Why are planets spherical? Why are molecules spherical?

[jamiestem]

Some publications say its the sheer mass pulling in on itself, others say its due to the orbit and spin?

 

And also can the Milky Way be compared to a hurricane? What is in the eye? We are one of many solar systems and other debris flying around at what speed? Do we know that?

[Sisyphus]

Some publications say its the sheer mass pulling in on itself, others say its due to the orbit and spin?

 

Planets are spherical because they are pulled that way by their own gravity. Spin has nothing to do with that, but it can make the sphere bulge around the equator. The Earth, like most planets, is a slightly flattened sphere called an oblate spheroid. Molecules are not spherical, generally, but if and when they are it is not because of the same forces. Only very massive objects get pulled into spheres by gravity.

 

And also can the Milky Way be compared to a hurricane? What is in the eye? We are one of many solar systems and other debris flying around at what speed? Do we know that?

 

The Milky Way and a hurricane are formed by very different forces and have very different structures, and there's no particular reason to compare them. There is no "eye" in the Milky Way - in fact it is densest at the center. Different stars orbit the center at different speeds. Our solar system is currently orbiting the center at about 780,000 km/h (485,000 mph)

[jamiestem]

Cool. So if the sun orbits the Milky Way center @ 485,000 miles an hour and we orbit the sun @ about 67,000 miles an hour and the moon orbits earth at about 2,288 miles an hour hen how fast does are the orbitals of an atom? Wouldn't it all be relative to size?

[Sisyphus]

Cool. So if the sun orbits the Milky Way center @ 485,000 miles an hour and we orbit the sun @ about 67,000 miles an hour and the moon orbits earth at about 2,288 miles an hour hen how fast does are the orbitals of an atom? Wouldn't it all be relative to size?

 

No, it's not relative to size. The orbitals of an atom don't really have a speed in the same sense that the Earth has a speed going around the sun. "Orbitals," despite the name, are very different from orbits.

[lemur]

Cool. So if the sun orbits the Milky Way center @ 485,000 miles an hour and we orbit the sun @ about 67,000 miles an hour and the moon orbits earth at about 2,288 miles an hour hen how fast does are the orbitals of an atom? Wouldn't it all be relative to size?

I think that according to relativity, all objects move relative to the photons/light and other particles they interact with directly. For Earth to be moving at some speed relative to something outside the Milky Way, you would have to consider the trajectory of photons/light between the two, which could be red/blue shifted or length-contracted depending on their speeds relative to each other and in the direction perpendicular to the line connecting them (I hope I am getting relativity right). I believe there's also the problem, associated with the dark matter/energy hypothesis, that galaxies do not appear to be rotating at a speed that would conform to their estimated mass and size. So my question becomes what are the actual speeds of things relative to each other and are they really relative to each other or to other factors such as spacetime variabilities.

 

 

 

[Cap'n Refsmmat]

I think that according to relativity, all objects move relative to the photons/light and other particles they interact with directly. For Earth to be moving at some speed relative to something outside the Milky Way, you would have to consider the trajectory of photons/light between the two, which could be red/blue shifted or length-contracted depending on their speeds relative to each other and in the direction perpendicular to the line connecting them (I hope I am getting relativity right).

I don't think that works, because photons move at c relative to everyone. All objects move relative to all other objects, and velocity can only be measured in reference to some agreed-upon reference frame; using light as a reference frame would not make sense.

[mooeypoo]

I think that according to relativity, all objects move relative to the photons/light and other particles they interact with directly. For Earth to be moving at some speed relative to something outside the Milky Way, you would have to consider the trajectory of photons/light between the two, which could be red/blue shifted or length-contracted depending on their speeds relative to each other and in the direction perpendicular to the line connecting them (I hope I am getting relativity right).

You're mishmashing a few issues here.

 

First, that's not quite what relativity says, but it's okay enough simplified (as long as we stick to a simplified version of *special* relativity).

 

You can calculate the speed of the Earth relative to something else, i'm not quite sure how the speed of light fits into here other than, again, by the calculation itself (lorentz transform, etc) between the frames of reference. You will need to simply choose a frame of reference. What you should be careful of, though, is movement in an orbit, which is more complicated.

 

The speed of light *cannot* be a reference frame.

 

The redshift/blueshift is a RESULT of relative motion, it's not a detail you include to figure out the relative motion. So are length contraction and time dilation. You can calculate them out of the equations.

 

HOWEVER

 

You cannot use Special Relativity for the movement of the Earth around the galaxy -- movement in a circle includes acceleration, and special relativity is for motion *without* acceleration. General relativity depends on mass, and in the case of the Earth there are two main issues that arise - first, compared to the center of the galaxy the earth is extremely tiny mass, and second, compared to the entire galaxy "circumference" that the Earth (and the solar system) is moving through around the center of the galaxy, the Earth moves *REALLY* slowly.

 

So, it would be like using relativity to calculate the speed of a turtle down a ramp. It's too slow for it, relatively.

 

 

 

I believe there's also the problem, associated with the dark matter/energy hypothesis, that galaxies do not appear to be rotating at a speed that would conform to their estimated mass and size. So my question becomes what are the actual speeds of things relative to each other and are they really relative to each other or to other factors such as spacetime variabilities.

Either way, we can figure out the actual velocity for a reference frame.

Other than that, if I remember correctly, Dark Matter is actually OUTSIDE the galaxies (surrounding them) and not interacting within the galaxy with normal matter.

But on that particular point I'm not entirely sure.

 

I don't know what "Spacetime Variabilities" are.

 

~mooey

[Cap'n Refsmmat]

You cannot use Special Relativity for the movement of the Earth around the galaxy -- movement in a circle includes acceleration, and special relativity is for motion *without* acceleration.

Well, if you accept GR's version of gravity, orbits are just a straight-line (geodesic) path through curved space. Hence orbits can be inertial frames.

[mooeypoo]

Planets are spherical because they are pulled that way by their own gravity. Spin has nothing to do with that, but it can make the sphere bulge around the equator. The Earth, like most planets, is a slightly flattened sphere called an oblate spheroid. Molecules are not spherical, generally, but if and when they are it is not because of the same forces. Only very massive objects get pulled into spheres by gravity.

Just adding onto that --

 

If we take stars, for instance, which have an "easier" time rearranging their shape (because they're made of gas and plasma rather than solid rock) - the "best" shape for equal distribution of forces all around you in 3 dimension is a sphere. No matter which direction you look (from the center), the distance to the edge is the same (radius) and hence the FORCE that is applied on all the matter is equally distributed.

 

That's why the most "natural" or "comfortable" choice of shape is a sphere. If you take water, for instance, in space, where they don't experience the one-direction force of gravity to destroy the natural shape, they appear in perfect spheres. That's because the surface tension of the water is distributed EQUALLY over the entire mass -- and the only way to achieve that is by creating a perfect sphere.

 

While water and gas can more or less rearrange themselves and so are easy to get "perfectly spherical", rocks and more dense liquids are tougher; so the initial tendency for the object is to become a sphere, but then other forces apply (as Sisyphus said) and push the center outwards and make it "fat" around the equator. The Earth has a solid crust, though, that prevents it from "settling" on the originally "natural" perfectly-spherical shape; it has mountains and valleys and craters, etc.

 

So the Earth is not a perfect sphere, but gravity pulls all its parts (and similarly other planets and stars) inwards, and creates as close-to-a-sphere shape as possible before it solidifies and continues to have internal effects that change its shape a bit.

 

~moo

 

Well, if you accept GR's version of gravity, orbits are just a straight-line (geodesic) path through curved space. Hence orbits can be inertial frames.

Yeah, but then you have a problem with the mass, it's just too small in this case. You probably could calculate it using GR and then add some perturbation or something like that, but it sounds a bit odd to do that... it's too small and too slow, I think it's probably best to calculate it directly.

[lemur]

I don't think that works, because photons move at c relative to everyone. All objects move relative to all other objects, and velocity can only be measured in reference to some agreed-upon reference frame; using light as a reference frame would not make sense.

But how can objects move relative to other objects without moving relative to the photon streams between them?

 

You're mishmashing a few issues here.

 

First, that's not quite what relativity says, but it's okay enough simplified (as long as we stick to a simplified version of *special* relativity).

 

You can calculate the speed of the Earth relative to something else, i'm not quite sure how the speed of light fits into here other than, again, by the calculation itself (lorentz transform, etc) between the frames of reference. You will need to simply choose a frame of reference. What you should be careful of, though, is movement in an orbit, which is more complicated.

 

The speed of light *cannot* be a reference frame.

 

The redshift/blueshift is a RESULT of relative motion, it's not a detail you include to figure out the relative motion. So are length contraction and time dilation. You can calculate them out of the equations.

Maybe I am mistaken, but my general understanding of the logic of special relativity is that all things move relative to the speed of light. So that means that nothing can travel faster than light/photons in its own frame. So no matter how fast two objects are moving toward each other, the only effect can be for the light waves between them to compress (i.e. blue shift) until they merge or pass each other. So I think a reasonable extrapolation of this is that objects' motion is always relative to light and particles as they enter into direct contact with them, and every other frame in which objects are construed as moving relative to each other's position is more of an abstraction that ultimately rests on the direct interaction between them. I.e. speed is not really measurable in abstract distance but only in rate of blueshift/redshift of the light connecting two objects. Ok, your point about SR not being relevant for relatively slow-moving objects is valid from the point of view of applying analytical frameworks appropriately, but aren't all objects, however fast or slow, always moving relative to the photons they are absorbing and emitting and isn't this the deeper point of SR (or am I making too much out of it?)

 

 

Other than that, if I remember correctly, Dark Matter is actually OUTSIDE the galaxies (surrounding them) and not interacting within the galaxy with normal matter.

But on that particular point I'm not entirely sure.

I tried to be as conservative as possible in what I said about dark matter, since I don't know the specific details of various discussions. I just wanted to make the point that there's an issue of incongruence between the observed rotational speed and estimated mass of one or more galaxies. I don't know the details, and I hope I'm not perverting them. I just wanted to point out that we may not really know how fast a galaxy is rotating or what speed even means at that scale.

 

I don't know what "Spacetime Variabilities" are.

In other words, dark matter/energy like any other matter/energy acts to shape the contours of spacetime. So speeds, shapes, etc. are all subject to variability from what is observed from a given vantage point, correct? In other words, how much can we really know about the extent to which spacetime curvature is dynamic by observing celestial motion as it appears?

Edited March 30, 2011 by lemur

[Cap'n Refsmmat]

But how can objects move relative to other objects without moving relative to the photon streams between them?

Because no matter how fast you are moving, you will measure the speed of photons to be exactly c relative to you. If I compare my motion to photons, I will always get exactly the same answer.

 

However, I suggest you open a new thread if you want to discuss these issues further, or I can split things off starting at post 5.

[mooeypoo]

But how can objects move relative to other objects without moving relative to the photon streams between them?

They move relative to one another. The speed of light is constant no matter which reference frame you're looking at it.

 

Maybe I am mistaken, but my general understanding of the logic of special relativity is that all things move relative to the speed of light.

The reason we use "1/5th of the speed of light" or such in calculation is because it's a good mathematical reference, but in terms of actual reference frame, you can't look *from* from frame of light, because light remains constant in ALL reference frames. If I hold out a flashlight ahead of me and I move at 90% of the speed of light relative to you, then for me, the light beam moves at C, and for you, the light beam also moves at C.

 

There's no change in the speed of light between reference frames.

 

So that means that nothing can travel faster than light/photons in its own frame.

Nothing with mass can move faster than light. There are theoretical particles that are massless that, theoretically, can move at the speed of light.

So no matter how fast two objects are moving toward each other, the only effect can be for the light waves between them to compress (i.e. blue shift) until they merge or pass each other.

Not quite.

 

Say you hold a flashlight and move away from me at a certain speed. The light of your flashlight beam moves at the same speed - C, the speed of light - as I see it. What changes is the frequency. In that sense you are right that the waves are "compressed" or "stretched", but that's not affecting the speed of light, it's affecting the frequency of the light -- hence the "blue" and "red" shift. The frequency is "shifted" up or down and I see a different color.

 

So I think a reasonable extrapolation of this is that objects' motion is always relative to light and particles as they enter into direct contact with them, and every other frame in which objects are construed as moving relative to each other's position is more of an abstraction that ultimately rests on the direct interaction between them. I.e. speed is not really measurable in abstract distance but only in rate of blueshift/redshift of the light connecting two objects.

No. Read up.

 

Ok, your point about SR not being relevant for relatively slow-moving objects is valid from the point of view of applying analytical frameworks appropriately, but aren't all objects, however fast or slow, always moving relative to the photons they are absorbing and emitting and isn't this the deeper point of SR (or am I making too much out of it?)

Again, not exactly. The relative velocity will change the *frequency* of the emitted (or bounced) light, but never its speed relative to ANY frame.

 

I tried to be as conservative as possible in what I said about dark matter, since I don't know the specific details of various discussions. I just wanted to make the point that there's an issue of incongruence between the observed rotational speed and estimated mass of one or more galaxies. I don't know the details, and I hope I'm not perverting them. I just wanted to point out that we may not really know how fast a galaxy is rotating or what speed even means at that scale.

Yeah, that's how dark matter was discovered; the expected mass didn't fit what was actually observed. I suggest you stop mixing dark matter and dark energy, though. Those are completely two different issued that were discovered completely separately and have different effects.

 

In other words, dark matter/energy like any other matter/energy acts to shape the contours of spacetime. So speeds, shapes, etc. are all subject to variability from what is observed from a given vantage point, correct?

Again, split dark matter and dark energy.

 

Dark matter is some mass, so in that aspect, it "curves" spacetime just like any other mass does. We don't quite know what sort of particles comprise dark matter - it's a bit of an unusual thing since it seems to not be interacting with normal matter the way we expect it to. However, as far as I understand (and this isn't my expertise, by far), dark matter does have a gravitational effect.

 

However, I would be more comfortable if one of the experts could chime in here. I'm not really knowledgeable about dark matter. It was my understanding that it isn't *inside* a galaxy but rather surrounds it, for some reason it's "pushed out" of the galaxy boundaries. It's affecting the rotation, but not like the mass of stars and planets do. That is my understanding.

 

In other words, how much can we really know about the extent to which spacetime curvature is dynamic by observing celestial motion as it appears?

I'm not sure I understand that question? We know by our theories and the fact that our observations support these theories. When the observations do not support the theory, we re-examine said theory. That's how dark matter came to be known; something didn't fit, and scientists examined what is happening.

 

We can always say that there are things we don't know, but there are things we do know and quite a lot of them. We can use these to infer and to make calculations - those usually come out right, or, at least, give us a ballpark idea of what goes on.

 

In the case of this particular question about the earth's movement around the center of the galaxy it depends how accurate you intend to be. I assume that here we are probably going to be satisfied enough with general estimates, so in that case we don't really need to venture too far, it's probably enough to estimate the movement as circular (even if it's probably not QUITE that) and calculate directly from the known (or estimated) speeds.

 

If you want a more accurate result, we should likely mix in general relativity around our trajectory - other stars we might pass or stuff like that that will affect the solar system's trajectory in general. It seems to me to be extremely small corrections though. That's what I meant by using perturbations. You get a general equation for the motion of the planet and then "correct it" with estimated perturbations.

 

Since the earth (and solar system as a whole) is so incredibly tiny compared to the galaxy these corrections will likely be extremely small - so small that it's just as well to simply get the "general" result by calculating it directly.

 

~moo

[lemur]

Because no matter how fast you are moving, you will measure the speed of photons to be exactly c relative to you. If I compare my motion to photons, I will always get exactly the same answer.

 

However, I suggest you open a new thread if you want to discuss these issues further, or I can split things off starting at post 5.

I wasn't trying to change the topic. My point wasn't that photons change speed because I know they always move at C. My point was that an object's speed relative to another object is ultimately an amount of redshift/blueshift in the light connecting the two objects. So their distance/speed relative to each other really comes down to the amount the light waves between them are being compressed or expanded. No matter how fast or slow they move toward or away from each other, light will always be traveling at C between them, correct? Looking at the thread title, though, I suppose this is moving off topic, but it was a response to a comment by the OP about relative motion between subatomic particles, the Earth's revolution, and galactic motion all adding up to a massive total speed. That's why I felt it relevant to point out that everything moves relative to light. Is that incorrect or impertinent?

 

 

 

[Cap'n Refsmmat]

I wasn't trying to change the topic. My point wasn't that photons change speed because I know they always move at C. My point was that an object's speed relative to another object is ultimately an amount of redshift/blueshift in the light connecting the two objects. So their distance/speed relative to each other really comes down to the amount the light waves between them are being compressed or expanded. No matter how fast or slow they move toward or away from each other, light will always be traveling at C between them, correct?

Right, so you can tell the relative velocity of any light source by determining the redshift of its light.

 

That's why I felt it relevant to point out that everything moves relative to light. Is that incorrect or impertinent?

I don't think this is the right way to phrase your idea. Everything moves at the same velocity relative to light. However, redshift and blueshift can measure the relative velocity between two objects. It does not measure any sort of absolute velocity.

[lemur]

Thanks, you've explained this stuff more clearly than I did.

Yeah, that's how dark matter was discovered; the expected mass didn't fit what was actually observed. I suggest you stop mixing dark matter and dark energy, though. Those are completely two different issued that were discovered completely separately and have different effects.

 

Again, split dark matter and dark energy.

Sorry, I really didn't know they weren't related. I haven't read anything interesting enough about either to really learn them as meaningful concepts.

 

In the case of this particular question about the earth's movement around the center of the galaxy it depends how accurate you intend to be. I assume that here we are probably going to be satisfied enough with general estimates, so in that case we don't really need to venture too far, it's probably enough to estimate the movement as circular (even if it's probably not QUITE that) and calculate directly from the known (or estimated) speeds.

I guess my point is that it's not really necessary to assume any congruency between the way we model supergalactic 'space' in terms of something visualizable on Earth and the way spacetime actually behaves "out there." With all the unknown gravitational topography that can't be observed directly, I just think it makes more sense to conceptualize all motion as being relative to the geodesics that connect objects via their photons. That's the reality of the direct relationships between things, no?

 

If you want a more accurate result, we should likely mix in general relativity around our trajectory - other stars we might pass or stuff like that that will affect the solar system's trajectory in general. It seems to me to be extremely small corrections though. That's what I meant by using perturbations. You get a general equation for the motion of the planet and then "correct it" with estimated perturbations.

 

Since the earth (and solar system as a whole) is so incredibly tiny compared to the galaxy these corrections will likely be extremely small - so small that it's just as well to simply get the "general" result by calculating it directly.

Like I said, that all makes sense to me in terms of calculating estimates accurately. But what about approximating some grasp of what's really going on between massively distant observables? To do that, doesn't it make sense to acknowledge that spacetime has topography that can't be directly observed? How can we really know how much frames are accelerating and decelerating in areas where only light is traveling and who knows what forces are present to affect that light's path? How can we know, for example, that spacetime curvature has a lower limit and that space doesn't go on expanding, contracting, and bending beyond centers of observable matter? In other words, why assume that spacetime is flat when it's not curved instead of just assuming that light reaches us by some path of undefined topography? I'm not arguing to stop interpreting the sky as representing real objects - just that there may be a lot going on that goes unseen (isn't that basically the same premise as dark matter/energy theories are predicated on?) I'm just being more generic about the underlying premise because I'm not into the specific concepts and theories.

 

 

 

Right, so you can tell the relative velocity of any light source by determining the redshift of its light.

I know, but then you're still giving primacy to the motion between the sources without regard to the light-distance between them being absolute in the sense that no moving object can ever "skip over" photons between itself and its target, right?

 

I don't think this is the right way to phrase your idea. Everything moves at the same velocity relative to light. However, redshift and blueshift can measure the relative velocity between two objects. It does not measure any sort of absolute velocity.

But couldn't you just say that spacetime consists of a certain number of light-waves between you and your target-destination? If you speed up, the waves shorten, but the distance is really just measured in light-waves apprehended, right? I mean, you could accelerate to the point that infrared waves were shorted to x-rays and time would have slowed down for you considerably, but the number of waves would be whatever they were by the time you arrived. The same waves would have appeared much longer from the perspective of their source that for you, because you encountered them as x-rays, but both observers would observe the same number of waves passing between departure and arrival, regardless of the measured distance from either vantage point, right?

 

 

[Cap'n Refsmmat]

But couldn't you just say that spacetime consists of a certain number of light-waves between you and your target-destination? If you speed up, the waves shorten, but the distance is really just measured in light-waves apprehended, right? I mean, you could accelerate to the point that infrared waves were shorted to x-rays and time would have slowed down for you considerably, but the number of waves would be whatever they were by the time you arrived. The same waves would have appeared much longer from the perspective of their source that for you, because you encountered them as x-rays, but both observers would observe the same number of waves passing between departure and arrival, regardless of the measured distance from either vantage point, right?

I suppose. However, that can't be used as an absolute reference point, since you can't know the original frequency of the waves when sent by the light source. (If you tried to measure them with your clock, you'd get a difference answer than the sender.)

 

This is quickly getting out of my depth, however.

[lemur]

I suppose. However, that can't be used as an absolute reference point, since you can't know the original frequency of the waves when sent by the light source. (If you tried to measure them with your clock, you'd get a difference answer than the sender.)

I think you estimate that based on assumptions about the sender. I agree it is a problem I wonder about, along with you you can estimate distance without relying on shifted light as well.

 

This is quickly getting out of my depth, however.

Mine too. That's why I post things tentatively and welcome correction and/or elaboration.

 

 

[mooeypoo]

Thanks, you've explained this stuff more clearly than I did.

 

Sorry, I really didn't know they weren't related. I haven't read anything interesting enough about either to really learn them as meaningful concepts.

It's okay, I was trying to make sure that the answer to the OP is a bit more clearer and that we're not getting confused (and confusing everyone else too).

 

This, by the way, is out of *my* depth too. I learned special relativity and how it was derived, but I am far from being an expert in it. Still, some of the basic concepts you raise show a bit of a misunderstanding of the basics. No offense meant here, I'm just trying to see how to make things clearer.

 

I guess my point is that it's not really necessary to assume any congruency between the way we model supergalactic 'space' in terms of something visualizable on Earth and the way spacetime actually behaves "out there." With all the unknown gravitational topography that can't be observed directly, I just think it makes more sense to conceptualize all motion as being relative to the geodesics that connect objects via their photons. That's the reality of the direct relationships between things, no?

I am sorry, but I don't understand your point. We don't "assume" really, we test things out... observational corroborations are strong evidence.

 

It's not like we're throwing wild guesses to the air, or even "educated guesses". We have a theory that is tested on and off Earth. We use it practically and get accurate results for our calculations repeatedly. I think you may have a bit of trouble conceptualizing this, but that doesn't mean that the physics is unsound or inaccurate.

 

Physics isn't always intuitive. It's a problem

 

Like I said, that all makes sense to me in terms of calculating estimates accurately. But what about approximating some grasp of what's really going on between massively distant observables? To do that, doesn't it make sense to acknowledge that spacetime has topography that can't be directly observed?

How does that help you, though?

We know that there are things we don't yet know. We also know where our theories have failings, and we try to find ways to solve for these - either adjust the theories or find new ones.

 

Other than that, the knowledge that space-time might have a topography we're missing is not really relevant. The calculations are successful, so if we ARE missing something, it's likely realatively small. We probably do, so the knowledge serves to remind us there's much more to learn, but other than that, you can't really drop all your available tools and say you can't be accurate because it's possible you don't know everything.

 

You know enough to be as accurate as you can. It's supported by observation and evidence. We use our lack of knowledge to remind us to look for more.

 

What more can we do?

 

 

How can we really know how much frames are accelerating and decelerating in areas where only light is traveling and who knows what forces are present to affect that light's path? How can we know, for example, that spacetime curvature has a lower limit and that space doesn't go on expanding, contracting, and bending beyond centers of observable matter?

 

In other words, why assume that spacetime is flat when it's not curved instead of just assuming that light reaches us by some path of undefined topography? I'm not arguing to stop interpreting the sky as representing real objects - just that there may be a lot going on that goes unseen (isn't that basically the same premise as dark matter/energy theories are predicated on?) I'm just being more generic about the underlying premise because I'm not into the specific concepts and theories.

Woah. Okay, we're on a totally different ballpark here. This isn't really about relativity anymore, this is about the topology of spacetime.

 

<call to higher powers>Martin!!!!!</call>

(that's his domain)

 

Oh and I'm not sure I understand what "lower limit" of spacetime curvature means. Maybe Martin does.

 

And this *IS* out of the scope of this thread. Maybe we should be splitting it right about now.

(up until now we were more or less answering the 'how to calculate the speed of the earth around the galaxy' part. No longer.)

I know, but then you're still giving primacy to the motion between the sources without regard to the light-distance between them being absolute in the sense that no moving object can ever "skip over" photons between itself and its target, right?

What? I'm confused. I don't understand what you're asking.

 

But couldn't you just say that spacetime consists of a certain number of light-waves between you and your target-destination? If you speed up, the waves shorten, but the distance is really just measured in light-waves apprehended, right?

 

I wouldn't say that, no... It's like saying that the distance between New York and New Delhi consists of a certain number of oxygen molecules. That's not strictly wrong, but it's not really useful.

 

I mean, you could accelerate to the point that infrared waves were shorted to x-rays and time would have slowed down for you considerably, but the number of waves would be whatever they were by the time you arrived.

What do you mean by "number of waves"? What changes is not quantity (there's no 'quantity' really.. intensity, maybe, you mean?) but rather FREQUENCY. There's a difference. I think you might be getting confused over what light *is*. It's true that it's a dual nature (wave/particle) but you seem to be mishmashing it to a point of confusing yourself.

 

Photons move at the speed of light. Never faster.

Light has a few properties we defined to describe it; intensity, frequency, amplitude, phase, etc.

 

When you move fast - reaaaaally fast - the photons still travel at the same speed. No matter how fast you move.

The properties of the wave of light, however (its frequency, sometimes its phase) can change.

 

That's what happens in a red shift or a blue shift. The photons still travel the same speed. The wave of light is *seen* with a new frequency.

 

It's hard to imagine. What I always found a bit more simpler to *try* and think about it is actual waves at sea. Think of a boat at rough ocean. The waves of water travel with a certain speed. The boat itself, however, is just standing there, the only displacement it has is up and down (I'm simplifying matters and neglecting winds or currents that usually drag boats away... I just want to relate to *waves* at the moment, so bear with me).

 

This isn't really an equivalent to light particle/wave duality, since the photons *do* travel at the speed of light, but it might give you a notion of how the WAVE part and the PARTICLE (photon) part are separate in behavior.

 

 

The same waves would have appeared much longer from the perspective of their source that for you, because you encountered them as x-rays, but both observers would observe the same number of waves passing between departure and arrival, regardless of the measured distance from either vantage point, right?

 

This is easily calculated. I'm not sure I understand your example so I can't see where to correct it. If you want, I can show you the calculation (it's not too hard) and you will see how things work out just fine to either observer and color shift.

 

~mooey

 

I suppose. However, that can't be used as an absolute reference point, since you can't know the original frequency of the waves when sent by the light source. (If you tried to measure them with your clock, you'd get a difference answer than the sender.)

 

This is quickly getting out of my depth, however.

Not only that, but there is no absolute reference point.

 

That's the point, absolutely.

 

Har. Har.

 

~mooey

[lemur]

Other than that, the knowledge that space-time might have a topography we're missing is not really relevant. The calculations are successful, so if we ARE missing something, it's likely realatively small. We probably do, so the knowledge serves to remind us there's much more to learn, but other than that, you can't really drop all your available tools and say you can't be accurate because it's possible you don't know everything.

Well, the other side of that is to say that everything we would assume on the basis of appearance is valid even though we know that spacetime curvature is universal and contradicts what we assume based on what we see. There's no basis for saying that "if we are missing something, it's likely relatively small," because that implies that we know what we don't know, which we can't.

 

What more can we do?

Re-formulate our assumptions about celestial spatial relations in terms of space as a network of geodesic paths instead of assuming it to be a homogenous framework that more or less resembles as straight-line 3D grid?

 

Oh and I'm not sure I understand what "lower limit" of spacetime curvature means. Maybe Martin does.

I mean that we assume that spacetime reaches maximum "flatness" where it's not curved. Why not assume that it keeps expanding as distance from matter increases?

 

What? I'm confused. I don't understand what you're asking.

It wasn't so much a question as an issue; i.e. that distance between observer and observed is relative to the motion of light so distance is best expressed in terms of light waves traversed, regardless of how contracted or expanded those waves are for either observer.

 

I wouldn't say that, no... It's like saying that the distance between New York and New Delhi consists of a certain number of oxygen molecules. That's not strictly wrong, but it's not really useful.

That's because oxygen molecules don't change size according to the speed of the observer.

 

What do you mean by "number of waves"? What changes is not quantity (there's no 'quantity' really.. intensity, maybe, you mean?) but rather FREQUENCY. There's a difference. I think you might be getting confused over what light *is*. It's true that it's a dual nature (wave/particle) but you seem to be mishmashing it to a point of confusing yourself.

Light frequency isn't measured in wavelength?

 

 

 

[Janus]

However, I would be more comfortable if one of the experts could chime in here. I'm not really knowledgeable about dark matter. It was my understanding that it isn't *inside* a galaxy but rather surrounds it, for some reason it's "pushed out" of the galaxy boundaries. It's affecting the rotation, but not like the mass of stars and planets do. That is my understanding.

~moo

 

Dark matter does exist inside the galaxy. The way to look at it is that DM forms a large spherical "cloud" of nearly uniform density and the galaxy is "embedded" in this cloud.

[Cap'n Refsmmat]

Re-formulate our assumptions about celestial spatial relations in terms of space as a network of geodesic paths instead of assuming it to be a homogenous framework that more or less resembles as straight-line 3D grid?

Geodesic paths in what? Geodesics are heavily used in general relativity.

 

That's because oxygen molecules don't change size according to the speed of the observer.

Yes they do. Length contraction.

 

 

Light frequency isn't measured in wavelength?

Wavelength is inversely proportional to frequency. You could count wave peaks as they go past you, yes, and the number of peaks you count would be the same as the number of peaks sent by the light source. They would take a different amount of time to pass you and would pass at a different frequency. The frequency shift will tell you the light source's velocity relative to you.

[lemur]

Geodesic paths in what? Geodesics are heavily used in general relativity.

In other words, there could be many different paths between any two objects with very different distances and shapes. How can we know that spacetime remains relatively homogenous far away from pronounced gravity-wells? Maybe spacetime only appears integrated in the vicinity of mass because mass functions to organize spacetime into a relatively logical grid. Maybe spacetime consistency starts breaking down and fragmenting in strange ways in between distant masses.

 

Yes they do. Length contraction.

Maybe what I should have said is that there isn't a constant stream of oxygen molecules between distant objects the way there is light-waves.

 

Wavelength is inversely proportional to frequency. You could count wave peaks as they go past you, yes, and the number of peaks you count would be the same as the number of peaks sent by the light source. They would take a different amount of time to pass you and would pass at a different frequency. The frequency shift will tell you the light source's velocity relative to you.

Right, but to know the shift, you would have to know the frequency of the original waves, right? Regardless of what the original wavelength was, though, the number of peaks remains constant so that is a neutral unit for amount of (spacetime) separation between two points, no?

 

 

[Cap'n Refsmmat]

In other words, there could be many different paths between any two objects with very different distances and shapes. How can we know that spacetime remains relatively homogenous far away from pronounced gravity-wells? Maybe spacetime only appears integrated in the vicinity of mass because mass functions to organize spacetime into a relatively logical grid. Maybe spacetime consistency starts breaking down and fragmenting in strange ways in between distant masses.

Large-scale astronomical observations would seem to rule this out.

 

Right, but to know the shift, you would have to know the frequency of the original waves, right? Regardless of what the original wavelength was, though, the number of peaks remains constant so that is a neutral unit for amount of (spacetime) separation between two points, no?

No. The number of peaks varies depending on the source wavelength and frequency. Consider a stationary source and observer; longer wavelengths will have fewer wave peaks between the two. And the source frequency is dependent upon which frame you measure it from.

[lemur]

Large-scale astronomical observations would seem to rule this out.

Why? Why shouldn't light take multiple paths between source and observer and re-integrate to form a coherent image at the point of the observer? Isn't this the way electrons travel, i.e. as lighting bolts?

 

edit: when electricity travels through a consistent conductor, the transmission appears integrated and stable, but if the same current has to flow across a distance of air or other insulating medium, it can fragment and take erratic and multiple paths.

 

No. The number of peaks varies depending on the source wavelength and frequency. Consider a stationary source and observer; longer wavelengths will have fewer wave peaks between the two. And the source frequency is dependent upon which frame you measure it from.

I know that. I was assuming a situation where it would somehow be possible to trace the same beam(s) of light identifiable by both source and receiver. E.g. if a constant radio signal was present throughout the entire journey, the number of waves could be counted as the same regardless of how length-shifted they would be by any observer.

 

 

Edited March 31, 2011 by lemur

[mooeypoo]

Well, the other side of that is to say that everything we would assume on the basis of appearance is valid even though we know that spacetime curvature is universal and contradicts what we assume based on what we see. There's no basis for saying that "if we are missing something, it's likely relatively small," because that implies that we know what we don't know, which we can't.

Of course there's a basis, our observations *FIT* our predictions. There is no discrepancy. If there's no discrepancy, then either the discrepancy is EXTREMELY small, or it's nonexistent.

 

That's how you support scientific theories, lemur. You make predictions and then you check to see if these are supported.

 

Re-formulate our assumptions about celestial spatial relations in terms of space as a network of geodesic paths instead of assuming it to be a homogenous framework that more or less resembles as straight-line 3D grid?

Okay, seriously, you are losing me. Are we even discussing physics any more?

 

*YOU* seem to assume. I said no such assumptions about spacetime. I used the wave example as a conceptual example, not as a basis to construct our view of the universe. Spacetime isn't 3 dimensional, either, so obviously it's not a 3D grid.

 

Can you try to at LEAST stick to ONE subject. Please. I beg you. I'm getting insanely confused about what the subject matter is anymore.

 

I mean that we assume that spacetime reaches maximum "flatness" where it's not curved. Why not assume that it keeps expanding as distance from matter increases?

I'll leave any and all questions bout the nature of cosmological topology to the experts. I have no clue.

It wasn't so much a question as an issue; i.e. that distance between observer and observed is relative to the motion of light so distance is best expressed in terms of light waves traversed, regardless of how contracted or expanded those waves are for either observer.

Meh.

 

Okay, I apologize, but I am not sure where to start... you're making assumptions that are just not right, and I'm trying to see how to start explaining it without making everything just that much more complicated.

 

First, and once again, there is no reference frame for light. Hence, nothing is "relative to the motion of light". This statement makes no sense no matter how many times you'll say it I assume, at this point, that it is accidental. Do try to notice this, however, since it just serves to confuse me as to what, exactly, you're trying to ask.

 

Second, the distance doesn't contract, it's the length of the object that contracts and time dilates - as a result it SEEMS to the traveler that he(or she) traveled in less time than what the observer (in another frame) thought it took that traveler. Nothing actually happened to space - it's the relative perception that is changed...

 

That's because oxygen molecules don't change size according to the speed of the observer.

Yes, they do, length contraction, as Capn pointed out. As far as your answer to him;

Maybe what I should have said is that there isn't a constant stream of oxygen molecules between distant objects the way there is light-waves.

Sure there is. Unless an object is in a vacuum, there is constant stream of oxygen molecules on it.

I don't get the importance of a stream of light waves on an object, though. Other than making sure we see it later, what's the relevance? We can only see things that have light. That's about it for the relevance. No?

 

Light frequency isn't measured in wavelength?

It could be, sure, but wavelength is another property of the *WAVE*, not the particles.

 

~mooey

 

I know that. I was assuming a situation where it would somehow be possible to trace the same beam(s) of light identifiable by both source and receiver. E.g. if a constant radio signal was present throughout the entire journey, the number of waves could be counted as the same regardless of how length-shifted they would be by any observer.

 

 

 

lemur, PLEASE, stop inventing terms. There's no such thing as a "number of waves". You are making it very confusing to understand what you want to ask.

 

"Number of waves" makes no sense. Do you mean 'number of peaks' perhaps, as in higher frequency? do you mean more intensity? What number of waves? Wavelength? Wavenumber (1/wavelength) ?

 

I know you think I'm being a dork about this, but the terms exist for a reason. They define things clearly. I can't understand what the problem is if I don't understand what it is you are asking.

 

~mooey

 

Dark matter does exist inside the galaxy. The way to look at it is that DM forms a large spherical "cloud" of nearly uniform density and the galaxy is "embedded" in this cloud.

 

Interesting, I was under the impression Dark Matter was in "halos" around galaxies? Again, this is totally not my field, and completely and utterly above my head. http://www.universetoday.com/960/dark-matter-halo-around-the-milky-way/