(fwd) Where Do We Go From Here?

What defines which is the 'fixed' frame of reference? Since velocity is relative, can't we say that the rocket stood still and the earth sped away at high velocity and then returned, leaving the earth-bound twin the younger?

1. Plot devices for SF stories.
2. Investigating the limits of theory.

The second reason is useful. Frank Tipler's "Rotating Cylinders and the Possibility of Global Causality Violation" showed that even in non-extreme situations, GR permits time travel. Now, non-extreme involves a cylinder probably made of neutronium rotating at about half c, if I remember right. It's an interesting definition of non-extreme .

There's a couple ways that time travel would be theoretically possible according to QM.

Why study them? The tachyon particle was considered one for a while, but because it permitted some form of time travel, it was investigated. A flaw was found in the theory that, when corrected, eliminated the possibility of tachyons.

The EPR paradox has gotten a lot of press. By studying the statistics of an EPR system, quantum entanglement has been shown to occur over space-like distances, while prohibiting useful information exchange at speeds greater than c.

It's like baseball. Someone who does not know the rules about baseball and who's trying to learn them by watching the game might wonder "What happens if the scheduled batter is on base?" That might be a great puzzle to the person, and they might wonder for years. Eventually, as they discover the "laws" of baseball, they would discover that the vast majority of rules in baseball are aimed toward avoiding having the scheduled batter be on base.

In the same way, one of the constraints on physical laws might be that time travel is not permitted. All laws, including particle interactions, might be aimed at absolutely preventing time travel. This might put an upper constraint on neutronium - as the rotating cylinder approaches the speed at which time-like orbits are possible, the neutronium breaks down and mass is lost from the cylinder.

The story of physics is like that. Why aren't single quarks found? We now understand that - but back in the '60s and early '70s, there were amazing searches for fractionally charged particles.

Black holes, as they were thought to behave, were pathological. Thermodynamics requires that all pathways be reversible - not that the reverse has to be easy. Matter was thought to only go into black holes, but not out. A lot of folks ignored the pathology, and so when Stephen Hawking announced that black holes can evaporate, a lot of folks thought he was suffering from ALS-induced dementia. ALS does not induce dementia, and if they'd known their medical pathology, they'd have known Hawking was onto a breakthrough that would eventually lead to a clue about quantum theory and something known as the "holographic principle."

Even in medicine, pathology is fun. What happens when you knock out the "Harry Potter" gene? A company studying "orphan" genes (genes that had no known function - they name the genes after famous orphans) knocked out the "Harry Potter" gene in mice. They looked for pathology, and found it: puberty did not take place. They were left wishing they'd named the gene "Peter Pan." Of course, since the latest Harry Potter book has Harry suffering the hormonal rages of puberty, maybe it's not such a bad name.

Even in rocketry, we have learned a lot about pathological behavior in rockets, and things like the Barrowman equations and Rocsim have become popular as a result.

CG before CP.

Zooty

Looking back on it, when I started studying relativity in 8th grade, I had the same confusion Chris did.

I, too, must have been a pain in the ass. Looking back on it, I remember my science teachers and my Dad getting frustrated with my questions.

Fortunately, by 9th grade, I could do the math and follow the logic.

Perhaps that's why Chris bugs me. He reminds me of myself when I was younger.

It's a wonder someone didn't stuff me in a 55 gallon drum with cement and throw me in the Yough.

Zooty

(And no, Chris, that's not a threat. You wouldn't fit in a 55 gallon drum. Now, neither would I.)

snip, giggle.

Hey Mike, You gunna be at 60 acres, Sunday? Hope to see you there. Maybe you can demonstrate this stuff for me?

steve

Yeah, but Chris is like, late 20's?

I have a 12yo son who is rapidly approaching the "I think I know more than you do stage". It's like seeing storm clouds on the horizon just prior to a doozy of a thunderstorm...

I'm pretty sure all of us rocket geeks were like that at that period of our lives... God knows *I* was...

tah

So you LIKE math (from a distance) but have not LEARNED how to DO what you say you LOVE.

I have a suggestion. Since you define yourself as a "math lover", simply learn more of it.

Jerry

You APPLIED yourself. That may not apply to other persons referenced here.

It's never too late:)

This was a difficult question for a while, which was answered by General Relativity.

Under Special Relativity, you can never get the two twins back together, so you can never actually show the age differences side by side.

In General Relativity, you can account for the four accelerations that the one twin undergoes: Accelerating up to light speed, accelerating from lightspeed to a stop At Alpha Whateverstaryouwannaname, accelerating up to light speed again, and accelerating to a stop once again at Earth.

The twin on earth undergoes the much milder acceleration of gravity, and so the two reference frames are never equal.

Zooty

I moved so fast, I was already there! By the way, you guys are going to have had a great launch, but watch out for the tall poplars and the pooled water.

(Unfortunately, I won't be able to make it this weekend, either for the joint SeaNAR/BEMRC launch on Saturday or the one on Sunday.)

I shouldn't have assumed what I did after reading your Alpha Centauri example and your statement that the measured flight time of the traveler was absolute; I assumed you were unfamiliar with relative time.

The main point, however, is that time is fluid in our current understanding. Other than conceptual paradoxes (being in two places at once), I am not aware of any science that forbids time travel, per se. People always discuss velocities and time simply because our best description of space-time relates them. Things like causality violations are inherent to the assumptions of the theories in which they occur; they occur in relativity because it assumes c is an absolute limit. I've seen nothing that indicates the Universe will cease to exist if my future self comes back and touches me, except in science fiction stories. (BTW, the Twins Paradox is the common name of the thought experiment, not a space-time paradox in itself.) That humans can't reconcile the apparent logical difficulties of time travel does not mean the Universe doesn't allow it. "Negative" time rears up in many theories.

The issue gets even more complex when the "strange" behaviors of quantum mechanics are involved. Observers, wavefunction collapse, quantum choices, and infinite futures all merge into a seemingly magical description of reality, one no more and no less viable than a Newtonian or Relativistic description; QM works, insofar as we can predict with it and build things that operate as we desire by using it. If you think time travel paradoxes are difficult, grab a book on QM for lay persons and consider the strange implications of a theory which has, essentially, solved solid state chemistry and optics problems (well, quantum electrodynamics is the particular theory). Truth (QM) is MUCH stranger than fiction.

Don't let time travel "paradoxes" be a hinderance to your investigations, especially if you like physics. Conceptualizing time as an attribute of reality, rather than as a human perception, may give you insight into other "strange" behaviors of Nature. The Omniverse does not perform for us, we simply strive to understand it.

RE: antimatter. There are some interesting reasons to consider it to be matter traveling backwards in time (as speculative as time travel, but interesting nonetheless). The fact that we can make it doesn't impact it's nature at all.

Now (at least now in my reference frame) I'm in my mid-40s, and I suspect my bass guitar/music theory instructor would laugh at this thread. He's always complaining about my habit of using minor second/ minor ninth intervals unsupported in songs I'm writing. And bass players are supposed to have more of a sense of rhythm than I've got.

I'm seeing myself in Chris. There's things I'm seeing that I don't like.

I'm going to have to admit my bass instructor is right, aren't I?

Zooty

Isn't that pretty much what relativity claims, that time is relative to the perception of the individual?

I don't recall hearing about that before. How do black holes evaporate?

Yeah, and that's why I was a geek and didn't start dating until I turned 15.

I'm not worth the effort.

Zooty

This was the question that made the Twins Paradox famous; at first glance, symmetry seems to imply that either twin can age differently. But there are not just two, symmetrical, reference frames, there are three: Earth, outbound leg frame moving at, say, .7 c relative to Earth, and the inbound leg moving at -.7 c relative to Earth. And not only is time dilated, distance (length) is foreshortened in the direction of travel. The foreshortend distance between Earth and the halfway point (both of which are in Earth's frame) caused by the velocities of the moving frames reduce the traveler's time to cross them in both directions, hence the traveler will experience a shorter round trip time relative to that measured by the Earth-bound observer who sees (measures) a longer distance.

(Note: most SR and GR problems throw around terms like "measure" and "distance" with little regard for what the scientists are actually measuring. A great amount of confusion is generated by the use of these terms without qualification. Since time and radial length are BOTH affected by relative velocity, some consistent measuring device is required in these kinds of problems. Usually, this is done with the only absolute SR has; the speed of light. Distances are defined in terms of time-of-flight of photons (light pulses). Time is "synchronized" with ideal stopwatches which are clicked as items in moving frames pass a defined point. Using rulers or yardsticks is NOT the way to explain or visualize this stuff, IMHO.)

An additional solution to the Paradox was provided by General Relativity and the incorporation of inertial (non-accelerating) and non-inertial (accelerating) reference frames applicable to real-life accelerations and decelerations required for a real trip. But the key to the GR solution relies on KNOWING which frame is affected by a gravity field and which frame experiences the same acceleration due to changing velocity. In essence, GR says you cannot tell the difference between a uniform gravitational field and the acceleration force which causes a continuous and constant velocity change.

very.......................... slowly..........................................

one........................ particle.............................. at...................................

a.................................... time............................................

Here. Let someone else try to explain this one:

Then again, a lot can happen in 10^106 years.

Zooty

Distance is NOT forshortenened unless the "distance" is moving.

the mass that is MOV>

That is exactly my point. we are basing out time travel reasoning on a human perception of what time is. I am betting our understanding of time is very "basic" to say the least.

as for antimater here is the problem. you need to define "moving back in time"

the conventional meaning of that phrase can be visualized as going from

if this is the case the moment we create it, it should vanish as it moves back in time.

If you think the phrase "moving back in time" means something else. well then things can get interesting :-)

Chris Taylor