6 ms·

Show HN: Relativity: A Modern Primer

204 points by fferen 5y ago

wanderingmind 5y ago
This is fantastic. I always wanted to learn relativity. But almost all the books I borrowed were so deep targeted towards students who will think about that day in and out. I think a lot of avenues of core sciences need books like these that are deeper than popular science but are accessible to someone who is not doing it for a living.
- ericbarrett 5y ago
  I recommend Sean Carroll’s “Biggest Ideas in the Universe” series on YouTube for modern physics. He’s a professor at CalTech. Each is 60-90 minutes with an accompanying 60-90 minute Q&A—don’t miss the latter, they’re just as interesting. You won’t get a PhD after watching them but he gets a lot mathier than e.g. a Kurzgesagt video while only assuming a high school education (basic calculus, trig, vectors).
- oefrha 5y ago
  (Physicist here.) Looked at TOC, then skimmed some chapters. The special relativity part of the book is about as deep and denser than most undergraduate introductions, with less motivations and intuitive discussions. The general relativity part is dense by necessity (after all 95% of it is differential geometry, no way around it), and this book apart from being way more condensed isn’t in any way diluted. It’s a compact book of essentials for mathematically minded people*. It’s not a shallower-than-textbooks accessible introduction one step up from pop science. * If you only ever took some linear algebra and calculus for engineering students, this probably doesn’t describe you.
  - fferen 5y ago
    This is fair enough. Hopefully some will still find it useful. I will note that I took a special relativity course in university that did things the "slow and intuitive" way, and many years later realized I still had some fundamental misconceptions about it. Eventually learned it the right way mostly by osmosis. More examples and motivation doesn't necessarily lead to correct understanding.
    - oefrha 5y ago
      > Hopefully some will still find it useful. Yes, I definitely think it will be useful for mathematically minded people who don’t find the “slow and intuitive” approach helpful. However, I suspect the fact that it is posted to HN where most of the audience would be people who only took some math for engineering students and that “popular physics book” is even mentioned on the homepage (it clearly says “unlike”, but for most textbooks this comparison isn’t even needed) might give rise to the misconception that it’s between pop science and other textbooks in terms of depth, as the root of this thread seems to demonstrate.
- MathMonkeyMan 5y ago
  Just read a textbook. It's not easy, but it's what you want: - rigorous treatment of the material - no bullshit fluff - exercises that you can do to think about the material and test your comprehension (often 50-100% of the answers are available online) - systematic treatment used to train real physicists In my experience, it's all about the homework. The rest just supports that.
- chisquared 5y ago
  I quite like this lecture series, and I normally hate learning from YouTube videos: https://www.youtube.com/playlist?list=PLFeEvEPtX_0S6vxxiiNPrJbLu9aK1UVC_ https://www.youtube.com/playlist?list=PLFeEvEPtX_0S6vxxiiNPr...
  - Koshkin 5y ago
    Dr. Schuller’s lectures is one of the few recordings of this kind on YouTube that I can listen to without getting irritated by one thing or another. (Another one being the widely known physics lectures by L. Susskind.)
powera 5y ago
I'm not sure who this is supposed to be useful to; I'm not sure how anybody can understand it unless they have completed an undergraduate degree in math AND already have a solid conceptual understanding of relativity. It is extremely dense.
- pvg 5y ago
  The same can be said for, say, Lifshitz & Landau's textbooks. A little bit closer to earth, Axler's Linear Algebra Done Right is well-liked and popular but of relatively little use to people trying to do linear algebra right for the fist time.
- goldenkey 5y ago
  I disagree. Most of relativity comes from the fact that all objects have an average velocity of their fundamental constituents. The highest average velocity is C, when the object is a beam of electromagnetic energy. This is why it takes infinite energy to accelerate an object to the speed of light, you can add more photons to it, but the average will always be less than C, you just get it slightly closer. The reason for time dilation and the other facets of relativity pretty much come down to the fact that objects either move or change, but cannot do both simultaneously. One can think of time passing inside a spacecraft or object as internal movement, as oppose to the external movement of the spacecraft throughout the cosmos. Each bit of energy provides h (Planck's constant) action, a measure of change of state. The more the object allocates toward external movement in space, the less it can allocate for internal movement / internal changes, which is what observer time really is. Even though I say external vs internal, I am not violating relativity. The reference frames are relative, we do not need an absolute reference frame. Doubly special relativity deduces most of these elegant derivations by assuming there is a smallest quanta of energy possible, but nonetheless, these concepts can be derived just by understanding the role of mass as loops of energy, energy as an allowance for change of state, and time as the usage of energy for internal change of state, and movement as the usage of energy for external change of state. Relativity is just the consequence of energy's connection to information and movement. https://en.wikipedia.org/wiki/Doubly_special_relativity https://en.wikipedia.org/wiki/Doubly_special_relativity
drewolbrich 5y ago
@fferen Thank you for sharing your work. This is an excellent resource. If you don’t mind, I have a question. If I drive in a straight line on the Earth’s surface without stopping and I ignore mountains and oceans and other obstacles, then after 16 days, I will arrive at my starting point. Why is this? It’s because the Earth is a sphere. This is a nice satisfying answer, whereas x^2+y^2+z^2=r^2, while perfectly accurate, is arguably less satisfying. Given this context, my question is, for special relativity, why do time dilation and length contraction happen? Ideally, I’m looking for an answer that has the same satisfying intuitive flavor as “Because the Earth is a sphere”, or at least is suggestive of that kind of answer.
- fferen 5y ago
  Thanks! I don't believe these two concepts are related. You get back to the same point because the earth is _globally_ a sphere. However, time dilation and length contraction are _local_ concepts: they happen even for very small motions. I guess a global object in spacetime analogous to a sphere in space is the hyperboloid t^2 - x^2 = r^2. Moving on this hyperboloid corresponds to changing boost velocity. But unlike a sphere, it is not closed, so moving in one direction does not get you back to the same point. May add to this answer later.
  - drewolbrich 5y ago
    Apologies, I didn't mean to suggest that the two concepts are related. For SR, I'm looking for an answer to "why?" that only has same satisfying flavor as the sphere question. I want the same "aha!" feeling. For example, if I stand up from the sofa and walk across the room and come back and sit down next to my friend, I want a deep intuitive sense that of course it must be the case that less time has passed for me than the amount of time that my friend has experienced. Why does this happen? An answer like "t'=t/sqrt(1-v^2/c^2) describes what happens", while correct, is not satisfying. Similarly, if I wave my hand in front of my face, I want it to seem obvious to me that less time must have passed for my hand than for the rest of my body. Given your experience writing the book, you must have developed an intuitive sense for the behavior of the effects of relativity and why they happen, so I am wondering how you would translate that into words for a general audience. Imagine the context where a random person with a minimal math background at a party was to ask you why less time passes in the sofa scenario, using an actual sofa to demonstrate it. They stand up and walk away from you and return and sit back down next to you and they want you to explain to them why less time has passed for them. They want you to explain why the room around them got shorter in the direction that they were walking. These are effects that, while undetectably small, really happened. They want to know why. How would you answer their question?
motohagiography 5y ago
Every question I have is predicted on ignorance of the complete picture, but I have dozens of questions. This is the perfect format for going through and re-formulating and re-writing it in ones own notes to hack through it.
musgravepeter 5y ago
I had a quick look and this seems like a solid, concise intro to GR. The article https://people.carleton.edu/~nchriste/PTO000041.pdf https://people.carleton.edu/~nchriste/PTO000041.pdf gives an overview of the ways and books to learn GR. It does not cover some more recent intro books. If you like "physics first" then I am a big fan of Hartle's book.
monday_ 5y ago
This is great and way better then the more long-winded textbook explanations. Math is around for a reason and it's good to see it at work by compressing a lot of material into still readable format. Sure hope there's a similar primer on QFT.
- Koshkin 5y ago
  I can recommend https://nononsensebooks.com/qft/ https://nononsensebooks.com/qft/ (as well as the rest of this excellent series of books).
  - monday_ 5y ago
    Thanks!
Deutscher 5y ago
Would you mind making this available as an EPUB?
wnoise 5y ago
I've always had a fondness for Taylor and Wheeler's "Spacetime Physics". https://www.eftaylor.com/spacetimephysics/ https://www.eftaylor.com/spacetimephysics/

204 points by fferen 5y ago

This is fantastic. I always wanted to learn relativity. But almost all the books I borrowed were so deep targeted towards students who will think about that day in and out. I think a lot of avenues of core sciences need books like these that are deeper than popular science but are accessible to someone who is not doing it for a living.

ericbarrett 5y ago
I recommend Sean Carroll’s “Biggest Ideas in the Universe” series on YouTube for modern physics. He’s a professor at CalTech. Each is 60-90 minutes with an accompanying 60-90 minute Q&A—don’t miss the latter, they’re just as interesting. You won’t get a PhD after watching them but he gets a lot mathier than e.g. a Kurzgesagt video while only assuming a high school education (basic calculus, trig, vectors).
oefrha 5y ago
(Physicist here.) Looked at TOC, then skimmed some chapters. The special relativity part of the book is about as deep and denser than most undergraduate introductions, with less motivations and intuitive discussions. The general relativity part is dense by necessity (after all 95% of it is differential geometry, no way around it), and this book apart from being way more condensed isn’t in any way diluted. It’s a compact book of essentials for mathematically minded people*. It’s not a shallower-than-textbooks accessible introduction one step up from pop science. * If you only ever took some linear algebra and calculus for engineering students, this probably doesn’t describe you.
- fferen 5y ago
  This is fair enough. Hopefully some will still find it useful. I will note that I took a special relativity course in university that did things the "slow and intuitive" way, and many years later realized I still had some fundamental misconceptions about it. Eventually learned it the right way mostly by osmosis. More examples and motivation doesn't necessarily lead to correct understanding.
  - oefrha 5y ago
    > Hopefully some will still find it useful. Yes, I definitely think it will be useful for mathematically minded people who don’t find the “slow and intuitive” approach helpful. However, I suspect the fact that it is posted to HN where most of the audience would be people who only took some math for engineering students and that “popular physics book” is even mentioned on the homepage (it clearly says “unlike”, but for most textbooks this comparison isn’t even needed) might give rise to the misconception that it’s between pop science and other textbooks in terms of depth, as the root of this thread seems to demonstrate.
MathMonkeyMan 5y ago
Just read a textbook. It's not easy, but it's what you want: - rigorous treatment of the material - no bullshit fluff - exercises that you can do to think about the material and test your comprehension (often 50-100% of the answers are available online) - systematic treatment used to train real physicists In my experience, it's all about the homework. The rest just supports that.
chisquared 5y ago
I quite like this lecture series, and I normally hate learning from YouTube videos: https://www.youtube.com/playlist?list=PLFeEvEPtX_0S6vxxiiNPrJbLu9aK1UVC_ https://www.youtube.com/playlist?list=PLFeEvEPtX_0S6vxxiiNPr...
- Koshkin 5y ago
  Dr. Schuller’s lectures is one of the few recordings of this kind on YouTube that I can listen to without getting irritated by one thing or another. (Another one being the widely known physics lectures by L. Susskind.)

powera 5y ago

I'm not sure who this is supposed to be useful to; I'm not sure how anybody can understand it unless they have completed an undergraduate degree in math AND already have a solid conceptual understanding of relativity. It is extremely dense.

pvg 5y ago
The same can be said for, say, Lifshitz & Landau's textbooks. A little bit closer to earth, Axler's Linear Algebra Done Right is well-liked and popular but of relatively little use to people trying to do linear algebra right for the fist time.
goldenkey 5y ago
I disagree. Most of relativity comes from the fact that all objects have an average velocity of their fundamental constituents. The highest average velocity is C, when the object is a beam of electromagnetic energy. This is why it takes infinite energy to accelerate an object to the speed of light, you can add more photons to it, but the average will always be less than C, you just get it slightly closer. The reason for time dilation and the other facets of relativity pretty much come down to the fact that objects either move or change, but cannot do both simultaneously. One can think of time passing inside a spacecraft or object as internal movement, as oppose to the external movement of the spacecraft throughout the cosmos. Each bit of energy provides h (Planck's constant) action, a measure of change of state. The more the object allocates toward external movement in space, the less it can allocate for internal movement / internal changes, which is what observer time really is. Even though I say external vs internal, I am not violating relativity. The reference frames are relative, we do not need an absolute reference frame. Doubly special relativity deduces most of these elegant derivations by assuming there is a smallest quanta of energy possible, but nonetheless, these concepts can be derived just by understanding the role of mass as loops of energy, energy as an allowance for change of state, and time as the usage of energy for internal change of state, and movement as the usage of energy for external change of state. Relativity is just the consequence of energy's connection to information and movement. https://en.wikipedia.org/wiki/Doubly_special_relativity https://en.wikipedia.org/wiki/Doubly_special_relativity

drewolbrich 5y ago

@fferen Thank you for sharing your work. This is an excellent resource. If you don’t mind, I have a question. If I drive in a straight line on the Earth’s surface without stopping and I ignore mountains and oceans and other obstacles, then after 16 days, I will arrive at my starting point. Why is this? It’s because the Earth is a sphere. This is a nice satisfying answer, whereas x^2+y^2+z^2=r^2, while perfectly accurate, is arguably less satisfying. Given this context, my question is, for special relativity, why do time dilation and length contraction happen? Ideally, I’m looking for an answer that has the same satisfying intuitive flavor as “Because the Earth is a sphere”, or at least is suggestive of that kind of answer.

fferen 5y ago
Thanks! I don't believe these two concepts are related. You get back to the same point because the earth is _globally_ a sphere. However, time dilation and length contraction are _local_ concepts: they happen even for very small motions. I guess a global object in spacetime analogous to a sphere in space is the hyperboloid t^2 - x^2 = r^2. Moving on this hyperboloid corresponds to changing boost velocity. But unlike a sphere, it is not closed, so moving in one direction does not get you back to the same point. May add to this answer later.
- drewolbrich 5y ago
  Apologies, I didn't mean to suggest that the two concepts are related. For SR, I'm looking for an answer to "why?" that only has same satisfying flavor as the sphere question. I want the same "aha!" feeling. For example, if I stand up from the sofa and walk across the room and come back and sit down next to my friend, I want a deep intuitive sense that of course it must be the case that less time has passed for me than the amount of time that my friend has experienced. Why does this happen? An answer like "t'=t/sqrt(1-v^2/c^2) describes what happens", while correct, is not satisfying. Similarly, if I wave my hand in front of my face, I want it to seem obvious to me that less time must have passed for my hand than for the rest of my body. Given your experience writing the book, you must have developed an intuitive sense for the behavior of the effects of relativity and why they happen, so I am wondering how you would translate that into words for a general audience. Imagine the context where a random person with a minimal math background at a party was to ask you why less time passes in the sofa scenario, using an actual sofa to demonstrate it. They stand up and walk away from you and return and sit back down next to you and they want you to explain to them why less time has passed for them. They want you to explain why the room around them got shorter in the direction that they were walking. These are effects that, while undetectably small, really happened. They want to know why. How would you answer their question?

motohagiography 5y ago

Every question I have is predicted on ignorance of the complete picture, but I have dozens of questions. This is the perfect format for going through and re-formulating and re-writing it in ones own notes to hack through it.

musgravepeter 5y ago

I had a quick look and this seems like a solid, concise intro to GR. The article https://people.carleton.edu/~nchriste/PTO000041.pdf https://people.carleton.edu/~nchriste/PTO000041.pdf gives an overview of the ways and books to learn GR. It does not cover some more recent intro books. If you like "physics first" then I am a big fan of Hartle's book.

monday_ 5y ago

This is great and way better then the more long-winded textbook explanations. Math is around for a reason and it's good to see it at work by compressing a lot of material into still readable format. Sure hope there's a similar primer on QFT.

Koshkin 5y ago
I can recommend https://nononsensebooks.com/qft/ https://nononsensebooks.com/qft/ (as well as the rest of this excellent series of books).
- monday_ 5y ago
  Thanks!

Deutscher 5y ago

Would you mind making this available as an EPUB?

wnoise 5y ago

I've always had a fondness for Taylor and Wheeler's "Spacetime Physics". https://www.eftaylor.com/spacetimephysics/ https://www.eftaylor.com/spacetimephysics/