Interested in Black Holes, Neutron Stars, and White Dwarf Stars

[Peter Cole]

How did you find PF?: duckduckGo search.

My math is very weak and I don't like explanations done using math. I read books with very little math. I try to use reason based on what I've read. My understanding is time slows down in gravity and it will actually stop at the event horizon (see "Einstein's Universe" by Nigel Calder where in the chapter "Shells of Time" on page 40 he says, "time itself stands still on the edge of a black hole."). This brings up other possibilities such as time is negative or imaginary inside a black hole, a free falling object goes the speed of light at the event horizon, the potential energy of a mass is zero at the event horizon, and there could be other possible strangenesses. I created a Facebook group called "Black Holes, Neutron Stars, and White Dwarf Stars" where we can get spaghettified.

 

[phinds]

Welcome to the forum.

I'm afraid your "understanding" is seriously flawed. Time does not stand still at the event horizon, it just APPEARS to do so to an observer who is far away from the black hole. AT the horizon, time just goes on ticking at one second per second and someone crossing the horizon sees nothing out of the ordinary.

You'll find that science without math is interesting but often wildly inaccurate and misleading.

 

[berkeman]

How did you find PF?: duckduckGo search.

My math is very weak and I don't like explanations done using math. I read books with very little math.

It's good to be open to learning at least a little math. A little math can go a long way. Welcome to PF.

 

[Keith_McClary]

Check out PF's list of Free Physics Books.

 

[Peter Cole]

Welcome to the forum.

I'm afraid your "understanding" is seriously flawed. Time does not stand still at the event horizon, it just APPEARS to do so to an observer who is far away from the black hole. AT the horizon, time just goes on ticking at one second per second and someone crossing the horizon sees nothing out of the ordinary.

You'll find that science without math is interesting but often wildly inaccurate and misleading.

What APPEARS to happen is very different than what actually happens according to the real world. Here is a thought experiment. There are two clocks far from a black hole that show the same time. One clock is lowered close to the event horizon of the black hole and the other stays far from the black hole. Both clocks start timing when the lowering stops. After a day of time on the far away clock the lowered clock is pulled back up. Both clocks stop timing at the start of the pull back. Now for the moment of truth. Are the clocks still showing the same time? If not then which clock is slower? I know that the answer is that the lowered clock will show that it was in slower time. This is not an illusion, this is a fact.

Now the next thought experiment is what happens to time at the event horizon? Either it is stopped or not. If somehow a clock could be lowered into the event horizon and pulled back out after a day then how much time would that clock have recorded? I know the answer is that the lowered clock will show that time stopped. This is not an illusion, this is a fact.

I don't care what an observer near or at the event horizon observes, I want to know what really happens according to the fact recorder who is still in the real world. If you still think that my "understanding" of time in a gravity field is seriously flawed then please tell me the flaw.

 

[italicus]

@Peter Cole

if you don’t like mathematics , I suggest to look for this scientist’s books and articles, there are a lot in the Internet:

https://en.m.wikipedia.org/wiki/Jean-Pierre_Luminet
t
I reconmend “ Black holes “ , written in 1987.

But don’t forget Stephen Hawking , Kip Thorne and Brian Greene popular books.
Anyway, I agree with others that a little math would be very helpful, almost unavoidable if you wish to get a sound knowledge of the subject.

 

[PeroK]

Now the next thought experiment is what happens to time at the event horizon? Either it is stopped or not. If somehow a clock could be lowered into the event horizon and pulled back out after a day then how much time would that clock have recorded? I know the answer is that the lowered clock will show that time stopped. This is not an illusion, this is a fact.

A massive object cannot hover at the event horizon. That said, you can theoretically hold the clock arbitrarily close to the event horizon.

Technically, the event horizon is a null surface. A point on the event horizon is more like the path of a light ray than a point in space. And a massive object cannot follow the path of a light ray. This is the same argument that goes against "time stands still at the speed of light". A massive object cannot move at the speed of light.

It's more accurate to say that time is not defined for a light ray or a null surface.

 

[MikeeMiracle]

Your example about the clock lowered to the event horizon. If you have two twins, one which stays with the clock outside the black hole and one which gets lowered down with the clock which sits just above the event horizon, the twin with the clock at the event horizon will still see his own clock ticking at exactly the same rate as the twin outside the black hole sees their own clock ticking.

It's called Relativity because what you measure is relative from what you are measuring and where you are measuring it from. It's only when either of the twins tries to measure the ticking of the others clock when separated that you see this rate of time changing.

If time truly stopped at or near the event horizon, then how does material fall into a black hole?

Saying you don't care what the observer at the event horizon will see is preventing you from grasping the very concepts you are trying to understand in my opinion.

 

[jbriggs444]

I want to know what really happens

This is your major problem.

The questions you are asking have no real answers. They are questions about coordinate systems, not questions about reality. A key lesson that needs to be taken away from special relativity before tackling general relativity is that synchronization is relative.

If you try to compare clocks at a distance, there is an element of arbitrariness to the comparison. You have to choose a coordinate system (or at least a synchronization standard).

If you have a situation where clocks start together, are separated and are then re-united, you cannot get too wild with your synchronization standard and still have a well behaved coordinate system. But if you have a situation (like a clock reaching the event horizon) where clocks are separated but then can never subsequently exchange signals then there is no way to say what has really happened to their relative tick rates. There is no underlying real answer.

You'll find a similar thing with the double slit experiment. There is no answer to which slit the electron really went through.

 

[Nugatory]

Now the next thought experiment is what happens to time at the event horizon? Either it is stopped or not. If somehow a clock could be lowered into the event horizon and pulled back out after a day then how much time would that clock have recorded? I know the answer is that the lowered clock will show that time stopped. This is not an illusion, this is a fact.

Saying “if somehow a clock could be lowered into the event horizon and pulled back” is somewhat like saying “if somehow a prime number could be factored” or “if it were mathematically OK to divide by zero” - if we start with a bogus premise we get bogus results, and “time stopping at the event horizon” is one of those bogus results.

 

[phinds]

@Peter Cole one of your problems is that you are confusing time dilation with differential aging. Time dilation is coordinate dependent but differential aging is not which is why you are getting confused when you try to compare the two.

 

[jartsa]

@Peter Cole one of your problems is that you are confusing time dilation with differential aging. Time dilation is coordinate dependent but differential aging is not which is why you are getting confused when you try to compare the two.

Would it be OK to say that aging is highly different between two dudes, when one dude hovers close to the event horizon of a black hole and the other dude is far from the black hole?Well, I can guess the answer. Something about dudes at different places not being able to compare times ages.

Although they can tell, by radio, that: "I'm this old now", or "I have gray hair now ".

 

[phinds]

Would it be OK to say that aging is highly different between two dudes, when one dude hovers close to the event horizon of a black hole and the other dude is far from the black hole?

No, it would not. They both age at a rate of one second per second. The fact that they appear to age differently is coordinate dependent. If you bring them back together they they will have aged different amounts, but not, as you seem to believe, because they aged at different rates but because they took different paths through spacetime.

Your attempt to compare them via radio signal doesn't work because the radio signals are affected by the same gravity as the light signals that make it look like the lower one is aging at a different rate.

 

[jartsa]

Your attempt to compare them via radio signal doesn't work because the radio signals are affected by the same gravity as the light signals that make it look like the lower one is aging at a different rate.

Sure, lower dude sees an appearance where there is the funeral of upper dude happening, although it's not really happening. (Sarcasm)No seriously, the two dudes can discuss normally if the black hole is a small one, there's just a small delay according to the upper dude, and even smaller delay according to the lower dude.

So the upper dude says to the lower dude: "I can see you moving through space-time in such way that after a while I'll be much older than you"

 

[Nugatory]

Would it be OK to say that aging is highly different between two dudes, when one dude hovers close to the event horizon of a black hole and the other dude is far from the black hole?

Sort of... That claim is much more OK than the similar-sounding claim about time passing more slowly for someone who is moving than stationary. But only sort of... Any attempt to analyze the situation in terms of the rate of aging obscures what's really going on, cannot be reliably generalized to other situations, and is pretty much guaranteed to lead to bad conclusions if taken too seriously.

(It works because in this setup just about any reasonable simultaneity convention leads to results that can be described as "far guy ages more quickly", so we can get away with careless failure to notice that we're making an assumption about some simultaneity convention. It is better to describe the situation in terms of invariants to either avoid the assumption altogether or make it explicit).

 

[PeterDonis]

"Einstein's Universe" by Nigel Calder

You should not be trying to learn actual science from pop science books.

This brings up other possibilities such as time is negative or imaginary inside a black hole, a free falling object goes the speed of light at the event horizon, the potential energy of a mass is zero at the event horizon

All of these are nonsense.

 

[Peter Cole]

You should not be trying to learn actual science from pop science books.
All of these are nonsense.

@Peter Cole one of your problems is that you are confusing time dilation with differential aging. Time dilation is coordinate dependent but differential aging is not which is why you are getting confused when you try to compare the two.

Are you trying to tell me that time dilation does not cause differential aging or that differential aging is an illusion that does not occur or that I'm using the wrong wording in my discussion about differential aging?

 

[PeroK]

Are you trying to tell me that time dilation does not cause differential aging or that differential aging is an illusion that does not occur or that I'm using the wrong wording in my discussion about differential aging?

Time dilation in general does not cause differential aging. Time dilation is in general a coordinate effect, which depends on your choice of global synchronisation and, in general, is not physically meaningful.

Differential aging is invariant (a physically meaningful quantity) - there is no ambiguity about how it is measured. The cause of differential aging is the different lengths of the paths two objects take through spacetime. The time measured on a clock is a measure of the length of its path through spacetime. If two clocks start and end at the same points in spacetime, then the time recorded on each represents the length of the path it took through spacetime.

In your example, an object can take a shorter path through spacetime by traveling towards a black hole, hovering near the event horizon and returning. This is a shorter path than remaining at rest relative to the black hole and a long way from it. To measure differential aging you read off the times on the clocks when they are colocated (in space and time) at the beginning and end of the experiment. That is unambiguous and coordinate independent.

That said, it's quite common for authors to be imprecise about the distinction between time dilation and differential aging.

 

[Ibix]

Are you trying to tell me that time dilation does not cause differential aging or that differential aging is an illusion that does not occur or that I'm using the wrong wording in my discussion about differential aging?

Neither. Differential aging is a real phenomenon, responsible for the differing ages in experiments like the twin paradox. Time dilation is a separate phenomenon, and is purely a coordinate effect. It can be removed or modified by a different choice of coordinates (although these coordinates are more complex to use). The two phenomena are related but distinct, a fact which does not come across in many presentations of relativity.

Gravitational time dilation and the differential aging due to traveling near a black hole and returning are also separate but related phenomena. Neither can be defined if you go to the event horizon.

I think Nugatory has said it clearest - you can get away with statements like "clocks tick slower nearer the event horizon" because of symmetries in Schwarzschild spacetime. But you are secretly making assumptions based on those symmetries, and they are not present at or below the event horizon.

 

[jartsa]

Neither. Differential aging is a real phenomenon, responsible for the differing ages in experiments like the twin paradox. Time dilation is a separate phenomenon, and is purely a coordinate effect. It can be removed or modified by a different choice of coordinates (although these coordinates are more complex to use). The two phenomena are related but distinct, a fact which does not come across in many presentations of relativity.

1: Twin that hovered near event horizon travels back up, learns that upper twin has just died, attends funeral.

2: Twin that hovers near event horizon stays there, learns that upper twin has just died, attends funeral remotely. (Uses webcam and microphone and some remote conference application)Differential aging in case 1.

Not differential aging in case 2. ?

 

[PeroK]

1: Twin that hovered near event horizon travels back up, learns that upper twin has just died, attends funeral.

2: Twin that hovers near event horizon stays there, learns that upper twin has just died, attends funeral remotely. (Uses webcam and microphone and some remote conference application)Differential aging in case 1.

Not differential aging in case 2. ?

The issue is not whether an event is in your past, but giving a precise, unambiguous age difference for two objects separated in spacetime.

 

[Nugatory]

Not differential aging in case 2. ?

Well, it is pretty clear that the death of upper guy is in the past light cone of lower guy at the event where lower guy joins the videoconference.

We also know that the proper time along the path through spacetime that runs from the separation event to the death event and then to lower guy joins videoconference event is greater than the proper time along lower guy’s path from the separation event to the joins videoconference event. And seeing as how “differential aging” is a term that can be informally applied to differences in proper time along different paths through spacetime... sure, you can call it that.

But do note that the question “How old was lower guy when upper guy died?” still has no answer. We can say “younger than when they joined the videoconference“ but to say more requires a simultaneity convention, and that’s an arbitrary choice.

 

[vanhees71]

Time dilation in general does not cause differential aging. Time dilation is in general a coordinate effect, which depends on your choice of global synchronisation and, in general, is not physically meaningful.

Differential aging is invariant (a physically meaningful quantity) - there is no ambiguity about how it is measured. The cause of differential aging is the different lengths of the paths two objects take through spacetime. The time measured on a clock is a measure of the length of its path through spacetime. If two clocks start and end at the same points in spacetime, then the time recorded on each represents the length of the path it took through spacetime.

In your example, an object can take a shorter path through spacetime by traveling towards a black hole, hovering near the event horizon and returning. This is a shorter path than remaining at rest relative to the black hole and a long way from it. To measure differential aging you read off the times on the clocks when they are colocated (in space and time) at the beginning and end of the experiment. That is unambiguous and coordinate independent.

That said, it's quite common for authors to be imprecise about the distinction between time dilation and differential aging.

Yes, and I think it's important to emphasize once more that in general relativity you can compare clocks only locally, i.e., in one point in spacetime. When discussing the twin paradox you thus have to synchronize two clocks when the twins are together at one event and then compare their clock readings when they meet again at another event (in the future). Otherwise there's no way to make physical sense of the question who aged more relative to the other.

 

[phinds]

2: Twin that hovers near event horizon stays there, learns that upper twin has just died, attends funeral remotely. (Uses webcam and microphone and some remote conference application)

As I have already pointed out in post #13, this is impossible because the radio waves or laser or whatever used for communication from the lower to the upper is bound by the same gravity that makes the lower guy look younger. If the time dilation is, say, 50 years between lower and upper at the time of upper's death then the communication signal would take 50 years to get up so no teleconference would be possible unless you wanted to wait 50 years between each statement and by the time of the first response the upper guy would have been dead for 50+ years.

 

[jbriggs444]

As I have already pointed out in post #13, this is impossible because the radio waves or laser or whatever used for communication from the lower to the upper is bound by the same gravity that makes the lower guy look younger. If the time dilation is, say, 50 years between lower and upper at the time of upper's death then the communication signal would take 50 years to get up so no teleconference would be possible unless you wanted to wait 50 years between each statement and by the time of the first response the upper guy would have been dead for 50+ years.

I am not convinced that you are saying here what you mean to be saying.

A 50 year accumulated discrepancy in "wristwatch age" between the lower twin and the upper twin at the time of the upper twin's death does not mean that there must be a 50 year delay in getting a round trip signal between the two.

More generally, "time dilation" should be measured as a unitless ratio while "differential aging" should be measured in units of time. A figure of 50 years for time dilation is not sensible.

As I understand the funeral situation, we are contemplating a small black hole so that round trip times are manageable along with a lower twin hovering very near the horizon so that arbitrarily great ratios for gravitational time dilation are achievable. [We are ignoring issues with fallen arches and varicose veins].

Note that, as @Nugatory suggests, "gravitational time dilation" has a coordinate-dependent aspect, but it is at least somewhat physical.

 

[Peter Cole]

Welcome to the forum.

I'm afraid your "understanding" is seriously flawed. Time does not stand still at the event horizon, it just APPEARS to do so to an observer who is far away from the black hole. AT the horizon, time just goes on ticking at one second per second and someone crossing the horizon sees nothing out of the ordinary.

You'll find that science without math is interesting but often wildly inaccurate and misleading.

For that one second to tick at the event horizon, how much time will have expired far from the black hole?

When I read books without math I assume the author has done the math or knows the math is correct for the statements he/she makes. For example in Stephen Hawking's book "A brief History of Time" (has no math) he discusses that black holes can radiate away their mass. However, he doesn't believe that will happen to solar mass black holes or larger anytime soon as the microwave radiation temperature is just too high. He didn't have to bore me with math that I would have never understood. His words were enough for me to understand the nitty gritty of the subject. However, I may not believe everything said or indicated. For example, in Stephen Hawking's same book is a wiggling singularity on Figure 6.1 where time increasing in the y direction. I've never read about a wiggling singularity anywhere so I assume this might indicate that he had a hard time drawing a straight line by hand. But then again it looked intentional.

 

[phinds]

For that one second to tick at the event horizon, how much time will have expired far from the black hole?

Since you are asking about something that is coordinate dependent rather than the proper time, which is what I was talking about, the answer is "depends on how far one is from the EH and how close the other is to the black hole". Pick any number > 1 and you can make it work.

 

[jbriggs444]

When I read books without math I assume the author has done the math or knows the math is correct for the statements he/she makes.

The difficulty is that it is difficult or even impossible to make understandable statements that match what the mathematics is actually saying. Essentially everyone posting in this thread has been trying to help you understand this.

The statements that you hear in popular science expositions and the questions that you ask about them make incorrect assumptions. Before the questions can properly be answered, the mistaken assumptions must first be addressed. The idea that there is an underlying fact of the matter on "how much time has elapsed far from the black hole" depends crucially on an assumption that there is one true standard of synchronization. That assumption is not correct.

 

[Peter Cole]

Since you are asking about something that is coordinate dependent rather than the proper time, which is what I was talking about, the answer is "depends on how far one is from the EH and how close the other is to the black hole". Pick any number > 1 and you can make it work.

You were the one who easily went through the event horizon as if nothing special happens there. I said to use a second at the event horizon as you said " AT the horizon, time just goes on ticking at one second per second." I wanted you to tell me how much time expires far from the black hole during that second assuming you understood that meant far outside the gravity of the black hole. That's what most people use to compare time when talking about time near a black hole. I expected you to tell me something like that hell would freeze over or that the Universe itself would have evaporated away into nothingness to include the total evaporation of the black hole itself. However, instead of telling me what happens to time far from the black hole you want me to tell myself what happened using something greater than 1 of what I haven't a clue. What you really seem to be telling me is that you can't answer my question.

 

[PeterDonis]

I read books with very little math. I try to use reason based on what I've read.

Unfortunately, this strategy will have limited usefulness at best for learning physics.

The problem is that, if you're reading books about physics with very little math, those books cannot give you a model you can reason from correctly. That's because there is no such model without math. Physicists don't use math because they want to make it harder for lay people to learn physics. Physicists use math because it's the only tool that works for building models of physical systems that you can reason from correctly.

So when you read a bunch of stuff in a book about physics that doesn't have math in it, even if it's a book written by a physicist, the stuff the physicist is telling you in the book is not anything you can actually reason from. If physics could be done that way, physicists would be doing it that way instead of using math, since using math is hard and requires a lot more training. What the physicist is actually doing when he writes a book like that is taking the underlying mathematical model that he already knows, extracting some interesting conclusions from it, and then describing those conclusions to you in ordinary language. But there's not enough information in what he's telling you to allow you to reconstruct the underlying model he's using to get those conclusions, and without that underlying model, you have no valid basis for further reasoning.

 

[Peter Cole]

Unfortunately, this strategy will have limited usefulness at best for learning physics.

The problem is that, if you're reading books about physics with very little math, those books cannot give you a model you can reason from correctly. That's because there is no such model without math. Physicists don't use math because they want to make it harder for lay people to learn physics. Physicists use math because it's the only tool that works for building models of physical systems that you can reason from correctly.

So when you read a bunch of stuff in a book about physics that doesn't have math in it, even if it's a book written by a physicist, the stuff the physicist is telling you in the book is not anything you can actually reason from. If physics could be done that way, physicists would be doing it that way instead of using math, since using math is hard and requires a lot more training. What the physicist is actually doing when he writes a book like that is taking the underlying mathematical model that he already knows, extracting some interesting conclusions from it, and then describing those conclusions to you in ordinary language. But there's not enough information in what he's telling you to allow you to reconstruct the underlying model he's using to get those conclusions, and without that underlying model, you have no valid basis for further reasoning.

So what you are trying to tell me is that none of you could write a book telling people like me what the heck you are talking about. My attempt at using this forum is not helping me so I will be deleting my account if I can. If I can't then you won't be hearing from me again.

 

[phinds]

I said to use a second at the event horizon as you said ... I wanted you to tell me how much time expires far from the black hole during that second assuming you understood that meant far outside the gravity of the black hole.

Reasonable assumptions. Answer is ... the simple answer is "an infinite amount of time". The more technically correct answer is, sort of, "the lifetime of the BH", so something like 10E60 to 10E80 years depending on the size of the BH. You can't really get to an understanding of the answer without math.

 

[Nugatory]

I wanted you to tell me how much time expires far from the black hole during that second assuming you understood that meant far outside the gravity of the black hole.

The problem here is that that question is not as well posed as you're thinking.

One second elapses on the wristwatch of the guy deep in the gravity well. For the sake of definiteness, let's say that it is the second between when their wristwatch reads 04:15:23 and 04:15:24. You ask how much time "expires far from the black hole during that second". To answer this we need a clock far from the black hole; we look at what that clock reads at the same time that the wristwatch reads 04:15:23; we look at it again at the same time that the wristwatch reads 04:15:24; we subtract the first reading from the second; and that difference is the time expired that you've asked for. This is pretty much how we determine how much time passes between any two remote events: we have a clock, we look what it says at the same time that the remote events happen, we compare the two readings.

But (and this is the part that trips up most people at first) this procedure crucially depends on how we define "at the same time". Your question does not have any meaningful answer until you provide a definiition of "at the same time". In the flat spacetime of special relativity where there are no gravitational effects there is a definition (at least in inertial frames) that is so natural that it would be perverse not to use it: Einstein clock synchronization. In curved spacetime there is no comparably natural definition and no answer to your question unless and until you tell us what you mean by "at the same time".

 

[Nugatory]

My attempt at using this forum is not helping me so I will be deleting my account if I can. If I can't then you won't be hearing from me again.

You might also try getting hold of the (now available online) book "Spacetime Physics" by Taylor and Wheeler. It's mathematically solid but well within the grasp of a high school senior, and a few tens of hours with it will be more rewarding than all the time you've wasted so far searching through the dumbed-down pop-sci treatments.

 

[PeterDonis]

In the flat spacetime of special relativity where there are no gravitational effects there is a definition (at least in inertial frames) that is so natural that it would be perverse not to use it: Einstein clock synchronization. In curved spacetime there is no comparably natural definition

Actually, while there is no natural definition of simultaneity in a general curved spacetime, in a static spacetime, like the one we're discussing in this thread (Schwarzschild spacetime), Einstein clock synchronization actually does work as a simultaneity convention. The difference from flat spacetime is that the elapsed time for two observers at different altitudes between two sets of "corresponding" events (events on each of their worldlines that happen at the same time) will be different--the one at the higher altitude will have more elapsed time. So in this case, unlike flat spacetime, Einstein clock synchronization doesn't actually "synchronize" clocks--clocks synchronized this way won't continue to run at the same rate, so the same readings on two separate clocks will not continue to occur "at the same time" according to the simultaneity convention that was established. All it does is provide a "natural" simultaneity convention ("natural" because it matches up with a symmetry of the spacetime). So "Einstein simultaneity" would be a better name for this process in a stationary curved spacetime.

(Note, btw, that similar remarks to the above apply to Rindler coordinates in flat spacetime--you can also define "Einstein clock synchronization" for a family of observers at rest in Rindler coordinates, but it will have the same limitations as above.)

(There is also a more technical definition of the "natural" simultaneity convention I'm describing for a static spacetime, in terms of the hypersurfaces that are orthogonal to the appropriate timelike Killing vector field. But that, of course, takes us far beyond "B" level. I mention it only to show that the "naturalness" of this convention does have some basis in an invariant geometric property of the spacetime; it isn't purely a matter of preference for a coordinate choice.)