**Ask a monkey a physics question thread**

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Originally Posted by Renton

Would the fuel mass be so prohibitive if it weren't solid rocket fuel, but fissile material or some other dense fuel? I mean you're already going to need to provide the thrust required to make the craft get to cruising velocity, as well as slow the craft upon arrival, it's merely a matter of whether that thrust happens all at once at the beginning and end (which seems deadly to humans tbh) or gradually throughout the journey. This is especially the case if we're talking interstellar travel.

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I think all of your points here are worthwhile.

Fissile material has vastly higher energy density. There is really no problem with creating nuclear fallout in space. There are designs for spacecraft which gain thrust by launching what are basically nuclear warheads out the back. The entire back end of the ship is an enormous radiation shield and shock absorber. The materials and technology to build such a ship have been around since the 70s. The trouble is getting it to space w/o detonating a dozen nuclear bombs in the atmosphere.

The deadly to humans part is complicated. It kinda reminds me of this XKCD What If?

Most manned rockets need to remain under ~3.5 g's if the humans are expected to be able to reach switches and controls (with considerable effort). During extreme conditions, especially a launch abort sequence, the manned capsule can reach over 20 g's for a short burst. This is pushing the limits, as 25 g's is the standard accepted threshold of permanent damage. There are a few recorded examples of humans surviving much greater accelerations.

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Originally Posted by Renton

edit: I just realized how quick the rate of acceleration of 9.8m/s/s is. You would reach the speed of light in less than a year. If you wanted to travel at just 10% of c you would make it in like 35 days and only be like 0.25% of the way to Proxima Centauri. It seems like this method of artificial gravity would only work for very short distances.

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@ bold: Hehe. No. You forgot about relativistic mass. The closer you are to the speed of light, the more massive you are. Meaning that it would take progressively more and more force to simply accelerate you the tiniest bit.

There is not enough energy in the universe to accelerate any massive object to the speed of light. Nor is there enough to slow any massive object even the tiniest fraction if it is already moving at the speed of light. Such an object would have infinite momentum.

Your estimate for 10% c is probably fine. Relativistic effects don't really show up until ~20 - 30% c. At 10% c, you'd expect your non-relativistic calculation to be off by ~0.5%.

Just to double check your calcs:

Spoiler: Show

Time to accelerate to 0.1 c at constant acceleration of 1 g:
Using c = 3(10)^8 m/s and g = 9.8 m/s^2
delta_t = 0.1 c / g = 3,061,224 s ~= 35.4 days

Distance covered during that time:
Using 4.37 ly as the distance to Proxima Centauri
x_final = 1/2 * (9.8 m/s^2) * (3,061,224 s)^2 = 4.58(10)^13 m ~= 0.005 ly
0.005 ly / 4.37 ly = 0.0011

After accelerating to 0.1 c at a constant 1 g, you would be 0.11% of the way to Proxima Centauri.
(This is the non-relativistic approximation.)

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Originally Posted by MadMojoMonkey

There is not enough energy in the universe to accelerate any massive object to the speed of light. Nor is there enough to slow any massive object even the tiniest fraction if it is already moving at the speed of light. Such an object would have infinite momentum.

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My point was that you couldn't maintain a constant rate of acceleration of 1g for very long (within interstellar travel time frames). For some reason when I first approached this problem I just assumed it would take 500 years or whatever for 9.8 m/s/s to add up to relativistic speeds. Not so.


One thing is that it seems like you could achieve reasonably high speeds (say 0.1c for example) by applying a small thrust over a period of years that would only require a steady stream of a (relatively) small amount of energy. Could solar panels approach the needs of this, or at least make a dent so not so much fuel is needed?

Solar panels face one huge problem... you're probably moving away from the sun. Blah blah something about inverse proportion - at twice the distance from sun to earth, one receives a quarter of the solar energy.

I just want to point out that slowing down is also accelerating. If you were speed up for a month to create a force of 1g, then rotate the craft 180 degrees, before slowing down at the same rate, then we're (mostly) maintaining a constant 1g force.

Fuel is the problem, as mojo points out. I guess we need a dense source of fuel that is available from target planets.

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Originally Posted by Renton

My point was that you couldn't maintain a constant rate of acceleration of 1g for very long (within interstellar travel time frames). For some reason when I first approached this problem I just assumed it would take 500 years or whatever for 9.8 m/s/s to add up to relativistic speeds. Not so.

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It's cool that you had the hypothesis and then tested it using theory. :)

The energy required to maintain the thrust would be the limiting factor.

No matter your speed, you can always accelerate. It's just that as your speed approaches that of light, your mass approaches infinity. So the amount of acceleration you get from the same force is ever decreasing as you go faster and faster. A constant force does not yield a constant acceleration when relativistic speeds are involved.

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Originally Posted by Renton

One thing is that it seems like you could achieve reasonably high speeds (say 0.1c for example) by applying a small thrust over a period of years that would only require a steady stream of a (relatively) small amount of energy. Could solar panels approach the needs of this, or at least make a dent so not so much fuel is needed?

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I was going to point out that they are terribly inefficient, but that's only in comparison to other ground-based methods. Wow, double oops, 'cause I was thinking in terms of monetary cost efficiency instead of mass cost efficiency.

The ISS has ~ an acre of solar arrays on it, producing 75 - 90 kW of power. The solar arrays (panels with many cells) do rotate to orient the cells, but the cells may have great longevity for their operation time. I.e. there is probably a bonus in the fact that the solar cells do not have any moving parts.

Ongbonga has a good point about the amount of light diminishing as you move away from the sun. The utility of a solar panel would be pretty limited as you move toward the outer planets, and practically nothing as you move past the gas giants.

I thought about solar sails being a roughly similar concept. I know that we use solar sail concepts to assist in maneuvering satellites and probes, but I don't think there is a probe which uses solar sails as primary thrust. It faces the same problem with diminishing returns as it moves from the sun.

If what you want is thrust, then a solar sail has got to be more efficient than a solar cell driving some engine.

Simple proof:
If you use a solar sail, you can reflect the photon off of it back in the direction it came. This means you gained the momentum of the photon as you stopped it, then you gained as much again as you threw it back behind you.

If you use a solar cell, the best you can do is match the first step in catching the photon, and absorbing its momentum. After that, there is no lossless system which transports / changes the form of that energy.

No matter what happens in the process of turning the captured energy of the photon into thrust, it cannot be as efficient as simply bouncing the photon back in the direction it came.

This model assumes you are oriented such that the desired direction of thrust is exactly away from your light source, but the conclusion is true for any angle.

Could the universe be considered a maching of perpetual motion?

What is the dark energy? I know this is not the easy question, but I really want to get the answer.

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Originally Posted by jwilliams

What is the dark energy? I know this is not the easy question, but I really want to get the answer.

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The farthest that my layman's brain could get on it is that it's the default tendency of empty space to expand when not acted upon by a dominant gravity field. It's more a property of space itself than it is actual stuff i.e. dark matter. Dark matter to me is far more mysterious.

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Originally Posted by BankItDrew

Could the universe be considered a maching of perpetual motion?

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This is complicated.

What do you mean by universe?

If it is a wholly self-contained system, inside which energy is neither created nor destroyed, and from outside which no energy is added or removed... then maybe. Otherwise, no.


What do you mean by motion?
The universe is expanding. If it keeps expanding above a certain threshold, then it overwhelms the gravitational attraction, and expands forever. This seems like a strong case for perpetual motion.

If it expands below that threshold, then the gravitational attraction will eventually overwhelm the expansion, halting it and turning it into contraction. This would, presumably, result in another Big Bang, and another universe. If the next universe starts with the same total energy as our universe, then this also seems like a strong case for perpetual motion. It must have the same (or more) energy to be perpetual. The more bit would likely explode it completely at some stage, but I think we can still call this a perpetual motion machine.

However, if the rate of expansion is just so, then it will approach an asymptote of some finite size, and never quite reach that size, ever slowing as it gets ever closer. This seems like a very weak case for perpetual motion. As far as we can tell, this final option is the case in our universe. That is, our universe is essentially "flat" from an GR point of view.

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Originally Posted by jwilliams

What is the dark energy? I know this is not the easy question, but I really want to get the answer.

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Well, it's easy in the sense that it's not too hard to say, "I don't know."

It's not just me, no one really knows what it is. So why are all these supposedly "smart" physics type people going on about it if they don't know what it is?

First of all, this is our normal state of being. Every answer leads to more questions (at least so far). It is common for people, even scientists, to lose sight of this when discussing the discoveries of the scientific process.

What we know that by our observations, the universe is not only expanding, but accelerating. Not only is it accelerating, which we would expect, due to all the gravitational acceleration sources about the place, but it's accelerating in the wrong direction... away from the attraction of gravity.

Expanding is easy enough to explain. There was a Big Bang... stuff is still 'sploding out from that bang. Did I mention it was Big? It was truly the Biggest of Bangs.

Now the universe is full of all this mass, and mass causes gravitational attraction. So we expect the rate of the expansion to be slowing down. However, we observe the rate of the expansion speeding up. That requires energy.

What is the source of this energy? We don't know. We have applied the name "dark energy" as a placeholder name in the mean time.

We don't know what it is, or even if it's a single thing or many things. We only know we observe a property of our universe which is beyond the ability of our current model to understand.

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Originally Posted by Renton

The farthest that my layman's brain could get on it is that it's the default tendency of empty space to expand when not acted upon by a dominant gravity field. It's more a property of space itself than it is actual stuff i.e. dark matter. Dark matter to me is far more mysterious.

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I'd love to know if you have a source for the underlined bit. As I understand it, this has not been shown.

The rest is hypothetical, but I hesitate to even argue with it, because it's at least plausible that you're on he right track.


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This kinda ties in with BID's question. If dark energy is a property of spacetime, then it seems like 'empty space' is ultimately a perpetual motion machine... While this is fine with me personally, the physicist in me is appalled. Violating the conservation of energy is kinda unthinkable to me at this point in my life. It's so evident to me that this conservation holds in our universe that I cannot truly believe that there is some mystical breaking of that law going on all around me.

It is a purely emotional response, though.

Wait... if the expansion of the universe is accelerating, how can it be asymptotically limited in size?

I'll get back to you after work. Lunch break over.

I really don't like the continually expanding universe model. It implies that the big bang was a one off. I don't think one offs happen.

I like this concept of a perpetual motion universe. I can get on with the idea of big bang, followed by expansion until gravity overwhelms, followed by big crunch, until singularity, then big bang etc. The problem here is the rate of expansion should be slowing down, but as I understand it the opposite is happening.

I'm sure we've discussed this already in this thread, but I like the donut universe model, one in which expansion and contraction are simultaneous, and one in which the big bang is a constant event at the centre of the universe.



I have no idea how much scientific credibility such a model has, but it's balanced and perpetual. I really have a problem with a "beginning" to our universe, because the concept of a "beginning" is inherently tied to our human concept of time. I prefer to think of the big bang as the centre, rather than the beginning. It's merely the beginning of our time.

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Originally Posted by MadMojoMonkey

I'd love to know if you have a source for the underlined bit. As I understand it, this has not been shown.

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Sorry, probably me not being a scientist and saying shit wrong. What I meant to say is that it seems to only expand intergalactic space. As I understand it, dark energy isn't causing the volume of the earth to expand, nor the space between the earth and the sun, nor the space within the galaxy. I guess the force is acting weakly in opposition to gravity. Am I correct?

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Originally Posted by NightGizmo

Wait... if the expansion of the universe is accelerating, how can it be asymptotically limited in size?

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The problem is that we do not know much about dark energy at all. We think we know that the universe started with the Big Bang, then gradually expanded for a while. Then it underwent expansion. Very little is known about this, but it makes sense of some observations about the large-scale structure of the universe. Then the expansion ended and the universe began to slow down in it's rate of expansion.

Now it's accelerating again.

If it keeps accelerating, then it's Heat Death for the universe, which is the idea that if gravity loses the fight and the universe keeps expanding, eventually all the stars will burn out, all the galaxies will spin apart, all the Black holes will have evaporated... and nothing but a thin atmosphere of ever-cooling atoms is left.

Bottom line is that we just don't know the time-dependent nature of dark energy.

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The notion that the universe is flat from a GR perspective is perhaps in question. That statement is based on the value of the cosmological constant... which is presumed to not change in time, and thus constant... but it is not clear whether or not Einstein's cosmological constant is the whole story behind dark energy.

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Originally Posted by Renton

Sorry, probably me not being a scientist and saying shit wrong. What I meant to say is that it seems to only expand intergalactic space. As I understand it, dark energy isn't causing the volume of the earth to expand, nor the space between the earth and the sun, nor the space within the galaxy. I guess the force is acting weakly in opposition to gravity. Am I correct?

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This is correct. The energy density of the dark energy is overwhelmed by gravitational attraction over these kinds of distances.

Perhaps over even larger distances. The distance between the Milky Way and Andromeda is not affected by dark energy. In fact, the entire Virgo Supercluster of galaxies may be unaffected by dark energy.

It is a very weak effect. It's just that the universe is quite big. Have I mentioned that? It deserves mentioning again.

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Originally Posted by OngBonga

I really don't like ...

I like ... I can get on with ...

The problem here is the rate of expansion should be slowing down, but as I understand it the opposite is happening.

I'm sure we've discussed this already in this thread, but I like ...

I have no idea how much scientific credibility such a model has, but it's balanced and perpetual. I really have a problem with ... because ... I prefer ...

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Yeah... this motivates science, but it is not science.

The universe is less intuitive to understand the farther you remove your observations from everyday life. When you look at the extremely tiny, or the extremely large, or vast or massive... intuition proves to be a poor guide.

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Originally Posted by MadMojoMonkey

If it keeps accelerating, then it's Heat Death for the universe, which is the idea that if gravity loses the fight and the universe keeps expanding, eventually all the stars will burn out, all the galaxies will spin apart, all the Black holes will have evaporated... and nothing but a thin atmosphere of ever-cooling atoms is left.

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Is "Heat Death" the new name for "Big Chill"/"Big Freeze"?

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and nothing but a thin atmosphere of ever-cooling atoms is left.

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Where is the heat going? If the atoms are cooling, what is warming?

Also, serious question... what the fuck is entropy?

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Originally Posted by NightGizmo

Is "Heat Death" the new name for "Big Chill"/"Big Freeze"?

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As far as I can tell, yes.

There may be some subtle distinction, but I didn't come across one in a quick search of both terms.

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Originally Posted by OngBonga

Where is the heat going? If the atoms are cooling, what is warming?

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I think this is my fault for poor word choice. The atoms aren't cooling, so much as they are all so spread out that they no longer interact with each other. So whatever temperature each one happens to be is locked in forever.

BUT (just in case it's not ONLY my poor word choice)
Maybe you're confusing heat and temperature.

Consider a piece of metal and a piece of wood, both at room temperature. You can use a thermometer to verify that both are the same temperature. If you touch the metal, it feels colder than if you touch the wood. Therefore heat and temperature are not the same.

Heat is the flow of temperature. Meaning that no matter what temperature a thing is, if it is not changing temperature, then it is neither giving off nor receiving heat (or the sum of the heat in and heat out is 0).

The universe is cooling (not the atoms) because the increasing volume means reduced pressure means fewer particle collisions... to the limit of no more particle collisions. So whatever temperature the atoms are, they're stuck there, because there is no particle to interact with.

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Originally Posted by OngBonga

Also, serious question... what the fuck is entropy?

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This is a tough one. It's tough for a lot of reasons. There are other tough forms of energy to wrap your head around in thermodynamics, but Entropy is central to the Laws of Thermodynamics, so it's an important hurdle to overcome.

It's actually a highly complicated topic to discuss with no background. Can you do me a solid and read a link or two on the subject?

I scanned the wikipedia page and it's, of course, very good, but highly technical.

Check out the Hyperphysics page on the 2nd Law of Thermodynamics.
Follow the links at the bottom on entropy and then ask me your questions, please.

Of course, you can post any link you like and I will try to answer your questions.

I'm sorry to give you homework, but this subject requires some background and I'm busy at work.

Am I right in thinking that "heat death" refers to thermodynamic equilibrium? This is where I'm struggling, because I think to understand this, one needs to understand entropy.

My problem with this model you describe here is there still seems to be a contradiction of the law of conservation. No more particle collisions? Where has all that energy gone? There must surely still be regions of the universe denser than others, and with that comes the distortion of spacetime, meaning gravity will still hold a subtle influence. If there's no other forces interacting with distant particles, then surely even the tiniest gravity is sufficient to influence? Is gravity able to influence at infinite ranges? If so, thermodynamic equilibrium can never happen imo. I just can't imagine a particle in this universe that is not under any influence whatsoever. Every action has its equal and opposite reaction. So for one particle to lose energy, another must have gained it. How does a system ever reach perfect equlibrium?

Thanks for that link, I'll take a look. I'm really struggling with entropy, like is it actually a thing, or an illusion? It's crazy that I think I understand gravity more (I consider that an illusion of distorted spacetime).

I do actually love the phrase, "I think."

When last I pointed it out to you, it was that you started with IDK, and then jumped straight to conclusions with no steps in between. I was only trying to illustrate that you were making extrapolations far beyond the data set.

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Yes, Thermodynamic equilibrium.

Recall that our original assumption on this line of thought is that the acceleration of the universe's expansion due to dark energy will remain positive over all time. Therefore, the rate at which space-time is expanding is increasing without bound.

Yes, all atoms are too far apart to interact gravitationally. Eventually all atoms are further apart than light can travel to connect them, since the extreme space between them, and its constant expansion, means there is no 'peculiar time' which any two atoms share.


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Nevertheless, gravitational action and the evaporation of black holes will eventually drive the universe to Thermodynamic Equilibrium, anyway.

We're talking a ludicrously long timeline, here.

I consider equilibrium to be similar to infinity. One can approach it while never actually getting there. Much like continual halving will approach zero while never getting there.

So the idea is that eventually the universe will be expanding faster than light speed? That's gonna take some time to wrap my head around. How can this ever happen? Does this mean new space is being created between two given particles? Or that one particle is moving away from another at >c, creating the illusion of "expanding" space?

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Originally Posted by OngBonga

I consider equilibrium to be similar to infinity. One can approach it while never actually getting there. Much like continual halving will approach zero while never getting there.

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Then we're talking about something besides thermodynamic equilibrium.

IRL, particles move around with random motions, and some of them overshoot the equilibrium point, then oscillate about a mean value. Except remember that in Thermodynamics, we're talking about the statistical average of usually at least an Avogadro's Number of particles. Those oscillations about the mean are frequently very tiny and are averaged out over the ensemble of particles.

Well... I mean... some equilibrium points are like you mentioned. Theoretically a critically dampened oscillation will behave as you describe. In practice, there is QM stuff that usually crops up before infinite time has passed.

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Originally Posted by OngBonga

So the idea is that eventually the universe will be expanding faster than light speed?

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Replace "eventually / will be" with "already / is"

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Originally Posted by OngBonga

That's gonna take some time to wrap my head around.

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OMFG, I KNOW, RIGHT?!

I still don't fully get it, but I'm pretty sure Veritasium did a video about it.

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Originally Posted by OngBonga

How can this ever happen? Does this mean new space is being created between two given particles?

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I think this is the current hypothesis, yes.

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Originally Posted by OngBonga

Or that one particle is moving away from another at >c, creating the illusion of "expanding" space?

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No, definitely not this.

That's the first I've heard about space expanding faster than light. wtf.... mind blown

Found the Veritasium vid:

When I measure the width of a poker chip, should I do it at the edge of the outermost atomic nuclei, or at the farthest trajectory point of their electrons?

Also, how hard do I need to squeeze the chip to force the electrons to clump together with the nuclei, and is this observed in nature? Neutron stars, black holes?

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Originally Posted by CoccoBill

When I measure the width of a poker chip, should I do it at the edge of the outermost atomic nuclei, or at the farthest trajectory point of their electrons?

Also, how hard do I need to squeeze the chip to force the electrons to clump together with the nuclei, and is this observed in nature? Neutron stars, black holes?

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There's a thing called electron degeneracy pressure - it's loosely electron's tendency to not occupy the same space in the same time. You pretty much need a supernova to squeeze atoms so close their electron shells break and you're left with just clumped neutrons - ie neutron stars.

The electron shell is what holds everything up, except for when it's defeated - neutron stars and black holes. It's neutron degeneracy pressure which holds up neutron stars. Which leads me to my question, MMM, what holds up black holes?

If space is literally expanding, then why is the distance from earth to sun not increasing?

As for entropy, I'd like to know something. If I have some food colouring, and put a droplet into a tub of water, we can expect to see the colouring spread out. Eventually it will be evenly distributed, it will be of equal concentration. When that point is reach, would it be possible (in theory, not practise) to use retroanalysis of the motion of the particles to determine exactly how much food colouring was added, when, from what height etc, or is all this information lost?

It seems to me that either a) eqilibrium is appraoched but never reached, or b) information is lost.

Where is the flaw in my thinking?

^I'm guessing we're close enough to the sun to be locked in its gravity.

I nearly asked about the space in me, but decided to scale up a little.

Nice ninja delete rilla.

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Originally Posted by CoccoBill

^I'm guessing we're close enough to the sun to be locked in its gravity.

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So gravity stops space from expanding?

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Originally Posted by OngBonga

Nice ninja delete rilla.

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MMM'll answer it better. But yes, all space everywhere is expanding. Even the space between you and you.

mojo can you give me any examples of perfect equilibrium in nature? Every case that I can think of is near perfect equilibrium, such as the Solar System. It can't be perfect equilibrium because none of the bodies in the system are perfect spheres, meaning gravity interactions will fluctuate, and there are influences from beyond the system.

Is there even any such thing as a closed system? Other than the universe itself.

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Originally Posted by a500lbgorilla

MMM'll answer it better. But yes, all space everywhere is expanding. Even the space between you and you.

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It's better to talk about how this is known, not what is known. Hubble figured this out. He looked at stars everywhere and realized there was such a thing as a standard star - called a standard candle. They all burned basically the same - size, light, rate, heat, pretty much alike. Well, he knew that just like a fire-engines siren shifts its pitch depending on if it's approaching you or moving away, so does the light of these standard candles. So he set about to figure out how the universe was moving around us.

And what he discovered was that everyone was moving away, everyone was red-shifted, everyone was sounding that lower pitch of a siren fleeing away. And not only that, those that were further away, we're even lower pitched than those closer. Without fail, the farther away a star was, the faster it was going away - what could explain that?

And for good measure, at heat-equilibrium, all information is lost. There'd be no local surplus of energy to accomplish anything. The universe would be perfectly lazy.

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Originally Posted by a500lbgorilla

Without fail, the farther away a star was, the faster it was going away - what could explain that?

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That the observable universe is expanding? He looked at a region of the universe, that is all. If the tide is coming in at one location, one doesn't assume it's coming in everywhere

My problem with space expanding is that every action has it's equal and opposite reaction. If space is expanding, what is contracting? Time? That's how it works with time dilation. Space actually contracts as one reaches light speed, and time expands; but only from the pov of an observer. Expansion and contraction of spacetime is relative. It's only happening from an observers pov.

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Originally Posted by a500lbgorilla

MMM'll answer it better. But yes, all space everywhere is expanding. Even the space between you and you.

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As far as I can tell, this is not the case, and dark energy is influenced by gravity.

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Originally Posted by CoccoBill

When I measure the width of a poker chip, should I do it at the edge of the outermost atomic nuclei, or at the farthest trajectory point of their electrons?

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Somewhere in-between, but very nearly at the outermost electron's <r>
<r> = The EV of the electron's r
r is the electron's time-averaged distance from the center of mass of the atom

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Originally Posted by CoccoBill

Also, how hard do I need to squeeze the chip to force the electrons to clump together with the nuclei, and is this observed in nature? Neutron stars, black holes?

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If you exert this much pressure, you will turn your poker chip into a (very tiny) white dwarf star:

https://upload.wikimedia.org/math/8/...70f7aa50d7.png
where h_bar is the reduced Plank constant, m_e is the mass of an electron, and rho_N is the number of free electrons per unit volume.

This is the electron degeneracy pressure. White dwarf stars are observed to exist.

Degeneracy pressure is an unexpected consequence of indistinguishable particles. If it were possible to tell electrons apart, like pool balls on a pool table, then this pressure wouldn't exist. There is no way to tell electrons apart because all of their intrinsic properties are exactly identical. The same is true for all fundamental particles. This gives rise to the Pauli Exclusion Principle.

The PEP states that no 2 identical fermions can be in the same state at the same time. Protons, neutrons, and electrons are all fermions. Photons are not fermions.

Neutron stars are a consequence of the pressure becoming great enough during a supernova to overcome the electron degeneracy pressure. The electrons are forced to no longer be indistinguishable particles of mass m_e within the volume they are constrained. It is emphatically not that they are "pushed into the nucleus." It's that the energy cost to be an electron + proton became greater than the energy requirement to become a neutron + neutrino + photon. So the particles "moved downhill" in terms of the energy required to fit the environmental conditions.

When the energy cost to be one thing is less than the energy cost to be another thing, and all else is equal, the universe tends to the lowest cost arrangement. <- Hints of Entropy.


Pretty cool, huh?

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Originally Posted by a500lbgorilla

There's a thing called electron degeneracy pressure - it's loosely electron's tendency to not occupy the same space in the same time. You pretty much need a supernova to squeeze atoms so close their electron shells break and you're left with just clumped neutrons - ie neutron stars.

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Yep, yep, yep.

I gave a more physical description of the "electron shells break" part in my prior post.

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Originally Posted by a500lbgorilla

The electron shell is what holds everything up, except for when it's defeated - neutron stars and black holes. It's neutron degeneracy pressure which holds up neutron stars. Which leads me to my question, MMM, what holds up black holes?

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Nothing, my rilla. Nothing at all. Or at least, we don't know what's going on beyond an event horizon.

A black hole is a region of spacetime which is so curved that there are no time-like paths which extend an arbitrarily large distance away.

Black holes are sneaky little things that wrap themselves in event horizon, so very tricksy to tell what's going on in there.

WARNING: All links are to wikipedia in this post. I'm lazy today.

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Originally Posted by OngBonga

If space is literally expanding, then why is the distance from earth to sun not increasing?

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Orbits are elliptical. Whether or not an object is moving toward or away from a local center of mass is going to vary over time.
Assuming you meant the average orbital distance, well...
The moon's average orbital distance is increasing at about 1.5 inches per year. This is nothing to do with dark energy.

Dark energy is (at least for now) a very weak effect, it only manifests as a dominant local energy over ludicrously vast distances. As I said in a prior post, the Milky Way is not affected by dark energy. The Milky Way and Andromeda Local Group is not affected by dark energy. The Virgo Super Cluster of galaxies in which the Milky Way exists is unlikely [citation needed] to be affected by dark energy.

Dark energy is projected to eventually tear apart the Laniakea Super Cluster, of which the Virgo Super Cluster is a member.
(I love the "you are here" in Russian on that link. :p )

Meaning that the expansion is currently acting on only the largest scale structures in the universe.

EDIT: If you click those links and actually dwell on the scale of these objects, your mind will 100% be blown. Every step up is another mind blow. We didn't even start with considering the size of the solar system, or even Jupiter. The vastness of it all is beyond words. I run out of words at the first step... then the steps just keep going.

Diameter of Oort Cloud: ~ 2 light years (ly)
Diameter of Milky Way: ~100,000 ly
Diameter of Local Group: ~10,000,000 ly
Diameter of Virgo Supercluster: ~100,000,000 ly
Diameter of Laniakea Supercluster: ~500,000,000 ly

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Originally Posted by Renton

As far as I can tell, this is not the case, and dark energy is influenced by gravity.

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What makes you say that?

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Originally Posted by OngBonga

That the observable universe is expanding? He looked at a region of the universe, that is all. If the tide is coming in at one location, one doesn't assume it's coming in everywhere

My problem with space expanding is that every action has it's equal and opposite reaction. If space is expanding, what is contracting? Time? That's how it works with time dilation. Space actually contracts as one reaches light speed, and time expands; but only from the pov of an observer. Expansion and contraction of spacetime is relative. It's only happening from an observers pov.

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How's that?

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Originally Posted by OngBonga

As for entropy, I'd like to know something. If I have some food colouring, and put a droplet into a tub of water, we can expect to see the colouring spread out. Eventually it will be evenly distributed, it will be of equal concentration. When that point is reach, would it be possible (in theory, not practise) to use retroanalysis of the motion of the particles to determine exactly how much food colouring was added, when, from what height etc, or is all this information lost?

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Classically, this is a perfectly deterministic system, all information is preserved. Theoretically, you could do exactly what you propose.

Practically, since it takes time to solve each particle's information, and you must take finite steps to approximate the next positions... The task is beyond monumental for any macroscopic system. This demonstrates the need for the statistical approach of Thermodynamics.
Note that a classical particle is a point mass. It is NOT a wave-particle. It is an idealized infinitesimally small localized mass which is NOT a black hole. Classical means without invoking any QM or GR.

If we start to think about the system in terms of QM, then we have to deal with wave functions and the uncertainties in position and momentum make this problem intractable to analyze in that context. We are moved to talk about the combined wave functions of all the particles in the system, and we call this the state function. This encapsulates all the properties of the system, insofar as they can be observed. Solving this explicitly (not an approximation) for more than a handful of particles is already getting obscenely time intensive. We are getting better and better at making computer approximations which are accurate to within experimental uncertainties.

It is the cutting edge of physics to explore this "meso-scale" region where QM rubs up against Thermodynamics.

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Originally Posted by OngBonga

It seems to me that either a) eqilibrium is appraoched but never reached, or b) information is lost.

Where is the flaw in my thinking?

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I don't even see how the two are related. Entropy isn't a "loss of information." It is "unusable" energy. Think of it as if something is moved by an "accidental" process to a low-energy state, then that energy becomes "unusable," since it can not be extracted by our "intentional" process.

The total entropy of any closed system can not go down. If we are very clever, we can theoretically do things which hold the amount of change in entropy to 0.

Equilibrium is EVERYWHERE. Anything which is not accelerating is experiencing a static equilibrium of forces. Anything which is going through a constant mishmash of motion that all kinda blurs together such that it kinda isn't changing over time is in a state of dynamic equilibrium.

Asymptotic behavior is common among solutions to differential equations. The equations of physics are differential equations. So it's cool that this is on your mind. It shows an intuitive understanding of the solutions to Diff Eqs. Oscillations are also common among solutions. Mix these together and you get dampened oscillations - oscillations which decrease over time and approach an asymptote.

Practically, QM steps in before infinite time has passed.

Consider the charge on a capacitor in a simple circuit with a Voltage source, a Resistor, and a Capacitor. The voltage across the capacitor will approach the value of the voltage source, but the resistor slows the process down. As the process is slowed, the charge collecting on each capacitor plate reduces the potential difference between the capacitor plate and the voltage source. So the accelerating force acting on the charges (electrons) decreases as the process occurs. This gives rise to asymptotic behavior. But QM steps in and says... hehe, I'm only going to allow you to take charges in units of 1e-, so as soon as you're at least that close to the charge required for equilibrium, then you're close enough.

Does that make sense?

Since certain quantities which describe matter come in discrete lump sizes, if you are within a lump size of equilibrium, you are there. So the "infinite time to reach the asymptote" doesn't practically apply with quantized values.

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Originally Posted by CoccoBill

^I'm guessing we're close enough to the sun to be locked in its gravity.

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Replace "locked in" with "bound by" and you are speaking my language.

Really, it's the entire system that is bound by the entire system's local gravity. I.e. it's not merely that the Earth is bound to the Sun's gravity. It is also that the Sun is bound to the Earth's gravity.

I think this is nothing more than a nit-pick by me, though.

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Originally Posted by OngBonga

So gravity stops space from expanding?

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Both play a role in the energy acting to accelerate an object over distance, but one is >>> than the other, so it is dominant.

Gravity doesn't hold atoms together, or atomic nuclei. The electromagnetic energy is much stronger than the gravitational energy acting on an atomic system. The strong force holding a nucleus is called such because it is strong (~137 times) enough to force equilibrium with the electromagnetic force over nuclear distances. Needless to say, gravity is so much less than this and it was negligible at the atomic level.

No, it doesn't stop the space from expanding, but the frame drag you would feel as an accelerating force from this minuscule amount of dark energy expressed over such a volume is practically 0.

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Originally Posted by OngBonga

mojo can you give me any examples of perfect equilibrium in nature? Every case that I can think of is near perfect equilibrium, such as the Solar System. It can't be perfect equilibrium because none of the bodies in the system are perfect spheres, meaning gravity interactions will fluctuate, and there are influences from beyond the system.

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You're getting lost in your head-space. Equilibrium is everywhere.

Anything which is not accelerating is experiencing static equilibrium. Dynamic equilibrium has multiple contexts, but is broadly analogous to a steady-state of a system. Meaning that the system undergoes changes, but more-or-less this is a cycle which represents a single energy-state.

When you sit on your chair, you accelerate your body in a complicated set of ways which culminate in you sitting still in your chair. This is equilibrium. If you want to nit-pick about your breathing and other biological process going on which are not altogether cyclical on this time scale, then we can certainly note that those parts of the system do not seem to be experiencing equilibrium. However, the system as a whole is in a steady state w.r.t it's position. I.e. the average change in velocity of all the bits of you is 0 to within experimental uncertainties.

Whenever we choose a model, we accept limitations, or boundaries of applicability. It is excellent form for you to question which parts of the system are behaving in which way.

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Originally Posted by OngBonga

Is there even any such thing as a closed system? Other than the universe itself.

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It's all semantics. So long as you are accepting that stuff-n-things may play a role in putting boundaries on your control volume, you're free to run.

Consider a chemists experiment in a test tube. The closed system is pretty obviously the volume of the test tube, as far as the contents of the container and their chemical properties. If it is a well-insulated test tube, then that's one more property that can be considered to be isolated in the control volume.

IDK about the semantics, but control volume is the terminology used in vector mathematics. So long as you account for changes happening at the boundaries of the control volume, then it need not be a "closed system."

EDIT: This is the same for Thermodynamics. One of the fundamental equations of Thermodynamics is
Q_in - W_out = delta_E
The Heat in minus the Work out equals the change in energy of the control volume.

This tells you how to deal with the goings-on at the boundaries of the control volume.

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Originally Posted by a500lbgorilla

And for good measure, at heat-equilibrium, all information is lost. There'd be no local surplus of energy to accomplish anything. The universe would be perfectly lazy.

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-.-

:/

...

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Originally Posted by a500lbgorilla

The universe would be perfectly lazy.

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I'm so ahead of my time.

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Originally Posted by MadMojoMonkey

-.-

:/

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;-)

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Originally Posted by OngBonga

That the observable universe is expanding? He looked at a region of the universe, that is all. If the tide is coming in at one location, one doesn't assume it's coming in everywhere

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I think this is a fair critique. Now let's move away from metaphor and discuss the actual observation and what our current theory says about it. If that is inadequate to describe the observation, then we will hypothesize a new idea to fit not only this observation, but all previous observations, and hopefully predict a new observation.

Frankly, we're still pinning down Hubble's Constant, and we're not entirely certain if it's constant, and we're not certain if it's related to Einstein's Cosmological Constant (thought by Einstein to among his greatest mistakes to include in his theory, but seems like it wasn't), which we're not sure if it's constant, either.

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Originally Posted by OngBonga

My problem with space expanding is that every action has it's equal and opposite reaction. If space is expanding, what is contracting?

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You're referencing Newton's Third Law of Motion, but misinterpreting it.

Newton's Third Law says:
When one body exerts a force on a second body, the second body simultaneously exerts a force equal in magnitude and opposite in direction on the first body.

Again:
Object A expresses a force F on object B in direction x IF AND ONLY IF object B simultaneously expresses a force F on object A in direction -x.

Questions along this line should be: Our universe is acquiring energy. What is the source of this energy? What is losing energy to our universe?

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Originally Posted by OngBonga

Time? That's how it works with time dilation. Space actually contracts as one reaches light speed, and time expands; but only from the pov of an observer. Expansion and contraction of spacetime is relative. It's only happening from an observers pov.

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This has nothing to do with equal and opposite reactions. Furthermore, it took a few years to convince the physicists of the world that we can do this and we haven't violated any momentum or energy laws. So the notion that this is somehow a consequence of Newton's Third is not appropriate. If anything, Newton's Laws are consequences of the more broadly applicable laws of QM and GR.

I'm just about to go out, but I just want to ask you if you can tell me of a single thing in the entire universe that isn't accelerating?

I mean if I sit in my chair, I'm still. Well actually I'm not becuase I'm on a rotating planet, which is in orbit around a star. And not only am I breathing, but so are other people, which influences the atmosphere around me in the most subtle manner. All of this means that my equilibrium is constantly disturbed by other influences.

As for the information vs equilibrium thing, well I'm making the assumption that if a system is in perfect equilibrium, then all particle velocity interactions will have settled, all particles have identical temperature, pressure is perfectly uniform... but what is information? Isn't it momentum, temperature, pressure, velocity, angular momentum, rotation, vibration etc? If all of these factors are uniform, then where is the information?

Anyway, taxi's due soon. I'll read through more thoroughly tomorrow.

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Originally Posted by Renton

As far as I can tell, this is not the case, and dark energy is influenced by gravity.

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It IS the case (that dark energy is causing the space which you occupy to expand), but the influence of gravity is greater over this distance.

I mean... hypothetically.

According to the ideas that seem most widely accepted at this time.

The magnitude of uncertainty in any hypothetical measurement device dramatically overshadows the magnitude of the effects of dark energy on any realistic scale at our current level of technology and experimental methods.

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Originally Posted by OngBonga

I'm just about to go out, but I just want to ask you if you can tell me of a single thing in the entire universe that isn't accelerating?

I mean if I sit in my chair, I'm still. Well actually I'm not becuase I'm on a rotating planet, which is in orbit around a star. And not only am I breathing, but so are other people, which influences the atmosphere around me in the most subtle manner. All of this means that my equilibrium is constantly disturbed by other influences.

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If we're talking about Coriolis Force and Tidal Force due to Earth-Moon and Earth-Sun orbital interactions, then we clearly must move away from the Earth a bit to even think about it.

Once in orbit above the atmosphere, for instance, the ISS experiences the acceleration toward Earth, but within the ISS, there is only microgravity.

I guess if you want to be nit picky about it, there is nowhere in the universe which is unaffected by gravitational fields, but microgravity is easily attainable if there's no atmosphere or other significant thrust / drag on a spacecraft.

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Originally Posted by OngBonga

As for the information vs equilibrium thing, well I'm making the assumption that if a system is in perfect equilibrium, then all particle velocity interactions will have settled, all particles have identical temperature, pressure is perfectly uniform... but what is information? Isn't it momentum, temperature, pressure, velocity, angular momentum, rotation, vibration etc? If all of these factors are uniform, then where is the information?

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The information is spread out in the uniformity.

The total angular momentum is unchanged, the total energy is unchanged, the total charge is unchanged, barring some physical processes, the number of leptons is unchanged and the number of baryons is unchanged. All of the information in every interaction is conserved. It may change it's form of expression; it may be divided into multiple fields; it may be blurred across multiple states, but the total hasn't changed.

Any information you perceive to have been lost is not considered information in a physical sense.

@ong:
Consider a frictionless simple pendulum.

It oscillates back and forth about it's static equilibrium point. With no friction, it will always rise to the same height as it reaches its peak, on every half period. It will always have the same speed as it passes through its static equilibrium position every half period.

Over any period, it has exactly the same properties it had exactly one period before. Therefore it is in dynamic equilibrium.

If we look at the pendulum at the peak of its swing, it has no kinetic energy, but it has some positive amount of elevation relative to its static equilibrium position. So it has some gravitational potential energy. Call the amount of gravitational potential energy at this point x.
KE = 0 ; PE = x

If we look one quarter period later, it is at the bottom of its swing. We find that it has the same height as its static equilibrium position. Now it has no gravitational potential energy. We find by careful observation that it now has a kinetic energy exactly equal to x.
KE = x ; PE = 0

WTF? If it's in equilibrium, then why did any of these numbers change?
Because the information that we know is that the total energy is x. The form it takes, KE or PE is not the information. The information is SUM(Energy) = x.

KE + PE = x

(1/2)mv^2 + mgh = E_tot

We can now use this to determine the relationship between the height and the speed. We find that this fully describes the state of equilibrium for the frictionless simple pendulum, and we need not include any other energy terms.

Ok that's a really good explanation for why information isn't lost as a result of equilibrium, thanks.

I'm still struggling with entropy, and I still feel like nitpicking anything that comes close to "perfect equilibrium". But then again I'd argue there's no such thing as a perfect circle, and feel exactly as pedantic.

There was something I thought of relating to space expansion... if dark energy is what's at play here, and we assume our universe to be expanding at an accelerating rate, then surely this tells us a great deal about dark energy? In particular that it isn't uniform across the universe with a static energy value. It must either be decreasing in value as space expands, or non uniform (or both). For it to remain uniform and static in value, energy is being created from nothing, which is a clear violation of our laws of physics.

If we assume dark energy to be fixed in value and uniform cross the universe, and that energy cannot be created from nothing, then we must assume the universe as a whole is not expanding.

Does that make any sense?

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Originally Posted by OngBonga

Ok that's a really good explanation for why information isn't lost as a result of equilibrium, thanks.

I'm still struggling with entropy, and I still feel like nitpicking anything that comes close to "perfect equilibrium". But then again I'd argue there's no such thing as a perfect circle, and feel exactly as pedantic.

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Keep digging into the 2nd Law of Thermodynamics. Investigate the Carnot Engine (or Carnot Cycle). This is the theoretically most efficient possible heat engine.

Think about how broad a category of things is encompassed in the term heat engine. This includes refrigerators, ovens, auto and diesel engines, rocket engines, pressure vessels and hydraulic systems, pumps, fans, etc.

Quote:

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Originally Posted by OngBonga

There was something I thought of relating to space expansion... if dark energy is what's at play here, and we assume our universe to be expanding at an accelerating rate, then surely this tells us a great deal about dark energy? In particular that it isn't uniform across the universe with a static energy value. It must either be decreasing in value as space expands, or non uniform (or both).

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We don't know what it is. We just noticed it. We are scrambling to see if our current theory does or doesn't predict this by jiggering with some presumed "constants" in our equations. We don't know where it's coming from. We don't know if it's in violation of Conservation of Mass-Energy, or a consequence of it.

It does appear to be a property of space which acts over volume. The "new space" is identical to the "old space." I.e. spacetime is spacetime; when it expands, it doesn't become "stretched." It's not a material, no matter how much we love to hear about "the fabric of spacetime" in science fiction.

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Originally Posted by OngBonga

For it to remain uniform and static in value, energy is being created from nothing, which is a clear violation of our laws of physics.

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This is a better statement. However, it is only a clear violation of classical physics. Which we kinda expect. It is not at all clear if it is a violation of QM or GR. We are unclear of the source. It is premature to assume, "There is no source."

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Originally Posted by OngBonga

If we assume dark energy to be fixed in value and uniform cross the universe, and that energy cannot be created from nothing, then we must assume the universe as a whole is not expanding.

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Not this.

Observation: The universe is not only expanding, but accelerating (on the largest measurable scales).

Ong: So it's definitely not expanding, then.

:wall:

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Originally Posted by OngBonga

Does that make any sense?

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I hope I didn't misinterpret you.

We're still at the point of asking questions.

There is no explanation for dark energy in our current understanding of physics. It is well worth questioning if it is in violation of the Law of Conservation of Mass-Energy. It is not a good time to be drawing conclusions.

I hope I have impressed upon you how weakly the dark energy contributes to the total energy over even the scale of Super Clusters. There is reason to be skeptical about what our theories can say when we extrapolate far from the domain of the measurements which motivate them. Sometimes a theory predicts many things beyond it's initial assumptions. This was true for both QM and GR, but we're pushing the limits of those extrapolations to the point of adding new data which was not predicted.

This is not too different from what was going on during the early 20th century when QM and GR were being discovered. There is something going on which our theories are having trouble dealing with. As our measurement devices become more precise, we find counter-intuitive stuff which has us scratching our heads.

Not completely unlike the Ultraviolet Catastrophe (more boring than it sounds).

What do you think about the National Institute of Standards and Technology (NIST) results discussed at http://www.dailymail.co.uk/sciencetech/article-3317630/How-Einstein-got-WRONG-Second-paper-proves-spooky-action-distance-real.html, why is entanglement so weird?

Cliff notes on video:
John Bell proved there are no hidden variables


Loophole free Bell Test in the Netherlands.


For the first time, both loopholes were closed at the same time.


The measurements show the correct correlations 80% of the time.

The most important fact to add to the conversation is that John Bell spent most of his life emphatically saying that his famous Bell Inequality does not mean "there are no hidden local variables." I'm not sure what nuance he was setting straight, though. It's still a hotly debated topic among quantum physicists.

The very difficult part of this is that the definitions and language of QM have changed over the past 100 years. Particularly much so when it comes to every key term in QM as it was used by the famous scientist who said stuff about it. The vast array of disagreement over what constitutes "observation" is mind-boggling.

I'm not totally sure what, exactly, the Bell inequality is about any more. I know what I was taught, and now I know that Bell criticized that interpretation, so I'm playing catch up on this one.

***
As I read it, all that happened is that the thing we've known Einstein was totally wrong about (QM - the most robust theory humans have ever put forth) is still not wrong. Einstain hated a lot of the conclusions which came from QM, and his "spooky action at a distance" quote betrays his criticism was at least partly based on emotion, or meant to persuade as opposed to compel.

At any rate, this result is the predicted result of QM. It is a more precise demonstration than others before it, but nothing unexpected.

***
Entanglement is weird because it goes against any intuitive understanding of how macroscopic things behave. It is not necessarily clear that entanglement implies action at a distance.


The following are my hypotheses, and are not, strictly, the kind of things I can state with great confidence:

The way I understand it, there are properties which the particles have, which obey conservation laws, yet which also have a time dependence. That is, they are not constant over time, but they change. The conservation laws hold up over time, so that the properties change in a manner which maintains this relationship.

So the relationship was set before the separation, and the measurement of the property must be made simultaneously (in the relativistic sense) to show that there was no communication between them. To me, all this demonstrates is that they were already linked and whether or not they change is not really hard to understand, since they change in a manner that preserves their initial orientation.

All of which is predicted by the Schroedinger Equation in that the time-dependence is identical for identical particles. So it would make sense if the particles had a "hidden local variable" which described the time-dependence, and that this hidden variable would not involve any distant communication. It would merely be a property which is difficult to measure directly.

It may not be a "hidden variable" at all. It may merely be a consequence of the formation and propagation of wave-particles (I.e. solutions to the Schroedinger Equation) and there is nothing more to be read into it. After all, we already predicted this behavior in analyzing solutions to the SE, so we kind-of already understood it in that sense. Forcing an intuitive picture onto it may be the only folly.

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Originally Posted by MadMojoMonkey

Entanglement is weird because it goes against any intuitive understanding of how macroscopic things behave. It is not necessarily clear that entanglement implies action at a distance.

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Right, the conclusion that there is action at a distance is wrong imo. The particles are connected in a way that our minds don't understand so there is no need for there to be action at a distance. Maybe their connection is through hidden dimensions or something.

How do blind people know if they're gay or straight?

If dogs have such better sense of smell than we do, how come they have no problem sniffing their own shit right after doing it, while I can't even hack their farts?

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Originally Posted by oskar

How do blind people know if they're gay or straight?

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:? The same way all the other non-sociopathic people do.

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Originally Posted by OngBonga

If dogs have such better sense of smell than we do, how come they have no problem sniffing their own shit right after doing it, while I can't even hack their farts?

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I know you like to think that your shit don't stink.

I'm no expert on dogs or biology, but the way I understand it, dogs use their sense of smell like we would read. So a dog smelling something is like a dog reading an article about that thing's past. A dog stopping to smell stuff while you're walking them is akin to them spotting an interesting picture on the cover of a newspaper and stopping to read the headline.

Also, dogs eat poo, so I'm not convinced that my answer is complete.

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Originally Posted by MadMojoMonkey

:? The same way all the other non-sociopathic people do.

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Touch and gag?

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Originally Posted by oskar

How do blind people know if they're gay or straight?

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You've never taken to a voice on the radio?

They know people in a way and they fall for them in that way.

Nothing smells inherently bad or good, those are just interpretations of our brain. Shit might and probably does smell entirely different to them. Humans evolved to be disgusted by shit for survival purposes, perhaps dogs aren't so vulnerable to the bacteria and had no need for that. Or maybe dog poo does taste awesome.

https://www.reddit.com/r/science/com...gled_messages/ links to http://phys.org/news/2015-12-computi...me-travel.html which says things like the following:

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Around ten years ago researcher Dave Bacon, now at Google, showed that a time-travelling quantum computer could quickly solve a group of problems, known as NP-complete, which mathematicians have lumped together as being hard.
...
"Whenever we present the idea, people say no way can this have an effect" says Jayne Thompson, a co-author at CQT. But it does: quantum particles sent on a timeloop could gain super computational power, even though the particles never interact with anything in the past. "The reason there is an effect is because some information is stored in the entangling correlations: this is what we're harnessing," Thompson says.

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Do you understand what they're saying? Can it be explained another way?

I hesitate to call a paper published by a dozen claimed experts as purely sensationalized nonsense, but that's how it reads. There are far too many conditional statements in that paper.

If we can presuppose time travel, then I don't see why we can't say all problems are now solvable, even problems which would require an infinite number of steps to solve. Just write a computer program which performs one loop of a recursive algorithm, then the last step of the loop says, if you have the answer, then output, otherwise, send this result back in time as the initial input. Start the program. It runs an infinite amount of recursions in the time of a single recursion, and, so long as I fed it an algorithm which converges, spits out the result.

We can see if the sum of all the positive numbers is truly -12 after all. That's unless the size and power required for the memory is too high.


The bottom line is that entanglement is an observable property of reality. Time travel, as of yet, is not. (Well, controlled time travel - it seems easy to move forward in time at this seemingly constant rate).

I'd be happy to read the original publication to see exactly what are their assumptions. Without that, it's impossible to tell if they're talking physics or just math. At any rate, the amount of prevarication in the paper is enough to instill serious doubt about the reality of what is being discussed.

"Of course, the connection between OTCs and gravitational fields is still very speculative and might turn out to be wrong. We hope to flesh it out into a more complete theory in future research."
Jacques Pienaar, University of Queensland in Australia - One of the researchers who published this paper.

OTC = Open Time Curve

"The circuit itself is easy to build; it's coming up with an OTC that's the problem!" Pienaar said. "Strictly speaking, we would require an actual time machine in order to build that circuit, which we obviously don't have. However, if our analogy between OTCs and gravity is correct, then we could make do with just an ordinary gravitational field like Earth's. In that case, the circuit certainly can be built; there is already a group working on sending entangled beams of light up to a satellite in orbit. This would then provide an experimental test that could either prove or disprove our claim about gravity behaving the same as an OTC."
-Phys.org

so i hear that there is no edge to the universe and that physicists don't know if it's finite or infinite. doesn't the universe not having an edge mean it's infinite?

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Originally Posted by wufwugy

so i hear that there is no edge to the universe and that physicists don't know if it's finite or infinite. doesn't the universe not having an edge mean it's infinite?

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If it's finite but edgeless, then it wraps around.