Confused about black holes

[Nich]

The event horizon is defined as the sphere where the escape velocity is faster then the speed of light.

So if you started at infinity with 0 velocity and fell into a black hole wouldn't you be going at the speed of light as you hit the event horizon to an outside observer?

But I thought objects with mass could not go the speed of light?

Wouldn't your time relative to the universe accelerate to infinity?

As time of the outside universe accelerates to infinity and black holes loose mass via hawking radiation wouldn't you observe the black hole evaporate in front of you before before you could reach the event horizon?

Also wouldn't your mass increase to infinity making you more massive then the black hole at some point?

[ZL647]

46 minutes ago, Nich said:

So if you started at infinity with 0 velocity and fell into a black hole wouldn't you be going at the speed of light as you hit the event horizon to an outside observer?

Interesting questions.

First off, gravitational forces decline with the square of distance, so as distance approaches infinity, the gravitational attraction between the black hole and craft would be 0 thus you wouldn't fall into it. 

Okay so you aren't actually at infinity, let's change this up and say you slowly fall toward the black hole at a realistic distance where gravity would actually accelerate you. As you approach the black hole, the differential in force between the outside and inside of the object being pull in would become large enough to stretch the object in question. You would cross the event horizon in roughly the shape of a molecular/atomic noodle long before you reached the speed of light.

To an outside observer, I suppose this would appear as a redshift, eventually  bending the light out of the visual spectrum entirely, possibly before they ever even got to the event horizon, I'm not sure on the math.

Edited November 2, 2018 by ZL647

[Vanamonde]

Actual science being discussion, so the thread has been moved to the science subforum. 

[Xd the great]

No. Your time slows down, you get torn to pieces, but gravity has still not accelerated you to light speeds.

 

[K^2]

12 hours ago, Nich said:

So if you started at infinity with 0 velocity and fell into a black hole wouldn't you be going at the speed of light as you hit the event horizon to an outside observer?

Well, for starters, from perspective of outside observer, you never do fall in. You'd be falling faster and faster, and indeed, approaching light speed, but the time it requires for light to escape also increases. From event horizon, light takes exactly forever to escape. Everything that ever has fallen into a black hole appears to still be falling to any outside observer. To anyone who has fallen in, the trip is much, much shorter.

12 hours ago, Nich said:

But I thought objects with mass could not go the speed of light?

Again, no object is at the event horizon from perspective of outside observer. But yes, they can appear to be very, very close to light speed.

12 hours ago, Nich said:

Wouldn't your time relative to the universe accelerate to infinity?

As time of the outside universe accelerates to infinity and black holes loose mass via hawking radiation wouldn't you observe the black hole evaporate in front of you before before you could reach the event horizon?

Again, from perspective of the person falling in, the fall can be quite short. Moreover, person falling in never crosses event horizon. As you get closer, the event horizon appears to deform out of the way, then envelop you forming a bubble. That bubble then shrinks as you get closer to singularity. Then finally, at one moment, the black hole, the bubble, and you become a single point. That is the only singularity you'll actually encounter as the person falling in.

For a supermassive black hole, surprising length of that harrowing experience is survivable. You can be well bellow the event horizon and still not having encountered anything that would kill you outright. You will get to conditions no living being can survive well before you actually encounter singularity, though.

12 hours ago, Nich said:

Also wouldn't your mass increase to infinity making you more massive then the black hole at some point?

Back to the outside observer perspective, your rest mass appears to decrease, as your potential energy is converted into kinetic. The total mass actually stays constant throughout the fall, since no energy is added from any outside source.

In general, from perspective of outside observer, matter approaching event horizon starts becoming indistinguishable from light. It all travels at light speed and has no rest mass.

[Nich]

On 11/3/2018 at 4:24 AM, K^2 said:

Back to the outside observer perspective, your rest mass appears to decrease, as your potential energy is converted into kinetic. The total mass actually stays constant throughout the fall, since no energy is added from any outside source.

Most of what you said made sense except I thought the reason you cant go faster then the speed of light is because your mass increases to infinity as you approach the speed of light.

I was also thinking about traveling near the speed of light.  If I am traveling at fast enough that time dilation is 100/1 I would be able to make a 100 light year trip in ~1 year so from my perspective I was traveling at ~100c.  Yes I know the reality is it appears the distance shrinks 100/1.  So from an accelerating perspective this gets even weirder assume I accelerate at 1 g for ~32 years.  as I approach the correct speed my target galaxy would appear to go from 100 light years away to 1 light year away appearing as if I was going 99 light years in a matter of days or ~17,000c.  Then I would coast a year and decelerate for 32 years.  For the first couple days the Galaxy would appear to get further as the rate of expansion is greater then my velocity, still when they are equal, and finally getting closer again as expansion is less then my velocity.

[p1t1o]

Time is RELATIVE.

If it ever seems like something ought to be travelling >c, adjust the flow of time in the relevant frames of reference to taste and you will find you fall into line with theory very quickly.

 

 

 

[Lisias]

13 minutes ago, p1t1o said:

Time is RELATIVE.

If it ever seems like something ought to be travelling >c, adjust the flow of time in the relevant frames of reference to taste and you will find you fall into line with theory very quickly.

 

Gravity appears to be a weaknesses on your argument. I remember reading somewhere that the gravitational effects are "felt" faster than the speed of light - i.e. If would be possible to make the Sun to simply "vanish", we would break loose the orbit about 8 minutes before going dark. 

[Terwin]

58 minutes ago, Lisias said:

Gravity appears to be a weaknesses on your argument. I remember reading somewhere that the gravitational effects are "felt" faster than the speed of light - i.e. If would be possible to make the Sun to simply "vanish", we would break loose the orbit about 8 minutes before going dark. 

'Instant Gravity' vs 'Gravity Waves' were two competing theories, but if you notice that there are regular discussions about gravity waves detected from X, Y, and Z, you can guess as to which one is currently considered to be more correct.

[p1t1o]

1 hour ago, Lisias said:

Gravity appears to be a weaknesses on your argument. I remember reading somewhere that the gravitational effects are "felt" faster than the speed of light - i.e. If would be possible to make the Sun to simply "vanish", we would break loose the orbit about 8 minutes before going dark. 

As far as I know, current science has gravity in all its forms travelling at c.

But mixing in gravity with relativity, special relativity and quantum weirdness is definitely where things start to get sticky.

Google "quantum gravity" and see how far down the rabbit hole you can make it

 

 

<bit'o'googlin'>

The speed of gravity has been measured:

"As Jupiter moved between Earth and the quasar, the gravitational bending of Jupiter allowed us to measure the speed of gravity, ruling out an infinite speed and determining that the speed of gravity was between 2.55 × 10^8 and 3.81 × 10^8 meters-per-second, completely consistent with Einstein's predictions."

 

**edit**

This might also have been one of the things that has been firmed-up by observing that black hole merger a little while back:

https://en.wikipedia.org/wiki/Speed_of_gravity

 

 

 

Edited November 5, 2018 by p1t1o

[wumpus]

On 11/2/2018 at 4:32 PM, Nich said:

The event horizon is defined as the sphere where the escape velocity is faster then the speed of light.

So if you started at infinity with 0 velocity and fell into a black hole wouldn't you be going at the speed of light as you hit the event horizon to an outside observer?

But I thought objects with mass could not go the speed of light?

Wouldn't your time relative to the universe accelerate to infinity?

As time of the outside universe accelerates to infinity and black holes loose mass via hawking radiation wouldn't you observe the black hole evaporate in front of you before before you could reach the event horizon?

Also wouldn't your mass increase to infinity making you more massive then the black hole at some point?

First,  relativistic mass isn't considered a good idea anymore.  It made a lot of sense when Einstein wrote special relativity, and I'm sure he included it in his descriptions of it, but I think it was quickly obsoleted after looking closely at general relativity.  The obvious failure is that there exists some speed where anything would become a black hole if "relativistic mass" was a thing.  If it then decelerated below that speed, it would no longer be a black hole.  Also whether or not it was a black hole would depend on the speed of both the observer and the "black hole" in question.  It really doesn't work like that (well that's what I've been told, I have zero tensor calculus)

ma=dp/dt : still true after special relativity.

p=vm(1+v^2/c^2)   : true, but don't assume you can declare mass = mass(at rest)(1+v^2/c^2)

Other notes:

When you cross the event horizon your escape velocity is >=c (this gives you a quick and dirty way to calculate where it is.  It is the "wrong" way to do it, but for simple black holes it should be right), not your actual velocity.

Consider a beam of light crossing the path of a black hole: it will never change velocity, but it will change wavelength and bend towards the black hole (and if close enough will get trapped itself).

Consider a sci-fi spaceship capable of arbitrary acceleration: it could conceivably sit just outside the event horizon at full thrust (a bad ideal.  You'd have to push at "full thrust" for a long time (to beyond well over twice the even horizon's radius) before you could even think of escaping by some other means (presumably orbiting the black hole and spiraling out).

Edited November 6, 2018 by wumpus
K^2 pointed out it was completely backwards

[YNM]

28 minutes ago, wumpus said:

The obvious failure is that there exists some speed where anything would become a black hole if "relativistic mass" was a thing. 

That would actually help explain blackhole mass accretion ? Combined with freeze-frame (due to extreme time dilation) and fade out (due to extreme redshifting).

Edited November 5, 2018 by YNM

[Lisias]

On 11/5/2018 at 4:23 PM, Terwin said:

'Instant Gravity' vs 'Gravity Waves' were two competing theories, but if you notice that there are regular discussions about gravity waves detected from X, Y, and Z, you can guess as to which one is currently considered to be more correct.

The detection of Gravity Waves (GW from now on), IMHO, should not rule out 'Instant Gravity"(IG) - at least, without evidence against IG. Light behaves as a particle and as a wave at the same time.  

Of course, I ignore if such evidence already exists, I hooking my argument from where you left it.  

For the same of brainstorming: imagine a body roaming the Universe into a Star at 0.95c. From one side, a huge mass travels in his direction at 0.9c . On the exactly opposing side (to simplify calculations), an equally huge mass travels, also in his direction, at 0.98.c 

So we have 4 frames of reference: S (star), B (body), H1 (huge body at 0.9c) and H2 (huge body at 0.98c). H1 and H2 vectors are aligned with the vector of the movement from B into S (so H2 would be traveling at 0.95 + 0.98 into S, if that would be possible).

S              H1---0.9c(B)--->            <---0.95c(S)---B             <---0.98c(B)---H2        

Time will bend on the H1-B reference frame, but also will bend at B-H2 reference frame and also on H1-H2 reference frame.

If gravity follows the same restrictions of light, how S will be gravitationally affected by H1, H2 and B ?

Edited November 6, 2018 by Lisias
better phrasing; adding frames of reference on the speeds.

[K^2]

8 hours ago, Nich said:

Most of what you said made sense except I thought the reason you cant go faster then the speed of light is because your mass increases to infinity as you approach the speed of light.

You can think of it that way in Special Relativity. In General Relativity, you can go FTL. In fact, most of the universe is receding from us faster than light. GR is a bit more complicated.

4 hours ago, wumpus said:

First, rest mass isn't considered a good idea anymore.  It made a lot of sense when Einstein wrote special relativity, and I'm sure he included it in his descriptions of it, but I think it was quickly obsoleted after looking closely at general relativity.  The obvious failure is that there exists some speed where anything would become a black hole if "relativistic mass" was a thing.  If it then decelerated below that speed, it would no longer be a black hole.

Your first sentence is backwards. Rest mass is the invariant, so that's still in use. And it's not that relativistic mass doesn't make sense. It really does, and a lot of what we call rest mass is actually dynamic at subatomic level. But most modern equations, when they have 'm' in them, refer to rest mass, as that's a more fundamental quantity. So a modern physicist wouldn't write E = mc². The equation would correctly be written as E² = (pc)² + (mc²)²

[KG3]

21 hours ago, Nich said:

Most of what you said made sense except I thought the reason you cant go faster then the speed of light is because your mass increases to infinity as you approach the speed of light.

 

Try looking at it like this.  You build a ship and you put enough fuel in it to go twice the speed of light (2c).  I'm back on earth watching you go.  You've told me that you are going to burn 100 gallons of fuel a minute for 30 days to get up to 2c.  Everything is going great as far as I can tell until I notice that it's taking you longer to burn up that 100 gallons of fuel.  There appears to have been a change plan on your part.  That same 100 gallons of fuel is taking longer than a minute to burn. It looks like you're slowly taking your foot off of the accelerator and stretching that 30 day burn out to 60 then 90 days.  Not only that it appears that the exhaust gasses leaving your engines are going at a slower and slower rate making them less efficient the faster you go.  Even if you do mange to burn up all that fuel it still won't get you up to the speed of light never mind twice!  The only remedy for this is of course send the next guy with MORE BOOSTERS!!!

  Of course for you on the ship everything goes as planned and you get to Alpha Centauri in only 2 years. 

[wumpus]

13 hours ago, K^2 said:

You can think of it that way in Special Relativity. In General Relativity, you can go FTL. In fact, most of the universe is receding from us faster than light. GR is a bit more complicated.

Your first sentence is backwards. Rest mass is the invariant, so that's still in use. And it's not that relativistic mass doesn't make sense. It really does, and a lot of what we call rest mass is actually dynamic at subatomic level. But most modern equations, when they have 'm' in them, refer to rest mass, as that's a more fundamental quantity. So a modern physicist wouldn't write E = mc². The equation would correctly be written as E² = (pc)² + (mc²)²

Edited rest mass to relativistic mass.

[Terwin]

17 hours ago, Lisias said:

The detection of Gravity Waves (GW from now on), IMHO, should not rule out 'Instant Gravity"(IG) - at least, without evidence against IG. Light behaves as a particle and as a wave at the same time.  

Of course, I ignore if such evidence already exists, I hooking my argument from where you left it.  

Seeing as how Gravity Waves are based on Gravity traveling at or near c(as opposed to instant gravity), I do not see how you could have both gravity traveling at c and with infinite velocity(instant) at the same time.

Also, considering that the light we are using to detect potential sources of gravity waves is traveling at near c, it would be much more difficult to verify their sources if they traveled at a radically different speed.

*All* accelerating frames of reference experience time dilation, but below ~0.1c it is just so tiny that we generally ignore it.

[Lisias]

24 minutes ago, Terwin said:

Also, considering that the light we are using to detect potential sources of gravity waves is traveling at near c, it would be much more difficult to verify their sources if they traveled at a radically different speed.

Again, this is just food for though. I'm pretty sure the contents of the spoiler below applies, but...

Spoiler

There's a fantastic episode from Richard Faynman interview, and on one part, talking about Magnets, he literally calls the interviewer (and me, and almost everybody else!) of… dumbs!

"I can't explain [it] in terms of something else you are more familiar with […] because I don't understand it in terms anything else you are more familiar with."

 

 

Our observations from the Gravity Waves would not being affected (or even distorted) by our observations of the Light traveling with it?

We already have the Quantum Entanglement as an example of information apparently traveling faster than Light. At least, in our 4 Dimension limited view - you are aware that some scientists talk about a 10 Dimension Universe, not? 

Spoiler

Full video: Richard Feynman "Fun to Imagine", 1983
 

 

 

[Green Baron]

Nothing travels in quantum entanglement. If you send two cards to two different persons and tell them one is a king, the other a queen, they will immediately know what the other one has when looking at their own card.

M-theory has 11 dimensions (there were 13 before iirc). But afaik it has become a little out of fashion since a decade or so. And these dimensions may reveal themselves in a given environment ("universe") or not. And it is just mathemagics, not necessarily "real".

Valid until correction :-)

 

Edited November 6, 2018 by Green Baron

[lajoswinkler]

Let's assume:

- nonrotating supermassive black hole (to avoid spaghettification near it)

-  impactor "very far" from black hole

- impactor has a clock visible to observers outside

- starting point of impactor contains a stationary clock

- impactor is stationary relative to the hole, kept from falling by a teeny tiny thruster with a non-zero force

 

Then the thruster stops working and impactor starts to accelerate towards the hole very slowly. In Newtonian mechanics, regardless of the frame of reference,  impactor will move at c when it reaches event horizon, and keep accelerating towards the center.

In special relativity, frame of reference has to be pointed out, and claims have to be made in the form of "A does this and that, relative to B".

 

Now the question. What does the impactor witness? It sees the hole approaching faster and faster, but it can't approach faster than c because that's a constant in all frames of reference. That's what I find problematic. I'm not concerned about the outside observers. We all know they see the clock on the impactor slowing down and its image turning dim and redshifting.

Wouldn't, from the standpoint of this impactor, the journey end at the event horizon, because it would experience zero time from the moment of reaching c to the moment of reaching center where it's perfectly destroyed?

Also, while approaching the event horizon, impactor looking back into universe's clock should see its time accelerate to the point of witnessing the death of universe.

Even more puzzling, it should see the hole in front of him reach its end, right?

[Terwin]

51 minutes ago, Lisias said:

Our observations from the Gravity Waves would not being affected (or even distorted) by our observations of the Light traveling with it?

We already have the Quantum Entanglement as an example of information apparently traveling faster than Light. At least, in our 4 Dimension limited view - you are aware that some scientists talk about a 10 Dimension Universe, not? 

Yes, if propagation of gravity events were instant, we could still observe them just the same as we do now, but you will note I said it would be harder to 'verify their sources', mostly because if we saw an instant gravity effect, we might be centuries away from the light of that effect getting to us, and thus have difficulty predicting what happened since we will not even see things leading up to it for quite some time.  This would also apparently violate causality, as we would witness an effect before witnessing it's cause.(and remember, general relativity insists that every observation point is just as valid as every other, and thus the effect would have happened before the cause as far as that observation point is concerned, a big no-no in physics).

 

Quantum entanglement is great for shared secrets(knowing one result helps you know the other result, much like knowing that if the top of a coin is heads, the bottom must be tails), but cannot actually transmit any data(you cannot control the other result by manipulating your particle). 

[Lisias]

1 hour ago, Terwin said:

Quantum entanglement is great for shared secrets(knowing one result helps you know the other result, much like knowing that if the top of a coin is heads, the bottom must be tails), but cannot actually transmit any data(you cannot control the other result by manipulating your particle). 

That was a new. Last time I checked about this, they were theorizing that changing a particle would change instantaneously the other, no matter the distance...

20 hours ago, K^2 said:

In fact, most of the universe is receding from us faster than light. GR is a bit more complicated.

That was another new to me. 

[wumpus]

6 hours ago, Green Baron said:

Nothing travels in quantum entanglement. If you send two cards to two different persons and tell them one is a king, the other a queen, they will immediately know what the other one has when looking at their own card.

M-theory has 11 dimensions (there were 13 before iirc). But afaik it has become a little out of fashion since a decade or so. And these dimensions may reveal themselves in a given environment ("universe") or not. And it is just mathemagics, not necessarily "real".

Valid until correction :-)

My understanding is that the EPR paradox has been consistently shown "transmit information" no faster than light speed.  While some bit of the wavefront of a "signal" can creep past c, not enough to actually use as information does.

The other bit is that if you look at what is needed for "multiworlds", you have the same difficulty in that the 'tears' where worlds divide move faster than c (this is mostly contemplating it on my own, while others insisted it satisfied both relativity and quantum issues.  Not something I heard by people who've studied both more than an engineer).

[K^2]

23 hours ago, wumpus said:

My understanding is that the EPR paradox has been consistently shown "transmit information" no faster than light speed.  While some bit of the wavefront of a "signal" can creep past c, not enough to actually use as information does.

It's not just matter of "shown". There is a no-communication theorem that proves that no information can be sent via entanglement. It can, however, be used to "augment" a classical channel. So if you have a way to send classical information, you can use entanglement to send quantum information instead. See quantum teleportation for details.