First "Observation" of Hawking Radiation

KentuckyFC writes "Italian physicists are claiming the first observation of Hawking radiation, but not from a black hole. Instead they've spotted it streaming from a sonic horizon in a Bose Einstein Condensate (abstract on the arXiv). That's consistent with previous predictions but they're claiming the 'first' even though the experiment was only a numerical simulation. Does that really count?"

Comments

Maybe they read /.

[JeepFanatic]

... and they wanted to get First Post?

reverse engineer it

[OrochimaruVoldemort]

and you get a black hole. but, don't feed it.

Re:reverse engineer it

[TheMadcapZ]

Especially after midnight. Keep it our of the rain too.

Doesn't Count

[ObsessiveMathsFreak]

Does that really count?

No, no it does not.

A numerical model is little more than a highly specific and round off error prone implementation of existing analytical results. All these guys have done, at most, is shown the correctness of Hawking's analysis. If that.

Re:Doesn't Count

[MozeeToby]

Well, it does prove that either our models of BEC are very accurate or there has been one hell of a coincidence. I don't think that they can say they were the first to discover this, but you may argue that they have produced evidence that supports the theory.

If they had found no radiation, that would not have been proof of anything since a flawed simulation would produce a flawed result. However, the odds of this particular flawed result (producing hawking radiation under very specific circumstances)must be pretty low, lending support to the theory.

Re:Doesn't Count

[Beetle B.]

but you may argue that they have produced evidence that supports the theory. No, you can't.

Without knowing the details of both theories, it's hard for me to judge. Basically, if their formalism is more or less isomorphic to Hawking's (without their realizing it) - then all they've done is do Hawking's work over again.

If they used independent formalism to get Hawking radiation, then it's a good sign, and shows that their theory is consistent with Hawking's (and perhaps later someone will link the two).

In either case, they did not produce any evidence. At best, they're saying, "If you look at this our way, it is consistent with what Hawking predicted."

Re:Doesn't Count

[CrimsonAvenger]

but you may argue that they have produced evidence that supports the theory.

No, you can't.

I agree. Physics is an attempt to model the universe mathematically. The fact that two models agree says nothing whatsoever about whether either is an accurate map of the universe.

Only numerical simulation

[jayhawk88]

That accurately describes about 90% of theoretical physics doesn't it?

Re:Only numerical simulation

[Thanshin]

That accurately describes about 90% of theoretical physics doesn't it? Yes, the other 10% actually test empirically all their theories. They just keep the TP name because chicks dig it.

Re:Only numerical simulation

[TeknoHog]

They just keep the TP name because chicks dig it.

I am Cornhawkio! I need TP for my blackhole!

First Observation of The Meaning Of Life

[Thanshin]

6 * 7

It's just a numerical simulation, but everybody knows it counts anyway.

Re:First Observation of The Meaning Of Life

[Selfbain]

I think you mean 6 by 9.

Re:First Observation of The Meaning Of Life

[bugnuts]

I think you mean 6 by 9. There's nothing fundamentally wrong with my universe, thank you very much. Same thing, it's just base 13.

Thinking in circles anyone?

[bikin]

I am sorry, but I don't buy it... You have a theory how the world behaves. You do a numerical simulation based on that theory, and amazingly, it proves true. And you consider that a proof of your theory?
I guess I will make a theory stating that fairies exist... simulate that in a computer, and when fairies appear in my simulation I write an article that I have observed fairies. Mmmmhh, this certainly sounds like proving ID.

Re:Thinking in circles anyone?

[CmdrGravy]

Not quite, if you imagine the theory as a car then the scientists suspected it might have been a Porsche Cayenne 4x4 and once they had built their simulation ( imagine that as a carwash ) it turned out that it was indeed a Porsche Cayenne 4x4. However the amazing thing was that even though they hadn't considered the driver specifically in their simulation it did indeed turn out the simulated driver was a enormous wanker thus proving beyond doubt the truth of their simulation.

Re:Thinking in circles anyone?

[Chris Burke]

You have a theory how the world behaves. You do a numerical simulation based on that theory, and amazingly, it proves true.

Well that's not exactly the case. We have a theory of how the world behaves, and Hawking Radiation is a predicted emergent property of that theory. It's not an axiom, it's a predicted consequence, so it isn't a given based on the theory. Here, we have a detailed simulation that shows that yes, if the underlying theory is correct, then we should expect to see Hawking Radiation.

It is true that this is in no way a real-world observation that shows that the theory accurately models reality. However it does have a non-trivial and non-circular implication for our theory.

What an interesting question

[idontgno]

...though the experiment was only a numerical simulation. Does that really count?

If so, then many slashdotters are no longer virgins.

Slightly OT: Unruh effect

[TeknoHog]

My first thought from the headline was Unruh effect. It's a kind of Hawking radiation you can get in a particle accelerator. It just happens that with black holes, the acceleration is due to gravity, but other sources of acceleration also work. There are huge decelerations from c to nearly 0 at heavy ion collisions, for example.

I first heard of the effect when some fellow physicists were considering the idea of tiny black holes created in particle physics experiments. It turned out that the presence of Hawking-like radiation doesn't necessarily mean a black hole.

Well, it also turns out that this has nothing directly to do with the article, but might be +i, interesting nevertheless.

Black holes should radiate anyway

[LiquidCoooled]

If you consider the cusp of the gravity well of the earth to be a simplistic representation of the event horizon then:

an object travelling inside the well is doomed to never escape without additional energy.
it will spiral to its death after some time period.

If two objects at this same point collide and explode, then some of the matter will have gained additional energy and will escape the gravity well, the rest of the body will spiral to its doom.

Re:Black holes should radiate anyway

[gardyloo]

I'm being pedantic here:

      1. Cusp? What cusp? That's not how the term is used, in either popular language, nor in physics.
      2. If I understand "cusp" to mean the location at which an object released from rest will *just* fall to earth, there's no such location. ALL objects will, from any location (given no other mass or energy in the universe, of course).
      3. If one allows an initial momentum to your object, then the "cusp" location can be anywhere.

Re:Black holes should radiate anyway

[ExecutorElassus]

yes, but Hawking radiation is somewhat different, and requires no material in the vicinity. It works like this: there are always opposite particles (say, positron-electron pairs) that spontaneously appear at the subatomic level. But in the normal universe, they immediately re-collide, giving back to the local space whatever energy was used to create them. That's a bad explanation (because I'm not a physicist), but it gives a rough picture. At the edge of an event horizon, however, there is a small - though nonzero - chance that one or the other particle will get sucked down the gravity well before it can remerge with its opposite. Thus the one that survived ambles off into space, no doubt pondering its cosmic parthenogenesis. The energy of the particle is - for reasons unknown to me - taken from the black hole. Also, this process steadily accelerates as the black hole continues to lose mass through the process: it eventually pops out of existence in a burst of gamma radiation.

So, it's a little more complicated/interesting than you described; I'm sure it would be even better if someone here could describe it from an actual background in physics, instead of the armchair variety I can muster.

Re:Black holes should radiate anyway

[UncleTogie]

It works like this: there are always opposite particles (say, positron-electron pairs) that spontaneously appear at the subatomic level.

...and you wonder why the ID crowd looks annoyed when they're not allowed to use the same "well, it just appeared!" argument...

Re:Black holes should radiate anyway

[Anguirel]

The energy comes from the black hole because the mass comes from the black hole.

The easiest way to conceive of it, in very basic terms, is that the Electron/Positron pair spontaneously converts to mass from the energy surrounding the black hole. The positron falls into the hole, and annihilates with an electron's worth of mass already in the singularity. The electron from the initial pair escapes. The black hole has been reduced in mass/energy by the amount of one electron.

If the electron, instead, falls into the hole, the positron escaping will annihilate with an electron being pulled toward the hole (probably) and release a burst of energy, leaving a net gain of no mass for the black hole as a particle that would have added to it no longer reaches that point.

Re:Black holes should radiate anyway

[harryjohnston]

Not quite my understanding. Positron-electron pairs don't get created spontaneously as this would violate the conservation of energy. However, a pair of what are called "virtual" particles can appear spontaneously if one of them has a negative mass.

Now, a negative mass particle can't normally exist for very long, so it has to recombine in short order with the original particle and they cancel each other out.

However if the negative mass particle is trapped by the event horizon, "not very long" gets stretched out indefinitely by the time dilation, and the positive mass particle can escape. The total mass of the black hole goes down because the particle entering it has a negative mass.

Of course, this is all just a way of visualizing what goes on so that it seems to make sense. It doesn't necessarily correspond in any meaningful way to reality.

Just a simulation

[wolverine1999]

It's just a simulation... hawking radiation hasn't been observed in real life yet.

What's next?

[AbsoluteXyro]

How long until they "observe" a Fry, er, Hawking Hole? I could have swore I "observed" one on TV.

Shenanigans!

[multimediavt]

I'm sorry, but I'm with the "no way this counts" camp. Theories have to be tested in the physical world to be proved. Theoretical physics included folks. That's why we have supercolliders and Z-machines, duh! Numerical analysis can help predict physical behavior but it is not law until it is proved in the real world. Sorry guys.

Re:Shenanigans!

[huckamania]

This is the 'new' science. First you have a theory, then you promote your theory, then someone takes a poll and then it becomes fact.

Computer simulations are acceptable proof in the 'new' science. Even flawed computer simulations are acceptable proof as they prove that the simulations are getting better.

Ugh.

[stonecypher]

but they're claiming the 'first' even though the experiment was only a numerical simulation. Does that really count?

No. Observed means "in the real world." These people should be ashamed of themselves. Physicists are supposed to have standards.

God Exists!

[serutan]

We've got millions of highly vivid simulations!

Story is distorted (what a surprise)

[Doug Merritt]

The story is grossly distorted -- what a surprise. I was going to say that at least it was distorted by author of the linked-to news item, rather than by the /. submitter, but now I see they seem to be the same person ("KFC" and "KentuckyFC").

The abstract that is linked to merely claims "numerical evidence", not "first observation", and to get from that unobjectionable claim to the more sensational false accusation, one must distort the paper itself ( http://arxiv.org/pdf/0803.0507v1 ), which says:

...a recent work [11] has anticipated that the presence of Hawking radiation in this setting can be unambiguosly revealed from a very peculiar feature of the correlation function of density fluctuations.

Here we report numerical experiments that nicely confirm this prediction. Differently from most previous works on analog models [6], our calculations are based on the application of microscopic many-body techniques to an experimentally realistic system and never involve concepts of gravitational physics.

In this way, our observations can be considered as a first independent proof of the existence of Hawking radiation and rule out the frequent concerns on the role of short wavelength, "trans-Planckian" physics on the Hawking emission.

So for one thing, they never claimed "first observation", they said "first independent proof", which is sharply different.

For another thing, they softened even that claim; they said "our observations [of the simulation] can be considered" proof, not that it is proof.

At any rate, it's interesting in general; they're talking about predictions that Hawking-Unruh radiation might be found in many settings unrelated to domains involving gravity or acceleration, and that their simulation might be an independent confirmation of those predictions.