B Is Time Travel Possible Near Black Holes?

[L Drago]

TL;DR Summary

In GR time dilation there is a space time fabric. The more the mass the more the curvature in space time the more the gravity and slower the time. Time dilation above event horizon of black hole.

Time for the observer near black hole = Time for the observer near earth x square root of (1 - (2Gm)/(rc²)).

But time for that observer near black hole will still tick normally in clock will not stop. But several hours would have passed in the earth.

If I have any misconceptions or misunderstanding kindly point it out and explain it to me.

 

[Ibix]

What is your purpose with all these questions?

If you want to learn a random set of facts about specific scenarios in relativity then yes, what you wrote is close enough. If you want to learn relativity well enough to use it then no, your whole post is based on an array of misconceptions and you need to learn at least a text book of material on special relativity before answers to general relativity questions will start to make sense.

 

[L Drago]

What is your purpose with all these questions?

If you want to learn a random set of facts about specific scenarios in relativity then yes, what you wrote is close enough. If you want to learn relativity well enough to use it then no, your whole post is based on an array of misconceptions and you need to learn at least a text book of material on special relativity before answers to general relativity questions will start to make sense.

I am trying to clarify whether what I have written in this post is Correct or not as I am just exploring relativity and now not reading a detailed textbook as I am just a seventh grader.

 

[L Drago]

@PeroK Is the information described by me correct. I case there is still some misunderstanding kindly clear my mistakes.

 

[Ibix]

I am trying to clarify whether what I have written in this post is Correct or not as I am just exploring relativity and now not reading a detailed textbook as I am just a seventh grader.

That doesn't really answer the question, so I don't think I can do better than my last reply. It's correct in the very specific case of a clock hovering at a fixed altitude over a non-rotating eternal black hole in an otherwise empty universe and being compared to another clock at rest at infinity. In most other cases the notion of "gravitational time dilation" isn't well defined and your question doesn't have a clear answer - you need to use an entirely different approach. The maths you wrote may get you an answer that's good enough even in cases where it doesn't really apply, but without knowing why you want to know I can't say for certain.

 

[PeroK]

@PeroK Is the information described by me correct. I case there is still some misunderstanding kindly clear my mistakes.

As @Ibix indicates, you can't learn SR, GR and QM all at once. I'd leave GR for now and focus on SR.

 

[L Drago]

As @Ibix indicates, you can't learn SR, GR and QM all at once. I'd leave GR for now and focus on SR.

Okay so let me focus on SR first. Thanks @Ibix and @PeroK

 

[PeroK]

Okay so let me focus on SR first. Thanks @Ibix and @PeroK

For example, in James Hartle's book "Gravity: An Introduction to GR", the formula you quoted appears in Chapter 9(!). There are eight chapters of essential prerequisites before he tackles the Schwarzschild Black Hole geometry. Whereas, if you look at Morin's SR book that I linked to, he introduces the important aspects of SR immediately, in chapter 1.

 

[phinds]

In GR time dilation there is a space time fabric.

No, there is not. "fabric" is a very unfortunate description that started with a comment by Einstein but which is not found in serious texts. It is a pop-sci description. Space-Time is NOT a "fabric" which would be a thing, like an apple or a sheet of paper. Space-Time is geometry. Period. It is a place where physical objects and waves interact with each other. It is not a physical object itself.

You REALLY need to stop asking questions based on pop-sci "information". Start studding the actual science and then when you have a question cite the specific place where you got information that confuses you.

 

[L Drago]

No, there is not. "fabric" is a very unfortunate description that started with a comment by Einstein but which is not found in serious texts. It is a pop-sci description. Space-Time is NOT a "fabric" which would be a thing, like an apple or a sheet of paper. Space-Time is geometry. Period. It is a place where physical objects and waves interact with each other. It is not a physical object itself.

You REALLY need to stop asking questions based on pop-sci "information". Start studding the actual science and then when you have a question cite the specific place where you got information that confuses you.

Means whatever space time fabric I read from internet conluntless articles and artificial intelligence were wrong.

 

[L Drago]

For example, in James Hartle's book "Gravity: An Introduction to GR", the formula you quoted appears in Chapter 9(!). There are eight chapters of essential prerequisites before he tackles the Schwarzschild Black Hole geometry. Whereas, if you look at Morin's SR book that I linked to, he introduces the important aspects of SR immediately, in chapter 1.

I think I need to read James Hartle's book first to first have a grasp of basics of actual science and not be misguided by pop Sci as I am a seventh grader. Thanks @PeroK

 

[PeroK]

I think I need to read James Hartle's book first to first have a grasp of basics of actual science and not be misguided by pop Sci as I am a seventh grader. Thanks @PeroK

Hartle's book is advanced undergraduate, so you have to be patient and learn SR first. Two major prerequisites for learning GR are SR and Vector/Tensor Calculus.

Spacetime fabric is popular science speak. Like relativistic mass.

As you now know, proper textbooks are the only way to learn (although you can also find genuine university lectures online - they are another option). I think we'd all prefer to see you post specific questions about passages from a textbook; or, homework style questions or exercise, where you are actually trying to solve specific problems. That's the only way to learn actual physics.

 

[Orodruin]

As @Ibix indicates, you can't learn SR, GR and QM all at once. I'd leave GR for now and focus on SR.

Let’s also note that there is a reason there are no pre-university courses on GR …

Means whatever space time fabric I read from internet conluntless articles and artificial intelligence were wrong.

Stop this immediately if you actually want to learn the subjects and not just random and often misstated facts of facts that are only valid in very specific cases.

AI is particularly troubling.

 

[Frabjous]

as I am a seventh grader.

For an ambitious seventh grader, I would recommend looking at Gravity from the Ground Up by Schutz and General Relativity from A to B by Geroch. You might also look at A Journey into Gravity and Spacetime by Wheeler (only available used). They could be too advanced for you.

 

[L Drago]

For an ambitious seventh grader, I would recommend looking at Gravity from the Ground Up by Schutz and General Relativity from A to B by Geroch. You might also look at A Journey into Gravity and Spacetime by Wheeler (only available used). They could be too advanced for you.

Thank you

 

[phinds]

Means whatever space time fabric I read from internet conluntless articles and artificial intelligence were wrong.

Exactly. They are WRONG.

You have GOT to stop this nonsense of posting what you think you know based on pop-sci presentations. Learn some actual physics.

 

[L Drago]

Exactly. They are WRONG.

You have GOT to stop this nonsense of posting what you think you know based on pop-sci presentations. Learn some actual physics.

Means whatever I learnt till date by spending so many days watching videos was wrong. Now I have to read some real textbooks. I did not know internet and AI are so misleading

 

[FactChecker]

You are very good for a seventh grade student. One book that I would recommend for an introduction is Chad Orzel's "How To Teach Relativity To Your Dog". Please don't be offended by the title. It is entertaining, does not get into tedious math, and addresses many questions and misconceptions. If it seems too elementary for you, you can still breeze through it quickly and move on to more advanced material.

 

[PeterDonis]

In GR time dilation there is a space time fabric. The more the mass the more the curvature in space time the more the gravity and slower the time. Time dilation above event horizon of black hole.

This is not correct as you state it. Time dilation is not the same as spacetime curvature, nor is there a single direct relationship between the two.

You would do better to stick to the very simple statement you make next:

Time for the observer near black hole = Time for the observer near earth x square root of (1 - (2Gm)/(rc²)).

This is a reasonable statement (if we neglect the time dilation due to the Earth itself, and for that matter due to the Sun and the Milky Way galaxy and any other gravity wells that we are in but the black hole is not in--but those are minor corrections). But notice that there is nothing in there about "space time fabric", nor does spacetime curvature appear anywhere in the formula.

Also note that, as @Ibix said, this formula is only valid for one very specific type of observer, an observer who is "hovering" at rest at a fixed altitude above the hole.

time for that observer near black hole will still tick normally in clock will not stop.

Yes, that is correct.

several hours would have passed in the earth.

To make this comparison you need to do one of two things:

(1) Have the observer travel down close to the hole, hover there for a while, and then come back up and return to the Earth, and compare the elapsed time on his clock with the elapsed time on Earth clocks.

(2) Have the observer hovering close to the hole exchange round-trip light signals with Earth, and compare the round trip elapsed time on his clock with the round trip elapsed time on Earth clocks.

 

[L Drago]

You are very good for a seventh grade student. One book that I would recommend for an introduction is Chad Orzel's "How To Teach Relativity To Your Dog". Please don't be offended by the title. It is entertaining, does not get into tedious math, and addresses many questions and misconceptions. If it seems too elementary for you, you can still breeze through it quickly and move on to more advanced material.

Thank you

 

[L Drago]

This is not correct as you state it. Time dilation is not the same as spacetime curvature, nor is there a single direct relationship between the two.

You would do better to stick to the very simple statement you make next:


This is a reasonable statement (if we neglect the time dilation due to the Earth itself, and for that matter due to the Sun and the Milky Way galaxy and any other gravity wells that we are in but the black hole is not in--but those are minor corrections). But notice that there is nothing in there about "space time fabric", nor does spacetime curvature appear anywhere in the formula.

Also note that, as @Ibix said, this formula is only valid for one very specific type of observer, an observer who is "hovering" at rest at a fixed altitude above the hole.


Yes, that is correct.


To make this comparison you need to do one of two things:

(1) Have the observer travel down close to the hole, hover there for a while, and then come back up and return to the Earth, and compare the elapsed time on his clock with the elapsed time on Earth clocks.

(2) Have the observer hovering close to the hole exchange round-trip light signals with Earth, and compare the round trip elapsed time on his clock with the round trip elapsed time on Earth clocks.

If we send an observer near Event Horizon of black hole let them hover and make them come back on Earth. I have cleared my misconception that time does not stop for them, it ticks normally but several hours or days would have passed on Earth and when they will return, they would have time travelled to future. Kindly correct me if I am wrong.

 

[jbriggs444]

they would have time travelled to future. Kindly correct me if I am wrong.

We all "time travel" to the future all the time.

The amount of time recorded on our wrist watches during the trip may not always match the difference in time coordinates between the starting and ending events.

The time recorded on ones own wristwatch is known as "proper time"

The time kept by an array of clocks all synchronized and at rest in an inertial coordinate system is known as "coordinate time".

 

[PeterDonis]

If we send an observer near Event Horizon of black hole let them hover and make them come back on Earth. I have cleared my misconception that time does not stop for them, it ticks normally but several hours or days would have passed on Earth

Probably more like many, many years, given how far away the closest black hole from Earth is likely to be. But we can imagine a large starship, say, in an orbit far enough away from a black hole that time dilation is negligible, but close enough so the travel time down to the hole is not very long, sending a small exploring shuttle to go down near the horizon, hover for a while, and then come back to the starship. For a scenario like that, the shuttle's clock might only show a few hours elapsed while the starship clock when the shuttle returned would show many days.

when they will return, they would have time travelled to future.

Only in a very trivial sense.