Do we need escape velocity? - Page 1
Forum section: electronics / general

author: vlad777
author_rank: Frequent Contributor
timestamp: September 19, 2015, 05:00:41 pm
content: Do we need escape velocity to get back from crossing an event horizon?Wouldn't a thruster that is outputting more force than weight do?We would not be in zero G but could get high.(Like the space jump from that Austrian guy, Felix Baumgartner.He was not in zero G but he was in space, just because balloon was light enough.)Many thanks.

author: Balaur
author_rank: Unknown
timestamp: September 19, 2015, 05:12:20 pm
content: Quote from: vlad777 on September 19, 2015, 05:00:41 pmThen you have acceleration (or propulsion). If in the right direction, you will move away from Earth.The escape velocity is computed for objects without propulsion, as a function of the object's kinetic energy when modulo equal (opposite) to the gravitational potential energy.

author: John Coloccia
author_rank: Super Contributor
timestamp: September 19, 2015, 05:22:22 pm
content: Quote from: vlad777 on September 19, 2015, 05:00:41 pmThere's no escaping from the even horizon. That is specifically a term attached to black holes. In fact, due to frame dragging, escape velocity is not even enough near the horizon...or said a more precise way, good luck attaining it.Yes, you can just blast away, but usually when you're talking about spacecraft, you're talking about practical orbits, and just brute force blasting away doesn't get you that. Honestly, I'm not even really sure how important "escape velocity" is as a quantity, other than an interesting thing for your mind to consider. Launching things out of cannons, hoping escape the Earth's clutches, is just not something we ever do. On occasion that we do go off into space, escape velocity is again pretty meaningless since you're specifically looking to solve for an orbit that gets you to some specific location somewhere, not just some academic speed you have to attain just to not fall back down.Not sure if that answers your question but I tried!

author: vlad777
author_rank: Frequent Contributor
timestamp: September 19, 2015, 05:26:31 pm
content: I am thinking about a probe reporting back (from inside of a black hole), so I do not need to go in orbit.

author: continuo
author_rank: Frequent Contributor
timestamp: September 19, 2015, 05:34:37 pm
content: The probe, once fallen behind the EH, would need to accelerate to a speed higher than the speed of light, to escape the Black Holes gravitational pull. And that's not possible, regardless of how much thurst you use. At least that's how I understand it.

author: vlad777
author_rank: Frequent Contributor
timestamp: September 19, 2015, 05:38:12 pm
content: Quote from: continuo on September 19, 2015, 05:34:37 pmEXACTLY! That is what we are all told. But if you do not want to "escape the Black Holes gravitational pull", what if you just want to get high?Of course, I don't understand that little quirk about space falling in faster than speed of light.

author: continuo
author_rank: Frequent Contributor
timestamp: September 19, 2015, 05:46:53 pm
content: Well, you wrote you want the probe to report something back. And that's not possible, you can't get any useful information out of a Black Hole. Maybe someone with more insight will chime in but as far as I know we don't even know if the laws of physics, as we know them, have the slightest meaning after you've crossed the border...

author: vlad777
author_rank: Frequent Contributor
timestamp: September 19, 2015, 06:03:55 pm
content: Leonard Susskind and Stephen Hawking both said thatif you had the right kind of BH (meaning right size and not active accretion)you would happily cross the event horizon and not even know it.So in classical physics terms ,what kind (size) of black hole do we need for the probe weight to be practical?Edit:Probe can report everything it seen coming from the outside, and that would be significant information.

author: Mechanical Menace
author_rank: Super Contributor
timestamp: September 19, 2015, 06:19:58 pm
content: Quote from: vlad777 on September 19, 2015, 06:03:55 pmIf a black hole is massive enough yes you could enter the event horizon and not know, depending on which solution to combining quantum and relativistic physics you're using. You wouldn't notice any tidal forces if the event horizon has a large enough diameter so for all intents and purposes you wouldn't notice you'd gone too far. Until you try to retrace your steps.This of course assumes there are no 'firewalls.'

author: vlad777
author_rank: Frequent Contributor
timestamp: September 19, 2015, 06:25:49 pm
content: Quote from: Mechanical Menace on September 19, 2015, 06:19:58 pmWhat would be your weight?

author: SeanB
author_rank: Super Contributor
timestamp: September 19, 2015, 06:27:44 pm
content: You do not notice your crossing the event horizon, it is just that at that point that to get a signal out your gamma ray signal generator ( about the highest frequency you can generate that still will radiate as a photon) will be red shifted so much that the receiving end will be unable to receive it. In fact a while before the photons you emit back from the probe will be red shifted to close to DC, and will be non receivable at the other non near station in any form. Even launching a small probe back will not work, it will have to leave at close to C. Think of the problem of launching something physical at close to C when doing so will need the entire energy output of a large sun for a decade, to do so for anything over a few hundred thousand atoms, even for a dust mote. Past the event horizon your photons leave you at C going out ( relative to you), but your local time is distorted so much by the local gravity field that they go out but will still be travelling into the singularity at a velocity that from the outside is greater than C.

author: vlad777
author_rank: Frequent Contributor
timestamp: September 19, 2015, 06:34:33 pm
content: "it will have to leave at close to C"This is what I am not understanding/ questioning.C is the escape velocity at EH, but we do not need to escape (just move a little and hover).

author: continuo
author_rank: Frequent Contributor
timestamp: September 19, 2015, 06:41:52 pm
content: Quote from: vlad777 on September 19, 2015, 06:34:33 pmOf course you have to escape if you want to report something back. It doesn't matter if it's the probe itself or some sort of communication signal. It has to escape the EH to be recognised by an outside observer. Or maybe I don't understand your question

author: IanB
author_rank: Super Contributor
timestamp: September 19, 2015, 06:42:44 pm
content: Let's express this thought experiment a different way. Suppose you have a large enough black hole that the tidal forces at the event horizon are small enough not to destroy physical objects. Now suppose you have a probe attached to a long tether and you lower it down below the event horizon. What is to prevent you pulling on the tether and retrieving the probe?

author: Chris C
author_rank: Frequent Contributor
timestamp: September 19, 2015, 06:46:21 pm
content: Quote from: vlad777 on September 19, 2015, 06:03:55 pmI'm not familiar with that statement by Susskind and Hawking, but I'm pretty sure you've misinterpreted it. If "cross" means only passing near the black hole, and if it's small and not actively accreting, then that statement is correct; you may not realize you've passed it.But you cannot actually cross through (in and out) of the event horizon. The event horizon is the distance at which the gravitational pull accelerates things towards the black hole at the speed of light. Producing an opposing equal acceleration to hold position is impractical, and a greater acceleration to escape is impossible.Quote from: vlad777 on September 19, 2015, 06:03:55 pmSubatomic. Anything larger would be shredded by gravitational shear forces before reaching the event horizon.Even if that wasn't the case, a probe in the traditional sense would be useless, due to temporal effects near strong gravity wells. What kind of useful information can you transmit when the probe's 3 gigahertz processor, in its brief descent into expungement, looks to the rest of the universe like it's operating at 1 femtohertz?But quantum entangled subatomic particles may be able to transmit some sort of information out.

author: miguelvp
author_rank: Super Contributor
timestamp: September 19, 2015, 06:46:58 pm
content: Is this in reference to Stephen Hawkin's new black hole theory that he just recently published? (Edit: not sure about publishing but he at least announced it).I have not had a chance to read anything about it, but plenty to find with google.

author: vlad777
author_rank: Frequent Contributor
timestamp: September 19, 2015, 06:52:56 pm
content: Quote from: IanB on September 19, 2015, 06:42:44 pm(You are in a stable orbit above EH and have a probe on a tether, that could be equivalent.)Yes ,tell me please.Edit:Ok the tether breaks.

author: vlad777
author_rank: Frequent Contributor
timestamp: September 19, 2015, 06:53:34 pm
content: Quote from: miguelvp on September 19, 2015, 06:46:58 pmNo.

author: John Coloccia
author_rank: Super Contributor
timestamp: September 19, 2015, 07:03:58 pm
content: Quote from: IanB on September 19, 2015, 06:42:44 pmLet me put it this way. In YOUR time frame, you would never actually see the probe ever reach the event horizon. It would simply gradually fade, getting dimmer and dimmer...forever. To outside observers, time grinds to a halt at the horizon. The early name of black holes was "frozen star", because when it eventually collapsed, it would appear to freeze right at the event horizon.The first step in starting to understand what really happens here is to start understanding that gravity works as much because TIME warps as it does because SPACE warps. The event horizon is that special place where time simply stops from the reference frame of an outside observer. Very strange things happen, but it's also a beautiful result when you consider it hand in hand with some of the equivalencies between thermodynamics and black holes, information theory, etc etc. It seems very fitting that everything should be forever frozen on the surface of the event horizon.

author: vlad777
author_rank: Frequent Contributor
timestamp: September 19, 2015, 07:09:01 pm
content: Quote from: John Coloccia on September 19, 2015, 07:03:58 pmSo even if it could come back, it would be MUCH later, thanks I think I get this part.

author: vlad777
author_rank: Frequent Contributor
timestamp: September 19, 2015, 07:23:25 pm
content: Quote from: IanB on September 19, 2015, 06:42:44 pmTether is a deal breaker here, my probe has a thruster which makes it independent of the outside space-time.Which means it can pop up anywhere anytime.

author: vlad777
author_rank: Frequent Contributor
timestamp: September 19, 2015, 07:32:52 pm
content: OK I am a layman, but I am referring to the idea that has been drilled to our minds.The idea that something needs escape velocity to get out of a black hole, so since this is C, exiting a black hole is impossible.But you do not need orbital or escape velocity to exit a region.You need force. As long as this force is equal or grater than your weight you will ascend.

author: IanB
author_rank: Super Contributor
timestamp: September 19, 2015, 07:55:21 pm
content: Quote from: vlad777 on September 19, 2015, 07:32:52 pmThat's the point of the tether. The tether can apply an upward force to counteract the downward gravitational force.But the point of escape velocity is that it corresponds to the amount of energy required to lift a mass M out of a gravitational well. For instance if you wanted to remove a mass M from the Earth's surface to a distance far from Earth's gravitational field the energy required would be found by integrating force over distance, i.e. integrate the gravitational force on M from zero to infinity. Since gravitational force obeys an inverse square law this amount of energy will be finite.Now the kinetic energy of a mass M is given by ½Mv². If you equate the two energies and solve for v you get the initial speed you would have to give the mass to reach an infinite distance from Earth. This is the escape velocity. If you solve for v and v is greater than the speed of light, then you discover that you cannot escape the gravitational well. It would require infinite energy to do so, and that is physically unfeasible. Your thruster could never carry enough fuel.

author: John Coloccia
author_rank: Super Contributor
timestamp: September 19, 2015, 08:01:56 pm
content: Quote from: vlad777 on September 19, 2015, 07:32:52 pmJust because there are no drastic TIDAL forces at a supermassive black hole's event horizon doesn't mean there is no force. It's still an even horizon. It gets to be one because gravity is so strong that nothing can escape. In free fall, you feel nothing because you're in free fall (i.e. you're in an inertial reference frame). Try to fire some rockets and you will quickly find that there is no rocket powerful enough to resist the pull of gravity. Said another way, all space-time paths in a black hole lead to the singularity...period. There's no trick to get around it. If you're inside the event horizon, you're going to the singularity, do not pass go, do not hesitate, do not wander about. That's where you're heading.Tether does you no good. If you pull hard enough, the tether will eventually just break without making the slightest bit of progress to drag the other thing back out. If the tether doesn't break, it will simply drag in anything else attached to it.Ian's explanation is good because it touches on the idea of a required ENERGY to escape the gravitational well. Thrusters, tethers, escape velocity or not, it's the energy required that dooms you to a steady march into the singularity.

author: vlad777
author_rank: Frequent Contributor
timestamp: September 19, 2015, 08:09:23 pm
content: " Try to fire some rockets and you will quickly find that there is no rocket powerful enough to resist the pull of gravity."Does this mean that their weight is infinite?Edit:Do you mean they will not move at all, or do you mean that they will eventually come back?

author: John Coloccia
author_rank: Super Contributor
timestamp: September 19, 2015, 08:12:11 pm
content: Quote from: vlad777 on September 19, 2015, 08:09:23 pmNo. Weight is a measure of force. F=ma. Their weight will be their mass multiplied by whatever acceleration the rocket can manage. Without the rocket firing, their weight is 0.

author: vlad777
author_rank: Frequent Contributor
timestamp: September 19, 2015, 08:16:22 pm
content: So you mean that they will eventually come back. ?

author: vlad777
author_rank: Frequent Contributor
timestamp: September 19, 2015, 08:20:11 pm
content: All I care about is motion away from singularity. I don't care if the probe crashes eventually.Edit:So near the EH all directions are parallel to EH?Edit2:EH is infinitely far from the singularity? (because of curvature and stretching of space-time)

author: tszaboo
author_rank: Super Contributor
timestamp: September 19, 2015, 09:48:49 pm
content: Quote from: vlad777 on September 19, 2015, 05:26:31 pmThe only way I - the physics layman - know of, anything leaving a black hole is pair production. It happens when some energy is high enough, and suddenly it creates matter, usually a pozitron and an electron, but it can be other particles.Now, if one particle falls into the hole, the other can escape.On the other hand, I'm not sure if I would start building my rocket based on this theory just yet.

author: kolonelkadat
author_rank: Regular Contributor
timestamp: September 20, 2015, 01:04:30 am
content: Quote from: vlad777 on September 19, 2015, 05:26:31 pmhow would you get a signal out of the black hole? I dont know anything about space or physics, but wouldnt it be akin to trying to wring out a towel, eg the "information" would just sort of puddle up around the antenna?

author: Deathwish
author_rank: Unknown
timestamp: September 20, 2015, 01:13:14 am
content: I will ask Ambassador Spock and get back to you on this one

author: miguelvp
author_rank: Super Contributor
timestamp: September 20, 2015, 02:32:19 am
content: Quote from: kolonelkadat on September 20, 2015, 01:04:30 amDidn't read much of Hawkin's new theory (new as in just a month of less ago), but information can come out of black holes in what he calls "super translations", but it's so scrambled that you couldn't make sense of it.The analogy I did read was taking a car through a crusher, then an industrial shredder, a wood chipper and finally through a coffee grinder. You'll end up with all of the particles but the information on how to put it together would be long lost.Edit: of course, someone will come out with the math to get that data back to coherent form, in the future, but not possible right now.

author: vlad777
author_rank: Frequent Contributor
timestamp: September 20, 2015, 09:08:40 am
content: Quote from: Deathwish on September 20, 2015, 01:13:14 amHe won't tell you. (You know? That is the Vulcans policy with Earth)

author: Nerull
author_rank: Frequent Contributor
timestamp: September 20, 2015, 06:31:18 pm
content: http://arxiv.org/pdf/0705.1029v2.pdfHere's a paper examining the use of rockets inside a black hole. The results are pretty interesting - to a point you can slow your fall to the singularity, but after a certain point the more thrust you use to get away from the black hole, the faster you descend to the singularity.Black Holes are weird, and you aren't going to understand them by thinking with Newtonian physics. Even relativity only works up to a point.

author: knks
author_rank: Regular Contributor
timestamp: September 29, 2015, 12:40:14 am
content: Quote from: vlad777 on September 19, 2015, 05:38:12 pmYou can get high, but not higher than EH. The same applies to any photon you radiate.

author: knks
author_rank: Regular Contributor
timestamp: September 29, 2015, 12:45:42 am
content: Quote from: IanB on September 19, 2015, 06:42:44 pmGravitational force. Probe weight will be = infinity.