Yet another time travel question

I watched a Nova special about time travel last week and on it, Stephen Hawking admitted that it might be possible to build a time machine, but that you could not travel back any further than the invention of the time machine itself. So here’s the question(s): How do you test it? Assuming it works: You invent the machine on a Monday and decide to wait until Friday to travel back in time to Tuesday (all the days occur in the same week, BTW). Wouldn’t there be two of you? The Monday you and the Friday you that appears on Tuesday.

I’m trying to prevent my brain from jumping out of my head by thinking about this.

Trump, I think you’ve misinterpreted the concept.
I remember reading the idea in his book, A Brief History of Time, and what he said was, you could build a time machine, but you could only use it to open a gateway into the future.

No racoons were harmed in the making of this post.

Sterling’s right, that is my recollection as well. I think Sterling’s right is a better way of putting it though :slight_smile:

That stuff is really complicated for us normal citizens… but the future bit is OK though.


“You know how complex women are”

  • Neil Peart, Rush (1993)

Beside…what person in their right mind would want to travel back to Tues… when Friday finally was here… I love my Fridays too much to give them up for some silly scientific experiment!

“Only when he no longer knows what he is doing, does the painter do good
things.” --Edgar Degas

Sure. That’s why time travel is so paradoxical.

OK, here’s the thing. I should probably start some other folder for this, and I’m probably going to get fried by the people who have read this book and can spit some light speed relativity nonsense at me, but this is my view:
Time travel, in any direction, implies that TIME exists in some way other than our perception of it. In other words, it implies that the events of 5:52 CT on 10/25/99 are inherent to that specific time, and that if we were to somehow recreate or revisit that time, or otherwise artificially reach that “time”, the same events would be transpiring. Remember that time is essentially a human creation, and basically exists for societal purposes. Some wisenheimer might come back by saying that the circadian rythms of our bodies determine our perception of “time,” but the fact is that over the last few thousand years, our perception of time has determined our circadian rythms.
If by “time travel” you mean accelerating to a speed at which the bodies aging processes are retarded (i.e. Planet of the Apes), then I suppose you’re right. But that’s not really “time travel” in the generally accepted sense of the word, all that is is screwing with your body.

The IQ of a group is equal to the IQ of the dumbest member divided by the number of people in the group.

Rosseau, actually, moving at high velocities doesn’t make your body age slower. That’s a misconception. To understand why you would age slower, you neeed that that light speed/relativity nonsense.

You body ages at exactly the same rate as normally. Time itself is what slows down. There’s a specific rate, but I won’t list it becuase a) it would be hard to represent and b) you probably don’t care.
Maybe Undead Dude (a real physicist, instead of a relativeity entusoast like myself) can help straighten this out better.


Stephen Hawking also said that time travel into the past was probably impossible because we haven’t had any tourists from the future visit us (as far as he knew).

Either that, or the world ends before we have a chance to invent one.

Those who do not learn from the past are condemned to relive it. Georges Santayana

Time travel into the past is basically forbidden because it can lead to a paradox. If you went back in time and shot your parents, for example, you wouldn’t have been born. But if you hadn’t been born, then you couldn’t have shot your parents. But if you WERE born, you could shoot your parents, but then you couldn’t shoot your parents because if you shot your parents you would not have been born. . . and so on.

Since we have no clue what would happen in a situation like that (collapse of the space/time continuum? Alternate timeline? Or would you just destroy your own timeline and leave everyone else’s alone?) we just assume that it can’t be done-- you can’t go into the past.
– Sylence

And now, for my next trick, I will talk in spooky half-references.

Sylence, you’re a spoilsport.

Those who do not learn from the past are condemned to relive it. Georges Santayana

Well, if you want good time travel sports, I think you could take the “alternate timeline” thang and run with it. Every time you go back in time, you create an alternate reality, carrying with you all the history that happened up to the point you travel back to. So in fact, there is no time line per se, it’s really a “time tree”. You can carry reality from an alternate universe.

Let’s use letters to talk about different branches on the tree. Say the time machine is invented in 2005A (A is the original branch of the tree). You use it to go back to 1950, thus created 1950B. (Everything before 1950 is the same as it was on the A time line.) You can travel forward in time, but not back to the A branch, you would just go from 1950B to, say, 1990B. This would be more or less like being in a coma except that your body would be in some weird other dimension so they couldn’t DNR you.

If you want to go through time from 1990B, you can just sit around and go one day per day; you can use your time machine to go to 2010B; or you can go back to 1950C. If you want to go back and sterilize your parents in 1950C, that’s fine - it won’t mean that you weren’t born, because you were born in 1960A (for example). You can talk to the person who would have been your mother in 1965C, but you can’t go back to 1965A to talk to your five-year-old self.

No one can prevent you from doing this by killing you in 1965A, either, because no one can get back there. They can only kill you in 1965D or -E, in which case, there won’t be a you in those time branches, excepting models of you imported from lower-lettered time branches.

Don’t worry, I don’t believe this is possible, it’s just an SF system a couple of my friends and I made up to systemize the movies Terminator and Terminator II.

Nothing I write about any person or group should be applied to a larger group.

  • Boris Badenov

Another theory I came up with involved an entertaining misapplication of the conservation of mass principle. If there is a limited amount of mass in our universe, then our universe can only move through time, it can’t leave versions of itself at previous points in time. Therefore, you can go back in time to 1950 tomorrow if you want, but you will find only empty space, because tomorrow the universe will be located on October 27, 1999. Going ahead in time will create the same problem, although I suppose if you could stop time you could wait around for the universe to catch up. Just thought I’d muddy the waters further.

Nothing I write about any person or group should be applied to a larger group.

  • Boris Badenov

You don’t get it.

Time does NOT exist in the sense that you are talking about. Time is a man-made creation, based on the arbitrary speed at which our tiny little planet is rotating, that we use to know things like when your grandmother’s birthday is and when the Simpsons is going to be on. In order for time to obey any laws of PHYSICS (i.e. the “slowing down” nonsense) it would have to be something PHYSICAL, which it most definetly is not. It is a concept. “Time trees” and “alternate realities” are sci-fi (science FICTION) concepts that have no validity here in the real world. So while this may be an interesting debate, let’s try to keep one foot in reality here.

The IQ of a group is equal to the IQ of the dumbest member divided by the number of people in the group.

Rosseau, ignore Boris B’s post, then read mine again. Then go get a phsyics test book.
And wrap your mind around some facts.

** Time is real.

Time is physical.

Time slows down as you approach the speed of light. **

That’s science, not science fiction. ANY introductory physics book can tell you these things. It’s chapter 26 in mine. Einstein figured all this out. If you, in your brilliance, have discovered he was wrong, don’t hang out on the message board, get your brilliant research to the Nobel prize people immediately!!


This post contained no sarcasm. Neither does this sig line.

Dearest Rousseau, how can I convince you that

Do I have to say it twice? Maybe put it in bold face?

Or maybe you had a problem with my second post? I really couldn’t tell which post you were responding to, or even if you were responding to me since you didn’t include my name. In any case, would it be fair to characterize my second post as


In contrast to my last two posts, which were (overtly) tongue-in-cheek, I will say very seriously that I do not believe time is a man-made creation in any sense. How can it be based on the speed at which our planet is rotating? Isn’t speed equal to distance travelled divided by time taken to travel? Do you believe speed and distance are also man-made concepts? Can you really mix man-made and natural concepts so freely?

Nothing I write about any person or group should be applied to a larger group.

  • Boris Badenov

Time is not an arbitrary, manmade concept, the means by which we measure time are. Time does exist, and it only flows one way. Take a ball and drop it. It lands on the floor after you release it from your hand. It takes a certain amount of time to do so. You can measure it in seconds, microseconds, years, or an arbitrary unit of time you just made up, but it does not change the amount of time elapsed. It also does not change the fact that the ball hit the floor after you let go, not before or at the same time.

And this opens up another can of worms: thermodynamics. Entropy. The irreversability of certain processes. Could the ball have suddenly flung itself off the floor and landed in your hand? Certainly, but it is highly unlikely. Could the coffe cup have reassembled itself, scooped up the puddle of coffee, and landed back on the table? Even more unlikely, but still possible. However, once you get into subatomic physics, certain things become completely irreversible. There is a certain particle (can’t remember which one, will have to look it up), that forms out of two particles, but decays into three of the exact same particles (or vice versa). The arrow of time always points in one direction. You could never put it back the way it was.

There’s also the issue of black holes, and their singularities. A singularity is defined as something whose past can not be extrapolated from its present condition. If you took a compacted car, you could conceivably figure out whether it used to be a Honda or a Mercedes. No matter how much you crushed it, you would still be able to find out what it was before, up until it collapsed under its own gravity and became a singularity, at which point there is no hope. Essentially, it has been reduced to its basic properties, mass, angular momentum, charge, etc. What it used to be is a complete unknown.

Time travel into the past would violate both these principles, since from the observer’s frame of reference, thermodynamics would be disobeyed, and singularities would be spitting things back out. Unless some loophole is discovered (and a number of scientists are working furiously on finding one), time travel into the past is impossible in this universe as we now understand it.

(P.S. I do not claim to be an expert. If anyone finds any flaws in my argument, do point them out, so I can use it again, New and Improved. Like I even have to ask…)

Modest? You bet I’m modest! I am the queen of modesty!

Oh, yes. One more thing. Shouldn’t this be in the Great Debates?

Modest? You bet I’m modest! I am the queen of modesty!

<Q>jab1 wrote:

Sylence, you’re a spoilsport.</Q>


Oh. . . yeah, sorry.
– Sy “spoilsport” lence

And now, for my next trick, I will talk in spooky half-references.

By the way, sylence, where did your sig line come from? It sounds vaguely familiar.

Nothing I write about any person or group should be applied to a larger group.

  • Boris Badenov
What can I say, I’m a fan.

– Sylence

And now, for my next trick, I will talk in spooky half-references.