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a reply to: pfishy
In particular take note of Lee Smolin's comment around 22 minutes that the singularity prediction is general relativity screaming at us that it's incomplete, and I posted another video by Michio Kaku saying something similar.

originally posted by: Phantom423
One thing I wanted to clear up though - the cosmological constant, dark energy, vacuum energy - these are all the same thing? The reason I ask is because when I pull up research papers very often these terms seem to be interchangeable. So I just want to confirm that the three terms are the same thing.

We aren't completely sure but we think they are probably the same thing.

Dark energy is an observation based phenomenon, with data backing it up. It's an effect, and we guess at the cause.

Our best guess for the cause is vacuum energy, but we aren't completely sure.

In our cosmological model this accelerating expansion is represented by the cosmological constant, called "lambda"

So they aren't all identical, but if our guess is correct all three are probably closely related. Here is NASA's discussion regarding those three terms to give a little different perspective and some possibilities of what it would mean if those three terms aren't related:

science.nasa.gov...

What Is Dark Energy?

More is unknown than is known. We know how much dark energy there is because we know how it affects the Universe's expansion. Other than that, it is a complete mystery. But it is an important mystery. It turns out that roughly 68% of the Universe is dark energy.

One explanation for dark energy is that it is a property of space. Albert Einstein was the first person to realize that empty space is not nothing. Space has amazing properties, many of which are just beginning to be understood. The first property that Einstein discovered is that it is possible for more space to come into existence. Then one version of Einstein's gravity theory, the version that contains a cosmological constant, makes a second prediction: "empty space" can possess its own energy. Because this energy is a property of space itself, it would not be diluted as space expands. As more space comes into existence, more of this energy-of-space would appear. As a result, this form of energy would cause the Universe to expand faster and faster. Unfortunately, no one understands why the cosmological constant should even be there, much less why it would have exactly the right value to cause the observed acceleration of the Universe.

Another explanation for how space acquires energy comes from the quantum theory of matter. In this theory, "empty space" is actually full of temporary ("virtual") particles that continually form and then disappear. But when physicists tried to calculate how much energy this would give empty space, the answer came out wrong - wrong by a lot. The number came out 10120 times too big. That's a 1 with 120 zeros after it. It's hard to get an answer that bad. So the mystery continues.

Another explanation for dark energy is that it is a new kind of dynamical energy fluid or field, something that fills all of space but something whose effect on the expansion of the Universe is the opposite of that of matter and normal energy. Some theorists have named this "quintessence," after the fifth element of the Greek philosophers. But, if quintessence is the answer, we still don't know what it is like, what it interacts with, or why it exists. So the mystery continues.

A last possibility is that Einstein's theory of gravity is not correct. That would not only affect the expansion of the Universe, but it would also affect the way that normal matter in galaxies and clusters of galaxies behaved. This fact would provide a way to decide if the solution to the dark energy problem is a new gravity theory or not: we could observe how galaxies come together in clusters. But if it does turn out that a new theory of gravity is needed, what kind of theory would it be?

The last option that our current model of gravity is wrong is mentioned in the "before the big bang" video and one physicist has an unconfirmed new model, so there are other ideas out there. As I said we aren't really sure, but I usually treat them as though they were closely related, even if I'm aware there's some significant doubt about that.

a reply to: Arbitrageur

Well, I'm well aware that the use of "infinite" in a physics model is just another way of saying "this is where smoke started coming out of our theory's ears". Especially in relation to the Big Bang and the inner workings of a singularity. It is obvious that GR is inadequate to describe these two things in particular. Einstein greatly disliked the concept of 'infinite', which is partly why he devoted so much time to attempting a GUT.

originally posted by: pfishy
a reply to: Arbitrageur

Well, I'm well aware that the use of "infinite" in a physics model is just another way of saying "this is where smoke started coming out of our theory's ears". Especially in relation to the Big Bang and the inner workings of a singularity. It is obvious that GR is inadequate to describe these two things in particular. Einstein greatly disliked the concept of 'infinite', which is partly why he devoted so much time to attempting a GUT.

Id argue no your wrong infinite I think is more of an excuse then actually showing were wrong. Here's what I mean if I'm doing something and I don't know say the volume or it do not have all the variables all I do is say well space is infinite so it doesn't matter. Problem becomes in physics it takes away the ability to make predictions. And in truth that is what science is all about if this happens then it causes this. I mean in reality we have no observational evidence to assume in infinitely big or small even existing. But this is the assumption of many though not all.

Even the expansion of space in our model new space is created this goes on infinitely but reality tells us energy is finite in nature. It's like stretching a rubber band eventually it reaches it's limit and snaps. But back to your point science doesn't predict the infinite it is purposely used in models. It isn't failure of the model it's there by design.

Now my personal belief is there is no such thing as infinite for example I can make a computer model showing me weather patterns or even formation of the galaxies around us. This computer has a finite amount of information and capacity yet I can use it to make predictions. So why would the universe need to be infinite to accomplish the same thing? Ok I'm done my rant but you hit a pet peve of mine. I don't think physics gains anything from saying something is infinite on the contrary it loses a lot.

Here's a quote is thought is add.
“Infinity is merely a way of speaking” and “I protest against the use of infinite magnitude as something completed, which is never permissible in mathematics.”
Carl Friedrich Gauss

a reply to: dragonridr

I never said GR is wrong in general. Just that the use of infinites shows certain areas where it's ability to make accurate predictions breaks down.

a reply to: dragonridr
The post you replied to referred to infinite in the context of big bang and black hole singularities, so it refers to a particular type of result, infinite density. I seem to recall you making a post about the problem with singularities yourself.

Your post seems to have a different context when you refer to the possibility that "space is infinite". We don't know if the universe is infinite or finite in size and perhaps will never know, but the observable universe is finite so in some sense that's what we really have to explain, what we can observe.

Michio Kaku made a comment relevant to the quote you cited, about infinities being "permissible in mathematics", which was that mathematicians were fine with infinities in mathematics since they don't care if it corresponds to something in the real world or not; it's physicists that have a problem with infinite density because it doesn't seem like a real-world result.

Kaku and the other physicists in the videos I posted were objecting to the infinite density in a singularity specifically and they don't think that's a realistic prediction. I don't recall hearing a physicist say that a universe with infinite dimensions was impossible, only that our observations are limited to the observable universe.

originally posted by: Hyperia
a reply to: stormbringer1701

So lets get back to fission, whats the difference between a Nuclear power plant fission, and a Nuclear bomb fission?

The same difference as between a diesel generator and a fuel-air explosive: speed.

In particular, it relies on a very magic and lucky property of fissile uranium. During fissioning, uranium emits two neutrons immediately, within microseconds (nanoseconds? Not sure). If you arrange the system such that these can multiply and fission other uranium nuclei with more being produced each generation, then you get a fast chain reaction with a microsecond timescale. That's a weapon.

The resulting products also emit another neutron, but with a timescale of hours. A controlled reactor has a multiplication factor < 1 for the fast neutrons, but > 1 once you count the slowly emitted neutrons. So you can control and modulate the power output over hours instead of a microsecond.

originally posted by: mbkennel

originally posted by: Hyperia
a reply to: stormbringer1701

So lets get back to fission, whats the difference between a Nuclear power plant fission, and a Nuclear bomb fission?

The same difference as between a diesel generator and a fuel-air explosive: speed.

In particular, it relies on a very magic and lucky property of fissile uranium. During fissioning, uranium emits two neutrons immediately, within microseconds (nanoseconds? Not sure). If you arrange the system such that these can multiply and fission other uranium nuclei with more being produced each generation, then you get a fast chain reaction with a microsecond timescale. That's a weapon.

The resulting products also emit another neutron, but with a timescale of hours. A controlled reactor has a multiplication factor < 1 for the fast neutrons, but > 1 once you count the slowly emitted neutrons. So you can control and modulate the power output over hours instead of a microsecond.

Figured t is would be interesting for people to watch gives you some background and discusses the next steps.

a reply to: mbkennel

I know alot, but, please dumb it down a bit, like when you tell a kid a story =)

originally posted by: dragonridr
Figured t is would be interesting for people to watch gives you some background and discusses the next steps.

Great, now everybody who watched that video gets added to a watch list, though most ATSers were probably on one already.


That video mentioned one factor which prevents the use of nuclear weapons is the radioactive fallout which even occurs with fusion bombs because they are triggered by fission. Then it says if we can eliminate that residual radiation it will make fusion bombs easier to use without all the fallout. I'm not so sure that's a good thing, meaning maybe it's good that we can't use the weapons now. It can't lead to anything good if we start using nukes.

originally posted by: Hyperia
a reply to: mbkennel

I know alot, but, please dumb it down a bit, like when you tell a kid a story =)

This video explains fission and fusion power production and bombs in simple terms in 6 minutes. It's dumbed down quite a bit but the actual reaction for fusion of hydrogen atoms in the sun is more complex than what's shown, so whenever something is dumbed down there are tradeoffs with accuracy. For example most hydrogen atoms don't already have a neutron as shown in this dumbed down version, but the final result of the helium atom with two protons and two neutrons is correct.

Fission and Fusion

a reply to: mbkennel

The neutron emission happens in one "shake', or 10 nanoseconds.

a reply to: Arbitrageur

Speaking of neutrons and hydrogen, anyone got an estimate of the hydrogen / deuterium ratio in the Sun?

originally posted by: pfishy
a reply to: Arbitrageur

Speaking of neutrons and hydrogen, anyone got an estimate of the hydrogen / deuterium ratio in the Sun?

The solar atmosphere probably has about the same ratio as Jupiter, which has been measured from 22-26 deuterium atoms per million hydrogen atoms. That's about the ratio in the proto-sun before fusion began.

Here's a paper with some ratios which includes the proto-sun, but the interior of the sun now can't be measured directly. In the sun, there's probably very little deuterium because it reacts so quickly it won't last long IN the sun, so all I can say is it's probably way less than 20 per million but this paper has no estimate for that and I don't have one. It has lots of other measurements and estimates though.
Solar System Deuterium/Hydrogen Ratio

a reply to: dragonridr
Not really sure about the use of a penning trap to make a anti-matter trigger... the hold time for the best we have is pretty... low.

Also, radioactive fallout is bad... but you all know that huge neutron flux and gammas are... pretty damn bad too, both would create a LOT of radioactive material.

Guess what happens when you pump neutrons into concrete? You make obscenely radioactive concrete. Problem... solved?

Great, now everybody who watched that video gets added to a watch list, though most ATSers were probably on one already.

Wouldnt worry so much, had one of the RA's basically give an exact run down as covered in that short video... for a public seminar that lasted an hour. That was in the early 2000s

originally posted by: ErosA433

Great, now everybody who watched that video gets added to a watch list, though most ATSers were probably on one already.

Wouldnt worry so much, had one of the RA's basically give an exact run down as covered in that short video... for a public seminar that lasted an hour. That was in the early 2000s

He was joking most physics students students could build a bomb. As they say that genie is out the bottle. The only thing that prevents it is refinement and designing the trigger mechanism. Thanks to computers simulations can be run and tested to be honest I'm surprised that most countries don't have a simple version.

a reply to: dragonridr

I'm fairly sure that if I had the necessary materials and access to the right machining equipment, I could build a crude 2 stage device myself. Wouldn't have any conceivable reason to ever want to, but that genie is certainly free-range these days. The plans are surprisingly not too difficult to come by. Luckily for all of us, certain components and fuels are extremely difficult to obtain, and require a very high level of precision to assemble properly.

originally posted by: pfishy
Luckily for all of us, certain components and fuels are extremely difficult to obtain, and require a very high level of precision to assemble properly.

Only if you want it to be efficient. If you don't care a lot, and have material to waste, it's possible to use single point ignition and a flyer plate with readily available plastic explosives and some metal work you can do in a high school shop to make a really crappy nuke with a few kT yield. It's really awful, and leaves crap everywhere, which actually might be preferable for certain uses.

a reply to: Bedlam

Are you referring to the Uranium gun design?
I was talking specifically about a boosted fission or thermonuclear design. PU pit, tritium injection, etc.
And I certainly wouldn't be assured that it would be efficient. But I'm pretty positive I could at least achieve boosted fission. Almost guarantee it wouldn't be a fizzle.

a reply to: pfishy

Not ever going to try, to be clear. But the basic designs have been in the public domain for a long time.

originally posted by: pfishy
a reply to: Bedlam

Are you referring to the Uranium gun design?

Nah, even easier - a shape changer. You can start off with an obround and hammer it into a partially shielded sphere with very minimal externals. It's small, and straightforward, and likely the inspiration of various "briefcase nukes".

We used it very briefly for howitzer shells. But it's good for the occasional well-equipped nutcase.