# Gravity and light **URL:** \ **Category:** Factual Questions\ **Created:** [May 10, 2009, 8:34am UTC](https://boards.straightdope.com/t/gravity-and-light/495931 "2009-05-10T08:34:54Z")\ **Posts on this page:** 12\ **Page:** 2
**Author:** ![Critical1](https://avatars.discourse-cdn.com/v4/letter/c/3e96dc/32.png) [@Critical1](https://boards.straightdope.com/u/Critical1)\ **Post date:** [May 11, 2009, 6:36pm UTC](https://boards.straightdope.com/t/gravity-and-light/495931/21 "2009-05-11T18:36:40Z")
the Speed of light question is simple (if bizarre) light moves at c, period, end of story. you however can move at any speed up to 99.99999% of c (if you have a really fast car anyway) but the faster you move the more time dilates (as in slows down relative to other stuff not moving at your speed) so the faster you move the slower time is moving for you thus at 90% of c your time dilation is also 90% (simplified but you dont want to get into that) so when you hit the headlights the light is moving away from you at a mere 10% of c but from your perspective it is 100% of c. that at least is my understanding. ---
**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:** [May 11, 2009, 6:49pm UTC](https://boards.straightdope.com/t/gravity-and-light/495931/22 "2009-05-11T18:49:41Z")
> [@](#): > > you however can move at any speed up to 99.99999% of c (if you have a really fast car anyway) This seems to be implying that you can’t go faster than 99.99999% of c, but really, you can get as close to c as you like. The highest speed ever recorded for a massive object, the [Oh-my-God particle](http://www.fourmilab.ch/documents/OhMyGodParticle/), was 0.9999999999999999999999951 c relative to Earth, and in principle there’s no reason you couldn’t go faster than that, yet. ---
**Author:** ![EdwardLost](https://avatars.discourse-cdn.com/v4/letter/e/bbe5ce/32.png) [@EdwardLost](https://boards.straightdope.com/u/EdwardLost)\ **Post date:** [May 11, 2009, 7:13pm UTC](https://boards.straightdope.com/t/gravity-and-light/495931/23 "2009-05-11T19:13:52Z")
> [@Chronos](#): > > This seems to be implying that you can’t go faster than 99.99999% of c, but really, you can get as close to c as you like. The highest speed ever recorded for a massive object, the [Oh-my-God particle](http://www.fourmilab.ch/documents/OhMyGodParticle/), was 0.9999999999999999999999951 c relative to Earth, and in principle there’s no reason you couldn’t go faster than that, yet. News articles about particle accelerators will say they accelerate particles to “almost the speed of light” - makes me roll my eyes. That’s a meaningless phrase when you’re talking about relativistic velocities; they’re all at “almost the speed of light”. (Another eye-roller is reading that a lab has cooled some atoms to “almost absolute zero”.) ---
**Author:** ![Critical1](https://avatars.discourse-cdn.com/v4/letter/c/3e96dc/32.png) [@Critical1](https://boards.straightdope.com/u/Critical1)\ **Post date:** [May 11, 2009, 7:24pm UTC](https://boards.straightdope.com/t/gravity-and-light/495931/24 "2009-05-11T19:24:37Z")
sorry I wasnt in anyway trying to imply anything fixed and super factual in that post, its just a very simplified explanation of the strange thing that is light. ---
**Author:** ![Frylock](https://avatars.discourse-cdn.com/v4/letter/f/ce7236/32.png) [@Frylock](https://boards.straightdope.com/u/Frylock)\ **Post date:** [May 11, 2009, 7:28pm UTC](https://boards.straightdope.com/t/gravity-and-light/495931/25 "2009-05-11T19:28:24Z")
Hey, I got a question about gravity and light. It’s my understanding that when gravitational force is parallel but exactly reverse of a photon’s direction of movement, the photon isn’t slowed but is instead red-shifted. What about a photon inside a black hole, heading directly from the center of the black hole outwards? It has to go as fast as c, right? So what does it mean to say it can’t escape? And what happens to its frequency? It gets red shifted to oblivion? What would that amount to exactly? ---
**Author:** ![Sapo](https://avatars.discourse-cdn.com/v4/letter/s/ec9cab/32.png) [@Sapo](https://boards.straightdope.com/u/Sapo)\ **Post date:** [May 11, 2009, 8:34pm UTC](https://boards.straightdope.com/t/gravity-and-light/495931/26 "2009-05-11T20:34:48Z")
> [@danielgjerde](#): > > A black hole has gravity of such magnitude, that light can not escape. So it sems that light do get affected by gravity. Now to the question: Is the gravity of the earth big enough to affect light, and if it is; how does it work. Does light bend around the earth, or even worse from a nothern habitat: does the sides of the earth (as viewed from space) attract more sunlight? (yes, that would make me a little bit jealous, since norway could do with a little bit more sun) Although people with much more knowledge of all this have already answered, allow me to offer an analogy. Imagine you are shooting bullets past a magnet. The magnet will act on the bullet and deviate it a little bit. The Earth is a rather weak magnet so although it does pull on bullets, it makes very little practical difference. Most definitely not enough to make a difference for our Norwegian friends in need of more sunlight. ---
**Author:** ![Ring](https://avatars.discourse-cdn.com/v4/letter/r/6a8cbe/32.png) [@Ring](https://boards.straightdope.com/u/Ring)\ **Post date:** [May 11, 2009, 9:00pm UTC](https://boards.straightdope.com/t/gravity-and-light/495931/27 "2009-05-11T21:00:45Z")
> [@](#): > > It’s my understanding that when gravitational force is parallel but exactly reverse of a photon’s direction of movement, the photon isn’t slowed but is instead red-shifted. To an outside observer it is also slowed. Of course locally its speed remains c. > [@](#): > > What about a photon inside a black hole, heading directly from the center of the black hole outwards? It has to go as fast as c, right? So what does it mean to say it can’t escape? And what happens to its frequency? It gets red shifted to oblivion? What would that amount to exactly? If the photon is traveling radially outward it will hover at the horizon which is a trapped surface. If it is not traveling radially outward it will join the singularity. To be infinitely red shifted is to possess no energy and in effect no longer exist. ---
**Author:** ![Ring](https://avatars.discourse-cdn.com/v4/letter/r/6a8cbe/32.png) [@Ring](https://boards.straightdope.com/u/Ring)\ **Post date:** [May 11, 2009, 9:09pm UTC](https://boards.straightdope.com/t/gravity-and-light/495931/28 "2009-05-11T21:09:35Z")
> [@](#): > > So what does it mean to say it can’t escape? There are different ways to approach this. To a faraway observer time comes to a stop at the EH, so obviously nothing changes or moves. To a local observer at the EH both the horizon and a radially moving photon are traveling outward at c. ---
**Author:** ![Frylock](https://avatars.discourse-cdn.com/v4/letter/f/ce7236/32.png) [@Frylock](https://boards.straightdope.com/u/Frylock)\ **Post date:** [May 11, 2009, 11:06pm UTC](https://boards.straightdope.com/t/gravity-and-light/495931/29 "2009-05-11T23:06:20Z")
> [@Ring](#): > > To an outside observer it is also slowed. Of course locally its speed remains c. Wait. I thought light travels at c from all frames of reference. ---
**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:** [May 11, 2009, 11:50pm UTC](https://boards.straightdope.com/t/gravity-and-light/495931/30 "2009-05-11T23:50:20Z")
> [@](#): > > That’s a meaningless phrase when you’re talking about relativistic velocities; they’re all at “almost the speed of light”. Or looked at another way, none of them are at “almost the speed of light”, since they’re all just as far away from the speed of light as we are. > [@](#): > > What about a photon inside a black hole, heading directly from the center of the black hole outwards? It’s not heading directly from the center outwards. Inside a black hole, “r” and “t” switch roles, such that r becomes a time coordinate, while t becomes a space coordinate. Inside a black hole, you could no more aim a flashlight towards the outside than a person in normal space can aim a flashlight towards last Thursday. ---
**Author:** ![Ring](https://avatars.discourse-cdn.com/v4/letter/r/6a8cbe/32.png) [@Ring](https://boards.straightdope.com/u/Ring)\ **Post date:** [May 11, 2009, 11:53pm UTC](https://boards.straightdope.com/t/gravity-and-light/495931/31 "2009-05-11T23:53:14Z")
> [@Frylock](#): > > Wait. I thought light travels at c from all frames of reference. That’s true in special relativity but not GR. Time slows near a gravitating body, so to a non local observer the photon would have to slow due to the fact that its moving through an area of slow time. (Sorry for the awkward sentence but my wife is calling me impatiently for dinner.) ---
**Author:** ![Ring](https://avatars.discourse-cdn.com/v4/letter/r/6a8cbe/32.png) [@Ring](https://boards.straightdope.com/u/Ring)\ **Post date:** [May 12, 2009, 12:18am UTC](https://boards.straightdope.com/t/gravity-and-light/495931/32 "2009-05-12T00:18:18Z")
> [@Chronos](#): > > It’s not heading directly from the center outwards. Inside a black hole, “r” and “t” switch roles, such that r becomes a time coordinate, while t becomes a space coordinate. Inside a black hole, you could no more aim a flashlight towards the outside than a person in normal space can aim a flashlight towards last Thursday. Yes. I should have said "emitted radially outward exactly at the horizon. But of course that wasn’t the question asked. I hang my head in shame. [Previous page](https://boards.straightdope.com/t/gravity-and-light/495931.md?page=1)