Since high school I was always confused by the concept of energy, in recent years I tried multiple times to understand what it actually is and I failed to grasp it from the common definitions (why does the joule have so many definitions on so many apparently unrelated physical phenomena?).  Recently I stumbled upon this quote from Feynman, addressing exactly what my problem was.     There is a fact, or if you wish, a law governing all natural phenomena   that are known to date. There is no known exception to this law – it   is exact so far as we know. The law is called the conservation of   energy. It states that there is a certain quantity, which we call   “energy,” that does not change in the manifold changes that nature   undergoes. That is a most abstract idea, because it is a mathematical   principle; it says there is a numerical quantity which does not change   when something happens . . . it is a strange fact that when we   calculate some number and when we finish watching nature go through   her tricks and calculate the number again, it is the same. It is   important to realize that in physics today, we have no knowledge of   what energy “is.” We do not have a picture that energy comes in little   blobs of a definite amount. It is not that way. It . . . does not tell   us the mechanism or the reason for the various formulas.   source: The Feynman Lectures on Physics Vol I, p 4-1   So my question is: Feynman died in 1988, do we today, 30 years later, have any better insights on what energy really is? does this quote still hold?
When lecturing about conservation of energy in the 1960s, Richard Feynman remarked:     It is important to realize that in physics today, we have no knowledge of what energy is. We do not have a picture that energy comes in little blobs of a definite amount. It is not that way. However, there are formulas for calculating some numerical quantity, and when we add it all together it gives “28” —always the same number. It is an abstract thing in that it does not tell us the mechanism or the reasons for the various formulas.    50 some years later, have we made any progress in terms of understanding the fundamental nature of what energy is?
This is a question that arose in the comments here:   I normally hide footnotes from the TOC by doing this:  \section[title]{title\footnote{This is a footnote.}}.   This works in a lot of document classes (memoir, article, ...), but not in all of them. amsbook, for example, uses the optional argument only for the running heads so the footnote mark still ends up in the TOC. So what is the 'universal solution'? In viewing the weekly list of rep change, I see  on top: razpeitia2 5 16  member for: 2 years, 7 months link #1 week rank +8529 change 727 total reputation 2,286 week reputation    How can someone with 727 total have 2286 for the week with no bounties?  Call me curious.  Is this a bug or something else here? What is a "callable"? download.01.org certificate verification failed - CRLfile missing? So I had intel-Linux-graphics-installer on my computer  but lately when I   sudo apt-get update   I get the following error  Err https://download.01.org trusty/main amd64 Packages                          server certificate verification failed.  CAfile: /etc/ssl/certs/ca-certificates.crt CRLfile: none   Tried reinstalling intel-linux-graphics-installer but nothing happend. And various solutions found here on askubuntu.com on certificates problems but didnt end up anywhere.  Any ideas on how to get that CRLfile? Also is this problem on my side or on the servers side? How to match all occurrences of a regex Definition of $\mathbb{R}^\infty$ Question: Why is the topological space $\mathbb{R}^\infty$ defined to be the subset of $\prod_{i=1}^\infty \mathbb{R}_i$ consisting of sequences $(a_i)_{i=1} ^{\infty}$ such at most finitely many $a_i\neq 0$? Why does one insist on the condition that $a_i\neq0$ for at most finitely many $i$? Duplicate questions that use different words to describe the same problem are important for people to find the content via search engines. But the user experience when an anonymous user arrives at a duplicate is less than ideal.  They have to notice the duplicate link and understand that the answer they are looking for is likely behind that link. I don't think this is all that obvious to someone without experience with an SE site.  I propose to just redirect users to the duplicate automatically if the closed duplicate has zero answers. In that case they need to follow the link to the duplicate in any case, so we might as well redirect them to it in the first place.  I could imagine this being problematic with search engines, as we would be displaying different content than what the search engines see. If that is too problematic I would suggest to show a very obvious link to the duplicate in the answers section for anonymous users. Problem :   Solve in $\mathbb N$ :   $8n+9\equiv 0\pmod{1163}$  The mathematics give me : $n=435+1163k$ , $k\in\mathbb N$   $1163$ is a prime number   $8n\equiv -9\pmod{1163}$   $8n\equiv 1154\pmod{1163}$   I don't know any ideas to complete my work ?   I have to see your hints Why isn't temperature frame dependent? How did the phenomenon of doubling words come about? I am referring to phrases such as: "Do you like her, or do you like like her." Can someone provide an explanation of this? There are many more examples but none come to mind at the moment. Rogue DHCP Server Can't be found Over the last 3-4 weeks I have been trying to find a rogue DHCP server on my network but have been stumped! It is offering IP Addresses that do not work with my network, so any device that needs a Dynamic Address is getting one from the Rogue DHCP and then that device stops working. I need help to find and destroy this thing! I think it might be a Trojan of some sort.  My Main Router is the only valid DHCP Server and is 192.168.0.1 which offers a range of 192.160.0.150-199, and I have this configured in my AD as Authorized. This ROGUE DHCP claims to be coming from 192.168.0.20 and offering an IP Address in the range of 10.255.255.* which is messing up EVERYTHING on my network unless I assign a static IP Address to it. 192.168.0.20 does not exist on my network.  My network is a single AD Server on Windows 2008R2, 3 other physical servers (1-2008R2 and 2 2012R2) about 4 Hypervisor VM's, 3 laptops and a Windows 7 box.  I can't ping the rogue 192.160.0.20 IP, and I can't see it in the ARP -A output, so I can't get its MAC address. I'm hoping that someone reading this post has come across this before. How does matter know to obey physical laws? First of all, you may say that, this is a "why" question and in physics we don't ask such a question and instead we ask about "how". However, I want to discuss this with you.  We model the behavior of matter by physical laws, for example by quantum mechanics laws. I want to know how the matter, say electron, knows that it must obey such a law. What forces the electron to obey quantum mechanics laws?  If you consider a simulation of the universe in a computer, in the view of someone  simulated in that computer the physical laws are just there. They may not know what keeps  in check the simulated matter to obey physical laws, but in our perspective those simulated physical laws are there because we implemented them as zero and ones in a hard disk. If we wanted, we could change those physical laws and in the eye of the simulated person everything would be still fine.  Is there a scientific attempt to discuss this issue? so that I can read about it. How do you use a variable in a regular expression? I would like to create a String.replaceAll() method in JavaScript and I'm thinking that using a regex would be most terse way to do it.  However, I can't figure out how to pass a variable in to a regex.  I can do this already which will replace all the instances of "B" with "A".  "ABABAB".replace(/B/g, "A");   But I want to do something like this:  String.prototype.replaceAll = function(replaceThis, withThis) {     this.replace(/replaceThis/g, withThis); };   But obviously this will only replace the text "replaceThis"...so how do I pass this variable in to my regex string? I figured out how to use a normal image as the background while using an HDRI for environmental lighting. But I can't figure out how to use a different HDRI as the background.  Please note I already read this answer:  but this appears to be different for Eevee in 2.8.  Thanks! Is Force lightning from the feet plausible? In all the canons of Star Wars, there has not been one instance of a Force-user projecting Force lightning from their feet. Why? Is it implausible? Disable chat migration notification if one of the users has insufficient rep? Calculating days of week given a week number Given a week number, e.g. date -u +%W, how do you calculate the days in that week starting from Monday?  Example rfc-3339 output for week 40:  2008-10-06 2008-10-07 2008-10-08 2008-10-09 2008-10-10 2008-10-11 2008-10-12 Plot $|z - i| + |z + i| = 16$ on the complex plane Plot $|z - i| + |z + i| = 16$ on the complex plane   Conceptually I can see what is going on. I am going to be drawing the set of points who's combine distance between $i$ and $-i = 16$, which will form an ellipse. I was having trouble getting the equation of the ellipse algebraically.  I get to the point:  $x^2 + (y - 1) ^2 + 2\sqrt{x^2 + (y - 1)^2}\sqrt{x^2 + (y + 1)^2} + x^2 + (y+ 1)^2 = 256$  $2x^2 + 2y^2 + 2\sqrt{x^2 + (y - 1)^2}\sqrt{x^2 + (y + 1)^2} = 254$  It seems like I'm and doing something the hard way. Multiple Image in Canvas ItemsControl Should I not answer my own questions?