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Rethinking the origins of the universe

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Re: Rethinking the origins of the universe

#81

Post by aroberge »

phkahler wrote: post #18

Now that all the physicists are reading, I have a question: Are the trajectories of light beams following the curvature of space-time reversible? In other words, if I reflect a light beam back on itself, does it return to its source? Because if that's true (and my feeble understanding is that it is true), then light can't enter a black hole or the reverse path would allow light to escape. What am I missing here?

Physicist here. Imagine a fish swimming down a river. Down the river, there is a water fall, with water falling faster than the fish can swim. Past the point where the water is falling faster than the fish can swim, the fish can not turn back and swim up the waterfall, back into the upper part of the river.

The same thing happens in spacetime. As you get passed the event horizon, there is no way to move through spacetime so as to emerge back through it.

Re: Rethinking the origins of the universe

#82

Post by kijin »

lutusp wrote: post #60
StefanKarpinski wrote: post #32

This paper claims that black holes can't come into existence, which is subtly different from claiming that they cannot exist. In particular, black holes can exist as long as they've always existed – it's just that new ones can't come into existence. Maybe the universe just has a fixed number of singularities. [I am not a physicist – this just struck me as an inaccurate implication in the article.]

> 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 singularity that ever existed was the Universe at the moment of the Big Bang. After the One Singularity has blown itself apart, no other singularity can come into existence. I don't see any inconsistency in that scenario.

Re: Rethinking the origins of the universe

#83

Post by alchemism »

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…

And with that, physics discovers Pandeism.

Re: Rethinking the origins of the universe

#84

Post by lisper »

antognini wrote: post #30
phkahler wrote: post #18

Now that all the physicists are reading, I have a question: Are the trajectories of light beams following the curvature of space-time reversible? In other words, if I reflect a light beam back on itself, does it return to its source? Because if that's true (and my feeble understanding is that it is true), then light can't enter a black hole or the reverse path would allow light to escape. What am I missing here?

If the spacetime is static, then yes, the trajectory of a light beam (or anything else) is reversible. The question you pose is an excellent one and is one of the reasons that many physicists believed that black holes could not exist after Karl Schwarzschild discovered the Schwarzschild metric that describes spacetime around a spherically symmetric black hole. So what happens if you direct a beam of light radially in…

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.

Re: Rethinking the origins of the universe

#85

Post by squozzer »

tjradcliffe wrote: post #14
AnimalMuppet wrote: post #9

If I understand correctly, black holes do not play a large role in our model of the origins of the universe. So this doesn't (directly) lead to "rethinking the origins of the universe". Does this work call the big bang into question (or even our current models of the big bang)? Or is it just details like galaxy formation that it makes us rethink? Another question: Something very massive and very small is at the cente…

Your first point is correct: the relationship between black holes and the initial state of the universe is not clear. Whether or not there was a primordial singularity or something else is an open question. With regard to the question "If not black holes, then what?", this is a problem. There is a strict upper limit on neutron stars of about 1.4 solar masses. Beyond this, gravity is stronger than the repulsive core o…

So it seems a cycle of collapse and Hawking radiation will occur until the star no longer has enough mass to collapse.

Re: Rethinking the origins of the universe

#86

Post by acjohnson55 »

lutusp wrote: post #71
dhimes wrote: post #50

Earlier quoted context omitted.

I imagine that the metaphor "black hole" will be here for a while. After all, we still use the term "big bang" for the 'first moment' of the creation of the universe, even though the "Big Bang Theory" isn't generally believed any longer.

> even though the "Big Bang Theory" isn't generally believed any longer. Really? Given that science doesn't turn on belief but evidence, still, the Big Bang remains the best explanation for existing evidence. It's the prevailing cosmological theory of the universe' beginnings. Like all scientific theories, it's subject to replacement as new evidence appears and as new theories are crafted, but it's a good match at th…

I think it's a bit naive to say that science doesn't turn on belief. Scientist aren't robots, mechanically and dispassionately ingesting evidence and spitting out theory. They're humans with imaginations, beliefs, agendas, and egos, guided (hopefully) by a methodology that leads to discovery. I think belief is what drives science, and it's not all that bad of a thing.

Re: Rethinking the origins of the universe

#88

Post by deciplex »

lutusp wrote: post #25
z3t4 wrote: post #11

What many people seem to overlook is that gravity is linked to time (space time)! So instead of the "black hole" being massive, it could be that time is running fast!

> So instead of the "black hole" being massive, it could be that time is running fast! I would have put it the other way around. Time passes more slowly near large masses relative to other locations.

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?

Re: Rethinking the origins of the universe

#89

Post by chakademus »

linguafranca wrote: post #87

My trust in metaphysical and other such sciences dropped significantly on the day I realized that most modern conclusions are just theories and half of them have already been proven wrong.

metaphysics: the branch of philosophy that deals with the first principles of things, including abstract concepts such as being, knowing, substance, cause, identity, time, and space.

astrophysics: the branch of astronomy concerned with the physical nature of stars and other celestial bodies, and the application of the laws and theories of physics to the interpretation of astronomical observations.

Much of the nature of the cosmos is indeed a mystery, but that doesn't diminish the importance of the multitude of things we've learned about our universe from various physical observations and measurements.

Re: Rethinking the origins of the universe

#90

Post by calinet6 »

linguafranca wrote: post #87

My trust in metaphysical and other such sciences dropped significantly on the day I realized that most modern conclusions are just theories and half of them have already been proven wrong.

That is science at its best. What, indeed, is the alternative to admitting we were wrong when confronted with new and more accurate information? There is no "trust" in science, there is only truth, and you must always leave room for doubt.

Astrophysics appears volatile simply because there is so much we do not know, but that is also what makes it fascinating, and of critical importance.

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