[HN Gopher] Researcher suggests that gravity can exist without mass
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Researcher suggests that gravity can exist without mass
Author : bookofjoe
Score : 35 points
Date : 2024-06-09 19:16 UTC (3 hours ago)
(HTM) web link (phys.org)
(TXT) w3m dump (phys.org)
| monocasa wrote:
| Haven't we known about massless particles warping spacetime for a
| while, hence how kugelblitzes work?
|
| It's not just mass that warps spacetime, but mass or energy.
| mr_mitm wrote:
| We've known for over one hundred years that all forms of energy
| influence the curvature of spacetime: mass, radiation,
| momentum, pressure. The source of the gravitational field is
| literally called the energy-stress-tensor.
|
| This, however, is about topological defects.
| monocasa wrote:
| It concludes with
|
| > "But it is the first proof that gravity can exist without
| mass."
| mr_mitm wrote:
| It's wrong
| monocasa wrote:
| Right, and it's a direct quote from the author of the
| paper in question.
| mr_mitm wrote:
| I know. He must be oversimplifying. It's pretty basic
| stuff, the way it's phrased it's definitely wrong.
| Valgrim wrote:
| Here's what I understand of this, correct me if I'm wrong.
|
| If 1 kg of matter and 1 kg of antimatter annihilate each other,
| their entire mass is converted into photons, which are massless.
| If said photons are entirely absorbed by a black hole, the mass
| of the black hole will increase by 2 kg, and so will the
| gravitational pull of the black hole.
|
| In that case, wouldn't it be safe to assume that photons, even if
| they're massless, create a gravitational field equivalent to
| their energy level?
| monocasa wrote:
| Yep
|
| https://en.wikipedia.org/wiki/Kugelblitz_(astrophysics)
| dustingetz wrote:
| iiuc the photons need to be confined (e.g. by a mirror box) for
| the _box-photon system_ to exhibit mass greater than the sum of
| its constitutents
| mitthrowaway2 wrote:
| What better box is there than a black hole event horizon?
| thfuran wrote:
| You can even theoretically make a black hole out of nothing but
| photons, though I don't think there's any plausible process
| that would ever put enough photons in one place to form a
| kugelblitz.
| hughesjj wrote:
| Possibly during the big bang era?
| mitthrowaway2 wrote:
| Is there a theoretical reason that a sufficiently-gigantic
| ultrafast pulse laser with a sufficiently-massive lens
| couldn't put enough photons at the focal point?
| ben_w wrote:
| It's a practical constraint rather than a theoretical one.
|
| If your photons are 1 nm wavelength, you're making a black
| hole of at least 3.37e17 kg:
|
| http://www.wolframalpha.com/input/?i=0.5nm%20%2A%20c%5E2%20
| %...
|
| Which is about 3e34 joules.
|
| The longer the wavelength, the more energy you need.
|
| You have to also make sure they're timed precisely enough
| to all be within the (target) horizon at the same time.
|
| IIRC there's also a quantum mechanical limit on energy
| density distributions (I have a lower confidence feeling
| that this is Heisenburg uncertainty?), but I don't know
| enough about it to get Wolfram Alpha to calculate it for
| me.
| fsmv wrote:
| Einstein's equation includes all forms of energy not just rest
| mass. Photons have 0 rest mass not 0 energy.
| zackmorris wrote:
| Yes, the effective mass of a photon is determined by E=mc^2
| even though it has no mass, but it's sort of by definition
| because nothing with mass can reach the speed of light.
|
| Also anything moving at the speed of light doesn't perceive the
| passage of time, because from its perspective we're so time-
| dilated that the universe comes and goes in an instant as it
| crosses that expanse. So an argument could be made that only
| one photon exists, forming a frozen 4D crystal with the shape
| of every photon's path through the universe.
|
| A black hole's average density decreases by increasing radius.
| So theoretically a black hole could be formed from a gas like
| air, and in fact IRAS 20100-4156 with a mass of 3.8 billion
| suns and a diameter of about 10 billion km (according to the
| video below) has the same density as air:
|
| https://www.youtube.com/watch?v=71eUes30gwc
|
| Also from the video, if we take the average density of the
| universe and consider it a uniformly distributed gas, we find
| that it fits within a black hole 10 times larger than the
| radius of the visible universe, which is 45 billion light
| years. So I believe that we're inside of a giant black hole and
| that the expansion of the universe is driven by Hawking
| radiation at the event horizon causing it to evaporate, which
| lowers the radius, which from our perspective looks like
| galaxies slipping away from us faster than the speed of light
| as they pass that velocity due to the Hubble constant
| multiplied by that distance.
|
| So technically a black hole could be created by just photons
| with a mass energy equivalence similar to air. It would be
| curious to see what the radius of a black hole filled with the
| cosmic background radiation would be, and how that correlates
| to the ~5 hydrogen atoms per cubic meter from the video. It
| wouldn't surprise me if they're equivalent or correlated so
| that the background radiation represents how much matter is
| inside the universe, not just its age since the Big Bang.
|
| In the first 4 minutes of this video, Neil deGrasse Tyson is
| worried about the end of the universe trillions of years from
| now, when the last galaxies have slipped outside the observable
| radius, so that future life will think that the Milky Way
| galaxy is the entire universe:
|
| https://www.youtube.com/watch?v=rFVdueuDD_o
|
| But I think that the inward rush of the universe's event
| horizon will continue forever and that even our galaxy won't be
| spared, even though right now it looks gravitationally bound.
| Eventually everything will be pulled apart in a big pop as even
| the force between nucleons won't be enough to resist Hawking
| radiation within a small enough black hole. Also the universal
| black hole will catch up to the Sagittarius A* supermassive
| black hole at the center of our galaxy, pulling even it apart.
| So eventually everything pops back out into the surrounding
| universe.
|
| So probably the universe can be thought of as perhaps an
| infinitely wide nested swiss cheese of black holes. There may
| be one principle formula that states how that shape comes from
| quantum mechanics where the long tail of probabilities leads to
| energy and particles arising (I'm speculating since my studies
| didn't take me that far), or maybe the physics of the child
| black holes can vary from their parent universes because
| they're separated by an event horizon so that our universe
| evolved physics favorable for life to observe its own
| existence. Which takes us back to the Anthropic Principle which
| doesn't really tell us much.
|
| TMI I realize, but hey, it's Sunday.
|
| -
|
| Edit: I forgot to add why I was writing this. At large scales,
| the universe forms bubbles with matter along the surfaces where
| they meet, and large voids in between. Which suggests that
| something caused matter to be attracted to those boundaries. I
| wonder if this is similar to how something like dark matter
| causes galaxies to rotate faster than they should for how much
| mass is at their perimeters. It suggests that space is moving
| there without having an attractor. As if space itself can
| acquire momentum without mass.
|
| This is sort of how space looks to us as it flows into Earth,
| causing us to perceive an accelerating reference frame which we
| feel as gravity. If the ground disappeared and we began
| falling, then we would no longer perceive gravity or its
| associated time dilation. Which suggests that someone falling
| into a black hole would remain weightless and stay synchronized
| with our frame of reference, even as they passed the speed of
| light from our reference frame and appeared close to frozen to
| us.
|
| So it might be possible to travel from outer to inner black
| hole. Although the speed of expansion by Hawking is so large
| that probably the falling observer would become suspended at
| the equilibrium point where expansion matches inward
| acceleration. So in a very real way, the amount of space within
| a black hole could be much larger than its apparent radius from
| our frame of reference. Giving some credibility to the idea
| that child universes exist within black holes.
|
| Also I don't know if there is a gravitational lens effect on
| photons arriving to us as they cross the heavier space at the
| edge of galaxies:
|
| A) If there is, then the mass exists there in the form of dark
| matter, neutrinos, WIMPs, etc.
|
| B) If there isn't, then space appears to be moving without an
| attractor, meaning that something like a Hubble constant might
| be missing from our relativity equations or work something like
| MOND, or that sci fi stuff like warp drives might be possible.
| I like to think that our universe evolved the conditions for
| sci fi, and that stuff like dark matter is a wink to us so we
| can figure out how to do it through something like
| electrogravitics.
| nico wrote:
| It was posted yesterday with a different title (the title of the
| paper about topological defects)
|
| In a way it's the same concept as Einstein's geodesics. And it
| doesn't mean that reality is different, just that we can model it
| in another way that might make the mathematical models and
| predictions easier to work with
|
| I'm all for having better models, but they don't need to all
| explain everything in the universe
|
| It's ok if we have one model for each different phenomenon we
| want to describe, even if they don't agree with each other
| outside their intended scope
| stouset wrote:
| The issue is we already _have_ models that describe different
| phenomena quite well. What we lack are models that describe
| things accurately when these phenomena are intertwined.
| nico wrote:
| Which phenomena specifically are you talking about?
| poopsmithe wrote:
| This isn't news to me. We've known for awhile that light circles
| around black holes.
| monadINtop wrote:
| So it's an absolute moonshot that posits that all observed
| discrepencies of galactic rotation curves is actually due to a
| coincidental and highly unlikely distribution of topological
| defects (that are not consistent with the full field equations)
| that have not been observed, and also fail to explain every other
| feature of dark matter at various different scales. nice.
|
| Its fine for a paper but doesn't really make sense as an article
| unless you are more interested in ad revenue from titles like "we
| don't need mass for gravity" than the ideas being presented.
| These science article websites live solely off of people who
| aren't equipped with the prerequisite background to actually
| critically parse the ideas that are being presented.
|
| This is probably the reason why everyone ~online~ (and not in
| departments) loved string theory two decades ago, and virulently
| hates it today, and also think that dark matter "is just a bunch
| of BS from the physics establishment" and that things like MOND
| or this are even remote competitors to the general consensus
| (which is basically just - we have no idea but we probably just
| need an extension to the standard model / quantum gravity at
| somepoint in the far future).
| RegW wrote:
| > ... a thin outer layer of negative mass ...
|
| Brilliant! My antigravity boots are nearly here.
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