The Stand-Up Physicist: Time Reversal and Spin

Part of the Stand-Up Physicist series, half hour chats by Doug Sweetser about his theoretical physics research. The laws of physics, mathematically, don't care about the direction of time, yet our experience has a clear arrow. By shifting the question a bit, it becomes reasonable to expect a direction to the arrow of spacetime, because space has a handedness. Half-integer spin appears as a mystery, unless you look at a beer can in a new way.

Category:

Science & Technology

Tags:

• Theoretical
• Physics
• Time
• Spin
• Quaternions

[356Spunky]

Thanks for your videos Doug. I've been finding them really helpful in trying to understand some of the more elusive concepts of QM. They're great for creating a visual image of the events in question.

[sweetser]

That was the goal. Do note that my work has not been embraced by anyone with a proper education :-) If you do go for a more formal education, these pictures may give you a technical edge.

[WisdomVendor]

when you speak of a "local" transform, are you referring to a situation similar to a "closed system"?

[sweetser]

By local, I mean the transformation depends on the values of t, x, y, and z. The Lorentz transformations are global and only depend on the velocity, not on location. In standard physics, time reversal is a global transformation, using the member of the Lorentz group that has on the diagonal {-1, 1, 1, 1}. The -1 flips time's sign wherever. For low velocity, my approach mimics this to a high degree. Yet the mimic is not perfect, which was the point I tried to make.

[ajhrockerboy6]

perhaps death is the entrance to a time-symmetrical universe in which a person is born old and dies a baby. i know it sounds like sci-fi and bizarre concepts but if you truly think about the origin of the universe this could theoretically happen.

[pandpinjapan]

You are the man!

[LANN98]

please email me your equations. i can barely see this choppy video.

[LANN98]

please email me your equations. i can barely see your video because it is so choppy.

[shanelejts]

thank you for taking time out of your life to help explain time reversal =D.

[dougsweetser]

Hello:

Nearly all laws in physics are invariant under time reversal. The exception is the second law of thermodynamics. People who study this sort of thing think that the asymmetry is "put in by hand." With quaternions, there is a reason for the asymmetry.