# Backward or forward in time?

1. Jan 13, 2005

Hi all,
i know that this has been mentioned in a lot of threads here.
But im a beginner in the physics and specifically in the relativity, and im very very interest in the relativity and specifically time travel.

I read a lot of threads here that talked about the same topic, but i didnt get what it means very well, so, im asking you to tell me a clear explanation for a beginner to help understand it well please

I would like to know,

If i travelled faster than the speed of light, is this will lead me to travel backward or forward in time?? and how?? and why ??

I read about the twin paradox, i really dont know if i understood it well or no, but i will say here what i understood.

That, if i travelled faster than the speed of light for, say, a day, 24 hours hour=60 mins., min=60 secs.
This day will be equal to, say, 10 days on eath, so if i landed on earth, i will be in the future, because i stayed 10 days in a day.

I tried to think about and calculated it.

The light from our sun to the earth takes 8 minutes to reach the earth and as we all know that the speed of light is 300,000 KM/Sec.

So i was thinking that if i travelled faster than the speed of light by two times (600,000 KM/Sec.) and from a distance equal to the distance from Earth to sun, so i will take it in 4 mins. (the half time that sun light takes to reach earth).

So, let say, The sun light began to travel at 9:52 (according to earth time) and reached earth at 10:00 (earth time).
And me will begin at the same time 9:52 and reach earth at 9:56.

So, here, i reached the earth in a past event, not a future, because i arrived before the light itself.

So, i would like to know where is the truth? am i think right or wrong?
So, all i want to know here, is, how can we make the day in a spaceship travelling at a high speed equal to, say, 10 days on the earth?
I hope you can get what i mean.

Please, make it very very clear as long as you can because as i said, im a beginner.

I have another question which is:
I read that, speed of light is constant to all observers. ... How ???!!

I think that means, that even if i was moving at the half of speed of light, it will still 300,000 KM/Sec. according to me!
So what is that??

Thank you.

2. Jan 13, 2005

### NeutronStar

Based on the rest of your post I can assure you that you did not understand it well.

In your post you keep talking about traveling faster than the speed of light. I would recommend that you begin at the very beginning of relativity theory. Start with the postulates. Then learn what must follow from them. After you understand why there can be no such thing as absolute simultaneity, and you are comfortable with the concept of time dilation, then you will be prepared to read about the twin paradox. There are tons of books on relativity in the book stores.

I can assure you that you aren't going to learn it by reading posts on a forum. You need to work through examples meticulously and think about them carefully. No one can bless you with instant knowledge in a post or two. I wish learning was that easy.

3. Jan 13, 2005

### JesseM

The reason has to do with the fact that in relativity, different observers define simultaneity differently--two events that have the same time-coordinate in one reference frame will have different time-coordinates in other frames. If there is a timelike separation between two events (ie a signal travelling slower than the speed of light could bridge the two events), all observers will agree on which event came first and which came second, but if there is a spacelike separation between two events (you'd need an FTL signal to bridge them), then different frames will disagree about which came first. Thus, if I send an FTL signal from one point to another, then even if in my frame the signal was received at a later time than it was sent, there will be some other frame in which it was recieved before I sent it.

According to the principle of relativity, the laws of physics work the same in all inertial reference frames, so if I am free to send an FTL signal arbitrarily fast in any direction in my own reference frames, the same must be true of other reference frames. So, if you're travelling away from me slower than light, I can send you a signal which moves FTL in my frame and backwards in time in your frame, then as soon as you recieve it you can send a reply that moves FTL in your frame and backwards in time in my frame, which opens up the possibility that I will recieve your reply before I send my original message, violating causality. There's an illustration of this using spacetime diagrams here:

http://www.theculture.org/rich/sharpblue/archives/000089.html

If you jettison the principle of relativity, and say that the FTL signals can only go arbitrarily fast in all directions in a certain preferred reference frame, then you can avoid the possibility of causality violation. In this case, if you're not in this preferred reference frame, then it will look like the signals can go backwards in time in one direction but there's an upper limit on their speed in the opposite direction. For example, in the year 3000 I might be able to send a signal to a star 10 light-years away that would reach them in 2999 in my reference frame, but then they could only send a reply in my direction at speeds less than 10 times the speed of light (again, in my reference frame), so their reply couldn't arrive before I sent the signal.
No, the twin paradox has nothing to do with travelling faster than light. In fact, relativity says that is impossible for any object travelling slower than light to accelerate to a velocity faster than light, because it would take an infinite amount of energy to do so.

Relativity says that if you are moving at a slower-than-light velocity v relative to me, then I will see your clock slowed down by a factor of $$1/\sqrt{1 - v^2/c^2}$$ (c is the speed of light). So, for example, if you are travelling at a velocity of about 0.866c, then to me it will appear that you are moving in slow motion--your clocks will be slowed by a factor of 2 in this case. What confuses some people is that from your own perspective, it will be my clocks that have been slowed down by a factor of 2, since from your perspective it is me who is moving at 0.866c away from you. The apparent "paradox" is that if you travel away from me at 0.866c, then turn around and return at 0.866, then when we reunite, we will both agree that it is you who has aged less--if the journey took two years from my point of view, it only took 1 year from your point of view, so in a sense you have travelled 1 year into the future. The resolution is that relativity only says that two observers moving at constant velocity will both see the other's clocks slowed down, if one changes direction or speed then the symmetry of the situation is broken.

Last edited: Jan 13, 2005
4. Jan 15, 2005

### Hyperreality

Twin paradox, is in fact not a paradox, it is merely misunderstood.

The first postulate of relativity is "The laws of physics are the same in all inertial reference frames". In other words, special relativity only applies for inertial reference frame (reference frame free of all external forces).

In the the twin paradox, it says one of the twin brothers travel to and back from a distant planet close to the speed of light would less considerably less than his twin brother on earth. The paradox argues that because the spaceship is moving away from earth, it is equivalent to the earth is moving away relative to the spaceship, so why should the man on the spaceship ages less than his twin brother and not vice versa?

The major flaw of the twin paradox is the spaceship must accelerate and decelerate in the journey, the two processes represents a change from inertial reference frame into an non-inertial reference frame and conversely. Since the case involves non-inertial reference frame, it cannot be solved by special relativity, it was left to general relativity to solve the time dilation effect due to acceleration and decleration; general relativity did indeed predict time dilation for the man on the spaceship thus solving the twin paradox.

5. Jan 15, 2005

### JesseM

Accelerated motion can certainly be dealt with by special relativity, it's just that it has to be analyzed from within the reference frame of an inertial observer, not the frame of the accelerating observer. For example, if I am an inertial observer who sees you travel at velocity $$v_1$$ for $$t_1$$ seconds in my frame, and then you accelerate almost instantaneously to velocity $$v_2$$ and remain at that velocity for $$t_2$$ seconds in my frame, then I know that during the first leg of your journey, since my clock ticked forward $$t_1$$ seconds, your clock must have ticked forward $$t_1 \sqrt{1 - {v_1}^2 / c^2}$$ seconds. Then during the second leg of your journey, your clock must have ticked forward an additional $$t_2 \sqrt{1 - {v_2}^2 / c^2}$$ seconds, so in the $$t_1 + t_2$$ seconds it took for you to take this trip in my frame, I know your clock registered that $$t_1 \sqrt{1 - {v_1}^2 / c^2} + t_2 \sqrt{1 - {v_2}^2 / c^2}$$ seconds elapsed. This method can be extended to a path through spacetime made up of many short line segments, and of course any curved path can be approximated by such a path. Using calculus, we can consider the limit as the number of line segments approaches infinity, giving a line integral which an inertial observer can use to calculate the number of seconds that will elapse on a clock travelling between two points on a curved path through spacetime. The curve will look different in different inertial frames (and thus so will the form of the line integral), but all inertial observers will get the same prediction for how much time elapsed on the clock travelling along this curve.

See The Twin Paradox: The Spacetime Diagram Explanation from the sci.physics FAQ for more on how the twin paradox is dealt with from the perspective of inertial observers in special relativity. You can also see some other ways of dealing with it on the Twin Paradox section from the same FAQ.

Last edited: Jan 15, 2005
6. Jan 15, 2005

### Garth

The Twin Paradox is resolved by realising that it depends on what you think is happening 'now' 'somewhere else', and that what is 'now' 'somewhere else' depends on your frame of reference; i.e. the relativity of simultaneity.

Accelerating the clock does not alter its momentary clock rate, although the subsequent change of velocity will eventually do so when it is compared with a non-accelerated clock.

Where the paradox arises is in the distant 'travelling' twin thinking that the home twin's clock is behind in time 'now'. She is correct in this outward going coordinate frame of reference.

However when she turns round her time-like hyper-surface of simultaneity, all that is happening in the rest of the universe at her 'now', and not in her past or future, changes, the hyper-surfaces 'swing round', and do so again when she makes Earth-fall and decelerates to get back home. As a consequence of these swinging hyper- plains of simulataneity her Earth bound twin will be seen to advance into the future and end up older than the travelling twin.
The Earth bound twin will think all along that she is the older one and so there is no paradox.

Garth

Last edited: Jan 15, 2005
7. Jan 24, 2005

### cen2y

Regarding traveling backwards in time, using any method, I'd say faces the following problem:

Taken from an earlier discussions of mine found at http://forums.philosophyforums.com/comments.php?id=12797&highlight=c&findpost=251440#post251440. If the above was not clear enough please read post 18 of that topic as well.

8. Jan 24, 2005

### JesseM

Remember that in relativity observers following different paths may disagree on the amount of time a given journey took. So, travelling to the past means that in the future as measured by my own clock (my 'proper time'), I will be at a point in spacetime where some other set of clocks read an earlier time then the point I left.

It may also be helpful to think in terms of the "static spacetime" picture rather than the "moving present" picture of time...as I said in this thread:
In a similar vein, see this article on different ways of thinking about time by physicist paul davies:

That Mysterious Flow

9. Jan 24, 2005

### pervect

Staff Emeritus
According to relativity, an observer moving faster than light would appear to move backwards in time according to some observers, and forwards in time according to others. If this doesn't make a lot of sense, consider that relativity does not permit normal matter to travel faster than light.

Hypothetical particles that could travel faster than light ("tachyons") but could never travel slower, would be compatible with some aspects of relativity, but would play heck with causality. Tachyons have never been observed, and if they existed would have a host of other problems.

If you believe in both relativity and causality, you probably can't believe in FTL travel, with the possible exception of wormholes.