6 ms·

Non poetic but hopefully bit more understandable explanation of what it tries to say: After crossing the event horizon the radial direction becomes like time.

Non poetic but hopefully bit more understandable explanation of what it tries to say: After crossing the event horizon the radial direction becomes like time. And what used to be time becomes like space. What this means that you will inevitably travel to the center, just in the same way as you travel into the future in normal life. Inside the event horizon the center is not at any place that you can see, instead it is in your future.

psychoslave 6y ago
To be fair, the universe will never confront you to such a puzzling situation, as it will kindly squash you under heavy gravity before you come close to any object so massive.
- kryptiskt 6y ago
  Not necessarily, a supermassive black hole can have quite bearable gravity at the event horizon (as the event horizon radius depends linearly on the mass and the gravity declines quadratically with distance). You still wouldn't be able to communicate your experiences though.
  - ckosidows 6y ago
    Doesn't an event horizon imply a gravitational pull so strong not even light can escape? Isn't the effect of gravity the same at the event horizon for all sizes of a black hole? I thought the size only affected the "acceleration" of the gravitational pull based on the distance from the singularity? A supermassive black hole means you will take longer to be spaghettified, but it will still happen and it will happen before you cross the horizon, won't it?
    - radioactivist 6y ago
      A uniform force doesn't do anything harmful to you. You get stretched into spaghetti by the difference in force between the ends of your body. In large black holes that difference can be small even as you pass through the horizon (where the total force is large).
    - gizmo686 6y ago
      In some sense, there is no such thing as a gravitational pull. What there is is a curvature of space-time that causes an apparent force in some coordinate systems [0]. For a sufficiently large black hole, gravity is effectively constant in the vicinity of the event horizon; which means there exists a coordinate system where the space around (a small region of) the event horizon is essentially flat space. From the perspective of an outside observer, you would appear to flatten as you approach the event horizon, but this is a consequence of the coordinate system. You don't feel any effect at that point. The reason light cannot escape from past the event horizon is that that is the point where space-time is so warped that what you consider a perfectly normal time axis is, from the perspective of an outside observer, now pointing in a spatial dimension directly towards the center of the black hole. In contrast, regardless of how small you are, there is some point where you do experience tidal forces and feel yourself getting spaghetti. For a small black hole, this is before you pass the horizon, for a large black hole, it is after. [0] The distinguishing feature between a real force like gravity, and a truly fictitious force like the centrifugal force, is that for the later, you can construct a coordinate system where the force disappears at all locations. For a real force, you can construct a coordinate system where the force goes away at any given point; but there will always be some point with an apparent force.
  - Isinlor 6y ago
    What is supposed to happen with different types of chemical bonds? If half of a molecule is inside a black hole and half is outside, the half inside is necessarily constrained to the black hole, while a molecule outside does not have such constraint. The chemical bonds should be also necessarily broken at least for a moment, since an atom inside black hole can not exert electromagnetic effects on an atom outside black hole. Wouldn't it create a bias across your whole body with some really weird atomic / chemical consequences?
    - btilly 6y ago
      In General Relativity, locally all reference frames are flat. This does not change at the event horizon, chemistry continues to work normally. It is just that in the time it takes for half the molecule to affect the other half, the other half will have slipped across the event horizon. There is a classic analogy of light traveling through space-time being like ants crawling on a balloon. A black hole is a spot where the balloon is being sucked in. Faster and faster as you get closer. The event horizon is where the speed at which the balloon moves matches the speed of the ant. There is nothing special from the ant's immediate perception about this line, but if the ant crosses, it is doomed.
    - yk 6y ago
      Nothing happens (locally) at the event horizon, everything works as normal. The analogy that is often used is fish upstream from a waterfall. At some point the current of the stream becomes so strong that the fish can no longer escape from the waterfall, but actually at that point nothing happens from the point of view of the fish, since it is carried by the current, you need a global view to notice that the fish is doomed past that point. And similar in case of a black hole, you are falling through the event horizon and nothing happens, it is just that calculating what you need to do to escape, you note that it is too late. To expand a bit on the molecule situation, when one of the atoms is inside the black hole, then parts of its electric field are still outside and influence the other atom. To have the other atom escape to infinity, you would need to break the part of the bond "still outside" and the gravity of the black hole.
    - a1369209993 6y ago
stallmanite 6y ago
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.
centimeter 6y ago
I have heard of space becoming timelike, but have never heard of time becoming spacelike in a black hole. Do you have more information on that?
- sharpneli 6y ago
  PBS Space Time is pretty decent and understandable source if one doesn’t want to wade trough the maths. I heavily recommend the whole black holes playlist. https://www.youtube.com/playlist?list=PLsPUh22kYmNBl4h0i4mI5zDflExXJMo_x https://www.youtube.com/playlist?list=PLsPUh22kYmNBl4h0i4mI5...
  - centimeter 6y ago
    I do want to wade through the maths, and I have to some degree, and I've never come across time becoming spacelike (which would imply rotation past the null vector, which afaik is not something that happens in a black hole).
frereubu 6y ago
Thanks, that's much more helpful, although still pretty mind-bending.

psychoslave 6y ago

To be fair, the universe will never confront you to such a puzzling situation, as it will kindly squash you under heavy gravity before you come close to any object so massive.

kryptiskt 6y ago
Not necessarily, a supermassive black hole can have quite bearable gravity at the event horizon (as the event horizon radius depends linearly on the mass and the gravity declines quadratically with distance). You still wouldn't be able to communicate your experiences though.
- ckosidows 6y ago
  Doesn't an event horizon imply a gravitational pull so strong not even light can escape? Isn't the effect of gravity the same at the event horizon for all sizes of a black hole? I thought the size only affected the "acceleration" of the gravitational pull based on the distance from the singularity? A supermassive black hole means you will take longer to be spaghettified, but it will still happen and it will happen before you cross the horizon, won't it?
  - radioactivist 6y ago
    A uniform force doesn't do anything harmful to you. You get stretched into spaghetti by the difference in force between the ends of your body. In large black holes that difference can be small even as you pass through the horizon (where the total force is large).
  - gizmo686 6y ago
    In some sense, there is no such thing as a gravitational pull. What there is is a curvature of space-time that causes an apparent force in some coordinate systems [0]. For a sufficiently large black hole, gravity is effectively constant in the vicinity of the event horizon; which means there exists a coordinate system where the space around (a small region of) the event horizon is essentially flat space. From the perspective of an outside observer, you would appear to flatten as you approach the event horizon, but this is a consequence of the coordinate system. You don't feel any effect at that point. The reason light cannot escape from past the event horizon is that that is the point where space-time is so warped that what you consider a perfectly normal time axis is, from the perspective of an outside observer, now pointing in a spatial dimension directly towards the center of the black hole. In contrast, regardless of how small you are, there is some point where you do experience tidal forces and feel yourself getting spaghetti. For a small black hole, this is before you pass the horizon, for a large black hole, it is after. [0] The distinguishing feature between a real force like gravity, and a truly fictitious force like the centrifugal force, is that for the later, you can construct a coordinate system where the force disappears at all locations. For a real force, you can construct a coordinate system where the force goes away at any given point; but there will always be some point with an apparent force.
- Isinlor 6y ago
  What is supposed to happen with different types of chemical bonds? If half of a molecule is inside a black hole and half is outside, the half inside is necessarily constrained to the black hole, while a molecule outside does not have such constraint. The chemical bonds should be also necessarily broken at least for a moment, since an atom inside black hole can not exert electromagnetic effects on an atom outside black hole. Wouldn't it create a bias across your whole body with some really weird atomic / chemical consequences?
  - btilly 6y ago
    In General Relativity, locally all reference frames are flat. This does not change at the event horizon, chemistry continues to work normally. It is just that in the time it takes for half the molecule to affect the other half, the other half will have slipped across the event horizon. There is a classic analogy of light traveling through space-time being like ants crawling on a balloon. A black hole is a spot where the balloon is being sucked in. Faster and faster as you get closer. The event horizon is where the speed at which the balloon moves matches the speed of the ant. There is nothing special from the ant's immediate perception about this line, but if the ant crosses, it is doomed.
  - yk 6y ago
    Nothing happens (locally) at the event horizon, everything works as normal. The analogy that is often used is fish upstream from a waterfall. At some point the current of the stream becomes so strong that the fish can no longer escape from the waterfall, but actually at that point nothing happens from the point of view of the fish, since it is carried by the current, you need a global view to notice that the fish is doomed past that point. And similar in case of a black hole, you are falling through the event horizon and nothing happens, it is just that calculating what you need to do to escape, you note that it is too late. To expand a bit on the molecule situation, when one of the atoms is inside the black hole, then parts of its electric field are still outside and influence the other atom. To have the other atom escape to infinity, you would need to break the part of the bond "still outside" and the gravity of the black hole.
  - a1369209993 6y ago

stallmanite 6y ago

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.

centimeter 6y ago

I have heard of space becoming timelike, but have never heard of time becoming spacelike in a black hole. Do you have more information on that?

sharpneli 6y ago
PBS Space Time is pretty decent and understandable source if one doesn’t want to wade trough the maths. I heavily recommend the whole black holes playlist. https://www.youtube.com/playlist?list=PLsPUh22kYmNBl4h0i4mI5zDflExXJMo_x https://www.youtube.com/playlist?list=PLsPUh22kYmNBl4h0i4mI5...
- centimeter 6y ago
  I do want to wade through the maths, and I have to some degree, and I've never come across time becoming spacelike (which would imply rotation past the null vector, which afaik is not something that happens in a black hole).

frereubu 6y ago

Thanks, that's much more helpful, although still pretty mind-bending.