Is there a digital time-stamping scheme that does not rely on a time-stamping authority? Given a bit string, is it possible to assign it an immutable time stamp that is verifiable as correct without assuming a (one or more) trusted time-stamping authority? A time-stamp is correct if it is within delta milliseconds of the creation of the (instance of the) bit string, for some fixed but small delta. The creation time of a bit string is chosen by its creator / owner (but it is immutable once stamped). 

To start off the discussion, I suspect that there will be a need to connect to some sort of physical information source (a la atomic clocks, resistor noise, radioactive half-life etc.) and make some clever use of it.

If it is not possible, I would like an argument that exposes the fundamental barrier.
If it is possible, I would like an exposition of the main idea and links to papers or patents for details.

I apologize if my question has a simple answer that my novice mind has  missed. 

Thank you for considering my question and I appreciate your spending any  cycles on a stranger's query.

Update 1: When I say that the creator of the bit string may choose the timestamp, I mean that they can choose when to timestamp the bit string. If they decide to do so at some time t, then the bitstring must be time-stamped to show time t (within delta). It does not mean that the creator can choose any timestamp in the past or future. Such arbitrary stamping must be impossible in the scheme, of course.
If being decentralized with no trusted timestamping authority that could backdate things is a must, then the solution is Bitcoin's blockchain (or a similar cryptocurrency's blockchain). OpenTimestamps is a project for efficiently using it to timestamp files.



Most of the design for a possible basic decentralized cryptocurrency has been known since public key cryptography was invented (have a shared ledger of public keys with associated balances, and anyone can transfer currency from their balance to another by signing a transaction with their private key), but it had always been blocked by the double-spend problem (Alice could sign a transaction sending currency to Bob to buy a physical object, and then once it's in her possession, she could broadcast a second transaction with an older timestamp that sends that same currency to a 3rd address also controlled by herself instead, and try to convince everyone else that it came first and should be honored instead of the transaction to Bob). Essentially, without a decentralized timestamping service, there's no way for a decentralized cryptocurrency to get everyone to agree on the same order of transactions.

The double-spend problem was finally solved in 2009 by Bitcoin through its use of a proof-of-work blockchain. Each block contains a proof-of-work based on a hash of the rest of the block, the hash of the previous block, and a list of transactions. People ("miners") are incentivized to compute the blockchain proof-of-works by receiving newly minted bitcoins and bitcoin transaction fees. Each block contains a timestamp, and the proof-of-work difficulty automatically scales so that the proof-of-work (and therefore a new block) can be made every 10 minutes. Every Bitcoin node knows to use the longest blockchain. For an attacker to backdate a transaction N blocks into the past so that it would be before a conflicting transaction, they would have to calculate N proof-of-works, but the while they are computing these proof-of-works, the rest of the miners are on the network are still building the blockchain by creating more proof-of-works. The attacker would have to have more computing power than the entire rest of the miners on the network in order to race them and create a bigger blockchain.

Now to rewind a bit: to actually use this for timestamping files, then you would just want to encode a file (or merkle tree) hash into a transaction in the Bitcoin network. OpenTimestamps is a project for doing this efficiently. (There are other such projects, but OpenTimestamps is much more efficient than most and is made by a Bitcoin developer.) You might be wondering "why involve Bitcoin at all?" and want to extract out its blockchain, but that can't work as Bitcoin's blockchain only works because the miners have a cryptocurrency incentive to contribute, and a blockchain without a significant number of miners like Bitcoin's already has could be easily attacked.