Cramer's time travel experiment

In summary, there has been recent publicity about John Cramer's experiment attempting to prove retrocausation or backward time travel. The experiment is based on Cramer's transactional interpretation of QM, which suggests that the outcome of the experiment will depend on the validity of his interpretation. However, this does not align with the fact that all interpretations of QM make the same empirical predictions. Cramer also suggests that the experiment would only work if there are small nonlinear effects deviating from pure QM. This raises concerns about potential risks, as once a time machine is created, anything could come out of it. However, it is possible that TI could offer a better understanding of quantum mechanics. The experiment itself is similar to a Bell test
  • #1
Ontoplankton
152
0
There was some publicity recently about John Cramer not being able to get funding for an experiment that he claims could prove "retrocausation" or backward time travel. I'm confused as to what the experiment is supposed to show. Cramer seems to be saying the outcome will depend on whether or not his transactional interpretation of QM is true, but that can't be the case, since all interpretations are supposed to make the same empirical predictions. Elsewhere, he seems to suggest the "sending information into the past" part works only if the world deviates from pure QM with small nonlinear effects. Does anyone understand what's going on?

I certainly don't think QM let's you send information into the past, but the experiment still bothers me. Let's say that Cramer is right, and this sort of experiment really does let you set up a causal loop. If this is something Nature hasn't safely tried out yet (and I don't know if that's true -- anyone?), couldn't there be all sorts of risks we don't know about? For one thing, it seems to me that once you have a time machine, it's consistent with the laws of physics for absolutely anything to come out, so long as it goes back in later. People have made some predictions of risks (like black holes and strangelets eating the Earth) associated with particle accelerators, but at least we have strong evidence that those risks aren't real, since they would already have happened naturally due to cosmic rays. Do we have some similar argument why we know successful time travel experiments wouldn't blow up the world?

If the answer is, "Oh, don't worry, there's only a 1 in 1000 probability that this experiment will destroy the universe", I don't find that very reassuring. Is doing the experiment worth one thousandth of a universe?

On the other hand, if you can destroy the world with a 20,000 dollar experiment, I guess that in the long run we're screwed anyway.
 
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  • #2
I have not heard of this one but ...wait a minute. So what is he proposing to actually do in the experiment? He might try to prove something fancy, but what is the apparatus? What is really expected to be observed? What is he doing that doesn't happen every day on our little planet, Earth, and why are we all of a sudden at risk of being sucked into the void over a $20,000 experiment?

Hey, I can do an experiment (say $19999 worth, with spitballs and cotton and see if time stops.
But don't lose sleep over this one...
 
  • #3
gammamcc said:
What is he doing that doesn't happen every day on our little planet, Earth, and why are we all of a sudden at risk of being sucked into the void over a $20,000 experiment?...

I have looked at this, and it DOES happen every day on our planet. So not to worry... :tongue:

Cramer's is a essentially a Bell test setup attempting to measure entangled photon interference. There is no signalling to the past any more than there is instantaneous signalling in a Bell test. Only when the results at both sides are compared does a pattern emerge.

In no ways does this experiment invalidate anything about oQM in favor of Cramer's Transaction Interpretation (TI). TI definitely has its own baggage, somewhat akin to the baggage of Many Worlds or Bohmian Mechanics. In this case, there are "offer waves" and "acceptance" waves.

Actually, there are some things to like about TI. In many ways it does help to make sense of what seems to occur. The thing I like most about it is that it postulates physical effects going from the future back to the past. This is sometimes referred to as retrocausality. However, this effect is limited in such a way as to prevent the kind of paradoxical loops that are the stuff of sci-fi books.
 
  • #4
DrChinese said:
I have looked at this, and it DOES happen every day on our planet. So not to worry... :tongue:

Cramer's is a essentially a Bell test setup attempting to measure entangled photon interference. There is no signalling to the past any more than there is instantaneous signalling in a Bell test. Only when the results at both sides are compared does a pattern emerge.
This description sounds a bit like the delayed choice quantum eraser, is Cramer's setup similar to that?
 
  • #5
JesseM said:
This description sounds a bit like the delayed choice quantum eraser, is Cramer's setup similar to that?

It does look very similar to my eyes as well. The following link provides a much more detail description of the basic experiment. The ones to look at especially are Figure 3 (which is the Dopfer experiment) and Figure 2, which is the double slit with entangled photons. The author is Anton Zeilinger, so you know the analysis will be accurate.

http://www.physik.fu-berlin.de/~simons/Publikationen/RevModPhys99.pdf [Broken]

You can see more easily that nothing about the HUP is violated in this experiment. The same is true when you insert a optical fiber delay loop on one side. The only time any interference can be seen is in a subset of detections containing no "which way" information. This part is like the quantum eraser, as you point out.

Is it retrocausality, as Cramer says? That is a matter of interpretation, in my opinion. I personally think that oQM must exhibit something like retrocausality in order to explain the mechanics of entanglement - but I would emphasize that is just one viewpoint.
 
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  • #6
The question is..Can QM have NOn-linear effects ??...by imposition and correspondece principle with Classical mechanics Schröedinguer equation is purely linear ,anyway why can you (according to cramer) send info to the past but not to the future ??..
 
  • #7
I suppose you've got to http://science.slashdot.org/science/07/06/12/154248.shtml", so many crackpot ideas get stuck at the "it's a conspiracy denying me any funding" stage. I just can't convince myself, if there were any shred of question in this, that Zeilinger wouldn't have done it already.
 
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  • #8
I don't care what the experiment is or even if it has the slightest amount value, he has raised a public interest in physics and that is the greatest result of all.

CraigD, AMInstP
www.cymek.com
 
  • #9
Now that it's going to happen, I have to ask.
DrChinese said:
I have looked at this, and it DOES happen every day on our planet.
How?
 
  • #10
Ontoplankton, you could always just email Cramer and ask him your questions. He is a pretty nice guy and I'm sure that if you kept your question brief and coherent (i.e., non-crackpot-esque) he would be happy to answer.
 
  • #11
Here's a new(?) explanation of the experiment: http://faculty.washington.edu/jcramer/Nonlocal_2007.pdf

OGP: I don't know, I might, but I'm reluctant to say something amounting to "prove your experiment for sure won't blow up the world" without a good idea what I'm talking about. I'd still be obliged if someone here could confirm/explain that the experiment is not somehow going into unexplored physics territory. (Thanks, everyone, for the answers so far.)
 
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  • #12
Look at fig. 1 in the http://faculty.washington.edu/jcramer/Nonlocal_2007.pdf" [Broken]you linked. The top right part of the apparatus is designed to produce (then separate) pairs of entangled red photons, which are collinear and "momentum" entangled, which is presumed to mean that if a photon goes through slit A at S1, the entangled partner photon must go through the corresponding slit A' at S2, and so forth.

Cramer's idea is that by choosing (at S2) to either measure "which slit the photons go through" or to not make that measurement, he thinks he can affect whether the camera (at S1) registers an interference pattern (as would occur if there was no way in principle to determine which slit the partner photon went through) or no pattern (as occurs if the "which-path" information is measured).

I'm not the first to predict this will fail in the following way: regardless of what happens at S2, there will no net interference pattern at S1 (because it would be possible in principle to determine the which-path information at S2). Moreover, the optical combiner C is measuring and then classically discarding available information about the superposition of "which-path" states that the VLP/"idler", information which would be necessary to reconstruct the interference pattern from classical correlations with the HLP/"signal" photon.

You don't need to trust my word on this, compare with the experiment published with the title Delayed "Choice" Quantum Eraser. If you look at the raw data from that experiment, you'll conclude Cramer's camera will measure "1" all of the time, never "0".

Now, if this wasn't the case, then sure, the experiment could be a way to transmit information (from S2 to S1) "instantly" (or even backwards in time, violating causality, the very paradox of which might be hypothesised to destroy reality). But quantum physics just doesn't seem to work that way (and if it did, you'd expect those world-destroying causality-violations would already be constantly occurring, due to quantum entanglements amongst the air particles around us, without ill effect). And of course, Kim, et al, have already done basically the exact same experiment, which I'm surprised Cramer hasn't even referenced (I think I'll try emailing him.. it'd be a shame if he just hadn't seen the other work, surely it would be worth explaining the difference if he thought there was one, after all the time-symmetric interpretations of QM seem otherwise promising).
 
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  • #13
Ontoplankton said:
OGP: I don't know, I might, but I'm reluctant to say something amounting to "prove your experiment for sure won't blow up the world" without a good idea what I'm talking about. I'd still be obliged if someone here could confirm/explain that the experiment is not somehow going into unexplored physics territory. (Thanks, everyone, for the answers so far.)

Hopefully his experiment *is* going into unexplored territory to some small extent... otherwise it would not be a cool experiment.

P.S. I can promise you that Cramer's experiment is certainly not going to blow up the world. If you are worried about the world blowing up then I encourage you to act pragmatically and work for nuclear non-proliferation. :)
 
  • #14
ma teory

So basicaly they are two teories for teletransportation:the one that copies u and then kills u, and the other one that transformes u into a puzzle and then remakes u. But i have another theory that i came in here for u all to say if its right or wrong.So i first thought in framerates in games, if u have low framerates and u walk frontways u disappear and appear somewhere infront instantly.Then i thought if real life movement was done by small teleportations that human eye sore them like continuous movement and if so, would it be posible of making those small teleportations bigger, in the game example, have slower framerates.The only bad thing about this is inertia, u need to teletransport that before u teletransport the object, basicaly we need to invert inertia. posibly it sounded like nonsence to u but... I am only 15 o:).I think posibly is out off topic but i think u need teleportation for time travel.which is posible because if u travel in time to kill ur dad u just cant, because u weren't destined to do it.and viseversa.:approve:
 
  • #15
cesiumfrog said:
Look at fig. 1 in the http://faculty.washington.edu/jcramer/Nonlocal_2007.pdf" [Broken]you linked. The top right part of the apparatus is designed to produce (then separate) pairs of entangled red photons, which are collinear and "momentum" entangled, which is presumed to mean that if a photon goes through slit A at S1, the entangled partner photon must go through the corresponding slit A' at S2, and so forth.

Cramer's idea is that by choosing (at S2) to either measure "which slit the photons go through" or to not make that measurement, he thinks he can affect whether the camera (at S1) registers an interference pattern (as would occur if there was no way in principle to determine which slit the partner photon went through) or no pattern (as occurs if the "which-path" information is measured).

I'm not the first to predict this will fail in the following way: regardless of what happens at S2, there will no net interference pattern at S1 (because it would be possible in principle to determine the which-path information at S2). Moreover, the optical combiner C is measuring and then classically discarding available information about the superposition of "which-path" states that the VLP/"idler", information which would be necessary to reconstruct the interference pattern from classical correlations with the HLP/"signal" photon.

You don't need to trust my word on this, compare with the experiment published with the title Delayed "Choice" Quantum Eraser. If you look at the raw data from that experiment, you'll conclude Cramer's camera will measure "1" all of the time, never "0".

Now, if this wasn't the case, then sure, the experiment could be a way to transmit information (from S2 to S1) "instantly" (or even backwards in time, violating causality, the very paradox of which might be hypothesised to destroy reality). But quantum physics just doesn't seem to work that way (and if it did, you'd expect those world-destroying causality-violations would already be constantly occurring, due to quantum entanglements amongst the air particles around us, without ill effect). And of course, Kim, et al, have already done basically the exact same experiment, which I'm surprised Cramer hasn't even referenced (I think I'll try emailing him.. it'd be a shame if he just hadn't seen the other work, surely it would be worth explaining the difference if he thought there was one, after all the time-symmetric interpretations of QM seem otherwise promising).

Did John Cramer make the experiment at last? Anyone knows what happened with this issue?
 
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  • #16
Ontoplankton said:
There was some publicity recently about John Cramer not being able to get funding for an experiment that he claims could prove "retrocausation" or backward time travel. I'm confused as to what the experiment is supposed to show. Cramer seems to be saying the outcome will depend on whether or not his transactional interpretation of QM is true, but that can't be the case, since all interpretations are supposed to make the same empirical predictions. Elsewhere, he seems to suggest the "sending information into the past" part works only if the world deviates from pure QM with small nonlinear effects. Does anyone understand what's going on?

I certainly don't think QM let's you send information into the past, but the experiment still bothers me. Let's say that Cramer is right, and this sort of experiment really does let you set up a causal loop. If this is something Nature hasn't safely tried out yet (and I don't know if that's true -- anyone?), couldn't there be all sorts of risks we don't know about? For one thing, it seems to me that once you have a time machine, it's consistent with the laws of physics for absolutely anything to come out, so long as it goes back in later. People have made some predictions of risks (like black holes and strangelets eating the Earth) associated with particle accelerators, but at least we have strong evidence that those risks aren't real, since they would already have happened naturally due to cosmic rays. Do we have some similar argument why we know successful time travel experiments wouldn't blow up the world?

If the answer is, "Oh, don't worry, there's only a 1 in 1000 probability that this experiment will destroy the universe", I don't find that very reassuring. Is doing the experiment worth one thousandth of a universe?

On the other hand, if you can destroy the world with a 20,000 dollar experiment, I guess that in the long run we're screwed anyway.
No offense, but I'm just wondering how much research and thought is behind what you wrote. Cramer knows a good bit of physics, but he's a crackpot, of sorts, with an agenda. Take it with a grain of salt, and, really, don't worry.
 

What is Cramer's time travel experiment?

Cramer's time travel experiment is a thought experiment proposed by physicist John Cramer in 1988. It explores the concept of retrocausality, which is the idea that future events can influence past events.

How does Cramer's time travel experiment work?

In Cramer's thought experiment, a particle is sent through a time machine and back to its past self. The future self of the particle sends a message to its past self, causing it to change its behavior in a way that alters the future. This creates a paradox and raises questions about the possibility of changing the past.

Has Cramer's time travel experiment been tested?

No, Cramer's time travel experiment is purely a thought experiment and has not been tested in real life. However, some scientists have conducted experiments on retrocausality in quantum systems, which could have implications for time travel theories.

What are the implications of Cramer's time travel experiment?

If Cramer's experiment were possible in real life, it would challenge our understanding of causality and the arrow of time. It could also have potential consequences for the concept of free will, as changing the past could potentially change our present and future choices.

Is Cramer's time travel experiment considered a viable theory?

Cramer's time travel experiment is still a hypothetical concept and is not widely accepted as a viable theory. While it raises interesting questions about the nature of time and causality, it is currently not supported by scientific evidence or technology.

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