No Time Travel, Sorry 888
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."
Drinking to much funny-juice (Score:5, Insightful)
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.
I'm no physicist (Score:5, Insightful)
Let's play: spot the Loony (Score:5, Insightful)
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
Slashdot allows any bullshit site now? (Score:1, Insightful)
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.
Re:Drinking to much funny-juice (Score:5, Insightful)
Not really. Now it's now, and that's all that is. You remember yesterday, but that is a memory occuring now. The past doesn't physically exist. Nor does the future. The only real (i.e. existing physically) part of our time perception is now.
Textbook strawman arguments. (Score:5, Insightful)
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.
Re:Slashdot allows any bullshit site now? (Score:4, Insightful)
Re:Let's play: spot the Loony (Score:3, Insightful)
Re:Drinking to much funny-juice (Score:3, Insightful)
The funny thing about the time travel theories is that, they are all based on a specific "definition" of time, when time by itself does not exist, it is just another metric that we mere mortals created (no I do not believe in god
[Un]fortunately, the only thing that gives us a sense of the past is the memory, that way, if yesterday someone implanted in my memory that I was going to be reading this story on slashdot, I may believe that I've traveled to the past (no dupe jokes please).
Re:Let's play: spot the Loony (Score:3, Insightful)
This page is probably there because the author enjoys making people who know better and are uptight about it get hot under the collar. In other words, he's trolling. If that's the case, the author would have to have a pretty decent grasp of the concepts he is mocking in order to know exactly which buttons to push.
People overlooking markers? (Score:3, Insightful)
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.
Re:Let's play: spot the Loony (Score:5, Insightful)
Mach is the ratio of two speeds. Doing such produces a number in which the units cancel out. Speed/Speed = dimensionless. This also brings up another property of a dimensionless number, in that the value *does not change* in any unit of measurement. Mach 1.5 is the same in SI, FPS, or any other system.
Don't believe me? Here [wikipedia.org]. "As it is defined as a ratio of two speeds, it is a dimensionless number." I'd hate to think you believe what you said... as others are taking you seriously.
Please don't respond back... I don't want to debate this further and you are most utterly wrong. I hope you don't work on... anything.
Re:Let's play: spot the Loony (Score:3, Insightful)
Mach is not a speed, it's a ratio of your speed (measured in distance per time) to the speed of sound in whatever medium you happen to be travelling in (also measured in distance per time). It's only used because it is very convenient to use such a proportion when making calculations about compressible fluid flow (shock waves and the like)--properties of the flow are identical at identical Mach numbers, regardless of speed. Since the speed of sound varies with temperature and fluid composition, your speed at any given Mach number could vary with the temperature and type of fluid you are travelling through. In order to find our speed from Mach number, it is necessary to define additional information and introduce dimensions to the dimensionless Mach number.
That being said, I agree with your larger point about needing more than a first-year Calculus course to debunk the world's leading physicists. However, if we're going to discuss how rigorous math is needed to express complex ideas, then we must use rigorous math ourselves, and not fall into the trap of calling a dimensionless ratio a "speed", which is a number with dimensions. It may be useful to think of a ratio as a speed, but in reality it is not.
AeroIllini
B.S. Aerospace Engineering, University of Illinois, 2003
need strict definition of time travel (Score:1, Insightful)
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!
Re:Drinking to much funny-juice (Score:5, Insightful)
All well and good, except that we've already proved in practice [metaresearch.org] that time has a different rate of passage for different people. Quote: "For GPS satellites, General Relativity predicts that the atomic clocks at GPS orbital altitudes will tick faster by about 45,900 ns/day because they are in a weaker gravitational field than atomic clocks on Earth's surface. Special Relativity predicts that atomic clocks moving at GPS orbital speeds will tick slower by about 7,200 ns/day than stationary ground clocks."
The difference is about 38,000 ns/day. Since the speed of light is about one foot per ns, if relativity were wrong (because time passed at the same rate for everyone), GPS would accumulate an error of about 7 miles per day. Such an error would be blindingly obvious to everyone using the system, and wouldn't require any fancy equipment to measure.
I'm interested to hear Mr Savain give an alternate explanation for how GPS works.
Re: Really? A tie? (Score:3, Insightful)
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.
Re:Drinking to much funny-juice (Score:2, Insightful)
That's a good insightful argument backed up by some godawful math.
The function for slope will be a simple constant for *any* linear function.
A linear function can be expressed as:
f(x) = mx + b
Its slope is the derivative:
f'(x) = m
which is simply the constant m, (and if m happens to be zero the line is horizontal.)
The case of a vertical line is special because it isn't a linear function. Hell, it isn't even a function. It has no defined slope at all.
That doesn't mean vertical lines can't exist, of course, merely that its nonsensical to try and determine their slope on the xy axes.
Similarly, as you said, it doesn't mean that time travel is impossible any more than vertical lines are impossible, just that the usual equations for velocity aren't applicable.
Re:Drinking to much funny-juice (Score:3, Insightful)
tm
Re:But I time travel every day! (Score:3, Insightful)
Re:No time travel into the future? (Score:3, Insightful)
Re:Let's play: spot the Loony (Score:3, Insightful)
Hmmm...point taken. I think technically it's supposed to be "mach number", rather than what I said, which is that it's a speed.
I don't know. Mach number is clearly dimensionless...but it gets larger in magnitude when you go faster, so it is a speed in a way. It's a dimensionless speed? That seems contradictory.
I perhaps typed incorrectly, that mach number is not a speed. However, my basic original objection still stands; the web site is playing dumb games with numbers and dimensions and making conclusions without really understanding them.
casuality is the key (Score:5, Insightful)
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.
Re:Let's play: spot the Loony (Score:0, Insightful)
ALL HAIL WIKIPEDIA!
spacetime is NOT a general 4-space (Score:3, Insightful)
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.
Re: Really? A tie? (Score:4, Insightful)
_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.
Re:Drinking to much funny-juice (Score:2, Insightful)
Re:casuality is the key (Score:4, Insightful)
I'm not sure conversation of energy is my first worry though, not in the strictest sense. Mass worries me more. Does the god fill in the missing particles that was Marty with something that won't be missed from some far corner of the universe? Now, I know that mass and energy are convertible to each other, especially in something as outlandish as all of this, so it's kind of a nitpick on my part. I wonder just how much energy (besides the mass of Marty/Delorean) would be needed to roll the clock back 30 years, if it's calculable.
The real killer is probably information entropy though. The "snapshot" itself isn't allowed, even if you postulate a god sitting in a metaverse/metatime playing with the Universe on his VCR.
Bad science? (Score:2, Insightful)
Re: Really? A tie? (Score:3, Insightful)
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.
Where is John Titor When You Need Him? (Score:2, Insightful)
Re:Of course time travel is possible! (Score:5, Insightful)
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.
Maybe time travel is the easy part (Score:2, Insightful)
Re:Textbook strawman arguments. (Score:3, Insightful)
Just to add what doesn't really need to be added, to really spell things out in case anyone isn't following this:
The guy's saying that velocity in space is measured in such-and-such a way (dx/dt, or change in position relative to change in time) and so velocity in time is impossible because it would be dt/dt, or change in time divided by change in time, which is 1, so we're always moving forward at the same rate. But all this really means is that he's arbitrarily chosen a nonsensical measure of time travel, and then says that time travel's impossible because the measurement system he arbitrarily choose doesn't make any sense.
Beyond that, it's not even entirely clear that the arbitrary system he choose doesn't make any sense. For example, if we ask the question "how far did you move in space compared to me," it presents the perfectly valid math problem dx/dx. For example, you walked two miles down the road, I walked one mile down the road, the relationship between how far we walked is given by 2 miles / 1 mile, or 2. The miles divide out- there is no unit to it, it's just a multiple - you walked twice as far as I did.
So if someone's traveling in time, you might say "how long did the time traveling experience last for you?" And they might reply 1 minute. And you'd say, "how far did you travel," and they might reply 100 minutes. So they traveled in time 100 minutes in 1 minute, and (100 minutes)/(1 minute) = 100. The multiple at which time was passing for them, as opposed to normal, was 100 x. Had they traveled in time for 2 minutes at the same rate, one might have expected them to go 200 minutes back in time. I don't see how it's "nonsensical" to divide time by time, or distance by distance.
Re:Drinking to much funny-juice (Score:2, Insightful)
Ah, this is the first time I have the honour to tell this to a /.er.
RTFA
I'm being unfair, since it was on an unlinked page on the same site
"Time does not dilate for the simple reason that time, by definition, cannot change. The slowing of clocks is more likely due to energy conservation principles that come into play when a huge number of particles are interacting locally." http://www.rebelscience.org/Crackpots/devil.htm [rebelscience.org]
Re:casuality is the key (Score:1, Insightful)
Consider a two state system and an observer:
|obs>(a |0> + b |1>)
Now, through time evolution, let the state of the observer become entangled with the system:
(a |obs0> |0> + b |obs1> |1>)
The observer will "split" as it becomes entangled with the system, and the two parts will now evolve completely seperately. One version of the observer sees 0 and the system behaves as though it is in state 0. The other version sees 1 and the system behaves as though it is in state 1. Thus, the collapse of the wavefunction.
This is precisely equivalent to the many worlds hypothesis and explains all observed properties of measurement, while being a necessary result of quantum mechanics.
Re:casuality is the key (Score:3, Insightful)
Nonsense. The universe in its past state that Marty McFly becomes part of when he travels back in time can simply fork() and make a private copy for Marty McFly to make his changes in. This avoids causality loops at the cost of believing the universe can fork(). Still, even though you might point out that the universe fork()ing is a wild and fantastic notion, I would argue that the universe's having come into existence in the first place is an equally wild and fantastic notion.
Another possibility is that the past simply isn't immutable. Sure, we're very much used to the idea that it's not, but perhaps that's just because we haven't ever observed the mutability of the past. And it's not wonder, because if time travel is impractical, it may be that it has never happened, and it would be necessary to travel time or observe someone traveling time in order to see that it's not immutable.