# I The velocity of a moving frame of reference

#### RossBlenkinsop

also anyone who is aware of the basics of time dilation would never claim that the source of light was under thier nose as it is apparently impossible to tell if your frame is moving or not and therefore no reasonable minded person would conclude their frame was not moving or was ....at best they can conclude "maybe it is, maybe it isnt ...i really have no clue"

Now if it is moving then the light source is back there somewhere where my frame came from

further this is confirmed in the explanations given by the so called experts

In fact I think it is even confirmed by experiment where a very accurate "moving" clock apparently lost time ,,,,,despite that being apparently impossible as there is no such thing as a moving clock

#### Ibix

my question has still not been answered
It has been answered repeatedly in many different ways. You just aren't grasping it.
a point in space or in space time (you choose)
Your failure to distinguish between these two things is the basis of your issue here.

If you mean a point in space then this is a frame-dependent notion because "space" is a frame-dependent notion.

If you mean a point in spacetime (usually called an event, and it will help immensely if you use standard words) then it is not anywhere at all except at the time when the flash was emitted.
also anyone who is aware of the basics of time dilation would never claim that the source of light was under thier nose as it is apparently impossible to tell if your frame is moving or not and therefore no reasonable minded person would conclude their frame was not moving or was ....at best they can conclude "maybe it is, maybe it isnt ...i really have no clue"
Um. I'm not sure where to begin correcting this. The choice of a frame includes (among other things) the definition of "not moving" that is in use. That other people use other definitions doesn't invalidate your choice. And "the basics of time dilation" require a frame, so that implies a definition of not moving.

I rather suspect you need an actual textbook - I recommend Taylor and Wheeler's Spacetime Physics.

#### weirdoguy

then according to all frames light will always trace back to a point in space or in space time (you choose) that does not move over time , over all time
There is no such point since movement is relative, even in Newtonian mechanics.

#### jbriggs444

Homework Helper
if the speed of light is invariant, then according to all frames light will always trace back to a point in space or in space time (you choose) that does not move over time , over all time
You still need to work on your terminology.

The light traces back to an event in space-time. Events in space-time are neither moving nor unmoving. The notion is simply irrelevant since an event has no duration.

If you try to walk over to the place where that event occurred, the place you walk to will depend on your standard of rest. It is not that the place moves. It is that the place is ambiguous until you specify what standard of rest you are using.

#### Nugatory

Mentor
also anyone who is aware of the basics of time dilation would never claim that the source of light was under thier nose as it is apparently impossible to tell if your frame is moving or not
Many posts back in this thread we cautioned you about using the phrase "a frame is moving" - it is easily misunderstood and you have misunderstood it here. A frame is a mathematical convention for assigning coordinates to points in space and events in spacetime, and mathematical conventions don't move.

and therefore no reasonable minded person would conclude their frame was not moving or was ....at best they can conclude "maybe it is, maybe it isnt ...i really have no clue"
If the spatial coordinates of an object, as assigned by a particular frame do not change with time then it is correct to say that the object is "at rest using coordinates assigned by that frame" (usually shortened to "at rest in that frame"). There's no question about whether an object is at rest or moving using any given frame nor that the point at the tip of my nose is not moving if we use a frame in which I and my nose are at rest.

So no reasonable-minded person would conclude that "their frame was not moving or was", because that would be like concluding that quadratic formula is moving south or that the Pythagorean theorem is purple and weighs 20 kilograms - it's a nonsensical conflation of words that don't go together.

However, a reasonable-minded person can choose to use any frame they want to assign coordinates to coordinates to an object (we often choose the frame in which we are at rest); that choice will determine whether the object is moving and if so, how fast.

#### Nugatory

Mentor
In fact I think it is even confirmed by experiment where a very accurate "moving" clock apparently lost time
Presumably you mean the Hafele-Keating experiment, and if so you have misunderstood the result. It is demonstrating the twin paradox in which two clocks are synchronized, one of them flies somewhere else, and then when they are brought back together less time has passed on the travelling clock. It's tempting (but wrong) to explain this as time dilation and "time slows down for the moving clock"; but this leads to a paradox because we could just as well say that the on-the-ground clock is the moving one and therefore it's the one that should have lost time. The resolution of the "paradox" is that time dilation isn't what's going in this experiment.

However, you probably don't want to take on the problem of understanding the twin paradox until you have corrected your basic misunderstandings about what a frame is and the distinction between points in spacetime and points in space. For that, I will second @Ibix's recommendation of Taylor and Wheeler - their presentation gets to these issues early and thoroughly.

#### RossBlenkinsop

if the speed of light is invariant, then according to all frames light will always trace back to a point in space or in space time (you choose) that does not move over time , over all time

it could be a single point, according to multiple people in multiple frames or multiple points according to multiple people in multiple frames , it is immaterial. What is of interest is the points appear to be in the same point in space or the same point in space time, over time, over all time. Ie if the same experiment of tracing light back to its source is conducted over and over and over , over a time period , by the same person in the same frame, they should get the same result if the speed of light in in fact invariant.

if they get a different result then the speed of light is variant ?

#### RossBlenkinsop

Many posts back in this thread we cautioned you about using the phrase "a frame is moving" - it is easily misunderstood and you have misunderstood it here. A frame is a mathematical convention for assigning coordinates to points in space and events in spacetime, and mathematical conventions don't move.

this "convention" is based on the premise that a person within a frame cannot determine the absolute velocity of that frame, should it have an absolute velocity or only a relative velocity

as no one can determine if the frame which they are in has an absolute velocity or not no one can say it definitely does not have a velocity or it definitely does

I believe a frame is a mathematical convention that is based on a real world phenomena. It seems somewhat pointless to have a mathematical representation of something that simply cannot exist for example a mathematical model of the physiology of a unicorn or a mathematical model of the pressure differentials in the feet of Yetis

#### RossBlenkinsop

one of them flies somewhere else, and then when they are brought back together less time has passed on the travelling clock.

which clock was travelling ? what does "travelling" mean ?

the experiment as I understood it is they synchronized some very accurate clocks, had one clock in a frame, put another clock in another frame and another clock in another frame again, the frames moved relative to each other. Later they all met back at a point where all clocks were in the same frame and compared the clocks. The results were time passed at a different rate in the frame that the experimenters deemed was moving !!

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#### PAllen

my question has still not been answered

if the speed of light is invariant, then according to all frames light will always trace back to a point in space or in space time (you choose) that does not move over time , over all time

on what basis do any of you assert that absolute movement, or lack of absolute movement aka absolute stillness, is not possible ? or is somehow a freakish outcome
Since both of your statements are trivially false, there is no logical answer. You might as well ask:

Since 1+1 is 3, how can you say there are no pink flying pigs?

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#### PAllen

one of them flies somewhere else, and then when they are brought back together less time has passed on the travelling clock.

which clock was travelling ? what does "travelling" mean ?

the experiment as I understood it is they synchronized some very accurate clocks, had one clock in a frame, put another clock in another frame and another clock in another frame again, the frames moved relative to each other. Later they all met back at a point where all clocks were in the same frame and compared the clocks. The results were time passed at a different rate in the frame that the experimenters deemed was moving !!
No, as has been explained many times. If between two points on a plane, you draw two different length lines, each will be at an angle to the other along various comparison lines you may choose to draw between them. However the difference in length is simply a fact of the difference in curves you draw. Similarly, between two spacetime paths between separation and meeting events, each path is in motion relative to the other (different spacetime angles), but one path is simply longer in time than the other. In plane geometry, the shortest possible line is a geodesic. In special relativity, the longest possible time is a geodesic, which means an inertial path.

The result of comparison on meeting again has absolutely nothing to do with with the experimenter rest frame, or any frame at all. It is a frame independent fact.

#### PeterDonis

Mentor
a point in space or in space time (you choose)
This is nonsense. A point in space is not the same as a point in spacetime.

You have received repeated correct explanations. Enough is enough. Thread closed.

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