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Time Reversal? Think History vs. Economics

November 11, 2008

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Shahn Majid

So far in these blogs I have focussed on hard science verifiable by experiment. But it is also part of the background to my multiauthored volume On Space and Time that to proceed further with fundamental science may need revolutionary new ideas for which science is still grasping. So this week we are going to let our hair down and extrapolate from what is understood into what is definitely, well, speculative.

Incidentally, I did run these ideas here past a BBC producer for Horizon a few years ago when he called me asking about the possibility of time travel, and obviously I was not controversial enough as he never called me back.

What I propose, as a motion for debate, is:

The direction of time is a spontaneously broken symmetry, in the same way as which side of the road to drive on is a spontaneously broken symmetry.

Let me explain the analogy first. For the sake of argument, let's say that either driving on the left or driving on the right is equally good. At some point, with enough drivers crowding the road, you have to break the symmetry and decide somewhat arbitrarily ('spontaneously') on one side or the other. But once enough of you have bought right handed cars and started driving them, you are pretty much locked into that choice in your region.

Now for the arrow of time. This is not controversial at a subatomic level and at the level of fundamental equations of physics; there is a symmetry between, say, t and -t in the eqations i.e. between increasing and decreasing time. For example, the relativistic wave equation that governs the simplest particles involves (d/dt)2 which does not change under such a change of variables. In physics the actual symmetry is PCT -- it means left-right reversal ("parity"), particle-antiparticle interchange ("charge conjugation") and time-reversal. This is what led the legendary physicist Richard Feynman once to say that in his view a positron (an anti-electron) is just an electron traveling backwards in time.

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Comments

Newton's third law

November 11, 2008 by Anonymous, 1 year 1 week ago
Comment id: 32809

While I think the concept is nice, and it is true that the equations used in physics could care less if t is + or -...reality dictates that reactions FOLLOW actions, making the universe flow in one direction only, right? If a positron was an electron moving backwards in time, would it not be absorbed in beta decay, rather than emitted?

-roland

More re: Newton's third law

November 12, 2008 by Fred Bortz, 1 year 1 week ago
Comment id: 32818

Roland,

The "action-reaction" terminology is misleading. I always explain that Newton's Third Law notes that forces always exist in equal and opposite pairs. Calling one an "action" and the other a "reaction" can lead to a misunderstanding that one causes the other when in fact they exist as a pair. You aren't alone in misinterpreting the terminology in that way.

In the introduction to my history of twentieth-century physics, I explain the historical importance of conservation laws, noting that Newton's Second and Third Laws lead to conservation of momentum as follows.

The third law, commonly known as the law of action and reaction, states that forces always occur in equal and opposite pairs. Whenever one body exerts a force on another, then the second body exerts the exactly same amount of force on the first, but in the opposite direction. For instance, while the Earth's gravitational attraction holds the Moon in its orbit, the Moon's gravity pulls back on the Earth with equal intensity. Because the Earth is so much more massive than its satellite, the effect of the Moon's gravity on Earth is not to create an orbit but rather a wobble, which is most notable in the ocean tides.

Newton's second law states that when a force acts on a body, it produces a change in a quantity called momentum, which is commonly expressed mathematically as the product of mass and velocity. The longer the force acts on a body, the greater the change in the body's momentum. When two bodies exert equal and opposite forces on each other, they produce equal and opposite changes in momentum. Thus if no other forces are acting on them, the change in total momentum of the two bodies must be zero.

(Reading that now, I probably overstated the case for the Earth not orbiting the Moon. The fact is that they both orbit their common center of mass, which is inside the Earth, and that makes the Earth's orbit around that c.m. a wobble.)

Fred Bortz -- Science and technology books for young readers (www.fredbortz.com) and Science book reviews (www.scienceshelf.com)

Re: Newton's third law

November 12, 2008 by Halliday, 1 year 1 week ago
Comment id: 32816

Roland:

In Newton's third law, there is no requirement, whatsoever, that reaction "FOLLOW" action. In fact, other than the "spooky action at a distance" cases, the action and reaction take place at the same point in space and time! (In subatomic, Quantum Field Theoretical [QFT], terms there is no action at a distance, all actions/reactions occur at single spacetime points, where particle reaction vertices occur.)

Of course the matter/antimatter imbalance of the universe suggests some additional symmetry breaking. However, even though we have C, and P, and CP symmetry breaking, the CPT (or PCT, as used by the article) symmetry appears to be too strong to allow for sufficient matter/antimatter imbalance to explain what we observe. (The fact is that if CPT symmetry is broken, then Feynman's "sum over histories" approach to QFT cannot work, since probabilities would then be complex.)

Another time reversal dilemma yet to be explained.

David

temporal security?

November 11, 2008 by Anonymous, 1 year 1 week ago
Comment id: 32807

so it seems entangled photons proove faster than lightspeed anomalous behavior (upto 100,000x normal lightspeed) i'm sure the theory applies with other RF fields, the subjective question is left to our imagination now.



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