moving away from the light source will indeed have different experiences, though as far as the observer in the center is concerned, nothing has changed from the static example.

#### ible

##### professional vim user
you are hung up on the fact that you think you can only have 1 frame of reference. there is more than 1 way of looking at the universe. your two pictures are just two of the ways to look at the moving train (one as observers on the train, and one as observers outside of the moving train watching it go by).

a basic postulate of relativity is that there is no way to determine your velocity (in an absolute sense), so if you think you have found a way to do so, you are not working within relativity anymore. (in fact, you are attempting to break one of the most time-tested theories ever.) you can only measure velocity with respect to something else, not within your own reference frame of things moving at the same speed as you.

#### Kuro

##### Still Fresh
Grench in your second ASCII art the observer IS moving in relation to the train, as there is no way that the walls get closer to you if you are not moving relative to them. what is not moving in relation to the train is the light source/point of return after reflection.

This is what i think you are not understanding correctly, the light source does not need to be your refrence frame.

#### Grench

you are hung up on the fact that you think you can only have 1 frame of reference. there is more than 1 way of looking at the universe. your two pictures are just two of the ways to look at the moving train (one as observers on the train, and one as observers outside of the moving train watching it go by).
Neither is correct. Break away from your training for a minute and consider that my observers (plural) are both onboard AND have an accurate measurement of time independent of the light pulse. This is not the classical example - which relies on mirrors and a single central (and perpetually oblivious to motion) observer.

Two, very precise, independent but highly synchronized observers is the key.

a basic postulate of relativity is that there is no way to determine your velocity (in an absolute sense), so if you think you have found a way to do so, you are not working within relativity anymore. (in fact, you are attempting to break one of the most time-tested theories ever.) you can only measure velocity with respect to something else, not within your own reference frame of things moving at the same speed as you.
So, if this works will I get a Nobel prize for Physics?

I am measuring velocity Vs something else. The point origin of a light source. That is a static point of reference. Calculating where it -was- requires (at least) two detection points at known distances. (Linear model used in example. A 3D model would require 4 observers and work better with 6 observers).

Grench in your second ASCII art the observer IS moving in relation to the train, as there is no way that the walls get closer to you if you are not moving relative to them. what is not moving in relation to the train is the light source/point of return after reflection.

This is what i think you are not understanding correctly, the light source does not need to be your refrence frame.
Again, missing a key factor. There is not one observer but two, who are IN the train, who are NOT at the center. The classic model for this as taught to physics students everywhere relies on a central observer either in or out of the system.

/***************/
Maybe if we slow things down a lot and simplify this to a Newtonian friendly 2D model.
Boat A is moving toward an underwater bomb.
Bomb B is not moving.
Boat C is moving away from an underwater bomb.
The two boats have synchronized clocks.
Both boats are moving at the same speed relative to B.

The bomb goes off when Boat A and Boat C are equidistant from the bomb B.
The shock wave moves outward at an even rate in all directions.
Boat A gets closer. Boat C gets further away.
Shock wave strikes Boat A. Clock is noted.
Shock wave strikes Boat C. Clock is noted.

Combining the times from the 2 boats allows calculation of a line in the sea upon which the bomb must have existed.

IF the boats and the bomb are known to be directly in a line ABC at the point of discharge AND IF the velocity of the wave is known AND IF AB = BC at the point of discharge AND IF the two boats are known to be moving at the same velocity in the same direction, then the velocity of the boats can be calculated relative to the point of discharge. (Reduction to a 1D model.)

Referencing this back to my crude ASCII drawings:
Length AB can be easily measured and recorded by bouncing a beam of light from B-A-B.
Length of BC can be easily measured and recorded by bouncing a beam of light from B-C-B.
The speed of the wave (light) is c in all directions from the point of origin.
The points ABC lie on a line by design.
The points of detection are independent, but synchronized.

Synchronizing the two detectors is relatively (haha) easy. Declare one as a reference (A). Measure distance by bouncing a beam from A-C-A, divide by 2. Send a calibration pulse from A to C. At C, subtract time taken for light to travel a distance of A to C. Two independent synchronized atomic clocks.

Two observers in motion, known to be traveling at the same velocity.

To translate the thing into 3D space from the linear model above, 6 detectors and a point of emanation.

The big hitch/catch/issue: I don't know if we can measure distances accurately enough or if existing atomic clocks are accurate enough or if two detectors can be made similar enough to enable enough measurement accuracy. A net measurement error of 0.0001% would still be 0.000001 * c or roughly 300 m/s.

#### klapse

##### Central Scrutinizer
Give Hilbert credit for G.R. pls.

#### Kuro

##### Still Fresh
In your boat example the bomb is moving relativel to the boats, that is not the same as both your previous ASCII art examples as in them the line formed by the light source/detector is parallel to the walls. But in relativity it makes no diference if it is just a light source and you dont use it after the moment it emits.

Now lets supouse the bomb also emits light and the boats are moving close to light speed relatively to the bomb and they both have synchronized clocks with unlimited precision that they set before the trip. Assuming the bomb explodes emiting light when the distance between boat A and bomb and boat B and bomb is the same and equal to x, booth boats see light travel at speed c, so obvously the light takes c/x seconds to hit each boat, so they both record the light at the absolute same time. If they dont record the same time it means that either they don't see light travel at c or that the distances between them and the bomb are diferent.

Note about the momentum of photons: if the light from the bomb was green the boat moving away from the bomb would see it red and the other boat would see it blue, this is known as red and blue shift.

Last edited:

#### Nintendo

##### Nintendo Switch
I just want to let you all know that Stephen Hawking is now in another universe.
There's no such thing as death.

#### ible

##### professional vim user
Neither is correct. Break away from your training for a minute and consider that my observers (plural) are both onboard AND have an accurate measurement of time independent of the light pulse. This is not the classical example - which relies on mirrors and a single central (and perpetually oblivious to motion) observer.

Two, very precise, independent but highly synchronized observers is the key.
two observers on either sides of the train (moving at constant velocity), with synchronized clocks, will see the light hit at the same time. the clocks can be as precise as you want, the train can be moving as fast as you want (less than c, of course).

here's a quote from the big man himself:
Einstein said:
Events which are simultaneous with reference to the embankment are not simultaneous with respect to the train, and vice versa (relativity of simultaneity). Every reference-body (co-ordinate system) has its own particular time ; unless we are told the reference-body to which the statement of time refers, there is no meaning in a statement of the time of an event.
http://www.gutenberg.org/files/5001/5001-h/files/ch09.htm

special (and general) relativity is a very well-tested theory. you're suggesting that the foundational principle is broken, but that is not where relativity breaks down. (it's been tested before. check out Michelson--Morley: https://en.wikipedia.org/wiki/Michelson–Morley_experiment). you're asking to go backwards from relativity, not forwards.

the only known place that relativity breaks down is at the singularity in a black hole. (general relativity, to be precise.)

#### error

##### Very Active Member
For your delight and amusement, discussion of space, time, etc, has been moved here:
From your perspective it has been moved, but to an observer more than 28 light hours away from Earth, it hasn't been moved yet.

#### Grench

From your perspective it has been moved, but to an observer more than 28 light hours away from Earth, it hasn't been moved yet.
Oh, it has been moved. That event -has occurred-. What hadn't happened yet is for the information to have propagated. The observer 28 light hours away would see the signal, know the travel distance and say, "This happened 28 hours ago.".
[doublepost=1527783347,1527778286][/doublepost]So I see you like Apples. Have an Orange.

two observers on either sides of the train (moving at constant velocity), with synchronized clocks, will see the light hit at the same time. the clocks can be as precise as you want, the train can be moving as fast as you want (less than c, of course).
Your statement does not hold true in our universe.
It would require that the signal from a radio beacon on Earth would be detected on every planet simultaneously regardless of their motion or position in the solar system. We know that to be false.

From the point in time where a light impulse is created, it expands ever outward at a constant rate of speed c. It does so from the exact moment and place in space and time where it was created. It does not care if the object creating the light is moving. It does not care if other objects are moving. It simply is.

When light from Alpha Centauri reaches Earth, it shows us where Alpha Centauri was 4.7 Light Years ago and strikes Earth where it is now. Just as Alpha Centauri has moved over the course of those 4.7 LY, so has Earth. We encounter that light at our position 4.7 Light Years after it's creation, not the point that Earth was at 4.7 LY ago when it was generated.

Thus is the propagation of light. The Lorentz equations purpose is to describe this propagation.

In my model above, the points of detection have changed position in the time between the impulse and the times of measurement.
Just as the light from Alpha Centauri arriving now shows where Alpha Centauri was 4.7LY ago, the light from the source in the experimental form shows where the source was a very tiny fraction of a second prior to the time of detection.

The point where light emanates is constant. Everything else is in motion. When we see light from an object, we see the light as was emitted it's light-distance & time away.

Between the time of impulse and the time of detection, the approaching detector itself has moved toward the source point by a very tiny fraction of (insert any distance measure here).

Between the time of impulse and the time of detection, the receding detector itself has moved away from the source point by a very tiny fraction of (insert any distance measure here).

We have atomic clocks that are far more accurate than anything of Einsteins time. I don't know if they are accurate enough.

We know how to synchronize the detectors and effectively tare them to account for the distance between them. The detectors will each consider the other to be 'wrong'.

here's a quote from the big man himself:

Einstein said: said:
Events which are simultaneous with reference to the embankment are not simultaneous with respect to the train, and vice versa (relativity of simultaneity). Every reference-body (co-ordinate system) has its own particular time ; unless we are told the reference-body to which the statement of time refers, there is no meaning in a statement of the time of an event.
Note: "unless we are told the reference-body."
The reference-body in the design above is the point of emanation of light. That IS the reference point. Just as it is in stellar systems, it is in our shoe box or rail car. The flash of a pulsar sweep is indifferent to the light from a supernova or an LED or lantern flame. All light propagates outward at a constant rate in a straight line (unless acted upon) from the point at which the light source existed at the time of emanation (ignoring the obvious gravitational lensing, etc.).

special (and general) relativity is a very well-tested theory. you're suggesting that the foundational principle is broken, but that is not where relativity breaks down. (it's been tested before. check out Michelson--Morley: https://en.wikipedia.org/wiki/Michelson–Morley_experiment). you're asking to go backwards from relativity, not forwards.

the only known place that relativity breaks down is at the singularity in a black hole. (general relativity, to be precise.)
Sigh - You keep bringing up Michelson--Morley. Their experiment in 1887 to see if they could find Aether by using light interferometric techniques. They were NOT testing motion with respect to the emanation point of light. Their experiment has nothing to do with the experimental system laid out above.

In order to use interferometry, the light source has to return to a common point - requiring a return trip which effectively washes out the travel distance of the system. That is why my design above deliberately avoids interferometry by having two opposing points of measure.

In my hypothetical scenario, the lack of return trip for the light is the key. You cannot base the measurement off of a return trip off of a mirror to an also moving measurement point. The average of the out and back trips is the same as in a non-moving construct. By just measuring the out, a difference should be seen. To make that measurement will require a level of measurement accuracy that may still be beyond current technology. It was far beyond Michelson--Morley's available technology.
[doublepost=1527787080][/doublepost]Finding an accurate enough clock...

Accurate to 1 second every hundred million years:
https://gizmodo.com/5834937/the-worlds-most-accurate-clock-ever

Accurate to 1 second every 12 billion years or so, but can only exist for 10 seconds at a time:
https://gizmodo.com/scientists-just-built-the-most-precise-clock-ever-to-he-1819174378

Would need two of them in line of sight to each other, preferably a line of sight in a vacuum. Otherwise the actual distance itself is irrelevant. Better would be two of them physically linked together floating through space for easy reorientation.

#### levi

##### Still fresh, damnit!
the only known place that relativity breaks down is at the singularity in a black hole. (general relativity, to be precise.)
Also in the scale of the very small - the quantum domain. Specifically Einstein's despised 'spooky action at a distance' of entangled particles, which seemingly allows information to be exchanged at faster than light speed over a distance between two subatomic particles. Although for our purposes, you can probably ignore that because as far as I know entangled particles have thus far only been spotted in our labs, and not in nature (though it's fair to say that even if all the atoms on earth were entangled with others, beyond the reach of light, or were even just outside the solar system, or were somehow already all in the kuiper belt, then we'd probably have no idea they were there), plus as far as I'm aware, quantum science doesn't say anything about the motion of particles - that's covered by Einstein's theorems to this day.

#### Kuro

##### Still Fresh
Grench let me just clarify that what you re saying is that there is an absolute refrence frame from which light moves at c relative to, and other refrence frames that move at some speed x relative to such frame see light travel at c+x in one direction and c-x in the oposite one.

Is this correct?

#### ible

##### professional vim user
not sure why you're thinking light has to travel infinitely fast for my perspective to work out. light travels at c in all reference frames. which is why the light hits both sides of the train simultaneously, in the reference frame of the train. (the walls aren't moving in the reference frame of the train.)

you think there's an absolute picture of what's happening, that there's only one "truth." (light hits both sides of the train at different times, watching the train go by. this is the case in the reference frame of the "earth" or "embankment" terminology used by Einstein.) but there's a second, equally valid "truth" -- that light hits both sides of the train at the same time, and this is true for observers in the train. (even with infinitely accurate clocks.)

Michelson--Morley was the best shot at proving there exists an "absolute velocity" (i.e., being able to keep a single "time" and understand things in an absolute context, but still "relative" to the aether). they failed, in a brilliant way. there is no sense of an absolute velocity; any such thinking goes backwards from relativity (in the sense that you're going back to Newton), and doesn't explain anything moving forwards from relativity (i.e. new phenomena).

it's like saying that phlogiston is really what makes fires burn, honestly.