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Comment by hinkley | Hacker News Reader

> But the extra photon wasn’t created by that special process, so instead of disappearing when you turn back time, its wavelength will eventually get impossibly small, concentrating its energy so greatly that the photon collapses into a black hole. This creates a paradox, absurdly implying that — in this fictional, expanding universe — microscopic black holes convert into photons. The thought experiment suggests that a naïve mashup of unitarity and cosmic expansion doesn’t work.

The Laws of Thermodynamics state that you can’t create a photon from nothing, so isn’t introducing an “extra photon” into the universe already a paradox? Who would be the least bit surprised that paradoxes create paradoxes?

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I don't know whether there was some clever construct to the thought experiment that was lost by the time of printing, or whether it's just a bizarre case of "I added in something that violates conservation of energy and lookie here we can make a paradox out of this!" like this is some double negation being used to "prove" something.

If you imagine something physically implausable as your setup, then yeah it's likely to have physically implausible consequences; and in this case seems rigged for paradox, since we're creating a photon in one moment, but then supposing it already existed and had a history, like, this just seems too awkward for a theoretician to actually propose.

Our theories are just maths that we CAN use to describe reality, when reality is suitably captured and represented within it, but can ALSO describe something that has no bearing on reality, just because "the types fit", as it were, to borrow from programming. The theory is still "good" because it works for experimental predictions, even if the implausible can happen if you tried it "outside" reality - if those issues creep into reality, then it breaks down in circumstances where it is "bad" or "incomplete. I'm sure Giddings understands that, so I'd lean more towards that this was just a bad translation from a more robust mathematical thought experiment that wasn't said in English well, or that it just didn't make it to press unscathed from the writer's and perhaps editor's misunderstandings, despite best intentions and efforts.

Because casting aspertions would not be fair when we are not experts and do not have the full context before us. I hope that Quanta releases clarification and edits to help us understand that section better.

His later statement that plausible states are only thus if they "evolve backward without generating contradictions" which is a truism if I ever heard one. You mean plausible states come from a prior plausible state, and can't just be created willynilly? Wow! That would be pretty damning for your thought experiment as presented!

Your thought experiment photon not being created by "special processes" like any of the interactions of quantum mechanics (such as virtual pair annihilation or from atoms) means that it presupposes an origin for such a photon, and you find it comes from an evaporated micro black hole? That doesn't feel like a paradox, that feels like the conclusions from what may be an implausible setup. Conservation of energy demands the photon comes from somewhere, and you found that it was emitted by a black hole, just as it vanishes; so perhaps it's less implausible than imagined, or perhaps not? Either way, the only paradox is the demand that we be able to place a photon in space without any cause and then work backwards to assume where in the vaccuum this photon sprung up from to somehow make this implausible scenario plausible - and then, when you find it, and don't trace back further to ask what made that, you then assert it's a paradox because you don't think a black hole makes sense? I still find Biddings sections in this article very confusing and in need of clarification. But, I doubt the universe is in the habit of retconning, though arguably we wouldn't be able to tell either way, so oh well.

The idea is that they add a magically a photon now, but they add in a very magical way that will change the future and also the past, so the new universe from the beginning to the end is consistent. If the laws of physics are reversible as we currently think, this magic operation make sense. You can't do it actually in the real world, but it's fine to imagine a new universe.

The problem is that the photons appear/disappear when they interact with a charged particle, but you can imagine a universe with a unlucky photon that never collides with a charge particle in the future, and that is so unlucky that when you pass the universe like a reverse movie, it also never collides(collided) with a charge particle in the past.

> you can imagine a universe with a unlucky photon that never collides with a charge particle in the future

Have we proven that though? I thought there was still some debate about how to think about photons. From the photon’s perspective it is a line in a four dimensional space, from source to sink with no time component, no curve (straight line through curved space).

I took away from that example that, even though unitarity clashes with cosmology to an extent, you can’t just naively break it or you get “even worse” paradoxes like this one. Plus, conservation of energy doesn’t apply to the universe as a whole, if it’s expanding (space has energy.) The assumption which is being violated by adding the single photon is really unitarity and not the laws of thermodynamics. (But it’s been a while since grad school for me, could be wrong here.)

> Plus, conservation of energy doesn’t apply to the universe as a whole, if it’s expanding (space has energy.)

I don’t believe that, but we either may never be able to prove it, or discover we live in a bubble of space where physics has different properties. We don’t know where the space is “coming from”. It’s the same problem laypeople have with the Big Bang Theory. Well where did the Bang come from? What happened before? There is no Before. The Bang and Time were born together.

We may well find that we live inside a black hole. The space inside a black hole grows. One theory says that infinite curvature means it grows at the speed of light. If we live in a black hole then perhaps the energy to create the additional space is coming from outside, our “experiments” are not within a closed system, and look like Free Energy but are not.

Or maybe we live in a physics bubble where different rules apply. Which might solve the whole, “how will we ever explore this Universe in time?” ‘dilemma’ because any ship we send outside that bubble starts to disassemble itself. The floor is lava.

Energy is guaranteed to be conserved if the Hamiltonian has time symmetry. The Hamiltonian of the universe may not have that property, and probably doesn’t. You can Wikipedia Noether’s theorem for more info.

Afaik particles can seem to appear “out of nothing”, random in nature. They are called virtual particles.

I believe the effect is very pronounced in the Casimir Effect https://en.wikipedia.org/wiki/Casimir_effect

The analogy is imprecise though, according to people know the maths/physics behind it, maybe someone can answer it better than me.

The Casimir effect is better explained by the zero point energy of the quantum vacuum, rather than "virtual particles". The latter is not necessary to explain it

Virtual particles are not particles.

https://profmattstrassler.com/2011/10/10/virtual-particles-n...

Probably completely wrong, but the mnemonic I use for virtual particles, ever since I read Hawking as a wee lad (read: before decades of more literature has been published), was that matter and space are part of a whole, any time there's space (pun not intended), things can get wobbly, and variation has a sane average but the outliers can be weird. Like a rogue wave in the ocean if the wobbles line up they 'look' like something but usually they fade back into nothing before they smash into anything that would observe them.

So virtual particles near an event horizon can interact with matter, but most of the time they just don't.

I have persisted with this mnemonic because nobody has made a headline of "Hawking was wrong!" so far.

Hawking Radiation is the process where virtual particles escape annihilation, and the surviving particle steals that energy from a black hole. If the black hole lives in space devoid of enough matter, it could eventually lose its event horizon, and explode.

If the supposed experimental conditions cannot plausibly occur in nature I don't think it's a valid thought experiment.

Conservation of energy is not fundamental. Noether’s Theorem shows that Conservation Of Energy only arises in systems that exhibit “Time Translation Symmetry” i.e. conducting an experiment now, and conducting an experiment later always are the same.

If you fling a charged particle like an electron through an increasing magnetic field, it will appear to gain energy.

Locally our universe seems to obey Time Translation Symmetry and thus also Conservation of Energy. However the expanding universe is not Time Translation Symmetric, at the scale of galactic clusters new “energy” is created all the time just like the electron in the magnetic field. That’s what “dark energy” is.

All the different conservation laws have roots in symmetries of the system.

And on the scale of the universe as a whole, energy continually decreases. Expansion entails increase of the wavelength of all light, which implies decrease in energy.

I don’t think the answer is so clear for the universe as a whole. The energy of the photons decreases due to the expansion - but the expansion can have other additional effects.

Energy doesn't decrease. Entropy increases.

> If you fling a charged particle like an electron through an increasing magnetic field, it will appear to gain energy.

“If I place a particle into an energetic system, it gains energy.” This is an even worse example than the one in the article. You’re trying to define a closed system that is stealing energy from outside the “closed system”. That’s the same logical fallacy that leads people to think they have discovered a Perpetual Motion Machine. How do people get into advanced topics like Time Translation Symmetry without understanding (and being able to explain) the basics?

I have a Physics degree for what it's worth. Though I'll admit I didn't articulate what I meant clearly.

The electron example was assuming that the increasing magnetic field was part of this hypothetical toy universe that violates time translation symmetry. Obviously constructing such experiment in our universe it's not surprising that the electron gains energy because as you said, it's not an isolated system. When considering the system generating the magnetic field it becomes obvious that the overall system does not gain energy.

However our actual universe, at the galactic scale, is just like the toy example. New Space / Energy is created out of nothing as the universe's expansion accelerates. There is no paradox here because conservation of energy is not fundamental but emergent from symmetries via Noether's theorem. Space is a "material" that is not "diluted" by expansion and contains energy.

The inflaton field at the heart of hyperinflation / big bang theory also expands without dilution.

Perhaps there is some larger system our universe is embedded in that does respect time translation system / conservation of energy and the expansion is being driven by a larger process loosing energy but we can't observe it.

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