gravity and nuclear forces

[drwayne]

Probably the highlight was getting to meet Dirac many years ago. I was still a clueless
undergraduate, and he was 90 something. Boy was I dwarfed.

In graduate school, we hosted a Society of Physics Students (SPS) zone meeting.
I got a chance to meet and have a few beers with Mertzbacher.

That's about it, living in the dimly reflected lives of others.

Wayne

 

[yevaud]

That's about it, living in the dimly reflected lives of others.

Wayne

Standing on the shoulders of Giants for certain.

Oh. one more which has nothing to do with Physics: I was in my first C++ course, when a ditzy giggly woman accidentally barged into the room. Her name is Helen Rose, AKA "Trillian," a very early-on famous Internet character and one of the founders of the IFF. Having seen her in action, I can only assume the founders of the IFF included her for comedic relief.

 

[MeteorWayne]

Actually, his non-fiction is notably excellent.

Wayne, a story back to you. Years ago, I used to take walks all the time along Boston's Esplanade, along the Charles river. Quite by accident, I struck up an acquaintance with Dr. Phillip Morrison and his wife Phyllis, who would take him there almost daily in his wheelchair. He was a very nice guy, piercing blue eyes, and much to say.

I also once shared a bottle of Champagne with Dr. Margaret Geller and a Grad. student, up on the BU observatory roof deck on the 4th of July.

Doc, any stores of your own...?

I can only imagine meeting Dr Morrison in person. those eyes came right through the TV secreen. What was the name of the TV series that was on PBS? I never missed an episode and found it very enlightening....so much so I can't remember the name...

 

[yevaud]

"Ring of Truth." I remember it so well, I had to Google the name. <-- back at ya on the "memory thing."

In fact, it was shortly before I met him that on that show, he made a demonstration by pouring a drop or two of oil onto the waters of one of the lagoons along the Esplanade - showing how the oil spread, quieted the water, and wasn't that a wonderful example! In fact, it was that scene in the series that got me to walk up to him and introduce myself.

 

[DrRocket]

Actually, his non-fiction is notably excellent.

Wayne, a story back to you. Years ago, I used to take walks all the time along Boston's Esplanade, along the Charles river. Quite by accident, I struck up an acquaintance with Dr. Phillip Morrison and his wife Phyllis, who would take him there almost daily in his wheelchair. He was a very nice guy, piercing blue eyes, and much to say.

I also once shared a bottle of Champagne with Dr. Margaret Geller and a Grad. student, up on the BU observatory roof deck on the 4th of July.

Doc, any stores of your own...?

Several. Here is one. If you like this one I have more.

As a graduate studen I was in a mathematics meeting, listening to a talk on ergodic theory by Harry Furstenburg, who is one of the "big guns" in the subject. I was having a tough time following him, basiclly lost. But sitting next to me was an older oriental gentleman who not only seemed to be keeping up and asksing a lot of good questions, but was apparently staying ahead of Harry. When the talk was over I saw my advisor at the door of the classroom and basically asked him "Who is is that masked man ?" He looked over and said "Oh, Thati's Kakutani !"

I don't know if that name means anything to you, but he was one of the premier mathematicians at the time -- one of the guys who didnT write text book but whose work filled them.

 

[lukman]

Is it heat or gravity creates fusion? if both, which is the key role here? if it was gravity, then we can imitate it using centrifugal force, like the one in astronaut g force training, can we fuse hidrogen that way? how many g force needed?

It is basically getting enough energy to cause the nucleii to get close enough together for the strong force to overcome the repulsion of the positively charged nucleii. That is usually accomplished by having the nuclei traveling really fast (temperature).

To force nucleii together with gravity takes a LOT of gravity. If you had enough centrifugal force in the centrifuge for astronaut training you would wind up with REALLY flat astronauts (the Maryland experiment that drwayne linked is trying to confine plasma in a centrifuge not directly cause fusion, and reemember that plasmas are hot).

Still not quite get it, sun at 28 earth G can sustain fusion, a formula 1 car crashing into a wall was heard generating more than 100g, no fusion?

 

[drwayne]

A couple of things to keep in mind:

(1) In the car crash, the material in the car is not constrained

(2) The material involved is critical. Iron for example will NOT undergo fusion, or for that matter
fission, because of its position on the stability curve.

Wayne

 

[DrRocket]

Is it heat or gravity creates fusion? if both, which is the key role here? if it was gravity, then we can imitate it using centrifugal force, like the one in astronaut g force training, can we fuse hidrogen that way? how many g force needed?

It is basically getting enough energy to cause the nucleii to get close enough together for the strong force to overcome the repulsion of the positively charged nucleii. That is usually accomplished by having the nuclei traveling really fast (temperature).

To force nucleii together with gravity takes a LOT of gravity. If you had enough centrifugal force in the centrifuge for astronaut training you would wind up with REALLY flat astronauts (the Maryland experiment that drwayne linked is trying to confine plasma in a centrifuge not directly cause fusion, and reemember that plasmas are hot).

Still not quite get it, sun at 28 earth G can sustain fusion, a formula 1 car crashing into a wall was heard generating more than 100g, no fusion?

The sun is pretty hot. As drwayne noted the nuclear species involved are also important -- binding energy per nucleon. Hydrogen is relatively susceptible to fusion. Iron is not. It is not just gravity that makes the sun work.

 

[lukman]

The sun is pretty hot. As drwayne noted the nuclear species involved are also important -- binding energy per nucleon. Hydrogen is relatively susceptible to fusion. Iron is not. It is not just gravity that makes the sun work.[/quote]

So, what is the relationship between heat, pressure and gravity in fusion? i mean like for example, sun fuse hydrogen at 3000 degree celcius at 28G, if we increase the G to 280, is that mean that we need less heat? can we contain hydrogen at room temperature and slam it hard to mimic a huge G force and starts fusion? sorry for the kindergarten question, i am only a fans of physics but know nothing at all but curious

 

[Saiph]

good questions those. Fusion is a matter of taking two particles, and moving them towards eachother with a high enough energy to overcome the natural repulsion (electromagetic forces between the protons) to be bound by the strong nuclear force (attraction between protons/neutrons). So you can have a single pair of atomic nuclei fuse if you smash them together just right. Doing this in a particle accelerator you can have very low densities, and relatively low energy (just enough to fuse).

That's it. The other factor comes into play is how often you want a fusion event. If you want the rate of fusion events to increase, you have to have more atoms. If you cram them in a smaller space the atoms will have to travel less distance before they get another chance to smack together. This is where the pressure comes in.

If you increase the pressure you increase the number of chances an atom has to fuse. Every failed attempt changes the energy of the atom, sometimes increasing it, sometimes decreasing it, as the momentum is swapped between it and the other atoms it hits. Eventually it'll get enough energy that the next atom it hits will be with enough energy to fuse. Increasing pressure increases this probability.

The other way you can increase the probability of a fusion event is to boost the temperature. Temperature is a measure of the average kinetic energy of the atoms being observed. Increase it, and the atoms move faster, meaning it's more likely for them to smack and fuse than smack and bounce.

Gravity kicks in because it acts like a lid. A ball of gas, heated up, expands and the pressure drops. Without some 'outside' force acting to compress the ball of gas it just spreads out and fusion can't occur. In the case of a star the gravity of the gas itself is enough to compress part of the gas (that in the core of the star) to keep the pressure up, like the lid in a steam cooker. More gravity means a higher pressure can be maintained, meaning a higher rate of fusion...

Get the probabilities high enough, via manipulating these two values, and you can get a sustained fusion reaction, where it generates enough heat on it's own to maintain the temperature required.

To a certain extent you can have a trade off between the two, though the temperature has to have a minimum value for fusion to work at all. This can produce various rates of fusion. Small stars, despite smaller reserves of hydrogen, burn for billions of years. Massive stars (like Sirius or Betelguese) have huge reserves of hydrogen, but their fusion furnaces burn so fiercely that they only last on the order of a hundred million years!

 

[MeteorWayne]

VERY nice explanation Saiph. Take a cheese sammich out of petty cash!

 

[Saiph]

Om nom nom nom!

 

[lukman]

excellent explanation, very layman, thank you very much. in a h bomb, it uses fission to ignite fusion, but do you think it is possible to use a natural occurred lightning bolt?

 

[kg]

I came accross this interesting bit

http://en.wikipedia.org/wiki/Hydrogen_fusion
"At the temperatures and densities in stellar cores the rates of fusion reactions are notoriously slow. For example, at solar core temperature (T ≈ 15 MK) and density (160 g/cm³), the energy release rate is only 276 μW/cm³—about a quarter of the volumetric rate at which a resting human body generates heat.[13] Thus, reproduction of stellar core conditions in a lab for nuclear fusion power production is completely impractical. Because nuclear reaction rates strongly depend on temperature (exp(−E/kT)), then in order to achieve reasonable rates of energy production in terrestrial fusion reactors 10–100 times higher temperatures (compared to stellar interiors) are required T ≈ 0.1–1.0 GK."

 

[Saiph]

so the best way to generate heat isn't fusion, but to cram large numbers of people together! The matrix was right!


All joking aside, that's actually a very interesting figure, thanks for sharing kg!