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Rethinking the origins of the universe

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Re: Rethinking the origins of the universe

#101

Post by deciplex »

lutusp wrote: post #99
deciplex wrote: post #88

Earlier quoted context omitted.

FWIW, it's always been easier for me to think of it as 'moving' more quickly along the time axis - which would appear slowed down to someone farther away from the source of the gravity. Perhaps that's what the parent was getting at?

> FWIW, it's always been easier for me to think of it as 'moving' more quickly along the time axis - which would appear slowed down to someone farther away from the source of the gravity. In reality, it's the other way around. At the bottom of a gravity well, time passes more slowly. An astronaut on the moon observing a laser beam from earth's surface would see it as red-shifted compared to a local reference, reflect…

It's not the other way around.

If you're deep in a gravity well, you observe physical processes happening outside that well to go along more quickly, not more slowly. And the reverse is true if you are outside a gravity well, observing something in it.

Take some observer, Alice, far away from a massive body, who sees some process occurring near it take one second to complete. A second observer, Bob, nearer to the massive body, will see the same process take 1 - ε seconds to complete (and Alice will say Bob's clock is slow). In effect, Bob has passed through one second of (Alice) time in only 1 - ε seconds of (local, to Bob) time.

That's what I mean when I say "pass through time / move along the time axis more quickly". To think of it another way, ask who ages faster, Alice or Bob?

Re: Rethinking the origins of the universe

#103

Post by imaginenore »

tericho wrote: post #39

Ethan Siegel just posted a detailed response to this. https://medium.com/starts-with-a-bang/yes-virginia-black-hol...

> If we want to escape its gravitational pull, we’d need to accelerate ourselves up past the escape velocity, or the speed necessary to climb out of the gravitational potential well that Earth’s mass creates. We can (and have) accomplished this, in fact, but it would take a speed of around 11.2 km/s (or 0.004% the speed of light) to make it so.

This whole paragraph is wrong. You don't need to move at the escape velocity to leave our planet, you can move as slow as you wish, as long as you have enough fuel. Escape velocity is just a minimum velocity of a fuel-less projectile enough to leave the gravitational pull.

Re: Rethinking the origins of the universe

#104

Post by trhway »

darkmighty wrote: post #40
trhway wrote: post #33

Ok. To start she addresses only "Gravitationally Collapsing Star"s. Interesting result, may be many star lifecycles don't end in a black hole. The "before-black-hole" state she describes still would involve significant time dilation and thus what we see as Hawking radiation from black hole may just be Hawking radiation from "before-black-hole" star. Looks the same :) The result doesn't say that black hole is impossib…

You seem to be knowledgeable, so is there a plausible bridge between the impossibility of this kind of collapse and complete impossibility of black holes? I mean, what if could construct a collapse to your desire, e.g. smashing dense cold chunks, would those necessarily radiate enough energy to prevent collapse too?

>is there a plausible bridge between the impossibility of this kind of collapse and complete impossibility of black holes?

the shown impossibility of star collapse to BH is somewhat questionable for massive stars because - as i cited wikipedia in another comment - the Hawking radiation carries away less mass/energy than cosmic background brings in - the net is positive for the collapsing star.

>smashing dense cold chunks, would those necessarily radiate enough energy to prevent collapse too?

we associate "collapse/smashing" with BH i think mostly because of collapsing stars. One can scale this process out to a lot of small chunks of cold matter as you suggest flying simultaneously toward common center. Imagine the sphere with radius 1000 times radius of Sun filled with material of density of water - this is already a black hole as its Schwarzschild radius is a bit larger than those 1000 radii of Sun. Now lets say we scale the sphere radius additional 10 times - the mass scales 1000 times and the Schwarzschild radius increase 1000 times too - thus all this water may be separated by a lot of space and still be a black hole. Now imagine that all this water is outside the Schwarzschild radius and spread on some imaginary sphere and simultaneously released to fly toward the center - once all these drops cross the schwarzschild radius, even without touching each other, and with each drop feeling only pretty small gravitation from the rest of the water, they nevertheless become a BH. What kind of miracles would happen in the moment these drops cross the imaginary sphere of Schwarzschild radius? Nothing. The only change here is that any light radiated by the drops close to crossing would reach far observers as a very redshifted and any light radiated at the moment of crossing and after would never be seen by the far observers. The article suggests that the loss of mass due to Hawking radiation would be fast enough to shrink the Schwarzschild radius faster than the drops can fall into. All this cold drops flying in empty cold space would supposedly cause very intense radiation close to the schwarzschild radius border? Nope. As mentioned above in the wikipedia citation Hawking radiation is pretty cold, low intense, for that size of black hole, and in particular it is much colder than CMB, ie. the losses are less than what is brought in by CMB. Though may be in the absence of CMB if the math in the article is right the losses (given the time slowing) would be enough to shrink the Schwarzschild radius fast enough. Yet not in our real Universe with CMB.

Re: Rethinking the origins of the universe

#105

Post by lisper »

antognini wrote: post #91
lisper wrote: post #84

Earlier quoted context omitted.

You have this backwards. To a distant observer, light going into a black hole is red-shifted, not blue-shifted. If this were not the case, sending even a single photon into a black hole would produce infinite energy.

No, any photons that are emitted from matter close to the event horizon and travel toward the distant observer will be redshifted. Photons falling into the potential well of the black hole will be blueshifted. Of course, the distant observer is not able to directly detect photons falling into the black hole because he is far away from it. All he can do is calculate what he would expect the energy of the photons to be…

Ah. OK, I misunderstood what you were saying. This is right.

Re: Rethinking the origins of the universe

#106

Post by wmt »

faizshah wrote: post #36

I'm not at all acquainted with astrophysics but I remember seeing an article state that there was a different physicist who proved this same thing: https://news.ycombinator.com/item?id=7191324 A quick google shows his response to this: https://twitter.com/abhasmitra/status/514804101024997376 His response: "Hawking radiation is unproven. And particle physicists are not aware that even without any Quantum Mechanics, cl…

The Quora reply was very illuminating: the astrophysicist mentions that Einstein and Eddington already suggested the same solution as Mitra did long before him, but most people today don't think it's the correct one. I guess Einstein also lacked the CAMBRIDGE AFFILIATION to get any recognition for that solution.

Re: Rethinking the origins of the universe

#107

Post by eyko »

kijin wrote: post #82
lutusp wrote: post #60

Earlier quoted context omitted.

> In particular, black holes can exist as long as they've always existed ... This doesn't work in physics. If you say they can only exist if they've always existed, then you're obliged to say how they came into existence earlier -- the deep meaning of "always".

The article implies that since singularities can't come into existence now, there can't have been a singularity at the moment of the Big Bang, either. That sounds implausible. Lots of things that cannot possibly come into existence at the present time did exist at the moment of the Big Bang (and shortly afterward), because the conditions back then were so radically different from they are now. Perhaps the only singul…

There is the idea that there could be multiple, infinite number of singularities (and therefore universes).

Re: Rethinking the origins of the universe

#108

Post by kijin »

lutusp wrote: post #98
kijin wrote: post #82

Earlier quoted context omitted.

The article implies that since singularities can't come into existence now, there can't have been a singularity at the moment of the Big Bang, either. That sounds implausible. Lots of things that cannot possibly come into existence at the present time did exist at the moment of the Big Bang (and shortly afterward), because the conditions back then were so radically different from they are now. Perhaps the only singul…

> The article implies that since singularities can't come into existence now, there can't have been a singularity at the moment of the Big Bang, either. Yes, that's true, but if the paper's thesis is correct, then there aren't any black holes or singularities at all, at any time. If the thesis is false, then there can be black holes at the universe's beginning, and they can form today, given the right conditions. > L…

No, the article only claims that a singularity cannot arise from a collapsing blob of matter.

We have no evidence that the singularity that existed at the moment of the Big Bang was the result of a collapsing blob of matter. Therefore, we have no reason to think that it could not have occurred.

In fact, it might not even make sense to think of that initial singularity as the result of anything at all, since causality requires time and there was no time before the Big Bang. Time is asymmetric, and this is especially important when we're talking about t = 0. You cannot simply imitate it from the other direction.

Re: Rethinking the origins of the universe

#109

Post by jorgis »

vorg wrote: post #29
tfgg wrote: post #13

"In 1974, Stephen Hawking used quantum mechanics to show that black holes emit radiation. Since then, scientists have detected fingerprints in the cosmos that are consistent with this radiation, identifying an ever-increasing list of the universe’s black holes." Er, what? I don't think this is true, or Hawking would have his Nobel prize by now. Maybe consistent in the sense that the predictions are that Hawking radia…

> "Stephen Hawking used quantum mechanics to show that black holes emit radiation." I don't think this is true, or Hawking would have his Nobel prize by now Hawking not getting a Nobel by now doesn't mean much. Einstein never got one for relativity (either Special or General), and when he did get his Nobel he was told not to mention "relativity" in his speech.

Einstein guy his for his work with Brownian Motion. Nobel prizes aren't typically given for science that lacks a body of supporting evidence, which was the case at the time for Relativity.

Re: Rethinking the origins of the universe

#110

Post by aruggirello »

scott_s wrote: post #51
tericho wrote: post #39

Ethan Siegel just posted a detailed response to this. https://medium.com/starts-with-a-bang/yes-virginia-black-hol...

I don't think that is a response to this work. That is dated February 1. He links to this paper's author, Laura Mersini-Houghton, but he links to a paper and blog post of hers from January. Reading that blog post ( http://backreaction.blogspot.com/2014/01/if-it-quacks-like-b... ), what she says seems to be in contradiction with the more recent paper she wrote ( http://arxiv.org/abs/arXiv:1409.1837 ). In her blog post…

I'm confused here. The mere existence of an event horizon - albeit a temporary one - should make a black hole! So "More specifically, we find that collapsing stars slow down their collapse right outside their horizon," by implying that a horizon DOES exist, simply doesn't cut it. Am I missing something?
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