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Dead Stars Don’t Radiate

johncarlosbaez.wordpress.com

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Re: Dead Stars Don’t Radiate

#141

Post by verzali »

deepsun wrote: post #81

Should it be a big embarrassment for Phys. Rev. Lett., a big dip in their reputation? The whole point of respectable journals is that they filter out bad quality papers.

That should be the real issue.

Bluntly, science journalism is not that well paid and most people won't have given the articles he calls out more than a moments thought. Of course journalists are going to be lazy if they aren't paid not to be. (Which is a problem itself, but I don't see many people advocating for better paid science journalists).

But journals should be a kind of guard rail against that. If their peer reviewers were too lazy to find experts or too arrogant to admit they didn't understand what they were reviewing, then that is a real problem.

Re: Dead Stars Don’t Radiate

#142

Post by verzali »

NKosmatos wrote: post #66

Ah yes, our favorite HN “entertainment”. Scientists, quantum physicists in our case, having a beef about Hawking radiation :-) Besides some high level ideas, which even us normal people can understand, there are so many details linked in the original post that you need an MSc/PhD to fully understand them. For the time being, let’s just keep that the universe has a few extra trillion years, and isn’t expected to decay…

A few extra trillion years would look like a drop of water against the ocean that is 10⁷⁸ years...

Re: Dead Stars Don’t Radiate

#143

Post by bencyoung »

ldunn wrote: post #139
bencyoung wrote: post #138

Earlier quoted context omitted.

You can do plenty of experiments to see if you are falling, e.g. hitting the surface of a planet you are falling towards. The event horizon is a surface like any other with a location in space and you can definitely see when you hit it (it's the bit where no light is coming out). And once you've crossed it, literally no EM radiation can move further from the singularity

I agree that if you are freely falling and then you are suddenly not freely falling because you hit the surface of a planet and experienced a huge acceleration, you will notice. That doesn't have anything to do with anything I said, but it is undeniably true. An event horizon is not like the surface of a planet - you will not be accelerated as you pass through it. It is, once again, irrelevant that light cannot propa…

Well, I disagree. Light literally can't move in a direction that makes it further from the singularity once inside the event horizon. I don't see what space being flat or not locally has to do with that. Check https://en.wikipedia.org/wiki/Event_horizon#/media/File:BH-n... for an example.

If your head is further from the singularity than your feet then you can't see them.

Happy not to discuss further!

Re: Dead Stars Don’t Radiate

#144

Post by ldunn »

bencyoung wrote: post #143
ldunn wrote: post #139

Earlier quoted context omitted.

I agree that if you are freely falling and then you are suddenly not freely falling because you hit the surface of a planet and experienced a huge acceleration, you will notice. That doesn't have anything to do with anything I said, but it is undeniably true. An event horizon is not like the surface of a planet - you will not be accelerated as you pass through it. It is, once again, irrelevant that light cannot propa…

Well, I disagree. Light literally can't move in a direction that makes it further from the singularity once inside the event horizon. I don't see what space being flat or not locally has to do with that. Check https://en.wikipedia.org/wiki/Event_horizon#/media/File:BH-n... for an example. If your head is further from the singularity than your feet then you can't see them. Happy not to discuss further!

It doesn't have to move in such a direction! Look at a spacetime diagram and think about the trajectory of your head and feet! Read a book on GR! Do literally anything except have strong opinions about GR when you don't know any GR!

Re: Dead Stars Don’t Radiate

#145

Post by bencyoung »

ldunn wrote: post #144
bencyoung wrote: post #143

Earlier quoted context omitted.

Well, I disagree. Light literally can't move in a direction that makes it further from the singularity once inside the event horizon. I don't see what space being flat or not locally has to do with that. Check https://en.wikipedia.org/wiki/Event_horizon#/media/File:BH-n... for an example. If your head is further from the singularity than your feet then you can't see them. Happy not to discuss further!

It doesn't have to move in such a direction! Look at a spacetime diagram and think about the trajectory of your head and feet! Read a book on GR! Do literally anything except have strong opinions about GR when you don't know any GR!

Apart from my MSci in Physics... Perhaps you could post some links to the spacetime diagrams you are talking about?

Re: Dead Stars Don’t Radiate

#146

Post by mr_toad »

bencyoung wrote: post #143
ldunn wrote: post #139

Earlier quoted context omitted.

I agree that if you are freely falling and then you are suddenly not freely falling because you hit the surface of a planet and experienced a huge acceleration, you will notice. That doesn't have anything to do with anything I said, but it is undeniably true. An event horizon is not like the surface of a planet - you will not be accelerated as you pass through it. It is, once again, irrelevant that light cannot propa…

Well, I disagree. Light literally can't move in a direction that makes it further from the singularity once inside the event horizon. I don't see what space being flat or not locally has to do with that. Check https://en.wikipedia.org/wiki/Event_horizon#/media/File:BH-n... for an example. If your head is further from the singularity than your feet then you can't see them. Happy not to discuss further!

I was considering a stationary observer inside the event horizon, but that’s not possible. ldunn is correct that a free-falling observer will catch up with the photons reflected from their feet.

Space being flat locally is important because if the gravitational gradient is too high (i.e. you get too close to the singularity) your feet will be accelerated much faster than your head.

Re: Dead Stars Don’t Radiate

#147

Post by floxy »

ldunn wrote: post #144
bencyoung wrote: post #143

Earlier quoted context omitted.

Well, I disagree. Light literally can't move in a direction that makes it further from the singularity once inside the event horizon. I don't see what space being flat or not locally has to do with that. Check https://en.wikipedia.org/wiki/Event_horizon#/media/File:BH-n... for an example. If your head is further from the singularity than your feet then you can't see them. Happy not to discuss further!

It doesn't have to move in such a direction! Look at a spacetime diagram and think about the trajectory of your head and feet! Read a book on GR! Do literally anything except have strong opinions about GR when you don't know any GR!

So when will we be able to just run general relativity numerical simulations on our desktop machines? So that you could set up Observer A at some point, and Observer B at some other point and and mass distribution, etc, then just crunch the numbers to see what each observer could see/measure as time evolves for each observer. Seems like the differential equations are straight forward enough(?). Is the possibility of singularities at the center of a black hole the hard part? What if you just simulated something that was 99.99% of the density needed to get a black hole? I suppose that you'd need a 4 dimensional matrix to hold the simulation (three space coordinates plus a time coordinate)? Is it that we just don't have enough RAM and storage yet in consumer machines? If your simulation did a 1,000 points in ever dimension, that would be 1e12 points. If there are 10 components of the tensor at each point and we are using 64-bit doubles per parameter, that means our simulation takes up ~80 TB. Or is it a that we don't have enough processing speed? Or are there still some philosophical issues that need to be decided when you program up the simulator? How many lines of code is a numerical general relativity solver using something like Euler's method? Is the core of a naive version less than, say 500 lines of C? I can see an optimized CUDA version being significantly larger of course.

Re: Dead Stars Don’t Radiate

#148

Post by ldunn »

bencyoung wrote: post #145
ldunn wrote: post #144

Earlier quoted context omitted.

It doesn't have to move in such a direction! Look at a spacetime diagram and think about the trajectory of your head and feet! Read a book on GR! Do literally anything except have strong opinions about GR when you don't know any GR!

Apart from my MSci in Physics... Perhaps you could post some links to the spacetime diagrams you are talking about?

The diagram on the Wikipedia page for Kruskal-Szekeres coordinates[1] does the job. There you see the trajectory of some infalling observer along with some future light cones[2] of points along that trajectory and the event horizon marked as the dashed line. The usual Schwarzschild r and t coordinates are also shown as the pale hyperbolas.

Say the trajectory that's drawn on the diagram is the trajectory of your feet. Now consider a second trajectory which begins slightly displaced "outwards" (that is, rightwards at t=0 on the diagram) from this first one - that's your head. Hopefully you agree that the head-trajectory would have to do something pretty strange to avoid crossing through the future lightcone of your feet, even behind the horizon. This doesn't require signals from your feet to travel "outward" - it's just that your head is travelling "inward".

K-S coordinates make it pretty clear that nothing drastic happens to the structure of spacetime at the event horizon - everything is perfectly regular. It's just that once you cross the horizon, the singularity (the thick hyperbola at the top of the diagram) is inevitably in your future: there is no trajectory within any future lightcone behind the horizon that doesn't run into the singularity. You're doomed to run into it in finite time, and all your future lightcones lie entirely behind the horizon.

[1]: https://en.wikipedia.org/wiki/Kruskal%E2%80%93Szekeres_coord...

[2]: A useful feature of K-S coordinates is that lightcones are always at +-45 degrees

Re: Dead Stars Don’t Radiate

#149

Post by floxy »

jpmattia wrote: post #132
nothrabannosir wrote: post #131

Earlier quoted context omitted.

Purely out of pedantic interest: is that a meaningful distinction, or is it just the same thing for a different audience? I'm reminded of chess youtubers who give similarly "click baity" titles to their videos which are only click bait to people who watch chess videos. Isn't it the same? All the power to them by the way. It's the crushing power of the algorithm. No hard feelings, just something I've been wondering.

Well, you got me thinking about "What exactly is clickbait?" So full disclosure: I've directly interacted with John Carlos Baez only in social media, with the topics as disparate as music and observational astronomy. My own QFT & GR background is grad course level but with little actual usage in my career. (I've done more solid-state + high-speed electronics work, with a bunch of programming as well.) With that backg…

I hope he recovers from the stock market not crashing

Re: Dead Stars Don’t Radiate

#150

Post by nabla9 »

quantadev wrote: post #37
nabla9 wrote: post #35

Earlier quoted context omitted.

> I'm someone who thinks the 'surface' of an event horizon is where the laws are preserved, I don't think this is a good way to think it. If black hole is big enough, there is nothing strange happening in the event horizon, no significant length contraction, nothing.

Some "infinities" of singularity are at the center sure, but all the maximal Relativistic effects are at the EH surface. It's even proven that the entropy (informational content roughly) is equal to the EH area divided by the number of planc-length square areas, as the amount of quantum arrangements of information that are allowed "inside". That is a HUGE hint everything's remaining on the surface. For example, when…

I think you misunderstand what Holographic principle says.

Even if all information is encoded in two-dimensional surface that forms the Bekenstein bound, that does not mean that anything changes when you cross the area. It only means that radiation which black holes emit can't emit information of the matter it has absorbed.

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