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Posts posted by md65536

  1. 23 hours ago, DimaMazin said:

    If they fall directly toward each other then what mass would be after such collision?

    There are additional replies better than I can give here: https://astronomy.stackexchange.com/questions/32445/head-on-collision-of-two-black-holes

    23 hours ago, DimaMazin said:

    Why creation of gravitational waves ,before their collision, takes only their KE and no significant mass?

    Why creation of gravitational bubble,at the collision, takes only their mass and no significant KE?

    If energy is lost from a system (in its CoM frame), the system loses mass. The binary system is losing mass equivalent to the gravitational wave energy radiated away, before the collision.

    If an individual component of the system doesn't radiate energy itself, that component don't lose rest mass.

    Radiating gravitational waves can reduce angular momentum, so the remnant BH should be decreasing its angular momentum during merger and ringdown.

    After they're merged, the system only has the one object in it. Your question is basically asking about the components of the system before the merger, and the system as a whole after, that's the main difference. Also, I think the energy radiated while merging is vastly greater than that radiated during the inspiral phase.



  2. 13 hours ago, DimaMazin said:

    These are inspiral orbiting black holes. Yes, I see the numbers in the wiki add up to zero kinetic energy included. However if you look at the references eg. [4], the numbers don't add up exactly, and the uncertainties in the estimates of the black hole masses are so huge that it's not possible to give an exact figure for other energy in the system from the masses. The equation describing this would be more like, remnant BH mass Mf = m1+m2+everything_else_ignored - radiated_energy/c^2. It's likely that whatever is ignored is small on a scale of solar masses.

    These aren't two masses falling directly toward each other, they come together because their orbits decay due to energy lost to gravitational waves. The kinetic energy is being radiated away as they approach. When they merge they'd generally have net angular momentum, resulting in a spinning black hole. The rotational energy of a black hole contributes to its mass (https://en.wikipedia.org/wiki/Kerr_metric#Mass_of_rotational_energy). Certainly then, some kinetic energy of the orbiting black holes contributed to the mass of the resulting black hole. In this case, at about 8 solar masses of energy radiated, much much more than any energy not accounted for is lost to gravitational waves.

  3. 14 hours ago, DimaMazin said:

    Then the rest mass after collision should be = m1 + m2 - energy of gravitational bubble /c2 +KE/c2

    But really mass=m1 +m2 - energy of gravitational bubble /c2 + 0   Why so?

    What are you basing this on? You're saying it should be one thing but is "really" another, where do you get the latter from?

    If you apply your equations to quarks forming a proton, it really does include the KE. From https://en.wikipedia.org/wiki/Proton "The mass of a proton is about 80–100 times greater than the sum of the rest masses of its three valence quarks, while the gluons have zero rest mass. [...] The rest mass of a proton is, thus, the invariant mass of the system of moving quarks and gluons that make up the particle, and, in such systems, even the energy of massless particles is still measured as part of the rest mass of the system."

    In the case of a black hole you can't measure or assert any internal motion, but the externally measurable rest mass is still there and the energy is still conserved.

  4. 11 hours ago, Markus Hanke said:

    So the correct question would be why inertial frames are related via hyperbolic rotations in spacetime - that’s a very valid question, but it isn’t one that any of our present theories can answer. So to make a long story short, we don’t have an explanation of why this happens, only a description of it. That’s not the same thing at all.

    It's easy to explain the hyperbolic rotation relation in terms of other things. If you assume that the speed of light is constant, and consider a path of light c*tau long in one frame, then consider it in another frame (as commonly done with a light-clock at rest and moving, in thought-experiments used to explain time dilation), chosen to form a right triangle with a sides x and c*tau and hypotenuse c*t (right? am I messing up the details?). Then by the Pythagorean Theorem, the value of x^2 - (ct)^2 is constant for a given tau. That equation happens to describe a hyperbola. You don't need to explain why it's described by a hyperbolic rotation, if you instead explain why the speed of light is constant and why Pythagoras' theorem holds. "Hyperbolic rotation" is just an equivalent mathematical description of the relationship. But likewise, a constant speed of light and Pythagorean theorem could also be just descriptions of what's happening, equivalent to some other things. Maybe there's some simplest explanation of what's "really" happening, or maybe they're all just equivalent descriptions of what we observe.

    I think we have the same point, that you don't need to explain a given description of a phenomenon to explain the phenomenon itself. Likewise, you don't need things to "really" be rotating for a mathematical equation for rotation to be a valid description.

  5. 21 hours ago, geordief said:

    Is this true ?Have there only been statistical observations and never a one to one observation?

    As far as I know (which is not much and hopefully someone else can explain or correct me), whether an experiment is measuring the distribution of many particles or a single entangled pair doesn't matter much because the results are consistent. However, even with a single observation, I think there is always an inherent randomness to them (at least where probability wave functions are involved), which makes drawing a conclusion from it probabilistic. A single photon doesn't show a diffraction pattern in a double-slit experiment, it just follows the probability distribution of the pattern.

    For example say you have an experiment to see if some action maintains the entanglement of a pair of particles, or breaks it. Suppose a measured event is expected to happen 70% of the time for entangled particles, and 50% of the time for random particles. If you observe a single pair and the event happens, were they entangled, or was it random chance? If the event doesn't happen, were they not entangled, or a random entangled outlier?


    My under-educated impression is, anything you do to make a result 100% certain ("collapsed wave function") removes the interesting effects you only see with the interaction of probability wave functions.

  6. 9 hours ago, Lorentz Jr said:

    Not directly, no. But entanglement suggests the possible occurrence of FTL communication within coherent systems, and that suggests at least the conceivability of harnessing such a process in some other way.

    Speculation isn't the same thing as magic. Lots of theoretical research introduces new concepts. An example of magic would be the relativistic explanation for time dilation in the twins paradox

    "Magic" wasn't the right word for me to use, especially since we're taking it to mean opposite things. Yes, entanglement gives some the idea of FTL communication, but when you look at the details of what's actually being measured, there's nothing there to actually do it. Like a trick illusion, where you get the impression of something extra happening, but if you look closely you see it didn't. The idea can still remain, because it's based on nothing real and can continue as such regardless of real measurements. Real measurements of entanglement show correlation (or at least statistical correlation), but not necessarily communication. The geometric explanation of the twin paradox can be measured using real measurements, so I disagree that it's magical.

    There are no quantum effects that have suggested any future experiment in which we would expect to see FTL communications. Speculatively, it's possible that some future experiment does show that it's possible. That speculation is effectively that there's something new that we haven't yet discovered that in some way goes against the observations and understanding we have so far.


  7. 7 hours ago, Lorentz Jr said:

    By "using entanglement" I meant using entangled particles to send an instantaneous signal, as the OP had asked about (or, more speculatively, possibly using whatever process underlies entanglement sometime in the future, if such a process exists and we can learn how to use it).

    Entanglement doesn't imply or even suggest the possibility of ever sending a signal faster than light. It is consistent with relativity in that sending such a signal seems theoretically impossible.

    You can interpret entanglement in such a way that some signal is sent faster than light, but you're basically adding your own "magic," something made up that hasn't been or maybe can't be experimentally observed. Magic could be something reasonable, or not, it's made up. Then if you ask, "Is magic forbidden by relativity?", the answer is probably no. Relativity has nothing to say about magic. A faster-than-light signal could break causality, which along with relativity can lead to paradoxes, so I suppose faster-than-light communication is forbidden by consistent causality plus relativity.

    Besides, special relativity doesn't forbid faster-than-light particles, it forbids accelerating particles from slower than light to faster than light. We haven't seen faster particles and probably shouldn't expect them to exist. It's other parts of reality as we know it that forbid them or using them for communication.


    Suppose you have a pair of sealed boxes each with a coin in it, and they have the property that the first time a coin is looked at, whether it's heads or tails, it's 100% certain that the first time the other coin is looked at, it will be the opposite. An equivalent device should be possible with entanglement. Say you have as many of them as you want. How would you use it to send a message from one coin holder to the far distant other, say a short message like 0 or 1 decided independently by someone else? Both coin holders can agree to a strategy beforehand. Anything anyone does to examine whether a coin is currently heads or tails counts as an observation.

    If you can find a way to send a message, it's certain that this is a bad analogy for entanglement, and not that entanglement allows sending messages.

    This is just an analogy, but there should be no way to force a coin to be either heads or tails without first observing what it currently is, and no way to know what's happened to the other coin, including whether it's been looked at. The weirdness here, if such a device existed, would be that you could shake both boxes, then open them to see that they're opposite, or you could shake both boxes, assume that they'd be opposite if you looked at them, and then purposefully flip one of them over, then look at them and see that they're only now opposite. This is an analogy only, and the analogy is that if you have 2 entangled particles, and you do different things to them but maintain their entanglement, they'll still maintain their entangled properties. In reality, excess manipulation will probably break the entanglement.

    It's mysterious and unintuitive, but not magic.

  8. 18 hours ago, swansont said:

    Net zero kinetic energy? If they each have KE, the system has KE.

    Yes, I mixed up KE and momentum. The kinetic energy of its parts contribute to the system's rest mass. In its center-of-momentum frame, the system has net zero momentum, and the total energy of the system is equivalent to its rest mass.


    18 hours ago, swansont said:

    Since it was measured by humans, it seems a given that it was from our reference frame.

    This is galaxy PG1211+143 they're observing, about a billion light years away. Wouldn't they compensate for cosmological redshift? Wouldn't they describe the speed of stuff falling into a black hole relative to the black hole, and not the Earth?

  9. 18 hours ago, DimaMazin said:

    let's consider next example when black hole whith minimal mass is falling into black hole with minimal mass.

    If their  kinetic energy > 2 millions minimal mass of black hole *c2       then where does the energy go after their collision?

    If they fall directly toward each other, it goes into the resulting combined black hole. Consider a simpler example of light "falling" into a black hole. Photons have no rest mass, yet adding light to a black hole increases its mass.

    Your system of 2 black holes has a rest mass in the frame of the center of momentum of the system, where the total energy (including mass and kinetic energy of the 2 black holes) is the mass of the system, and the system as a whole has net zero kinetic energy. Assuming no energy is radiated away as gravitational waves etc., the mass of the system is the same before and after they collide.

    On 11/11/2022 at 6:06 PM, swansont said:

    What’s ambiguous about “30% of the speed of light”?

    The coordinate system. Is it clear to you what coordinates or frame of reference they're talking about?

  10. Maybe OP can clarify? But in the meantime, a search shows headlines like, "Matter clocked speeding toward a black hole at 30 percent the speed of light". https://astronomy.com/news/2018/09/matter-clocked-speeding-toward-a-black-hole-at-30-percent-the-speed-of-light

    Is the meaning of that speed understood, or does it need more details? It seems ambiguous. I'd assume they mean different things depending on how far from the black hole the matter is.

  11. On 11/8/2022 at 9:11 AM, Markus Hanke said:

    spacetime between them isn’t flat, so these frames are not related by simple Lorentz transformations. Hence, asking about what the gamma factor between these frames will be is meaningless

    You could do Lorentz transformations for two local inertial frames at a location the event horizon passes. (However the horizon itself is lightlike so you can't use it as an inertial frame or even describe the speed of the object relative to the horizon except in some other given frame of reference.)

    Relativistic effects due to high speeds still occur in curved spacetime. I'm not sure how the effects combine but it seems easy to set up an example with A and B "nearby" and related by a Lorentz transformation, and a distant C related to B by gravitational time dilation say, and if light from A passes through B and reaches C then the total redshift between A and C will be the Doppler shift between A and B multiplied by the gravitational redshift between B and C (you can see this by letting B be a signal repeater and consider what it observes of A and sends to C).

    In this way you could separate the relativistic effects due to SR and GR in some specific example, and have "gamma factor" be meaningful, but not only would you have to define frames of reference and/or what the free-falling object's velocity is relative to, but depending on what one means, such as in my example, one might also have to define gamma if it's not standard.


    On 11/6/2022 at 4:49 AM, DimaMazin said:

    What is gamma factor of object, which is freely falling into black hole, on event horizon?

    Regardless of whether there's any sense in what I wrote above, you'd at least have to say what frame of reference you're using, and depending on that, different objects could have different velocities as they cross the horizon, so that would also need to be defined.


  12. 21 minutes ago, AbstractDreamer said:

    Yet despite all your responses you have repeated failed to address the important points; either due to a failure to understand my point, or you are blinded by a desire to be right and steadfast refusal to be helpful.   

    I feel like you're running around in circles. Altogether the response has been something like 1) There's no reason to either accept or 2) refute your claims because you're not making any testable predictions at all, and 3) you'd need some equations or quantification of effects that make a useful prediction and 4) it would need observational evidence that fits it for it to be treated seriously or accepted, and maybe 5) other stuff I'm forgetting.

    You're jumping back and forth attacking individual points without considering them all together. It's like it's a puzzle with many missing pieces and you're asking "why can't we ignore this one piece and work on the rest of the puzzle?" and others are asking to see one piece, any piece, show something, and each time they do, that's the piece you want to ignore next.

    This idea is probably perfectly acceptable except that it's missing all the pieces. Missing pieces doesn't mean it's wrong, just that there's nothing there that can be evaluated to see if it's right.

  13. On 9/30/2022 at 2:32 PM, joigus said:

    I still think this expansion would make sense in that it would reflect deviations from a model of universe with just matter density and spatial curvature. [...] The fact that something is very difficult to measure doesn't necessarily mean it cannot be measured, at least in principle.

    Yes, that seems valid. It doesn't matter whether they would measure it or find meaning in it, just that it's possible they could. It seems their meaning would basically be, "if there were other things not bound by gravity, the space between them is expanding."


    Just to add to what it all means: From https://en.wikipedia.org/wiki/Expansion_of_the_universe


    A metric defines the concept of distance, by stating in mathematical terms how distances between two nearby points in space are measured, in terms of the coordinate system.

    [few paragraphs later...]

    Specification of a metric requires that one first specify the coordinates used.

    Expansion of our universe is stated in terms of comoving coordinates where general galaxies (or "comoving observers") that have not been accelerated by forces or gravity, have fixed spatial coordinates. Their distances would be considered at a common comoving time, where "The comoving time coordinate is the elapsed time since the Big Bang according to a clock of a comoving observer and is a measure of cosmological time."

    So for example if you had 2 galaxies not gravitationally bound to each other, on a line, one located at (0) in comoving coordinates and the other at (1), we say their locations are fixed and that the measure of distance between them is increasing. Say we call the distance between them "one intergalactic unit", which I just made up. Here, the meaning of "the measure of distance itself (in these coordinates) is increasing" is that an intergalactic unit is equal to an increasing number of lightyears, over time. The measure of a lightyear can remain fixed. It doesn't mean that every possible measure of distance is increasing, which would be meaningless (see several other threads) because a change in a measure of distance is relative to something else.


  14. 6 hours ago, joigus said:

    If not for the loss of mass, the slightest anisotropy, or assymetry between both, would be enough to make the situation unstable for the galaxies, and they would be able to tell --eventually-- that spacetime is expanding.

    I agree, but then the topic's question could be reframed by that. Our universe and this toy universe are expanding due to vacuum energy or dark energy, but it would collapse if gravity was increased (put everything closer together, and/or add more mass). So if there was still this vacuum "expanding" influence on everything, but the universe collapsed, is there any way in which it could be said that the space itself is still expanding? Or is the metric still expanding?

    I no longer think this is a scientific question because the definition of expansion (as far as I know it) just doesn't apply. I think the question might be philosophical, and similar to asking "what is the true nature of the universe beyond what can be measured?"

    Conversely, for example say you have a toy universe with vacuum energy but where everything is gravitationally bound and collapses, except for one particle that is distant and not gravitationally bound, and separates at an ever-increasing rate. This fits the definition of expansion and I'd say that the space between the particle and the rest of the universe is expanding. This might be a stretch because the definitions are based on a universe that appears homogeneous.


  15. 11 hours ago, joigus said:

    the CM of both would be placed exactly the same.

    Yes, but the effect of gravity would diminish as the energy was carried away. This is what I thought you were talking about when you first mentioned photons. In a Newtonian analysis, a symmetric "shell" of outbound photons (or an expanding shell with mass equivalent to the photons' energy) would stop attracting a mass to the center of the shell as soon as the mass was inside the shell (shell theorem, which also implies the gravitational effect would not diminish before that happened). Based on that, you could reason something like, the photons from the first mass that pass by the second mass stop attracting the second mass toward the first, and start attracting it away.

  16. 21 hours ago, zapatos said:

    But expansion involves space, not matter, correct? Matter is not moved by expansion. For expansion to not occur on smaller scales seems to imply that matter is stopping expansion. Just because gravitationally bound objects don't separate due to expansion, I don't think we can conclude that space in that area is not experiencing expansion.

    But what is space without matter? Isn't it just distance, and specifically distance between things? Isn't the vacuum itself observer-dependent? Space isn't made of some "stuff" that is measurable independent of other things, I think.

    Anyway I think scientific answers don't depend on those questions, because science deals with definitions and measurements. If expansion is defined as "expansion of the metric", and not of "space", then it doesn't matter what space is. If expansion is defined only for gravitationally unbound objects, and defined as their increasing separation, then I think there's no expansion without that separation, and no way to apply it to gravitationally bound masses.

    But yes, it doesn't make sense that matter would "stop" expansion. For example, consider 2 comoving distant galaxies that are separating due to expansion. Then say there are 2 gravitationally bound small masses somewhere between them. There is metric expansion between the 2 galaxies. It seems fine to say that space is expanding everywhere throughout the distance between them. What is happening between the 2 small masses? The definitions can answer that! In comoving coordinates, the galaxies have fixed coordinates, and the measure of distance itself is increasing. In these coordinates, for the 2 smaller masses to remain a fixed distance from each other in their local coordinates, one or both must be changing location in comoving coordinates. That gives the answer, that space is expanding between the gravitationally bound masses and they're moving through that space (at the very least mathematically!).


    But then, we can also consider the local coordinates of the gravitationally bound masses, where in this example they're stationary. You really can say that the masses aren't moving through space (which means nothing more than that they're not changing location in these coordinates).

    The metric is covariant but I'm still not sure if expansion would be something that is an absolute part of the metric, or dependent on the coordinates.

    I think it's reasonable to say "space is still expanding here" but with the implication that it's referring to space generally, or in other coordinates, not space in local coordinates.


    19 hours ago, joigus said:

    A bunch of more photons than average escape in the direction towards the "neighbouring" galaxy, and bam, the galaxies start separating forever.

    Interesting, I hadn't thought of the physics of the model beyond being an example. I think that even a bunch of photons scattered in all directions would start separation because of decreased mass density.

  17. 9 hours ago, AbstractDreamer said:

    That is, the redshift shows that the receeding exceeds the speed of light, which violates GR, so there must be another mechanism at play;

    It doesn't violate GR. It is a prediction of GR given the values of several measured variables.

  18. Well I changed my mind again. Geodesics aren't some set of lines that remain fixed relative to absolute space, because there is no such thing. Whether something's moving or not will always (I think) involve a choice of coordinates. Also, whether 2 masses are gravitationally bound depends not just on their locations, but their relative velocity. So for example, two comoving fairly distant galaxies can be unbound and separating due to expansion, while two similarly located galaxies can be gravitationally bound just by giving them enough velocity toward each other (basically an "unescape" velocity to overcome expansion).

    So I think the answer to whether some specific effect is expansion or not, can depend on a choice, and doesn't have a single answer. To get around that, the definition exists only where it is applicable, eg. per wikipedia "The expansion of the universe is the increase in distance between any two given gravitationally unbound parts of the observable universe with time." As well, to say whether something is moving or not, "comoving coordinates" are used, not because that represents absolute motion or anything, but because most of the observable universe approximately fits them pretty well.

    I think whether expansion is occurring on smaller scales between gravitationally bound masses is literally undefined, and a definitive answer would require some extra definitions and that would necessarily involve making some arbitrary choices of what's moving and what's not.

  19. 4 hours ago, zapatos said:

    The analogy I use (don't know if it is necessarily a good one...) is imaging a ball bearing on a table with a magnet to the right of it and them representing the two masses, and a fan to the right of both and blowing toward them. The breeze is analogous to expansion and it always has an effect on the ball bearing, trying to push it away. The closeness of the magnet and ball bearing determine whether the ball bearing moves away, remains in place, or rolls toward the magnet.

    I think it's confusing because the expansion "breeze" would have to blow in all directions and not push on the ball bearing at all. But, the idea of expansion contributing and gravity contributing to the whole, makes sense.

    As for atoms expanding, what I've read is that that would only happen if the vacuum energy increased boundlessly, so that the scale at which expansion dominates gets smaller over time, approaching zero. But that would be speculative.


    I thought of 2 analogies that help me make sense of this.

    1. Imagine the Earth is a perfect sphere and it's expanding uniformly. A rigid tectonic plate or island could represent a gravitationally bound region. This is analogous to universal expansion at all scales, since even though the island doesn't change in size, the underlying size of the Earth's surface does. Note that for the island to stay a fixed size, some other part of the crust would have to expand more than the average expansion, for the whole Earth to expand uniformly.

    2. Imagine a patch of live skin cells that all slowly divide over time, so that any given area of skin doubles over some long time. A scab or patch of dead cells on it could represent a gravitationally bound region. In this case it's not needed for another area to expand faster to make up for the scab, because the whole isn't expanding with global uniformity and the distortion caused by the scab would be like curvature of spacetime.

    I think the second is a closer analogy to expansion in the universe, because it's attributed to local vacuum energy rather than some global expansion of the volume of space. I think the island analogy is wrong in that it confusingly represents space both as the crust (including the island) but also as some extra underlying thing that doesn't have a real analog in the universe.


    But beyond analogies as thought experiments, I think the technical answer needs to be based on the actual definition of expansion. Expansion refers to "metric expansion" which is a change in the measure of distance itself, rather than just a change in the distance between specific things. Well that's not helpful to think about, but what does it mean? The closest I've seen to a description of the technical meaning of expansion is that it involves a divergence of geodesics over time. I think in the case of a static gravitationally bound region, geodesics wouldn't diverge. It wouldn't matter how the rest of the universe was expanding, it would be like tattooed lines on a rigid scab.

    The geodesics would be like the fan and magnet contributing to a whole, or vacuum energy and gravity contributing to one metric. It would not be like the Earth analogy where one could imagine separate geodesics for an expanding Earth and a static island through the same place.

  20. 14 hours ago, AbstractDreamer said:

    I cannot show the alternative fits.  I'm asking why it cannot fit.

    I don't think anyone said it cannot fit? Just that it is meaningless. In fact there are published papers that propose exactly what you're saying, they're just not accepted as mainstream science, mainly because they don't account for everything that the accepted science does. Accepted science is always changing (usually slowly) but it changes to better account for actual observations, not imagined alternatives (unless it's a useful improvement). A lack of evidence doesn't prove your alternative doesn't fit, just that it doesn't improve anything.

    14 hours ago, AbstractDreamer said:

    I'm not dispensing of anything within GR.  Whatever mathematics are used to describe space expansion, such as the scale factor of expansion, or the cosmological constant, I am re-interpreting what some of the variables actually represent, rather than the  equations themselves.

    What it looks like to me, is that you're suggesting the possibility that instead of using GR to account for the universe as we observe it (where it fits very well for cosmology and large scales), we use it to account for something different than what we observe, and add in your proposals to account for the difference.

  21. The cosmological constant (CC) is associated with vacuum energy and its corresponding expansion, which is the accelerating expansion that dominates on the largest scales. There's also expansion (or contraction) in the Einstein field equation (EFE) without the CC (as in the original form of the EFE which didn't have the CC. The CC was first added to try to avoid expansion/contraction and allow for a steady-state universe). So, not all of expansion is an 'add-on'.

    That might provide the answer I was looking for. If the vacuum energy is contributing an expanding influence to the curvature of spacetime that is still present even at small scales, but the Einstein tensor overwhelms it and the net effect is no expansion or even contraction at small scales (eg. within a galaxy), then there's simply no expansion. So expansion is the end result, meaning space between comoving stuff is actually increasing, rather than just a component such as the influence of vacuum energy alone.

    I think my mistake is in thinking of space as a thing that is independent of the stuff in it, where for example the vacuum can expand without the stuff moving apart. I think I should consider it just as coordinates, but quickly get lost trying to think of expansion that way.

  22. On 9/23/2022 at 12:59 PM, MigL said:

    The fact is, the universe is only expanding at certain scales; below those scales, galaxies may actually be coming together, such as our galaxy and Andromeda, because the gravitational coupling exceeds the 'dark energy' of the expansion.

    IOW, expansion is distance ( scale ) dependent.

    I was about to reply to this saying basically that I think that space is expanding at all scales, but on smaller scales (even between Milky Way and Andromeda), gravity overwhelms expansion so that the galaxies are "moving through" space relative to each other faster than expansion can separate them. A google search shows only results that disagree with my view, and seem to suggest that if two things (in an otherwise empty universe, say) aren't separating, then I guess there's no meaningful way to say that that space is expanding? If two more-distant galaxies are separating at an increasing rate due to expansion, people use phrases like, "gravity has no effect at those distances" due to expansion, which is also not what I thought.

    Say for example you have 2 masses in an empty universe where their gravitational attraction exactly balances expansion of space between them, so they remain at a fixed distance.

    Would you say, space is expanding between them, but gravity accelerates them through that space at a rate that keeps them at the same distance? Maybe even there is a measurement that shows that gravity still applies and that expansion is also happening.

    Or would you say there is no expansion of space and no gravitational effect between the 2 masses in this system? It's nonsense to say the masses are moving through space or accelerating, because those only make sense relative to something else, and they're not moving relative to anything. There is no way to distinguish expansion and gravity, because any effect (like redshift) of expansion that would otherwise cause the 2 masses to separate, would be exactly cancelled out by an opposite effect of gravity that would otherwise cause the 2 masses to converge, and so no such effects are measurable.

    Is either correct?

    Could you also say that "gravitational effect" and "expansion" are just emergent effects of the metric, and are not fundamental and separate parts of the metric or the universe? It is the metric that has these two masses relatively stationary, and gravitational attraction and expansion are simply zero for these 2 masses? If so, then I think you could model or label the system so there is both a gravitational effect and expansion of space between the 2 masses, but you would only do that if you had a reason to separate them, otherwise it is simply an unnecessary complication.

    It seems, I shouldn't think of expansion as some intrinsic process that's happening throughout the universe, but is just a measurement that is a consequence of our universe's particular metric tensor?


  23. 11 hours ago, Mitcher said:

    time is the duration measured by a clock of a mobile covering a certain distance with velocity v. This is the best we have to define space and time.

    What is the time measured by a clock moving 0 m with velocity 0 m/s? How would you define an hour measured with a stationary clock? If time can be measured without motion, why would the "most basic" definition require motion?

  24. http://curious.astro.cornell.edu/about-us/102-the-universe/cosmology-and-the-big-bang/the-big-bang/586-did-time-go-slower-just-after-the-big-bang-advanced

    has the closest to an answer that I've found so far. From there:


    "clocks run slower in a deep gravitational field than they do in a shallower gravitational field". So it's true that if you compare a clock in the present epoch to a hypothetical clock in the early universe, just after the Big Bang, you'll find that time ran "slower" in the early universe because the mass density was so high. But does such a statement have any physical meaning?

    Not much. At the time when it was so dense that time went appreciably "slower", the whole Universe had that high density. So some hypothetical observer wouldn't notice anything strange, as they'd have no way of comparing their clocks to someone in a low density universe.

    It seems everyone agrees there's not a lot of physical meaning to comparing such clocks.

    I don't understand "clocks were slower because mass density was so high" enough to believe it. A "shallow gravitational field" doesn't make sense to me, I think he means higher relative gravitational potential, but like in the first post I'm not sure that gravitational potential even makes sense here.

    It seems that to compare the "gravitational depth" here, it would be more correct to compare "the metric tensor of the universe in local coordinates of a clock in the past, to the metric tensor in local coordinates of a clock in the present." Is that the right way to say it? It still makes no sense to me, because if you could compare those, why can't you just compare the clocks?

    But basically it sounds like evolving from a high density uniform universe to a lower density uniform universe due to expansion of space, can be considered to be moving to a higher gravitational potential --- and when I write it like that it sounds just plain wrong because there's nowhere to fall to, where the gravitational potential is relatively lower. However it seems like it would be measured as if it was the same, for example light from an older clock would redshift as it traverses through expanding space, losing energy as if it were climbing out of a gravitational well. So, I'm still confused.


    Edit: Or is it, the metric in local coordinates of a modern clock, could tell you the gravitational (only) time dilation factor between the two clocks? Or is it pointless to talk about metrics without a better understanding first?

  25. 8 hours ago, swansont said:

    Same is true for comparing a clock with itself.

    Does this mean it could be done (compare a clock with itself in the past while accounting for any effects related to the location of everything in the universe relative to the clock)? The implication is that any expected difference in the rate of the clock over time can be attributed to a change in the universe relative to the clock, leaving nothing that could be attributed to a change in time itself, in accepted theory?


    8 hours ago, AbstractDreamer said:

    What I mean by this is that the "rate that time propagates" is different at two different values of time, relative to a theoretical frame of reference that is "outside" or "independent" of time.  Now clearly there is no such frame of reference in reality.

    If it's made up and can't be measured, how can you possibly draw a useful conclusion from it? What's stopping you from imagining it to be something where the rate of time varies wildly relative to what we measure, and someone else imagining it to be something where the two measures are the same?

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