Relativistic mass of photon
How can a photon have no mass and still travel at the speed of light?
I was reading French's Vibrations &amp; Waves where he discusses Huygens-Frensel Principle.   The principle talks about how secondary sources give rise to secondary wavelets to form the displaced wavefront. However, any secondary source can form two wavelets one moving forward &amp; other towards the original source as pointed by French:     [...] The Huygen's construction would define two subsequent wavefronts , not one. In addition to a new wavefront farther away from the source, there would be another one corresponding to a wavefront back toward the surface, But we know this does not happen.    Then he writes:     If the Huygens way of visualizing wave propagation is to be acceptable, it must introduce the unidirectional property of a traveling wave. This can be achieved by requiring that the disturbance  starting out from a given point in the medium at a given instant is not equal in all directions. Specifically, if $O$ is the true original source ,&amp; $S$ is the origin of Huygens wavelet, &amp; $P$ is the point at which the disturbance is being recorded, then the effect at $P$ due to the region near $S$is a function of $f(\theta)$ of the angle $\theta$ between $OS$ &amp; $SP$.     I am not understanding how his reasoning actually averts the possiblity of formation of back-waves. Can anyone help me visualise what he is talking? How does his reasoning maintain the unidirectional propagation of waves? Can anyone explain his argument? Is it possible to use text-overflow:ellipsis on multiline text? Java: Prefix/postfix of increment/decrement operators? How do I use the Chinese remainder theorem to find all the square roots of 11 in $\mathbb Z_{35}$? Eclipse error: "The import XXX cannot be resolved" Dimensional analysis, and the notion that quantities with different units cannot be equal, is often used to justify very specific arguments, for example, you might use it to argue that a particular formula cannot possibly be the correct expression for a particular quantity. The usual approach to teaching this is to go "well kids, you can't add apples and oranges!" and then assume that the student will just find it obvious that you can't add meters and seconds.  I'm sorry, but... I don't. I'm not convinced. $5$ meters plus $10$ seconds is $15$! Screw your rules! What are the units? I don't know, I actually don't understand what that question means.  I'm specifically not convinced when this sort of thing is used to prove that certain formulae can't possibly be right. Maybe the speed of a comet is given by its period multiplied by its mass. Why not? It's a perfectly meaningful operation - just measure the quantities, multiply them, and I claim that the number you get will always equal the current speed of the comet. I don't see how "but it doesn't make sense to say mass times time is equal to distance divided by time" can be a valid counterargument, particularly because I don't really know what "mass times time" is, but that's a different issue.  If it's relevant, I'm a math student and know extremely little about physics. Taking a spin off of this question:  in a new direction for 2014.  Taking into account a basic Windows infrastructure:   domain controllers Exchange 2007/2010/2013 Sharepoint SQL File Servers / Print Servers AD Integrated DNS AD authenticated 3rd party devices (let's say 802.1X for networking and maybe some content-filtering, etc.) AD/LDAP authenticated "administrative" functions on IT apps/hardware/etc. perhaps some KMS stuff throw in a CA if you'd like home grown apps 3rd party in-house apps   Now, let's rip it all out and decide we are going to the cloud.  We've contracted to move Exchange/Sharepoint/File Services to Office 365.  SQL will now be hosted as well on  something like Azure.  We've gotten away from the need for AD-DNS and simply run everything via a simple Windows DNS server.  We still need 802.1X and would like SSO if possible to our various cloud apps.  Home grown and 3rd party in-house apps would likely stay, but have the ability to use internal user databases instead of AD authentication  The question is...do we really need Active Directory at all?  Or more to the point, AD on-premise or even hosted via Azure or similar (ADFS) or running ADDS on a hosted VM through Azure or similar.  Could/Should we look to something else like a 3rd party SSO option such as  or similar that can provide SSO functionality even if it is as simple as LastPass or similar for each user?  What kind of legitimate needs does AD fulfill if everything else in the cloud?  Could a MS-centric infrastructure get away with not having AD at all if they move everything that previously relied on AD to SaaS offerings that didn't rely on AD authentication? Is Ubuntu based on Debian unstable or Debian testing? which of the two it is? Firefox - Websites with custom fonts not displaying What is the maximum party size and the corresponding monster buffs? What is the maximum party size (if there is one) in Diablo 3?  Additionally, how much of a buff do monsters get when adding party members?     "A new ally has joined, but the minions of Hell grow stronger." how to show image in table cell through json parsing Java Round up Any Number My Sophix tablet was working without the SD card, but now the camera will not work. I get the following error message:     Insert a SD card before using. For a project I'm working on, I need to represent network flows over 1,000 zones with directionality.    Any suggestions on displaying the information in a nice and non overwhelming way?  Any examples on displaying network flows in heavy urban areas? Color suggestions and thickness schemes? any "not to dos" when representing network flows? Does voltage drop occur with zero load and zero resistance Exploring ocean depths: is pressure an issue? Should it be? An upcoming adventure will have the party exploring the depths of the ocean. Breathing underwater shouldn't be an issue for them, as there are both items and rituals to take care of that, however, I was wondering what to do about about pressure, since none of these options mention it. I'll have to split my question into two, since they are closely related.   Is pressure an issue in 4e adventures? Is it mentioned in any of the published material? Has it been ruled anywhere that options that enable water breathing also grant immunity to pressure? Should it be an issue? This is somewhat subjective, but I really am wondering what other people have done. Would it be unresonable to assume that these options grant immunity to pressure? Should the players work to find another solution? Mathematical symbol for "and" I have found some pretty complete lists (I think) of mathematical symbols  and , but I don't see a symbol for the word "and" on either list.  A person could easily just write the word "and" or use an ampersand, but I was wondering if there was an actual mathematical symbol for the word "and".  Also, if anyone knows any lists that are more complete than the ones I have linked to please provide a link. Does iOS support a built-in firewall like macOS does? I don't see anything about it in Settings. How to get the size of a JavaScript object? LibreOffice Calc - Disable rounding There are a  on this topic, but no simple and definitive answers.    Can someone please tell me how to permanently disable cell rounding on the .csv file which is created by script?  Basically, '377635637222436751' is being converted to '377635637222437000' automatically and that isn't really acceptable for my needs.  I've tried this: Tools->Options->Calc->Calculate "Precision as shown"   and this: Format > Cells > Numbers > Category > (scroll down for) Text. (but considering my script creates the .csv -- this option isn't very good since it must be done after the file is created)  And neither is working.  The number stays rounded -- even after formatting the column after the file is created.  Does anyone have any suggestions?