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No Time Travel, Sorry
MOBE2001 writes "The bad news is that time does not change. Spatial velocity is given as dx/dt. Velocity in time(dt/dt) is nonsensical. As simple as that. In other words, no time travel to the past or the future, no motion in space-time, no wormholes and no hanky-panky with your great, great grandmother. There is only the changing present, aka the NOW. The good news is that distance is an illusion and we'll be able to travel instantly from anywhere to anywhere."
That's weird because I could have sworn when I went to bed last night it was yesterday and now its today.
Nevertheless...this is fun. Looking at the equation from which all his arguments flow, it seems he is only demonstrating that it doesn't make sense to talk about one's velocity through time. I would agree. If I hop in my time machine and zip off to tomorrow, it doesn't make much sense for you to ask how long it took to get there. Or if you and I both have time machines and we decided to race to 1:00 pm tomorrow it would be always be a tie. But this is a far stretch from demonstrating that it is impossible. By this same logic we could define slope as the change in x over y or s = dx/dy. Does this definition make it impossible to move along the y axis because then the slope of our movement would be dy/dy? No. but it does say that if you move along the y axis your slope will be a constant.
But in all my readings, I have learned one thing about physics. Nothing is "as simple as that".
I'm sorry, but if you're going to put up a web page in which you call all the foremost theoretical physicsts in the world frauds, then you'd better have more evidence than some undergraduate-level pseudo-calculus and verbal smoke screens.
The t-axis or time-axis velocity component is 1, a dimensionless number. Now there are relativists who will insist that it is perfectly acceptable to express velocity in time with a dimensionless number but the rest of us with our head on our shoulders, know that it is not true. We know that a dimensionless number such as 1 has absolutely no meaning in as far as expressing velocity.
Not true. Normalized velocities are perfectly reasonable things to express. Mach 1.25 is a perfectly well-defined speed that does not violate any laws of physics, and what do you know--it's a dimensionless number.
I'm sorry, but this page is really quite embarassing for the author's parents and any physics teacher's they've ever had. This sort of reminds me of people that read things like A Brief History of Time, a perfectly excellent book, and then try to tell me that the physics is really great and it would be so much better unencumbered by the mathematics.
I don't think real time travel, a-la Dr. Who is physically possible. But the "arguments" on this web page don't really make sense, much less prove all those physics wrong.
Craig Steffen
Ph.D. Physics, Indiana Unversity, 2001
Craig Steffen
http://www.craigsteffen.net
So, I can start up any old domain and post some random crap, and it'll get posted as the truth on Slashdot?
Of course "time travel" is possible. Of course in one's own frame, their time will always be the "present". But, other people have different frames, and if you move relative to them, your "present" won't match up with theirs. So, if you leave the Earth at a high speed, turn around, and come back, you'll be in the Earth's future, but it will still be your present.
Thanks for the bullshit links, though, I'll be sure not to read them.
Spatial velocity is given as dx/dt. Velocity in time(dt/dt) is nonsensical.
That would be a lovely argument if changes in position were measured in velocity.
You describe spacial travel as the dx, not the dx/dt. It stands to reason that you would describe time travel with the dt, not as some rate of travel we haven't come up with yet.
"Joke" foot on article: check.t mp ace
Suspicious URLs:
http://www.rebelscience.org/Crackpots/notorious.h
http://www.rebelscience.org/Crackpots/nasty.htm#S
Check.
Comments taking the article 100% seriously: check.
This must be Slashdot.
two points:
one: this is just a simple objection but it seems that the author is putting too much emphasis on notation and how well we are able to apply a given mathematical model.
two: to even begin to discuss time travel a strict definition of what time travel is or means must be set out. the author is correct that, according to his definition, time travel is a meaningless notion. however, all his argument shows is that his definition of time travel is not suitable to to describe the notion we have in mind. time is much more complicated than he makes it out to be (ie just like there is no absolute frame of reference when it comes to space, so it is with time. though really the distinction is only made for clarity of exposition since what we really have is that there is no absolute frame of reference for space-time). one way to define time travel, then, would be to say that there is change in something we can call "personal time" with respect to "everything else" (though this is also a bit of a simplification). at this point his one line proof breaks down since we are talking about change in one parameter with respect to another, which is perfectly acceptable. moreover, this definition captures more precisely what we mean when we say "time travel" (and also will allow us to make sense of racing to some other time [cf a previous response]).
one final point, and this is just a pet peeve: we are not all time travelers since we move through time at 1 second per second since there is no change in our personal time with relation to external time. start with definitions!
It took less time as you observe it. To use one of the standard ways of explaining relativity: there are two astronauts. One stays on Earth to train, while the other goes on a mission, zipping around the solar system near the speed of light. When he returns, he aged 1 month, while the astronaut on Earth aged 1 year. What gives? Well, the same amount of time "happened." Both spent one year on their individual tasks. The one that went on the space missions feels like one month passed, but that's just because of how he observed time. Time is constant. Time did not pass at different speeds.
Another way of thinking about it is driving along a highway, watching a mountain, forest, clouds, or some other large object at a distance. Nearby objects appear to move faster relative to your car, while the farther away objects appear to move slower. The road sign and the mountain are both standing still, but appear to move at different speeds relative to your car. This is similar to time. You may observe different events at different speeds, or two astronauts may observe the same event at a different speed, but time itself is constant.
24 beers in a case, 24 hours in a day. Coincidence? I think not!
That only works if you never accellerate. Otherwise, your frame is non-inertial, and cannot be so defined.
If "time travel" just means the usual physics definition of "tracing a trajectory backward" then of course you can regard it as happening all the time. Positrons (anti-electrons) can easily be regarded as electrons traveling "backwards" in time, so that a positron-electron annihilation event is nothing more than an electron traveling "forward" in time, then reversing itself and traveling "backward" in time. Obviously we (traveling steadily forward in time) see two particles with opposite properties converge and disappear. Whoopee.
However, I think what most people mean by "time travel" is something different, a causality loop. That is, they mean you do something (which they call "time travel") and this something lets you become your own grandpa, or influence the outcome of the Civil War, and so forth. Since, of course, those things influence the you that's influencing them (otherwise the story is not interesting), this makes a nice little loop of cause and effect: you influence x which influences you who influences x, and around and around.
Whether or not the physics of the universe allows such a thing, I can't see any obvious reason why it would cause big problems -- or even be interesting. Certainly it could not manifest itself the way it's shown in the movies, in which you see the loop first one way (Marty McFly's parents marry and produce him), and then another way (Marty's parents fail to marry, because McFly travels back in time and interferes with their meeting). That's logically impossible. If the loop exists at all, it must have one unchanging form.
That is, if Marty McFly does go "back in time" he obviously can't (or rather doesn't) prevent his parents from marrying and having him, because they actually did. Whatever he does "back in time" is already part of history. His "changes" already exist, and have always existed. Indeed, they can't even logically be regarded as "changes" because nothing really changed. Although...it's possible McFly, with his imperfect knowledge of the past, could have assumed something about the past was different than it actually was (e.g. he thought his parents met at the dance, instead of afterward, when some strangely-dressed clown introduced them). Therefore, when he "changes" history (by interfering with his parents meeting during the dance, and then "fixing" things up by introducing them afterward), he might be under the illusion that he is really "changing" history instead of simply causing it to happen as it actually did.
I suppose we could now argue about whether Marty's sense of free will (as well as our own) is therefore just a big fat self-delusion, but, ugh, not before a pint or two.
One thing I think I should point out about how spacetime doesn't fit the general model of a 4-space, and it's simple:
Object do not pop in and out of existence as time progresses.
If time were simply a velocity in a dimension in a 4-space, that could happen. Instead, we see a continuity in 3-space, where an object might move, but there is a relationship between where it "is" and where it "was" and where it "will be".
So, it makes sense to model spacetime as a 4-space, but not as a general one.
_Special_Relativity_ says that two observers moving past each other in unaccelerated reference frames will each perceive that the other's clocks are running more slowly. The observations are consistent. To get the two observers into the same reference frame, one or both of them would have to accelerate, and general relativity conveniently works it so that the paradox vanishes.
In the actual example, one observer is remaining stationary, while the other is accellerating to high velocity, then accelerating to change velocity ("around the solar system") and then presumably accelerating a third time to return to the original reference frame. All three accellerations, plus travelling at high velocity relative to the rest frame, will cause the traveller's clock to run more slowly, as observed by someone in the rest frame.
I believe that you may be labouring under a misunderstanding of spacetime. I suggest that you look for the text "Spacetime Physics" by Wheeler et al. as it explains this from first principles in simple language and backs this up with (initially) easily comprehensible mathematics. Time dilation occurs and is why synchrotrons are able to produce the high frequency output that they do. It is measurable, repeatable science and is now used for a commercial purpose. We are well past experiment or theory on this. Time dialtion is also critical in the explanation of several observed astronomical phenomenon. But we should be thankful for the website. Any challenge to the accepted is a good thing and allows us to grow in our understanding.
That's not my point. I'm saying that while time appears to go slower, it didn't actually speed down. Sure, the tree grew more slowly, but that doesn't change the fact that time itself did not.
24 beers in a case, 24 hours in a day. Coincidence? I think not!
I wonder what his crackpot index would be?
Why is motion in spacetime impossible? It has to do with the definitions of space and time and the equation of velocity v = dx/dt. What the equation is saying is that, if an object moves over any distance d x, there is an elapsed time d t. Since time is defined in physics as a parameter for denoting change (evolution), the equation for velocity along the time axis must be given as v = dt/dt which is self-referential. The self-reference comes from having to divide dt by itself. dt/dt always equals 1 because the units cancel out. This is of course meaningless as far as velocity is concerned.
Does the impossibility of motion in spacetime invalidate Einstein's relativity? The answer depends on whether one takes spacetime to be physically existent (as relativists do) or as an abstract, non-existent, mathematical construct for the historical mapping of measured events. If one chooses the former, one is obviously a crackpot or a fraud, or both. If one chooses the latter, then general relativity is to be seen as a mere math trick: the physical mechanism of gravity is still out there and it is incumbent upon physicists to find it.
This guy seems like an idiot to me. If you make the step to say that this is a mathematical construct that best describes our limited understanding of reality, which I believe to be true, you'll never be able to describe OR refute a more complete understanding of reality using that construct. You often need to discard and rethink the original concept or adapt it for it to improve. None of our knowledge, scientific or otherwise, is fully and completely right. Not one bit, it's just the best abstract model we've got. Everything we know will eventually be demonstrated to be incomplete, inconsistent or wrong. Which means you can't use any existing models to refute a new one. You can use them as a guide, you can say that the old and the new are inconsistent, but to refute them you need to go to the real world.
-1 Uncomfortable Truth
But since the universe is expanding at such an incredible rate, when somebody goes back in time, it is not to the same place on earth, which is moving at something like 300,000 kps along with the universe, but they stayed right where they were. Just a thought. Everybody always assumes time travel is always tied to a relative location not an absolute one.