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krrazypassions's avatar

Is it possible to make a time machine? If yes, how? If no, why?

Asked by krrazypassions (1332 points ) May 11th, 2011

We feel that we live in a three-dimensional space and the fourth dimension of time. We can move around in the three-dimensional space freely. But feel that we are traveling linearly in time at a constant rate in the same direction (that we call the future).

Einstein’s special theory of relativity leads to Twin’s paradox and simultaneity and such other paradoxical situations- where the relative space-times of two people can change drastically if they relatively travel at speeds comparable to the speed of light. Of course, such high speeds require huge amounts of energy. Mass also becomes heavier at high speeds.

Although very expensive, at least we know how we can travel to the FUTURE- We just have to travel in a spacecraft at high speeds comparable to that of light.
But then, what about traveling to the past?

Also, what about teleportation in the three dimensions- that is, jumping from one point to another without passing through intermediate points? Do you think we need to solve teleportation problem before we can think of solving time-travel?

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34 Answers

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Fyrius's avatar

Of course, travelling into the future is trivial, since everyone is doing it all the time already. You might be able to move faster in time by moving faster in space, or by living in high orbit for a while¹, but you can achieve the same effects with cryogenic stasis. Get frozen up, take a nap, thaw, wake up. Congratulations, you are now in the future.
Moving through time slower would sound like a more worthwhile thing to try, if for some reason you want to age faster than the rest of us. (Who knows, maybe you just really need to catch a deadline.)

I recall reading somewhere – I think it was in a theoretical physics popular science book, probably Michio Kaku’s Hyperspace – that it would be theoretically possible to create a time machine that takes you into the past, but you could never travel further back than the moment the machine was created.
_____
¹ If I’m not mistaken. I always forget if being close to a heavy object makes time go faster or slower for you, so maybe it’s the other way around.

marinelife's avatar

If it was possible, someone would have done it. (Or perhaps they will yet do it, but we don’t know it yet.)

ragingloli's avatar

Well, you could theoretically build a machine that is capable of warping spacetime in such a specific way, that it loops in on itself, so while you move along this warped spacetime you would end up in the past.
http://en.wikipedia.org/wiki/Closed_time-like_curve

tedd's avatar

They theoretically know how to make one.

Time travels slower the higher the gravity is where you are. They have tested this with nuclear clocks at low and high elevations on earth (the further from the center you are the less gravity you feel), and it is very true. Sooooooooooo

You need a massive gravity well, I mean MASSIVE. Our sun would not even do the trick. The one used in the theory is a black hole. Now at this black hole you want to put one end of a wormhole. Now take the other end of your wormhole, fly it out to the middle of nowhere space, and lock it into place there.

Since time travels so incredibly slowly at the exit of the wormhole that has the black hole, it would effectively always be the exact moment you placed the wormhole there. So if you traveled through your wormhole after putting it in the middle of nowhere, you would emerge to meet yourself dropping off the first end of the wormhole.

talk about a paradox huh?

Qingu's avatar

FYI, photons (particles of light) do not experience time. If you were a photon, you would experience every point of your travel through space as happening at the same point in time. There is no past or future for a photon.

Even so, photons take time to go from place to place. But that’s from our perspective, as beings with mass.

Also, CPT symmetry. That’s Charge-Parity-Time symmetry. A particle traveling normally through time is indistinguishable from an antiparticle (i.e. with opposite charge), traveling in a mirrored direction in space (parity), and backwards in time. Pretty weird, huh?

Re: the actual question, I guess you could say that anything is possible, but I think it would be stretching the bounds of physics. The reason is that entropy seems to undergird our entire concept of time. The reason we can remember the past but not remember the future is because entropy—the direction of entropy is the same as our sense of time’s direction. So “time travel” would need to involve messing with the laws of entropy. But entropy may be the most fundamental aspect of physics entirely. So, no easy task.

For individual particles, like photons or anything involving CPT parity, entropy doesn’t actually exist. A box with just one particle has no measure of entropy, for the same reason that there is no such thing as a one-sided dice. Entropy only comes into play when we are talking about multiple particles. So, while you can probably send an individual particle backwards in time, a complex system of particles (like a human, a brain, or an AI) probably can’t.

Skaggfacemutt's avatar

Wow, @Qingu , I am impressed. I have never heard it explained so thoroughly before. My simplistic explanation sounds puny next to that. I was going to say, the way it was explained to me anyway, is that time does not exist. It is a measurement we invented and use to keep track. The past and the future do not exist and therefore cannot be traveled to. No matter what you do, mess with, or discover, it is really and truly an impossibility. It would be interesting to find out if we can “see” the past by sending a camera far enough out into space to take videos of the earth. It would have to be sent out several times faster than the speed of light so that the video will be of the earth in the past. In other words, to capture images that were emitted from the earth in the past.

Qingu's avatar

I don’t know if I’d agree that time doesn’t exist… you can say the same thing about space. You can also think of space (like time) as a measurement or as a relation between two or more objects or events. But that relation exists.

Also, everything we see is the “past,” because light takes time to travel. The photons of light that we see as distant galaxies come from billions of years in the past; we see the galaxies as they existed billions of years ago.

Physicists talk about light cones that encompass everything about the universe that we, as observers, can see or experience. There is a whole universe that lies outside our Earthling light cone, but we will never know anything about it. Time travel would involve going in and out of our light cone.

Skaggfacemutt's avatar

The measurement of time exists, and the measurement of space exists. Space is even a tangible thing, but time is not. You can have a cup full of space, but not a cup full of time.

erichw1504's avatar

All you need is a flux capacitor.

krrazypassions's avatar

@erichw1504 I guess you are referring to the “Back to the Future” method of time travel?

King_Pariah's avatar

theoretically, yes. But since we’re not seeing any visitors from the future, I guess we don’t.

Qingu's avatar

@Skaggfacemutt, how do you measure space?

Edit: to avoid being rhetorical, I’ll answer my own question. You measure space the same way you measure time: with a repeating pattern.

In the case of space, that pattern can be an orderly, repeating structure, such as the number of atoms in a crystal, or the number of notches in a ruler. In the case of time, it’s an orderly, repeating event, such as the number of orbits of a planet.

erichw1504's avatar

@Qingu Multiply the length times the width times the height.

Qingu's avatar

@erichw1504, what is width? What is height? These words have no meaning without some sort of repeating pattern of matter.

s321scba's avatar

every new dimension are infinite continuous consecutive parallels of the previous oriented linearly perpenicular to the previous
3fundamental-forces momentum gravity magnetism
why is entorpy so highly regarded
how many inches to a second

krrazypassions's avatar

@Skaggfacemutt String Theory demands 10 dimensions to explain everything.
10-dimensions seem wierd to imagine.
Someone thought it could be explained like this-> 10-dimensions explained
If you see the video, you can associate how 2 dimensions can fold through the higher third one. Similarly , maybe 4 dimensions can fold through the higher fifth! So, we can keep your time in a 5-dimensional cup.
Ok, that sounds crazy! But still, 10-dimensions help String Theory to explain everything. Also, atleast the 5 or 6 dimensions beautifully explain how we always have a choice at every moment and that we aren’t the slaves of our fate. That we perceive only the resultant four-dimensions which are infact one selection of infinite possibilities that exist in higher dimensions. Alternate realities, parallel worlds are all predicted by the quantum physics and string theory- and it certainly is fun to watch it in the recent Source Code movie.

Anyway, if String Theory is right, then a)we have our Theory unifying laws of quantum physics and Relativistic physics, b)10-dimensions exist, c)and maybe just there, we will also find a solution for time-travel (maybe even travel in higher dimensions too- so that we can move about through the Alternate Worlds and Parallel Universes)
Ok, its a really long shot for now, but still, its crazy enough to fit the requirements of 21st Century New Science!

erichw1504's avatar

If I knew how, I wouldn’t be here right now.

Qingu's avatar

Some other weird facts about dimensions (tangentally related to string theory):

Fractals do not have integer-value dimensions.

AdS/CFT correspondence — physics in 3-dimensional space, with gravity, basically works the same as physics in the 2-dimensional boundary of this space (i.e. the surface of a sphere), without gravity.

I am probably mangling that second one.

Nullo's avatar

We kinda, sorta already have one, in a roundabout way. The net effect is to remove distance from the equation, making it theoretically possible to receive a signal before it could have arrived via radio.

thecaretaker's avatar

I think time travel is possible the day they also figure out perpetual motion, it all looks good on paper but in the end it doesnt work.

Nullo's avatar

It should be noted that time travel isn’t worth anything without some hefty space travel to go with it. The Earth is rotating about its axis while careening around a star at about 107,306 kph, and the whole mess is itself moving about the galactic core, and the Milky Way itself is moving away from everything else.
Going back an hour might not cause too much trouble, but a week? A year? You’d be breathing vacuum.

s321scba's avatar

what if position is “drawn” through time by matter

Nullo's avatar

A correction to my previous post: one hour’s distance would leave you well behind Earth.
@s321scba Then you would be limited by the inverse square law.

LostInParadise's avatar

I am surprised that nobody has mentioned the grandfather paradox. If you go back in time and kill your grandfather, that would mean that you were never born. The only way around this that I know of is the many worlds interpretation of quantum mechanics

ragingloli's avatar

@LostInParadise
It is not the only way.
The other way is that whatever you do after you go back in time is already a part of your own history. That means, the fact that in your history, your grandfather never died, means, that you are going to fail in any attempt to kill him.

King_Pariah's avatar

Here are two theories

1) (This is seen in “Paycheck”) Convert matter into energy (laser) shoot it towards the edge of the universe, when it hits the edge it’ll curve, comeback in a loop, energy returns to earth in two separate times but same space, convert energy back into matter, matter exists both in future timeline and the present.

2) Take a space ship capable of near light speed, approach a black hole, take a dive right before reaching the event horizon, hold your breath cause God knows what’s going to happen.

LostInParadise's avatar

Apart from the grandfather paradox, there are other aspects of time travel which, if not paradoxical, would be quite bizarre. Suppose you went back in time and then traveled a few days ahead. Would there now be two copies of you? Could you do this multiple times and form a small army? If there could never be more than one copy of you, then you must be constrained to act in the exactly same way for each past moment that you visit. How could this constraint be enforced, assuming you have memory of all your time traveling behavior?

krrazypassions's avatar

@LostInParadise could it be that traveling to the past would erase our memory, so that at any given time, we will remember everything only up to that current ‘present’ moment!
So one wouldn’t remember that one had time-traveled! So, maybe I might have invented the time machine and traveled back in time, but i cant remember it because its not there in my memory!
So if I go back to the time before I was born, maybe i would cease to exist!
Or perhaps (if rebirths are true), then I might become my previous birth. Now that’s a perspective on time-travel, isn’t it?

Feels like we aren’t meant to invent time machine after all! Maybe, “words are like arrows, once shot, you cant retrieve them” and “time and tide wait for no one” are the ultimate laws of the universe! :p

krrazypassions's avatar

So, the only way is to get out of the earth’s gravitational field, and travel at near-light speeds for a few years. And then, you might have grown only 5 yrs older but everyone on earth would have grown 50 yrs older, and so, for you, you would be in the future. (Same could be done by freezing yourself for whatever time you want to travel ahead in time- though as shown in Forever Young movie, one may age rapidly once brought back into normal living conditions, and so ageing cannot be avoided after all!) So, maybe traveling at near-light speeds in the only possibility.

chocolatechip's avatar

@Skaggfacemutt The measurement of time exists, and the measurement of space exists. Space is even a tangible thing, but time is not. You can have a cup full of space, but not a cup full of time.

Tangibility as you describe it is a property of space, not time. You can have a cup full of space, because the property of being “full” or “empty” is a property of space. You can’t have a cup full of time because being full or empty is not a property of time. By the same token, you can sleep for four hours, but you can’t sleep for four metres, because duration is a property of time, not space.

s321scba's avatar

what is calculated to happen at greater than light speed

krrazypassions's avatar

@s321scba What are you heading towards? Nothing can travel faster than light in our relativistic space-time. However,

1) see this
and then see this
2)
The expansion of universe is thought to be happening at rates greater than light speed! we deduce that when we observe very far-away galaxies- Their redshift value is 8, while maximum relativistic redshift value of a galaxy near to us can only be 1.4

So the only thing calculated to happen at greater than light speed is universal expansion.

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