# Event Horizon **URL:** \ **Category:** Factual Questions\ **Created:** [March 20, 2003, 4:16am UTC](https://boards.straightdope.com/t/event-horizon/162726 "2003-03-20T04:16:53Z")\ **Posts on this page:** 12\ **Page:** 1
**Author:** ![Enola\_Straight](https://avatars.discourse-cdn.com/v4/letter/e/dec6dc/32.png) [@Enola\_Straight](https://boards.straightdope.com/u/Enola_Straight)\ **Post date:** [March 20, 2003, 4:16am UTC](https://boards.straightdope.com/t/event-horizon/162726/1 "2003-03-20T04:16:53Z")
When an object falls through a black hole, does gravity accelerate the object to lightspeed once past the event horizon? I ask this since the known laws of physics breaks down in the immediate vicinity of a singularity. ---
**Author:** ![Exapno\_Mapcase](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/exapno_mapcase/32/1051_2.png) [@Exapno\_Mapcase](https://boards.straightdope.com/u/Exapno_Mapcase)\ **Post date:** [March 20, 2003, 4:39am UTC](https://boards.straightdope.com/t/event-horizon/162726/2 "2003-03-20T04:39:47Z")
I shouldn’t be posting right before bed, but the singularity is usually thought of as a point source. There can be considerable distance between the event horizon and the singularity. Because this region oputside the singularity is still subject to the laws of physics we know about, then I’d say that massed particles still cannot accelerate to the speed of light. ---
**Author:** ![JasonFin](https://avatars.discourse-cdn.com/v4/letter/j/4af34b/32.png) [@JasonFin](https://boards.straightdope.com/u/JasonFin)\ **Post date:** [March 20, 2003, 5:53am UTC](https://boards.straightdope.com/t/event-horizon/162726/3 "2003-03-20T05:53:35Z")
I don’t think that your question has a straight answer. From the perspective of an external observer, nothing can ever enter the event horizon of a black hole. The reason is that time runs more slowly closer to the horizon. At the horizon itself, time doesn’t run at all: objects approach the horizon at an exponentially decreasing rate and never cross it. On the other hand, an observer who is actually falling into a black hole will pass straight through the horizon with no observable effect (other than the effects normal for being very close to a black hole, that is). I think that, for an observer on the inside of a black hole event horizon, time and space in the external world aren’t really well-defined. I don’t believe one can meaningfully say that they’re traveling faster than light with respect to anything in particular. ---
**Author:** ![Ficer67](https://avatars.discourse-cdn.com/v4/letter/f/c57346/32.png) [@Ficer67](https://boards.straightdope.com/u/Ficer67)\ **Post date:** [March 20, 2003, 11:53am UTC](https://boards.straightdope.com/t/event-horizon/162726/4 "2003-03-20T11:53:01Z")
Tidal forces rip apart any matter that falls toward a black hole. Because of this, eventually, a particle falling towards a black hole is turned into energy, not because it is accelerated, but because all of the atoms in the particle are ripped apart. Nothing ever falls into a black hole. ---
**Author:** ![CalMeacham](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/calmeacham/32/35_2.png) [@CalMeacham](https://boards.straightdope.com/u/CalMeacham)\ **Post date:** [March 20, 2003, 11:58am UTC](https://boards.straightdope.com/t/event-horizon/162726/5 "2003-03-20T11:58:36Z")
> [@](#): > > Tidal forces rip apart any matter that falls toward a black hole. Because of this, eventually, a particle falling towards a black hole is turned into energy, not because it is accelerated, but because all of the atoms in the particle are ripped apart. Nothing ever falls into a black hole. Tidal forces aren’t necessarily grotesquely huge at the event horizon – it depends upon the mass of the black hole. You can, given a big enough hole, pass across the Event Horizon perfectlyintact. What happens when you get closer to the singularity (or how fast you end up going once inside) I do not know. ---
**Author:** ![MC\_Master\_of\_Ceremonies](https://avatars.discourse-cdn.com/v4/letter/m/bb73d2/32.png) [@MC\_Master\_of\_Ceremonies](https://boards.straightdope.com/u/MC_Master_of_Ceremonies)\ **Post date:** [March 20, 2003, 12:23pm UTC](https://boards.straightdope.com/t/event-horizon/162726/6 "2003-03-20T12:23:51Z")
Yes the event horizion in someways is almost an arbitary point on a gradient, it marks the point of no return, but for most massive particles this already been realistically reached just before the event horizion. For a super massive black hole, it’s possible that you could past the event horizion without even realizing it. ---
**Author:** ![MC\_Master\_of\_Ceremonies](https://avatars.discourse-cdn.com/v4/letter/m/bb73d2/32.png) [@MC\_Master\_of\_Ceremonies](https://boards.straightdope.com/u/MC_Master_of_Ceremonies)\ **Post date:** [March 20, 2003, 12:26pm UTC](https://boards.straightdope.com/t/event-horizon/162726/7 "2003-03-20T12:26:10Z")
Also should add the normal laws of physics still apply past the event horizion, it’s only at the point of the singularity that they cease to apply. That is to say that objects past the event horizion are still shiled from the singularity. ---
**Author:** ![eburacum45](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/eburacum45/32/8690_2.png) [@eburacum45](https://boards.straightdope.com/u/eburacum45)\ **Post date:** [March 20, 2003, 3:00pm UTC](https://boards.straightdope.com/t/event-horizon/162726/8 "2003-03-20T15:00:12Z")
> [@](#): > > \*Originally posted by JasonFin \* > \*\*From the perspective of an external observer …time runs more slowly closer to the horizon. At the horizon itself, time doesn’t run at all: objects approach the horizon at an exponentially decreasing rate and never cross it. > > . \*\* This is true, but is is an illusion- all it means is the light emitted or reflected by the infalling object will take longer and longer to reach the observer (and become more redshifted) and the light of the object crossing the event horizon itself _never_ reaches the stationary observer. ---
**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:** [March 20, 2003, 11:56pm UTC](https://boards.straightdope.com/t/event-horizon/162726/9 "2003-03-20T23:56:19Z")
You can’t exactly say that it’s an illusion, since there is no way to possibly measure the time from outside which will not run into this complication. Science makes it a point to avoid questions which cannot (in principle, at least) be answered by observation. But suppose that the observer is falling into the black hole, and also suppose that the hole is sufficiently large that tidal effects at the horizon are comfortably small. In any local frame of reference, you will not be exceeding the speed of light. If you take an external reference frame and try to blythely extend that reference frame to the interior of the hole, then in that reference frame, you will exceed the speed of light. However, such a reference frame isn’t particularly relevant to the person falling in, and any answers that frame might give you aren’t really meaningful. ---
**Author:** ![SPOOFE](https://avatars.discourse-cdn.com/v4/letter/s/c5a1d2/32.png) [@SPOOFE](https://boards.straightdope.com/u/SPOOFE)\ **Post date:** [March 21, 2003, 12:12am UTC](https://boards.straightdope.com/t/event-horizon/162726/10 "2003-03-21T00:12:38Z")
> [@](#): > > For a super massive black hole, it’s possible that you could past the event horizion without even realizing it. Can someone expand on this? Chronos told me this once, and since then I’ve been thinking of interesting ways to use this in a science fiction story (like having a starship accidently passing too close to a black hole, and the slower time makes a few seconds for them actually be a few centuries to the rest of the universe). Things I want to know: Gravity is a force of acceleration, isn’t it? Wouldn’t that require that, at an “event horizon” (where light can’t escape) the force of acceleration would need to be universal? ---
**Author:** ![SlowMindThinking](https://avatars.discourse-cdn.com/v4/letter/s/dc4da7/32.png) [@SlowMindThinking](https://boards.straightdope.com/u/SlowMindThinking)\ **Post date:** [March 21, 2003, 12:26am UTC](https://boards.straightdope.com/t/event-horizon/162726/11 "2003-03-21T00:26:08Z")
Chronos is correct. The event horizon is easier to understand when you realize that the apparant singularity at the event horizon _is_ an illusion. The coordinate system we all know and love, the one that looks like spherical coordinates outside of the black hole, has a singularity. The universe does not. Coordinates with singularities are common. Think about latitude and longitude on the Earth’s surface. Longitude is not defined at the two poles, even though there is nothing special about the earth at those points. Similary, the “radial distance coordinate”, r, and the “time coordinate”, t, are not defined at the event horizon, but spacetime is fine. At the event horizon, r and t switch meanings - within the event horizon r becomes timelike and t spacelike - so the coordinate system is undefined. If I remember my physics history correctly, this wasn’t proven until the 60’s, so its not like this was easy to figure out. As far as, > [@](#): > > On the other hand, an observer who is actually falling into a black hole will pass straight through the horizon with no observable effect (other than the effects normal for being very close to a black hole, that is). I think that, for an observer on the inside of a black hole event horizon, time and space in the external world aren’t really well-defined. I don’t believe one can meaningfully say that they’re traveling faster than light with respect to anything in particular. I don’t think so. It seems to me a hapless observer falling in, but looking out, would continue to be able to see photons from all the same sources before and after falling in. As Chronos says, the observer might mistakenly assume something is moving faster than light, and so that he is moving faster than light relative to that thing, but only because of sloppy physics. ---
**Author:** ![SlowMindThinking](https://avatars.discourse-cdn.com/v4/letter/s/dc4da7/32.png) [@SlowMindThinking](https://boards.straightdope.com/u/SlowMindThinking)\ **Post date:** [March 21, 2003, 12:30am UTC](https://boards.straightdope.com/t/event-horizon/162726/12 "2003-03-21T00:30:12Z")
I meant to say that it wasn’t proven that there is no singularity at the event horizon and that only the coordinate system is singular.