# Information Loss in Photon Absorption?

• jacksonwalter
Then all I can say is for you to read Lene Hau's ability to "stop photons" (Nature 409, 490-493 (2001)), and how difficult that was to preserve the information in such a system. It clearly implies that other phenomena or technique that we have simply can't do that, including shining light on a "black"...

#### jacksonwalter

How can I tell the difference between a free electron that's absorbed one photon at $$E = hf$$ and a free electron that's absorbed two photons at $$E = .5hf?$$

If a photon at frequency $$f$$ represents a '0' and a photon at frequency $$.5f$$ represents a '1', then I could send out '11', have an electron absorb both photons, and reemit a '0'. What happened to my bit?

jacksonwalter said:
How can I tell the difference between a free electron that's absorbed one photon at $$E = hf$$ and a free electron that's absorbed two photons at $$E = .5hf?$$

If a photon at frequency $$f$$ represents a '0' and a photon at frequency $$.5f$$ represents a '1', then I could send out '11', have an electron absorb both photons, and reemit a '0'. What happened to my bit?

What about a free electron makes you think it is a suitable medium for information storage and retrieval? It has a continuous distribution of momentum states, so it doesn't seem to make sense to try to use those states as a way to store binary information.

Or am I missing something ... ?

SpectraCat said:
What about a free electron makes you think it is a suitable medium for information storage and retrieval? It has a continuous distribution of momentum states, so it doesn't seem to make sense to try to use those states as a way to store binary information.

Or am I missing something ... ?

If an electron absorbs a photon, it would gain momentum from that photon. Measure the momentum, you can determine the frequency of the photon that was absorbed to a precision > hbar/2, and retrieve the information.

...am I missing something?

Absorption is not a unitary process, but involves a state reduction (collapse). A non-unitary process does not need to conserve information.

jacksonwalter said:
How can I tell the difference between a free electron that's absorbed one photon at $$E = hf$$ and a free electron that's absorbed two photons at $$E = .5hf?$$

If a photon at frequency $$f$$ represents a '0' and a photon at frequency $$.5f$$ represents a '1', then I could send out '11', have an electron absorb both photons, and reemit a '0'. What happened to my bit?

Didn't this thread started off by making a faulty assumption? A free electron cannot absorb a photon.

Zz.

Last edited:
ZapperZ said:
Didn't this thread started off by making a faulty assumption? A free electron cannot absorb a photon.

Zz.
That is true, but not essential. Namely, a non-free electron (e.g., an electron in an atom) can absorb a photon. Moreover, it can also absorb more than one photons (although the probability is much smaller). One can again wonder where the information is gone. And I think that my post above provides the answer.

Demystifier said:
That is true, but not essential. Namely, a non-free electron (e.g., an electron in an atom) can absorb a photon. Moreover, it can also absorb more than one photons (although the probability is much smaller). One can again wonder where the information is gone. And I think that my post above provides the answer.

But is this relevant to the original question, which asked about free electrons?

I've personally done experiments on multiphoton photoemission, so I'm quite aware of not only photon absorptions in solids, but multiphoton absorption in solids as well.

Zz.

ZapperZ said:
But is this relevant to the original question, which asked about free electrons?
Maybe we should ask him, but I think it is relevant because I presume that his main point was the information loss, while the free electron only served a purpose of a simple example.

ZapperZ said:
I've personally done experiments on multiphoton photoemission, so I'm quite aware of not only photon absorptions in solids, but multiphoton absorption in solids as well.
So what would you say about the information loss in that context?

Demystifier said:
So what would you say about the information loss in that context?

I would say that that is off-topic for this thread.

Zz.

Demystifier said:
Maybe we should ask him, but I think it is relevant because I presume that his main point was the information loss, while the free electron only served a purpose of a simple example.

I'm fine with a thread hijack, my question was answered. Carry on.

jacksonwalter said:
I'm fine with a thread hijack, my question was answered. Carry on.

Then all I can say is for you to read Lene Hau's ability to "stop photons" (Nature 409, 490-493 (2001)), and how difficult that was to preserve the information in such a system. It clearly implies that other phenomena or technique that we have simply can't do that, including shining light on a "black" surface.

Zz.

ZapperZ said:
Then all I can say is for you to read Lene Hau's ability to "stop photons" (Nature 409, 490-493 (2001)), and how difficult that was to preserve the information in such a system. It clearly implies that other phenomena or technique that we have simply can't do that, including shining light on a "black" surface.

Zz.

Thanks. The other assumption that I had made which may or may not be true was that information is always conserved. I've read a little about the Hawking-Susskind debate and I guess it's now commonly accepted that Susskind was right, information is never lost in a black hole. His main motivation seemed to be that information conservation is one of the most fundamental postulates of physics and without it all is lost.

jacksonwalter said:
Thanks. The other assumption that I had made which may or may not be true was that information is always conserved. I've read a little about the Hawking-Susskind debate and I guess it's now commonly accepted that Susskind was right, information is never lost in a black hole. His main motivation seemed to be that information conservation is one of the most fundamental postulates of physics and without it all is lost.

Information preservation is the property of a Unitary system. A non-Unitary system, as Demystifier has already said, doesn not necessarily conserve information.

By the way, by no means is Susskind presumed to be correct. That is one question that is very much up for debate. Either Unitarity and therefore SQM takes a huge hit, or GR does past the EH of a BH. This is in no small part why people have formulated conjectures and theories such as The Holographic Principle, or the notion of ever-collapsing-but-not-past-an-EH supermassive 'thing' that is in fact a BH.

The Beckenstein Bound just took the wind out of the sails of people who believed there could be a remnant containing information when the BH evaporates, and other theories have come and gone. Basically, if HR is detected at the EH of a BH (or analogue) it would be strong evidence that Hawking is correct. However, even if that is the case, there are ways for that to occur and still have the information encoded on the EH or similar manner.

Demystifier said:
For the list of various proposals concerning the problem of conservation of information in black holes, with their main advantages and disadvantages, see

Wow... they still have proponants of a Planck-Scale remnant?! :lol:

Frame Dragger said:
Wow... they still have proponants of a Planck-Scale remnant?! :lol:
What is your favoured approach to the BH information paradox? And how do you deal with the disadvantages of this approach?

Demystifier said:
What is your favoured approach to the BH information paradox? And how do you deal with the disadvantages of this approach?

I'm a fan of The Holographic Principle myself, over the notion of a remnant that violates the BB. It seems to me that either that is correct, or BHs ARE roach motels for anything outside their EH, and this is a case where Unitarity is broken, just as GR fails to predict beyond a certain point at a BH.

EDIT: To answer your second question, I accept that it might be hard to prove or falsify, and that it has massive implications for cosmology. Given that, you'd think and hope it would be more than a solution to a paradox that may or may not exist. Then again, that is at LEAST the problem with any other theory to explain the IP.

Last edited:
Demystifier said:
What is your favoured approach to the BH information paradox? And how do you deal with the disadvantages of this approach?

See my signature. What information? There isn't any information transmitted in finite time. This is equivalent to the usually meaning of "never".

jacksonwalter: wonderful question. Well posed in practical terms in the context of information theory. I don't give a rip if you said electron instead of atom. I wish I could have thought of it.

Last edited:
Phrak said:
See my signature. What information? There isn't any information transmitted in finite time. This is equivalent to the usually meaning of "never".

jacksonwalter: wonderful question. Well posed in practical terms in the context of information theory. I don't give a rip if you said electron instead of atom. I wish I could have thought of it.

Where are the photo-graphs of BLACK HOLES. Photo, from the greek Phos "Light", and Graph "To draw". So... even though you're making a rhetorical point you begin by asking for something which requires light, to be drawn from the ABSENCE of light? Didn't we all have this precise series of debates in another thread? Does everyone just hijaack threads here to further their own pet positions?!

jacksonwalter said:
I'm fine with a thread hijack, my question was answered. Carry on.

Frame Dragger said:
Where are the photo-graphs of BLACK HOLES. Photo, from the greek Phos "Light", and Graph "To draw". So... even though you're making a rhetorical point you begin by asking for something which requires light, to be drawn from the ABSENCE of light? Didn't we all have this precise series of debates in another thread? Does everyone just hijaack threads here to further their own pet positions?!

It was not intended to be rhetorical. You seem to be frustrated at that I haven't made an attackable post on the Hawking Nobel Prize thread. I'm sorry, things have been busy for me in secondary work, I don't know how to put things into well forms phrases and it's more complicated than it may at fist appear, and I really, really need down time, because I really have a high pressure job.

Phrak said:
It was not intended to be rhetorical. You seem to be frustrated at that I haven't make an attackable post on the Hawking Nobel Prize thread. I'm sorry, things have been busy for me in secondary work, I don't know how to put things into well forms phrases, it's more complicated than it may at fist appear, and I really, really need down time, because I really have a high pressure job.

Ok, I assume that you mean it was meant to be rhetorical and not literal. Anyway, when you use your signiture as the centerpiece of your argument, then you should probably word it more exactly. As a rhetorical flourish it is lacking. As for the thread-jacking, there's a difference between meandering off topic once the main issue is closed and simply changing the topic entirely.

Here's a thought: Take your time, and I mean that, to formulate your thoughts and put them into English. There is no shame in that at all. Then, START a thread based on your ideas, which would be a perfectly sensible thing to do and you could debate this view to your heart's content, without needing to flail your signiture around this way.

By the way, the Nobel for Hawking thread was on-topic for your point of view. I don't agree with you, but that WAS a central part of that thread... very different from here.

Frame Dragger said:
By the way, the Nobel for Hawking thread was on-topic for your point of view. I don't agree with you, but that WAS a central part of that thread... very different from here.

No. I refer to transmitted and received information. Specifically, measurable information rather than the unmeasurable information of an evolving wave.

Thanks for the props, Phrak, although I don't really find it too great a question as it already seems to have a well founded explanation.

In regard to Phrak's signature, maybe it'd be better phrased as 'where's the experimental evidence' rather than photograph.

I agree that while all this is very interesting and I've learned a few things, there's no reason why completely new topics without relation to the original thread should carry on here, even though I personally really don't really care.

From what I know, it sounds like it'd be a pretty extreme extrapolation to try and apply GR to black holes, or to the entire observable universe as a whole. GR should be pushed to the breaking point, but aren't BHs just that, the ideal situation to be described by a new theory of quantum gravity?

Note: For future reference, please don't take anything I say as assertions or positions I'm attempting to prove and defend. I have enough humility to know that as a first year undergrad I'm not the person to be making outlandish claims based on the physics I know and have read about. There are huge gaps in my knowledge but I'm interested and am really just looking for an explanans when I think of things that don't make sense within the framework of my current state of knowledge.