# Information Loss in Photon Absorption?

1. Jan 22, 2010

### 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?

2. Jan 22, 2010

### SpectraCat

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 ... ?

3. Jan 23, 2010

### jacksonwalter

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?

4. Jan 23, 2010

### Demystifier

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

5. Jan 23, 2010

### ZapperZ

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

Zz.

Last edited: Jan 23, 2010
6. Jan 23, 2010

### jacksonwalter

7. Jan 25, 2010

### Demystifier

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.

8. Jan 25, 2010

### ZapperZ

Staff Emeritus
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.

9. Jan 26, 2010

### Demystifier

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.

10. Jan 26, 2010

### Demystifier

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

11. Jan 26, 2010

### ZapperZ

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

Zz.

12. Jan 27, 2010

### jacksonwalter

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

13. Jan 27, 2010

### ZapperZ

Staff Emeritus
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.

14. Jan 29, 2010

### jacksonwalter

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.

15. Feb 1, 2010

### Frame Dragger

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.

16. Feb 1, 2010

### Demystifier

17. Feb 1, 2010

### Frame Dragger

18. Feb 2, 2010

### Demystifier

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

19. Feb 2, 2010

### Frame Dragger

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: Feb 2, 2010
20. Feb 2, 2010

### Phrak

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: Feb 2, 2010