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The Nobel Prize in Physics 2020

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Re: The Nobel Prize in Physics 2020

#41

Post by kryptiskt »

psychoslave wrote: post #32
sharpneli wrote: post #13

Earlier quoted context omitted.

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 i…

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.

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.

Re: The Nobel Prize in Physics 2020

#42

Post by T-A »

frereubu wrote: post #9
tchalla wrote: post #2

The press release has a popular science background - https://www.nobelprize.org/uploads/2020/10/popular-physicspr... Highly recommend the illustrations on the press release for a quick overview Cross section of a black hole (pdf) - https://www.nobelprize.org/uploads/2020/10/fig2-phy-en-cross... The Milky Way - https://www.nobelprize.org/uploads/2020/10/fig3-phy-en-milky... Stars closest to the centre of the Milky Way…

An illustration in the press release contains this phrase: "When a massive star collapses under its own gravity, it forms a black hole that is so heavy that it captures everything that passes its event horizon. Not even light can escape. At the event horizon, time replaces space and points only forward. The flow of time carries everything towards a singularity furthest inside the black hole, where density is infinite…

Here's a simple explanation, with pictures:

http://visualrelativity.com/LIGHTCONE/schwarzschild.html

And here's a more technical one:

https://en.wikipedia.org/wiki/Penrose_diagram#Black_holes

Re: The Nobel Prize in Physics 2020

#43

Post by pacman128 »

immmmmm wrote: post #17
jb1991 wrote: post #7

> Einstein did not himself believe that black holes really exist Wow.

Einstein did not believe in gravitational waves either. He also famously added the cosmological constant because he didn't like the idea of an expanding universe.

Do you have a source for this? I thought they were a direct prediction of GR and this link says the same: https://www.ligo.caltech.edu/page/what-are-gw

Re: The Nobel Prize in Physics 2020

#44

Post by stallmanite »

sharpneli wrote: post #13
frereubu wrote: post #9

Earlier quoted context omitted.

An illustration in the press release contains this phrase: "When a massive star collapses under its own gravity, it forms a black hole that is so heavy that it captures everything that passes its event horizon. Not even light can escape. At the event horizon, time replaces space and points only forward. The flow of time carries everything towards a singularity furthest inside the black hole, where density is infinite…

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 i…

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?

Re: The Nobel Prize in Physics 2020

#45

Post by cb321 »

fsnowdin wrote: post #38
nabla9 wrote: post #29

Earlier quoted context omitted.

We already know it because Genzel and Ghez discovered it. Their work goes back 30 years.

When I heard about this, I seriously thought we just only discovered Sagittarius A* this year and that I must have fucked up my memories. Thanks for the clarification!

The Penrose-Hawking singularity theorems go back 55 years to work starting in 1965. No joke. This has be to near a record in time lag between the work the Nobel committee recognized and when that work was first published. Better late than never I guess (but too late for Hawking, as has been mentioned elsewhere..) EDIT - a detailed analysis: https://physicstoday.scitation.org/do/10.1063/PT.5.2012/full...

Re: The Nobel Prize in Physics 2020

#46

Post by pa7x1 »

stallmanite wrote: post #44
sharpneli wrote: post #13

Earlier quoted context omitted.

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 i…

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?

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.

Re: The Nobel Prize in Physics 2020

#47

Post by sharpneli »

stallmanite wrote: post #44
sharpneli wrote: post #13

Earlier quoted context omitted.

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 i…

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?

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.

Re: The Nobel Prize in Physics 2020

#50

Post by ISL »

xioxox wrote: post #30
stargazer-3 wrote: post #25

Earlier quoted context omitted.

Back in the day they each led research to detect irrefutable evidence of it. This isn't anything new and groundbreaking.

I think that's rather unfair. The measurements are still ongoing with ever-increasing accuracy. In 2018 they managed to record the closest approach of the star S2, measuring a velocity of 7000 km/s at its maximum. This was after mapping the full 16 year orbit. I know that Genzel is still actively researching this topic with his group, see e.g. https://www.mpe.mpg.de/7433286/news20200416

Ghez is very active. Both her group and Genzel's group are at the very leading edge.

I haven't found an updated GIF, but even when I first saw images like this around 2002, it was clear she was chasing something awesome.

https://www.physicscentral.com/explore/people/images/2008orb...

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