Let’s say I have a time machine, and an orange. I place the orange on the time machine and send it back to five minutes ago.
After this event occurs, and the orange is upon the time machine of five minutes ago, wouldn’t it no longer need placing on the machine? Wouldn’t it have been impossible for me to have put the orange on the time machine, as it was already there?
Are there people who seriously believe time travel is feasible, given time and technological advancements? Aren’t issues such as this pretty clear indicators that it is not possible?
General scientific consensus at this time as far as I know is that it is impossible. Still, your scenario isn’t a problem if you assume that changing the past is possible in the first place. Another possibility is that time travel creates a new time line; in this case, a timeline where an orange appeared on the time machine.
I guess some people believe that during those 5 minutes, something happens to cause the orange to be removed from the time machine, and that this is unavoidable in order to avoid a paradox.
Your hypothesis depends on the presumption that the orange, having travelled back in time 5 minutes, remains in the same location for five minutes. An alternative hypothesis might say that the orange can’t possibly remain in the same location for five minutes, and during those 5 minutes something happened to remove it.
If you don’t put the orange in the past because the past has an orange from the future, then the past won’t have an orange from future so you will put it in as before which results in you not.
Basically you’ve initiated an infinite loop cycling between the orange being there, or not. Infinite loops are not logically impossible, as anyone who’s ever forgetting to increment the variable in their while loop knows.
Another possible way to look at it is the Novikov consistency principle. Briefly, it means that, in a universe in which time travel exists, nothing can be achieved in a trip to the past that is inconsistent with the traveller’s history (which is, of course, part of the history of the universe of the universe “before” the trip in question; any seeming alteration is simply the revelation of new information, not the creation of new events. F’instance, I can travel back to 1865, shoot Lincoln, and frame John Wilkes Booth for the crime (including making him think he did it), but I cannot stop Lincoln from being shot. Any attempt to do so must inevitably fail.
Incidentally, for fiction that is the time-travel setup I prefer.
In the OP’s scenario, if he attempted to send the orange back to a place where he knew it was not, then either the time travel machine would fail in that instance, or something would intervene to remove the orange from the spot he sent it to before hte past him was able to see it.
If time travel can occur, when you go back in time you split off another universe. The original one continues unaltered.
Heck, you don’t even have to have spliting universes…just that if you go back to 1:00 April 2 1865 then your altered events proceed forth at 1 second per second…so the original timeline is still there…it’s just that it’s history has been altered but they don’t know this and it has no affect on their reality.
Right. A program can function that way, but can the universe? Also, as a program is going through those iterations, it’s happening in time, which is quite a bit different.
But that depends on any time travel to the past having already happened. So, unless Doc Brown has already been seen appearing spontaneously in the middle of town, he will never do so in the future. Which is kind of my point, I guess. If I don’t see the orange appear seemingly out of nowhere five minutes before I run my experiment, then the experiment clearly will not work/didn’t work. So, what’s the point of trying anyway?
Well that’s a good question. Can time change? Change is a function of time. Is there a meta-Time, that the time line its self can change in?
Don’t know, but if you postulate time travel it seems like it’d be logically required. Things from the future don’t materialize in a vacuum, even disturbing a single electron from a path it was previously destined to go, would require the time line to accept change.
Even empty space, that was previously destined to be empty, or holding something else, now holding an orange would require the time line to accept change.
I prefer the Lost strategy of time travel. When you go back in time, it’s not really back in time. It’s your future, filled with events from the universe’s past. So instead of imagining an arrow traveling along a time line and all of a sudden looping back to a previous spot (the arrow goes back in time), instead imagine the timeline being severed and the past moving in front of the arrow (time goes to the arrow).
So you can still die. You’ll just die in the past, as is/was your destiny all along.
Actually, it doesn’t. In at least some views of time, what we perceive as the passage of time is an illusion, a matter of perspective. Doc Brown always time traveled; the universe came into being with all of the past and future already existing as a seamless unchanging whole.
You go to place an orange in the machine, but you see an orange from the future there. So if:
a) you don’t ever remove the future orange from the machine, it’s always the future orange going back in time with each iteration. Putting the present orange in the machine never happened because the first time you used the machine you changed history forever.
b) you remove the future orange and put the present orange in the machine. You have a future orange sitting around.
c) of course even my limited understanding of physics tells me that you can’t do this because you would be creating a new orange in a universe where matter cannot be created or destroyed. That’s why the concept of single universe time travel never works at all. If you aren’t sending a future orange to the past to co-exist with the present orange, i.e., it must be the same orange, all you’ve done is make a machine that teleports things in the past. Your orange will suddenly jump from your hand onto the time machine because of your future actions. So you haven’t time traveled at all.
I recall that in a recent discussion one of the more physics minded posters mentioned that that doesn’t hold because the law of conservation of energy is a local law, not a global one.
Right. Since space and time are intertwined in spacetime, time travel doesn’t actually create energy from nothing. You’re simply moving energy from one spacetime location to another. So while it violates relativity, it doesn’t violate thermodynamics.
Speaking of thermodynamics, one of the big problems with the scenario in the OP is what happens to the orange if it’s stuck in an infinite time loop. From the point of view of the orange, time is still passing normally, so it’s going to rot and degrade over time. Each time it goes back to 5 minutes ago it will be a slightly different orange, until at some point it will rot away and not be an orange anymore. Then what happens?
Does the machine travel with the orange? If not, you have nothing to worry about-you’re just looking in the wrong place. In those five minutes the earth has revolved and orbited, our solar system has moved through our galaxy, our galaxy has both spun a bit and traveled in some unknown direction etc.
Anyone want to guess where that orange is now?
Seems there’s quite a bit of Schrödinger in here. I postulate that even if we can manipulate time, we will never know if we were successful or not. Case in point:
In my mad scientist lab, I have two clear cylinders. The first cylinder should send objects back in time five minutes to the second cylinder. Let’s say, for argument’s sake, that I’ll know my experiment was successful five minutes before I flip the switch if an orange appears in the second cylinder. I then have a choice: I can choose whether or not to proceed with the experiment. If I don’t, I create a paradox. And the universe will prevent me from creating that paradox, so my experiment is a failure.
However, suppose I can’t see what’s inside either cylinder until I flip the switch. The orange will either be in the first cylinder or the second, but I won’t know until I open them. Therefore, I’ve deprived myself of the ability to create a paradox five minutes before I flip the switch. It now doesn’t matter whether my orange traveled time or not: If it’s in the first cylinder, then the experiment is a failure. If it’s in the second cylinder, all I can be sure of is that I teleported an orange. Whether or not it was there five minutes ago is impossible to know.
Ergo, even if we could invent a time-travel device, we’ll never know if it works or not. Q.E.D.
