What is time?

[AnssiH]

Originally Posted by Boerseun

* Not really - a photon, travelling at the speed of light, experiences no time at all - the origin and end of the universe is one and the same moment for a photon - but it illustrates the point.

I'm glad you added that comment because I was just about to open my mouth when I read what you had said there. It's very important to understand the relativistic definition of photons properly, otherwise one is just talking self-conflicting nonsense.

Coming back to my post about "up" and "down" being as fundamental as time itself, the same "mystery" as to what actually contracts, apply to space, as well.

It's fundamental. It's space that's contracting, and time that's dilating.

Now that I think is a bit bad way to put it. Maybe you could say "it is the notion of space that is "contracting" and notion of "time" that is "dilating". But that can be somewhat confusing as well, so can I just ask the reader to think out something from this perspective:

Think of any situation mapped in terms of any arbitrarily chosen inertial frame.

Then think about your ability to freely choose any other inertial frame, completely arbitrarily so, and map the exact same situation in there (you will do this transformation in accordance to relativity because you know that transformation produces valid picture).

What you've mapped will have different lengths and timewise evolution to it, but it's still the exact same situation. Can you say that your decision about which inertial frame to use actually somehow contracted space and dilated time? I think it should be clear to all that the decision about which inertial frame to use does not actually change anything in reality out there (and I think the stone in Michael's shoe is that the terminology that is commonly used does imply that reality actually does care).

In other words, it is part of our definitions of "lengths" and "time" that they behave the way they do in a transformation that we know to be valid. I.e. relativistic effects occur in our worldview, in our head.

Now that is not to say that time dilation between 2 clocks is illusionary. I.e. they do actually display different reading, and there's a good reason for that, having nothing to do with ontological reality but everything to do with how we model reality. What I'm saying is that those relativistic effects on space and time arise from the definitions of multitude of things in our worldview. We do know relativistic transformations are valid, and then the real question is, why are they valid. Why is it that when you change the presentation from one coordinate system to another, the transformation affects our notion of time and space?

I hope you could wrap your head around the above, because that answer already exists...

And both space and time can only be said to exist because it can be measured. It cannot be defined in any lower-level terms, because it's fundamental. If there was any "constituent" lower-level elements with which it could be defined, then that would've been fundamental.

...epistemologically speaking there are of course underlying definitions to the modern definition of "proper time" (or just "time" as these concepts are sometimes lumped together a bit incoherently) and space, and I think you should be interested of looking into this issue as it will answer that question "why are relativistic predictions valid" in completely explicit manner, without postulating an ontological existence of some hypothetical entity.

I.e. it is explained why relativistic time relationships arise from underlying definitions of things like "mass" and "massless entities" (which will come to behave exactly in the same elusive manner as modern definition of photons), and from carefully performing a self-coherent transformation from one coordinate system ("inertial frame") to another. There are very good self-coherence reasons why relativistic transformation is required, and it would be valuable thing if more people understood this.

I'm of course talking about DD's presentation of the subject here:
http://hypography.com/forums/philoso...elativity.html

Understanding that touches directly the questions, concerns and undefendable claims that have arose in this thread recently yet again. I think if you are interested of really understanding the underlying mechanisms, you should pay attention to that thread.

One aspect of "time" in that presentation, what DD is also commenting on few posts above, is that a world model requires a notion (a parameter) for timewise-evolution of entities, and any idea of how a clock works (a massless oscillator between mirrors) is already using that sort of parameter by definition, as part of our model/expectations about what sort of measurement the clock is to produce.

I.e. there must exist an evolution parameter that is fundamental part of our definition of a clock, and it is very different parameter from what the clock measures (upon the final analysis). What the clock measures is exactly what is said to "time dilate", and if you really want to understand why that arises from the underlying definitions, please follow the thread.

Also if you had not yet noticed, I did have a stab at trying to explain little bit what is going on in that thread, for those who are not very familiar with what an epistemological analysis even is;

http://hypography.com/forums/philoso...tml#post271521

I really really would not want to scare anyone away, or make some claims and then hide behind a barrage of scary looking math. It is not half as scary as it might at first seem, and if I've been able to follow it, I think anyone can, with enough patience. I recommend.

-Anssi

[modest]

Originally Posted by AnssiH

Can you say that your decision about which inertial frame to use actually somehow contracted space and dilated time?

Does your decision about which inertial frame to use actually somehow change the kinetic energy of a particle?

~modest

[AnssiH]

Originally Posted by modest

Does your decision about which inertial frame to use actually somehow change the kinetic energy of a particle?

Hey not a bad example actually! Nicely done.

Yeah, people tend to use the kind of terminology where they say that a spaceship "gains kinetic energy when it gains speed", just like they say a spaceship length contracts when it gains speed. Of course, that situation is symmetrical to saying that instead of letting the spaceship gain speed, you just describe the same spaceship from different inertial frame (ignoring acceleration for the purpose of discussing special relativistic effects only).

Good suggestion as an example, as it is very easy for everyone to see how the statement about its kinetic energy is more properly a statement about what "would happen" if the spaceship hit an immovable wall, which is is at rest in such and such inertial frame. Choose a different wall, and you assign different kinetic energy to the spaceship. Even though we are talking about a measurable variable, it never was something that ontologically existed inside the spaceship somehow, it always was just something we defined as part of our worldview.

I think if you look up any explanation about kinetic energy, they will almost without fail talk about how it is the particle that is "moving" and via that motion "posesses kinetic energy".

Yup, sure enough Wikipedia chooses its words as thus; "The kinetic energy of an object is the extra energy which it possesses due to its motion. It is defined as the work needed to accelerate a body of a given mass from rest to its current velocity. Having gained this energy during its acceleration, the body maintains this kinetic energy unless its speed changes."

Anyway, I just wanted still to say, that DD's work does actually resolve relativistic time relationships in similar manner, via underlying definitions that are necessary for self-coherency reasons. So far all the nay-saying I've heard have been analogous to "of course kinetic energy is real and exists for any moving particle; we can measure it". I.e. coming from undefendable perspective and inability to see exactly how that perspective is undefendable.

-Anssi

[Michael Mooney]

As "cool hand Luke" famously said (in the movie), "What we have here is a failure to communicate."

I wrote in post 592:

We measure how long it takes events to happen and call it time. The reality is that events are always happening everywhere. As observers, we select certain *designated events* to measure the duration thereof. The reality is that earth is rotating, orbiting, etc. The measurement we call time is based on the duration of our monitoring of these events...
None of the above makes "time" into an entity. Yet we all find it meaningful and useful to say "It takes earth 24 hours of time to complete one rotation."

...and, in post 595:

This galaxy would still revolve the same, with the same "event duration" for one revolution whether there were an earth or not ( Ed: populated with measuring animals and their devices) to calibrate that great cycle in terms of the number of"earth orbits around the sun."

Closer to home, one earth revolution would take the same amount of "time" ( event duration) whether or not humans had ever evolved and invented all our various time-keepers.

Then Boerseun wrote, in post 599:

It's fundamental. It's space that's contracting, and time that's dilating.

And both space and time can only be said to exist because it can be measured.

The first sentence does not address at all the "fundamental question" ("what is space that it (an entity) can contract (or expand, as in the "inflation" version of cosmic genesis?), nor does it address the "fundamental question" "what is time, that it (an entity) can "dilate.) You, Boerseun, assume time and space to be actual entities with such properties, prior to any ontological basis for claiming they are such entities.

The second sentence seems to ignore my "thought experiment" above which posits a cosmos without humans or their clocks, (or measurements!) I ask then, without clocks or measurements, "what is time, in the real world, as it exists independent of human investigation and measurement? The latter (measurement) is posited as creating "time"... which makes "it" a human conceptual artifact, not an extant entity independant of human measurement.
("Measuring bioplasma and human aura-energy emissions" with various kinds of "aura-meters" does not establish the existence of auras, for instance.)

This argument seems to be consistently ignored in this forum. It will not go away until it is directly answered.
And again "time is time" (and it has the intrinsic prpperty of malleability, dilating, etc.) adds exacly nothing to the discussion of "what is time?"

Michael

[TheBigDog]

Just a not Mike, but it was the sadistic captain in Cool Hand Luke who said, "What we've got here is... failure to communicate. Some men you just can't reach." He was saying it because Luke would not obey his rules.

I always marvel at these conversations because truth be told, understanding the true nature of time doesn't change life any. It may be useful in a theoretical physics sort of way, but what is the practical purpose of clarifying our understanding of time?

For me it would be to solidify a truth, a formula, that would then be applied to understanding other concepts that still allude us. So, yeah, I guess there is a purpose. Never mind me. As you were.

I guess what I had was a failure of understanding.

Bill

[Little Bang]

Time can explain every phenomena in the universe, gravity, inertia, radiation, mass and probably charge. I just haven't yet figured out how it is responsible for charge.

[TheBigDog]

Originally Posted by Little Bang

Time can explain every phenomena in the universe, gravity, inertia, radiation, mass and probably charge. I just haven't yet figured out how it is responsible for charge.

Time is used as a frame of reference for all those things, that I can see, but how is it the cause?

Bill

[Little Bang]

Go here Bill, http://hypography.com/forums/physics...tml#post275849

[modest]

Originally Posted by AnssiH

Good suggestion as an example, as it is very easy for everyone to see how the statement about its kinetic energy is more properly a statement about what "would happen" if the spaceship hit an immovable wall, which is is at rest in such and such inertial frame. Choose a different wall, and you assign different kinetic energy to the spaceship. Even though we are talking about a measurable variable, it never was something that ontologically existed inside the spaceship somehow, it always was just something we defined as part of our worldview.

I think if you look up any explanation about kinetic energy, they will almost without fail talk about how it is the particle that is "moving" and via that motion "posesses kinetic energy".

[my bold]

Yup, I agree. As every good relativist would tell you, it is not the particle which takes on energy from velocity. It is, rather, the particle's path relative to something else. This is pointed out well-enough by Taylor and Wheeler in the book Spacetime Physics saying “In reality, the increase of energy with velocity originates not in the object but in the geometric properties of space-time itself.”

Likewise—it is not the physical length of the ship which contracts nor the mechanics of the ship's clock which dilates when the ship is given velocity relative to something else. It is, rather, a property of spacetime itself—a purely geometric effect. Space and time in that inertial frame literally are contracted and dilated relative to our own frame.

I think that's what Boerseun was saying and I agree.

~modest

[AnssiH]

Originally Posted by modest

Yup, I agree. As every good relativist would tell you, it is not the particle which takes on energy from velocity. It is, rather, the particle's path relative to something else.

Well, I would say that's a bit careless statement as well; whether one assign the energy to particle's path or particle itself, it has still got exactly the energy one simply decides it has got (which inertial frame one happens to choose for plotting the path in).

I don't doubt your understanding of what "kinetic energy" properly means, I'm just bothered by careless communication. However you were to put it, it's important to point out that the values we assign to these parameters are entirely dependent on the inertial frame we happen to choose. I.e. parameters occurring inside our mind, not in reality.

I know those "good relativists" mean to say that once you have a particles path, it is a statement about the velocity of the particle in a given frame (no need to say "relative to something else"; if you have a path, it is by definition relative to the inertial frame it is plotted in ), and since it is a statement of velocity, it is tautologous to the statement about the kinetic energy of the particle on every point of that path (via the definition of kinetic energy)...

But it should still be pointed out to those not so fluent with relativity that, like velocity, that energy is also entirely dependent on the frame on which you plot the path. i.e. not something real, but something we assign to the particle in our head, as a shorthand reference to "what would happen" if the particle hit something in given frame.

(you might notice from the length of my response that I'm trying to be really careful with tying parameters to the underlying definitions, to show they are epistomological by nature, not ontological... something people just tend to skip without blinking their eyes

This is pointed out well-enough by Taylor and Wheeler in the book Spacetime Physics saying “In reality, the increase of energy with velocity originates not in the object but in the geometric properties of space-time itself.”

That I could take as a good statement, except for the first two words

I.e. I could take it as a good statement about the definitions of spacetime, but not very objective as an ontological idea.
See... ->

Likewise—it is not the physical length of the ship which contracts nor the mechanics of the ship's clock which dilates when the ship is given velocity relative to something else. It is, rather, a property of spacetime itself—a purely geometric effect. Space and time in that inertial frame literally are contracted and dilated relative to our own frame.

Believe me, I do understand how people look at relativistic spacetime as an explanation exactly to that conundrum with "parameters that are dependent on our choice of inertial frame".

It seems that, in those people's minds relativistic spacetime is an actual ontological entity (nevermind you are restricted to see it as a static construction*), as that would explain why relativistic time relationships are valid, i.e. they see it as entirely rational to think it actually and really is something that does exist.

What I was trying to point out earlier was that there are some very good underlying reasons as to why our definition of "length" and "time" behave that way, and it's got absolutely nothing to do with the existence of a hypothetical entity called relativistic spacetime (nor ontologically real "relative simultaneity").

If people cared to look, they would understand that the idea of relativistic spacetime is consequential to those underlying definitions, and also the statement "space and time in that inertial frame literally are contracted and dilated relative to our own frame" is exactly like saying "the particle literally gains kinetic energy relative to our own frame". (both statements are true if it means "literally in our head", but false if it means "literally in reality")

I.e., likewise with the statements of kinetic energy, I could take the statements about how spacetime contracts and does this and that as rational, if and only if it was made explicitly clear that spacetime is the construction inside our head, and it does all these things, inside our head, as a function of whatever inertial frame we choose to plot any given situation in.

Relativistic spacetime turns out to be a shorthand reference to the behaviour of rational definitions of "length" and "time", having nothing to do with how reality exists, but everything to do with how "lengths" and "time" have been defined. (If that sounds impossible, I think you are squarely omitting the definitions that must exist before any idea of "measurable length" and "measurable time" can be meaningful, most of all a rational definition of "entities that have got persistent identity to themselves".

None of that is to say that newtonian definition of space is ontologically correct. Once you understand the epistemological reasons for why relativistic transformations are valid (having nothing to do with spacetime), you also understand exactly why newtonian definition of space and time is invalid as a rational model of the underlying raw data. Without anything mysterious to the issue really.

Oh did I meantion it unifies quantum mechanics with relativity too? In that it becomes quite clear that the apparent conflict between them has to do with exactly the extraneous assumptions that people have in their minds, incl. spacetime.

-Anssi

*I think many of those same people also understand the consequential problems that arise from the idea that reality is a static spacetime, and they then keep the idea sufficiently vague in their head... I think those people especially should be interested in understanding DD's work, and thus resolve the ontological conundrums in their head.