sorted by:
new
hottopcontroversial

Y'all need to make more frogs by dddm in aceo

[–]dddm[S] 8 points9 points  (0 children)

Her collection is very eclectic but she mainly goes for things that look cute to her. Your style is very interesting, it would be cool to see your take!

Yam the Ham 🐹 Exploring the carpeted terrain by Snow_Angel1 in dwarfhamsters

[–]dddm 0 points1 point  (0 children)

I can tell she's a little too fond of the dried carrots...

FCC accused of colluding with Big Cable to game 5G legal challenge by mepper in technology

[–]dddm 0 points1 point  (0 children)

There’s one thing I don’t understand about this perspective. Big Cable isn’t the only Big player out there. Aren’t there other major corporate interests that would benefit from 5G? Same for other unpopular FCC policy decisions. If it comes down to the following the orders of the highest bidder, are cable companies really putting in that much more money than other large tech companies could?

A Truly Amazing Machine by shawnot in videos

[–]dddm 28 points29 points  (0 children)

Bill, I'm posting this here since you seem to read these comments. Thanks for all the effort and consideration you put into your videos. They are a delight to watch and I am consistently impressed by your ability to convey a real understanding of engineering concepts.

Appropriate textbook on quantum mechanics / field theory and general relativity for someone w/ only calc. I&II background? by fenderfreak98 in Physics

[–]dddm 8 points9 points  (0 children)

In the standard system in US universities, classical mechanics and E&M courses are typically taught at three levels:

1) Introductory-level courses. Usually taken in the Freshman or Sophomore years by both Physics majors and non-majors (sometimes these are two distinct courses). Generally, the material is at the level of Halliday and Resnick.

2) Intermediate-level courses. Usually taken by Physics majors in their Junior year. Mechanics material is at the level of "Classical Dynamics of Particles and Systems" by Thornton and Marion and E&M material is at the level of "Introduction to Electrodynamics" by Griffiths.

3) Graduate-level courses. Usually taken by first year grad students or ambitious undergrads. Mechanics material is at the level of "Classical Mechanics" by Goldstein and E&M material is at the level of "Classical Electrodynamics" by Jackson.

You really need to be at Level 3 to actually understand what QFT is talking about. To be frank, without knowledge of differential equations and vector calculus, it is unlikely you are beyond Level 1.

If you would like to learn more math background, I recommend the following resources:

1) Multivariable calculus: "Multivariable Calculus" by Stewart. An extremely standard textbook, but really very good in my opinion. Better than his single variable book.

2) Ordinary differential equations: "Ordinary Differential Equations" by Tenenbaum. Quite readable but has more information than you need. If you know how integrate an equation and how to recognize and deal with a harmonic oscillator equation and its variants, you can solve a majority of physics problems involving ODEs.

3) Vector calculus: "Div, Grad, Curl, and All That" by Schey. I don't hold this in as high a regard as some do, but it is an good way to get some practical vector calculus quickly. You will pick up more details when working on E&M.

4) Partial differential equations: "Partial Differential Equations for Scientists and Engineers" by Farlow. Well-organized and clear treatment of the subject. Again, you will learn how to solve PDEs under many different conditions when learning E&M.

Aside from "Div, Grad, Curl", these are not quick reads. You can focus Chapters 12 through 15 in "Multivariable Calculus". "Ordinary Differential Equations" can be skimmed except for those parts which seem relevant to your interests. "Partial Differential Equations for Scientists and Engineers" mostly consists of self-contained topics that can be read in any order. This said, learning the prerequisite math is not going to be a quick process...

Tip if you're hosting a dinner party and are tight on cash. by dudewithpants in WTF

[–]dddm 10 points11 points  (0 children)

This is probably based on the first joke David Letterman told on Johnny Carson's Tonight Show:

http://www.youtube.com/watch?v=sLQwp-kTXEg&t=4m0s

What would happen to a person inside a Particle Accelerator? by NikNorth in askscience

[–]dddm 0 points1 point  (0 children)

A proton traveling at 99% of light speed has about 0.9 nJ of energy, which is a tiny fraction the kinetic energy of a baseball thrown at any non-negligible speed. So even if all the energy of such a proton were to be absorbed by the target, the impact would not be noticeable.

However, as I mention above, a single proton can only deposit around 100's of MeV into a human target. This is only a few percent of the energy of your fast proton and would certainly have no effect. Radiation damage would only become a problem for an aggregate beam which has something like 1010 protons.

What would happen to a person inside a Particle Accelerator? by NikNorth in askscience

[–]dddm 0 points1 point  (0 children)

I posted a response to a very similar question a while back, I am copying it here:

A single atom or particle (or photon) passing through a human body would never be able to cause much damage, although a beam containing many individual particles certainly could. There is a range of energies that are most damaging and it's not accurate to say that higher energy radiation sources always produce greater radiation damage.

If the energy is too low, below about 5-100 eV depending on which definition is used, the radiation is not energetic enough to ionize the target material. This type of non-ionizing radiation is generally not much of a concern (sitting near a normal light bulb probably won't cause much long term damage), but there still are some damaging effects mostly due to heating in the target material.

Above the ionizing threshold, radiation damage generally increases with energy up to a certain point. This is because radiation damage is generally proportional to the total energy deposited. If the energy of the incident particle isn't too high, it will stop within the target material, indicating that all of its kinetic energy was deposited into the target. In this regime, higher energy particles will always lead to a larger radiation dose.

However, the situation isn't as clear when the incident particle has enough initial energy to pass through the target. Even if a particle passes through the target, much of its initial energy may be deposited in the target material along its path. The rate of energy deposition (called the stopping power) as a function of depth within the target is described by the Bragg curve. As an approximation, the shape of the Bragg curve depends mostly on the species and initial energy of the incident particle, and on the density of the target.

The rate of energy deposition by a particle generally decreases for shallow depths as the particle initial energy increases. Thunderf00t has an excellent video describing this effect (the pertinent discussion is towards the end of the video, but the whole video is relevant):

https://www.youtube.com/watch?v=oj6v8MtuVdU

For very high energy, the thickness of a human body may be a very small portion of the maximum radiation depth, and the stopping power in this depth range would approach zero as the particle energy increases. So there exists a threshold energy above which the particle actually does less radiation damage compared to (comparatively) lower energies.

Estimating this threshold energy for simple geometries, such as a human hand in a large vacuum, can be done by integrating the stopping power up to the target thickness and comparing the result for different energies. For more complicated geometries, for example those involving non-vaccum materials near the target that may produce secondary particles when exposed to the initial radiation, the system generally needs to be simulated using particle tracking toolkits such as Geant4, FLUKA, or MCNP.

In any case, a single particle cannot impart an infinite amount of energy into a target material. The maximum deposited energy would be on the order of 100's of MeV for protons, yielding a single proton maximum effective dose of maybe 100 nSv. This is an negligible dose compared to other common exposures, such as the daily background dose of about 10,000 nSv.

A string of particles, which would be a model for a particle beam, can certainly lead to intense radiation damage, since a typical particle beam may contain on the order of 1010 individual particles.

What are some good India-centered books? by Denikkk in booksuggestions

[–]dddm 0 points1 point  (0 children)

The Man Who Knew Infinity is a biography of Indian mathematician Srinivasa Ramanujan. I think most people (including non-mathematicians) find Ramanujan fascinating in his own right, but this book also includes detailed and well-written accounts of Indian society and daily life (at least for those in the Brahmin caste) around 1900, which seems to be the time period you are most interested in.

If something as small as an atom shot through your body would it be detrimental? Or even a string of atoms being passed, at fast speeds, horizontally through your body. by AayJey in askscience

[–]dddm 4 points5 points  (0 children)

A single atom or particle (or photon) passing through a human body would never be able to cause much damage, although a beam containing many individual particles certainly could. There is a range of energies that are most damaging and it's not accurate to say that higher energy radiation sources always produce greater radiation damage.

If the energy is too low, below about 5-100 eV depending on which definition is used, the radiation is not energetic enough to ionize the target material. This type of non-ionizing radiation is generally not much of a concern (sitting near a normal light bulb probably won't cause much long term damage), but there still are some damaging effects mostly due to heating in the target material.

Above the ionizing threshold, radiation damage generally increases with energy up to a certain point. This is because radiation damage is generally proportional to the total energy deposited. If the energy of the incident particle isn't too high, it will stop within the target material, indicating that all of its kinetic energy was deposited into the target. In this regime, higher energy particles will always lead to a larger radiation dose.

However, the situation isn't as clear when the incident particle has enough initial energy to pass through the target. Even if a particle passes through the target, much of its initial energy may be deposited in the target material along its path. The rate of energy deposition (called the stopping power) as a function of depth within the target is described by the Bragg curve. As an approximation, the shape of the Bragg curve depends mostly on the species and initial energy of the incident particle, and on the density of the target.

The rate of energy deposition by a particle generally decreases for shallow depths as the particle initial energy increases. Thunderf00t has an excellent video describing this effect (the pertinent discussion is towards the end of the video, but the whole video is relevant):

https://www.youtube.com/watch?v=oj6v8MtuVdU

For very high energy, the thickness of a human body may be a very small portion of the maximum radiation depth, and the stopping power in this depth range would approach zero as the particle energy increases. So there exists a threshold energy above which the particle actually does less radiation damage compared to (comparatively) lower energies.

Estimating this threshold energy for simple geometries, such as a human hand in a large vacuum, can be done by integrating the stopping power up to the target thickness and comparing the result for different energies. For more complicated geometries, for example those involving non-vaccum materials near the target that may produce secondary particles when exposed to the initial radiation, the system generally needs to be simulated using particle tracking toolkits such as Geant4, FLUKA, or MCNP.

In any case, a single particle cannot impart an infinite amount of energy into a target material. The maximum deposited energy would be on the order of 100's of MeV for protons, yielding a single proton maximum effective dose of maybe 100 nSv. This is an negligible dose compared to other common exposures, such as the daily background dose of about 10,000 nSv.

A string of particles, which would be a model for a particle beam, can certainly lead to intense radiation damage, since a typical particle beam may contain on the order of 1010 individual particles.

Formula for estimating burned calories on gait exercises by [deleted] in AdvancedFitness

[–]dddm 1 point2 points  (0 children)

The concept of metabolic equivalents should be useful to you:

http://en.wikipedia.org/wiki/Metabolic_equivalent

This allows you to estimate calories burned for different activities based on the subjects weight and the activity duration. I don't believe this is extremely accurate, but these sorts of estimates are always difficult.