Question about time and measurement

[Dale]

So if two objects wanted to tell which object was actually moving they could just compare how much time accumulated for each of them since the astronaut left the earth, or the Earth left the astronaut. no?

No, because they could both be moving. There is no such thing as actually moving. There is only moving wrt some frame.

 

[questionator89]

So then how does time dilation appear in reality at all if what you say is true.
This means that if I were to travel around at very close to the speed of light, and come back to earth, no discrepancy between minutes felt will be notable because the Earth also could have been the object speeding around at close to the speed of light?

So time dilation does not actually appear in reality?

The big difference being between the object which actually moved is that it accelerated, traveled fast for a while, then returned to the same velocity for clock comparison.
So firstly the Earth would not have felt any acceleration, and would not have experienced any time dilation by accelerating.

If time dilation does not effect reality then why is it being discussed at all

 

[Drakkith]

Drakkith,
I am comparing Earth's motion against a clock which is at rest, not orbiting. Just like what I have been talking about for the past 60 posts and over 2 threads.
Each time I post I am pretty sure I mention this single scenario.

Which would be fine if we were talking about the exact same resting frame the whole time, but we are not. You cannot just say the clock is at rest when we have different conditions for different observers. You must specify what frame of reference you are using or things will not make sense and the conversation will be very confusing. As this thread has shown.

Drop a clock into space at rest, do a one year orbit around the sun, find how much more time accumulated for the resting clock compared to the clock on Earth which was moving.

I assume the clock in space is kept at rest with respect to the Sun's frame?

So another scenario,
An astronaut leaves Earth at an incredible rate for a period of time then stops.
You say the Earth could view it as speeding away from the astronaut, but the astronaut will view it as he is speeding away from earth. They cannot tell who moved away from who.
But if the astronaut were to speed back to Earth and compare a difference between time dilation, he would have experienced much less time in comparison to earth. Is this wrong?

It's not wrong, it's just missing the key point in that the astronaut was accelerated while the Earth was not. Both still moved away from each other, but only the astronaut was accelerated. So while he's accelerating he can observe the Earth, do the calculations, and say, "Hey, I'M the one causing us to get further apart." Thus, when the two meet back up, the astronaut is the one who has experienced less time because he accelerated to a frame of reference that was no longer at rest compared to us and then returned back to the Earth's frame.

So if two objects wanted to tell which object was actually moving they could just compare how much time accumulated for each of them since the astronaut left the earth, or the Earth left the astronaut. no?

Kind of. Both objects are "actually moving" with respect to each other. But the difference lies in the fact that only one was accelerated. Plus, since this motion isn't steady, you cannot know exactly what the accelerated observer did just by comparing clocks when they return. You could only get an average velocity.

 

[Dale]

The word "move" means "change position over time". Mathematically, that is dx/dt, which is velocity. So "move" means velocity (specifically nonzero velocity). Motion (velocity) is relative, but acceleration is not. So while it is incorrect to say that one object "actually moved" it is not incorrect to say that one object "actually accelerated". The object that actually accelerated will accumulate less time.

 

[questionator89]

OOOOH .. Kay i get exactly where this translation was lost.
So if we were to all of the sudden become conscious and another object were all of the sudden to become conscious, they would have no idea which one was moving right?
But what I can't grasp is, one of these objects was accelerated to begin with and would be experiencing slower time, while the other one experienced faster time.
If when they became conscious they threw a signal to the other guy to start some ticker that accumulated ticks ( and both tickers were built so if they were at the same momentum they would experience the same rate of ticks) would the one that was actually moving have his ticker tick less? Right as they pass each other they flash the number of ticks each object experienced.

So one object would say your object is ticking fast and mine is ticking normally, while the other object thinks my ticker is ticking normally and yours is ticking slow?

Could we not, without knowing who was accelerated toward who, compare time dilation to find who accelerated?
or at least how much faster one is moving in the case that they are both not at rest.

Drakkith you are right we could only get an averaged velocity. But could we not use this to find out how fast our sun is moving (on average) around our galaxy?
And to answer your q from post 73 the clock is at rest, and the sun is at rest, and the Earth orbits the sun for one year and picks the clock up in what the Earth and the sun perceive to be the same location the clock was originally placed in.

 

[questionator89]

In that last post the part about the tickers, were going to assume that the tickers receive the signal and are automated to account for the time it took for the signal to reach the object and the starting distance.
the signal is RF so it travels at lightspeed and it won't matter how fast each one may or may not be moving

 

[Drakkith]

But what I can't grasp is, one of these objects was accelerated to begin with and would be experiencing slower time, while the other one experienced faster time.
If when they became conscious they threw a signal to the other guy to start some ticker that accumulated ticks ( and both tickers were built so if they were at the same momentum they would experience the same rate of ticks) would the one that was actually moving have his ticker tick less? Right as they pass each other they flash the number of ticks each object experienced.

You've already started on the wrong premise. BOTH objects are time dilated equally, regardless of who accelerated. This is because you have no frame other than the two which are in motion relative to one another to observe from. As such, you cannot claim that one is experiencing absolute faster time than the other. There is no "absolute" time.From wiki: http://en.wikipedia.org/wiki/Time_dilation

The green dots and red dots in the animation represent spaceships. The ships of the green fleet have no velocity relative to each other, so for the clocks onboard of the individual ships, the same amount of time elapses relative to each other, and they can set up a procedure to maintain a synchronized standard fleet time. The ships of the "red fleet" are moving with a velocity of 0.866 of the speed of light with respect to the green fleet.
The blue dots represent pulses of light. One cycle of light-pulses between two green ships takes two seconds of "green time", one second for each leg.

As seen from the perspective of the reds, the transit time of the light pulses they exchange among each other is one second of "red time" for each leg. As seen from the perspective of the greens, the red ships' cycle of exchanging light pulses travels a diagonal path that is two light-seconds long. (As seen from the green perspective the reds travel 1.73 (\sqrt{3}) light-seconds of distance for every two seconds of green time.)

The animation cycles between the green perspective and the red perspective, to emphasize the symmetry.

Could we not, without knowing who was accelerated toward who, compare time dilation to find who accelerated?

Nope. Both of you would measure equal time dilation.

Drakkith you are right we could only get an averaged velocity. But could we not use this to find out how fast our sun is moving (on average) around our galaxy?

You can, but you'd have to have a clock which is at rest with respect to the center of mass of the galaxy to measure compare with.

 

[Dale]

OOOOH .. Kay i get exactly where this translation was lost.
So if we were to all of the sudden become conscious and another object were all of the sudden to become conscious, they would have no idea which one was moving right?

Yes, although the anthropomorphization is unnecessary.

But what I can't grasp is, one of these objects was accelerated to begin with and would be experiencing slower time, while the other one experienced faster time.

Why should one of them have necessarily accelerated to begin with? Muons are created in the upper atmosphere all the time which live out their entire brief life at relativistic speed in the Earth's frame without ever significantly accelerating.

Indeed, for any object at there is a reference frame where it was at rest initially, but there are infinitely many other reference frames where it was moving initially. Thus, it is not generally true that "one of these objects was accelerated to begin with".

If when they became conscious they threw a signal to the other guy to start some ticker that accumulated ticks ( and both tickers were built so if they were at the same momentum they would experience the same rate of ticks) would the one that was actually moving have his ticker tick less?

Again, the phrase "actually moving" is simply incorrect.

So one object would say your object is ticking fast and mine is ticking normally, while the other object thinks my ticker is ticking normally and yours is ticking slow?

No. No clock ever ticks fast in an inertial frame. I thought that we had already covered this. Clocks at rest tick normally and moving clocks tick slowly. This is true in any inertial frame.

 

[questionator89]

Yes, although the anthropomorphization is unnecessary.

I agree.

Why should one of them have necessarily accelerated to begin with? Muons are created in the upper atmosphere all the time which live out their entire brief life at relativistic speed in the Earth's frame without ever significantly accelerating.

Well in the scenario the distance between them is shrinking. so either one is moving towards the other or they are both moving towards each other.

Indeed, for any object at there is a reference frame where it was at rest initially, but there are infinitely many other reference frames where it was moving initially. Thus, it is not generally true that "one of these objects was accelerated to begin with".

Ok so I can't tell, and you can tell, and I can tell that you can tell, that I do not understand something about this principle.
Is what I do not understand is that time dilation is just an optical illusion? Or can an astronaut actually travel at a fast velocity and experience less time than the planet he just left?

Again, the phrase "actually moving" is simply incorrect.

How would you word it then? We are using a scenario where there are two objects. One is moving and one is not. How do we figure out which one is actually moving.
If this is improper english I am sorry, but I do not know how to convey what I mean any better

No. No clock ever ticks fast in an inertial frame. I thought that we had already covered this. Clocks at rest tick normally and moving clocks tick slowly. This is true in any inertial frame.

 

[questionator89]

Thanks for your efforts you guys but I am unsubscribing from these threads.
I feel more insulted then helped anymore and have gotten no farther for the effort
I will read what I can on my own

 

[Drakkith]

Is what I do not understand is that time dilation is just an optical illusion? Or can an astronaut actually travel at a fast velocity and experience less time than the planet he just left?

It's no illusion, it actually happens. GPS satellites and particle colliders must deal with relativistic effects like time dilation all the time.

How would you word it then? We are using a scenario where there are two objects. One is moving and one is not.

No, you cannot say that one is moving and the other is not. Each one is moving according to the other's perspective. And they are both correct. Movement is relative!

 

[Drakkith]

Thanks for your efforts you guys but I am unsubscribing from these threads.
I feel more insulted then helped anymore and have gotten no farther for the effort
I will read what I can on my own

Well, thanks for wasting hours of our time. (or at least mine) I'm sorry you refused to listen.

 

[TumblingDice]

I've followed questionator89's threads and thread hops. Forum members - y'all have tried SO hard to help him get on track. My grandpa liked the saying, "you can lead a horse to water, but you can't make him drink."

My impression is that Q89 blew past excellent facts and references everyone worked to help with every time they didn't jive with the bad assumptions he started with. He used his own faulty reasoning to twist facts into an even worse understanding.

Q89 - I think you believe in absolute speed. I think you believe that somewhere in the universe there is a true reference frame that moves at absolute 0, and fastest reference frames move at c. That's incorrect and you have to let that misconception go before the rest makes sense. The word relative is "in relationship to something else" and relativity is about reference frames. If you can force yourself to accept there is no 'slowest' moving frame (no zero speed where time clicks fastest) and there is no absolute speed anywhere. Even c should not be thought in the sense of absolute in the classical sense. I think forum members even prefer the word invariant rather than constant, because we speak of the 'measurement' of c, and that never varies.

When I 'let go' of my classical handcuffs that caused similar frustrations for me, I found that the things I felt wouldn't make sense actually did. If you commit yourself to start with a foundation instead of trying to understand everything all at once, you'll have a much more enjoyable time learning what you seek.

 

[Drakkith]

Q89 - I think you believe in absolute speed.

Yes, I think we finally pinned down the ultimate source of his confusion just at the end of this thread and in another one right before that. I just wish we could have hammered this point in earlier, we might have avoided 4 pages of confusion.

 

[harrylin]

[..] Q89 - I think you believe in absolute speed. I think you believe that somewhere in the universe there is a true reference frame that moves at absolute 0, and fastest reference frames move at c. That's incorrect and you have to let that misconception go before the rest makes sense. [..]

TumblingDice that misconception is yours: Lorentz believed in absolute speed and everything made sense to him, as a matter of fact he also taught SR and GR and he explained it rather well. Different metaphysical interpretations work to make sense of the phenomena that SR describes, just as with QM. :tongue2:

 

[TumblingDice]

TumblingDice that misconception is yours: Lorentz believed in absolute speed and everything made sense to him, as a matter of fact he also taught SR and GR and he explained it rather well. Different metaphysical interpretations work to make sense of the phenomena that SR describes, just as with QM. :tongue2:

This is a good example of why I've read the forum daily for six months w/o posting. I found that quite often threads involve interpretations. I come here to learn, and keep an open mind when facts can be presented.

I have ideas I'd like to float that go deeply into the concepts and layers of time, but not for a while - not until i feel that I've learned enough to contribute further based on science. I think absolute speeds may play a key role in a larger picture. But, if I understand the math of reference frames, relative motion and time, there's no way to setup an experiment to measure. Just like trying to measure a one.way speed of light. (Hope i don't ruffle feathers with that. <grin>

So I'll wait to see if other members have anything to offer regarding your correcting what I wrote. Is a new thread in order?

 

[harrylin]

This is a good example of why I've read the forum daily for six months w/o posting. I found that quite often threads involve interpretations. I come here to learn, and keep an open mind when facts can be presented.

I have ideas I'd like to float that go deeply into the concepts and layers of time, but not for a while - not until i feel that I've learned enough to contribute further based on science. I think absolute speeds may play a key role in a larger picture. But, if I understand the math of reference frames, relative motion and time, there's no way to setup an experiment to measure. Just like trying to measure a one.way speed of light. (Hope i don't ruffle feathers with that. <grin>

So I'll wait to see if other members have anything to offer regarding your correcting what I wrote. Is a new thread in order?

Indeed there is no way to measure something like that, and heated debates about interpretations regularly get people banned from this forum (check the Rules!). In the QM forum discussions about interpretations are more tolerated, perhaps because in publications on QM such interpretation issues are still discussed but not anymore in publications on relativity. The topic came up in several threads before which you can check out:
www.physicsforums.com/showthread.php?t=595021
www.physicsforums.com/showthread.php?t=574624

PS: As you see in the Rules, this forum allows discussion of the historical development of SR and GR, as found in the literature; but floating personal ideas or theories is against the rules.

 

[Drakkith]

I'm not sure I see the issue Harry. How is believing in an absolute speed not a misconception? Where do interpretations come into this?

 

[harrylin]

I'm not sure I see the issue Harry. How is believing in an absolute speed not a misconception? Where do interpretations come into this?

Did you read through the past discussions to which I linked? Believing in an absolute speed (Lorentz; impossible to disprove metaphysics) must not be confounded with believing that measured speeds are absolute* (Maxwell; disproved by experimental confirmation of Lorentz and Einstein).

*[edit. rephrasing: believing that absolute speed can be measured]

 

[questionator89]

Yah I checked in,
So even if there is not absolute speed, at the speed of light time stops. Drakkith will disagree with this by saying that we can't reach the speed of light so there is no way to know.
If we are following a pattern where time rate is slowing down, eventually it must stop.;
just like my misconception about us on Earth not be resting. The Earth (bad example) cannot go slower than 0.
and time cannot go slower than 0 after and at 100%c(unless you believe you can go back in time).

Drakkith continually misread what I was posting. Every post was more of an argument than a lesson.

YOu must realize by now that I am not trying to determine absolute speed or absolute rest or time.

I am trying to figure out if there was a way to tell you were in motion, if you already were in motion and not accelerating or decelerating

A couple smart people once said "If you cannot explain something to a 5 year old and have him comprehend it then you do not understand it fully"

Im not passing blame and I really do appreciate all the responses.

As of right now, nobody can truly tell me what it is that I do not comprehend.
And Dice, its not absolute speed or rest. But I do believe we would lose the ability to be sentient at the speed of light, and our spaceship wouldn't take the TIME to propel itself and likely wouldn't take the TIME to slow down or speed up anymore to get you out of this state of frozen time. So I wouldn't do it

Great input tumbler, its really awesome that you followed my posts the whole time and jumped in once I left and criticized me without any input whatsoever

 

[questionator89]

And also if we read Drakkiths first post even, he sounds frustrated already.
Why do you even try Drakkith if it just aggravates you to talk to people so obviously less intelligent than you are

 

[questionator89]

Harrylin you were helpful

Drakkith you were helpful twice

Dalespam
all you do is argue about how I worded my posts. No helpful input

Im off PF good luck with your quest for knowledge

 

[Nugatory]

You must realize by now that I am not trying to determine absolute speed ...
I am trying to figure out if there was a way to tell you were in motion, if you already were in motion and not accelerating or decelerating

The bolded text above is pretty much a definition of "absolute speed", so whether you realize it or not, you're still pursuing that notion. That may be why you feel like we're yelling at you...

You are always at rest relative to yourself, and everything that is not at rest relative to you is moving relative to you. And the same is true of everything else in the universe; every object in the universe is at rest relative to itself. Therefore, there is no way to determine whether something is "in motion", you can only determine that it is in motion relative to something else, and then its speed can be zero (no motion at all) or anything up to almost the speed of light (very fast motion indeed), depending on what something else you choose to compare with.

 

[TumblingDice]

So even if there is not absolute speed, at the speed of light time stops.

At the risk of not being perfect and hoping any "mistakes" are in my terminology.

Regarding the idea that time stops at the speed of light: If you examine the math, it's only valid with relative speed less than c. That's because if you plug 'c' as a reference speed into equations, you get an "undefined" result. That's because the function only "approaches" the axis where you might suggest that relative time stops. It cannot reach the axis where "time stops" because of standard rules of mathematics. We can toss around the concept of "infinity", but that's another example of something that requires proper perspective. Infinity is a concept, not a value. If it were a real value, the equations come out undefined, for example, the denominator of the result becomes zero, and dividing by zero is meaningless.

To be clear, anything with invariant mass can never travel at the speed of light. You can accelerate in your own reference frame until the cows come home and try your best to go faster. But as you do your thing to accelerate, the theory of relativity keeps "raising the bar". You can experience constant acceleration, but the math always confines you to only "approaching" c.

 

[Samshorn]

Believing in an absolute speed... must not be confounded with believing that measured speeds are absolute...

That's a mis-use of the word "measured". The confounding that you are referring to is not between measured and un-measured speeds, nor is it between observed and un-observed speeds. Relativity is not a subjectivist theory. It's propositions refer to perfectly well-defined objective attributes of the external world, regardless of whether they are measured/observed. The correct distinction is between defined and un-defined conceptions of speed. For example, the "one-way speed of light" can have any value we choose, because it is an undefined concept, whereas the "one-way speed of light in terms of a system of coordinates in which the Newtonian equations of mechanics hold good (to the first approximation)" is unambiguously equal to c. This is objectively true for any pulse of light, whether that particular pulse's speed is measured or not. So the distinction isn't between measured and unmeasured quantities, it's between defined and un-defined quantities. Or, to put it another way, the distinction is between knowing what we are talking about, and not knowing what we are talking about.

Needless to say, given the set of all possible inertial coordinate systems (defined as above), we are free to select one of them and declare it to be the "plamange" one. Or we could substitute the phonetic sound "true" in place of "plamange", without of course attaching any well-defined conceptual significance to the sound, but this kind of silliness takes us outside the bounds of physics (as Lorentz admitted to Einstein in his "universal spirit" confession), and of course it does nothing at all to explicate quantum phenomena (as Bell admitted when pressed on his "cheapest solution" comments). So calling it metaphysical is actually giving it too much credit. It's utterly pointless.

 

[ghwellsjr]

That's a mis-use of the word "measured". The confounding that you are referring to is not between measured and un-measured speeds, nor is it between observed and un-observed speeds. Relativity is not a subjectivist theory. It's propositions refer to perfectly well-defined objective attributes of the external world, regardless of whether they are measured/observed. The correct distinction is between defined and un-defined conceptions of speed. For example, the "one-way speed of light" can have any value we choose, because it is an undefined concept, whereas the "one-way speed of light in terms of a system of coordinates in which the Newtonian equations of mechanics hold good (to the first approximation)" is unambiguously equal to c. This is objectively true for any pulse of light, whether that particular pulse's speed is measured or not. So the distinction isn't between measured and unmeasured quantities, it's between defined and un-defined quantities. Or, to put it another way, the distinction is between knowing what we are talking about, and not knowing what we are talking about.

You lifted that quoted phrase from Einstein's 1905 paper (first article) where he proceeded to argue that the "one-way speed of light" requires an additional definition. Do you know something that Einstein didn't know?

 

[questionator89]

Right. I will lay a thought experiment out there to clearly define my question. I want to be clear that I am taking into consideration everything you say TumblingDice, but you should realize that, other than this one question, the concept as a whole is something I can grasp. Except infinity. But when I use this word I use it only for something that has lost all sentience or consciousness or meaning of time. And I have a feeling this perspective could comprehend infinity. and comprehend nothing at all ever.

My thought experiment:

We as a fourth observer are watching a spaceship move from left to right, viewed directly in front of us.

This spaceship launches two clocks out, at exactly the same speed.
The clock which it launched behind itself is seemingly at rest to our perspective as the fourth observer, because the exact rate at which the spaceship was moving was canceled out.

To the spaceship, both clocks zoomed away from it in exactly the same way, and the spaceship would see both clocks ticking at exactly the same rate. which is slower than its own rate of time.

The spaceship already knows how fast each clock left its surface. The spaceship knows what distance is between the ship and the two clocks always.

after 10 minuits each clock sends a signal back to the ship how much time was accumulated during the 10 mins to the ship. let's say that the clocks both accumulated 8 seconds

What you guys (all of you) are trying to convey to me is that to the ship, both clocks have experienced less time, because to the ship both clocks zoomed away from it.

Fine.

Lets think of it this way.
Now let's say this ship was already traveling at 99.9%c. The ship does not know it is traveling at this speed.

So to us as the fourth observer we know that the propellants in the clocks can't break this speed. We would expect from watching that the clock launched in front, would not be able to be launched in front.
And the clock launched behind would indeed be launched behind, and would be at a slower momentum relative to the ship. From our perspective.But what is conveyed to me is that, A) the ship has no idea that is traveling this fast, and cannot see anything around it including us as the fourth observer, except for the clocks.
B) Both these clocks will zoom away from the perspective of the ship at the same speed and seem to experience less time

So let's leave absolute speed completely out of this.
We know anything with invariant mass can never travel at the speed of light. Or our math tells us this?

So from the perspective of the spaceship both clocks leave and are zooming away.
But to the perspective of the fourth observer watching this all happen, the leading clock went faster than the speed of light, and the trailing clock is at rest and experiencing the same rate of time that we are?

How is it that, because the spaceship has nothing to reference from and does not know that he is breaking the this law, this is able to happen?

Wouldnt the spaceship notice that the clock ahead of him never sent another signal again?

To us as the fourth observer the trailing clock is at rest and experiencing the same time rate as us, which is much faster than the ship and the forward clock.

But to the ship this trailing clock is speeding away, and experiencing time dilation compared to the ships clock.
This means the the trailing clock sends a signal which would be identical to the leading ship, were it experiencing time.
Us as the fourth observer know that this leading clock is traveling faster than light (if it left the spaceship at all)

Would the ship only receive one signal? or Because the ship has no frame of reference , would the clock go faster than c?

Lets say the spaceship didnt use clocks. Let's say he used lasers.
Would his leading laser leave his spacecraft ? Would it appear to leave his spacecraft to the spaceship but not move to us as the fourth observer?

If light cannot break the speed of light, could you not at any velocity, throw a disco ball up and see which way the light goes slower?

So if you are already moving, you do not accelerate you do not slow down, you cannot know how fast(if at all) or in what direction you travel, you appear to be at rest.

And anything you launch away from yourself you assume is speeding up, because you believe you are at rest.

But isn't the speed of light completely independent from your speed/

When we are driving at 80mph and we throw a baseball at 80mph we know when this ball leaves our hand it is traveling 160mph

Does light follow this rule? If not wouldn't time dilation also not follow this rule?

 

[Samshorn]

You lifted that quoted phrase from Einstein's 1905 paper (first article) where he proceeded to argue that the "one-way speed of light" requires an additional definition. Do you know something that Einstein didn't know?

I said the "one-way speed of light" (with no further specification) is undefined, and hence requires an additional condition to be well defined and meaningful. You point out that Einstein said the same thing, and then you ask if I know something Einstein didn't. That's a bit of a non-sequitur, isn't it?

I think what you're missing (and what Einstein didn't emphasize as clearly as he might have in his initial paper, although in subsequent writings he did clarify) is that the extra condition represented by the definition of "inertial coordinate systems" in terms of Newton's laws (to the first approximation) amounts to stipulating the isotropy of mechanical inertia, and this is equivalent to the extra condition represented by stipulating isotropy of light speed. (This equivalence is unavoidable if you accept the inertia of energy.) Indeed this is what justifies the claim that the latter condition yields the inertial coordinate systems, just as does the former condition. That's why Einstein wrote that "With the given physical interpretation of coordinates and time, this [stipulation of isotropic light speed in terms of inertial coordinates] is by no means a merely conventional step, but... can be experimentally confirmed or disproved". You can read about this in any good book on relativity.

Again, the "one-way speed of light" is an undefined and arbitrary conception, but the "one-way speed of light in terms of coordinate system in which the equations of Newtonian mechanics hold good to the first approximation" is unambiguously equal to c, and this is an empirical fact that can be (and has been) confirmed experimentally. This is fundamental to a genuine understanding of special relativity.

 

[harrylin]

[..] Regarding the idea that time stops at the speed of light: If you examine the math, it's only valid with relative speed less than c. That's because if you plug 'c' as a reference speed into equations, you get an "undefined" result. [..]

Plugging in c doesn't work and of course it's impossible to reach that speed, but taking the limit gives you zero clock frequency. Einstein phrased the equivalent length contraction as follows:
the greater the value of v, the greater the shortening. For v=c all moving objects—viewed from the “stationary” system—shrivel up into plane figures." - the greater the value of v, the greater the shortening. For v=c all moving objects—viewed from the “stationary” system—shrivel up into plane figures.
- §4 of http://www.fourmilab.ch/etexts/einstein/specrel/www/

 

[harrylin]

That's a mis-use of the word "measured". The confounding that you are referring to is not between measured and un-measured speeds, nor is it between observed and un-observed speeds. Relativity is not a subjectivist theory. It's propositions refer to perfectly well-defined objective attributes of the external world, regardless of whether they are measured/observed. The correct distinction is between defined and un-defined conceptions of speed. [..].

I thought to make clear in that post, among other things, that SR is about making predictions of observations and not about metaphysics. But indeed I did not phrase one sentence well enough. Rephrasing: Maxwell thought that absolute speed can be measured.

 

[harrylin]

[..] The correct distinction is between defined and un-defined conceptions of speed. For example, the "one-way speed of light" can have any value we choose, because it is an undefined concept, whereas the "one-way speed of light in terms of a system of coordinates in which the Newtonian equations of mechanics hold good (to the first approximation)" is unambiguously equal to c. This is objectively true for any pulse of light, whether that particular pulse's speed is measured or not. So the distinction isn't between measured and unmeasured quantities, it's between defined and un-defined quantities. Or, to put it another way, the distinction is between knowing what we are talking about, and not knowing what we are talking about.[..].

You seem to argue here that Newton and Lorentz proposed un-defined quantities and didn't know what they were talking about. No further comment. (anyway it's all off-topic).

 

[ghwellsjr]

My thought experiment:

We as a fourth observer are watching a spaceship move from left to right, viewed directly in front of us.

This spaceship launches two clocks out, at exactly the same speed.
The clock which it launched behind itself is seemingly at rest to our perspective as the fourth observer, because the exact rate at which the spaceship was moving was canceled out.

To the spaceship, both clocks zoomed away from it in exactly the same way, and the spaceship would see both clocks ticking at exactly the same rate. which is slower than its own rate of time.

The spaceship already knows how fast each clock left its surface. The spaceship knows what distance is between the ship and the two clocks always.

after 10 minuits each clock sends a signal back to the ship how much time was accumulated during the 10 mins to the ship. let's say that the clocks both accumulated 8 seconds

What you guys (all of you) are trying to convey to me is that to the ship, both clocks have experienced less time, because to the ship both clocks zoomed away from it.

Fine.

Here are some spacetime diagrams to illustrate your scenario. First is the rest frame of the spaceship. The spaceship is shown in black with dots indicating the passage of each minute of time. The spaceship launched the red clock to the left and the blue clock to the right and the passage of minutes are also shown by their dots. The clocks are traveling at 0.6c:

Note that at the Coordinate Time of 10 minutes, the two clocks send a signal to the spaceship saying that their own clocks were at 8 minutes. These are shown as the thin red and blue lines and they arrive simultaneously at the spaceship when its own clocks is at 16 minutes.

Does this make perfect sense so far?

Now we transform the scenario to our rest frame, which also happens to be the rest frame of the red clock:

Now you will note that the red clock experiences no Time Dilation but the spaceship has the same Time Dilation that the clocks had in the previous frame and the blue clock has even more Time Dilation but still the signals sent at the speed of light arrive at the spaceship at its time of 16 minutes.

Are we clear on this?

Lets think of it this way.
Now let's say this ship was already traveling at 99.9%c. The ship does not know it is traveling at this speed.

Now you want to see what it looks like in a frame where the spaceship is traveling at 99.9%c. Unfortunately, this would require a very large screen or I would have to reduce the image to the point that the entire scenario would look like a bunch of lines along the 45 degree diagonal and you wouldn't be able to see any details. So how about we do it at 90%c, OK:

​

Now you can see that both clocks and the spaceship have different amounts of Time Dilation (because they are traveling at different speeds) but still the signals travel at c and are sent and received just like they were before.

Are you getting this? If so, I think that you should be able to answer most of the rest of your questions in the rest of your post.

 

[TumblingDice]

Plugging in c doesn't work and of course it's impossible to reach that speed, but taking the limit gives you zero clock frequency.

I'm not disagreeing with you, only trying to help the OP find a handle. You've jumped into the thread with facts that many of us already have a grasp on, and can digest with our own grain of salt. I don't think they're helpful to the OP. I understand limits - in this sense it's a convergence that can never be reached, like saying you can use X amount of energy to gain one-half of the difference between your relative speed and the speed of light. You can keep adding X, but halving the difference means you will never reach it. "Limit" is an acceptable mathematical term, and yet when you decided this had to be mentioned, I feel you detracted from helping the OP, choosing to add information that just blows smoke at this point.

I say this after embracing videos of Richard Feynman that I've watched that I learned of here on PF. One aspect I like is that Feynman was "all good" with keeping complications out of lectures when getting new students on board, freely explaining that "it's more complicated, but I'll tell you that later". To choose my own words, "A leader should never get too far ahead of their followers."

I'm more than game to discuss issues more deeply. Just don't think that's a good thing while we try to help questionator89.

 

[harrylin]

I'm not disagreeing with you, only trying to help the OP find a handle. You've jumped into the thread with facts that many of us already have a grasp on, and can digest with our own grain of salt. I don't think they're helpful to the OP. [...] I feel you detracted from helping the OP [..]

The OP made clear (or so I understood it) on that same line that he knew that the speed of light cannot be reached; just ask him if your arguing that the speed of light cannot be reached so that he should not phrase it like Einstein did was any help to him.

 

[Drakkith]

So from the perspective of the spaceship both clocks leave and are zooming away.
But to the perspective of the fourth observer watching this all happen, the leading clock went faster than the speed of light, and the trailing clock is at rest and experiencing the same rate of time that we are?

No, velocities do not add that way. If your ship, which is moving at 99% c according to the 4th observer, sends out two probes at 99% c relative to itself, the one sent to the front is now moving at something like 99.99% c with respect to the 4th observer.

If light cannot break the speed of light, could you not at any velocity, throw a disco ball up and see which way the light goes slower?

Nope, from your frame the light goes equally as fast in all directions.

When we are driving at 80mph and we throw a baseball at 80mph we know when this ball leaves our hand it is traveling 160mph

Does light follow this rule? If not wouldn't time dilation also not follow this rule?

Actually the baseball is not traveling at 160mph. The velocity would be very very slightly under 160 thanks to the way velocities add together in SR. It's just that 160 mph is so slow compared to c that the relativistic effects aren't able to be seen without exceedingly accurate measuring devices. The differences between SR and Classical physics at this speed are so small that we don't bother using SR. Classical physics is accurate enough for practically everything at this scale.