Is time travel possible?

[C_K_Dexter_Haven]

NOTE FROM MODERATOR:
Welcome to the Straight Dope Message Boards, Gaydope, we’re glad you found us. For future ref, if you’re commenting on one of Cecil’s columns, it’s helpful to other readers to provide a link. I’ve found a column that I assume you’re commenting on, and have added the link. If I’m incorrect, please let me know. If you’re not actually commenting on that column, but asking the question independently, it would get better response in a different forum. No biggie either way, and, as I say, welcome!

[Asympotically_fat]

@C_K_Dexter_Haven

NOTE FROM MODERATOR:
Welcome to the Straight Dope Message Boards, Gaydope, we’re glad you found us. For future ref, if you’re commenting on one of Cecil’s columns, it’s helpful to other readers to provide a link. I’ve found a column that I assume you’re commenting on, and have added the link. If I’m incorrect, please let me know. If you’re not actually commenting on that column, but asking the question independently, it would get better response in a different forum. No biggie either way, and, as I say, welcome!

Or for past ref, depending on which way you’re travelling.

[John_W.Kennedy]

@Gaydope

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.

You have just proven that if time travel is impossible, then time travel is impossible.

[Asympotically_fat]

@Asympotically_fat

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.

btw to add ‘hyperbolic’ in this context isn’t referring to a geometric property of the spacetime, it means that it can be stated in terms of an initial value problem or in other words knowing what happening at a given point in time allows you to determine what happened/will happen at any other point in time.

[John_W.Kennedy]

@epbrown01

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.

And the entire phenomenal universe, not to mention all of science, disappears in a puff of logic…

[Civil_Guy]

Somebody’s got to do it, though I’m not sure just why:

So we’re mostly saying that there’s no way to get Descartes before the hearse, right?

[Gaydope]

@epbrown01

time travel isn’t possible because there’s no such thing as time - we made it up.
…there’s no previous moment to go back to, there’s no new moment on the horizon. There’s just an endless now.

I fully concur. Time is an illusion, albeit a very realistic one. It is our ability to possess continuity of thought that allows us to perceive what we call time, but actually all that really exists is what we generally refer to as “now”, an infinitely divisible barrier between what we can imagine as the “future” as what we can recall as the “past”. The spinning of the earth, giving rise to day and night and its orbit around the sun, giving rise to the seasons of the year only serve to reinforce the misconception that there is the passage of time. Although the universe is undergoing a continual process of change in state, progressive disorder if you like, all of these changes in state actually occur within the same infinite moment we call “now”.

Having said that, the concept of time plays a very real and measurable role in physics, mathematics and life in general, so no harm done in falling for this illusion.

[Ranger_Jeff]

xkcd: time travel

[sfbiker]

@epbrown01

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.

Doesn’t this same logic mean that distance is not real either since we just made up the units? We said that a foot is the length of a human foot but we could have just as easily divided it up in other ways, like we could have said that a meter is one ten millionth of the distance from the equator to the north pole.

The passage of time can be expressed in physical properties just as distance can, such as “a second is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom”

And both measurement of time and distance vary based on the reference point of the observer. If a 1000 foot long spacecraft travels past Earth at close to the speed of light, an observer on earth may measure it to only be a fraction of that length. Likewise, if the observer on the spaceship looks out the window at the clock on the Big Ben Clocktower, he’ll see the hands on the clock face move faster than the miniature Big Ben replica on his desk.

[aNewLeaf]

@sfbiker

Doesn’t this same logic mean that distance is not real

Yes, you’ve proven that neither time nor space is real.

[Irishman]

@Gaydope

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).

Minor quibble - water is not* an atom*, it is a molecule. It is a specific arrangement of two hydrogen atoms and one oxygen atom. Technically, that molecule may be broken down by chemical reactions into component molecules, which are then separated and never the more to meet. That said, the hydrogen molecules and oxygen molecule remain to be continuously cycled through eternity.

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.

This is an incorrect interpretation. Traveling in time does not violate one molecule being in two places at one time. That molecule is in one place at two different times, that just happen to overlap.

If I get up, walk out of my house, drive to the bank, go to the grocery store, drive back to the bank, then go home, I have been to the bank twice, but I was in the same bank. That is not being in two places at one time. Time travel would move the future me to the past, so the future me (and all my molecules, atoms, whatever) are no longer in the future, they have been relocated to the past. They are not a second incarnation of the past me bits.

This may not be possible, but your stated reason does not apply.

@epbrown01

Second - time travel isn’t possible because there’s no such thing as time - we made it up…

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.

This is gibberish. Time is the medium that allows chemistry to progress, that allows motion to occur, that allows the future to be different than the past.

@Gaydope

I fully concur. Time is an illusion, albeit a very realistic one. It is our ability to possess continuity of thought that allows us to perceive what we call time, but actually all that really exists is what we generally refer to as “now”, an infinitely divisible barrier between what we can imagine as the “future” as what we can recall as the “past”. The spinning of the earth, giving rise to day and night and its orbit around the sun, giving rise to the seasons of the year only serve to reinforce the misconception that there is the passage of time. Although the universe is undergoing a continual process of change in state, progressive disorder if you like, all of these changes in state actually occur within the same infinite moment we call “now”.

You appear to be talking about human consciousness. We are limited to the perpetual experience of a now. All our past is mere memory and all our future is mere wish and dream. We only experience a constant, changing now. But that’s the point, the now is changing, because there is a physical reality of time that is changing around us. Reality is not constrained by how we experience it.