[HN Gopher] Scientists create matter from pure light, proving th...
___________________________________________________________________
Scientists create matter from pure light, proving the Breit-Wheeler
effect
Author : was_a_dev
Score : 319 points
Date : 2021-09-14 08:46 UTC (14 hours ago)
(HTM) web link (www.bnl.gov)
(TXT) w3m dump (www.bnl.gov)
| pretadank wrote:
| Curious, can this process be reversed? Also, is momentum
| conserved, then if it is wouldn't the positron and electron be
| ejecting at the speed of light (and wouldn't that violate the
| principle that matter cannot reach the speed of light)? If not
| what is slowing them down / where is the energy going? Could this
| be theoretically a way to power something? Either by the ricochet
| of the particles off something, or by using them to energize
| something?
| tsimionescu wrote:
| Momentum is mass times velocity. Momentum is conserved, but
| you're going from (probably) mass-less photons moving at c to
| mass-ive electrons/positrons moving at <c.
| drdeca wrote:
| when you say mass times velocity, I take it you mean
| relativistic mass? I assume you know all the following, and
| I'm just saying it to explain my reasoning for the above / to
| elaborate for other readers. One can certainly pick a frame
| of reference such that the total momentum of the electron and
| positron afterwards is non-zero, and therefore the total
| momentum of the photons beforehand (in that reference frame)
| will also be non-zero , despite photons having (or, believed
| to have, but I believe it also) 0 rest mass.
| whatshisface wrote:
| All quantum-mechanical processes can be reversed. In fact, this
| process is actually a lot more common in the other direction; a
| positron and an electron making a pair of photons is matter-
| antimatter annihilation and it happens all the time. This is
| because a couple photons are thermodynamically preferred over
| an electron-positron pair at room temperature. It's kind of
| like how allotropes of solids will form and reform as you
| change the heat and pressure.
| SeanFerree wrote:
| Very cool!!
| r721 wrote:
| Better article: https://www.bnl.gov/newsroom/news.php?a=119023
| smurpy wrote:
| It looks like one of the authors is on HN with us. Can you offer
| any insight into the cause of the confusing language in the
| "Science News" piece? It reads like it was written by a bot not a
| human.
|
| In the sentence "With the theory physicists Gregory Breit and
| John Wheeler were able to prove that when two high-energy photons
| collide, a positron and an electron arise, i.e. matter is formed"
| shouldn't the word be "predict" not "prove"?
|
| A more flagrant example of a strange word choice for a human
| science writer to make is "A direct conversion would require a
| laser that emits gamma-ray photons in a highly concentrated
| steel." Shouldn't the word "steel" instead be "beam"? This seems
| like the sort of thing an uncomprehending bot might do, conflate
| those two words.
|
| Are my the nits I've picked, above, unfounded? Does the author of
| the original paper have any information which might suggest that
| an actual human wrote the "Science News" piece?
|
| If not I would suggest that we've got a bot on the loose! Eeek!
|
| Further, I think I'm seeing rather a lot of "content" floating
| around recently which smacks of machine origins.
|
| Also, to Daniel... Great paper. Amazing stuff!
| raducu wrote:
| How can two photons collide? Aren't photons force
| carriers/bosons?
| eigenhombre wrote:
| Yes, photons are bosons / force carriers, but they interact
| with charged particles, and in this case produce e+ e- pairs
| via this Feynman diagram [1]. By rotating the diagram in
| spacetime, you get different known interactions: pair
| production (this topic), pair annihilation (same diagram
| running the other way in time), and, if memory serves,
| Compton scattering.
|
| [1] https://en.wikipedia.org/wiki/Two-
| photon_physics#/media/File...
| bee_rider wrote:
| "Rotate the diagram in spacetime" would make for some top-
| tier technobabble (not expressing doubt that it is a real
| thing, just wow, it sounds so cool).
| coldtea wrote:
| Well, "rotating the diagram in spacetime" is also exactly
| what you do when you move your hands while holding the
| printout of a diagram...
|
| (In this case though it's about rotating what's shown in
| the diagram, that is the interactions pictured, not the
| diagram itself :-))
| dr_dshiv wrote:
| So, can photons interact and produce matter without
| particles to interact with?
| BizarroLand wrote:
| One of the consequences of E=MC^2 is that there is a
| strong implication that energy can be converted directly
| into energy and vice versa.
|
| Photons, although mass-less, have energy and can
| therefore be converted into matter under some
| circumstances.
| adamc wrote:
| I think you mean "directly into matter".
| BizarroLand wrote:
| Yes. Thank you!
|
| Apparently my well intentioned ramblings can be directly
| converted into brain farts.
| acchow wrote:
| Although in propositional logic we can say:
|
| x [?] y
|
| which means "x proves y".
| ravel-bar-foo wrote:
| Automated synonym replacement is the current cutting edge in
| plagiarism, or more generously, avoiding automated plagiarism
| detection. There was recently a large scandal in which some
| large number of scientific papers were traced to a ghostwriting
| house via their automated synonyms.
|
| So my guess would be that the science news "author" is
| rewording things to avoid stepping on another author's
| copyright.
| AnimalMuppet wrote:
| > So my guess would be that the science news "author" is
| rewording things to avoid stepping on another author's
| copyright.
|
| And doing it badly...
| perihelions wrote:
| It's Google's machine translation of a German-language article,
|
| https://www.forschung-und-wissen.de/nachrichten/physik/mater...
| jdbburg wrote:
| Author here, thanks! I am not familiar with this website
| (science-news.co) so I cannot comment on the quality of the
| writing (or human origin). I agree with you that "prove" should
| be "predict" - they did not prove anything except that from a
| theory standpoint, this process can happen in quantum
| mechanics.
|
| The sentence about lasers is also strange, I have no idea what
| is meant there. My only guess is that it might be trying to
| describe some laser experiment that uses lasers to "heat" a
| hohlraum to produce a field of photons. Then some other high
| energy photon beam is used to collide with the photons inside
| the hohlraum.
|
| edit: Original article from DOE press release is here with a
| bit more info: https://www.bnl.gov/newsroom/news.php?a=119023
| dang wrote:
| Ok, we've changed the URL to that from https://science-
| news.co/scientists-create-matter-from-pure-l.... Thanks!
| perihelions wrote:
| Dang, the old URL is the Google translation of a German-
| language magazine article (not attributed, so it's
| functionally plagiarism):
|
| https://www.forschung-und-
| wissen.de/nachrichten/physik/mater...
|
| You've previously asked for examples [0] of this type of
| fraud on HN: so this is one example! Credit to 'smurpy' in
| this thread, who noticed the 'bot'-like writing style.
|
| [0] https://news.ycombinator.com/item?id=27295943
| Tepix wrote:
| Fascinating.
|
| Could two cooperating civilizations a and b use this effect to
| create matter (perhaps even something self-replicating?) at a 3rd
| location in space? incoming ascii art
| (a)---(c) | |
| | (b)
| adrian_b wrote:
| It is very unlikely that teleportation could ever be achieved
| by generating matter from light.
|
| There are 2 problems.
|
| First is that the energy required for generating a human-sized
| body is huge, several hundreds times greater than that of a
| nuclear bomb with the same amount of active material.
|
| Second is that even if you would be able to do that, you would
| make 2 bodies, one of matter and the other of antimatter and
| you would not be able to dispose of the antimatter one except
| by causing an explosion equivalent with hundreds or thousands
| of nuclear bombs.
|
| In general, for now it appears to be completely impossible to
| do teleportation towards an arbitrary place.
|
| At most, it could be imagined a method that would work between
| 2 large installations existing at the end points, where the
| destination could manufacture a replica of what was sent from
| local materials (e.g. by placing one atom after another until
| an exact replica is made; extremely unlikely, but not
| completely impossible).
| Tepix wrote:
| The idea would be to build a nanoassembler that can receive
| instructions via light.
| jamiek88 wrote:
| Wouldn't the no cloning theorem make an _exact_ replica
| impossible?
| adrian_b wrote:
| The "no cloning theorem" refers to replicating the same
| quantum state.
|
| It does not apply to making a replica of a body that is
| composed by the same kinds of atoms linked in the same way
| between themselves, which can be considered as identical
| with the original for any practical purposes.
|
| In a very rudimentary form, such technology exists today,
| it is possible to remove one atom at a time from a certain
| position of a body and identify it with a mass spectrometer
| and it is possible to grow a structure a few atoms at a
| time.
|
| It is likely that the precision of such techniques could be
| improved well enough to be able to replicate some extremely
| small microscopic objects.
|
| The most difficult limitation to overcome is the speed of
| the process.
|
| To be able to make some human-sized object atom-by-atom and
| molecule-by-molecule in a reasonable time would require a
| many orders-of-magnitude higher speed than possible with
| the current technologies.
| AmericanChopper wrote:
| > First is that the energy required for generating a human-
| sized body is huge, several hundreds times greater than that
| of a nuclear bomb
|
| I thought this was a bit short, I'm pretty sure it's closer
| to:
|
| 1 gram = 90000000000000 joules
|
| 1 average us man = ~89000 grams
|
| Google tells me 1 Hiroshima bomb = 63000000000000 joules
|
| So the average us man should be ~127,142 Hiroshima bombs, or
| ~110,000 Hiroshima bombs for the average us woman.
| goohle wrote:
| This amount of energy is required to CREATE mass from
| scratch. IMHO, it's much easier to use transmutation of
| atoms to print an object layer by layer instead.
| adrian_b wrote:
| I have said that a factor of hundreds applies for the same
| weight of active material.
|
| Most nuclear bombs have just a few kilograms of active
| material, much less than a human.
|
| Also the number that I have given was for ideal conversion
| and for a fusion bomb, which is more efficient per weight.
|
| The fission bombs, like that from Hiroshima, are limited by
| the excess energy of uranium or plutonium, which is only
| about 0.13% of mass compared to iron.
|
| Because we cannot actually split plutonium into iron but
| into nuclei with higher energy, the available energy from
| fission is slightly less than 0.1% of mass.
|
| Taken into account that only a fraction of the active
| material is consumed, let's say 10% and that the active
| material of a bomb is ten times lighter than a human, then
| an annihilated human mass would be equivalent to 100
| thousand fission bombs, so your numbers match this.
|
| For a fusion bomb, the efficiency is much higher, about
| 0.5% of mass becomes energy and also the mass of active
| material can be much higher, so an equivalence with less
| than one thousand nuclear bombs is possible.
|
| In any case I have given just the most optimistic limit,
| i.e. a few hundreds nuclear bombs of maximum energy, with
| smaller bombs, of course you need more.
| throw1234651234 wrote:
| Teleportation has that issue of "the reprinted is not the
| original" - ironically, turning someone into light and then
| re-assembling them into matter seems closer to "true sci-fi"
| teleportation. Of course, it still wouldn't be the same
| person since you essentially converted all the constituents,
| kind of like melting down a sword and reforging.
| rockbruno wrote:
| Given that we don't know how sentience works, I like to
| think that you could achieve true teleportation if it turns
| out that the "you" are not the connections of your brain,
| but something else that exists in a fourth dimension.
| whatshisface wrote:
| I have seen a few science fiction stories where everyone
| is happily running teleporters until someone accidentally
| turns off or breaks the part of the machine that
| annihilates the guy standing on the sending terminal.
| tiborsaas wrote:
| Imagine engineering teams from different civilizations trying
| to collaborate to the precision of a few atoms :) That's a sci-
| fi I'd watch.
| pacija wrote:
| Matter (aether) is constant, it cannot be created or destroyed.
| Mass is created in places where matter's (aether's) density gets
| slightly higher or lower than its baseline density. Photons don't
| exist. You're welcome.
| chana_masala wrote:
| As a Saivite, this perfectly aligns with both my religious and
| scientific understanding of the universe.
| tablespoon wrote:
| > A direct conversion would require a laser that emits gamma-ray
| photons in a highly concentrated steel. However, research has not
| yet been able to develop such a laser.
|
| What does "highly concentrated steel" mean here?
|
| All my Googling is getting hits for the metal, but I feel like
| something else is meant here.
| r721 wrote:
| This domain was never submitted to HN before, so I suspect this
| is some other article which was put through the so-called
| "synonymizer" software.
|
| Here is an article from Brookhaven Lab website:
| https://www.bnl.gov/newsroom/news.php?a=119023
| tablespoon wrote:
| > This domain was never submitted to HN before, so I suspect
| this is some other article which was put through the so-
| called "synonymizer" software.
|
| If that's true the domain should be banned. There's no point
| in reading things that garbled plagiarizations.
| lmilcin wrote:
| Now the real question is, when can I order my replicator and
| dilithium crystals to power it?
| eloeffler wrote:
| This induces some severe Star Trek replicator fantasies. Though I
| assume the result of this compares to having a cup of earl like
| opening a lego drawer vs. a visit to Legoland
| newpavlov wrote:
| You forget about one smaaaal issue: disposal of created
| antimatter. Also why bother creating matter from energy when
| even today we are able to create stockpiles of raw elements
| (although quite inefficiently and with almost no control over
| isotopic composition). Assuming that waste will be converted
| back to those elements, you don't need those stockpiles be too
| big and energy consumption will be modest when compared to
| creation of matter directly from energy.
| TeMPOraL wrote:
| I've been thinking about it a lot, and I feel that the road to
| Star Trek replicator is not through matter synthesis. Instead,
| a better way - perhaps the only way - is through some sort of
| universal feedstock (or few of those) and a library of
| carefully designed chemical processes (possibly involving
| nanoengineering).
|
| It seems nice at first - the idea of turning raw energy into
| things. But the amount of energy required is just ludicrous,
| going through antimatter intermediary is extremely dangerous,
| and... there's really no point. It isn't buying you anything.
|
| If you tried to store the amount of energy needed to replicate
| a kilogram of stuff in some advanced batteries, you'd quickly
| discover that those batteries now weigh a kilogram more. E=mc^2
| works both ways - storing energy adds mass. So, unless you can
| literally fly through a star and suck it dry as you replicate
| your tea, replicators won't be reducing weight of a starship.
| It turns out, _matter itself_ is the ultimate energy storage
| device.
|
| And then, turning energy into matter just gives you elementary
| particles. You need more work to assemble that into useful
| stuff. This is nuclear work, and then chemical work. So why not
| just ditch the matter/antimatter intermediary, and start from
| some feedstock that already has the necessary elements, and is
| optimized to be easy to react into the stuff you want (and then
| to be recycled from that stuff)?
| unclewalter wrote:
| Of course I don't think your statement's motive is to
| understand how we replicate the exact process of what is in
| Star Trek in the real world. More of a functional equivalent.
| However, because I have the poster, if you look at plans of
| the Enterprise, it contains a large volume that is reserved
| for "bulk matter". I believe it's this, along with some
| transporter technology enables the replicator to create a
| variety of combinations of chemicals and materials.
| TeMPOraL wrote:
| Oh, nice! I never saw that in any of the supplemental
| materials I stumbled upon. Which Enterprise is on the
| poster? Do you have a link handy?
| dougmwne wrote:
| Poop, gentlemen. The bulk matter is poop.
| andrewf wrote:
| Spoiler for S3 of Star Trek: Discovery -
| https://www.youtube.com/watch?v=GIIYb04HGbQ
| unclewalter wrote:
| dougmwne's right. Though I can't find my poster online
| (for the life of me), the technical manual is pretty
| clear:
|
| [...]The various waste sludges recovered from the water
| recycling processes are a valuable resource. The organic
| waste processing system subjects the sludge to a series
| of sterilizing heat and radiation treatments. The waste
| is then electrolytically reprocessed into an organic
| particulate suspension that serves as the raw material
| for the food synthesizer systems. Remaining byproducts
| are conveyed to the solid waste processing system for
| matter replication recycling. [...] Material that cannot
| be directly recycled by mechanical or chemical means is
| stored for matter synthesis recycling. This is
| accomplished by molecular matrix replicators that
| actually dematerialize the waste materials and
| rematerialize them in the form of desired objects or
| materials stored in computer memory. While this process
| provides an enormous variety of useful items, it is very
| energy intensive and many everyday consumables (such as
| water and clothing) are recycled by less energy intensive
| mechanical or chemical means. Certain types of
| consumables (such as foodstuffs) are routinely recycled
| using matter replication because this results in a
| considerable savings of stored raw material (See: 13.5).
|
| From tngtm section 12.5:
| https://xaeyr.typepad.com/files/franchise-star-trek-tng-
| tech...
| Razengan wrote:
| And red shirt corpses
| imglorp wrote:
| I think this is right. And this, plus unlimited power (coming
| soon) gives you the route to the Roddenberry economy where
| everyone works to improve themselves and society and there is
| no money.
|
| Think about it. If you put dirt in the replicator and you get
| a Rolex or a hamburger or a garment, you don't need money for
| anything: everyone is equal and they can go off and study or
| work on whatever they want. Wealth is a false proxy for
| social status anyway.
| 14 wrote:
| I dream of a post scarcity world. Live long and prosper as
| they say.
| layer8 wrote:
| > you don't need money for anything
|
| Except for intellectual property, and possibly information
| in general. Which probably means that protecting it would
| become even more of a focus than it is now.
| TeMPOraL wrote:
| Yup. The current intellectual property regime pretty much
| shuts down any dream of replicator-driven utopia, even if
| a 24th century starship suddenly showed up in orbit and
| beamed down working replicators along with full specs.
|
| It's not really a theoretical consideration - as we get
| better at turning information into matter, intellectual
| property laws literally centralize the means of
| production away from the people!
| goohle wrote:
| When replicator will be ready to use, you will be able to
| replicate iphone XXX freely, because copyright for iphone
| XXX will expire at that time.
| layer8 wrote:
| Unless the legislators go the Disney route and
| effectively extend copyright and patents indefinitely.
| nynx wrote:
| If that happened, people would just break intellectual
| property laws left and right until they became
| unenforceable.
| aaron-santos wrote:
| I'm not sure I follow. Private property exists right now
| to enable the accumulation of wealth. In a post-scarcity
| economy, what is the value of wealth accumulation?
| ctdonath wrote:
| "Post-scarcity" is a myth. There will always be some
| limited-supply thing humans will value.
|
| Consider how utterly dissatisfied people are with
| abundant cheap food, clothing, water, fuel, housing, etc.
| today vs not all that long ago, having integrated new
| norms into mundane sustenance making abundance seem
| unattainable. An hour's minimum wage today buys medieval
| luxury but for regulations demanding even better.
| layer8 wrote:
| Power and status. People want something to bargain with,
| to show off, and to control other people. If money is not
| an option, information will do (and possibly
| cryptocurrency, which in some way is both).
| aaron-santos wrote:
| Sorry, still struggling to understand here. Today power
| and status is derived from scarcity. What is an example
| of power or status not derived from scarcity?
| ctdonath wrote:
| Energy is not free. Humans can always find something
| desirable/influential that requires more energy than is
| cheaply available.
|
| Ex.: the Star Trek replicator requires staggering amount
| of energy. Owning one, and affording the fuel, would
| provide power and status. Were energy cheap enough to
| give everybody one would enable something next level
| scarce and desirable/influential.
| CalRobert wrote:
| We're already there, nearly. Calories are incredibly cheap
| and a modest dwelling can be produced very inexpensively as
| well. Humans will find scarcities to focus on, or barring
| that, make them. (Housing cartels come to mind).
|
| Access to healthcare remains genuinely scarce, admittedly,
| and Baumol's cost disease is a real problem when you need
| to buy someone else's time.
| lippel82 wrote:
| Real Estate! That'll be the only valuable thing in such an
| economy.
| White_Wolf wrote:
| we'll all have a pocket universe to design as we wish :D
| monkeybutton wrote:
| That's something that always bugged me. Why does Picard
| get to have an entire, private vineyard to himself?
| layer8 wrote:
| Maybe when beaming down to earth they really beam down
| into a holodeck (or the Matrix).
| CalRobert wrote:
| Primogeniture, apparently. Great thread athttps://mobile.
| twitter.com/sarahtaber_bww/status/12002306275...
| TeMPOraL wrote:
| That's a great (and long) thread. Reminds me of a pet
| theory I have: that the post-scarcity utopia of the
| Federation is based on expansionism, and not sustainable.
|
| They've managed to turn Earth into a paradise and give
| (mostly) everyone great life quality, but they did this
| by doubling down on exponential growth, which manifests
| in the constant need to get more and more planets to join
| the Federation.
|
| You can see the expansionist vibes all over the show, and
| they've even been hinted at by various aliens in DS9
| (although without any mention of economic implications).
| swalsh wrote:
| I really think this is right, it would be nice to have a
| Star Trek like sci fi series that actually tried to
| tackle the issues. As a series it's been a source of
| inspiration for decades. It's a shame modern Star Trek is
| just generic sci-fi drama.
| choeger wrote:
| That thread points out some interesting observations
| about typical Star Trek inconsistencies (another one
| would be to think about _why_ someone should move to a
| colony in the first place or rather about why not
| everyone can simply travel between planets as they like).
| But the objection against desalination is pointless. We
| can manage that problem already today (e.g., by drying up
| the brine or by pumping it into old oil wells). There is
| absolutely no reason to assume water would be scarce in
| the 24th century of TNG.
| elihu wrote:
| They might have some sort of immigration quotas for the
| Earth, and if the population is approximately what it is
| now, then there's going to be a lot of open space,
| farmland, and wilderness still. That's assuming you have
| cities with a lot of people packed close together, and
| rural areas with sparse population. That seems much
| better than the alternative of turning the entire land-
| surface of the Earth into a uniform-density suburb.
| swalsh wrote:
| There's plentiful space all over the universe if you're a
| multiplanetary species.
| kamarg wrote:
| There's a pretty big difference between being able to
| colonize the universe and being able to colonize planets
| near to you. As an example, the Federation wasn't even
| able to map out a single galaxy nevermind colonize it.
| swalsh wrote:
| > Wealth is a false proxy for social status anyway
|
| Wealth is just the current proxy. I suspect you'll never be
| able to fully eliminate a social hierarchy. Many animals
| compete physically, humans compete socially, and without
| money we'll find new ways to socially standout. The game
| just changes, it's not going to go away.
| allemagne wrote:
| Maybe nobody has to starve, work, or even die from anything
| besides a freak accident ever again, but not all the 100
| billion humans who want to can possibly live in Neo-
| Manhattan (at least until the Mega Dome is finished in 50
| years, but then not everyone can live on the Upper Rim).
| Not everybody can meet the coolest celebrity in the
| metaverse, or all have the same clever instagram handle.
| Not everyone can control who is allowed to visit and build
| on holy sites or nature preserves.
|
| I think people can always invent or discover new things
| that are functionally finite, and their claims on these
| kinds of things and the way they decide to distribute it
| can always theoretically be quantized into something we can
| understand as currency or wealth. Even if it's not done
| explicitly, it will simply be done in something like an
| extremely advanced algorithm or even an opaque shadow
| economy of networks of favors and patronages.
| TeMPOraL wrote:
| It's true that some things are naturally scarce (like
| land, or celebrity exposure, or attention), and that
| people can _make_ things scarce on purpose. It probably
| isn 't possible to work around this - but this doesn't
| mean we couldn't have a post-scarcity society in spite of
| it.
|
| The way I see it, if "nobody has to starve, work, or even
| die from anything besides a freak accident ever again",
| if everyone has free access to as much varied, healthy
| and tasty food as they want, if they can pursue almost
| arbitrary hobby[0], or do interesting and challenging
| service adjacent to their interests - then the bits that
| remain fundamentally scarce (like land or attention)
| won't be enough to justify having a money-based economy,
| and without it, people will lose the reason for creating
| artificial scarcity. Some people may end up trading IOUs
| over trivialities like clever Instagram handle -
| something that, on a global scale, _nobody_ cares about.
| But there would be no need to e.g. DRM movies or games
| anymore - the producers aren 't getting paid for it, they
| do it for fun/reputation/self-actualization - so what's
| the point of making infinitely-copyable good artificially
| scarce?
|
| Post-scarcity doesn't have to be absolute for it to be a
| money-less utopia - it's enough to make enough goods too
| cheap to meter that money becomes irrelevant for everyday
| life of average citizen.
|
| --
|
| [0] - There will always be limits to what individuals can
| do - post-scarcity is a practical concept, not an
| absolute one. Even Star Trek societies wouldn't be able
| to afford people having a hobby of "detonating warp cores
| to use the gamma ray flash for interplanetary Morse
| code".
| xwdv wrote:
| What do you mean you don't need money? How will you pay for
| services where people must spend their time to serve you?
| Any time you start tracking who owes something to someone
| you have the beginnings of currency. The post-scarcity
| economy being free of money never really made sense.
| TeMPOraL wrote:
| If everyone's needs and even all non-extravagant desires
| can be met for free (or "too cheap to meter" free; this
| is the "post-scarcity" part), then the _need_ for
| accurate tracking of value flow vanishes. Everything else
| can be mostly handled on a pay-it-forward basis, perhaps
| with a bit of "reputation economy" sprinkled in, and the
| remaining exceptions don't warrant a society-wide
| currency.
| allemagne wrote:
| I would expect the remaining exceptions to be enraging
| examples of unfairness to any post-scarcity society, and
| for civilization to really test the boundaries of "too
| cheap to meter".
| TeMPOraL wrote:
| Perhaps, but then I would expect people in a post-
| scarcity society to be fine with the idea that life is
| not fair - that it's not possible for everyone to have
| equal access to everything at the same time. I'd expect
| them to not care that much, having a comfortable life all
| needs taken care of.
|
| I'd also expect post-scarcity people to not even stress
| the "too cheap to meter" boundaries that hard - when
| comparing with people of today, it's worth remembering
| that most consumption of non-essentials is driven by
| artificial needs created by advertising, and advertising
| is predicated on scarcity. Post-scarcity people wouldn't
| be brainwashed to consume so much.
| bovermyer wrote:
| But Star Trek replicators don't work through matter
| synthesis, they work through matter transformation. It's a
| variant of transporter technology; a source fuel is
| deconstructed and then reconstructed in the desired
| configuration.
| ape4 wrote:
| Like 3D printing.
| TeMPOraL wrote:
| I know. Or at least that's how I interpreted various
| remarks during the show.
|
| Direct matter synthesis doesn't make sense in this context,
| but I've seen _a lot_ of people on the Internet assume this
| is what happens. The way they talk about it on the show
| could lead you to either theory, for both replicators and
| transporters.
|
| But to the extent we're discussing it as technology we
| could potentially have, and/or may want to make happen, I
| feel it's worth pointing out that transforming feedstock is
| a _better_ design, doesn 't require mastery of bulk
| antimatter processing, and is perhaps closer to possibility
| than one would otherwise assume.
|
| Hell, I'd go as far as saying that there are problems with
| reasonable "attacks"[0] on the road to a replicator -
| engineering, chemical and biotech problems we could be
| working on right now.
|
| -- [0] - In the Hamming sense, see https://www.cs.virginia.
| edu/~robins/YouAndYourResearch.html.
| noneeeed wrote:
| That's basically the idea in The Diamond Age by Neil
| Stephenson. It's years since I read it but it's set in a
| world of nanoengineering where whatever you need can be
| created out of common feedstock.
| TeMPOraL wrote:
| I've never read it. Thanks, I'm fast-tracking it to the top
| of my "to-read" pile!
| dTal wrote:
| Energy does have one big advantage over matter: you can send
| it at the speed of light.
|
| The implications of this in a world with both mass->energy
| and energy->mass conversion are left as an exercise for the
| reader.
| bovermyer wrote:
| Unless, of course, you take speed out of the equation by
| folding space to transport things.
| ptr2voidStar wrote:
| Let's not get carried away now - shall we? :)
| scarygliders wrote:
| Sounds a lot like the CHON (Carbon, Hydrogen, Oxygen,
| Nitrogen) food I've read about in some science fiction novels
| I've read.
|
| It would make sense that if you could "print" food from CHON
| then to restock during your inter-galactic travels you'd just
| need to find some nice asteroids with the requisite
| materials.
| ptr2voidStar wrote:
| Fascinating! I've never heard of CHON before. I like this
| idea ...
| TeMPOraL wrote:
| I'm thinking higher level than that, for practical reasons.
|
| Assembling complex things - particularly biologics - from
| raw atoms is a painful, slow, and energy-intensive work.
| The growth rates of various organisms on Earth give you a
| ballpark of how fast you can get with this. I for one would
| love replicators to work faster if possible :).
|
| I feel a better idea would be to make universal feedstock
| (or family of those) out of complex molecules - perhaps
| proteins, or protein-like nanostrutures. Molecules selected
| for having energy advantage in bulk reactions and precision
| nanowork required to assemble the most common things people
| replicate.
| whatshisface wrote:
| CHON food would not really work because it doesn't have any
| sodium or chloride, and who wants to live without salt?
| (But more seriously, we need our mineral nutrients to
| survive.)
| barbecue_sauce wrote:
| Well, then just add an S and a Cl in there.
| npongratz wrote:
| >> ... because it doesn't have any sodium or chloride ...
|
| > Well, then just add an S and a Cl in there.
|
| If we're talking sodium, wouldn't "S" instead be "Na"?
| Though I'm guessing sulfur has some importance as well.
| Khoth wrote:
| Much like how we can turn lead into gold but don't, this
| process isn't remotely cost-effective. There's plenty matter
| lying around that you can have for probably 20 orders of
| magnitude cheaper.
| api wrote:
| Unfortunately the energy requirements of those replicators
| would be ludicrous... like 10x the yield of the Tsar Bomba
| (largest H-bomb in history) to make a cup of tea. Replicator
| malfunction would be an extinction level event.
|
| Just do E=mc^2 on the mass of your cup of tea.
| Retric wrote:
| More like 1/10th the Tsar Bomba. 1KG = 9e16J, where the bomba
| was ~209 PJ or 2e17J and a cup of tea is only like 0.2kg so
| 2e16J.
| vanderZwan wrote:
| So only merely wiping a city off the map, nothing as
| dramatic as an extinction level event, then? ;)
| rainworld wrote:
| Twice that. One cup antimatter, one cup ambient matter.
| Retric wrote:
| Not everything that produces mater also produces
| antimatter otherwise the universe would be a 50:50 mix of
| each. Why Baryon asymmetry exists is unknown, but as long
| as we're talking magic tech we might as well make it
| efficient.
| tsimionescu wrote:
| > Not everything that produces mater also produces
| antimatter otherwise the universe would be a 50:50 mix of
| each.
|
| That's not known. It's also possible that the initial
| stuff of the universe, before inflation happened, had
| more particles than antiparticles. Since we have no
| physics for what happened before inflation started, this
| wouldn't break any theory at all. It's a bit
| unsatisfactory as theories go, but it is absolutely
| compatible with all physical observations so far.
| Retric wrote:
| That's just a turtles argument, whatever happened before
| the Big Bang now has the asymmetry.
| tsimionescu wrote:
| Well, technically whatever happened between the big bang
| and inflation. But, while unsatisfactory, this is not an
| invalid argument. It is very plausible that it is
| impossible to replicate the conditions of the big bang,
| or anything close to them, so it is plausible we will
| never be able to scientifically describe that process.
|
| As such, it is possible that the processes of the Big
| Bang (or whatever came before) somehow created more
| matter than anti-matter, but all of the physics that
| comes after is symmetrical with respect to charge. So, we
| have no reason to expect that we'll ever find a theory
| explaining why there is more matter than antimatter, just
| like we don't have any reason to expect to ever find a
| theory that explains why there are two kinds of
| electrical charge and not 5. So, it's not a promising
| avenue for research, and not a promising assumption.
|
| In contrast, there are reasons to expect we will some day
| find a theory of quantum gravity, since we know for sure
| matter does interact gravitationally and QM doesn't
| account for that.
| Retric wrote:
| Before or after the Big Bang you still get some process.
| The ability to replicate it is of course in question, but
| we are already talking about a device we have no idea how
| to build so that doesn't change anything.
|
| As to impossibilities, we already figured out how to
| break conservation of energy due to the expansion of the
| universe. Arguing about what is in isn't possible for
| some process we don't know about seems silly.
| ta988 wrote:
| I would prefer a cup of dark matter please.
| airstrike wrote:
| Joseph Mallozzi has you covered, then
| api wrote:
| Whoa... so that means the Tsar Bomba actually converted
| many _kilograms_ of mass into energy? Holy crap.
|
| Ahh... the YOLO days of nuclear physics...
| simion314 wrote:
| Star Trek will use existing matter to replicate stuff(but maybe
| not all the time) , otherwise it would cost a lot of energy.
| pretadank wrote:
| Curious if the reverse is true ... also is momentum conserved in
| this, then wouldn't the positron electron pair be shooting off at
| the speed of light, and if not where is the extra energy going?
| Or have I missed something?
| turdnagel wrote:
| At the risk of sounding like an idiot, why doesn't this happen
| when you point two flashlights at each other? Or point a
| flashlight at the sun? Or when light from one star reaches
| another? Etc etc.
| platz wrote:
| does your flashlight emit gamma rays
| choeger wrote:
| It's funny that you ask, but I have this flashlight that I
| found in my weird neighbor's garage/machine shop...
| stronglikedan wrote:
| Is your weird neighbor's surname, Smith, by any chance?
| turdnagel wrote:
| I also mentioned stars, but thanks.
| [deleted]
| Y_Y wrote:
| You need to have enough energy (aka. mass, physicist will use
| the two interchangably) to pay for whatever particle you want.
|
| An electron weighs about 10^-30 kg, or about half a
| megaelectronvolt, but because of cosmic balance you'll want to
| make a positron at the same time. So your total bill will be
| about 1 MeV. Visible light photons have about 1 eV of energy,
| if you want a photon with an MeV then you're past even hard
| x-rays and into gamma rays. This process is the same as matter-
| antimatter annihilation, except backwards.
|
| A proper explanation would take several years.
|
| By the way, you can use more than one photon at a time, but to
| my knowledge nobody has managed to fuse the million flashlight
| photons you'd need to do pair-production.
| jacinabox wrote:
| Matter from pure light huh, today's job market is getting
| competitive for sure.
| ivoras wrote:
| Can someone ELI5 the relationship between electrons and photons?
| So far we have:
|
| * Electrons emit photons when going from a high energy state to a
| low energy state (but remain electrons)
|
| * Two photons colliding produce an electron-positron pair (always
| exactly one?)
| tsimionescu wrote:
| There's no ELI5 answer. You can look at some Standard Model
| charts. In principle, any particles in the SM can interact or
| decompose into any other particle, as long as mass-energy,
| spin, charge, color charge etc are conserved. The probability
| of many of these interactions may be minuscule.
| dekken_ wrote:
| Except electrons aren't matter.
| fires10 wrote:
| This gives me a negative feeling.
| Positivelyfalse wrote:
| "The electron is a subatomic particle, (denoted by the symbol
| e- or b- ), whose electric charge is negative one elementary
| charge.[9] Electrons belong to the first generation of the
| lepton particle family,[10] and are generally thought to be
| elementary particles because they have no known components or
| substructure.[1] The electron has a mass that is approximately
| 1/1836 that of the proton."
|
| https://en.m.wikipedia.org/wiki/Electron
| SAI_Peregrinus wrote:
| Electrons are Fermions, and therefore matter. That's in
| contrast to Bosons, which are "energy" (more precisely force
| carriers).
| ironSkillet wrote:
| I am positive that electrons are matter. Never heard your claim
| before.
| dekken_ wrote:
| You are positive you were told that electrons are matter
| maybe. This is the standard (see: current) model, in QFT
| electrons are not particles, so are not leptons, so are not
| matter.
| drdeca wrote:
| Wikipedia says "Even restricting the discussion to physics,
| scientists do not have a unique definition of what matter
| is." (in the sense of contrasting with antimatter), and
| then later says "In a wider sense, one can use the word
| matter simply to refer to fermions.". (Presumably this
| means, fermions and things comprised of fermions. One would
| not say that Helium-4 is not matter just because it is a
| boson (and is comprised of an even number of fermions).)
|
| Electrons are fermions.
|
| I am therefore comfortable saying that electrons are
| matter.
|
| I don't know why you say that electrons are "not particles"
| in QFT. Sure, there's an electron field, and electrons are
| just, like, excitations in that field. There's also a quark
| field. What of it? If you are saying "because everything is
| fields, there are therefore no particles, and therefore no
| matter", you are using the word "matter" in a silly way.
|
| Also, isn't a "particle" just a representation of a compact
| Lie group? What's the problem?
| petrocrat wrote:
| Is the mass measurement of electrons an abberation then in
| the QFT interpretation of the empirical data? Or is it that
| energy can have mass prior to being converted under some
| conditions?
| dekken_ wrote:
| > Is the mass measurement of electrons an abberation then
| in the QFT interpretation of the empirical data.
|
| I'm not sure this is resolved. see: https://en.wikipedia.
| org/wiki/Quantum_electrodynamics#Conclu...
|
| > Or is it that energy can have mass prior to being
| converted under some conditions?
|
| Using https://en.wikipedia.org/wiki/Mass%E2%80%93energy_e
| quivalenc... you can (theoretically) convert between
| energy and mass, and then measure everything as
| https://en.wikipedia.org/wiki/Electronvolt. I have yet to
| see anything create protons from energy, so I am hesitant
| to do this.
| pelorat wrote:
| How do you think we got matter from the big bang?
| dekken_ wrote:
| I don't pay much attention to big bang things, as far as
| I'm concerned it's as unprovable as an infinite universe
| theory. I'm not sure we can ever know either way.
| TeMPOraL wrote:
| > _I have yet to see anything create protons from energy,
| so I am hesitant to do this._
|
| Won't a proton/antiproton annihilation also create two
| photons? If so, wouldn't that also work in reverse?
| dekken_ wrote:
| > Won't a proton/antiproton annihilation also create two
| photons?
|
| I expect it would make a lot more than just two photons.
| See: https://en.wikipedia.org/wiki/Annihilation#Proton%E2
| %80%93an...
|
| > If so, wouldn't that also work in reverse?
|
| It would have to be demonstrated.
| TeMPOraL wrote:
| Thanks, I didn't know proton/antiproton annihilation is
| so complex. But it makes sense.
| jcims wrote:
| I have had this mental question about 'what time is it when
| photons collide'. Given that a photon could have been travelling
| for a million years colliding with one that has only been
| travelling for a microsecond, and from the photon perspective
| there is no passage of time and no distance.
|
| I then kept reading things that tell me photons don't interact,
| which saddened me because I like the question. This now appears
| to not be the case, is there a specific condition under which
| photons can interact like this?
| bbarnett wrote:
| Don't feel sad. If you look at a set of all ever existing
| humans, the number that ever interact is incredibly small.
|
| Even if in the same vicinity, and time, eg same apartment
| building, subway, street, driving down the road, there is often
| zero interaction.
|
| Really, humans interact very rarely too. Yet, humans _do_
| interact, in meaningful ways. Thus, it is likely the same for
| photons.
|
| This theory also explains why, with the rarity of human
| interaction, I am unmarried. In fact, it probably explains it
| on multiple levels.
| jdbburg wrote:
| In classical electromagnetism photons (light) do not interact,
| period. But in quantum mechanics the photons can briefly
| fluctuate into electron + positron pairs which allow them to
| interact with each other. But the probability for interacting
| is very small AND you need enough energy to make the electron
| positron pair (for the Breit-Wheeler process), so it is very
| rare in practice.
|
| I am not sure about the "time" the photons collide, but the
| interesting thing is that the Breit-Wheeler process is what
| determines the opacity of the universe - since high energy
| photons traveling through the universe can hit low energy
| photons from the cosmic microwave background and convert
| (disappearing) into an electron positron pair.
| earthscienceman wrote:
| Wait. I did a physics undergrad but I never asked myself this
| question. How is it possible that the EM fields from photos
| can't interact with each other? Doubly funny, I did my
| undergrad research for the PHENIX experiment.
| whatshisface wrote:
| Phillips gave you a good answer, but another good answer is
| that Maxwell's equations are linear; forbidding any
| interaction between solutions _f_ and _g_ because if _L_ is
| a linear operator, L[ _f_ + _g_ ] must = L[ _f_ ] + L[ _g_
| ].
| acchow wrote:
| Where do they teach physics like this? Anywhere?
| whatshisface wrote:
| I feel like this would be mentioned in a typical
| undergraduate wave mechanics course taught in a typical
| American university, but I have not been to enough
| undergraduate wave mechanics courses in enough American
| universities to know, really.
| earthscienceman wrote:
| Oooo. Wow. This is a great answer, thanks for explaining
| it that way.
| jdbburg wrote:
| This is what we are taught in high school / college as
| the super-position principle
| at_a_remove wrote:
| Photons _are_ EM fields -- self-propagating ones, like a
| glider in the Game of Life. The electric field changes,
| creating a magnetic field, but the magnetic field changes,
| creating an electric field. Use sines for the functions and
| you can see how the function just keeps re-appearing.
|
| Now, combine that with the superposition principle ...
| photons pass through one another under most circumstances
| because of this (unlike gliders).
| leephillips wrote:
| Fields are abstractions that ease the calculation of the
| forces on charges caused by the positions and motions of
| other charges. The charges interact, but the fields don't;
| there is nothing "there" to interact. (This is classical
| EM.)
|
| If you are in a room with two charges, the electrostatic
| field at any point is the addition of the fields from each
| charge. There is no extra interaction term.
| vlovich123 wrote:
| Stupid question I'm sure, but do our estimates for the total
| amount of matter in the universe account for this effect?
| Could this effect end up being the source of dark matter? Or
| is too infrequent to generate enough matter to explain the
| discrepancy?
| simonh wrote:
| Best estimates are that photons only make up about 0.005%
| of the energy density of the universe. There's just not
| enough of them.
| whatshisface wrote:
| If by "this effect" you mean particles fluctuating so that
| every field has at least some role to play in every
| interaction, it also applies to empty space, which has a
| nonzero energy density due to its own fluctuations. In
| fact, empty space has an enormous energy density due to
| this stuff. It is so enormous that it's implausible, and
| that's considered to be one of the major unsolved problems
| in physics.
|
| If by "this effect" you mean radiation occasionally
| interacting to produce particles, that actually doesn't
| change the amount of gravity in the universe when it
| happens, because energy is what gravitates and it's
| conserved in particle interactions.
|
| https://en.wikipedia.org/wiki/Cosmological_constant_problem
| jdbburg wrote:
| Not a stupid question. However, I think that this process
| (whether it is included in calculations or not) shouldn't
| contribute to the dark matter issue. Electrons and
| positrons are "normal" matter and if they are anywhere near
| e.g. a star, they will contribute to the plasma around the
| star and therefore be visible. Also, even if it were not
| visible, my intuition is that the process is too rare to
| produce enough mass to account for the dark matter
| rootbear wrote:
| Thank you! I was curious about exactly this. But now I have a
| new question. If a photon is sufficiently energetic to be
| capable of briefly fluctuating into an electron-positron
| pair, does that imply that it would, over a long distance, be
| slower than c, since the e-p pair can't travel at c for the
| brief moment it exists?
| jdbburg wrote:
| The structure of the photon (here meaning the fact that it
| can fluctuate into an electron positron pair) is what leads
| to the speed of light being c and not something even
| larger. So your intuition is somewhat correct, except that
| the effect is "already taken into account" when we first
| learned what the speed of light was. Also, a photon can do
| this fluctuating even if it doesnt have a lot of energy,
| since the electron positron pair can be virtual - i.e. they
| have almost no mass and live only for an instant before
| annihilating back into a photon
| shoemakersteve wrote:
| That's really interesting. What's this effect called? I'd
| love to learn more about it. This topic fascinates me to
| no end!
| vlovich123 wrote:
| Hmm... is the right mental model then that the photon has
| a speed higher than C in its photon state but on average
| it's C due to the fluctuation into an electron/positron
| pair? I would imagine that's the wrong model because
| there's contradictions that arise (e.g. the speed of
| light would vary with the energy of the photons so that
| those not fluctuating would have a higher speed but
| that's not the case AFAIK). Is this just "quantum
| mechanical weirdness" or do we have an "intuitive"
| explanation for how the speed of light and this
| fluctuation interplay?
| tsimionescu wrote:
| I don't think this is right at all. C is a universal
| constant, and it is not in fact known for sure whether
| light travels at c or just below (whether photons have a
| mass). However, individual photons in certain conditions
| can absolutely move at speeds lower than c. If photons of
| different energy move at different speeds, this would be
| detectable by firing the photons at the same far-away
| mirror, and measuring how much time it takes for each
| photon to return (on average).
| jerf wrote:
| This is a good example of one of the errors that I often see
| in what is otherwise the best scientific writing, which is
| either forgetting to specify what a certain _model_ says, or
| simply straight up forgetting /confusing that a certain model
| isn't reality.
|
| Maxwell's equations say that light doesn't interact.
| Maxwell's equations are known to be wrong in that way.
| They're still a very good simplifying case that you can do
| very well with using, by all means, just like Newtonian
| physics in the right conditions, but they aren't the way
| reality works. Maxwell's equations also have no place to put
| a gravity term, yet gravity clearly affects light.
|
| The one I probably see the most often is articles about black
| holes confidently speaking about "what goes on below the
| event horizon" from an Einstein relativity point of view,
| which is where you get all the singularities and ring
| singularities that lead to different universes somehow, etc,
| again either forgetting to point out or simply forgetting
| entirely that those are the specific predictions of Einstein
| relativity, which is _known_ to be inadequate to describe the
| inside of a black hole. It is certainly fair to discuss that
| theory 's predictions, and whatever really is happening in
| there, relativity will certain shine some sort of light on
| it, but it is a mistake to present it simply as "what happens
| on the inside". The model is known to be broken here.
|
| I am working out how to phrase this in a way that makes sense
| to the HN crowd because this tends to ruffle feathers when I
| say it, but this is all what should be well-known stuff. It's
| not like I'm "denying science" when I say this; quite the
| contrary! It's "denying science" when you _insist_ the known-
| by-science-to-be-broken models are in fact not broken where
| the science is pretty clear that they are.
| tgb wrote:
| I find your comment confusing. What exactly is the "good
| example" you're referring to? The person you are replying
| to appears to have done exactly what you are saying to do,
| by my reading, so I'm wondering what you're referring to.
|
| 'The map is not the territory' is the phrasing I've heard:
| https://en.m.wikipedia.org/wiki/Map%E2%80%93territory_relat
| i...
| radicalbyte wrote:
| > or simply straight up forgetting/confusing > that a
| certain model isn't reality.
|
| That's partly to do with the way that high school science
| is taught. If we made the context clear at all times then
| people would have a better grounding.
| whatshisface wrote:
| > _Maxwell 's equations also have no place to put a gravity
| term, yet gravity clearly affects light._
|
| The place to put the "gravity term" is in the coordinate
| system that Maxwell's equations take place in, and the
| definition of the derivative which is implicitly brought in
| via the curls, divergences, etc. That's general relativity,
| and Maxwell's equations are already fully compatible with
| it.
|
| > _[this tends to ruffle feathers when I say it] ... It 's
| "denying science" when you insist the known-by-science-to-
| be-broken models are in fact not broken where the science
| is pretty clear that they are._
|
| People are probably taking issue with your use of the words
| "broken" and "wrong," because you're describing a car that
| says 90mph on the dealership's sticker but can't go 900mph
| as "broken," or a one pound lump of beef as "wrong,"
| because although the butcher said it weighed a pound, and
| you were charged for a pound, it'd be nice if it were two.
| jerf wrote:
| Yes, if you add relativity to Maxwell's equations, you
| get relativistic Maxwell's equations. Ultimately not
| particularly relevant here anyhow since it's QM
| describing what's going on here, not relativity.
|
| I don't deal in automotive metaphors because they rarely
| enlighten, so I'll just stick with, yes, they _are_
| broken in those places, and are not suitable for
| unqualified claims about the nature of reality. This isn
| 't about what would be nice if it were true or slight
| exaggerations, it's about the models being broken by
| being applied outside of their domain in an unqualified
| manner. That's exactly _not_ how they are wrong. They are
| wrong in a much stronger manner.
|
| And what's more, their strong brokenness is _scientific
| consensus_ , not some sort of whacky theory. Whack
| theorization is what you're doing when you take these
| models, apply them in a domain they are known to be
| broken in, then claim this is the absolute truth about
| what is going on.
| ChrisLomont wrote:
| > relativity to Maxwell's equations, you get relativistic
| Maxwell's equations
|
| Maxwell's equations imply (special) relativity, so
| there's nothing to be added. Maxwell's equations imply
| the speed of light is the same in all reference frames,
| which is all you need to derive special relativity.
|
| That is why people of the time were trying to understand
| how this can ben so, why the did things like Michaelson-
| Morely to look for invariance/ether, and why so many of
| the terms used in relativity predate relativity, since
| they were invented to handle that Maxwell's Equations had
| this invariance.
|
| Basically, Maxwell's equations, as written were
| relativistically invariant, thus compatible.
| tsimionescu wrote:
| Special relativity is (partially) predicted by Maxwell's
| equations, and they are fully compatible. They are
| instead incompatible with classical mechanics.
|
| GP claimed that Maxwell's equations are missing a term
| for gravity/mass, which would be the domain of General
| Relativity. This is more complicated, as it's true that
| they didn't predict gravitational lensing. But, they are
| still compatible with GR, as GR modifies the coordinate
| system, and Maxwell's equation in the GR curved space-
| time coordinate system do predict gravitational lensing.
|
| GP also pointed out that Maxwell's equations are not
| compatible at all with QM, as they incorrectly predict
| that photons can't interact. Here there is no way to save
| them - Maxwell's equations are just an approximation, and
| the actual laws governing the behavior of light are
| substantially different, only reducing to ME in certain
| approximations (just like classical mechanics is not
| compatible with either QM or SR/GR, except as an
| approximation of either of the two others).
| im3w1l wrote:
| Rest mass is conserved. So the "system" of the photons has to
| have sufficient rest mass to support the particle production.
| Rest mass depends on the energy of the individual photons, and
| the angle they are colliding with (head on is best, nearly
| parallell photons grazing is worst).
| jfengel wrote:
| Rest mass is not conserved. Total energy is conserved, which
| includes both rest mass and momentum (E^2=m^2+p^2). Photons
| have momentum but no rest mass; ordinary matter has rest mass
| but (usually) little momentum.
| hpcjoe wrote:
| Photons are not generally thought to have a rest mass, unless
| something has changed in the last 25 years or so. The
| conversion of energy into 2x electron masses (positron +
| electron). So the photon pair needs at least 2 x 911 MeV of
| energy. This would be around 1.82 GeV of photon energy. To
| give a sense of the frequency of this, a 911 MeV photon
| (colliding with another of the same energy) would have a
| frequency of 911 x 10^6 ev/(4.14 x 10^-15 ev/Hz) or 2.2 x
| 10^23 Hz, with a wavelength of 1.4 x 10^-15 m. That is, very
| hard gamma.
|
| But the photons really shouldn't have rest mass.
| im3w1l wrote:
| If you have two photons, then neither of them will have
| rest mass. But the system of both of them does. This
| happens because |p_system| < |p_1| + |p_2|.
|
| https://en.wikipedia.org/wiki/Energy-momentum_relation
| hpcjoe wrote:
| I'd suggest looking at the Invariant mass page. What you
| are talking about isn't rest mass of a photon, rather the
| center of mass invariant mass[1]. This arises from the
| equivalence principle, the relation between mass and
| energy, not because a photon (a massless particle) has a
| rest mass (it doesn't).
|
| It might be a pedantic argument on my part, but still, it
| is worth being clear on this point.
|
| [1] https://en.wikipedia.org/wiki/Invariant_mass#Collider
| _experi...
| im3w1l wrote:
| (Center of mass) invariant mass is synonymous with rest
| mass.
| AnimalMuppet wrote:
| Is it? It sounds like you are explicitly disagreeing with
| hpcjoe's position, but just a bare "no, I'm right" isn't
| informative. Can you give some reason or argument that
| supports your position over hpcjoe's?
| im3w1l wrote:
| > In physics, the energy-momentum relation, or
| relativistic dispersion relation, is the relativistic
| equation relating total energy (which is also called
| relativistic energy) to invariant mass (which is also
| called rest mass) and momentum. It is the extension of
| mass-energy equivalence for bodies or systems with non-
| zero momentum
|
| Those are the first few sentences in the article I
| already posted.
|
| > The invariant mass, rest mass, intrinsic mass, proper
| mass, or in the case of bound systems simply mass, is the
| portion of the total mass of an object or system of
| objects that is independent of the overall motion of the
| system. More precisely, it is a characteristic of the
| system's total energy and momentum that is the same in
| all frames of reference related by Lorentz
| transformations.[1] If a center-of-momentum frame exists
| for the system, then the invariant mass of a system is
| equal to its total mass in that "rest frame".
|
| Those are the first few sentences in the article he
| already posted.
| AnimalMuppet wrote:
| In the last sentence of the article he posted, it's
| unclear to me how that applies to a system with two
| photons in it. If I have two photons heading off in
| opposite directions, I'm in the center-of-momentum frame
| for that system. Then the invariant mass is equal to the
| system's "total mass" in that frame. For two photons, is
| that 0, or E/mc^2?
|
| I know what your position is. I'm just not sure that the
| quote from his article supports your position.
|
| I will grant you that if you take the energy-momentum
| equation and apply it to such a system _as a whole_ , and
| plug in the total energy (non-zero) and the total
| momentum (zero), then out comes a non-zero mass. I just
| am unclear on whether that's a valid thing to do. (I also
| don't know that it's _not_ valid.) Can you make a
| convincing case that it 's reasonable to apply that
| equation in that way?
| im3w1l wrote:
| Well there are a couple of things to address here. The
| first is that the invariant mass is invariant - it's the
| same in any reference frame. So the invariant mass of a
| system will either equal the sum of the masses of the
| particles or it won't, irrespective of frame. And
| equality will only hold if there is no internal movement.
|
| > Can you make a convincing case that it's reasonable to
| apply that equation in that way?
|
| Even hpcjoe did not object to that, only to the name
| "rest mass" for the result. We saw that the number you
| get out was useful upthread, because it allowed
| predicting if pair production can occur or not.
| alde wrote:
| Photon - photon scattering in vacuum is a known process, they
| can interact through virtual particles. The problem is that the
| probability of such an event is very low and requires very high
| photon energies.
| aero-glide2 wrote:
| "From the photon perspective there is no passage of time" What
| exactly happens when light travels through denser medium? Speed
| is less than c, so does that mean it experiences time?
| gumby wrote:
| > Speed is less than c, so does that mean it experiences
| time?
|
| There is no absolute clock in the world; we can only know
| about what happened where and when by looking.
|
| A way to understand this (and _special_ relativity) is from
| an information-theoretic perspective, and to think of c as
| the "Mach number" of free space, nothing more and nothing
| less.
|
| So imagine that you couldn't see but could only know about
| what happened where and when by hearing. All those phenomena
| (foreshortening, frequency shift, apparent time dilation)
| would be exhibited though the speed of information travel
| would be much lower. You have probably experienced the
| equivalent of blue shift/red shift yourself by listening to a
| high speed vehicle.
|
| Change the medium from ordinary air to something with a
| different density and all these phenomena will be detectable,
| even though the speed of sound will be different.
| aaaaaaaaaaab wrote:
| https://en.wikipedia.org/wiki/Ewald-Oseen_extinction_theorem
| eptcyka wrote:
| It bounces around, thus not travelling in a straight line,
| thus covering more distance whilst still moving at c.
| cygx wrote:
| That would imply light getting scattered willy-nilly as it
| crosses any medium, which it isn't in case of transparent
| media...
| monocasa wrote:
| There's no perfectly transparent media other than pure
| vacuum.
| jcims wrote:
| My understanding, which could be wrong, is that it doesn't
| 'bounce around' as much as it is absorbed and re-emitted.
| This process isn't instantaneous and accounts for the net
| propagation delay through the medium.
| monocasa wrote:
| Sort of. Refraction it better looked at from a wave
| perspective than a particle one, IMO. The wavefront
| expands at the speed of light, but the wave bounces off
| of the medium it's traveling through and self interacts
| in a way that means that the highest probability area
| moves slower than C despite the edge of the wavefront
| still moving at C. That smearing is also why the
| wavelength changes upon refraction. (And obviously
| leaving concepts like time dilation off the table for
| simplicity's sake).
| wruza wrote:
| Both ideas are incorrect. Bouncing around would be random
| and the outgoing light would be scattered (as in matte
| reflections) in all directions. Absorbtion simply
| destroys the original light. If photons are mostly
| absorbed, we call that "a wall" or "a fog", not "a
| transparent medium", which to be transparent _shouldn't_
| have electrons whose energy level differences correspond
| to a photon's wave length. The transparency and slowdown
| effect is a complex interaction between fields in the
| medium, which I honestly don't have enough courage to
| retell.
|
| https://youtube.com/watch?v=CiHN0ZWE5bk
| whatshisface wrote:
| There are two pictures that you could take of the situation.
| Classically, where the medium is smooth continuous stuff with
| a permeability and a permittivity, there are no photons, only
| waves. Quantum mechanically, where the medium is made up of
| lots of little particles, there are photons indeed, but no
| smooth medium, and no region where the speed is less than C
| (because there is no "region," only lots of separate
| particles). So there's one picture missing "photons," and the
| other missing "speed less than c," so you can't capture
| "photons in a region where the speed is less than c," in a
| single image.
|
| I guess if you wanted to pose that question in a framework
| where a medium where the speed was less than c, and photons,
| could exist simultaneously, you would want an effective field
| theory (maybe). But you will have to get someone else who
| knows more than me to explain that. :-)
| TedDoesntTalk wrote:
| > they collided gold nuclei accelerated to 99.99 percent of the
| speed of light.
|
| How did they achieve that?
| detaro wrote:
| Sentence before:
|
| > _Relativistic Heavy Ion Collider_
|
| https://en.wikipedia.org/wiki/Relativistic_Heavy_Ion_Collide...
| NiceWayToDoIT wrote:
| So, after all, it seems science fiction writers are prophets in
| disguise.
|
| List of Star Trek inventions that came true:
| https://qz.com/766831/star-trek-real-life-technology/
|
| Although, from "light into electron and positron" to "Tea, Earl
| Grey, Hot" there is a long way to go.
| whatshisface wrote:
| You can't give credit to Star Trek because I am pretty sure
| this possibility was understood before Gene got to it. ;)
| NiceWayToDoIT wrote:
| True, Star Trek initial release was October 14, 1986 while
| theory was there from 1934. Thanks for correction.
| neonnoodle wrote:
| We have matter at home.
| noobermin wrote:
| I was surprised because I thought the Breit-Wheeler Effect has
| been demonstrated already. It just looks like what they mean here
| is single photon (or really, two photon), not the so-called
| strong field Breit-Wheeler that was demonstrated at SLAC and with
| powerful lasers for decades now.
| jdbburg wrote:
| One of the authors here. You are correct, the famous SLAC
| result already demonstrated that light can be converted into
| matter, but they needed several photons >4, average of about 6
| - that is the so called non-linear Breit-Wheeler (BW) process.
| This result is the original linear BW process where only 2
| photons collide. The main difference is that for the linear BW
| you need much higher energy photons. Also, in this experiment
| we are able to measure the wavefunction of the two-photon
| system, which proves other aspects of the original predictions
| by Breit & Wheeler and others.
| suifbwish wrote:
| I was always curious before I heard of this today what would
| happen if you concentrated more and more light into a ultra
| fine point. It makes sense that at some point particles would
| begin to emerge as photons appear to have various
| electromagnetic properties and also carry a small amount of
| kinetic force (solar sail principle)
| Chris2048 wrote:
| So, a question: traditional objections to galactic travel (e.g.
| 1G accel) is the requirement for huge amounts of propulsion mass.
| Even with a method of perfect E=mc^2 of some special fuel matter,
| you still need a whole load of propulsion mass to chuck in the
| opposite direction, which in turn requires more energy to
| accelerate.
|
| the question is, could we get around that by converting from
| mass-less forms of energy, and generating mass on-the-fly,
| skipping the need to accelerate tons of acceleration mass; or is
| there some restriction that would disallow this?
|
| If I fire 2 photons at each other on a ship travelling at 0.5c,
| and this creates 2 particles e+ and e-; would the particles also
| be travelling at 0.5c along with the ship; would their
| velocity/momentum be in random directions etc?
| was_a_dev wrote:
| Publication link:
| https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.12...
| adrian_b wrote:
| Free link:
|
| https://arxiv.org/abs/1910.12400
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