Invariance.

[peter]

Here's an interesting way of seeing it: there is no difference between the speed you walk down the street at and the speed of light - it purely depends upon how much of that motion falls into the time part of space-time and our inability to perceive this as the four dimensional realitiy that it is. The faster an object (appears) to travel, the more of it's motion moves into the space part of the four dimensional matrix until you arrive at the top speed, that of light, where all (?) of the motion is in the space part and virtually none in the time. So far from being relative, motion is in fact invariant - it is everything else that is relative, which is why Einstein didn't approve of the name 'the theory of relativity', he thought it should be the theory of invariance.

Question; two boats travelling on a perfectly frictionless ocean, perfectly flat with no horizon, no stars and a featureless sky, (no magnetic poles either ), there is no experiment that can be performed that can determine their relative motions. True or false?

[Vraith]

Question; two boats travelling on a perfectly frictionless ocean, perfectly flat with no horizon, no stars and a featureless sky, (no magnetic poles either ), there is no experiment that can be performed that can determine their relative motions. True or false?

Hmmm...if they are moving in precisely the same way [[perfectly "parallel" in every relationship...including that the ocean/substrate is unmeasurable and/or unchangeable]]---probably not. But any and all differences provide at least some portion of measurement possibility.

These kinds of thought-experiments have the coolest [to me, anyway] dichotomy. They can be extremely valuable---provide insights and perspective that are nearly unattainable by other means.
Yet they are always across, on the other "side" of, a [so far] inexplicable/indescribable gap. It is impossible for there to be any physical/material universe in which that situation could arise. And yet---the thoughts and solutions can generate real, and relevant/usable, ideas in the material.
[[heh...sometimes the dichotomy annoys me, too---or at least the over-reach of applicability and/or the excess hierarchical elevation/ordering that occurs]].

[Zarathustra]

Here's an interesting way of seeing it: there is no difference between the speed you walk down the street at and the speed of light - it purely depends upon how much of that motion falls into the time part of space-time and our inability to perceive this as the four dimensional reality that it is.

There is also increase in mass and decrease in length. So it doesn't depend "purely" on the space/time issue as you describe (a description I've never heard before).

The faster an object (appears) to travel, the more of it's motion moves into the space part of the four dimensional matrix until you arrive at the top speed, that of light, where all (?) of the motion is in the space part and virtually none in the time. So far from being relative, motion is in fact invariant - it is everything else that is relative, which is why Einstein didn't approve of the name 'the theory of relativity', he thought it should be the theory of invariance.

I briefly tried tracking down the source for that, but couldn't find Einstein saying it himself. It's true that the speed of light is invariant, as well as the laws of physics, but nothing else is. Motion isn't invariant. I think 'relativity' is a better term, given how it upturned people's conception of a "fixed" reality where everyone measures things the same no matter which reference frame they're in.

Question; two boats travelling on a perfectly frictionless ocean, perfectly flat with no horizon, no stars and a featureless sky, (no magnetic poles either ), there is no experiment that can be performed that can determine their relative motions. True or false?

If they have relative motion, you can measure it relative to the ships themselves. If they are traveling parallel to each other at the same speed, they have no motion relative to each other, so there would be nothing to measure. If they are spinning, you could measure their spin in terms of centripetal force. If they were accelerating, you could measure their motion in terms of inertia.

[Fist and Faith]

Here's an interesting way of seeing it: there is no difference between the speed you walk down the street at and the speed of light - it purely depends upon how much of that motion falls into the time part of space-time and our inability to perceive this as the four dimensional reality that it is.

There is also increase in mass and decrease in length. So it doesn't depend "purely" on the space/time issue as you describe (a description I've never heard before).

Brian Greene's explanation of special relativity:

To get a fuller sense of what Einstein found, imagine that Bart has a skateboard with a maximum speed of 65 miles per hour. If he heads due north at top speed - reading, whistling, yawning, and occasionally glancing at the road - and then merges onto a highway pointing in a northeasterly direction, his speed in the northward direction will be less than 65 miles per hour. The reason is clear. Initially, all his speed was devoted to northward motion, but when he shifted direction some of that speed was diverted into eastward motion, leaving a little less for heading north

This extremely simple idea actually allows us to capture the core insight of special relativity. Here's how:

We are used to the fact that objects can move through space, but there is another kind of motion that is equally important: Objects also move through time. Right now, the watch on your wrist and the clock on the wall are ticking away, showing that you and everything around you are relentlessly moving through time, relentlessly moving from one second to the next and the next. Newton thought that motion through time was totally separate from motion through space - he thought these two kinds of motion had nothing to do with each other. But Einstein found that they are intimately linked.

In fact, the revolutionary discovery of special relativity is this: When you look at something like a parked car, which from your viewpoint is stationary - not moving through space, that is - all of its motion is through time. The car, its driver, the street, you, your clothes are all moving through time in perfect synch: second followed by second, ticking away uniformly.

But if the car speeds away, some of its motion through time is diverted into motion through space. And just as Bart's speed in the northward direction slowed down when he diverted some of his northward motion into eastward motion, the speed of the car through time slows down when it diverts some of its motion through time into motion through space. This means that the car's progress through time slows down, and therefore time elapses more slowly for the moving car and its driver than it elapses for you and everything else that remains stationary.

That, in a nutshell, is special relativity.

The faster you move in a space dimension, the slower you move through the time dimension.

[peter]

Is the speed of light the fastest speed there is, or is it just the fastest speed that anything can travel at (or indeed are these the same thing)?

If the sun were to disappear we would feel the gravitational effect instantly, but the light would take eight minutes to go out. How do we reconcile these two things: why is gravitational information transferred instantly but light information not?

(That is a great quote Fist. If only all popular science writing were as clear and lucid as that! )

[Vraith]

If the sun were to disappear we would feel the gravitational effect instantly

why is gravitational information transferred instantly

Unless I'm badly mistaken the answers here are:

We would not feel it instantly and

It isn't.

[Fist and Faith]

(That is a great quote Fist. If only all popular science writing were as clear and lucid as that! )

Yeah, he's pretty cool. I googled that one last night. Now that I'm looking at The Elegant Universe, I found the one I was actually thinking of:

When an object moves through space relative to us, its clock runs slow compared to ours. That is, the speed of its motion through time slows down. Here's the leap: Einstein proclaimed that all objects in the universe are always traveling through spacetime at one fixed speed - that of light. This is a strange idea; we are used to the notion that objects travel at speeds considerably less than that of light. We have repeatedly emphasized this as the reason relativistic effects are so unfamiliar in the everyday world. All of this is true. We are presently talking about an object's combined speed through all four dimensions - three space and one time - and it is the object's speed in this generalized sense that is equal to that of light.

[Zarathustra]

So it's another way to describe time dilation, I see.

I'm having a little trouble with the idea that everything moves at the same speed through space/time, merely with different proportions of that spectrum. The "speed of light" end of that spectrum seems easy enough: time slows down for objects approaching the speed of light, coming to a standstill when it reaches that speed. So there is a defined end, as well as an explanation why nothing can go faster. All observers in every reference frame will measure this speed the same, no matter how fast they are moving.

But when you move toward the other end of the spectrum, motion becomes relative again. Not every observer will measure velocity the same. Time will "speed up" for observers relative to how close they are getting to "absolute rest" (I suppose is the implication?). But there is no such thing as absolute rest when all motion is relative. So what the heck are they approaching? Could time be made to speed up infinitely, as we eliminate more and more relative motions (e.g. motion due to earth's spin, earth's orbit, sun's orbit of the galaxy, galaxy's motion in the local group ... etc.)? Or is there a terminus in this direction like there is in the other?

[Fist and Faith]

The "speed of light" end of the spectrum is one thing relative to another. So is the "absolute rest" end. Time speeds up, relative to each other, as they come closer to absolute rest, relative to each other. When they are moving at extreme speed, relative to each other, the time difference is noticable. As they approach absolute rest, time speeds up (either sees the other's time as speeding up), and the time difference can only be measured by extremely sensitive instruments. When absolute rest is achieved, time has sped up to the ultimate degree - simultaneity. All motion is in the dimension of time, and none is in any of the dimensions of space.

[peter]

So as with the speed of motion through space, there is a top limit to the rate of passage of an object though time, achieved when it attains absolute rest (Z's point about the relatively of motion notwithstanding {although is not what we are saying here is that all motion is invariant, not relative?})?

[Vraith]

So as with the speed of motion through space, there is a top limit to the rate of passage of an object though time, achieved when it attains absolute rest (Z's point about the relatively of motion notwithstanding {although

is not what we are saying here is that all motion is invariant, not relative?})?

Yea...umm...wouldn't the "object" have to take up zero space? How would it be an object? I mean..for consistency?? I'm not sure that follows...but at light speed time isn't just on pause, it doesn't exist, right??? So same principle at the other end?

on the invariance...aren't we saying something like the TOTAL is invariant---
so, the answer is always 4. but there are many arrangements of "numbers"/processes that can get you to it [[though it's really more of a topological and material arrangement than numerical/abstract.]]

[peter]

The four-dimension problem seems to be the key here; why do we experience the four dimensional entity of space-time as three of space and one of time. Why are they not four indistinguishable (in all but direction) dimensions like the three of space? And if they were - and I think this was the point being made in the book I was reading that prompted this thread - would all motion/stationary states being revealed to us in a single (ie. non-summational) state with he twisting and warping of the four dimensions of reality revealed to us?

[Vraith]

The four-dimension problem seems to be the key here; why do we experience the four dimensional entity of space-time as three of space and one of time. Why are they not four indistinguishable (in all but direction) dimensions like the three of space? And if they were - and I think this was the point being made in the book I was reading that prompted this thread - would all motion/stationary states being revealed to us in a single (ie. non-summational) state with he twisting and warping of the four dimensions of reality revealed to us?

See...I'm gonna improvise out my un-mental quasi-conscious butt this time, cuz I'm offended by your judgement of my mental butt...
that only works/matters IF space and time are fundamentally the same in some weird way...a way far stranger than the unification of the forces.
We "see," or "experience" them differently because they ARE different.
We experience light and sound differently because they ARE different---not because our brains sort/qualia-ficate them differently [though our brains DO do that] but because light waves and sound waves are not the same.

Suppose you are particle man. You magically become aware. You have a strange bunch of "knowledge" in your "head." This includes a thing that is "spacetime."
If nothing else is there...is there a difference between the space parts and the time parts? AND can you TELL the difference? Telling the difference gets into a huge post trying to delineate/explain billions of things--a post the size of the universe, probably. So pretend nothing else exists except you and the concept of spacetime, and a "mental" where/when map.

"You" are "in" Atlantis. You need to get to Babylon...which is Northeast of you. North is a dimension. East is a dimension. So Northeast is the "combination" of those. BUT--is it really? Or is it a dimension of its own? Is our space, or any space a particular "(number)D?" 0D, sure...1D...maybe...2D...are you SURE???

Nevermind that.

If you go North x distance, then East y distance, in any correct total of measured movements of north/-north and east/-east, you will arrive to witness the fire on Babylon.

SO---you can take a trip south to Caracas, or west to California, or "Up" to Capella [since you are by yourself ] and still get there.

The spacetravel doesn't matter [as long as nothing happens on the way].

BUT---if you were born thirty years ago in Dundee, then moved to Elysium when you were ten [ignoring other events]...and you decide to take a different route through time...will you EVER get to Babylon? And if "someone" does get there, will it be YOU? [[and will ANY of the reasons you needed to be there even exist anymore?]]

I mean, if you choose to be born in Flexville, a totally different mother will be birthing you, some other doctor will be slapping your ass and making you cry...and you won't even KNOW about Elysium, Dundee, Capella, Babylon, or Atlantis.

If you're listening to Nena song 99 Red balloons,---99, red, and balloons are RELATED---there are effects and affects between them [and you]. But they are not the same---you cannot perceive/experience them as the same unless you are insane. [[note--synesthetes don't ONLY hear a sound as a color...they hear the sound AND perceive the color]] Hell, they aren't even RED in the original. And that's just the WORDS.

King David is a useful aggregation. KingDavidness may well describe a "reality frame." Nevertheless they are also separate/distinct in multiple ways...Kinglyness doesn't become more Davidish [nor Davidness more Kinglish] by manipulating the grammar frame.

That was FUN!

[wayfriend]

Space and time ARE different. We have discovered some practical uses for a mathematical model in which they are similar, in certain ways, for certain applications. But this doesn't mean they are the same.

A model is like an analogy ... it's only good within certain limits.

[Zarathustra]

Space and time are different like matter and energy are different. They are two forms of the same thing, inextricably linked, such that one can be turned into the other. It's not just a mathematical model. Changes in velocity actually slow down time (as one's motion through time converts into motion through space). The same happens in gravitational fields: gravity slows down time (just as it warps space). This is not only true for certain applications, but true everywhere all the time in the whole universe. After a century, the theory has withstood an avalanche of verification in a plethora of different contexts. The continuum of spacetime is one of the most empirically verified facts of the universe. If it was just a useful mathematical model, with no corresponding reality "underneath" the model, it never would have overturned Newtonian mechanics.

[Vraith]

Space and time are different like matter and energy are different. They are two forms of the same thing, inextricably linked, such that one can be turned into the other.

Inextricably linked---that appears so at this point for both space-time and matter-energy, no matter what perspective/theoretical approach you take.

But linkage isn't fundamental sameness. And are matter-energy really just different forms of the same thing? Some say that, some times, for some purposes. But it isn't a known and absolute fact.

For instance, some physics forum I was at once upon a time noted this:
When a matter and antimatter particle meet each other they do not "release pure energy." The end "release" is two photons, both of which HAVE energy, neither of which IS energy.
All electrons are materially identical. But they can and do have different---and alterable---energy.
Another way: ice, water and steam are all different forms of the same thing. The difference between them is temperature---how much energy they have.

Even in QFT---which I've been looking at a lot lately---the 'field' vibrates locally---that's the particles. That comes close to creating an identity-except-for-form for matter/energy...but not really.

String views have their own oddities.

[peter]

But there was no corresponding reality underneath Newtonian mechanics, yet it also was able to pass the litmus test of experimental verification for a very long time. With ever more sophisticated methods of putting it to the test however, the cracks began to show, and ultimately it became, with the advent of relativity, exposed as just an instrument - a good one, but limited as Wayfriend observes, to a certain set of parameters beyond which it falls down. Isn't this failure we have of being able describe the math of relativity (which undoubtedly works) in a manner which corresponds to our lived experience of the world evidence of it's being just yet a further, albeit more sophisticated instrument that will ultimately be overturned by (possibly) the real deal. When we eventually get to this Shangri-la land of truth, won't all of the paradoxes be immediately resolved, such that we can slap our collective forehead and say " How could it have been otherwise!" Remember, Newtonian dynamics was also replete with absurdities; how could the influence of matter extend across empty space to effect the motion of other matter at a distance ..... nonsense!

[Zarathustra]

But there was no corresponding reality underneath Newtonian mechanics, yet it also was able to pass the litmus test of experimental verification for a very long time.

But there was a reality underneath Newtonian mechanics. His equations merely described that reality with less precision. Inaccuracy doesn't make a model an analogy. An analogy is a figurative similarity, rather than literal a description. A mathematical model that matches the evidence is a literal description of reality, not a figurative comparison between concepts. Accuracy in science is thus a measure of degree of experimental verification, not a measure of aptness of an analogy.

Now, it's true that the underlying metaphysical interpretation of Newtonian physics grossly mismatched physical reality. But that is distinct from his mathematical models, which were fairly accurate.* Einstein's insight was a metaphysical epiphany, the implications of which he later traced out in mathematical form. He realized that Newtonian metaphysics didn't logically make sense.

The great mystery of reality is that logical insights like this can then be used to explore implications in physical reality--implications that can be mathematically codified, predicted, and then measured in physical reality. That would not be possible on the "models are like analogies" view. In no sense is that chain of insight, logic, math, and empirical evidence an analogy. Analogies don't allow you to make mathematical, testable predictions. The confirmation of these predictions don't merely facilitate increased usefulness, they enable deeper understanding into reality ... into truth. [In fact, their usefulness in this context (e.g. technology) is a measure of their approximation of the truth.]

Einstein's insights were not merely the revelation of a more useful science, but one that necessitates a revision of our world view. The previous world view could not be true if relativity "works" (and it does). Its consequences for the nature of reality are not poetic license, but logical necessity.

*[Though Newtonian mechanics are consistent with a Newtonian metaphysics, they are not dependent upon it. His equations do not eliminate the possibility of relativity, they merely obscure that as a possibility. His equations can be made consistent with a relativistic universe. In fact, that's exactly what Einstein's modification of those equations achieve, so that Newtonian mechanics are subsumed as a subset of a more detailed, accurate description.]

[peter]

But isn't science just the history of one 'instrument' being overturned by another, each one being simply a better description than the last? The 'planatary' system of the atom, Ptolomy's system of cycles and epicycles, Lamarkism........, each one being believed as being 'the way things really are' - and work within the paradigm confirming it to be so - until a new scientific revolution occurs and the old paradigm comes tumbling down to be replaced by the new. Have we really reached the point where this process is at an end, where we are looking into the fundamental reality of being? I have my doubts: the Standard Model is a mess requiring hundreds of putative supersymmetric particles (none of which there is an iota of experimental evidence for) in order to prop it up, unification of the two key theories of physics stubbornly refuses to reveal itself and ninety five percent of the matter and energy required to explain the behaviour of the universe cannot even be seen! Something (methinks) is rotten in the house of Denmark ........and there are many cutting edge physicists who think the same.

[Vraith]

A mathematical model that matches the evidence is a literal description of reality

[[[SNIP]]]]

Its consequences for the nature of reality are not poetic license, but logical necessity.

Heh...I get your point...but analogies---even the purely figurative ones---are more than poetic license.
A mathematical model that matches evidence and describes reality is still an analogy. It's just more accurate and has a different purpose than that of "young Dawn with finger tips of rose lighting up the world."

Analogy has ITS fingers lighting up logic all over the place.

And logical necessity can easily lead to falsehood and disaster UNLESS it begins with some accurate analog to reality.

A quibble, I suppose. ..but it's a little peeve of mine. Similar to when someone says that some issue is "just," or "only" semantics.

Heh #2---but you might be glad/interested to know Elon Musk mostly agrees with you...he's said that one of the biggest problems with finding solutions [[in everything, not just tech]] is too many people/systems argue from analogy instead of from first principles.