# If light goes the speed of light... **URL:** \ **Category:** Factual Questions\ **Created:** [November 18, 2001, 6:54am UTC](https://boards.straightdope.com/t/if-light-goes-the-speed-of-light/94044 "2001-11-18T06:54:10Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![Speaker\_for\_the\_Dead](https://avatars.discourse-cdn.com/v4/letter/s/c77e96/32.png) [@Speaker\_for\_the\_Dead](https://boards.straightdope.com/u/Speaker_for_the_Dead)\ **Post date:** [November 18, 2001, 6:54am UTC](https://boards.straightdope.com/t/if-light-goes-the-speed-of-light/94044/1 "2001-11-18T06:54:10Z")
I was told vaguely that it would take an infinite amount of energy to go the speed of light, because your mass increases as you near it. How come photons don’t have this problem? ---
**Author:** ![The\_Ryan](https://avatars.discourse-cdn.com/v4/letter/t/7feea3/32.png) [@The\_Ryan](https://boards.straightdope.com/u/The_Ryan)\ **Post date:** [November 18, 2001, 7:04am UTC](https://boards.straightdope.com/t/if-light-goes-the-speed-of-light/94044/2 "2001-11-18T07:04:05Z")
It takes an infinite amount of energy to accelerate to the speed of light. But light doesn’t have to accelerate to the speed of light; it’s always traveling at the speed of light, right? Ha, got you there! 🙂 The other explanation is that it’s infinity times the rest mass. Since the rest mass of light is zero, you have zero times infinity, which is indeterminate, so light can any amount of energy. ---
**Author:** ![Cardinal](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cardinal/32/4000_2.png) [@Cardinal](https://boards.straightdope.com/u/Cardinal)\ **Post date:** [November 18, 2001, 7:07am UTC](https://boards.straightdope.com/t/if-light-goes-the-speed-of-light/94044/3 "2001-11-18T07:07:22Z")
As I recall, (we’re treading on thin ice with that statement), photons can’t go anything less than the speed of light in the current medium, and have no rest mass. (Even more shakily, I’ll say that) Photons are said to have mass because they have momentum, not because they have a mass that was gradually (or even quickly) accelerated to light speed. What we really need here is Chronos. ---
**Author:** ![Napier](https://avatars.discourse-cdn.com/v4/letter/n/ce73a5/32.png) [@Napier](https://boards.straightdope.com/u/Napier)\ **Post date:** [November 18, 2001, 1:44pm UTC](https://boards.straightdope.com/t/if-light-goes-the-speed-of-light/94044/4 "2001-11-18T13:44:45Z")
Photons don’t have any mass and cease to exist when they are going less than c, the “speed of light in a vacuum”. Objects that have mass when they are moving at less than c would take infinite energy to accelerate to c. Or, better to say, as you keep putting more and more energy into accelerating objects with rest mass, they keep going faster, but never as fast as c. This velocity c is a very fundamental part of the fabric of the universe, and might be thought of as a limiting velocity governing all relative motion between objects that have rest mass. For example, velocities don’t add when you compare different frames of reference. When a train is going 30 mph and you are walking 5 mph forward up the aisle of the train, you aren’t quite going 35 mph compared to the ground outside, but slightly less. The “slightly” becomes more important at higher speeds so that you can never get velocities higher than c between objects. And photons always move at c. They don’t move at slower velocities going through other media, such as glass. Photons actually die when they hit glass, and electromagnetic interactions plus actual physical vibrations of the charge carriers (especially the electrons) propagate the disturbance through the glass (at speeds approaching c, or certainly more than half c) to the far side, where photons are recreated to take the energy away. It is pretty practical and useful to think of the photons as travelling through the medium, but they don’t. ---
**Author:** ![Speaker\_for\_the\_Dead](https://avatars.discourse-cdn.com/v4/letter/s/c77e96/32.png) [@Speaker\_for\_the\_Dead](https://boards.straightdope.com/u/Speaker_for_the_Dead)\ **Post date:** [November 18, 2001, 6:20pm UTC](https://boards.straightdope.com/t/if-light-goes-the-speed-of-light/94044/5 "2001-11-18T18:20:03Z")
So a reeaallly thick piece of glass (like… miles and miles) would make it so if you turned on a light on one side, you’d have to wait to see it on the other? ---
**Author:** ![DSeid](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dseid/32/20194_2.png) [@DSeid](https://boards.straightdope.com/u/DSeid)\ **Post date:** [November 18, 2001, 7:07pm UTC](https://boards.straightdope.com/t/if-light-goes-the-speed-of-light/94044/6 "2001-11-18T19:07:30Z")
It starts to get very circular. Time is _defined_ by the speed of light, to some extent. As you approach the edge of a singularity time slows down relative to the rest of the universe, as light itself is slowed down relative to the rest of the universe, but it is still moving at c in its own frame of reference, isn’t it? My brain hurts. (It is the same problem I get with descriptions of gravity. Gravity _is_ the metric itself, that is distortions of the metric by mass. But gravity is also potentially carried by gravitron particles and shares the same wavicle characteristics of other forces, except that it spreads through all branes.) Ow. ---
**Author:** ![Podkayne](https://avatars.discourse-cdn.com/v4/letter/p/edb3f5/32.png) [@Podkayne](https://boards.straightdope.com/u/Podkayne)\ **Post date:** [November 18, 2001, 7:44pm UTC](https://boards.straightdope.com/t/if-light-goes-the-speed-of-light/94044/7 "2001-11-18T19:44:02Z")
> [@](#): > > \*Originally posted by Speaker for the Dead \* > \*\*So a reeaallly thick piece of glass (like… miles and miles) would make it so if you turned on a light on one side, you’d have to wait to see it on the other? \*\* There’s a science fiction story about “slow glass”. They take a plate of glass with a _very_ high index of refraction and place it in a pleasant locaton. Say it takes twenty years for glass to travel from through the glass. So they leave it there for twenty years, then send it to someone who installs it in their house. For the next twenty years, the light that leaves the glass is the light from the pleasant location, showing exactly what happened there all that time. Cool! Scifi Channel has it online: [“Light of Other Days” by Bob Shaw](http://www.scifi.com/scifiction/classics/classics_archive/shaw/shaw1.html). ---
**Author:** ![Podkayne](https://avatars.discourse-cdn.com/v4/letter/p/edb3f5/32.png) [@Podkayne](https://boards.straightdope.com/u/Podkayne)\ **Post date:** [November 18, 2001, 7:50pm UTC](https://boards.straightdope.com/t/if-light-goes-the-speed-of-light/94044/8 "2001-11-18T19:50:42Z")
Ack. ". . . twenty years for _light_ to travel through the glass. . . " of course. ---
**Author:** ![Derleth](https://avatars.discourse-cdn.com/v4/letter/d/b9e5f3/32.png) [@Derleth](https://boards.straightdope.com/u/Derleth)\ **Post date:** [November 18, 2001, 8:20pm UTC](https://boards.straightdope.com/t/if-light-goes-the-speed-of-light/94044/9 "2001-11-18T20:20:52Z")
> [@](#): > > \*Originally posted by Speaker for the Dead \* > \*\*So a reeaallly thick piece of glass (like… miles and miles) would make it so if you turned on a light on one side, you’d have to wait to see it on the other? \*\* Yep. And if you measured the speed of the light coming off of it, you would find it is a fraction of _c_. ---
**Author:** ![Ressl](https://avatars.discourse-cdn.com/v4/letter/r/bbe5ce/32.png) [@Ressl](https://boards.straightdope.com/u/Ressl)\ **Post date:** [November 18, 2001, 8:23pm UTC](https://boards.straightdope.com/t/if-light-goes-the-speed-of-light/94044/10 "2001-11-18T20:23:12Z")
I recall an anology of 2 sticks with marked ends and their reference to space and events in time. When one stick was moving in reference to the other, it was shorter than the other stick. But you guys are saying that things reach an infinite mass as they get faster or approach the speed of light? Aren’t time and space supposed to compress as they approach the speed of light? It seems somewhat confusing. ---
**Author:** ![Ressl](https://avatars.discourse-cdn.com/v4/letter/r/bbe5ce/32.png) [@Ressl](https://boards.straightdope.com/u/Ressl)\ **Post date:** [November 18, 2001, 9:21pm UTC](https://boards.straightdope.com/t/if-light-goes-the-speed-of-light/94044/11 "2001-11-18T21:21:22Z")
I’m sorry, but I have to add this seperately after reading the calculus post. We can’t go the speed of light, but we can go 0.9999…c. But doesn’t 0.99999… = 1 ? So if we were going 0.9999… the speed of light wouldn’t we really be going the speed of light? ---
**Author:** ![Ring](https://avatars.discourse-cdn.com/v4/letter/r/6a8cbe/32.png) [@Ring](https://boards.straightdope.com/u/Ring)\ **Post date:** [November 18, 2001, 9:53pm UTC](https://boards.straightdope.com/t/if-light-goes-the-speed-of-light/94044/12 "2001-11-18T21:53:54Z")
> [@](#): > > Photons actually die when they hit glass, and electromagnetic interactions plusactual physical vibrations of the charge carriers (especially the electrons) propagate the disturbance through the glass Visible light’s frequency doesn’t correspond to any of the allowed energy transitions of the atoms or molecules in the glass, so it cannot be absorbed or scattered. Nonetheless because of the Heisensberg Uncertainty Principle a photon can interact with the medium for a very short time, this is called a virtual interaction and although the interaction is very short it is still long enough to account for refraction. > [@](#): > > Yep. And if you measured the speed of the light coming off of it, you would find it is a fraction of c. Nope. As Napier stated earlier in the thread photons always travel at c. In fact they’re traveling at c while still inside the glass. > [@](#): > > I was told vaguely that it would take an infinite amount of energy to go the speed of light, because your mass increases as you near it. How come photons don’t have this problem? Relativistic mass isn’t a good concept for a number of reasons. First, it’s confusing. Mass is an invariant and therefore the term should only be used to refer to rest mass. Second, you wind up with an object that has a different mass when measured longitudinally versus transversely. In other words you turn a scalar quantity into some kind of weird pseudo vector quantity. Third, if you were standing on the object you would always weigh the same since the object would be at rest WRT you, so obviously it’s mass hasn’t actually changed. Fourth, if velocity could actually change an objects mass then I could collapse stellar objects into black holes just by moving past them at a speed close to c. And on and on. ---
**Author:** ![barking\_frog](https://avatars.discourse-cdn.com/v4/letter/b/96bed5/32.png) [@barking\_frog](https://boards.straightdope.com/u/barking_frog)\ **Post date:** [November 18, 2001, 10:41pm UTC](https://boards.straightdope.com/t/if-light-goes-the-speed-of-light/94044/13 "2001-11-18T22:41:40Z")
> [@](#): > > \*Originally posted by Ressl \* > \*\*I’m sorry, but I have to add this seperately after reading the calculus post. > We can’t go the speed of light, but we can go 0.9999…c. > But doesn’t 0.99999… = 1 ? > So if we were going 0.9999… the speed of light wouldn’t we really be going the speed of light? \*\* Yes .999…c=c, but we can’t accelerate an object to .999…c ©, because as we accelerate it to .9c, then to .99c and to .999c its mass increases; as said object approaches .999…c © its mass approaches infinity, and therefore we can’t accelerate an object to .999…c nor to c. What I wonder is this: what happens when a photon hits a mirror (or anything else that reflects light for that matter)? ---
**Author:** ![erislover](https://avatars.discourse-cdn.com/v4/letter/e/71e660/32.png) [@erislover](https://boards.straightdope.com/u/erislover)\ **Post date:** [November 18, 2001, 10:49pm UTC](https://boards.straightdope.com/t/if-light-goes-the-speed-of-light/94044/14 "2001-11-18T22:49:19Z")
Not much, **Ian** , if you’re thinking what I think you’re thinking. Speed is a scalar, not a vector, so it is not affected by direction. ---
**Author:** ![Speaker\_for\_the\_Dead](https://avatars.discourse-cdn.com/v4/letter/s/c77e96/32.png) [@Speaker\_for\_the\_Dead](https://boards.straightdope.com/u/Speaker_for_the_Dead)\ **Post date:** [November 18, 2001, 10:50pm UTC](https://boards.straightdope.com/t/if-light-goes-the-speed-of-light/94044/15 "2001-11-18T22:50:35Z")
Velocity is, though. Right? _thinks he’s wrong… already been made fun of twice on here today_ ---
**Author:** ![erislover](https://avatars.discourse-cdn.com/v4/letter/e/71e660/32.png) [@erislover](https://boards.straightdope.com/u/erislover)\ **Post date:** [November 18, 2001, 11:00pm UTC](https://boards.straightdope.com/t/if-light-goes-the-speed-of-light/94044/16 "2001-11-18T23:00:18Z")
Yeah, velocity includes direction. ---
**Author:** ![barking\_frog](https://avatars.discourse-cdn.com/v4/letter/b/96bed5/32.png) [@barking\_frog](https://boards.straightdope.com/u/barking_frog)\ **Post date:** [November 18, 2001, 11:09pm UTC](https://boards.straightdope.com/t/if-light-goes-the-speed-of-light/94044/17 "2001-11-18T23:09:39Z")
What I’m wondering is: does the photon physically bounce off the mirror? If so, isn’t its speed at some point zero? ---
**Author:** ![g8rguy](https://avatars.discourse-cdn.com/v4/letter/g/7cd45c/32.png) [@g8rguy](https://boards.straightdope.com/u/g8rguy)\ **Post date:** [November 19, 2001, 12:04am UTC](https://boards.straightdope.com/t/if-light-goes-the-speed-of-light/94044/18 "2001-11-19T00:04:26Z")
Nope, the speed is never zero. When a photon bounces off of something, one of two things has occurred. Either: a) it got absorbed by the material and a while later another photon got emmitted, or b) it got scattered, which is what we normally think of when talking about things like this. But all this really means is that the path got bent around; it’s not an instantaneous turn but just a bend (although it can be, depending on the scattering angle, a fairly sharp one). ---
**Author:** ![barking\_frog](https://avatars.discourse-cdn.com/v4/letter/b/96bed5/32.png) [@barking\_frog](https://boards.straightdope.com/u/barking_frog)\ **Post date:** [November 19, 2001, 7:01am UTC](https://boards.straightdope.com/t/if-light-goes-the-speed-of-light/94044/19 "2001-11-19T07:01:02Z")
If light hits something at an angle, it has to bounce off at the same angle, right? So how does the remitted photon know in which direction to travel then, if the original one just gets absorbed? Also, why would a photon bend in the first place? What is it about a mirror that would bend the path of a photon? ---
**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\ **Post date:** [November 19, 2001, 2:55pm UTC](https://boards.straightdope.com/t/if-light-goes-the-speed-of-light/94044/20 "2001-11-19T14:55:48Z")
> [@](#): > > \*Originally posted by Ian Fan \* > **If light hits something at an angle, it has to bounce off at the same angle, right? So how does the remitted photon know in which direction to travel then, if the original one just gets absorbed?** Funny you should ask this as I asked a [very similar question](http://boards.straightdope.com/sdmb/showthread.php?threadid=94971) about a month ago. Here’s my question and the answer from **Ring**. NOTE: The answer may be hazardous to your mental wellbeing. Read at your own risk! > [@](#): > > _Whack-a-Mole wrote:_ > Why does an atom re-emit an absorbed photon opposite of the side the photon hit it from such that light passes straight through? > > > [@](#): > > > > _Ring responded:_ > > Well, Fermat’s principle of least time (least action) says that when light passes from one medium to another it will always take the path of least time. > > > > Electromagnetics says that incident light wave will polarize and magnetize the molecules of the medium and, amazingly, the resulting oscillating dipoles will create their own fields which combine with the original fields to create a single wave with the same frequency but different speed. (index of refraction) > > > > Quantum Electrodynamics says that the complex phases of the photons cancel out except for the paths of least time. (Feynman’s Sum of Histories path integrals.) This is probably closest to an explanation to what you asked, but in reality no one really knows what a photon or an electron is…. much less what is truly happening. > > > > This stuff really sucks. [Next page](https://boards.straightdope.com/t/if-light-goes-the-speed-of-light/94044.md?page=2)