Is time travel possible?

I so wish that time travel was possible. Not just because I wish I could go back in time and undo some of the mistakes I have made in my life, but because I would love to go back and undo some of the mistakes that others have made! For example, I would love to appear on the deck of the Titanic on April 15th 1912 and warn the Captain of impending doom. I would love to appear in the vicinity of Adolf Hitler just before his rise to power and put an end to his perverse ambition. I would love to appear in Dallas in 1963 to prevent the assassination of President Kennedy.

Alas, I am certain that time travel is not possible and the reason is as follows.

Like any of the physical things surrounding us, we are all made up of atoms. Protons, neutrons and electrons. The atoms that allow us to exist were not “born” at the same time we were born. They did not grow inside of us. They have existed, either as atoms or as other forms of energy, since the creation of the universe.

Whilst some atoms may undergo changes in arrangement from time to time, such as the chemical change from one compound to another, or undergo a sub-atomic change as occurs in the center of stars from one element to another, most atoms typically exist in one state for a very, very, very long time. An atom of hydrogen, for example, might bond with two atoms of oxygen to form a new atom of water, but the individual atoms of hydrogen and oxygen are still there. They have just changed their atomic arrangement to form a new substance with new properties.

As we grow and continue to live our lives, we accumulate atoms from the world around us, as well as lose atoms back to the environment. An atom of water, for example, was probably once part of an ocean, then evaporated into a could, falling as rain, possibly draining into a river, back into the ocean, another cloud etc until one day it was absorbed through the soil by a plant, then eaten by me or by an animal that I subsequently consumed. That very same atom is now (temporarily) part of me. There is an endless list of paths that the atoms that make up me may have taken before becoming me (or you).

Putting aside reality for a moment, imagine you could place a unique number on each atom in and around the earth, including the atmosphere, like the number on a pool ball. Suppose you could also then track specific atoms as they continuously move from one thing to another, observing the unique number on each atoms once it becomes something else in order to identify it. Of course this is a silly idea, but lets use it to illustrate a point.

Now, imagine you invented a time machine or found a stable worm hole that you could use to travel back in time and you attempt to do so. This is where the laws of physics will kick in, preventing success.

The reason is simple. It is impossible for you to exist at an earlier point in time, because the atoms that make up you (or anyone or anything else you wish to take back in time with you) already existed at the earlier point in history, as something or someone else. Using the imaginary illustration, if you were to make a list of all of the uniquely numbered atoms that constitute “you” and you were able to locate each numbered atom at the earlier point in time, you would discover those precise atoms were at various places across the planet existing as things other than you: perhaps part of certain bodies of water, various plants, rocks, minerals in the soil, animals, structures and even other people!

Unlike sub-atomic particles such as electrons, atomic particles can not exist in two different places at the same time. In other words, you can never exist at an earlier point in time when your atoms are already there, busy being something or someone else. Time travel is not possible due to a fundamental law of physics. Bummer.

I think this is the LINK TO COLUMN: Is time travel possible? - The Straight Dope See my Post below (#21). – CKDH


Why do you say this?

It will never be possible because they keep coming back to sabotage the research.

You are assuming that “you” are physical, “you” consist of specific atoms.
Some might say that “you” are a separate soul that gives life to those atoms.

Anyway, time travel is possible.
I’m doing it right now.

And I bet you have traveled here from the past and are thus unable to tip me any future sports scores?

You are either forgetting about or seriously underestimating the amount of radioactive decay that occurs in nature.
Powers &8^]

You’re overnitpicking. First, radioactive decay is limited to a subset of atoms and those atoms are not among the most abundant. The silicon and oxygen alone in the earth constitute “most” flat out. And you’re the one who is forgetting that the earth is not representative of nature, because it has a disproportional supply of heavy atoms that make up its solid interior. Other planets are going to be more gas, and nonradioactive gas. And interstellar gas and dust is probably 99.999% of all “nature” outside suns and is very unlikely to be radioactive. The OP mentions earth but doesn’t specifically limit time travel to our infinitesimal pinpoint of the universe.

The OP’s argument is a familiar one and not readily settled. The fact that the individual atoms he labeled would be at a later point in their time cone may be enough to allow their existence even if they were duplicates of others. He or she merely asserts that they can’t without evidence. I don’t remember if it has been mathematically studied either way.

Time travel to the future is probably out of the question. How can you travel down a road that hasn’t been planned, let alone built yet?

Time travel to the future absolutely, unquestionably exists. It’s time travel to the past that’s problematic.

I’m referring to anything faster than 1sec/sec, of course.

First - you sound like you would really screw crap up if you could do this. Thank goodness that…

Second - time travel isn’t possible because there’s no such thing as time - we made it up. We said 1 planetary revolution is a year - we could have just as easily divided it up other ways. If we said 50 revolutions is a year, Kennedy hasn’t been dead a year, and WW2 was a year and a half back. It’s just a reference system we made up to organize things for us, but it’s got no real relevance or substance.

Time is a single unit. All the time there is is just one unit - one moment. You can slice it however you want, claim this slice or dole out others as history or the future, but it’s still just the one moment - there’s no previous moment to go back to, there’s no new moment on the horizon. There’s just an endless now.

As a concept, I’m not saying it’s not handy. It helps us measure things, quantify things, etc. But don’t get caught up in it as some sort of universal absolute or assume it’s objective - it’s a commonly-accepted subjective measure, like saying this much land is an acre and that amount of distance is a mile.

Since this is the issue that people have debated for centuries, you don’t get to use it as an assumption or state it as a certainty. Until we have a full mathematical theory of time you can’t rule out time travel. It can be found in relativity (under extremely specialized and perhaps not physically realizable conditions).

You can read about one such proposal in Cecil’s column Is time travel possible, which is the one I assume the OP was referring to. It also explains why Chronos made the claim he did. Thorne et al. probably are better physicists than you and I, so if they say time does not conform to your limitations on it, we probably need to listen.

That’s slightly harder, but still straightforward and well-understood. No method is known for going slower than 1 sec/sec, but there’s still no inherent reason to believe it must be impossible.

I don’t think the OP considered relativity and quantum physics, but the idea Gaydope is trying to encapsulate (i.e. that at any given moment in time one and one only copy of each particle should be present, ignoring quantum effects such as creation/annihilation, indistinguishability, etc), has a technical name in general relativity - global hyperbolicity.

Global hyperbolicity is not actually a fundamental feature of general relativity though: there are plenty of spacetimes that obey Einstein’s field equations that are not globally hyperbolic. This is why the possibility of backwards-in-time travel is still an open question in physics. However, at the same time, it is quite normal to reject solutions that fail to be globally hyperbolic as unphysical and it’s conjectured that either limitations on the types of mass-energy that can exist (so-called energy conditions) or an undiscovered aspect of quantum gravity will essentially rule out time travel.

If you could travel to the future, where would you be going? Unless you are saying that the future is set in stone in such a way that if you went forward in time then came back, you would be compelled against your will to do exactly what the future timeline said you had already done, then there is in reality no “future” to go to.

Well, of course I can. I could also be wrong… the irony is, eh, so could the experts.

The idea is that you can time travel to the future by, for example, taking a near-c round trip on a rocket- starting and finishing on Earth- so that much less time has elapses between your departure and arrival for you than someone who stays on the Earth. However, without backwards-in-time travel, once you’re in the future you’re stuck there.

Doesn’t time dilation do it? If you accelerate, then everyone else advances 1 second, but you advance a mere 0.999 seconds.

Within your reference frame you always advance 1 sec./sec. It’s just that when you go into a different reference frame and compare clocks they may not match. But everybody everywhere at everytime advances 1 sec./sec.

That’s what I was understanding Czarcasm to mean by “faster”: You get there in less time.

And Czarcasm, there is no coming back. You can travel forward in time, but so far as we know, you can’t travel back.