Steady State Speed

Frame of reference.

Exactly!

There was a stupid bit on the news about the space craft going to land on that comet which is travelling at 300,000 mph or some such. Complete tripe of course because the only relevant speed it the speed of the craft relative to the comet, which is very slow.

It's all relative. Of course we tend to take the surface of the Earth as our frame of reference, but that has rotary motion as well as orbital speed around the sun.

But maybe the Sun is moving relative to the Great Cat of the Universe.

It's all relative to scale too... with my arrows I don't have to worry anout ant comsological effect... although gravity brings 'em down to earth pretty reliably. I don't think I'll achieve escape velocity with a longbow

Del

Now thats what we call "Delighthing"

Measure the wavelenght of the light emitted by that object at different points around it, any difference is an indication of motion, plus a mean to measure absolute speed and direction.

I meant to indicate that you are traveling with the object.

As such, all of your measurements will be in error,due to dilation,will they not?

If you are measuring it from outside,then there is a fixed frame of reference.

Steady state speed (or velocity) is relative, meaning that you have to specify a reference frame. For example, the person sitting in the seat in front of you in an airplane is at rest relative to you, but is moving at 500 mph with respect to someone standing at the airport. Both reference frames are equally valid. Specifying a velocity without a reference frame is meaningless, the same as stating that the drug store is 2 miles south (south from where).

Einstein's theory of relativity is only theoretically possible.

It is all relative though.

Does anybody really know how fast we are moving....? eh, relative to what...? wherever we are coming from....Well we know(or think we know) that we originated from the big bang, and have been accelerating away from that point 13 billion years ago, ever since....so the galaxy we are in is moving with great speed, the solar system is orbiting the center of the galaxy, and the Earth is orbiting the sun, and the Earth is rotating....

http://www.astrosociety.org/edu/publications/tnl/71/howfast.html

"Does anybody really know how fast we are moving....?"

I've been told that I move pretty slowly. Of course, that's an observation based on their frame of reference.

Your relative speed can be determined using the Doppler effect....If the horns you hear tend to go from low pitch to higher, you're going too slow....If on the other hand the horns you hear are increasing in pitch, you are going too fast.....or vice versa...?

"Does anybody really know how fast we are moving....? eh, relative to what...?"

Actually, yes. The simple reference point used by astronomers is the cold microwave background radiation, or CMB itsef.

The way that is done is to measure the CMB temperature from all directions around us. The CMB is relatively isotropic, so any velocity relative to that can be determined by its temperature shift.

So, if we are moving there will be a corresponding shift in temperature as seen from any two points 180° apart, which will also determine our direction. That is, looking in the direction we are moving we will see a slight shift positive and looking aft will show a slight shift negative in temperature as compared to 90° to any one side.

Now, Earth is moving around the Sun at about 30 km/s and the Sun is moving around the galactic core at about 200 km/s. However, if we ignore those velocities for a second, as they are cyclic in nature, our own galaxy has a motion of about 600 km/s (about .2% C) relative to the CMB.

This is how we determine our own vector relative to other galaxies around us and the universe itself. Pretty tricky, no?

I like it,, 1.34 million mph....we be zoomin'...

"In conclusion, galaxies experience neutral attractions on one other. Due to relativity, the speed of the Milky Way varies when compared with different objects in space. For example, I have learned from my research that the Milky Way and Andromeda galaxy are approaching each other with a speed of about 130 km/s, however the collision of these two galaxies will not occur for about 5 billion years (AstroFile). Another result I found was that our galaxy and neighbors are moving at 600 km/s in the direction of the constellation Hydra (Scientific American). Finally, I found that the Milky Way moves through space within the cluster of galaxies it is a member of, and this cluster in turn moves through space towards yet another larger cluster of galaxies off in the direction of the constellation Virgo. This speed is approximately 300 km/s (Ask the Space Scientist). Therefore, the speed of the Milky Way galaxy is not a single number, its value is relative to the speed of other objects."

http://hypertextbook.com/facts/1999/PatriciaKong.shtml

Correct.

Think of it this way. If the universe is a soccer field and all the players represent galaxies, then you can assign velocities for each player based on any frame of reference you want.

However, if you want to assign velocities to each player based on the soccer field as the frame of reference, then you would use the CMB to define your true velocity vector and calculate all the others based on their relative vectors to you.

I know I moved so fast through the '60's that I don't even remember being there!

If you can remember the '60's you weren't there.

I understand about relative speed,reference to another object,

but if one approaches the speed of light,does is not require a lot of energy,regardless of the surroundings?

What if one were in hypothetical "empty"

space,with no references,

(Except for you and your space vehicle)

Does space time itself influence the energy requirement?

Is it the "ZPEF"? that affects it?

I understand about Doppler shift of light,but without a reference from another point, it is meaningless.

Example: Two objects in space,both traveling at the 90% C,no other objects of reference in this hypothetical space.

One object accelerates to 99% the speed of C.

In reference to the other object, it is only going at 9% the speed of light.

Yet it requires a tremendous amount of energy to accelerate, a lot more than the first 9% required.

So how does space time "know" the object's speed unless there is some inherent

property of mass or space time that causes this?

OK, hypothetically you're almighty, so nothing stops you from sitting on that hypothetical moving object, surrounded by nothing (no reference frame) and try what I said in my post #2.

Take it or leave it, but tell me your reasoning.

Example: Two objects in space, both traveling at the 90% C, no other objects of reference in this hypothetical space.

If there is no other frame of reference then they aren't travelling at 90% C.

For example, If you were floating is space and could throw two objects in opposite directions at .999 C, each object would be travelling away from you at .999 C. The objects would also be travelling away from each other at about .9999 C.

I am not very good at explaining it. Maybe Jorrie will step in and clear things up.

Velocities don't add (or subtract) in Special Relativity. Two objects moving at .9C and .99C with respect to a reference frame are not moving at .09C with respect to each other.

Steady state speed is the point at which the number of words per minute uttered simultaneously by two or more women during a talk show discussion reaches the carrier frequency, without over-modulation.

Hilarious, very funny comment; but true, without a doubt. I've actually seen 'em going to saturation, when the number of words per minute surpass the carrier frequency thus you have no response beyond that point.

The reference point or inertial frame is really just an arbitrary point that we can assign to whatever is convenient for the measurement we are taking.

It is a little like the point on Earth that we assign as GMT. It could have been anywhere, but we standardized in Greenwich.

Now, steady state velocity with no acceleration can be determined by observing any other object around us and assigning the inertial frame wherever we want.

However, if you want to compare our velocity relative to the universe as a whole, then you would use the cold microwave background radiation (CMB) as an inertial frame. The CMB completely surrounds us since it is an isotropic part of the universe.

Any proper motion that we have relative to the universe can be determined by the variation of the CMB temperature around us. If we are moving we will see a slight shift positive in the CMB temperature in the direction we are moving and a corresponding drop in temperature behind us. This trick is how astronomers find not only our vector or direction we are moving, but the velocity of our movement relative to the universe.

Lastly, the energy required for an object to remain at a constant velocity is zero (assuming no other external forces are acting on it). It does not matter if that is .1% C or 99% C. The only time energy is required is when you change velocity.

Ok,I understand the CMB reference method,and that is a very good idea, but that still requires an outside reference.

Relative speed is well understood.But it is always referenced to another object.

I guess my basic question boiled down is why does an object increase in mass as it approaches the speed of light, and what is this speed (of C) referenced to?

It seems you're trying to apply the rules of physics of this universe to a totally different universe....The rules of physics could be completely different in another universe....

Ok, in our universe, an object traveling at near the speed of light(referenced to the CMB) elongates,and increases in mass,and time (as measured on or within that object) slows down.

Referenced to another object traveling at 1/2 C in the same direction, it is only traveling at 1/2C,relative to that second object.

Is the CMB expanding with the rest of the universe?

If so, the reference is changing,therefor any measurement is changing also?

Hi HTRN, to make your question more tractable, assume you are at rest relative to the CMB radiation (observing isotropic CMB temperature on average) and you observe an object moving relative to you at 0.9999c. Yes, you will observe the object to have a slowed down clock rate and as Lorentz contracted relative to you. Its mass does not increase, but relative to you it will have a relativistic momentum and inertia that is larger than the Newtonian value (by a factor 70.7). I guess you know all that.

Now if you consider another object moving in the same direction at 0.5c, the relative speed between the two is not 0.5c, but rather 0.9997c. You get this by the relativistic summation of velocities, which in this case is (0.9999-0.5)/(1-0.5*.9999)c=0.9997c.

On you final question, the CMB is not quite a reference point. It is the microwave part of the background noise of the universe. Once you have set yourself up as a reference point as in my opening sentence, then that reference will not change (in principle). What will change is the overall temperature of the CMB that will decrease over long timescales.

Does it help?

-J

I agree with you on those points,Jorrie,thanks!.

However,my question is somewhat hard to express.

Consider ,hypothetically,that you are on board a spacecraft that is moving through

totally empty space.

You are in a steady state,neither accelerating or decelerating.

You apply thrust with your hypothetical drive,and you begin to accelerate.

As your absolute speed increases towards C, it requires more and more energy to

maintain the same rate of acceleration,approaching infinity as you get closer and

closer to C.

Does this energy requirement not increase regardless of the reference point?

If not,why not?

Is not space time itself,regardless of other objects, creating this resistance to acceleration?

This would seem to imply (in my mind,anyway), an energy field of space time that

limits how many "lines of force" can be bent or broken by matter or energy,

similar to the stator/rotor(or armature/field) relationship of a motor or alternator.

A "slip" between the rotor and field is required to produce work or produce energy.

(Disregarding for the time being,small synchronous motors).

To attain the same speed as the field,an outside energy source must be applied.

In this case,energy not available in our universe.

There seems to be a big gap in my understanding here.

I believe, that if you are in "totally empty space", there is no reference point!

That is my point.

No reference point.

Speed in an absolute sense, not a relative sense.

I know the common definition requires the change in position of two points over a time interval.

Even if you could not measure your speed, by reference to an outside point,

if you accelerated at a constant rate,would you not eventually reach a point where

more and more energy was required to maintain the same acceleration,to the point

when you could no longer accelerate and be in a steady state,and by measuring this

energy requirement, you could determine your speed without reference to any

outside point of reference?

If you are talking just empty space in the real universe, there is always the CMB as a speed reference. In a hypothetical "empty universe" there is no speed in an absolute sense. Yes you can accelerate (as felt by the seat of your pants), but you will be going nowhere; just like if there was only one gravitational mass in the 'universe' and you were sitting static in its gravitational field and going nowhere...

-J

In this universe, with no means of observing any other object outside of your space

ship,you could still sense acceleration.

You could measure the amount of energy to sustain or increase or decrease this

acceleration,even if you had no reference to the outside CMB.

Whether or not there were any other objects around you for reference,as you

approached C, the energy required to maintain or increase acceleration would

increase dramatically.

So what I am saying is speed through space time is limited (to C) by some feature of

space time itself,not by reference to other objects.

So are there "lines of space time," analogous to magnetic field lines that impose the

speed limit"C" on all movement within it?

An object is only "allowed" to bend or break a certain number of these "space lines"

in a given time interval?

"Whether or not there were any other objects around you for reference, as you

approached C, the energy required to maintain or increase acceleration would

increase dramatically."

If you measure acceleration on-board (say by an accelerometer) and your hypothetical long-playing engine maintains constant thrust, then acceleration will increase as you use up fuel mass. It is only with reference to some inertial reference frame that you will approach C and the acceleration will decrease to almost zero, as measured by that frame.

In whatever inertial reference frame you may choose, the relative speed limit is C, as measured locally (where the object of interest is moving). Because the real, expanding universe is not flat spacetime, there is no inertial frame that covers large distances. As a result, there is no speed limit as measured over cosmic scales.

If the hypothetical spaceship keeps on accelerating forever, then relative to the reference frame of the "starting point", it will eventually exceed the speed of light in an expanding universe (the most distant galaxies are receding from us at twice the speed of light). Relative to its "local space", the accelerating spaceship will forever approach C, but never quite reach it. Now the question may be: "what is that local space?"

I guess it depends on point of view, but I think of it as the universal gravitational field, as experienced in that location. In that sense, it may be perhaps be stated as you wrote: "An object is only "allowed" to bend or break a certain number of these "space lines" in a given time interval". For this purpose the inertial observer that we used before, 'seeing' the CMB at the same temperature in all directions, is the perfect reference - being at rest relative to the local gravitational field. A whole horde of such observers may be used to represent the local "space lines".

-J

At some level I understand, but I am not totally comfortable with my understanding

of this matter.

Intuitively,there seems to be something amiss,or something that I have missed.

I feel like I have eaten a morsel instead of a full meal.

I can digest it, and it has nourished my curiosity, but it is still not satisfying on a

deeper level.

Of course I know one cannot trust intuition in matters of science,but something still

nags me for a better answer.

Like the four year old that asks a thousand times "Why?"

I guess that is the price of curiosity.

I will probably never be satisfied,but thanks to everyone for their efforts.

Another question pops up regarding your statement that the most distant galaxies are moving away from us at faster-than-light speed.

How could we tell,would they not be invisible?

Or is this an extrapolation based on the ones we can see?

It's a little off-topic, but we do observe the most distant galaxies and measure their cosmological redshifts at around z=7.5, giving them a proper distance* of around 29 billion light years (now) and a recession speed of around 2c. When the light we observe today originally left the galaxy, it receded at about 3.3c.

According to any credible cosmological model, the photons from the galaxy have always moved at v=c relative to the local gravitational field, initially being dragged away from us by the high expansion rate. As the expansion rate subsided, they entered a region of space receding from us at less than c and the photons could start to make headway towards us.**

If the spacetime (not space) was globally flat, the redshift z=7.5 would have translated to special relativity's recession speed of c=0.973. But spacetime is not flat if space is expanding, giving us the superluminal (v > c) possibilities.

Does this help ?

-J

* See the tooltip definition against "proper distance" in my Cosmo-calculator. You can put 7.5 into the redshift box, calculate and look at more results.

** The distance at which 'space recedes' from us at exactly c is called the Hubble radius.

Absolutely mind blowing! But let me add this: Does anyone really know what time it is? Does anybody really care? Oh my my! This thing about light dragging has me brain twitching (if that is possible).

Yes,it helps a lot!

Great link also.

Thanks!

But would you feel it in the seat of your pants? Doesn't that depend on inertia? From his rotating bucket experiment (as I'm sure you know) Newton concluded that the rotation of the water (causing the water to go up at the sides) was relative to what he called absolute space. I believe nowadays the idea of absolute space is frowned on and rotation (and linear acceleration) are taken as relative to distant matter in the universe, which gives rise to inertia. But how does it work? I believe an idea is that the effect varies inversely with distance, so the contribution from a shell of unit thickness at distance d, which contains mass proportional to d², varies as d. So the effect of distant galaxies overwhelms anything local.

But that implies in a very hypothetical "empty universe" there is no inertia and nothing to rotate or accelerate relative to. Do you know the current thinking? I'm sure you're more up to speed than me.

Incidentally, I believe there's some evidence that the whole universe is rotating, but relative to what? Maybe Newton was right all along!

Hi Codemaster.

Yes, good old Newton was probably not that far off the mark. The modern idea of global spacetime is that it is the universal gravitational field and spacetime geodesics relative to which things move; they just move somewhat differently from what Newton envisaged. So, in a sense it is the "distant matter", or rather "all the matter of the universe" that influence how things move.

Rotation and proper acceleration (measurable by a g-meter) are absolute, since they force the molecules of objects out of their present spacetime geodesics. Inertia manifests itself as the resistance of objects against changes to the (stable) relationships between spatial and time components of their spacetime vectors (loosely speaking).

So inertial movement can be defined as when objects maintain a constant internal space-to-time relationship, irrespective of any coordinate speed changes. This is perhaps the better answer to HTRN's original question: "what is steady state speed?"...

-J

OK thanks Jorrie

What about the Alcubierre warp drive...?

http://www.dailygalaxy.com/my_weblog/2013/10/our-star-trek-future-nasa-scientists-engineering-a-warp-drive-solution-for-faster-than-light-space-t.html

Try Amazon. ;-)

"I think the Alcubierre warp drive is not going anywhere soon..."

That statement is certainly worth a GA.

Agreed, the drive never moves through space under its own power, if I've read up on it correctly.

Maybe somebody will just stumble across a source one day....

....or maybe Dilithium crystals will be found to work somehow...who knows? But is it , according to the laws of physics as we now interpret them, possible that we could accelerate without any forces acting upon our person....0G, 0 drag..?

Yes, good old gravity does that, so if possible, the Alcubierre drive would also cause zero g on the person. I would prefer the 1g variety, which is actually very simple - an ordinary rocket engine will do just fine. The only problem is where to get the fuel, which should not be carried along, but somehow be harvested out of the vacuum of space.

To go to another star's planet, we accelerate at 1g for half the distance and then decelerate at 1g for the other half. And it can easily be done in a human lifetime, as experienced on board that rocket, with time depending on the distance of the star, of course.

-J

My understanding is that there are some experiments planned to test the physics of the idea on a microscopic scale.

However, as you pointed out, it looks like the resources to make a fully functional version is out of our technical reach and the need for exotic matter is another stumbling block (although I don't remember why exotic matter was required).

So THAT's how the big bang happened! Some guy in another universe was fooling around with +g matter and -g matter and somehow the two got together, and poof!

Prevailing theory is that in an alternative universe some chef accidentally poured pasta and anti-pasta into the same bowl about 14 billion years ago.

The rest is history.

Hmmm.... maybe if the whole universe was previously rotating rapidly enough it could provide a conceptually satisfying explanation for observations suggesting the rate of universal expansion continues to increase.

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Helpful metaphors could shift from raisins in rising bread pudding, to sloppy mud pie like big bangs being released in the center of a rapidly turning merry-go-round..... the mud expands outward at an increasing rate as it moves outward.

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The explanations of frame dragging always strike me as a bit comical when considering how much grief some people received for not dropping 'aether' theories quickly enough. Perhaps someone can elucidate the important distinctions that make the suggestions of an aether so wrong, but frame dragging reasonable.

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There is definitely no evidence for a rotating universe, but there is some direct evidence for frame dragging around rotating masses. Frame dragging is a general relativistic phenomenon and may perhaps be described as a manifestation of the "Einstein new aether".

In a sense it is the gravitational field that is dragged around the rotating mass, something that did not occur in the absolute "old aether" concept. They are mathematically and observationally totally different animals.

-J

Does this frame dragging create any resistance to the rotation of the object,similar to a

conductor in a magnetic field?

Is there a resultant energy created by the bending of the gravitation field?

With sufficient mass and rotation speed,could a wormhole be created this way?

Could there possibly be wormholes near rapidly spinning neutron stars?

Frame dragging creating resistance to rotation? Not intrinsically, unless interactions with other massive bodies cause rotational energy to be transferred by ejecting something from the vicinity. The "Penrose process" could be of interest. I have written something on it in GIGO-Garbage-In-Gigawatts-Out.

Wormholes might be associated with the centers of rotating black holes, but not neutron stars. I do not know much about wormholes...

-J

"Einstein's new aether"... thank you, I like the sound of that.

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"There is definitely no evidence for a rotating universe"... this one, though, doesn't sit quite right.

I read your comments with enthusiasm in part because I have become conditioned by the high frequency of receiving a reward in the form of thoughtful introduction to an interesting and previously unknown-to-me concept. I have come to trust and respect your commentary in part because you seem very conscientious about making distinctions between what is well vetted and what is, so far, just a neat idea.

Perhaps there is a way to know definitively that there is no evidence for something like rotation of the universe. I'm hopeful that I'm about to be rewarded with another introduction.

.

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I'm also hoping you might be willing to share your thoughts on a related hypothetical:

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In the hypothetical universe containing just the one space ship in which linear speed doesn't really mean much because there is nothing else to reference... hypothetically would the same hold true for rotation speed? Would the otherwise apparent centrifugal force be missing if there was no 'rest of the universe' in reference to which the sole object in the universe might spin?

I suspect that if the rest of the universe was absent (or on holiday), if you could still spin a bucket around your head. its contents would be secure in their place.

So, as long as the laws of physics have not changed, centrifugal force is still going to work like it does here on Earth as it would in an environment absent of all other mass.

As far as a spinning universe goes, there is anecdotal evidence to suggest that it might be based on studies that look at which way galaxies rotate.

If the universe is rotating, you would expect to see half the galaxies tending to rotate one direction in one hemisphere of the universe and the other hemisphere would be tending to be rotating in the opposite direction.

I think the jury is still out on this one, but it would give rise to a curious fact. That is, there would be a central axis to the universe, which would be a twist to the current theory that the universe is homogeneous and isotropic.

"....I suspect that if the rest of the universe was absent (or on holiday), if you could still spin a bucket around your head. its contents would be secure in their place....

So, as long as the laws of physics have not changed,..."

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A counter intuitive outcome might not require a change to our current understanding of the laws of physics.

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I do think you have changed the problem in a significant way by suggesting I swing the bucket around my head, since in that situation there is relative motion between the bucket and the rest of the universe, i.e. my head and attached parts not being swung around. I think we can still get to the crux via your construct....

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In this universe comprised only of me, the bucket, and its contents; will it work equally well as your suggestion, if I simply elevate the bucket to beside my head and then rotate myself, but not the bucket and is contents, around my vertical axis?

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If this universe, the answer is obvious, but in a universe comprised of only me, the bucket and it's contents, I'm not convinced the answer is so obvious. If space time is anchored to some degree to the mass in the universe, as frame dragging seems to suggest, centrifugal forces might seem apparent whether you spin up a single object, or spin up the rest of the universe around that object. Unfortunately, half of the experiment to test that hypothesis isn't really feasible.

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Suggesting that there is a meaningful distinction between:

A) an object spinning around an axis relative to everything else in the universe

and

B) everything else in the universe spinning counter around that same axis relative to that object;

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sounds a little like aether theory might be making a come back.

"In this universe comprised only of me, the bucket, and its contents; will it work equally well as your suggestion, if I simply elevate the bucket to beside my head and then rotate myself, but not the bucket and is contents, around my vertical axis?"

No. You can change the inertial frame of the experiment to consider different points of view, but the real problem with the second case is that the bucket does not change velocity (as a vector) relative to its own inertial frame, so there can be no acceleration, where there is for you.

As long as the laws of physics in this universe are valid in your hypothetical void universe, the outcomes will be the same. That is, any change in a vector will result in acceleration.

Even in an empty universe there is an inertial frame based on current and past positions of the object in space-time; even if that initial frame is arbitrary, which it really is in both our universe and the hypothetical universe anyway.

I understand that is your point. I appreciate your enthusiasm for your belief. What I wish you had included was some reasoning for this belief.

Spinning of anything is considered to be an absolute effect, with certain effects that are independent from which inertial frame they are observed in. The 'g-meter' on the spinning centrifuge will tell you there are centrifugal forces and it will be there even if the centrifuge was all that existed in otherwise empty space. The old Mach-principle is no longer considered valid.

The resistance to change in angular momentum (or moment of inertia) will also still be there. Again no theory or evidence saying otherwise.

On the universe spinning: as AH has said, there is no reliable evidence and neither is there a theory that tells us how a potentially infinite entity might spin. If we believe inflation theory (and there are more than one, but all saying similar things), it would have killed any rotation that the primordial universe might have had.

-J

Frame dragging, space time being pulled around with rapidly spinning massive bodies, would seem to strongly suggest that what is considered spinning, or actually 'not spinning' is affected by the surrounding matter.

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It would also seem to be a remarkable coincidence the surrounding stars and planets just happened to line up so well with the 'not spinning' state, but had little to actually do with each other.

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This is not my field. You certainly know better than I do. I guess I was just hoping for the clear logical explanation that usually accompanies your assertions.

I understand that you are stating there is no evidence, I'm just not sure why frame dragging and other phenomena are disqualified. I get the feeling these are ideas that have fallen into disfavor, perhaps that anyone discussing them should be discouraged lest they be discredited as a nutcase. Perhaps mass dependent relative spinning rather than absolute spinning could provide a decent explanation for the discrepancies between the observed and predicted motion of galaxies without having to resort to conjuring unobserved dark matter/energy just to make the numbers work. I'm guessing not. You probably even know why.

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I understand that you are stating the Mach Principle is not considered valid. My understanding is that Mach's Principle is not well defined, that there are many interpretations that differ considerably, and that Mach himself never made it clear exactly what the principal was. I appreciate you letting me know something is no longer considered valid, I'd appreciate it even more if you took it one more step to define what exactly that something is.

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I do recall that some important thinkers have help their version of Mach's Principle in high regard and have even drawn important inspiration from it....

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"....turns out that inertia originates in a kind of interaction between bodies, quite in the sense of your considerations on Newton's pail experiment... If one rotates [a heavy shell of matter] relative to the fixed stars about an axis going through its center, a Coriolis force arises in the interior of the shell..."

-Einstein in a letter to Mach.

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Of course, the advice of E.P. Box is probably more useful here:

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"All models are wrong, some are useful"

Frame dragging (Lense-Thirring effect) is but one observable effect near (say) a rotating black hole. By studying the path of light at various points, the orbits of particles and the magnetic field from some inertial frame outside the BH, we can deduce that it does not have a Schwarzschild (metric) spacetime around it. For me this is enough evidence that the BH is spinning in an absolute sense.

If that "absolute sense" is determined by all the matter in the universe (or more precisely, by the local manifestation of the total gravitational field), so be it. We do not have a way to test anything without that field being present.

Special relativity (SR) hypothetically 'whisks away' that gravitational field and then we have a flat spacetime where there are some unlimited number of inertial frames possible and each of them will spot a small rotating object; they will also spot any accelerating object. And we know what SR's observational status is - no discrepancy found so far.

-J

Thank you.

If you can't measure your speed, you can't have any acceleration...that requires a reference point.

Sure you can.

Acceleration still produces a force.

It's like standing in an elevator. At rest or at constant velocity you only feel 1 G. If it ascends or descends at a changing rate you feel more or less force imparted on your feet due to acceleration. You may not know how fast the elevator is moving, but you can easily determine and measure the acceleration.

Could the act of observing change this steady state speed?

If you happen to be standing in the way.

Just be grateful that we don't observe or rather perceive the CMB, and have no perceptual reference for hurtling along at 600 km/s. Small mercies.

This is all very simple. The steady skate speed is the speed at which you can maintain the smoothest rhythm on the ice.

What?

Oh,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,, never mind.

Escape velocity....

Looks like it was only half-way.

Speaking of relatives, Did I ever tell you about my great uncle Izzy?.....

There is no absolutely stationary body in space. It exists only in relative movement w.r.t. to other bodies. So any body not under acceleration in space can be considered as stationary or in uniform motion w.r.t. other bodies according to whether it is stationary or in relative motion w.r.t. them. The max. relative velocity a body can achieve w.r.t. any other body is C.

Well talking of relative speed, i do wish some of my relatives would show some speed and go away...

wait, except that woman, with some curves... maybe i would like her to show some speed in coming towards me.

"...i do wish some of my relatives would show some speed and go away...

wait, except that woman,..."

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so....you are wishing that one of your female relatives, specifically the one with the curves, hasten her approach to you?

"So if a body is moving at say,90% C, steady state, with no reference points for guidance or measurement ,how can one determine speed?" (Emphasis added)

The simple answer is: one cannot.

The long answer is: Relativity expands on Newton's Laws of Motion. The First law of Motion is usually described as:

'An object in motion tends to stay in motion, an object at rest tends to stay at rest, unless acted on by an external force.'

Taking Relativity into account, the First law of Motion can be described as:

'A steady state of motion is indistinguisible from a state of rest, unless measured against another object.'

Getting back to your question: with no reference points for guidance or measurement, the object cannot be described as either moving or at rest, it is in a 'steady state.' Only when it receives an acceleration force can it be described as moving, and only relative to the external force. Or if it acquires the view of another object, then the relative motion between the two objects can be described, but only against each other.

adreasler, I agree mostly, except this little bit:

"Only when it receives an acceleration force can it be described as moving, and only relative to the external force."

How do you suggest one measures movement relative to the force?

-J

The external acceleration will have its own velocity vector relative to the body, and what will be measured will be the difference between the two velocities until such time as the force stops or the body achieves an equilibrium with the force.

If the external force is completely external, like a wind or the flowing of a stream, you will be able to measure the speed of the force until the body matches the forces inertial frame, when the two will not be able to detect any change in the other anymore.

If the external force is bound to the body, such as a rocket engine firing, then the body will feel acceleration from the engine until the engine cuts out.

Due to the initial constraint that the body has no reference points to measure off of, the problem has been one of pure math, as there is no point within the physical universe that a body can exist without being able to observe some reference point. Even inside the event horizon of a black hole, one could still 'see' out, as there is nothing preventing more light from entering.

Technically, an observer would never be 'inside' the event horizon of a black hole, since from the observer's point of view, the horizon would always be some ratio of the distance between the observer and the singularity. Light from the universe would be coming at the observer from almost all sides, including light what wraps around the black hole due to gravity lensing, do the event horizon would keep shrinking away as the observer gets closer.

This is just looking at the basic gravity functions, not the time dilation or spaghettification that would be occurring.

I'm kinda wandering off-topic near the end, but the start is still on-topic enough for me to not self-OT.

"If the external force is bound to the body, such as a rocket engine firing, then the body will feel acceleration from the engine until the engine cuts out."

Yea, I think this is essentially what HiTekRedNek originally asked about. If you are in a windowless spaceship with that rocket thrusting, you cannot say if you are sitting static in a gravitational field or being rocketed away (equivalence principle). I think HTRN's issue was that if it was the latter, one must eventually approach the speed of light and the acceleration must decline.

The answer obviously still is that the acceleration as measured by an accelerometer will not decline, given that the thrust/mass ratio is kept constant. It is only an external inertial observer that will observe a decline in acceleration.

I do not want to throw in here what would happen to "steady state speed" on a cosmic scale, but it's an interesting issue (maybe worthy of a Blog posting).

-J

The thing that bothers me about all of this is what if it all comes to a screeching halt.

The good news is you won't hear the screech in a vacuum.

So "The big bang" is metaphorical?

Except on TV.