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09 Mar 2019 12:54

Hewerton22 wrote:
Source of the post It's like the "atmosphere" of the object, right? But it's very dense.

No, it's just a boundary of no return.
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09 Mar 2019 20:14

Hewerton22 wrote:
Source of the post It's like the "atmosphere" of the object, right? But it's very dense.

A black hole is completely empty (vacuum of space) except for some very small and dense point at the very center (the singularity).  There's no solid surface or anything at the event horizon.  In fact if you fall through the event horizon, you don't encounter or experience anything strange at all.  There isn't even an obvious visual cue for crossing it.
 
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11 Mar 2019 13:10

I was thinking about dinosaurs that have two legs and stand upright that could have eventually evolved hands like we have :)  I didn't know they already had opposable thumbs- that was an amazing advancement!  As far as intelligence, before the mass extinction event they'd probably evolved the intelligence of highly intelligent birds like Amazon Grey Parrots and Magpies which are known to recognize their reflections as such and can even understand the concept of 0, about a 5 year old level of intellect.

About a toroidal shape, can't we construct a toroidal form that averages out to a flat curvature?  But that would still contradict the principles of isotropy and homogeneity for the universe.
 
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11 Mar 2019 13:49

Levels of intelligence in animals are difficult to compare.  Ability to reason one specific way does not imply general intelligence.  A fascinating example is how chimpanzees can beat humans in memory tests:

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16 Mar 2019 15:15

Okay, I was reading more about special relativity and quantum mechanics, and came to this (possibly incorrect) conclusion. I read that mass can distort spacetime and "stretch" it, if there is enough of it, (the field model) or, if it has a high enough velocity (time dilation) (i.e anything that has or "adds" enough energy). I also read that quantum fluctuations have an energy of 10e+113 joules per cubic meter, and if all that I read here is true, does that mean that that without quantum fluctuations spacetime would "shrink"? because of all that energy coming from the fluctuations distort spacetime so much? Does this mean that the entire universe is in slow motion?   
 
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16 Mar 2019 17:58

Propulsion Disk, very interesting questions, and you're on nearly the right track thinking about them. :)

In simple terms, mass "bends" spacetime, in the sense of making it curved (but in a four-dimensional way which is difficult to visualize, though made precise in the math of general relativity).  Curved spacetime changes the way that objects move through it, which is the effect of gravitation.  Mass curves spacetime, and curved spacetime tells mass how to move.

This means that any energy density associated with the vacuum (vacuum energy) will distort the geometry of spacetime as well.  Since vacuum energy is the same everywhere, the curvature must also be the same everywhere, throughout the entire universe.  We refer to this vacuum energy on a cosmological scale as the "cosmological constant", and its effect is to accelerate the universe's expansion (also called "dark energy").  

Observations indicate the dark energy is real, but its strength is in very poor agreement with our understanding from quantum field theory.  The actual energy density of the dark energy is only about 6x10-10 joules per cubic meter!1  This sounds very small, but over very large distances the effect adds up to something measurable, and in fact it is the most significant form of mass/energy in the universe!  (69% of the total density, compared to 26% which is dark matter and 5% regular matter.)

If the universe had no vacuum energy, then the result would be no additional acceleration.  The expansion rate of the universe would instead slow down due to the combined gravitational effects of matter and radiation.  It would never slow all the way down to zero though (and never collapse in a Big Crunch) because there isn't enough matter and radiation to halt the expansion completely.


Using the cosmology code (see here for details), we can figure out exactly what the differences in evolution would be between our real universe (called the ΛCDM universe), and one containing no dark/vacuum energy.  I think the results are perhaps most easily visualized on a spacetime diagram, which I've plotted here:


With vacuum energy:

Image


Without vacuum energy:

Image



Time is shown horizontally in billions of years since the Big Bang, and space is shown vertically (by "proper distance" in billions of light years).  The present time is marked by a vertical gray bar.  The locations of galaxies are traced through time by the dotted white lines, with a thicker dashed white line marking the location of the most distant part of the universe that we can now see (the "particle horizon").  Finally, a solid yellow path extends backwards from our location at the present to the Big Bang, representing the path taken by light that is just now reaching us.  This is our past light cone.


With vacuum energy, we see that the universe's expansion initially slows down, but then starts speeding up again after about 10 billion years.  Galaxies are driven apart ever more rapidly in the future.  Without vacuum energy, it instead continues to slow down.  The galaxies recede from one another, but at a diminishing rate.


Another tremendous difference is the present age of the universe.  Without vacuum energy, the current age of the universe must be younger: 11.6 billion years instead of 13.7!  This is because it would take less time for the expansion rate to have dropped to the currently observed value without dark energy speeding it up.


The size of the observable universe would also be smaller, by about 40%!  This is because of both the reduced expansion rate, and the decreased age of the universe, so that light would have less time to reach us.


One other subtle effect is that the curvature of the universe would be different -- precisely because of what we said earlier about how matter and energy curve spacetime.  The curvature of the real universe is what we call "flat", in the sense that the geometry is Euclidean.  Without the vacuum energy, the spacetime would instead be negatively curved.  This means that parallel beams of light would spread away from each other, and the sum of angles in a triangle would be less than 180°.  The "radius of curvature" for this vacuum-energy-less universe would be about 17 billion light years, which would be very noticeable in observations of the cosmic microwave background radiation!



1:  To calculate the energy density of dark energy, we can multiply the total density of the universe which is very close to equal to the "critical density" of (3H2)/(8πG), multiply it by the fraction which is dark energy (about 69%), and then multiply by c2 to convert to energy density.
 
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16 Mar 2019 19:10

Sorry I can't see your image, something on my computer is blocking it, makes a few of the things you said confusing. But anyway, so mass BENDS spacetime and doesn't stretch or shrink it, is that what the "relativity of simultaneity" is? i'm guessing so, I still don't fully understand it yet, but i'm researching! Also, you say that 6x10-10 is the energy density of quantum flux, which is hard for me to believe, I understand that there is a discrepancy between two different values for the energy density, but I say that there is something that "cancels out" most of the energy of the flux that gets your value, but in truth, the undisturbed energy density I say is 10e+113 J/m3 because if not, the zero-point energy of the flux is wrong, and any well proven theories that involve ZPE are now wrong also. But thanks for the explanations, they really make you think, I guess relativity does that though.  
 
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16 Mar 2019 21:18

Propulsion Disk, the image was an animated gif, so perhaps that was the problem.  I replaced with two static images, see if it shows up for you now.

Propulsion Disk wrote:
Source of the post But anyway, so mass BENDS spacetime and doesn't stretch or shrink it, is that what the "relativity of simultaneity" is?

Not quite.  The relativity of simultaneity is a feature of special relativity, not requiring any mass or curvature.  It basically means that two observers who are moving with respect to one another will disagree on whether two events are simultaneous, because of how space and time transform between their reference frames (the Lorentz transformations).  

Example: If we imagine a spaceship 100 meters long (as measured by those on board) moving past us at 90% of the speed of light, with clocks on the front and back of the ship that are synchronized according to those on the ship, then we will observe that the clocks at the front and back are out of sync by 300 nanoseconds (with the clocks in the front of the ship lagging the ones in the back).  Again this has nothing to do with the mass of the ship, but only its velocity relative to us.

A visual way to think about this is that on a spacetime diagram, transforming to the other reference frame will cause the time and space axes to "rotate into each other", each shifted by the same angle towards the 45° line representing the speed of light.  By rotating the axes, this changes the sets of events that we say are simultaneous:






Propulsion Disk wrote:
Source of the post Also, you say that 6x10-10 is the energy density of quantum flux which is hard to believe, I understand that there is a discrepancy between two different values for the energy density, but I argue that there is something that "cancels out" most of the energy to get your value, but in truth, the undisturbed energy density I say is 10e+113 J/m3 because if not, the zero-point energy of the flux is wrong, and several well proven theories are now wrong also.

6x10-10 joules per cubic meter is the density of whatever sort of energy that is consistent with the observed acceleration of the universe.  We don't know what that energy is exactly, but the simplest working model is that it is a sort of vacuum energy, because it has the same density everywhere and that density appears to remain constant over time despite the expansion.  In general relativity, any energy density associated with the vacuum can be represented as a cosmological constant, which is that capital Lambda (Λ) in the ΛCDM model.

We (cosmologists) are also confident that the value for the vacuum energy for the universe cannot be as large as 10113 J/m3.  If it was, it would break the universe very badly.  A vacuum energy of even just 10-8 J/m3 would be inconsistent with the universe starting out dense enough to form the CMB and also having its current age and expansion rate. This high of an energy density would also make the spacetime so strongly positively curved (like the 3D version of the surface of a sphere, or hypersphere) that light would make multiple circulations around the universe.  We would see multiple images of the same objects all across the sky, as if living in a hall of mirrors.  Going as high as 10113 J/mbecomes ridiculous.  My computer can't even handle simulating it!  :D  

So I agree completely with you, I think the solution to this problem must be that the vast majority of the vacuum energy density predicted by quantum field theory somehow cancels out to be consistent with cosmological observations.  It's not clear yet what corrections to the physics must be made in order for that to to happen -- this is currently a significant research question.  PBS Space Time has a great episode discussing this as well, if you're interested:





Propulsion Disk wrote:
Source of the post But thanks for the explanations, they really make you think, I guess relativity does that though.

Absolutely!  I think relativity is one of the most interesting topics in physics, especially for how much it breaks from our intuitions.  We can retrain our brains to make a lot of it more sensible, but it is challenging, and stimulating, I think. :)
 
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16 Mar 2019 23:59

Watsisname wrote:

This particular video is probably the most important one to ever see if you want to understand causality and relativity, Propulsion Disk. If it doesn't make any sense, just keep re-watching it 'till it does.
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17 Mar 2019 00:02

Stellarator wrote:
Source of the post If it doesn't make any sense, just keep re-watching it 'till it does.


And ask questions, of course.  :P
 
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17 Mar 2019 00:03

Watsisname wrote:
Stellarator wrote:
Source of the post If it doesn't make any sense, just keep re-watching it 'till it does.


And ask questions, of course.  :P

More specifically, just ask Watsisname ;).
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17 Mar 2019 08:42

Watsisname wrote:
Source of the post Propulsion Disk, the image was an animated gif, so perhaps that was the problem.  I replaced with two static images, see if it shows up for you now.

Nope... they still don't work.
Watsisname wrote:
Source of the post Propulsion Disk wrote:Source of the post But anyway, so mass BENDS spacetime and doesn't stretch or shrink it, is that what the "relativity of simultaneity" is?Not quite.  The relativity of simultaneity is a feature of special relativity, not requiring any mass or curvature.  It basically means that two observers who are moving with respect to one another will disagree on whether two events are simultaneous, because of how space and time transform between their reference frames (the Lorentz transformations).  

Yeah I was reading more about it, I understand now. But thanks for clearing it up even more for me!
Stellarator wrote:
Source of the post This particular video is probably the most important one to ever see if you want to understand causality and relativity, Propulsion Disk. If it doesn't make any sense, just keep re-watching it 'till it does.

I'm pretty sure I got it now, i'm mainly intrigued by the "valley model" at the end of the video, it brings everything into a more imaginable perspective, but I have one question about the spacetime diagram, if you could somehow make time go faster, how would that look on the diagram? is that what he meant by "reversing the direction of your changing spacetime interval" and it would be plotted on the "uphill" section of the valley model? or would it be drawn out some other way.
Watsisname wrote:
Source of the post 6x10sup {vertical-align: super; font-size: smaller;}-10 joules per cubic meter is the density of whatever sort of energy that is consistent with the observed acceleration of the universe.  We don't know what that energy is exactly, but the simplest working model is that it is a sort of vacuum energy, because it has the same density everywhere and that density appears to remain constant over time despite the expansion.  In general relativity, any energy density associated with the vacuum can be represented as a cosmological constant, which is that capital Lambda (Λ) in the ΛCDM model.We (cosmologists) are also confident that the value for the vacuum energy for the universe cannot be as large as 10sup {vertical-align: super; font-size: smaller;}113 J/msup {vertical-align: super; font-size: smaller;}3.  If it was, it would break the universe very badly.  A vacuum energy of even just 10sup {vertical-align: super; font-size: smaller;}-8 J/msup {vertical-align: super; font-size: smaller;}3 would be inconsistent with the universe starting out dense enough to form the CMB and also having its current age and expansion rate. This high of an energy density would also make the spacetime so strongly positively curved (like the 3D version of the surface of a sphere, or hypersphere) that light would make multiple circulations around the universe.  We would see multiple images of the same objects all across the sky, as if living in a hall of mirrors.  Going as high as 10sup {vertical-align: super; font-size: smaller;}113 J/msup {vertical-align: super; font-size: smaller;}3 becomes ridiculous.  My computer can't even handle simulating it!    So I agree completely with you, I think the solution to this problem must be that the vast majority of the vacuum energy density predicted by quantum field theory somehow cancels out to be consistent with cosmological observations.

Ah yes, this topic, I watched the video you put up for this as well, and it gave some interesting thoughts, such as this. I read that electroweak energy comes from the interaction between negative and positive particles, (but that might be wrong,) and because of that, the energy density must also be positive and negative because mass is related to energy according to relativity, (E=mc2), the video said that an equal amount of -ZPE and +ZPE would cut down the energy density of quantum flux, but keep it above zero (equaling 6x10-10 J/m3), so in conclusion I think that quantum flux's energy density is being constantly "beat down, and helped back up again" by electroweak energy, but then again I don't know much about electroweak energy so please tell me if I made any mistake in my reasoning.  
But yeah that pretty much does it, probably the longest post I've ever made, but whatever, thanks for the videos and more explanations WHN!
 
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18 Mar 2019 01:20

Propulsion Disk wrote:
Source of the post is that what he meant by "reversing the direction of your changing spacetime interval"

No, that sentence is merely describing the speed at which you are accelerating to (+/- c). Since you are traveling 'uphill' on the curve when traveling faster then light, you alter your trajectory down the Minkowski diagram space/time.

I could answer this question more thoroughly, but at risk of messing up my physics and embarrassing myself, I'll let Watsisname explain it instead. Suffice to say that I don't think a faster procession of time for one observer would radically change the space-time diagram, aside from shortening the intervals between events for one worldline by the amount it is ahead of other worldlines, since all time is relative.
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18 Mar 2019 06:46

Stellarator wrote:
Source of the post I could answer this question more thoroughly, but at risk of messing up my physics and embarrassing myself, I'll let Watsisname explain it instead. Suffice to say that I don't think a faster procession of time for one observer would radically change the space-time diagram, aside from shortening the intervals between events for one worldline by the amount it is ahead of other worldlines, since all time is relative.

I know that it wouldn't CHANGE the spacetime diagram, I was asking, "how would that (be plotted) on the diagram?" would it go uphill on the diagram? or possibly on the other "downhill" slope opposite from ours (probably not), a picture would help the most out of anything.
 
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18 Mar 2019 19:47

Propulsion Disk wrote:
Source of the post if you could somehow make time go faster, how would that look on the diagram?

There's a subtle difficulty with this question.  What do we mean by "making time go faster"?  Time can refer to two different things: coordinate time and proper time.  

Coordinate time is what we mark along the time axis of our spacetime diagram, representing the passage of time measured by anyone who as at rest in our chosen frame of reference.  If we build our spacetime diagram to have us be at rest in space at the origin, then we move along the time axis at 1 second per second, representing the ticking of our clock.  There is no sense of changing this rate because we can only experience our clocks ticking at 1 second per second in our own reference frame.

Proper time is the time elapsed along some chosen path through spacetime, as measured by any clock fixed to the person or object following that path.  It is directly related to the spacetime interval, Δs2, along that path.  (Rigorously, the proper time, Δτ, is the square root of -Δs2.)  Because Δs2 = Δx2 - c2Δt2, for any slower-than-light speed, increasing the speed makes Δx larger for the same amount of coordinate time Δt, which makes Δs2 less negative, and therefore decreases the amount of proper time experienced on that path.  If the speed equals that of light, then Δs2 = 0 and there is no proper time experienced at all.  For speeds faster than light, Δs2 > 0, and the proper time is imaginary, which is another way of saying you can't go faster than light.  

So what we can change is the relationship between coordinate time and proper time.  We can do this by changing our velocity, or by changing the geometry of spacetime.  Moving clocks tick more slowly, and so do clocks in a gravitational field.  Of course this is just the effect of time dilation.  On the spacetime diagram, we can make a fast-moving clock tick off more seconds of proper time for the same amount of coordinate time by reducing its velocity, which also shifts its path closer to our coordinate time axis.  But I feel like this might not be what you have in mind by "making time go faster".

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