# Speed of light is hard... **URL:** \ **Category:** Factual Questions\ **Created:** [November 23, 2011, 4:33pm UTC](https://boards.straightdope.com/t/speed-of-light-is-hard/603880 "2011-11-23T16:33:50Z")\ **Posts on this page:** 20\ **Page:** 2
**Author:** ![Leaffan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/leaffan/32/299_2.png) [@Leaffan](https://boards.straightdope.com/u/Leaffan)\ **Post date:** [November 23, 2011, 7:03pm UTC](https://boards.straightdope.com/t/speed-of-light-is-hard/603880/21 "2011-11-23T19:03:38Z")
> [@John\_Mace](#): > > Everything is correct except the last part-- you can’t travel at c unless you are light. Are you light? 🙂 No, in fact I’ve put on a few pounds. ---
**Author:** ![leahcim](https://avatars.discourse-cdn.com/v4/letter/l/b4bc9f/32.png) [@leahcim](https://boards.straightdope.com/u/leahcim)\ **Post date:** [November 23, 2011, 7:05pm UTC](https://boards.straightdope.com/t/speed-of-light-is-hard/603880/22 "2011-11-23T19:05:26Z")
> [@Leaffan](#): > > So, I’m parked and light from my headlights is coming out of the bulbs at C. > > Now I’m traveling at 100 KPH and light is still coming out of my headlights at C, because it can’t travel at C + 100 KPH > . A stationary observer sees the light emitted by your headlights travelling at c. Your 100kph observer would see the light moving at 100kph less than c, except that time is moving slower for it in exactly the right way so that the speed of the light with respect to the moving observer’s time is also exactly c. ---
**Author:** ![John\_Mace](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/john_mace/32/185_2.png) [@John\_Mace](https://boards.straightdope.com/u/John_Mace)\ **Post date:** [November 23, 2011, 7:09pm UTC](https://boards.straightdope.com/t/speed-of-light-is-hard/603880/23 "2011-11-23T19:09:40Z")
> [@leahcim](#): > > A stationary observer sees the light emitted by your headlights travelling at c. Your 100kph observer would see the light moving at 100kph less than c, except that time is moving slower for it in exactly the right way so that the speed of the light with respect to the moving observer’s time is also exactly c. Not really. You sort of imply that the “stationary” reference frame is the correct one. Time only slows down in the moving reference frame when observed from the stationary one. Either reference frame could be considered “stationary”, with the other one moving. ---
**Author:** ![Punoqllads](https://avatars.discourse-cdn.com/v4/letter/p/d2c977/32.png) [@Punoqllads](https://boards.straightdope.com/u/Punoqllads)\ **Post date:** [November 23, 2011, 7:11pm UTC](https://boards.straightdope.com/t/speed-of-light-is-hard/603880/24 "2011-11-23T19:11:55Z")
> [@Leaffan](#): > > So, I’m parked and light from my headlights is coming out of the bulbs at C. > > Now I’m traveling at 100 KPH and light is still coming out of my headlights at C, because it can’t travel at C + 100 KPH. > > Now I’m travelling at 0.5 C and light is still coming out at C. > > Now I’m travelling at C, therefore no light can be emitted. > > But I know that’s not correct. > > Please correct what actually happens. One of the things about relativity is that “traveling” is not an inherent property of something, only traveling _relative_ to a given observer. Hence, _relativity_. So one observer sees you moving at velocity 0.5 c and you, by symmetry observe that observer moving at velocity of -0.5 c – that is to say, 0.5 c in the opposite direction. Your headlight then emits a photon in the direction you’re traveling. After 1 second has passed, you see that it has traveled exactly one light-second (approximately 300,000,000 meters). But time and distance are relative to an observer. You observe one second pass, but when you mark that point in time t and the distance the photon traveled x, the observer has seen t’ seconds pass, where t’ = gamma \* (t - v \* x / c[sup]2[/sup]) where v is your velocity = 0.5 c, and they’ve seen the photon move distance x’, where x’ = gamma \* (x - v \* t). In both formulas there is a factor called gamma = 1 / sqrt(1 - v[sup]2[/sup] / c[sup]2[/sup]). These equations are called [Lorentz transformations](http://en.wikipedia.org/wiki/Lorentz_transformation) and gamma is called the [Lorentz factor](http://en.wikipedia.org/wiki/Lorentz_factor). ---
**Author:** ![Inigo\_Montoya](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/inigo_montoya/32/124_2.png) [@Inigo\_Montoya](https://boards.straightdope.com/u/Inigo_Montoya)\ **Post date:** [November 23, 2011, 7:12pm UTC](https://boards.straightdope.com/t/speed-of-light-is-hard/603880/25 "2011-11-23T19:12:50Z")
As far as the time thing changing from an observer’s perspective, I think this was fairly well-explained in Young Einstein. He muses about what a clock face would do (from your perspective) if you were traveling away from it. Light bounces off the clock face and into your eye at the speed of light. But when you’re moving away from it, it takes longer for those photons to reach your eye the closer your speed approaches that of light. When you reach the speed of light, the clock will appear to have stopped. From this I have to ask two things: 1. wouldn’t the clock face also seem to _dim_? After all, there is only one set of photons bouncing off of it at any given moment, once those hit your eye, there are no more if you’re traveling away from them at the same speed they’re moving toward you. 2. If you approach something at the speed of light, would time (at the target) appear to speed up from the perspective of the traveler? ---
**Author:** ![Musicat](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/musicat/32/20189_2.png) [@Musicat](https://boards.straightdope.com/u/Musicat)\ **Post date:** [November 23, 2011, 7:22pm UTC](https://boards.straightdope.com/t/speed-of-light-is-hard/603880/26 "2011-11-23T19:22:32Z")
Just my 2 cents worth…while it seems counter-intuitive that the beams of light from passing cars would travel at the same speed when the generating vehicles are converging or diverging at high speeds, _it is a necessary consequence of our observations._ If light is not constant from/to all observers, we have an awful lot of 'splainin to do, Lucy. Perhaps it’s a bit like the [Ptolemaic solar system,](http://en.wikipedia.org/wiki/Geocentric_model) which, using epicycles upon epicycles, was able to (almost) compute planetary positions accurately. The [Copernican system](http://en.wikipedia.org/wiki/Copernican_heliocentrism) removed the need for the complicated compensations by simply adjusting the frame of reference. I see a parallel here to relativity, which _simply_ explained a number of failures and observations, like the [Michelson-Morley Ether](http://en.wikipedia.org/wiki/Michelson%E2%80%93Morley_experiment) experiments. _The speed of light may not be constant,_ but assuming that it is sure answers a lot of questions, explains a lot of otherwise puzzling stuff, and is the best computational system we have found so far. ---
**Author:** ![RealityChuck](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/realitychuck/32/195_2.png) [@RealityChuck](https://boards.straightdope.com/u/RealityChuck)\ **Post date:** [November 23, 2011, 7:41pm UTC](https://boards.straightdope.com/t/speed-of-light-is-hard/603880/27 "2011-11-23T19:41:11Z")
> [@Washoe](#): > > Bolding mine. I thought this was demonstrated by Maxwell’s equations. Wasn’t this established prior to the publication of Einstein’s paper? 😕 I think you mean the Lorentz Transformation Equations, which do predate Einstein. Lorenz was commenting on the Michaelson-Morley Experiment, which showed that the speed of light was a constant. No one understood the results; Lorenz came up with equations that would have explained it, but it was only an intellectual exercise which left scientists saying, “well, yes, but why would they work that way?” Einstein explained why and thus Lorenz’s equations became more than an exercise. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [November 23, 2011, 8:06pm UTC](https://boards.straightdope.com/t/speed-of-light-is-hard/603880/28 "2011-11-23T20:06:41Z")
> [@](#): > > Bolding mine. I thought this was demonstrated by Maxwell’s equations. Wasn’t this established prior to the publication of Einstein’s paper? Yes and no. Maxwell’s equations yield a single unambiguous value for the Speed of Light, but are completely mute on the question of what this speed is relative to. Before Michelson and Morley, the prevailing consensus was that there was some substance called the “ether” which pervaded the Universe, and that the speed of light was relative to the ether. Einstein, however, interpreted it differently: The speed of light is relative to _everything_, all at once, with no ether necessary. Oh, and **RealityChuck** , Lorentz and Lorenz were, confusingly, two completely different turn-of-the-century physicists, who occasionally even worked together. There’s actually something called the Lorentz-Lorenz Effect which is named after both of them. The one you meant to be referring to throughout that post was the one with the T. ---
**Author:** ![Keeve](https://avatars.discourse-cdn.com/v4/letter/k/f07891/32.png) [@Keeve](https://boards.straightdope.com/u/Keeve)\ **Post date:** [November 23, 2011, 9:23pm UTC](https://boards.straightdope.com/t/speed-of-light-is-hard/603880/29 "2011-11-23T21:23:23Z")
> [@Leaffan](#): > > So, I’m parked and light from my headlights is coming out of the bulbs at C. > > Now I’m traveling at 100 KPH and light is still coming out of my headlights at C, because it can’t travel at C + 100 KPH. This is exactly correct. But I’ll fill in some missing pieces: I’m standing outside your car. From my point of view, I see you moving at 100 KPH, and I see the photons of your headlights moving at C. I do some mental math, and I laugh, and I say, “He thinks his car is giving an extra push to those photons, and that they’re going at a speed of C + 100 KPH! But that’s impossible! They’re still going only C relative to me, and they’re going C minus 100 KPH relative to him!” But that’s not accurate. Relative to you, they really are going at C. That’s because time and space have adjusted slightly. Time is slowing down for you - relative to me. Therefore, even though I think the photons should appear to you as moving slightly slower than usual, your own slowed time compensates for that, and the result is that the photons appear totally normal to you. Now, suppose you say, “NO! I know my car is moving at 100 KPH, so what you’re saying doesn’t make sense.” But you’re not allowed to say that, because it constitutes cheating. It is cheating, because when you say “my car is moving at 100 KPH”, you are saying that from MY point of view. You can’t have it both ways. “Relativity” always means “Relative to what?”, and you can’t change the rules in the middle of the game. ---
**Author:** ![The\_Hamster\_King](https://avatars.discourse-cdn.com/v4/letter/t/8edcca/32.png) [@The\_Hamster\_King](https://boards.straightdope.com/u/The_Hamster_King)\ **Post date:** [November 23, 2011, 10:15pm UTC](https://boards.straightdope.com/t/speed-of-light-is-hard/603880/30 "2011-11-23T22:15:04Z")
> [@Inigo\_Montoya](#): > > Light bounces off the clock face and into your eye at the speed of light. But when you’re moving away from it, it takes longer for those photons to reach your eye the closer your speed approaches that of light. When you reach the speed of light, the clock will appear to have stopped. No. Propagation delay is not the mechanism that causes time dilation. Because if it were (as you correctly conclude) clocks should run faster if we’re approaching them. Here’s the easiest way I’ve found to understand it: EVERYTHING moves at the speed of light ALL THE TIME through 4-dimensional space-time. When something appears to be stationary, it’s because all of its motion is in the time dimension, and none of it is in any of the three space directions. When something appears to be moving, it’s because some of its motion is in a spatial dimension. But since it’s always traveling the same speed in 4-D space-time, any increase in motion in space reduces it motion in time – i. e. clocks run slower for moving objects. However (and this is the big point) what constitutes a “time dimension” and what constitutes a “space dimension” varies from observer to observer. From my perspective, I’m stationary and moving through time normally. You’re moving through space, so you’re moving through time more slowly. But from your perspective, you’re stationary and moving through time normally, while I’m the one who’s moving through time more slowly. The direction of the time axis through 4-D space-time varies from observer to observer. It’s not that clocks slow down for moving things. It’s that when two observers are moving relative to each other, they perceive the arrow of time as pointing in different directions. ---
**Author:** ![Askance](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/askance/32/8281_2.png) [@Askance](https://boards.straightdope.com/u/Askance)\ **Post date:** [November 24, 2011, 1:06am UTC](https://boards.straightdope.com/t/speed-of-light-is-hard/603880/31 "2011-11-24T01:06:16Z")
[Relativity in 5 minutes](http://www.youtube.com/watch?v=KYWM2oZgi4E) on Youtube. ---
**Author:** ![Andy\_L](https://avatars.discourse-cdn.com/v4/letter/a/c67d28/32.png) [@Andy\_L](https://boards.straightdope.com/u/Andy_L)\ **Post date:** [November 24, 2011, 1:14am UTC](https://boards.straightdope.com/t/speed-of-light-is-hard/603880/32 "2011-11-24T01:14:58Z")
> [@Chronos](#): > > Yes and no. Maxwell’s equations yield a single unambiguous value for the Speed of Light, but are completely mute on the question of what this speed is relative to. Before Michelson and Morley, the prevailing consensus was that there was some substance called the “ether” which pervaded the Universe, and that the speed of light was relative to the ether. Einstein, however, interpreted it differently: The speed of light is relative to _everything_, all at once, with no ether necessary.. Although, if you attempt to combine Maxwell’s equation with the idea implicit in Galilean relativity that the laws of physics ought to be invariant with constant motion, you arrive at relativity, so in that sense, Maxwell’s equations implied relativity. ---
**Author:** ![Nametag](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/nametag/32/406_2.png) [@Nametag](https://boards.straightdope.com/u/Nametag)\ **Post date:** [November 24, 2011, 7:23am UTC](https://boards.straightdope.com/t/speed-of-light-is-hard/603880/33 "2011-11-24T07:23:01Z")
> [@John\_Mace](#): > > Everything is correct except the last part-- you can’t travel at c unless you are light. Are you light? 🙂 He ain’t light, but he ain’t heavy. He’s my brother ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [November 25, 2011, 8:16pm UTC](https://boards.straightdope.com/t/speed-of-light-is-hard/603880/34 "2011-11-25T20:16:48Z")
> [@](#): > > Although, if you attempt to combine Maxwell’s equation with the idea implicit in Galilean relativity that the laws of physics ought to be invariant with constant motion, you arrive at relativity, so in that sense, Maxwell’s equations implied relativity. Yes, though this is clearly not as obvious as it seems to us, given that it took Einstein to realize it. ---
**Author:** ![Inigo\_Montoya](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/inigo_montoya/32/124_2.png) [@Inigo\_Montoya](https://boards.straightdope.com/u/Inigo_Montoya)\ **Post date:** [November 25, 2011, 8:45pm UTC](https://boards.straightdope.com/t/speed-of-light-is-hard/603880/35 "2011-11-25T20:45:33Z")
> [@The\_Hamster\_King](#): > > EVERYTHING moves at the speed of light ALL THE TIME through 4-dimensional space-time. When something appears to be stationary, it’s because all of its motion is in the time dimension, and none of it is in any of the three space directions. > > When something appears to be moving, it’s because some of its motion is in a spatial dimension. But since it’s always traveling the same speed in 4-D space-time, any increase in motion in space reduces it motion in time – i. e. clocks run slower for moving objects. This hurts. So basically I can’t think about very fast motion without also thinking about time? Am I getting hung up because I think of thing in Newtonian terms where stuff can always go a little faster and weigh a little more with no practical upper limit? Would it be more appropriate to think of the universe as having a resistance to motion akin to water? Cuz when you move through water, you get little resistance at low velocity and can do pretty much what you want, but higher velocities require you to overcome the resistance/inertia of the water that’s in your way. At some point it seems there would be a maximum speed at which you simply can’t overcome it because the density of the water directly in front of you exceeds your own and you start to flatten against it. I’m sure that’s the wrong way to think about high speeds in the near vacuum of space, but how wrong is it? ---
**Author:** ![naita](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/naita/32/5862_2.png) [@naita](https://boards.straightdope.com/u/naita)\ **Post date:** [November 25, 2011, 8:56pm UTC](https://boards.straightdope.com/t/speed-of-light-is-hard/603880/36 "2011-11-25T20:56:48Z")
> [@Inigo\_Montoya](#): > > Would it be more appropriate to think of the universe as having a resistance to motion akin to water? Cuz when you move through water, you get little resistance at low velocity and can do pretty much what you want, but higher velocities require you to overcome the resistance/inertia of the water that’s in your way. At some point it seems there would be a maximum speed at which you simply can’t overcome it because the density of the water directly in front of you exceeds your own and you start to flatten against it. I’m sure that’s the wrong way to think about high speeds in the near vacuum of space, but how wrong is it? Very wrong. You’ve almost stumbled back into Aristotelian physics where continued motion requires continued force, and you’re also close to the concept of an ether. What you need to grasp first is that there is no such thing as high speeds, except in relation to other objects. And no objects are more equal than others. ---
**Author:** ![Alka\_Seltzer](https://avatars.discourse-cdn.com/v4/letter/a/c89c15/32.png) [@Alka\_Seltzer](https://boards.straightdope.com/u/Alka_Seltzer)\ **Post date:** [November 25, 2011, 9:01pm UTC](https://boards.straightdope.com/t/speed-of-light-is-hard/603880/37 "2011-11-25T21:01:30Z")
> [@Chronos](#): > > Oh, and **RealityChuck** , Lorentz and Lorenz were, confusingly, two completely different turn-of-the-century physicists, who occasionally even worked together. So, which one is the anti-particle? ---
**Author:** ![Bryan\_Ekers](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bryan_ekers/32/183_2.png) [@Bryan\_Ekers](https://boards.straightdope.com/u/Bryan_Ekers)\ **Post date:** [November 25, 2011, 9:01pm UTC](https://boards.straightdope.com/t/speed-of-light-is-hard/603880/38 "2011-11-25T21:01:49Z")
> [@John\_Mace](#): > > Are you light? 🙂 Well, he won’t be after Thanksgiving. ---
**Author:** ![OpalCat](https://avatars.discourse-cdn.com/v4/letter/o/8edcca/32.png) [@OpalCat](https://boards.straightdope.com/u/OpalCat)\ **Post date:** [November 25, 2011, 9:11pm UTC](https://boards.straightdope.com/t/speed-of-light-is-hard/603880/39 "2011-11-25T21:11:13Z")
It’s hard for me to understand as well. I have this additional question: if I turn on a light, it is automatically traveling at the speed of light, regardless of how much energy went into the light being created, right? So it wouldn’t take any more or less energy to make light travel at the speed of light if I’m standing still (relative to whatever reference point) vs. traveling at .9999 the speed of light. How is it that it takes the same energy to make something accelerate to the speed of light from 0 as opposed to .9999 c? ---
**Author:** ![Candyman74](https://avatars.discourse-cdn.com/v4/letter/c/ad7895/32.png) [@Candyman74](https://boards.straightdope.com/u/Candyman74)\ **Post date:** [November 25, 2011, 9:15pm UTC](https://boards.straightdope.com/t/speed-of-light-is-hard/603880/40 "2011-11-25T21:15:52Z")
> [@OpalCat](#): > > It’s hard for me to understand as well. I have this additional question: if I turn on a light, it is automatically traveling at the speed of light, regardless of how much energy went into the light being created, right? So it wouldn’t take any more or less energy to make light travel at the speed of light if I’m standing still (relative to whatever reference point) vs. traveling at .9999 the speed of light. How is it that it takes the same energy to make something accelerate to the speed of light from 0 as opposed to .9999 c? I don’t know the answer to this question, but I’ll make a guess: light has zero mass. Anything multiplied by zero is zero. Or something. [Previous page](https://boards.straightdope.com/t/speed-of-light-is-hard/603880.md?page=1) [Next page](https://boards.straightdope.com/t/speed-of-light-is-hard/603880.md?page=3)