So on several websites, including phys.org and physics.stackexchange, I have read that photons do not experience time. And that if you travel at the speed of light, time and distance shrink to 0.  Is this really the case? Or is this simply one of those analogies gone wrong type thingy?  If this is the case, I have a couple of questions:   If light emitted from a star x lightyears away reaches earth, has it truly not experienced time, or any change?  Does this mean that from its point of view, as soon as it left x, the earth was already there? This seems very counterintuitive, since then for light the future is already set in stone, leading to a deterministic universe? What happens if at the time it leaves x, I put something in its path to earth? Yet before it reaches it, I put it away again. Will that object never have existed? How does it deal with the time it took me to put the obejct there and remove it?  Given 3, is there simply no space and time between begin and endpoint? Does it simply not exist between these points? Since speeds are relative, one could just as easily take a perspective that the photon is fixed in space and everything else is moving relative to it.. So where does from its perspective the earth get its energy from to attain this speed? What is a good book to read on relativity, or about the maths needed to understand this? Because I get a feeling it's always the case that the explanations are faulty when there was no math involved. Can we even take the reference frame of a photon? Because since it appears if we can it would lead to all these problems above, so I'm guessing for some reason we can't? If so, why not?   Feel free to answer one, multiple or all questions. I'm not afraid of math although my understanding is lacking, so explanations in that regard would be much appreciated. :)  Bonus question (really out there and more philosophical what-if than semi-serious pondering): 8. If all reference frames are equally valid (perhaps not the photon), and assuming spacetime is grainy at the planck scale, could this not lead to an apparent continuous spacetime since all dilations and contractions would be equally valid? In a sort of integer to rational numbers type difference?
To an an external observer it appears that time has stopped for photon. But this relation is reflexive, so for an observer travelling with the photon it appears the universe has stopped everywhere.  Is this right?  Space also gets distorted parallel to the direction of motion, but not perpendicular to it.   Does this mean that for an observer travelling with a photon sees spacetime as a flat plane?  Note 1: I'm using language vividly not literally when I say a photon experiences space and time. Not that I'm against idealist or panpyschist interpretations of matter or energy come to that.  Note 2: Since it's been pointed out that an inertial frame can't be boosted into the frame of a photon and moreover there is no limiting behaviour here - no matter what frame you are boosted into, light will always appear to travel at c. Nevertheless, when an external observer watches a train chasing a photon by gradually increasing its speed, he will note that the train speed is approaching the speed of the photon (but will never match it) and when he looks at the clock inside the train he will not too that it is gradually slowing down (but never actually stops). Using this picture, we can attempt the thought experiment - if not one that can be carried out - of what a world looks like to a photon.
I know the definition of Airplane mode is all communications and radio signals are blocked. But is it possible to turn off the "calling/receiving" part of the phone so your phone is not connecting to a cell tower and only allow Wifi? I've looked into a couple of apps in Google Play but they're either not working or out of date with newer versions of Android.   I know there is strong opinions on both sides of the issue but I'm worried about radiation from my phone since i use it a lot in bed and I hold the phone close to my head and face. At night I'm not worried about receiving calls, so I just want to turn off the phone part and  use WiFi. Is there a way? Or am i just crazy and paranoid? Setting up PyCharm to reference ArcPy? How would I setup  to recognize import arcpy?   I'd like to use PyCharm to develop and debug geoprocessing tool. Calculate integral $\int_{-1}^{1} (1-x^2)^n dx $ Performance Advantages to Iterators? What (if any) performance advantages are offered by using iterators. It seems like the 'Right Way' to solve many problems, but does it create faster/more memory-conscious code? I'm thinking specifically in Python, but don't restrict answers to just that. How to show hidden files in Nautilus 3.20.3 Ubuntu 16.10? How to show hidden files in Nautilus 3.20.3 Ubuntu 16.10? I'm trying to get a Schengen Visa from my first port of entry, France. I am trying to book an appointment with the French Consulate in SF, and the URL I am using is the following:   I've been checking this page periodically for the past few weeks, and I always see a dialog that that says "We are currently at full capacity. Please try again later." Their calendar provides no direct option for going to the next month. (Using the in-browser developer tools, I noticed that the button for next month is hidden. Un-hiding it and clicking on the Next Month button displays no available appointments either. I checked all the way until September 2016).  My guess is that there are absolutely no appointments for the next 6 months. Or, more likely, their page isn't functioning properly. I'll be traveling in July. Does anybody have any alternate suggestions to book an appointment? Why can't the indefinite integral $\int\frac{\sin(x)}{x}\mathrm dx$ be found? If gravity is a pseudoforce in general relativity, then why is a graviton necessary? I'm having trouble distinguishing between the concepts of scedasticity and stationarity. As I understand them, heteroscedasticity is differing variabilities in sub-populations and non-stationarity is a changing mean/variance over time.   If this is a correct (albeit simplistic) understanding, is non-stationarity simply a specific case of heteroscedasticity across time? Is there a criterion to differentiate "thriller" from "horror"? The genres, in my opinion, have a very slim boundary separating one from another. I see a lot of movies being categorized as thriller or horror almost interchangeably, which might seem like lack of thought or criteria. However, is there a true criterion used to differentiate between the two film genres? Why every map $f : S^n \to T^n (n>1)$ has topological degree zero? Why every map $f : S^n \to T^n (n&gt;1)$ has topological degree zero?  I don't know anything about covering spaces, and has been told to me that this assertion comes from this theory!  I do appreciate any help. 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! Approximation of measurable function by simple functions Black holes are thought to originate when a star burns out and collapses to a smaller physical size. This compresses it's matter, but would that increase it's gravity? And would it be enough to capture light? If that star's light could have been observed prior to it's collapse, how could it's burning out and collapse enable it to become a black hole and capture light? UK PLAB Visa Refusal due to unexplained bank deposits I had applied for a PLAB (Doctors) Visa for the UK to appear in PLAB test. I got refused on the basis of two recent large deposits in the bank statement I provided (my mothers). In reality, these are my own savings which I had deposited in her account to make it more strong. The only objection I got was that I must provide evidence for the source of these deposits. Other than this, I have previously travelled to the UK and USA as well as other parts of the world and my travel history is fine. Can someone please advise on how to proceed and re-apply? A single word for someone who spends their wealth very foolishly How to implement Bitcount using only Bitwise operators? How can I prove $\sqrt{2} ^{\sqrt{2}}$ is irrational? I`m using the Newton-Raphson method for obtaining MLE for parameters for maximizing my objective function. At each iteration, I want to check that is the Hessian matrix negative definite or not and I see the Hessian matrix is not negative definite at some iterations. So, I want to use the Fisher scoring algorithm ( of derivation) In this situation, the Fisher information matrix is indefinite!! Is it possible? What are the reasons for this? Are the second-order derivatives wrong? Or are values of expectations incorrect? Show if a user is online, idle, or offline on their account page? I want to display the status of the user on the account page. I added the following code in the theme I am using. It works, but users are never displayed offline. Why? What is wrong in the code? user.html.twig &lt;div class=&quot;bs-field-status&quot;&gt;   {% if status == 'Online' %}     &lt;i class=&quot;user-online fa fa-circle fa-lg&quot;&gt;&lt;/i&gt; Online   {% elseif status == 'Absent' %}     &lt;i class=&quot;user-absent fa fa-circle fa-lg&quot;&gt;&lt;/i&gt; Absent   {% else %}     &lt;i class=&quot;user-offline fa fa-circle fa-lg&quot;&gt;&lt;/i&gt; Offline   {% endif %} &lt;/div&gt;  bootstrap_subtheme_front_office.theme &lt;?php /**  * @file  * Bootstrap sub-theme.  *  * Place your custom PHP code in this file.  */  use Drupal\Core\Database\Database;  /**  * Implements hook_entity_presave().  */ function bootstrap_subtheme_front_office_preprocess_user(&amp;$variables) {   // get user object   $user = $variables['elements']['#user'];   //- The user has logged in at least once   if ($user-&gt;getLastLoginTime()) {     if (account_is_logged_in_less_then_thirty_minutes($user-&gt;id())) {       $status = 'Online';     }     else {       $status = 'Absent';     }   }   else {     $status = 'Offline';   }   $variables['status'] = $status; }  /**  * @param $uid  *  * @return bool  */ function account_is_logged_in_less_then_thirty_minutes($uid) {   $connection = Database::getConnection();   $query = $connection-&gt;select('sessions', 'sessions')     -&gt;fields('sessions', ['sid', 'uid', 'timestamp'])     -&gt;condition('sessions.uid', $uid, '=')     //- chef if the user was online in 30 minutes (60 * 30)     -&gt;condition('sessions.timestamp', \Drupal::time()         -&gt;getRequestTime() - (60 * 30), '&gt;')     -&gt;execute();   //- Get result.   $results = $query-&gt;fetchAll(\PDO::FETCH_OBJ);   return (count($results) &gt; 0) ? 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