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69 /r/AskPhysics comments

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131 points

·
7th Aug 2021

I try to ELI5 as best as I can.

In quantum mechanics, particles behave like waves. Keep this in mind.

Look at this wave (it's supposed to go off to the left and to the right like this into infinity). You can easily tell me what wavelength it has - just count the distance from one peak to the next one.

But where is this wave? This question doesn't even make sense, the wave goes off like this on both sides to infinity. It has a very well defined wavelength (~velocity), but the position is totally unclear.

Now look at this wave. If I ask you "where is it?", you can imemdiatly say "it's at (or around) 0". But what is its wavelength? There isn't another peak you can measure the distance to.

This wave has a very clearly defined position, but the wavelength is undefined.

And you can make up all sorts of "mixed waves" between those two examples, but you will always trade uncertainty in one for exactness in the other.

33 points

·
1st Nov 2021

not sure what you mean by complicated

in physics it doesn't matter how subjectively beautiful a theory is to you. it's important that it describes nature.

see here for the why do we need it http://www.scholarpedia.org/article/Quantum_gravity_as_a_low_energy_effective_field_theory

26 points

·
30th Nov 2021

I think a good explanation is to use the length/time contraction formula and and energy consumption.

So when we observe objects(with mass and dimensions) travelling close to the speed of light the length of objects start to get compressed or get shorter or closer and closer to zero, their clocks start to tick slower and the amount of energy needed to get closer and closer to the speed of light gets more and more tending to infinity.

Since we are made of matter we would need an infinite amount of energy when at C.

There are a few online calculators that you can play with that show this like https://www.omnicalculator.com/physics/length-contraction

26 points

·
7th Nov 2021

>I imagine people would think the same about distance and length.

yeah we have no reason to assume discreteness of space.

>They probably don't believe in significance of the Planck length I assume...

.. ? i think you're misunderstanding what the Planck length is

Planck length does not represent a pixel size of space. it has a very different meaning, it indicates a scale at which you need to consider quantum effects of gravity.

mentioned here http://www.scholarpedia.org/article/Quantum_gravity_as_a_low_energy_effective_field_theory

20 points

·
21st Oct 2021

In addition to the fact that the Planck length is not a "reality pixel," "quantizing" a field does not mean making it discrete.

Lack of a known particle is not what makes quantum gravity hard -- in fact, we basically already know what the particle should be, and we can construct a working theory of quantum gravity in the low-energy limit. When we try to expand this to high energies (which are the cases that are actually interesting), *that's* when we run into problems.

15 points

·
20th Jun 2021

The problem is actually how does chaos arise from quantum mechanics. Quantum mechanics is governed by the linear Schrödinger equation and thus does not display the sensitivity to initial conditions found in chaotic systems. There's a whole field of research dedicated to studying the issue.

14 points

·
22nd Jul 2020

No it does not. You get a theory of gravitons when you quantize GR.

http://www.scholarpedia.org/article/Quantum_gravity_as_a_low_energy_effective_field_theory

This whole idea that "gravity being the curvature of spacetime and not a force" being "incompatible" with gravity being "mediated by quanta" (often misunderstood as saying that particles that interact through that force will send these quanta back and forth between them, which is not the case.) is out there and quite common but wrong.

14 points

·
2nd May 2019

There is a website called mentimeter where you can post some questions and students and answer them or just respond them using their phones or laptops. You can use it as a way to get some interactions or make the session more lively.

For mentimeter, you can flash the live update (statistics) of the answers your audience is responding. Pretty interesting if used correctly.

I recently did a short presentation on planetary motion and used it to ask the audience what they think will happen if I were to change some conditions, most of them responded with the wrong answers and they became more interested in finding out the content I will present later on.

14 points

·
14th Apr 2018

12 points

·
21st Oct 2021

>(I don't know much about quantum physics, so bear with me)

cool but you need to actually study a ton of general relativity and quantum theory to even understand the problem of quantization of gravity.

> gravity is the distortion of space around any object with mass, and evades quantification due to lack of a known particle (say, a graviton)

that's not the reason.

http://www.scholarpedia.org/article/Quantum_gravity_as_a_low_energy_effective_field_theory

> since the planck is the smallest possible distance between two points of space

it's not. Planck length is the scale at which quantum effects of gravity become important.

>, wouldn't that mean that space (and thus gravity) is quantized with the planck length being a "reality pixel" like a particle?

it's nothing to do with discreteness of spacetime. not a Pixel size.

and discreteness isn't the same thing as quantization.

9 points

·
20th Sep 2020

Physics for scientists and engineers by giancoli book I took college level physics 10 years ago with that book. I'm retaking physics now to apply for grad school. My professor is using a different book that is not well illustrated and cuts out steps and explantations. I didn't realize how good the giancoli book is until now. It has really good reviews online.

8 points

·
28th Jul 2020

As a university Physics student, I can also recommend The Physics of Superheroes for a very non-rigorous introduction to ideas of physical science and all that. It sounds childish, but it actually does a great job of explaining all sorts of topics without all of the mind-bending math.

7 points

·
24th Nov 2015

According to https://www.quora.com/Why-is-light-affected-by-gravity-if-photons-are-massless the geometry of space responds to energy density (not just mass). So the gravity field would stay the same until the energy in that region moves away at the speed of light.

7 points

·
29th Aug 2017

First watch this video about the unit circle -- make sure to pay attention to how the triangles are drawn: https://www.khanacademy.org/math/algebra2/trig-functions/unit-circle-definition-of-trig-functions-alg2/v/unit-circle-definition-of-trig-functions-1

Then watch this desmos which relates the unit circle on the left to the graph of sin on the right: https://www.desmos.com/calculator/kxfekf0kgb

What you are discovering here is the beauty of math. Don't think "wtf this doesn't make sense". Think instead "Holy balls! These two things are connected mathematically???" Then maybe down the road you can think "I wonder what else is connected mathematically??" and next thing you know you'll be a physics major.

7 points

·
6th May 2019

I used to use TeXstudio but now I prefer to use overleaf.

It is a LaTeX environment that you use through your browser and all your documents are accessible online. It's particularly nice for collaborative projects because you can share the document and each edit it.

7 points

·
20th Sep 2020

I wouldn't start with Feynman Lectures. Those are better to read after you've gone through the material once already.

I like Knight's textbooks, but any of the major ones will do.

6 points

·
16th Jan 2017

Planets **do** have conserved angular momentum. This is the reason why Kepler's laws work, and the reason why comets increase their velocity when closer to the orbitee. Here is a fun game that nicely illustrates this concept. You can set the initial angular momentum of each planet, and although you can see their speeds change, their angular momenta doesn't.

5 points

·
28th Jan 2017

this paper discusses *exactly* this. They ultimately claim the singularity becomes naked. This really shouldn't happen and they speculate a bit on what this means in the last part.

5 points

·
19th Jul 2021

I mean you would have to study GR and QFT and start from the advanced sections in textbooks, QFT in curved spacetime and work your way into topics like string theory to get a good overview over that research area. here's some basic info too

5 points

·
15th Feb 2021

Most physicists don't care or are agnostic on the issue.

Physicist Victor Stenger wrote a book arguing against the fine-tuning argument.

4 points

·
14th Nov 2012

Knowing nothing else about what you've seen, I would definitely recommend Feynman's Six easy pieces; they're good for getting your feet wet. There's a sequel which is also pretty cute, and explains some simple stuff well. Both books are basically excepts from his lectures on physics, which I haven't actually read through, but I've been told are also excellent. http://www.amazon.com/Six-Easy-Pieces-Essentials-Explained/dp/0465025277

If you want some additional accessable works, Brian Greene's works are interesting. But if you want to really understand what's going on, you're going to need to pick up some mathematics. It's essential if you want to really understand what's going on in physics, and to some degree the other sciences. As for resources on intro math, I don't know too many, but I've heard http://www.khanacademy.org/ does a pretty good introduction, albeit at a very introductory level. Other than that, I would suggest rooting around the internet, reading wikipedia, and then finding problems and working through them.

4 points

·
8th May 2019

Yes, by topological near-field vs. far-field arguments. It's easier to think about the gravitational potential than the acceleration.

From far away, the 3 masses "look like" a single mass. A contour of the gravitational potential will be roughly circular, centered on the center-of-mass.

From near any mass, that mass will dominate the potential value. A contour will be a circle centered on that mass. There will be other contours around the other masses with the same potential value (3 contours total).

As you look at regions in between the near-field and far-field, the 3 contours will have to merge into one contour. This will happen as either one merger or several. It's just a matter of whether the contours merge simultaneously or separately.

Here's a Desmos graph that allows dragging a point around and draws the corresponding equipotential contour. When contours "cross", that's a saddle point that is one of the places where the force is zero. https://www.desmos.com/calculator/kjjrjtzkkt

Here's one with the masses and geometry set to be symmetric. Interesting! You can find the unstable equilibrium in the middle, where there's a floating fourth contour. https://www.desmos.com/calculator/rna8xhghns

4 points

·
8th Jul 2021

You're kind of fighting a losing battle here, because you're trying to get useful work out of waste heat. Thermodynamics isn't on your side. Even best case scenario suggests that you're only going to get ~13% efficiency from that (already fairly small) temperature differential.

MSI had a Stirling engine motherboard cooler back in 2008. I don't think anything ever came of it, and I can't find any references to more recent attempts.

At best, I could imagine a liquid cooling setup that used the Stirling engine to circulate the cooling fluid, but that may not even be much better than a passive liquid cooler.

4 points

·
27th Jun 2021

>They're trying to figure out how far gravitons (each vertex of spacetime) are spaced out naturally?

No not at all. Quantizing a theory doesn't mean you get a "pixelisation" of spacetime vs gravitons are not vertices of spacetime.

see here

http://www.scholarpedia.org/article/Quantum_gravity_as_a_low_energy_effective_field_theory

>Or they're trying to confirm dark matter/energy?

A different problem that has nothing to do with quantum gravity per se

>I'm pretty sure they're trying to confirm gravitons, and I'm pretty sure gravitons are similar to higgs but instead of colliding with things they transfer energy.

Not really similar, at least not more so than they are similar to electrons and photons.

> What I've come up with is this idea

You need to study quantum field theory and general relativity before you start studying the problem of quantum gravity and even then you don't come up with useful ideas for a while.

Physics isn't just making stuff up on the go without background knowledge. That's not what theories are

>In other words, c (speed of light) isn't the speed of light, it's the rate of energy going from a particle to gravitons. the "speed" of photons is instananeous, but the transfer of energy isn't. And either (hypothetically, I have no idea yet) 1/c or 1/2c is the distance from each graviton.

The units don't even fit here...?

> so please don't criticize me too much.

You have to be criticized for just making stuff up.. that's not science

4 points

·
11th Dec 2014

There are a whole lot of Free Coursera Physics Courses which are very well taught. You can just go through the video lectures. I took from "Great Courses" since particle physics I needed a refresher because my degree was from the 80's and woe, have things changed.

4 points

·
15th Aug 2021

Here are two possibilities:

- Provide each topic with a navigation bar to display content. At the moment, you have to scroll through the page to find what is shown. For example, going to Pythagorean from the Sound and Hearing section brings you to a page with temperament, intervals, problems, and the whole tone, but you wouldn't know this without scrolling through the page.
- Provide a responsive website so that the content is easily readable for different screens without the need to zoom in. This is the philosophy behind Bootstrap. On my browser at 100% zoom (normal), the Hyperphysics webpages take up the left, and only the left, half of my screen.

4 points

·
5th Apr 2014

One of the gas giants entering the inner solar system is bound to have some sort of effect on the orbit of Earth but exactly what would happen depends entirely on the specific circumstances and timing. If you have access to Universe Sandbox you can load up a simulation of the solar system and try it yourself; most of the scenarios involving Jupiter I ran end up with all the inner planets being flung out into the void.

But as far as an effect of the planet hitting the Sun? Nothing. The Sun has something like 99.8% the mass of the solar system, the planets are nothing but rounding errors when you're talking about the Sun.

4 points

·
23rd Nov 2020

It’s not really necessary to use E= mc² to estimate the energy released by nuclear fission, since you can also do it based on the stored electrostatic energy. A nucleus represents a lot of positive charges in a small volume, and this repulsion is converted to kinetic energy of the fragments when fissioning occurs.

This was the method used by Robert Server when he originally wrote the Los Alamos Primer, as a handbook to introduce incoming scientists to the work being done at the Manhattan project.

3 points

·
7th Nov 2021

See the graph in the abstract for this paper.

3 points

·
30th Apr 2021

The cable would be made out of carbon nanotubes. As a normal cable wouldn’t be able to support it’s own weight, it should be tapered. That means that the cable at geostationary orbit, made of carbon nanotubes with a density of 1300kg/m^3 and a tensile strength of 63GPa, will be around 2.7 times as wide as the cable at the base (the taper ratio is 2.7). At higher altitudes than geostationary orbit the cable will again be narrower. This calculation was made by putting the previously named density and tensile stress values in equation (7) in this paper.

3 points

·
2nd Nov 2015

It looks like you aren't using the full formula for R.. If the Christoffel symbols are non-zero, there are extra terms you need to account for.

3 points

·
20th Nov 2021

Hmm, I don't think this is true in general. The gravitational force decreases with the square of the distance to the centre of mass of the planet, so when you're higher up, you would actually feel less gravity.

This source calculates a decrease in g to be 0.027 m/s².

I think you want to account for the mass of Mt. Everest itself. I found this page which estimates the mass of the Mt. Everest. There are two estimates, but I think the order of magnitude is the most important. We can calculate g due to the Mt. Everest and compare it with the decrease in g due to the height.

g = G*M/r² = 6.674*10^(-11) * 6.399*10^(15)/6636²

The 6636 m is approximately the distance from the top of the Mt. Everest to its centre of mass, approximating it as a cone. The answer is

g = 0.01 m/s²

Which is still less than the decrease due to the increase in height. Now of course, the above calculation may be lacking, but I think it's still more likely that g on top of the Mt. Everest is smaller than that at sea level, than that it is greater.

3 points

·
13th Nov 2020

>I might know barely nothing about general relativity and what I have understood might be worng. >I think it explains more or less that gravity does not really exist

If that is your idea of what GR does you've got it wrong. Have you been watching that veritasium video?

it's utter nonsense to say gravity doesn't exist. GR says that gravity is curvature of spacetime.

This video is misleading crap.

>So if gravity does not exist

it does.

>why there is still a need for a quantum gravity and a unification with the "real" forces, strong and weak nuclear forces and electromagnetic force?

http://www.scholarpedia.org/article/Quantum_gravity_as_a_low_energy_effective_field_theory

3 points

·
3rd Sep 2014

As u/Bellgard pointed out, the conservation of energy in an isolated system is a result of symmetry in time translation. If you're interested in a more rigorous proof/understanding of this as well as how other conservation laws emerge from symmetries you can look up Noether's Theorem. Personally, it's one of my favorite theorems in physics. :)

EDIT: Wikipedia Link

3 points

·
17th Mar 2020

Power supply, resistors, capacitors, inductors, solderless breadboard, gaussmeter, signal generator, multimeter, impedance analyzer, oscilloscope, vector network analyzer, spools of wire, steel laminations, soldering iron and solder, laser cutter, etc. Build whatever you are asked about and measure it.

I'm mostly joking, but if you had time, it might actually be useful to check your work on some resistor network problem by building it and testing it, and resistors are dirt cheap.

There are also free circuit simulators that are pretty easy to use for similar checking of your circuit analysis work. http://www.falstad.com/circuit/ is the most basic intro-level one.

3 points

·
3rd May 2018

Here is a way to test your answers.

You should find that your answers for i), iii) and v) are correct but the other two are wrong - double-check your application of Kirchoff's laws.

3 points

·
1st Feb 2014

I agree with /u/iamiamwhoami's method, and this question sort of piqued my interest so I went with it.

Incidentally, I believe the answer is ~70.53 degrees, or arcsin[2*sqrt(2)/3].

A heuristic idea of what's "going on" here is shown in this graph that I've attached. Note the red line is the answer. These lines plotted are spaced about five degrees each.

For the full solution, see the attached link.

3 points

·
6th Jul 2021

Mr. Tompkins in Paperback is awesome. It presents intuitive descriptions of most modern physics, through allegorical short stories about a somewhat bumbling British banker. A delightful read with 100% accurate physics.

3 points

·
30th Jun 2021

These powered gyroscopes are super cool - not sure if a battery violates the "outlet principle" =)

3 points

·
4th Jan 2021

Google Scholar is often pretty good, though you might need an academic subscription for full access (or you could use, erm, certain shady websites). I've bought a couple collections of classic papers: I've got this book and this book on my shelf, and they're both pretty good. Oh, I have a copy of Newton's Principia with lots of modern annotations too (still pretty hard to read though).

3 points

·
16th Nov 2020

Farr's Physics for Medical Imaging is a technical but approachable reference on this.

​

https://www.amazon.co.uk/Physics-Medical-Imaging-Penelope-Allisy-Roberts/dp/0702028444

3 points

·
8th May 2017

This is the book you want. On the Shoulders of Giants. It has the work of Copernicus, Galileo, Kepler, Newton, Einstein. It is a "slim" 1280 pages. And there are some used copies on Amazon for less than a cup of coffee!

3 points

·
27th Jul 2020

Meh. You’ll use university physics 5ever, I kept my thermo book, I kept Griffith’s E&M, and I would advise keeping any bvp, numerical analysis, and complex analysis books you get. Oh and BOB if you go into Astro.

3 points

·
29th Mar 2019

Fermilab made a video about this: https://www.reddit.com/r/MemeEconomy/comments/b6z3gl/kick_your_losses_to_the_curb_with_this_oc_invest/?utm_source=share&utm_medium=ios_app

To cut to the chase, the reason is that the electrons are driven by the wave and create a secondary wave that superposes with the incoming wave making for a slower wave. It’s sorta like how you can make a standing wave by adding two waves going opposite directions together, except you can make it go anywhere in between stopped and full speed, and it’s more complicated. Because sure now we’ve explain why it slows down, but not thoroughly why the wave turns in detail and without analogy. That I have yet to learn about in my EnM course, but I do have a book on Dielectrics and Waves that discusses this stuff that is very useful: https://www.amazon.com/Dielectrics-Artech-Microwave-Library-Paperback/dp/1580531229

2 points

·
9th Oct 2013

Professional physics is a horrendous amounts of maths, undergrad is too for that matter. You need to be very competent with calculus, linear algebra etc.

Sounds like you've only just been introduced to that sort of thing so give it time before you say you struggle with it, could just be you need more time to get your head round.

I'd recommend http://www.khanacademy.org/ for an insite into some slightly higher level physics and the maths required. Start from where you know and build up. Importantly, everyone struggles with learning physics at some point so don't be deterred!

2 points

·
23rd Jan 2015

I'm pretty sure there isn't a name for that phenomenon (but I may be wrong). If you want a nice explanation of why it happens, then check out Ethan Siegel's article "Does water freeze or boil in space?"

2 points

·
13th Feb 2015

There's this. My ~~friend has it~~ (welp, just bought it, now I have it), and as someone who enjoys manga/anime, this was a joy to read.

http://www.amazon.com/Manga-Guide-Physics-Hideo-Nitta/dp/1593271964

>Megumi is an all-star athlete, but she's a failure when it comes to physics class. And she can't concentrate on her tennis matches when she's worried about the questions she missed on the big test! Luckily for her, she befriends Ryota, a patient physics geek who uses real-world examples to help her understand classical mechanics-and improve her tennis game in the process!

>In The Manga Guide to Physics, you'll follow alongside Megumi as she learns about the physics of everyday objects like roller skates, slingshots, braking cars, and tennis serves. In no time, you'll master tough concepts like momentum and impulse, parabolic motion, and the relationship between force, mass, and acceleration.

>You'll also learn how to:

> * Apply Newton's three laws of motion to real-life problems * Determine how objects will move after a collision * Draw vector diagrams and simplify complex problems using trigonometry * Calculate how an object's kinetic energy changes as its potential energy increases

>If you're mystified by the basics of physics or you just need a refresher, The Manga Guide to Physics will get you up to speed in a lively, quirky, and practical way.

Oh my, how lucky are we? It's in a humble bundle, right now!

https://www.humblebundle.com/books

If you don't know how humble bundle works, these are all eBooks, so if you want an actual copy like I probably will, you can buy one later but at least you can try it here first.

You pay whatever you want. Literally. You choose what you want to pay for the bundle. If you pay more than the average (currently $13.42), then you get the second tier of books. If you pay more than an always-constant amount of $15, you get the third tier of books.

2 points

·
1st Jul 2019

E-readers are never good for textbooks and pdf's in general. If you need a tablet purely for reading purposes, there are cheap ones with low ROM (16, 32 GB) and RAM (1,2 GB) and battery over 8000mah.

You can use this tool to find what tablet suits your budget and requirement.

2 points

·
20th Sep 2018

2 points

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21st Jun 2021

I found some information on the theory behind this on Wikipedia and here on Hackaday.

This paper on the Levitation of an Aluminium Disc in a Magnetic Flux Well, even though is way too advanced, has some figures that show some graphical drawing of the forces at play. Specially Fig. 1 and Fig. 2.

I could not find the specific reason on why two magnets are used in most of those levitating platforms.

2 points

·
15th Aug 2012

Have a look at Khan Academy. This video is an intro to the ideal gas law. Look through the rest of the chemistry videos for more detail on certain areas, maybe doing some thermodynamics once you've got a good understanding of gases.

2 points

·
19th Jun 2015

Orbit, in astronomy, path of a body revolving around an attracting centre of mass. - From the Encyclopaedia Britannica, link: http://www.britannica.com/science/orbit-astronomy

There is apparently no official scientific definition, I thought the IAU might have something, but apparently not, so I went with the most reliable authority.

2 points

·
15th Aug 2021

As stated by others, focus on school. Otherwise, I have two main suggestions.

First, I strongly recommend **learning how to program**. I'd say that Python and C++ are the best all-around languages for physics, but Julia looks like a promising contender. If you find that you enjoy doing this, it will be a very powerful skill.

Second, start **writing lab reports and essays with LaTeX**. Specifically, go to overleaf and learn how to use it. LaTeX (pronounced "luhtek") is how academic papers and textbooks are written since it is a simple way to write pretty mathematical equations. Furthermore, learning how to use BibTeX, the bibliography manager, will make your life much easier in college.

2 points

·
26th Nov 2021

Summary here

http://www.scholarpedia.org/article/Quantum_gravity_as_a_low_energy_effective_field_theory

and in short, for some reason, most laymen seem to think that these two things (gravitons and gravity being curvature of spacetime) somehow contradict each other, but they don't. most likely because it's based on an already flawed (popsci) understanding of quantum field theory as well, like for instance here:

>Do massive particles exchange virtual gravitons in order to mediate gravitation? And why haven’t we detected them yet, if they exist?

Virtual particles aren't real. they aren't actually exchanged between two interacting particles. Not for gravity, not for electromagnetism or the other fundamental interactions. There's nothing to detect between them. That's even before looking at gravity in particular.

GR and QFT are both graduate topics so you have a lot of reading to do to understand the problems around them.

2 points

·
27th Oct 2021

> What happens when you use the equations of the Standard Model that predict the other bosons and where can one SEE this math?

Here's a good one: http://www.scholarpedia.org/article/Quantum_gravity_as_a_low_energy_effective_field_theory

2 points

·
2nd May 2021

Special and general relativity are not themselves quantum in any way.

Einstein's physics is more than SR / GR though and he's contributed to quantum theory as well.

And there are relativistic quantum mechanics, relativistic quantum field theory and attempting to quantize general relativity is ongoing research (see here).

2 points

·
15th Apr 2018

The traces from the green and purple dots can be used to more easily get the coordinates of an event in the moving frames' coordinates. The "n" slider can be used to divide the event coordinates' tick marks into smaller divisions for more accurate readings.

The next thing I want to figure out is how to calculate and return those coordinates instead of having to estimate them. But one step at a time.

2 points

·
29th Aug 2017

For a right triangle, the sine just happens to be the ratios of the sides. The sine is a function, so we can plot it like any other function.

2 points

·
9th Jun 2020

Hi, I used deepl.com to translate your question, maybe you can use the same method to convert the answer back to German.

Rotating black holes do not have point-like singularities. The reason for this is, that rotating black holes have angular momenta. Since point-like objects cannot have angular momenta, it is theorized, that the singularities of rotating black holes are ring-shaped – as rings are the simplest structure that are able to carry angular momentum.

When we say that black holes grow, we usually don't mean the actual singularity. Rather, the radius of their event horizon gets larger, the bigger the mass of the black hole is.

I'm not sure if I answered all of your questions, so let me know if I missed something.

2 points

·
7th Jan 2015

I'd just like to add that not __everything__ spins even though it orbits around a parent. Sometimes they are tidally locked which means the same side of the planet/moon always faces its parent body.

Our moon is tidally locked, as is recently discovered planet Gliese 581 c.

2 points

·
11th Jun 2015

Temperature is not a thing on the microscopic level. It is a macroscopic quantity which relates to the velocity of the particles in the system. So the ELI5 is actually just what it is. I mean you can go in depth about it and I'd suggest a good statistical physics book looking at the link between microscopic degrees of freedom and how they relate to macroscopic parameters such as temperature

Heat relates to a transfer of energy wikipedia

2 points

·
18th Jan 2014

Presuming the speedometer was 95 mph, which is 153 km/h or 42.47 m/s.

The jump seems to be roughly the length of the car, and at the highest point looks about the height of the ambulance which according to Ask.com would be 2.79 meters. Converted into meters, the Ford Mustang is 4.76 meters length (which is the hypotenuse of the jump thingy). This gives us a jump angle of 35.88 degrees.

Using this information and equations of motion (x = x_0 + v_0*t +(1/2)*a*t^2), we get an air time of 5.08s. His horizontal speed being 34.44 m/s, he could cross 175 meters in that time.

If his speed was in km/h, the speed was 26.39 m/s; air time would be 3.16s, for 67.57 meters.

Very, very very rough lower bound estimate based on distance between cables of the Vincent Thomas Bridge would give a jump length of, say, 50-60 meters.

Of course, no air friction, no fancy yada-yada. Overall, as incredible as it looks, it may be possible.

References: http://www.ask.com/question/what-are-the-dimensions-of-an-ambulance

https://maps.google.ca/maps?ll=33.749121,-118.269177&spn=0.032757,0.066047&t=h&z=15

2 points

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28th Apr 2020

It's very brief and to the point compared to eg. Boas and Riley, which both put more emphasis on qualitative explanations compared to Arfken. Thus, they are more suitable for someone who likely hasn't developed all that much "mathematical maturity" just yet. Arfken is a great reference though, and works great for catching up on some particular topic at late bachelors early graduate level (imo).

A 3rd option, which i have only briefly looked at myself is "Basic Training in Mathematics: A Fitness Program for Science Students" by Shankar. I'd recommend you download all 4 (*cough* https://libgen.is/ *cough*) and sample them before you decide to purchase physical copies of any of them.

2 points

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6th Jan 2020

You only need to send one electron at a time. A 'cloud' isn't required. The electron's wavefunction is what is interfering. I'd recommend reading Sakurai: https://libgen.is/book/index.php?md5=80E3F352ACB5E9025165158D4AF8999F

The theory you mention is a type of Objective collapse theory: https://en.wikipedia.org/wiki/Objective-collapse_theory

2 points

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18th Mar 2015

Try this one: http://orbit.medphys.ucl.ac.uk/

It is a much more realistic version of KSP, but much harder and more difficult to use. It is free was for example used for the soyuz launch movie from ESA. https://www.youtube.com/watch?v=AVvgpKt5uCA

2 points

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14th Oct 2020

This is now being asked every day. Just because a YouTube video made a big point of it to insist. :) imo not much is achieved by insisting on it not being a force.

It's not a force in GR but it is one in newtonian gravity. Even in GR it is still one of the four fundamental interactions.

A theory of gravitons reduces to GR and is not in conflict with this at all. you get a theory of gravitons by quantizing (linearized) Einstein's equation.

http://www.scholarpedia.org/article/Quantum_gravity_as_a_low_energy_effective_field_theory

Plus you're probably thinking of force carriers as particles being sent back and forth between interacting particles to communicate some force., That's another misconception in popscience.

2 points

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15th Dec 2019

some reading http://www.scholarpedia.org/article/Quantum_gravity_as_a_low_energy_effective_field_theory

Your question is kinda like asking why do we need photons (quantum electrodynamics) if we have classical electrodynamics / maxwell equations.

>and it's the best explanation otherwise black holes couldn't exude gravity as gravitons couldn't escape it

This is a misunderstanding of how quantum field theories work. Gravitons don't have to be emitted by masses to attract other masses and they don't have to leave black holes for gravity to work.

2 points

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2nd Aug 2015

Let's look over what's posted on Wikipedia

From easiest to hardest:

**0th law** An isolated black hole in an equilibrium state (stationary) will exert a constant gravitational force at its surface - the *event horizon*, beyond which not even light can escape

**3rd law** A black hole with a gravitational force of 0 at the event horizon cannot exist

**1st law** Small energy changes in an otherwise isolated black hole are directly related to changes in surface area, rotation (angular momentum), and the charge.

**2nd law** (let's take Hawking's version) The entropy of a black hole increases with time as the area and mass decrease (Hawking radiation). However, the total entropy increase of the black hole system is greater than the entropy decrease of the outside universe.

EDIT: Formatting and wording

2 points

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5th Apr 2015

> The temperature of the gas in the nebula is about 10,000 degrees Celsius

http://www.scholarpedia.org/article/Planetary_nebulae

>That gravitational potential energy turns into kinetic energy, which turns into heat at the center of this ball of matter.

Does matter in a ground state have potential energy?

2 points

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12th Apr 2014

I assume you mean the universe has systems that are casually disconnected? Dynamics in one system doesn't influence the other one?

If we just think about it in classical Newtonian sense and just think about gravity, the numerical solution of a N-body system goes as O(N^(2)) in terms of computational complexity.

http://www.scholarpedia.org/article/N-body_simulations_(gravitational)

If you set up a toy universe with, let's say, 100 particles this will give ~ 100^2 = 10000 (ignoring constants) if they're all connected by gravity. However, if you set up the universe such that you have 10 casually separated systems of 10 particles you get that each universe take 10^2 = 100. Since you have 10 of them, this gives you 1000. If you in general put sqrt(N) in each universe you get the complexity of O(N^(3/2)) which is clearly less than O(N^(2)).

So at least if you're only thinking about simulating the separate systems, the answer is yes.

2 points

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1st Mar 2016

That is correct. Take a quick look at table 2 of this paper:

This is some theoretical and experimental data for various materials illuminated to simulated solar spectrum (n(%) is efficiency). As you can see even though the current for germanium is much higher, the Voc and FF are much much lower, leading to lower efficiency.

The explanation your textbook gives is a bit unclear(unacceptable) to me since by their reasoning of "using widest range", you would be correct. I would ask your teacher for clarification or just make sure you're on the same page which is important for tests.

2 points

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14th May 2019

> So any given ion only exists for a tiny period of time?

As a general statement no. Solvated (see solvation) compounds hold their ionic state, ionic bonds hold their ionic state.

So there are cases in which ionic states can be stable. Generally you do not need to blast those substances with enough energy to ionize them though.

In cases where you have elevated the ground state via ionization, those cases are excited and want to return to their ground state. These will be incredibly fast.

In fact the light emitted by the ~~light~~ fluorescent bulb is this process, excitation of the electronic state and return to ground state. The returning to ground state is achieved by release of a photon. The photons wavelength will be determined by the electron orbitals and through which states they are transitioning. (see: Electron Transition).

This is why different molecules will produce different colors. The best examples are gases used for neon lights. The different colors are achieved with different gases, as their electronic structure is different meaning different electron transitions and different wavelengths.

2 points

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23rd Oct 2017

Nope, the voltage across the 5 Ohm resisstor is 5 Ohm * 0.5 A = 2.5 V.

Next, we need to calculate the equivalent resistance of 12 Ohm and 8 Ohm in parallel, which is 1/(1/12 + 1/8) = 4.8 Ohm. Then we calculate the voltage across both resistors: 4.8 Ohm * 0.5 A = 2.4 V.

So we get:

5 Ohm resistor: 2.5 V 8 Ohm resistor: 2.4 V 12 Ohm resistor: 2.4 V

2 points

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17th May 2015

I might suggest checking out the EEV Blog He has a bunch (over 700!) episodes. Maybe not exactly what you are looking for, however most of the time you do end up learning something important about how all the devices around us work.

As for practice, you could check this out

2 points

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6th Jun 2014

It may be too simple for you but https://www.udacity.com/course/ph100 is a fun free self paced course. Again, it may be too easy for you but I found it an entertaining review of a bunch of different ideas. Its self paced so you can go through it as quickly or as slowly as you like.

Coursera also has a bunch of free courses that may be of interest. https://www.coursera.org/courses?orderby=upcoming&search=physics&lngs=en

2 points

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14th May 2015

I am sure you know of the textbook by Goldstein. This quora thread is good for any resources you may need.

2 points

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24th Dec 2015

An inner product in special relativity is actually something called a metric contraction. This doesn't only happen between two four vectors; it is a more generalized tensor operation. However, for a contraction to produce a true scalar using a four vector and another object, that other object must be a four vector.

2 points

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1st Sep 2021

In Bohren's What Light Through Yonder Window Breaks? the first (tiny) chapter is about condensation on windows. It's actually a complex mechanism that involves temperature gradient and convection around the window.

But in the case of a double-pane window, things might get more convoluted, although those two mechanism will still be at play.

2 points

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29th Jul 2021

I'm also from Canada and started getting into physics around your child's age. (I'm now a grad student.)

If he's interested, you should get him *both* popular books and a good introductory textbook. I can't speak as to other provinces, but the physics curriculum in Ontario leaves much to be desired. For this reason, I think it's worth self-studying physics if he's interested. Irrespective of whether he pursued physics as a career, the skills you acquire from doing physics carry over to many different fields, and (more importantly) it's fun!

You've already received many good recommendations for popular books - I'll add QED: the strange theory of light and matter, by Feynman. Here's a link to it on Amazon.

As for textbooks, I'd get him a nice book on calculus and any introductory physics text. I learned calculus from this book, which is intended for self-study. It's a pretty old book, but it's great.

A bunch of good youtube channels have already been brought up in this thread, and the only one I'll add is 3blue1brown, who makes some beautiful and accessible videos about math.

2 points

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23rd Jun 2021

"From Falling Bodies to Radio Waves" by Emilio Segre isn't focused on electromagnetism, but its Chapters 3 and 4 (107 pages) deal with light and electromagnetism and are excellent (as is the rest of the book). If you want to see how Maxwell's equations came to be in the form they have today, "The Maxwellians" by Bruce Hunt is fascinating.

2 points

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15th Jun 2021

Avalanche diode noise generators are super cheap and easy to make. They make electronic noise for quantum processes. You can buy ready made ones.

2 points

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15th May 2021

You could go through the Feynman lectures https://www.feynmanlectures.caltech.edu/

If you want like an actual textbook, someone mentioned Young and Freedman, another option would be RHK volumes 1 and 2: https://www.amazon.com/Physics-1-Robert-Resnick/dp/0471320579

2 points

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25th Feb 2021

Assuming you don't have a background in physics, I recommend learning basic mechanics first. So basic orbital mechanics, Keplers laws, etc.

If you are already there then I recommend the book by Ryden:

This book takes you a step further without being TOO challenging.

2 points

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20th Jan 2021

Are you sure you can't buy them on amazon? I checked, and found this listing. It says kindle listing, but they show if they have hardcover and paperback versions for sale. The only one missing is volume 5, but I think it should be available because I bought my copy from amazon.

2 points

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3rd Dec 2020

As a follow-up suggestion: look for supplementary resources. MIT's Opencourseware has some really good lectures that I used quite a bit during grad school. You also might want to start looking for textbooks that will help bridge the undergrad texts with the grad ones. For instance, making the jump from Griffiths to Landau is very difficult. I used Zettili because it's better than Griffiths with the mathematical formalism, but it's much more approachable than Landau. There are books that fill the same role in other subjects, but QM is the only one I can think of offhand.

2 points

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1st Dec 2020

For deeper readings I recommend Variational Principles in Classical Mechanics (pdf link) as well as the highly recommended Variational Principles of Mechanics by Lanczos (amazon link).

2 points

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30th Nov 2020

A good undergraduate text on classical mechanics is Taylor's Classical Mechanics. This text gives a good intro to the Lagrangian and Hamiltonian formalisms that physicists use.

A more challenging text the staple for graduate courses by Goldstein, Safko, & Poole.

2 points

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2nd Sep 2020

That’s a way to think about it, no it would either still have one hole or no longer have one (if it cracked all the way up), and 50 Mathematical Ideas You Really Need to Knowis the only one I got that’s not masters level sorry. But numberphile should help

2 points

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11th Aug 2020

Have a look at Pais’ biography of Einstein. The book is definitely more of a scientific bio than a biography of his life.

Subtle Is the Lord: The Science and the Life of Albert Einstein https://www.amazon.com/dp/0192806726

2 points

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10th Aug 2020

University physics is usually a good start. It does spec rel at the end. As for cosmology... I can’t immediately think of a non graduate level book except BOB, but you might have to work on your math to get through that one. Good luck!

2 points

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7th Aug 2020

Check out the ARRL Handbook for Radio Communications. A classic reference book for radio hobbyists.

And you stay safe as well.

2 points

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28th Jul 2020

If you are learning first year physics as a physicist, look in your college library for an old version of Halliday and Resnick from around 1980 or just before. It will be black and white text. It has extraordinarily clear exposition of the physics. Here is an example: https://www.amazon.com/Physics-Parts-II-David-Halliday/dp/047134530X/ref=sr_1_7?dchild=1&keywords=halliday+resnick+1981&qid=1595905793&sr=8-7

My experience was that physicists never learn thermodynamics. Even if you take a class, if it is in the physics department, it tends to quickly dive down into statistical mechanics or, at the least, will not describe entropy without reference to number of states, which misses the point and beauty of thermo per se. I like an old book called Fundamentals of Classical Thermodynamics by Sonntag. Get the SI version. Mine is the 3rd edition and I think I got it for about $1.00 used on Amazon. This is not an easy book, but you will learn thermo from it. Real thermo. Cengel and Bowles is another option and is a bit easier.

I've never made it through the Feynman lectures. I dive in now and then. It is interesting when you already know the stuff because he describes things from a different perspective or goes back to basics, but not in a basic way.

Trust your professors, buy the books they say, then go to the physical library and snoop around in related books for other presentations. This will show you want you to get, if anything.

2 points

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28th Jan 2020

I know other people have already answered your question in depth but I have been able to grow tomatoes, basil, wild flowers, and most recently a grape fruit sapling with 2 of these cheep ass bulbs from amazon. I just built up a make shift cage that I could hang the bulbs from and taped a dollar store thermal blanket around the outside to reflect the light and to keep them from driving me insane. I always like to have plants around to remind me that winter will come to an end sooner or later and it will be spring again.

1 point

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22nd Dec 2022

Another book from an evolutionary biologist's perspective, The Selfish Gene by Richard Dawkins.

I think this question has so many different angles (from Fermi's paradox to the specific chemistry involved) and I think it's safe to say that no one has a fully satisfying answer for how inevitable self-replicating life is.

1 point

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22nd Dec 2022

Others here have given great answers. I will point you to a book by the physicist Erwin Schrödinger:

https://www.amazon.de/What-Life-Autobiographical-Sketches-Classics/dp/1107604664

It was written before the discovery of DNA but points heavily towards it and, in the end, Schrödinger discusses his thoughts on the relationship between physics and the mind. It was also written for the layman, so it should be accessible for you.

Erwin Schrödinger was one of the physicists who really kickstarted the quantum revolution - the equation we use to model matter on a quantum mechanical level is named after him. I think his thoughts on this matter might be of interest to you.

1 point

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12th Aug 2014

I don't have much time right now and only skimmed your question, but ->here<- are my notes from my DiffGeo class about that topic. I hope this helps.

1 point

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7th May 2013

Don't ever let anyone tell you a field of study is too hard. They might believe it's too hard for them, but what do they know about you?

An important point brought up is, if you're serious about pursuing physics, you'll have to know the appropriate math. For both, Khan Academy has some excellent sections. Check out the Math and Physics sections.

1 point

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26th Nov 2022

"University Physics" is a great book; current editions are expensive, but older ones can be cheap. I paid $30 for a used one in "very good" condition about 4 years go. There are "acceptable" copies for $10.

https://www.amazon.com/gp/product/0805391797/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

I found the book very interesting. I wore out a previous version and I had to buy this one.

1 point

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22nd Nov 2022

/u/mfb- recommends a book. I recommend specifically "Physics for Future Presidents" by Richard Muller. It has a 100-page section on nuclear energy that covers basically all this stuff. It's also got some good sections on the physics of terrorism, outer space, energy, and climate change.

https://www.amazon.com/Physics-Future-Presidents-Science-Headlines/dp/0393337111

1 point

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19th Nov 2022

My friends and I are building a rainbow laser, which uses that. We even have a working prototype. A single beam changes smoothly from red to blue and even does white.

look up dichroic cube.