Space explosion reveals possible hints of dark matter | Hacker News Reader

Space explosion reveals possible hints of dark matter

nautil.us

179 pointsrbanffy119 comments

>The burst even appears to have caused Earth’s ionosphere, the upper layer of Earth’s atmosphere, to swell in size for several hours.

For a gamma ray burst from an object that's 2.4 billion light years away, that seems pretty worrying. If it had been in our galaxy (50k light years across, so about 50,000 times closer) I'd assume the consequences would be serious indeed.

If I'm not mistaken, gamma ray bursts properly directed from a close enough entity are a realistic "wipe out all life on Earth" event, though extremely improbable.

Indeed, I think a supervolcano like Yellowstone erupting is much more likely.

That would not even kill all humans. It would possibly destroy all of civilization though. But even then, the coming generations would have artefacts to study and be inspired by.

Call me selfish but I don’t make much of a distinction since there’s a good chance me and everyone I know are dead.

Wait 80 years and you'll get the same effect.

Another question is, how selfish are your genes? Half-kidding. :)

I suppose people in North America have very slim chances either way, though.

Sure, but since we’ve used up all easily accessible oil and gas they’d have a hard time to find the energy required to level up a society.

That's an interesting thought. ACOUP did a blog post a while back [1] outlining the very specific circumstances that led to the Industrial Revolution and arguing that it's hard to imagine another way it could have happened. I suppose a rebuild would have to find a completely new route. Perhaps fields of Don Quixote esque wind mills connected to giant led-acid batteries rather than coal! With a couple hundred years gap between Armageddon and a substantial human population making progress, at least there would hopefully be decent timber reserves to work through the early phases.

[1] https://acoup.blog/2022/08/26/collections-why-no-roman-indus...

Using recycled copper from scavenged wiring, because there’s no high grade copper ore anymore.

All that copper went somewhere, though. A rising post-apocalypse civilization would probably mine our dumps and ruins, Rimworld-style.

In a rebuilding event, we'd have the advantage of knowing it was possible and desirable. This book, for instance, would be worth kingdoms in such a situation - https://www.amazon.com/Knowledge-Rebuild-Civilization-Afterm...

Just make sure to purchase the paper copy and not kindle

Or this one: https://www.amazon.com/Machinerys-Handbook-Toolbox-Erik-Ober...

> For more than 100 years, Machinery's Handbook has been the most popular reference work in metalworking, design, engineering and manufacturing facilities, and technical schools and colleges throughout the world. It is universally acknowledged as an extraordinarily authoritative, comprehensive, and practical tool, providing its users with the most fundamental and essential aspects of sophisticated manufacturing practice.

I think the level of scientific knowledge we have accumulated would be a tremendous short cut.

Even without books, the myths and obvious ruins of previous technological success would be a huge cultural guidepost for recovery.

Perhaps we would go through a 1000 year energy poor "dark age", and recovered populations wouldn't peak as high as ours, but I would expect that to be the worst case if an awareness of our history was not lost.

And maybe 10,000 years, after a complete cultural breakdown to hunter gathering with little functional memory of the past.

Given rewrites of first versions are often much improved for having seen previous failure modes, it would be interesting to pop into the future of a recreated civilization and see how they might do things better!

Even having this written knowledge available to us it might still be impossible to recreate the technologies; so much knowledge exists in an active state only.

Like the F1 engines on Saturn V. We couldn't just "recreate them" because all of the know-how was lost.

Written info is useless without the right sort of culture where the necessary experimentation and problem solving can occur to work out those undocumented details. That is all that matters.

However, that is also true today and the effects are visible through the variations in technological progress of different populations currently on earth.

If you can jump right to electric power, then hydro power can give you all the power you need to get civilization going again. I don't think that tech knowledge would be likely lost if people survive at all.

My guess is that all the required knowledge, including sufficient people, are squirreled away in the bottom of Cheyenne Mountain and probably a few similar locations around the world.

I guess it depends on how long before the collapse happens, but I think there's enough accessible fossil fuels left to sustain a less than 10-figure human population for under a century while it figures out what else to do. They'd probably have to live with far more local supply and distribution networks, but global communication should still be fine.

Probably shouldn’t worry much about that:

https://m.youtube.com/watch?v=fXb02MQ78yQ

They are. Luckily the only GRB candidate close enough to do that is eta carinae and it's probably pointing in the wrong direction.

Sounds like it's a perfect case where "probably" is not enough.

Enough for what? Useless worry? I don't think there's much we could do about it.

I mean, not much you can do about it, can you?

Either way, it's not likely to produce a GRB in the first place, it's too far away to really affect the earth in dramatic sci-fi story ways, and it's pointed in the wrong way.

It's a "GRB candidate" in the same way you're a "lottery winner candidate" if you buy a ticket.

What does “pointed in the wrong direction” mean? Isn’t it a sphere? Genuine question; I am ignorant of such things.

Stars have a magnetic field and rotate along their axis, similar to planets. The burst is emitted along the axis of rotation (the geographic north and south pole).

Many stellar objects rotate and/or have a magnetic field, and thus an "up", "down", and "side" in spite of being a sphere. Pulsars (extinguished stars that emit radio waves from their poles) are another famous example.

> What does “pointed in the wrong direction” mean?

Gamma-ray bursts [1] are a class of observations, not single event. (Like how we first saw pulsars, and then learned they're neutron stars.) Some GRBs may originate from relativistic jets [2] emitted by massive, spinning, charged objects colliding (e.g. black holes) or collapsing (supernovae). Those jets' intensity is not uniform, they emit from the poles. (See: spinning, charged.)

There are other proposed mechanisms that columnate emissions [3][4]. These involve a star's rotation creating a radiating column along the star's axis. (I'm not sure if the atoms in that column radiate with a bias.)

[1] https://en.wikipedia.org/wiki/Gamma-ray_burst

[2] https://en.wikipedia.org/wiki/Astrophysical_jet

[3] https://arxiv.org/abs/astro-ph/9808355

[4] https://www.plasma-universe.com/gamma-ray-bursts/

Stars, like planets, rotate around an axis. GRBs that are emitted by collapsing stars are believed to be directionally aligned with the axis of rotation.

> I mean, not much you can do about it, can you?

Work to shed our biology faster.

We'll get to that sometime in the next hundred years or so.

Mate I've got some bad news for you

>I mean, not much you can do about it, can you?

AKA: How I stopped worrying and learned to love the GRB

It's probably enough.

Your comment helped my worrying about this a lot, thank you.

Probably not? Half of the life on earth would probably survive the initial radiation burst, due to geography, and while there will be massive atmospheric chemistry changes that would be highly traumatic it seems like life would find a way, as they say.

The object is likely an active galactic core, so couldn't have been in our galaxy.

2.4 x 10^9 light years away, so not in our galaxy by a very large measure.

I think the meaning was more, that our galaxy can't produce such events, so that's one thing we don't have to worry about in our galaxy.

Edit: a (to me) random site [0] says the Milky Way does not have an active galactic nucleus.

0: https://socratic.org/questions/does-the-milky-way-have-an-ac...

Okay, missed that. https://en.wikipedia.org/wiki/Active_galactic_nucleus says

"An active galactic nucleus (AGN) is a compact region at the center of a galaxy that has a much-higher-than-normal luminosity over at least some portion of the electromagnetic spectrum with characteristics indicating that the luminosity is not produced by stars. Such excess non-stellar emission has been observed in the radio, microwave, infrared, optical, ultra-violet, X-ray and gamma ray wavebands. A galaxy hosting an AGN is called an "active galaxy". The non-stellar radiation from an AGN is theorized to result from the accretion of matter by a supermassive black hole at the center of its host galaxy."

Do we? https://en.wikipedia.org/wiki/Sagittarius_A\* says

"Sagittarius A*, abbreviated Sgr A* is the supermassive black hole[4][5][6] at the Galactic Center of the Milky Way"

Front row seat! Get in!

Sgr A* isn't a very active black hole (meaning that it's accreting only very little matter), compared to black holes at the center of AGNs.

Is that so? There is the theory that our galaxy could be a Seyfert Galaxy, we just haven't witnessed it so far.

https://link.springer.com/article/10.1007/BF00717870

https://earthsky.org/space/explosion-milky-way-center-seyfer...

https://www.bbc.com/news/science-environment-49955468

Edit: I meant to say that our galaxy could, and has had such fits of rage, not that this event could have come from it.

These types of GRBs are caused by the death of massive stars with low metallicity, not active galactic nuclei. There absolutely could be a GRB inside of the milky way galaxy pointed at us, but it is very unlikely.

From the article

"It’s also likely that the BOAT’s powerful jet was pointed toward us"

The energy given out is(?) very non-isotropic. A random shot got 'lucky' and hit us.

There's a hypothesis that Ordovician-Silurian extinction cca 450 milion years ago, one of the big five mass extinctions, was caused by a GRB.

Would the Earth shield one side of itself? If yes by how much?

Seems like that would only help if the duration of the blast was less than 24 hours

Or perhaps more like 12 hours, as half of the Earth would be exposed instantly.

But even then it depends on whether it's the direct radiation that's the issue or something more indirect (atmospheric changes, etc.)

doesn't this depend on the blast's origin relative to the Earth's axis?

The longest gamma ray bursts only last a few minutes.

Just how many nines reduction from peak intensity after a few minutes are we talking? Because I'd imagine 0.01% of a nearby gamma ray burst would still be bad news.

Rotation around the axis doesn't expose the northern hemisphere to the southern sky, nor vice versa.

That depends on the orientation with regard to the poles. If the source were directly over a pole, only fifty percent of the planet would be directly exposed. If it were over the equator, however, the whole planet would be, if it were a magic GRB that lasted way longer than usual.

So the upside is that anything that can get through 10k km of rock is very unlikely to interact with anything on the other side (eg neutrinos)

It will affect atmospheric chemistry. It will produce a ton of NO2 and make atmosphere unbreathable without equipment.

I imagine that killing half the planet is enough to send the "lucky" half into chaos and collapse.

Is there any data to back this up? (not attacking, just genuinely curious, i love learning about mass extinctions)

Only extremely circumstantial evidence. Some guesses about which species died off vs being protected, some climate change signatures that are consistent with a high UV burst.

About the best you can say for it is "you can't prove it didn't happen". Which, given that it was half a billion years ago, may be the best you can get. There isn't a real smoking gun.

The leading hypothesis is more that a long-term climate change caused secondary effects in a vicious cycle. We know the climate change was happening; there's very strong evidence. But it's not clear exactly what caused it or exactly how it led to mass extinction.

No, and the idea is widely derided by paleontologists because it's pretty much untestable.

Anton Petrov discussed this [1] a bit, but the essence was that there still would not be any obviously noticeable immediate effects, but that it would kick off a chain of changes like changes in atmospheric composition that would unfold over a longer time span.

[1] https://www.youtube.com/watch?v=4AWaqeUmQik

And this answers the Fermi Paradox. The universe is an extremely hostile environment for life to evolve.

This does not really answer anything. As far as we can tell (modulo the precision of our instruments), it's likely there are many, many stars like our sun and planets like Earth, even if they are not in the majority. It also seems like life sprang up almost immediately after the Earth cooled enough for it to be feasible.

If the universe is filled with violent incidents then it doesn't really matter how probable life is. It will take an extreme amount of luck to manage to survive through all those events. The Fermi Paradox doesn't exclude life, just the frequency of it. You could still have plenty of alien civilizations out there, but at a frequency of one every a thousand galaxies or so because everything else gets wiped out every so often.

Even with 'one every thousand or so' (which feels like a fantastically speculative number) the law of big numbers suggests this does not, in fact, 'answer the Fermi paradox'.

FWIW my (comparably uninformed by more than one data point) money is on mitochondria.

If you take the Drake equation and start speculating on a conservative way, the one in a thousand galaxies estimation is very probable. It could be mitochondria, or it could be that the universe is still too young to be safe for life on a mass scale. We're just lucky to be one of the first.

If I understand this right, they are hypothesizing that a super high energy photon encountering a magnetic field near its origin supernova got converted into a dark matter particle. That let it travel 2.4 billion light years to earth without interacting with stray photons during that intergalactic journey. Then somewhere near here, it hit another magnetic field and got converted back to a normal photon that set off all our detectors.

Does that mean we get hit with unconverted dark matter particles all the time? Do they just sail through the earth, like neutrinos supposedly do most of the time? Could they be converted to regular photons using magnetic fields? Could we reproduce the conversion to and from dark matter in the LHC, which reaches comparable energy levels to this photon? This is big brain stuff, I guess.

I'm confused at the magnetic field aspect. The point of dark matter is that it doesn't interact electromagnetically, at least not strongly. Is the idea that this interaction is such a rare one that it has escaped all the earth-based experiments so far?

It sounded like this interaction was special because of the super high energy of the photon and the super strong magnetic field near the supernova. At lower energies there would not have been an interaction. Presumably another occasion when those conditions existed was the Big Bang. So a lot of the universe's dark matter would have been created then, under this picture.

Axions can't be perfectly dark matter, they're very weakly interacting with electrons and the EM field.

They would be the Higgs field of the strong force (Axions were initially also named a "higglet") and would be an electrically neutral scalar particle like the Higgs, and that would interact with magnetic fields by the Primakoff effect: https://en.wikipedia.org/wiki/Primakoff_effect

Right, it said "axion-like particle" as distinct from an axion.

Yes. Dark matter is typically modeled as approximately dark… dark enough to have evaded experiments so far, and bright enough to have a footprint on phenomena (newly) observed at a fine level of sensitivity.

I’m glossing over important technical details, but roughly imagine that anything can typically interact with electromagnetism at least indirectly/weakly (very difficult to screen absolutely).

I love how we're able to debate the origins and life story of a single photon from 2.4 billion light-years away

It’s the ultimate counterargument to ”my actions don’t matter”.

It’s a very energetic photon though

It says, near the top of the story, "There were so many photons per second that they couldn’t keep up".

Not a single photon. Not easy (not possible?) to distinguish signal from noise with a single photon, afaik.

They detected many photons, but I read this as meaning that they had focused their attention on one specific photon that was detected by their instruments:

> Among those detections was a suspected high-energy photon at 18 teraelectron volts (TeV)—four times higher than anything seen from a gamma-ray burst before and more energetic than the highest energies achievable by the Large Hadron Collider.

In fiction: the solar flare of Inconstant Moon [1] and, very appropriately, the gamma ray burst of Diaspora [2]

[1] https://en.wikipedia.org/wiki/Inconstant_Moon

[2] https://en.wikipedia.org/wiki/Diaspora_(novel)

I remember reading [1] and enjoying the hard science (e.g. Jupiter taking its time to light up). Then the dawn...

Q: do gamma rays undergo red shifting as they travel across the universe? If so, did they start out as gamma rays or get shifted into gamma rays?

They absolutely do get red shifted (or blue shifted). In this regard they are no more special than any other electromagnetic wave. That said I don't believe we've got any kind of classification for higher energy EM waves than "gamma rays" so anything that red-shifted to gamma rays would be gamma rays originally anyway.

However if you travel at relativistic speeds it’s possible and likely for particles in the ligh cone ahead of you to be blue shifted to gamma. It’s a major thing we seem to leave of when talking about interstellar travel.

It’s one of the lovely things about the Alcubierre drive, if anyone ever figures out how to make it work that is. You’re traveling in a bubble that is moving at slower speeds, so the light cone is hitting you with fairly mild blue shifting instead of both barrels.

Maybe rather bad for your destination though:

"were an Alcubierre-driven ship to decelerate from superluminal speed, the particles that its bubble had gathered in transit would be released in energetic outbursts akin to the infinitely-blueshifted radiation hypothesized to occur at the inner event horizon of a Kerr black hole; forward-facing particles would thereby be energetic enough to destroy anything at the destination directly in front of the ship"

https://en.wikipedia.org/wiki/Alcubierre_drive#Damaging_effe...

Oh my. So 'back-in parking only' unless you want to end up in whatever the interstellar equivalent of The Hague ends up being.

Decelerating at sub-light speeds will also require massive high energy exhaust plumes if we’re using any traditional drives for that, which if a ‘warp’ drive is possible, we’d likely still need.

So death from both ends, basically.

Probably best to wait for us to land to say hi, or nuke us from far enough away the mess doesn’t get in their hair.

IIRC that is very similar to how C.J.Cherryh described the 'jump mechanism' in

https://en.wikipedia.org/wiki/Alliance%E2%80%93Union_univers... and

https://en.wikipedia.org/wiki/The_Chanur_novels

Though it mainly applied to matter caught in the 'field/bubble' during acceleration. In those fictional universes they got around this by designating entry- and exit-zones in the star systems, coordinated by automated buoys stationed there, to circumvent communication delays. If you didn't have the right 'passport'... err codes to authenticate, you'd fly blind, and possibly be hunted by the space traffic police for grave violation of all sorts of traffic rules. If you were military, you shat on it, and even used it as a weapon.

Ancient intergalactic war between two alien civilizations. Facing imminent defeat, Emperor Glurkz resolved to take his foe down with him, and detonated his ultimate weapon.

Could also be noise from the construction site of the Goe D. Glurkz Intergalactic Tunnel. A notice about this unfortunate disturbance was posted 7,700 years ago in our local municipality Galaxy Hall in downtown Mu Cephei.

Posted?!? I eventually had to go down to the cellar to find them. With a flashlight. And the stairs were gone! It was on display in the bottom of a locked filing cabinet stuck in a disused lavatory with a sign on the door saying ‘Beware of the Leopard'.

We never got to know if there was really a leopard in that room.

No way. It was clearly some intern testing in production the fancy new framework for handling remote total obliteration with no side-effects. Naturally, the side-effects were plenty and extremely local.

TL;DR: Scientists think the 18TeV reading was an axion. I'm sure Wikipedia will update itself soon:

https://en.wikipedia.org/wiki/Axion#Possible_detections

Some scientists think it was an axion. The articles says "one possibility". There is far from being a consensus on that.

Is there any estimation of the angular width of the cone for these sorts of gamma ray bursts?

Distance:

2.4 billion light years = (2.4 * 10^9) * (6 * 10^15) meters = 15 * 10^24 meters away! (approximately)

Area of emission sphere at contact with Earth:

4 * pi * (15 * 10^24 meters)^2 = 2 * 10^50 meters^2 sized sphere when passing Earth (approximately)

The amount of energy that must have been spread out over that sphere shaped wave, so any discernible signal reached Earth, much less interacted noticeably with our atmosphere ... simply incredible.

Err, looks like we were hit by a polar jet https://en.wikipedia.org/wiki/Astrophysical_jet so the energy doesn't radiate equidirectionally. But still mindblowing, and thanks for doing the maths.

I've an unrelated beginner physic question that make me lose sleep.

Say Im on earth with a clock and a photon - with a clock - is emitted from the sun towards me.

Is the following correct: when the photon reach me, my clock ticked for say 1 hour and I see photon's clock ticked for say 1 min. But the photon sees its clock at 1 hour and mine at 1min?

It should be noted that math sort of breaks down when you actually hit c. As I understand it, the math is a bit undefined as you're effectively past the asymptote. Technically, I've always read that light has no reference frame.

Basically, light doesn't experience time as such, so your question kinda doesn't make sense as written.

I'm glossing over details here, but if I'm wrong I would love to learn why from somebody with more knowledge!

What I think you're asking:

"The photon has a watch, and I have a watch. Both read 12:00.00 when the photon is emitted from the sun. What do the watches read when the photon reaches me?"

Photons take ~8 min to travel from the sun to the earth. Therefore MY watch reads 12:08.00.

The photon's frame of reference is at the speed of light though. So because of time dilation, the photon's watch reads 12:00.00.

The photon traveled all that distance without experiencing any time at all. In fact, if you, a human being, could travel at the speed of light (spoiler: you can't) without running into anything, you would live forever in reference to the universe.

The photon's frame of reference is at the speed of ligh

Thats the part I dont understand. From earth point of view the photon (or the spaceship) goes at speed of light (or close to it). But from the photon point of view, the earth is going at speed of light. If thats not the case, then speed isnt relative its absolute: photon goes at speed of light, earth does not.

Thank you for this framing. I haven't thought about this particular corner before!

Also, if it is the case that from the POV time is not passing at all, all time passes in no time, which would mean perceiving the entirety of time in one static juxtaposition,

but... can information reach a thing traveling at light speed? Does it matter the angle of incidence? (Nothing can catch up, but information-carrying particles like other photons could intersect or approach head on... but with their own relative velocity capped...?)

Time to fire up the Wikipedia...

Nothing can travel faster than light, so no, nothing can catch up to something traveling at the speed of light.

Really though the physics says nothing can _attain_ the speed of light, so light (or some other photon) could still intercept something with mass traveling at 99.9999999% the speed of light. And that thing with mass will never reach 100% the speed of light.

If you want to look deeper I suggest checking out the accelerating expansion rate of the universe, and what that means for information traveling throughout the universe over the next billions of years. Humanity is actually extremely lucky to have been born at a time when the universe was young enough to still be mostly visible. Civilizations that come about in the next 10s of billions of years might actually assume their galactic clusters are the only ones in existence (much like we thought initially until Hubble found another galaxy).

From the photon's perspective, the entire universe is compressed to a line of infinitesimal length, from the place it was emitted to the place it is ultimately absorbed, if any. In its perspective, it takes 0 time to travel through that line (really more of a dot).

The velocity of objects moving through spacetime is relative but there are certain quantities that aren't. The fact whether or not an object is traveling at the speed of light is one such quantity (called the interior product of the 4-velocity vector). That is, it is independent of the observer. So in all frames of reference Earth is traveling at sub-lightspeed velocities and the photon is moving at, well, the speed of light.

Second, a photon doesn't have an inertial frame of reference. There is no "photon point of view". The equations that relate the velocities of the same object as seen from different reference frames break down when you plug in the speed of light as relative velocity between the two frames. Put differently, all frames of references have relative velocities that are lower than the speed of light.

Hope this answers your question!

Nice. Thanks.

Not quite tbh. What if we are taking a fast spaceship instead of a photon. Spaceship starts from sun toward me. When it reaches me, I see my watch has ticked for an hour and I see the spaceship watch has ticked for 15 min. But from the spaceship point of view, earth was going toward it. From spaceship point of view, its watch must have ticked for an hour and it sees mine has ticked for 15min.

If things are relative why isnt it the case?

Right. So you see the ship taking an hour to travel from the Sun to the Earth, and the ship’s clock only counts fifteen minutes. But you turned this round and ask how much time the ship would see our clock tick during its journey? Now if everything is symmetrical then it would see our clock tick 3.75 minutes.

So, the thing the observers disagree in their different reference frames is what “now” means.

I thought A goes toward B at C is undistinguishible from B going toward A at C. So if A sees A:60min and B:15min, therefore B should see B:60min and A:15min. Anything other than that means something was absolute.

Damn its frustrating. Month I ask questions about that. Every time I get: velocity is absolute. Maybe it is, maybe thats the answer to my question, velocity is absolute, meaning the ship is going toward earth in "absolute". But then one could be at rest in the universe and then there is an absolute frame in the universe thats motionless. Therefore we could elect a center.

You’re so close, but missing the important thing. You talk about measuring time with the assumption that all reference frames can agree on whether events are simultaneous.

Let’s turn the problem round. The ship is leaving Earth at some high velocity. In Earth’s frame after 60 minutes have passed the ship’s clock reads as 15 minutes having passed. In the ship’s frame of reference when 60 minutes have passed the clock on Earth will have recorded 15 minutes.

Now this sounds impossible because you think we agree on when events are simultaneous, “A’s clock is at X so B’s must be at Y,” but this is only true when A and B are in the same place, if they aren’t in the same place then you can only say, “A’s clock is at X so in A’s reference frame B’s must be at Y.”

Essential you can divide space time into three regions for a point A. The future is the cone of space time that could be reached from A without exceeding the speed of light. The past is the cone of space time from which an object could reach A without exceeding the speed of light. The third region isn’t the future or the past, and for any point B in this region we can find an inertial reference frame where A and B are simultaneous, or where A happens before B, or where B happens before A.

We can only ever globally agree that points in A’s future cone are after A, and points in its past cobe are before A, and these remain true in every inertial reference frame.

Absolutely wonderful. Great explanation, thank you so so much. Months Im asking questions around.

For a 60 min travel between earth and ship, there is a point where ship reaches earth reading ship:60 and earth:15. And there is a point where earth sees the ship land and read earth:60 and ship:15 resp. Those are not the same points.

So when you travel at high speed in a frame A, you're kind of gonna "experience a different story" than objects traveling in low speed in frame A relative to each other.

Edit: do we know an object with a mass thats traveling at high speed in our frame?

Edit2: Oh but the universe is expanding so we do actually.

If a fast ship leaves earth and comes back. From earth point of view, when it comes back the passenger gonna have outlived us all whereas for the passenger, it only left for an hour.

Mind boggling really.

Correction I meant

From earth point of view, when it comes back the passenger has outlived us all whereas from the passenger point of view, when he comes back, he only left for an hour.

I realize this is one of the most difficult points to understand about relativity.

The question this paradox comes down to is, when are you comparing your watches? The two observers (the spaceship and you) won't be able to agree on what it means for two events to occur at the same time. Once you take this into account you will realize there is no contradiction and the POVs of the two observers are indeed symmetric as expected from the word "relativity".

For more on this check out e.g. Bernhard Schutz's "A First Course in General Relativity". IMO it's a rather terrible book as far as General Relativity goes but the first chapter, which is about Special Relativity (~25 pages), is fantastic and is was what helped me finally understand SR and, in particular, the paradoxes.

Also what I think is being asked:

...if the photon comes to rest on earth, and we compare clocks...

IIRC, the deceleration of the photon relative to me (as it comes to rest on earth as would a returning space traveler) would "reverse" the time dilation relative to me, so both our clocks would read 12:08.

Please, correct me if I'm wrong. I read this many years ago but never dove all the way into the math.

No. This difference in the clocks will remain even if you could somehow slow down the photon without it being absorbed. This is why if you sync two atomic clocks, bring one atomic clock up to orbiting speed (such as on a gps satellite), then decelerate it and return it to earth the clocks will still read different numbers.

Which is why if you could accelerate yourself to say 90% the speed of light, then decelerate and return to earth, you would have essentially traveled into the future and outlived everyone you ever knew.

Do photons have to slow down to change? Change requires time to pass doesn't it? So how does a photon change polarization (without slowing down)?

The photon actually experienced only one minute* and you one hour, that is what the clocks would show. The photon sees you one hour older and you see the photon one minute older.

It is like the waterplanet in the movie Interstellar (I think it was that planet), where they spent a short while on the surface and the person in space aged decades.

It is called time dilation [0] and there was a rather interesting experiment done by Hafel Keating [1].

* Interesting to know: time slows down to a halt at the speed of light, and the one second time lapse of the photon goes to zero no matter how long you wait on earth.

[0] https://en.m.wikipedia.org/wiki/Time_dilation

[1] https://en.wikipedia.org/wiki/Hafele%E2%80%93Keating_experim...

Finally, we have possible hints of Dark Matter! We may be able to silence skeptics once and for all with this data that may have a few percent chance of being defining proof of Dark Matter. Or maybe it's axions! Either way, tens of thousands of physicists, astronomers and cosmologists will be relieved they may have not wasted their entire professional careers.

I know that a combination of second-option-bias and someone-is-wrong-on-the-internet might make it seem like dark matter is an all consuming concern in physics but in the real world even most astrophysics people don't give much of a shit or bump up against it in their work.

The problem is Dark Matter, without any proof it exists, was elevated into the cosmological paradigm, taken for granted that it exists, when it may very well not exist. As part of the paradigm, it is incredibly difficult to remove. Someone could have figured out what the missing mass was 15 years ago, or that there was no missing mass - eliminating the need for Dark Matter - yet Dark Matter could continue remain part of the established paradigm for 50 more years or longer. Paradigmatic criteria should at the very least include positive proof of existence, saving the effort wasted in teaching, discussing and testing for something that never existed, as proving it doesn't exist is much more difficult if not impossible even if it doesn't exist, which I personally think is a pretty good bet at this point.

> Dark Matter. Or maybe it's axions!

This would be proof that dark matter, or at least some of it, is axions.

Few cosmologists doubt that dark matter exists. It’s quite well established that it does (multiple independent sources of evidence), we just don’t know what exactly it consists of. What this gamma-ray burst possibly hints at is that it might be axions.

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