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Ok that explains how space becomes timelike. But I never hear anyone address how time becomes spacelike. In what sense does it become spacelike? Can one mov

Ok that explains how space becomes timelike. But I never hear anyone address how time becomes spacelike. In what sense does it become spacelike? Can one move back and forth in time inside the event horizon?

pa7x1 6y ago
The radial direction of the inside of the Schwarzschild Black Hole is timelike. This means that; inside the event horizon, the inevitable passage of time becomes the inevitable move towards the singularity at the center of the Black Hole. To answer your question in clear terms: No, you cannot move freely in the radial direction. Only towards the center, in the same way you cannot move freely forward or backwards in time.
- pbhjpbhj 6y ago
  Considering a point particle, what constrains it from orbiting [which is what I'm assuming we're meaning here as all matter has to move radially to cross the event horizon]? If it had a tangential component to its momentum prior to meeting the event horizon (EH) wouldn't it continue to orbit past the EH?
  - pa7x1 6y ago
    Nothing forbids a slight tangential component but the movement of the test particle is inevitably decreasing in r. Closed orbits are impossible though, all test particles finish at the singularity in a finite amount of proper time.
  - entropicdrifter 6y ago
    I think by definition the event horizon can only be crossed in one direction (towards the center). By this I mean nothing escapes once it is past that point. It's defined by being the point of no return It is the point at which nothing, no matter how fast or massive, can possibly return from the gravitational pull. For instance: light, traveling at the speed of light in a vacuum, cannot escape once it has crossed the event horizon; it's pulled inevitably inward.
  - a1369209993 6y ago
    > what constrains it from orbiting Not quite what you seem to be asking, but the event horizon is the point where[0] the escape velocity is equal to the speed of light; the orbital velocity at that distance is greater (by a factor of ln 2 IIRC, so v_orbital ≈ 1.44c). There's a more distant distance, called the innermost stable orbit or the photon sphere, where the orbital velocity is equal to c (so photons will orbit if you emit them tangentially at this height), but the escape velocity is only ~0.69c. 0: It's not quite right to say that that's because the escape velocity is equal to c, though.
- srtjstjsj 6y ago
  You paraphrased the GP post about space becoming time, but didn't answer the Parent question about time becoming space: > In what sense does [time] become spacelike? Can one move back and forth in time inside the event horizon?
  - pa7x1 6y ago
    Sorry, I missed that. The time direction does indeed become spacelike inside the black hole. The most remarkable consequence of this is that the singularity is at the future of every test particle that crosses the horizon. This can be seen best in a conformal diagram or Penrose diagram (now that he has a Nobel prize might as well use the name of his creator). Here is the Penrose diagram of a star collapsing gravitationally into a Black Hole and its evaporation via Hawking radiation: https://fias.uni-frankfurt.de/~hossi/Bilder/BR/bhevap_l.jpg https://fias.uni-frankfurt.de/~hossi/Bilder/BR/bhevap_l.jpg You should track the r=0 line, initially it points upwards (it's timelike) as it chugs along at the center of the star as it collapses gravitationally. When the BH forms, it becomes horizontal (spacelike) and lies at the future of every test particle that enters into the BH. The singularity is inevitable for anything that crosses the horizon. The Black Hole will at some point have radiated all its energy in Hawking radiation at which point it disappears and r=0 becomes timelike again. Unfortunately it's hard to make it easier to understand without indulging in some math.
sharpneli 6y ago
It just becomes like space. You can freely move around it without any issues. It’s no longer time, just as the radial direction is no longer space. How it happens? Normal spacial direction has positive sign. Time has negative. When crossing the horizon the equations become such that t gets a positive sign and the radial direction a negative one. After that they behave as expected.
- foobarian 6y ago
  This reminds me of the games you can play flipping signs in relativistic equations when you make v > c. You can interpret those in a lot of fun ways too.
- srtjstjsj 6y ago
  Is that because you could move away from the center (a bit) to delay your progress toward the center singular?
  - sharpneli 6y ago
    You can’t move away from the center. You can move in direction around it (while still moving towards it at the same time). And you can move in the previously time now just space. Delaying your fall is precisely the same as trying to avoid tomorrow by moving around. Moving away from the center is like moving back in time. No matter how you move in the spatial dimensions you can’t do it. With the exception of faster than light travel. That allows you to move away from the center, and it also allows you to avoid tomorrow. It’s pretty much the same as time traveling in general relativity.
steve76 6y ago
>But I never hear anyone address how time becomes spacelike. Take a region of space, let everything that could exist absorb heat. Take the same region of space, and emit enough heat to create a voltage in anything that could look at it. Fall of the cliff of all things nature that want heat. Don't just have a parachute to slow your fall to non-existence, but a jetpack to absorb some heat of your own. Absorbing heat, you can now emit heat. Something asks "anything exists here?" you now have an answer. Nature has no internal mind, no distinction between time and space. Heat transfers through mating conductive surfaces, flowing convective mediums, or the nothingness of electromagnetic radiation, like sunlight going through the vacuum of space to warm the Earth. Special relativity provides a medium. It's easier to slow down the ticks of your own clock, move, and speed up your own clock again than it is to just move. At best, nature sees the photoelectric effect, brightness converting to internal voltage, which we see as "clock speed locality" and volumes of voltages. At worst, nature doesn't even make that distinction. It's easier still to have wiggle rooms in your answer. I slow down my clock when you say I'm going to move. I move when you say I'm going to slow down my clock. Looking back, both are true, whatever is in my advantage, because I hold the cards, and I make that potential a medium in of itself. The way I reason about black holes is that they tell more about what's around it than what's there. Everything that could exist is in agreement that we all dump heat there. I'm really interested in seeing if we can point a telescope at a black hole, and cause something elsewhere. Or point a telescope at everything, and see if the black hole does something, like pair production. Gravity and massless photons are such loaded terms promoting mysticism. Newton's idea of gravity, a cannon on top of a mountain shooting a ball that falls down as equally as it transverses lengths is a good place to start. No absolute position, everything is relative motion, and sum up yourself to all relative motion you are accelerating. Then move onto fields and Maxwell. Ticking clocks slow down and speed up for advantage. Before you get brighter, you need to charge up some internal energy. That's Planck, and Einstein brought the photon by relating that to relative time creating a voltage.

pa7x1 6y ago

The radial direction of the inside of the Schwarzschild Black Hole is timelike. This means that; inside the event horizon, the inevitable passage of time becomes the inevitable move towards the singularity at the center of the Black Hole. To answer your question in clear terms: No, you cannot move freely in the radial direction. Only towards the center, in the same way you cannot move freely forward or backwards in time.

pbhjpbhj 6y ago
Considering a point particle, what constrains it from orbiting [which is what I'm assuming we're meaning here as all matter has to move radially to cross the event horizon]? If it had a tangential component to its momentum prior to meeting the event horizon (EH) wouldn't it continue to orbit past the EH?
- pa7x1 6y ago
  Nothing forbids a slight tangential component but the movement of the test particle is inevitably decreasing in r. Closed orbits are impossible though, all test particles finish at the singularity in a finite amount of proper time.
- entropicdrifter 6y ago
  I think by definition the event horizon can only be crossed in one direction (towards the center). By this I mean nothing escapes once it is past that point. It's defined by being the point of no return It is the point at which nothing, no matter how fast or massive, can possibly return from the gravitational pull. For instance: light, traveling at the speed of light in a vacuum, cannot escape once it has crossed the event horizon; it's pulled inevitably inward.
- a1369209993 6y ago
  > what constrains it from orbiting Not quite what you seem to be asking, but the event horizon is the point where[0] the escape velocity is equal to the speed of light; the orbital velocity at that distance is greater (by a factor of ln 2 IIRC, so v_orbital ≈ 1.44c). There's a more distant distance, called the innermost stable orbit or the photon sphere, where the orbital velocity is equal to c (so photons will orbit if you emit them tangentially at this height), but the escape velocity is only ~0.69c. 0: It's not quite right to say that that's because the escape velocity is equal to c, though.
srtjstjsj 6y ago
You paraphrased the GP post about space becoming time, but didn't answer the Parent question about time becoming space: > In what sense does [time] become spacelike? Can one move back and forth in time inside the event horizon?
- pa7x1 6y ago
  Sorry, I missed that. The time direction does indeed become spacelike inside the black hole. The most remarkable consequence of this is that the singularity is at the future of every test particle that crosses the horizon. This can be seen best in a conformal diagram or Penrose diagram (now that he has a Nobel prize might as well use the name of his creator). Here is the Penrose diagram of a star collapsing gravitationally into a Black Hole and its evaporation via Hawking radiation: https://fias.uni-frankfurt.de/~hossi/Bilder/BR/bhevap_l.jpg https://fias.uni-frankfurt.de/~hossi/Bilder/BR/bhevap_l.jpg You should track the r=0 line, initially it points upwards (it's timelike) as it chugs along at the center of the star as it collapses gravitationally. When the BH forms, it becomes horizontal (spacelike) and lies at the future of every test particle that enters into the BH. The singularity is inevitable for anything that crosses the horizon. The Black Hole will at some point have radiated all its energy in Hawking radiation at which point it disappears and r=0 becomes timelike again. Unfortunately it's hard to make it easier to understand without indulging in some math.

sharpneli 6y ago

It just becomes like space. You can freely move around it without any issues. It’s no longer time, just as the radial direction is no longer space. How it happens? Normal spacial direction has positive sign. Time has negative. When crossing the horizon the equations become such that t gets a positive sign and the radial direction a negative one. After that they behave as expected.

foobarian 6y ago
This reminds me of the games you can play flipping signs in relativistic equations when you make v > c. You can interpret those in a lot of fun ways too.
srtjstjsj 6y ago
Is that because you could move away from the center (a bit) to delay your progress toward the center singular?
- sharpneli 6y ago
  You can’t move away from the center. You can move in direction around it (while still moving towards it at the same time). And you can move in the previously time now just space. Delaying your fall is precisely the same as trying to avoid tomorrow by moving around. Moving away from the center is like moving back in time. No matter how you move in the spatial dimensions you can’t do it. With the exception of faster than light travel. That allows you to move away from the center, and it also allows you to avoid tomorrow. It’s pretty much the same as time traveling in general relativity.

steve76 6y ago

>But I never hear anyone address how time becomes spacelike. Take a region of space, let everything that could exist absorb heat. Take the same region of space, and emit enough heat to create a voltage in anything that could look at it. Fall of the cliff of all things nature that want heat. Don't just have a parachute to slow your fall to non-existence, but a jetpack to absorb some heat of your own. Absorbing heat, you can now emit heat. Something asks "anything exists here?" you now have an answer. Nature has no internal mind, no distinction between time and space. Heat transfers through mating conductive surfaces, flowing convective mediums, or the nothingness of electromagnetic radiation, like sunlight going through the vacuum of space to warm the Earth. Special relativity provides a medium. It's easier to slow down the ticks of your own clock, move, and speed up your own clock again than it is to just move. At best, nature sees the photoelectric effect, brightness converting to internal voltage, which we see as "clock speed locality" and volumes of voltages. At worst, nature doesn't even make that distinction. It's easier still to have wiggle rooms in your answer. I slow down my clock when you say I'm going to move. I move when you say I'm going to slow down my clock. Looking back, both are true, whatever is in my advantage, because I hold the cards, and I make that potential a medium in of itself. The way I reason about black holes is that they tell more about what's around it than what's there. Everything that could exist is in agreement that we all dump heat there. I'm really interested in seeing if we can point a telescope at a black hole, and cause something elsewhere. Or point a telescope at everything, and see if the black hole does something, like pair production. Gravity and massless photons are such loaded terms promoting mysticism. Newton's idea of gravity, a cannon on top of a mountain shooting a ball that falls down as equally as it transverses lengths is a good place to start. No absolute position, everything is relative motion, and sum up yourself to all relative motion you are accelerating. Then move onto fields and Maxwell. Ticking clocks slow down and speed up for advantage. Before you get brighter, you need to charge up some internal energy. That's Planck, and Einstein brought the photon by relating that to relative time creating a voltage.