Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Bill S on 01/12/2017 17:48:07

Title: Is this really reversing time?
Post by: Bill S on 01/12/2017 17:48:07

https://www.sciencenews.org/article/arrow-of-time-reversed-quantum-experiment

An interesting experiment; worth reading; but wouldn't one have to make the assumption that the "arrow of time" was actually governed by the 2nd law in order to claim to have reversed time?

Then there is this.

Quote

“It’s not that it’s contradicting any laws of physics,” says Vlatko Vedral, a physicist at the University of Oxford not involved with the study. The standard second law of thermodynamics assumes that there are no such correlations. When the second law is generalized to take correlations into account, the law holds firm. As the heat flows, the correlations between the two nuclei dissipate, a process that compensates for the entropy decrease due to the reverse heat flow.

Title: Re: Is this really reversing time?
Post by: alancalverd on 10/12/2017 23:33:47

"Entropy is time's arrow" is a neat phrase but easy to misinterpret, apparently even by science journalists.

A road sign that points to Birmingham does not determine the position of Birmingham, but indicates it.

Time is what separates "before" (b) and "after" (a). We define the sign of time as t_a > t_b. The experimenters report that there is an energy transfer from P to Q  despite T_P < T_Q, so T_Q^a > T_Q^b. This is unusual but does not affect t_a > t_b.

Title: Re: Is this really reversing time?
Post by: Bill S on 11/12/2017 13:46:09

Thanks, Alan.  It’s always good to have some definitions to back up a “gut feeling”.

I’m a bit unsure about “We define the sign of time as t_a > t_b “  At first sight, this looks like “ t_a (present time) is greater than t_b  (some time in the future of t_a ).

Either my interpretation is wrong, or I need to ask in what sense the present time is greater than the future time.

Title: Re: Is this really reversing time?
Post by: Colin2B on 11/12/2017 14:52:03

@Bill S - I think your brain may have skipped over this bit:

Quote from: alancalverd on 10/12/2017 23:33:47

Time is what separates "before" (b) and "after" (a).

So event b precedes even a

Title: Re: Is this really reversing time?
Post by: Bill S on 12/12/2017 10:48:54

Oops!  In situations like that it's tempting to ask: "What brain would what be? :)

Title: Re: Is this really reversing time?
Post by: yor_on on 13/12/2017 16:02:01

There is a logical fallacy in presuming that thermodynamics is the arrow of time. Whatever weird quantum processes you can create they all are done under a local clock in a laboratory. The argument is one that correlates to the one in where you can argue that you have a 'real' two dimensional system embedded in our four dimensional. Just because its (the experiments) limitations makes it behave as a thought up two dimensional system is expected to behave.

Thermodynamics are processes inside a four dimensional continuum as far as I can see.

Title: Re: Is this really reversing time?
Post by: Colin2B on 13/12/2017 17:01:09

In our 4D macro world heat flows from a hot body to a cold body over increasing time eg holding an ice cube in your hand. But no-one expects there to be any connection (correlation) between the molecules of the ice and those of your hand, yet that is what was done in this micro experiment.
Correlated and uncorrelated systems behave differently. Correlated sound waves will interfere to create static dead spots in a room, uncorrelated will not. If someone steps down from a chair you would not expect someone 6’ away to rise into the air - uncorrelated system, but if someone steps down onto a seesaw you would not be surprised if the person on the other end goes up - correlated system.
As it says in the article all this was predicted many years ago, more proof that QM is highly predictive.