Why is then Time Travel possible?

[HIGHLYTOXIC]

Hi guys,

Relativity says (actually, proves) that nothing & absolutely nothing can be faster than light (assuming the body has mass)..

Now, its also said that we can reverse or slow down time, if we move with the speed of light or even approach it..

Its a proven fact that time slows down for an object that is approaching the speed of light, it stops when it reaches the value of c and starts reversing when we reach a velocity higher than c..

If we were to go in the past, we have to use the third part of the above statement..right? But then its just not possible, is it?

So, how can some scientists still say that Time Travel is possible? Is there any other way to move back in time?

Thanks..

 

[asdfjkl]

A few things.

It is possible to have particles that can only travel faster than light (they are called tachyons). I haven't heard of any experimental investigations into the existence of tachyons (but there probably are some constraints).

Usually when scientists talk about time travel it is by taking one end of a worm hole and moving it next to the other end. The movement causes the time read at one end to retard with respect to the other end. Therefore, if you were to step into one side you would come out of the other at a previous time. There are some problems with this that are usually brushed aside. 1) The acceleration of the worm hole (to get it from one place to another) is not taken into account (I think this negates the scenario but I'm not sure). 2) We don't know if worm holes exist. 3) If they do exist, we don't know if ordinary matter can pass through them.

 

[chroot]

Wormholes aside, relativity says we can only effective travel into the future, not the past.

Wormholes are mathematically valid, but the chances of such freaks of nature actually existing in the universe are, IMO, very small.

- Warren

 

[Parth Dave]

Einstein didn't say nothing can travel faster than the speed of light. He simply said nothing can accelerate to faster than it (ie if you are slower, you cannot make yourself faster).

 

[selfAdjoint]

And if you are faster (tachyonic) you can't make yourself go slower than light. In both cases there is an infinite singularity of energy at c. BTW, a tachyonic mass is pure imaginary, a multiple of $$\sqrt{-1)$$. That's how theorists recognize them, for example in bosonic string theory, you get a particle with $$mass^2 < 0$$.

 

[DW]

Hi guys,

Relativity says (actually, proves) that nothing & absolutely nothing can be faster than light (assuming the body has mass)..

Now, its also said that we can reverse or slow down time, if we move with the speed of light or even approach it..

Its a proven fact that time slows down for an object that is approaching the speed of light, it stops when it reaches the value of c and starts reversing when we reach a velocity higher than c..

If we were to go in the past, we have to use the third part of the above statement..right? But then its just not possible, is it?

So, how can some scientists still say that Time Travel is possible? Is there any other way to move back in time?

Thanks..

You are talking about special relativity, not general relativity and special relativity is only applicable in the absence of gravitational sources or in local limits, and general relativity is also not a quantum applicable theory. In short, when you include gravitation more is possible than what special relativistic physics would otherwise allow. If you are interested in general relativistic investigation into time travel see section 3 of chapter 12 at
http://www.geocities.com/zcphysicsms/chap12.htm#BM12_3

 

[Silverious]

I just finished reading a book by Nick Herbet called "Faster Than Light." In it he says that there are things in the universe that actually do travel faster than light. But most of these are either completely random, or completely monotonous. Meaning that we cannot encode them in order to communicate back in time. As for traveling back in time and reexperiencing the past, our best bet is to make a singularity. One example is Tipler's cylinder. If you were to rotate a large enough cylinder fast enough, it would eventually warp space so much that time would be reserved inside the singularity. Of course no material we have can witstand such centrifugal forces. But he suggests quasars could help us understand this more.

Good read go check it out.

 

[Rashad]

Speed of Light in what

Just a quick point which most of you are sure to know, intended for those as new to the subject as me - yeah, can matter travel faster than the speed of light? Well, a charged particle can move faster than light in specific media. I believe an experiment by a Russian physicist demonstrated this in water. I know that's not the topic here, but its a minor correction I would've felt itchy for not pointing out.

Of what I have read on the topic, time travel is possible because the time not being thing ("Newton, forgive me..."). Since velocity affects time, you can time 'travel' at a different speed from others based on your frame of reference. But there's the second question; what I just said only affects traveling in the 'right' direction, the future.

Or maybe I'm completely wrong? I apologize beforehand if I am.


~Rashad

 

[VantagePoint72]

Yes, that does only allow "time travel" to the future. In the original post it said:

Its a proven fact that time slows down for an object that is approaching the speed of light, it stops when it reaches the value of c and starts reversing when we reach a velocity higher than c..

Special relativity does NOT say that time reverses if you travel faster than the speed of light. If plug a value greater than c in for v in the Lorentz equation for time, the result is not a negative number that would denote time reversal, it's an imaginary number, simply illustrating that accelerating to faster than light is impossible.

You can "travel" to the future by traveling at a significant percentage of the speed of light, so that time passes more quickly for everyone on earth. This is what everyone learns in high school physics.

Travel to the past may or may not be possible. There are theoretical highly concentrated forms of energy called cosmic strings, thinner than an atomic nucleus but millions of kilometres long. As I said, their existence is theoretical. If they do exist, two cosmic strings circling each other could cause such a warp in spacetime than if you were to fly a circle around them, you could arrive back at your starting point before you left.

An important thing to remember is that it is commonly viewed that what we call the past and future are forever frozen in spacetime. Do to the prohibition of simultaneous events in all frames of references in SR, it's a requirement. The separation of events is also magnified by distance. If traveled at about 15 km/h toward the Earth at a distance of about 10 billion LY from the earth, your little trip would be viewed as simultaneous with something happening in around the year 2100 on Earth from your perspective (though you could never know for 10 billion years.) The point is, the past and future are there, how much we can access them remains to be seen.

 

[omin]

Time Travel

I am studying relativity for the first time. Time travel is something I'm struggling with. Here's how I understood time travel before I began hearing these strange relativity theories:

If a person wants to travel forward in time, they would have to do one of two things.

1. Apply a magnificient force to everything in the universe that would accelerate everything, simultaneously, in it's current direction, accept for the person wanting to time travel, and then apply an equal and opposite force that would perfectly deccelerate everything back to it's original velocity (relative to the time traveler) when the amount of time passed that the person wanting to time travel would want to travel.

2. Or, leave the universe alone and only perfectly slow the time traveler's matter and physiological processes down with another magnificiently accurate force acting upon every element of their being that would also be applied to accelerate the person's matter and processes up once the desired time of travel would have passed.

Is there anyway this easy but physically improbable understanding can help me understand the relativity time travel theory?

And, if 2 is somewhat accurate, it seems to contradict high speeds creating time travel, because in this analogy, when I sleep I travel through time, because I've slowed down while everything went by me.

Wake me up if you can, Omin.

 

[VantagePoint72]

Before answering your question, we need to define "time travel". Do you consider it "warping" out of your time and into another time, or in anyone getting to a time that would normally be beyond your lifespan? I can't say I have ever heard of method one, I doubt it has anything to do with relativity. Method two is only time travel depending on which definition you choose. If you consider time travel the second definition, then yes going to "sleep" would technically be considered this. Here's an exerpt from a previous post of mine explaining the theory of relativity's way of dealing with time travel:

According to Einstein's theory of Special Relativity, as you increase your speed relative to another observer, that observer will note time as passing more slowly for you than for them. There is an extremely good article at http://science.howstuffworks.com/relativity.htm that explains why this happens and does so in very easy to understand, non mathematical terms, if you would like to know more about it. Once you accept time dilation as it's called (which actually refers to what the moving observer sees), then you can see how it can be used to "travel" to the future. If you were to blast off in a spaceship and accelerate to 0.9999999999999 the speed of light, for every 1 second that passes for you, about 2236068 seconds would pass for everyone back on Earth (to anyone who knows how to use the Lorentz equations, yes I did calculate this, I left the math out for simplicity's sake). If you maintained this speed for 24 hours, over 6,000 years would pass back on earth. You could then return to Earth having hardly aged but millenia after you left, effectively "travelling" to the future. Unfortunately, reaching such fantastic speeds is no easy task. Another relativistic effect is that as you increase your mass, more and more energy is required to accelerate. This number becomes infinite at the speed of light (the Universe imposes many contraints making accelerating to the speed of light impossible). Particles in atom smashers are accelerated to speeds comparative to the one I mentioned, but accelerated a spaceship to that speed would be quite a feat indeed. Still, it's possible. Time travel to the past is a different matter. It's not simply a matter of traveling faster than light (not only is this impossible, but plugging such a value into Einstein's equation yield's an imaginary number, as opposed to the negative number that would suggest time reversal). There are possibilities though. There are objects called cosmic strings, their existence is purely theoretical but if they do exist they are highly concentrated strings of energy, remnants of the Big Bang, that are thinner than a nucleus of an atom but several million kilometres long. Two of these strings circling each other would warp spacetime (now we're dealing with Einstein's Theory of General Relativity) so significantly, that spaceship flying around them could theoretically arrive back at it's starting point before it even left! Again, the existence of these strings is purely mathematical. Still, it is possible. Special relativity says that events that are simultaneous in one reference frame aren't necessarily simultaneous in another (see the article I mentioned). That means one observer could see an event happen and it's in their past, while it has yet to happen for another observer, it's in their future. This suggests that what we call the past and future are forever frozen in spacetime and the present is just our fleeting impression of the Universe as it is as we hurdle through time. If that's the case, the past and future are always there, always part of the Universe, and it's only accessing them that's the question. Perhaps the methods I suggested are the ways of going about this.

It's important to note that the purpose of relativity (special and general) is not time travel, the "ability" to do so only comes from basic fact that time is relative. "Time travel" to the future is insanely more easy than travel to the fast. In fact, relativistic future time travel happens all the time, as particles that are traveling at extremely high speeds, come to rest in our frame of reference, thereby "jumping" to the future.

And, if 2 is somewhat accurate, it seems to contradict high speeds creating time travel, because in this analogy, when I sleep I travel through time, because I've slowed down while everything went by me.

Another important thing to note is that you are ALWAYS traveling through time. Trees grow, particles decay, people die...in the sleep case, nothing related to relativity is happening, as relativity's method of time travel involves changing your velocity through time. In this example, your velocity through time doesn't change, you really only increase your lifespan. So, "time travel" is really a misnomer, since we're always time travelling.

The whole concept can be pretty complicated, but I hope that helps.

 

[omin]

I define time as a specific distance an object travels in a linear motion across that mass. Time represents my experience of the usual movement of physical things in my world in general. And of course, if all things weren't moving, I wouldn't sense light, sound...the fives senses; therefore I wouldn't recognize time. Time is my sense of movement.

Travel in terms of time travel means an unusual amount of motion. If I could make the motion of a substance decay faster, I think this substance would then be moving unusually fast. Relative to me, I think I am time traveling into it's future.

I'll check out that web page.

 

[C3l7]

Just wondering.. even if you were faster then the speed of light "Time Travel" would be just looking at the light coming off of objects as it approaches you?..

 

[JD]

Just wondering.. even if you were faster then the speed of light "Time Travel" would be just looking at the light coming off of objects as it approaches you?..

It may be the case that you would "see" an object that was moving faster than light before it reached you.

 

[Richard Harris]

in a sense, you will be run down by the train, Then you will see the train about to hit you.

much like the way a supersonic jet can fly past you before you hear it.

 

[JD]

in a sense, you will be run down by the train, Then you will see the train about to hit you.

much like the way a supersonic jet can fly past you before you hear it.

Given that the train is operating (in one sense) in an opposite way, how could you perceive it? What might the end result be? Would you be run over first and then be untouched afterwards?

 

[Richard Harris]

as Percieved by the Victim, you will experience being run down, then (if still Able to) see the train moving backwards towards the location it started in.

It's being "(run down)-1" by a train that's moving backwards.

 

[Richard Harris]

edit: It's the oposite of being...

you would be Perfectly fine when you percieve the Train beind you. but injured when the train is in front of you.

If an object is moving faster than the speed of light, then it's most recent Activitys will be portayed first. and it's Previous movements will be shown afterwards.

 

[JD]

as Percieved by the Victim, you will experience being run down, then (if still Able to) see the train moving backwards towards the location it started in.

It's being "(run down)-1" by a train that's moving backwards.

Is there necessarily a size limit for FTL particles/objects?

 

[Richard Harris]

no known limitation, i'll leave that to those with Highschool or greater qualifications, Maths is greek to me.

 

[JD]

If traveling back in time is possible (given that it cannot exceed the point in time at which the TT equipment was produced), do options other than parallel universes opening up exist? There appear to be significant barriers (ie. the Grandfather paradox etc) to allowing such disturbances.

 

[Richard Harris]

i did not give proof for traveling back to the past, I am working off the assumption that despite the train traveling faster than light, the light given off or reflected by the train is limited to the speed of light. the train is not where it apears to be, but rather has moved past that point.

this observation is taken from the path of the train.

Person Y sees a train at point A start to Accelerate, this continues normally up until the speed of light is reached. At this point (b) (, what is visible to you becomes Inaccurate.

you see the train Jump to a point(C) Far ahead of where it was. (you can assume to be in the middle of these two points

The train Procedes to move backwards until the point(B) in which it Vanished. The train Vanishes again, this time you find it at the same far point(C), but moving fowards this time. the train then comes to a rest at point D

this observation is taken from some distance at a tangent to the path of the train.

You see the train start to Accelerate, this continues till the mid point where Person Y is standing. the train then starts to Deccelerate.

what you see is the train slowly Grow in length, the length of the train becomes so long, that in the Mid section, it extends from point B to point C.

both of these both assume you are capable of seeing changes that fast, ot have a recording device that can and then slow the playback down to a stage where it is possable to view.

 

[JD]

i did not give proof for traveling back to the past, I am working off the assumption that despite the train traveling faster than light, the light given off or reflected by the train is limited to the speed of light. the train is not where it apears to be, but rather has moved past that point.

Yes - sorry - I'm mixing the two areas up a little. What I'm wondering is whether the potential of time travel is held within the FTL area. If so, it will probably remain inaccessible to us.
I'm also wondering whether FTL objects can "exist" in the same realm. Do you think that an FTL object would actually give anything off that could be perceived? You have said that you have assumed this, but is it not the case that FTL objects cannot (slow to) reach the speed of light because the amount of energy required would be infinite? Would this not include any light effects given off - these would be moving at FTL speeds too.

 

[omin]

Time Travel Not Possible

Its a proven fact that time slows down for an object that is approaching the speed of light, it stops when it reaches the value of c and starts reversing when we reach a velocity higher than c..

I believe time travel is not possible. Here is my reasoning.

Let's say we have a spaceship that will travel the speed of light. In this spaceship we put a clock. This clock is a special clock. To make it tick one unit of time, it needs a specific unit of energy. We have a battery that has an exact amount of these units of energy. Let's say it has exactly one million units of this energy. We put the clock in the spaceship and the spaceship takes off for a thirty year speed of light trip. We also have another clock on Earth that is identical. (There are enough energy units so some will be in the batterys after the travel time.)

Some say, the clock in the spaceship will be slower than the clock on Earth when the spaceship returns, which means more energy units will be in the battery of the spaceship clock than the battery of the Earth clock. This means that energy expended for a clock at the speed of light has some resistence, which slows the procces of energy expended from the battery to the clock.

I do not agree. I say the clock will be the same. Energy will be expended at the same rate in both cases, because physical laws will behave the same in both cases. The clock will tick the same, because the physical laws will dictate it uses the circumstantial amount of energy (it's the same kind clock!).

A human being's physiological processes will expend energy the same in both cases and therefore would return without aging differently.

This time travel theory violates basic physics of the rate which energy is expended, therefore time travel in this case is not possible.

Correct me if I'm wrong.

 

[VantagePoint72]

If traveling back in time is possible (given that it cannot exceed the point in time at which the TT equipment was produced), do options other than parallel universes opening up exist? There appear to be significant barriers (ie. the Grandfather paradox etc) to allowing such disturbances.

The short answer is yes, there are other ways. Basically, special relativity seems to suggest that events in the past and future are forever frozen in spacetime. You cannot change what happens in a certain location of space at a certain point in time. In other words, if you go back in time to shoot your grandfather (as per the grandfather paradox), then you always existed at that point in spacetime. You didn't suddenly appear in it, that seems to be impossible. Along the same reasoning, since you can't change events in spacetime, you wouldn't be able to kill your grandfather. The fact that before you went back in time he existed after the moment you try and kill him in would forbid such an action. Perhaps your gun malfunctions. Maybe you suddenly lose the desire to kill him. Whatever happens, this logic suggests that the resolution to the grandfather paradox is that SR says it's an impossible situation, since you could never actually go back and kill your grandfather. As I said, the event of you popping into that point in spacetime and trying to kill your grandfather had always been in place. Maybe your grandpa once even recounted to you before your time traveling voyage how a young fellow dressed in funny clothes had once tried to kill him with some advanced weapon. Of course this is just one suggested resolution to the issue, albeit a very reasonable one. Perhaps the many worlds interpretation is correct, however that idea tends to bother me even more so than the fact that our actions are predestined as the previous example suggests. After all, I'm a Calvinist Christian, so I guess I kind of believe in this stuff anyways.

 

[VantagePoint72]

I do not agree. I say the clock will be the same. Energy will be expended at the same rate in both cases, because physical laws will behave the same in both cases. The clock will tick the same, because the physical laws will dictate it uses the circumstantial amount of energy (it's the same kind clock!).

You're looking at the situation backwards. The clock's ticking doesn't slow down therefore causing time to slow down, time slows down thereby causing the clock's ticking to slow down. You're absolutely right in saying energy will be expended at the same rate in both cases since physical laws will behave the same. However, your next step is incorrect. Say the clocks both expend energy at a rate of 1 J/s. This is true in both cases. However, more time passes for the clock on earth, therefore it expends more energy. It used energy at the same rate of the clock on the space ship, it just did so over a longer period of time.

Special relativity defines the idea of time dilation, and I'm sorry to tell you you're several decades too late to argue it. Time dilation has been proven experimentally. It is the ONLY explanation for several phenomena, and it explains them to perfect accuracy. While you may accuse me of being stubborn, special relativity (special only, general relativity is still in the proving phase) is as debatable as the fact that you see objects due to light bouncing off them and to your eyes. Whether you believe it or not, "time travel" this way is most assuredly possible.

 

[Richard Harris]

dont know if someone noticed, but i got a little muddled, both observers would see the train as going fowards as a long stretch of light, none woul see the train move backwards, as that would imply moving backwards,

 

[omin]

Motion over Distance is Time, Right?

The clocks individually move relative to themselves, which is an expression of energy and how we represent the rate of energy expended.

The stipulative energy in these clocks is neither increased or decreased, no matter what position in the universe they are in or what speed they travel.

For every action, there is an equal and opposite reaction, so Newton has assured us the rate of energy will be consistent, as long as there is no field or other type of resistive force upon the clocks. So... one clock is expending energy the same but it's in a ship that is moving it away at the speed of light. Physic laws behave the same in both places the same, right?

It(Earth clock)used energy at the same rate of the clock on the space ship.

Each clock is identical and no physical force is exerted upon them during the time passed, except their own isolated forces of the clock mechanisms. But it seems you infer time is a force...

The clock's ticking doesn't slow down therefore causing time to slow down[energy expended same rate], time slows down thereby causing the clock's ticking to slow down...

I understand time represents an object that travels a specific distance at a specific speed, and speed is based upon a standard unit of motion which is a specific speed across that same specific distance. Time is simply a quantity of motion over distance. I see time as more of a cognitive symbol, a mathematical theory that represents the physics, rather than something that causes things, slows down or speeds up things. I see cause deriving soley from the physical objects, and time is a symbolic property representing those changes depending upon the characteristics of the physical substances and their speeds.

Since both object's energy is expended at the same speed, relative to themselves, what is the force is exerted upon one of the clocks to cause:

One clock to travel farther[meaning the other expends less energy, but at same rate] or in the converse one to travel less[meaning the other expends less energy]. Remember,

time slows down thereby causing the clock's ticking to slow down[less distance, because rate of energy same]...

What's your definition of time? Is your definition of time this force that changes distance traveled? What entity causes the distance change?

It is less or more distance right, since energy is expended at the same rate?

 

[JD]

dont know if someone noticed, but i got a little muddled, both observers would see the train as going fowards as a long stretch of light, none woul see the train move backwards, as that would imply moving backwards,

This is one area where getting muddled may take us closer to finding answers!
I'm not convinced, though, that a FTL object could be perceived by our senses. Having said that, of course, it is feasible that an object could travel faster than light through certain media. So what might happen there (in terms of how we could perceive that FTL object, the speed of light being relative)?

 

[Mentat]

This is one area where getting muddled may take us closer to finding answers!
I'm not convinced, though, that a FTL object could be perceived by our senses. Having said that, of course, it is feasible that an object could travel faster than light through certain media. So what might happen there (in terms of how we could perceive that FTL object, the speed of light being relative)?

First off, an FTL object just wouldn't hang around long enough to be "percieved". It might interact with something, but would perish from the "present" of that inertial frame in a Planck's unit of time (or whatever the smallest incriment of time turns out to be).

Now, I also wanted to point out that backward time travel may seem really confusing, but had you ever tried to imagine negative travel along space? Movement is only ever perceived (by outside reference frames) when the object in question has a velocity greater than 0. What would we percieve if the object had a velocity less than zero!? :surprise:.

Something to think about, anyway.

 

[C3l7]

I do not think something could have a velocity less then zero. Because zero is stationary and less then zero would be negitive but depending on your point of view it would be positive. it is just a vector quantity.

 

[VantagePoint72]

omin,
I don't see where I implied I thought time was a force. What I meant by time slowing down causes the ticking to slow down was that since the clock's ticking is a measure of the rate that time passes, when time slows down the ticking must therefore slow down too. This doesn't require some phantom force, it just means that if one tick uses one unit of energy, then energy _from the perspective of the observer back on earth_ the clock is using energy at a slower rate. From the observer on the rocket, the laws of physics hold true. If I drove past you in a car traveling at a constant speed and threw an apple up, from my perspective I applied enough force to throw it straight up and then gravity brought back down. Newton's laws hold true. If you through an apple straight up, you would conclude the same for your own apple. However, if you were to argue that I must have applied more force since you saw my apple travel further (you would observe my apple travel vertically and horizontally), then you would be ignoring the movement of the car relative to you thereby giving the apple the inertia that causes the additional distance from your perspective. This is the core of your argument regarding the clock. The laws of physics hold true according to both observer in there own reference frame. You can't say that the laws of physics aren't being obeyed in another reference frame, one that's moving relative to you, it's a comparison of apples and oranges.

Time is simply a quantity of motion over distance. I see time as more of a cognitive symbol, a mathematical theory that represents the physics, rather than something that causes things, slows down or speeds up things. I see cause deriving soley from the physical objects, and time is a symbolic property representing those changes depending upon the characteristics of the physical substances and their speeds.

And now your making another impossible comparison: you're trying to solve a relativistic problem with classical thinking. Time IS something. Or more specifically, spacetime. Your velocity through one (space or time) affects your velocity throught the other. When your velocity through time slows relative to another observer, they will view events as passing more slowly for you. You see, your thinking is backwards...time isn't a measure of cause and effect, cause and effect are product of your journey through time. The clocks expend energy at the same rate in their own respective reference frame. When time passes more slowly for one observer cause and effect or, to be more concise, events pass more slowly for them _from the other observer's perspective_. And as for your arguments regarding distance travelled, length contraction is the complement of time dilation and makes sure both observers see the other moving at the same velocity. There is no force changing the expendature of energy, the rate stays the same. Again, in each observer's reference frame, the clock uses energy at the same rate. However, the trips take different lengths (and to complement this, is a different distance) from each observer's perspective so one clock uses more energy since it had more time to do so. As for your last statement, time is not distance, as I said before it's the complement of distance, or more generally, space. How do I define time? A specific component of a single entity, spacetime, that is the dimension defining when events take place. As individual observers, time is relative to each of us, meaning that what view as simultaneous may not be simulataneous to me do to the fact that I can break up spacetime differently than you. The passing of events is an illusion as time doesn't flow, events are forever frozen in spacetime and cannot be changed. It is only our own motion through spacetime that causes reality to appear as it does. Time is not simply a relationist's way of separated events, it is the entity that, like space, gives meaning to the events. To sum it up:
"Space is what we measure with a measuring rod and time is what we measure with a clock." - Albert Einstein