Hot answers tagged

261

In all probability, we, 21st century barbarians, will not be very well loved by our 23rd century descendants. Non-historians of that age will probably group us together with the generations of the late 20th century, and refer to us collectively in their high-school level history courses. We are the generations who baked the earth and poisoned the seas. ...


177

In the film Primer, there was a time travel machine that could only jump as far back as when it was first turned on. This puts a practical limit to how far back you can jump to the point where time travel was invented in the first place.


127

Because the earth orbits the sun at 30km a second, the solar system is moving at 20km a second and the galaxy is moving at 230km a second! Any time travel will result in the people being dumped out into the void of space... Maybe we need to set up some beacons to guide people to the earth through time and we can only set these up now...


80

Here's another option that is often forgotten: Time travel might need an exit system available at the destination. Since we don't have an exit system in our time, time travellers cannot visit us. For example, time travel might be done through a wormhole whose two ends are shifted in time relative to each other, so if you enter one opening, you come back ...


59

Viewing from a point of single, unbroken timeline - it doesn't matter how far are you from your parents. As silly as it sounds - you could even pop up in the room they are having sex in. Why? The key point here is the very first sentence in your question. If we assume there is only one timeline, killing you by sending you back in time will just not work. ...


50

There are no paradox concerns. Your system prevents modification of the past, so there is no way to cause changes which would lead you to not go back in time, or go back in time differently. Your system cannot see into the future, so you cannot see the effects of your actions and act differently. Really, it's more of a VCR than time travel. You can go ...


49

Two potential solutions: They are able to travel to the past, but are unable to alter it. This includes showing off their time machines or other future technology or knowledge, or anything else that might remotely change the progression of events. This is the Novikov self-consistency principle. They have come back and told us, but their influence is ...


46

The problem with causal loops is that they only happen because they happen (the fact that the loop happened is all the justification it needs in order to happen). The solution to causal loops is that they only happen because they happen. In other words, all you need in order to justify that a loop does not happen is that it doesn't happen. For example, say ...


42

I think there is a major flaw: Since there is no fast forward, you could never catch up with the present. For everyone else, time moves forward and you are always behind. Unless triggering the time travel freezes time for everyone else.


30

You have a whomping big paradox You stipulate that the traveller "cannot change anything that happened to [he/she]." That's a problem. You can jump back and relive time, but you can't change the fact that you jumped back to relive time. You'll never live another second of free will at all. Let's investigate this. Assumption Let's ignore the entire is-...


28

@chrisj hit on the grand problem of time travel: you have to travel in time and space. I'd love to expand on all the ways this ties time travel to space travel. The surface of the Earth is rotating at 0 to 465 m/s depending on your latitude. Fortunately momentum will be conserved (it must otherwise that would imply there is an absolute rest state for the ...


27

Ever noticed how people in time travel stories mess everything up? And then somehow by the skin of their teeth manage to put everything back to rights? Things probably don't actually work like that. About half of the time the calvary doesn't arrive in time, the answer to the riddle fails to pop into the hero's head, the match doesn't light, the green wire ...


25

Have you ever looked into a pair of mirrors that are facing each other? It looks like an infinite tunnel of mirrors, but because the mirrors aren't perfect the farther down the tunnel you look the more blurry it gets. You could think of mind reading as looking into a window. While you are reading someone else's mind, your window becomes a one-way mirror. So ...


23

Just because they can doesn't mean they will. Most people have phones; why do they not call people in other continents to tell them about their phone? Many people have cars, why don't they drive to places that do not have cars? Despite the simplicity of this argument, it gives you the answers; They don't care about people from the past. They do not want ...


22

You have a horrible, horrible problem. Memories are formed by strengthening or creating new synapses. That is a physical change. If nothing can change, then you cannot form new memories - you will re-live the period, but arrive in the present in the exact same condition you were the first time, with all the memories and everything. But that isn't the bad ...


21

Option 1: Time travel is a one-time deal Let's suppose time travel allows you meaningfully alter the past. By jumping back in time you're erasing the future you came from, leading to the good old fashioned grandfather paradox. Let's just sidestep that - You're physically in the past, so let's just assume you're now part of the past. Kill Hitler, save Lenin ...


17

While I do not see any issues with paradoxes with this "type" of time-travelling, there are some odd things that I'd question. First of all, your example of losing your wallet. While it's great to "re-live" your past and remembering that you put it in your couch, how would going back help if you were not conscious that you lost your wallet, by dropping it ...


16

I think there's something subtle missing from the "how do you account for Earth's motion" explanation: which reference frame does your time machine use? Sure, the Earth is rotating, but that's relative to the Earth's center. It's also orbiting the sun, but that's relative to the sun. That's also orbiting the galactic center, but that's relative to the ...


16

The paradox only occurs if you assume a single timeline. In which case it is pretty much identical to the 'grandfather paradox'. In the left fig. below, if there is a single timeline then there is a single 'time-path' for you (red line) - that doesn't in itself produce a paradox, but the time-machine also only has a single path (blue line) which by ...


15

We are the ALPHA-TIMELINE. Just as space expands, the dimension of time expands as well, the process of which we perceive as the passing of time. We are at the very edge of time, any new moment that happens hasn't existed before it happened. History is the coordinates of time we leave behind. As the progression of time only happens because of the "pressure"...


14

Assume a world with no multiverse theory, just a single unbroken timeline. If you were to go back in time to the town your parents lived in on day of your conception, the air displacement due to your presence in the world is going to prevent your own birth. The reason for this is simple. Semen is a liquid. The slightest alteration in the fathers movements is ...


13

You answered it yourself with your time travel rules: "Unlikely" doesn't come into play with time travel. It happened that way, it must have happened that way, so probability is 1.


12

Assuming that there are no mistakes or errors (like the division by 0 in the proof that 1=2) A whole slew of mathematicians would examine his working, work out exactly which axioms he was using and then describe in excruciating detail why his initial assumptions are wrong in the real world. A good example of a mathematical paradox arises in set mathematics,...


12

In terms of the biological mechanisms, you have two major mechanisms that come into play: The first is the random segregation of chromosomes during meiosis, the generation of the egg cell. The egg cell will contain only one of each chromosome, and you need it to be the "maternal" chromosome each time. This is unlikely (1/2^23 for each generation) but ...


11

Time travel to the past requires both a sending device in the future and a receiving device in the past. Without both in place, no backwards time travel is possible. It could be that no stable wormhole will form, or attempting travel without a receiver will always result in the traveler being atomized. No matter how far technology advances, nothing is able ...


11

Caveat: I've made some assumptions in order to make this work as a potential world. Like many have pointed out, a truly mad god acting without care would probably manage to kill off the human race pretty quickly if they acted often enough to have a tangible effect on the world (by triggering a catastrophic imbalance in the environment). The only way to make ...


9

There are various theories for this, the most common being Backward time travel creates a parallel timeline where it happened, the prime timeline remaining untouched. This prevents paradoxes, it also means that the primary timeline can never have examples of backward time travel. It also means that you can never go home, never undo the damage you've done,...


9

This sounds chaotic What you're referring to is chaos theory, popularly summarized by the essay title Predictability: Does the flap of a butterfly’s wings in Brazil set off a tornado in Texas?, subsequently known as the Butterfly Effect A chaotic system is defined mathematically in a complicated manner, but the basic idea of it is that a tiny change in the ...


9

It has happened before Well, the $\sqrt{-4}$ length hasn't happened before. But "things that were deemed impossible mathematically" has. The first example of this that comes to mind is Russell's paradox (you may have heard about its derivation: the Barber paradox). It showed the base axioms of Cantor's naive set theory where unsound. The only thing to do ...


8

I think you're vastly underestimating just how different a truly non-deterministic universe would be. Case A If the god's influence is sporadic enough that you can still discern a "default" set of laws that nature follows most of the time, then scientists would simply study those laws. Case B If the god's influence is so omnipresent that you can't ...


Only top voted, non community-wiki answers of a minimum length are eligible