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Quantum Physics, CERN and Hawking radiation
http://lifeboat.com/blog/2011/06/dear-dr-hawking
Hey guys, my quantum physics is not powerful enough to understand this guy... Can anyone help me out with this one?
Thanks LW
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http://lifeboat.com/blog/2011/06/dear-dr-hawking
Hey guys, my quantum physics is not powerful enough to understand this guy... Can anyone help me out with this one?
Thanks LW
Comments (66)
The author of that article, Otto E. Rössler, believes that the Large Hadron Collider experiment could create a stable black hole and destroy the world. I have no knowledge of the relevant physics that would enable me to determine whether this claim is plausible. He has been writing about risk from the LHC on the Lifeboat blog for some time now. This particular post claims that a mechanism which is believed to cause black holes to lose mass, Hawking radiation, might not exist.
I've been increasingly skeptical of Lifeboat Foundation, and have been meaning to write this up. Otto matches a number of items on the usual crank checklist, and his co-blogger Matt Funk matches even more. (Funk's writings are amazingly cranky; he doesn't mind posting rejections from ArXiv or including crank letters he's sent to prime ministers or Warren Buffet or Paul Allen, IIRC.) And then there's this stuff...
Black holes not being able to lose mass would violate reversibility. If something can fall into a black hole, other things must be able to come out again - since physics exhibits microscopic reversibility, and the known laws of physics are symmetrical under T=-T.
This is probably not correct. We know that CP symmetry is broken. There are good theoretical reasons to believe that CPT symmetry holds. Consequently T symmetry must be broken.
To quote from here:
...for the reason to think that it is T symmetry, not CPT symmetry that holds.
Charge and Parity are likely to be implemented using internal rotation or sequences - and so will reverse automatically if T=>-T. We don't have to manually reverse particle momenta or spin if we are running things backwards. That happens automatically. The idea is that Charge and Parity function like that too - due to how these phenomena work. For instance, Charge seems likely to be implemented rather like a bi-directional pump. Reverse time, and such a pump runs backwards automatically. You do not have to go around reversing all the charges - they reverse themselves automatically. It works like spin does - and reverses automatically for much the same reason.
T-symmetry is neater and simpler. We should prefer it - unless there are good reasons not to do so.
If so, then how can T symmetry hold? You seem to be saying that T symmetry implies CPT symmetry. But we know from experiment that CP symmetry is broken. If T symmetry holds, and CP symmetry does not hold, then CPT symmetry cannot hold.
Really, this looks pretty straightforward. The theory you quote has A->B. Experiment !B. Consequently, either !A or !(A->B).
Why do you think so?
Particle momenta, no; spin, yes. Although spin is angular momentum, it does not come about because particles are rotating about an internal axis, as you seem to have in mind. (To the best of anyone's knowledge, of course.) Consequently parity does not auto-reverse under time-reversal.
OK - so, you don't understand the idea. There is a much more detailed description of the associated model written by someone else here.
The punchline at the bottom reads:
Please let me know if that fails to sort you out - and you are still interested.
First, the theory rests on the airy assertion that reversing T automatically causes the reversal of spin and other quantum numbers as well. I found the argument given for this unconvincing. Second, and more importantly, you do not seem to have grasped that you cannot possibly have both T symmetry and CPT symmetry, because CP symmetry is experimentally excluded. It does not matter if you invent a special form of T symmetry that is 'equivalent' to CPT symmetry.
Take a physical system that exhibits CP violation; assume it is described by the kind of theory outlined in your link. Now reverse time. By the argument in your link, this also reverses CP. Because the system is not symmetric under CP, it exhibits different behaviour. Bing, T symmetry has been broken: There is a measurement I can make that tells me which way time is flowing.
Well, I don't have a watertight argument for the first point. I think it is more likely than not, but if your intuition is the other way around, I won't argue too much. What I object to is the idea that T-symmetry is wrong. In fact, T-symmetry is pretty plausible, IMO.
From your second point, (from my perspective) you still don't get the logic of the whole idea - and you have exhausted most of my resources on the subject, so I am not sure what more to do with you.
Assuming that charge and parity quanta involve moving parts internally, then they would both reverse automatically if time is reversed - producing what appears to be CPT symmetry as a result. That would be consistent with all known experiments, and physics would then by time symmetric.
You said: "Because the system is not symmetric under CP, it exhibits different behaviour." No, because you have also reversed time, (you just said so yourself) - and if C,P and T are all reversed, then symmetry is restored. So, then there is no measurement you can make that tells you which way time is flowing.
No. Start with a left-handed neutrino. Reverse T under your assumption. It is now a right-handed antineutrino going the other way; reverse space as well to restore the original direction, if you like, although the argument does not depend on this. Because CP is broken, right-handed antineutrinos do not behave exactly as left-handed neutrinos do. Therefore you can tell how many times T has been reversed. You don't get the full symmetry back except by applying CP another time.
Yes.
A parity flip, I presume you mean.
That is indeed true.
Well you only said you reversed it once - and then you flipped P, but not C, leaving things in a bit of a mess - and then you tried to make out the mess was something to do with me.
Reversing T an odd number of times changes everything. Reversing it an even number of times changes nothing. You can't distinguish between reversing T different numbers of times beyond that - under the hypothesis that reversing T automatically reverses C and P.
So, which of the hypotheses of the CPT theorem do you find less compelling than, er, the fact that it would be kinda neat if T symmetry were correct? (If there's any reason beyond that in the page you linked to, I failed to see it.)
Well, T symmetry is favoured by Occam's razor. We have pruned away the momentum quanta from needing to be replaced by their anti-particles. The idea that T-symmetry is true just takes this a bit further, becoming more elegant and neat in the process.
You didn't answer the question, nor did you give any grounds for thinking it doesn't need answering.
Occam's razor applies to theories, not to individual propositions about them. CPT (or T) symmetry isn't something you build into a physical theory by having an axiom like "CPT-symmetry holds"; it arises from the structure of the theory. Do you have any actual reason for believing that theories with T-symmetry but not CPT-symmetry are simpler than theories with CPT-symmetry but not T-symmetry? The CPT theorem seems to me to give good reason not to believe that.
Well, I think so, but maybe not in a format suitable for a short blog post. There are numerous small, simple CA with the property of being symmetrical under T=-T. For example, the BBM. My impression is that other means of reversal are correlated with automaton complexity. Then there's the idea of charge as a pump. That is appealing on other grounds - and pumps tend to have moving parts - which would then reverse automatically if T=>-T . Also, the possibility of simulationverse ideas would seem to favour ease of reversal, to some extent.
IMO, you should really not be counting the CPT theorem as evidence on the issue - one way or the other.
I certainly don't think people should be telling me that the known laws of physics are not symmetrical under T=>-T. IMO, it is more probable that they are symmetrical that way than that they are not. The idea that CPT symmetry illustrates that they are not is simply a popular misconception, with no basis in the facts of the matter.
The laws of physics as currently understood -- i.e., the laws in the best model we' ve got -- are in fact CPT-symmetric but not T-symmetric. (Because the best model we've got is a quantum field theory of the sort that the CPT theorem applies to; and because CP symmetry is violated (1) by that model and (2) in reality, according to the available evidence.)
Sure, there are plenty of small simple cellular automata with T-symmetry. And also with P-symmetry, which does not hold in the real world. So far as I know, CAs with PT symmetry are just about exactly as easy to make as ones with T symmetry. (And if you have CAs with a property corresponding to C, I bet CPT is as easy to arrange as T.) Why is any of this meant to mean that T-symmetry is simpler than CPT-symmetry?
You may find the idea of "charge as a pump" appealing; fair enough. I am at a loss to see why that is a reason for thinking that T symmetry is simpler than CPT symmetry.
Your argument, so far as you've provided one, seems to go like this: "I expect the universe to work like a cellular automaton. I have the feeling that T symmetry is simpler than CPT symmetry in cellular automata. Therefore the CPT theorem is irrelevant and we should expect T symmetry to prevail but not CPT symmetry". This strikes me as very strange since (1) we have very successful physical theories that are QFTs (to which the CPT theorem applies) and no successful physical theories based on cellular automata, and (2) the CPT theorem is an actual theorem governing QFTs, whereas your intuitions for relative simplicity are merely your intuitions.
You understand that the claim is that that is just a historical accident about the way the model was built? The idea that C and P reverse automatically if T is reversed does not make any new predictions that the old model did not. The idea that CPT symmetry is favoured by some kind of experimental evidence seems completely wrong to me. Since the two models are totally equivalent experimentally, this is an issue for Occam.
Well, if true, it is a reason to think T symmetry or PT symmetry holds - since then charge would reverse itself automatically if T was reversed.
That a time-inverted process doesn't contradict the fundamental laws doesn't mean that we could observe it with frequency basically comparable to that of the uninverted process. Think of thermodynamical irreversibility, for an example of class of processes which practically don't have inverted counterparts, even if these are perfectly compatible with microscopic physics.
Also, losing mass is not clearly defined process. If by mass of a BH one means the mass included below the horizon (or calculated from the diameter of the horizon) observed from constant distance, then black holes never lose or gain mass, since such an observer would never see anything cross the horizon in any direction: for an outside observer it takes infinite time for any falling object to reach the horizon.
If mass is calculated from the gravitational force measured in a constant distance, it may grow as mass gets attracted towards the horizon, or it may shrink if the mass near the horizon has enough outward momentum to overcome the gravity of the black hole. The latter scenario is quite unlikely to happen during typical black hole formation, at least I think so.
Can anybody explain how a black hole with mass of few dozens of atoms can gravitationally attract a significant amount of matter to grow? Is there some paper which quantitatively analyses how fast would such a tiny stable BH grow under some plausible circumstances?
If I take 1000 TeV, the expected energy of two colliding Pb ions on LHC, to be the mass of the produced BH (which is certainly a gross overkill), its classical Schwarzschild radius will be of order 10^-48 m. For comparison, the charge diameter of a proton is about 10^-15 m, the interatomic distances are of order 10^-10 m.
A black hole small enough that Hawking radiation is relevant is too small to be a danger to anything. As far as I can tell, a quite large black hole could sit at the center of the earth without doing anything noticeable. I think my calculation was that a 10 micron radius black hole at the center of the earth would have a doubling time of a billion years. That link is to Scott Aaronson's blog where he asks that exact question. I recorded 10 microns there, but otherwise my calculations have been lost. Various people do similar calculations and seem to get similar numbers, but don't all make the qualitative conclusion. One of the commenters linked to this paper, but I don't recall extracting an answer from it.
Essentially he's claiming that Hawking radiation either does not exist or will be at such small levels that it won't prevent black holes from evaporating. My impression from physicists is that we'd have to be pretty wrong for it not to exist or to exist at much lower than predicted levels. But there's also a more serious problem with this whole approach: the physics that predicts that black holes will go poof is very much connected to the physics that would suggest that it might be possible to make mini black holes in the LHC. If Hawking radiation doesn't exist then the chance that a black hole could form in the LHC goes down, not up. So the total danger attached to "The LHC will make black holes that won't go away" is very small.
In addition to the extraneous personal BS in the blog post, I looked up some of the papers mentioned, which were pretty bad. It's not my kettle of fish, but I'd say crackpottery, 99%.
It's not a reasonable concern. CERN addresses various safety claims here.
This is not related to your question, but I thought it is important (and interesting) to note that predictions in physics can turn out not to match reality very well:
Link: What exactly is the vacuum catastrophe and what effects does this have upon our understanding of the universe?
More: http://en.wikipedia.org/wiki/Vacuum_catastrophe
Strongly reminiscent of the "infinite energy from black-body radiation" problem that kicked off quantum mechanics; but if we're only counting finite disagreements between theory and measurement, the vacuum catastrophe still wins.
He is a crackpot, I'm 99.99% sure. Hawking radiation stems from the second law of thermodynamics, in a general relativity setting one cannot have the first without the second. Besides this, collisions between atoms of the energy produced at the LHC happens every time in baseball, between a ball and a bat.