# Backwards Time Travel

1. Dec 2, 2008

### Math Jeans

I had a thought that I wanted to run by physicsforums so all of you can tell me that I'm wrong and make me feel better.

One of the main issues with the concept of time travel into the past is that we don't know whether is breaks any physical laws in the process.

Here is what I believe might be a law that backwards time travel actually breaks:

Suppose you are examining a particle at one point in time. At that point in time, you figure out that particles momentum.

Then, you hop in your time machine, and travel back in time, and measure the particle's position at the exact time the you measured its momentum.

Wouldn't that suggest that backwards time travel can break Heisenburg's Uncertainty principle?

2. Dec 2, 2008

### Nick89

Who is to say that the particle behaves the same way after you traveled back in time than it did before you traveled back in time? In other words, perhaps you traveling back in time would change the 'reality' so that the particle behaves differently.

3. Dec 2, 2008

### Math Jeans

But would it really change it?

Suppose you traveled about a day into the past before the experiment was done, and you aren't the one doing the measuring.

You time travel back into your house (from someplace far away from the lab) which is far away from the experiment.

You call your labmate who is on his way to the lab and tell him to measure position instead of momentum.

The argument in measuring a particle twice in Heisenburg's uncertainty principle is that you aren't measuring the same particle, but if you are able to time travel such that you do not effect the experiment, wouldn't you be breaking that law?

4. Dec 2, 2008

### Math Jeans

My argument is that it might be possible.

5. Dec 2, 2008

### Nick89

Well, either time-travel is possible and the uncertainty principle is invalid, or the uncertainty principal holds and time-travel is impossible. We probably won't know until we have tried it.

6. Dec 2, 2008

### Math Jeans

And of course, which do you think is more pheasable?

7. Dec 2, 2008

### Nick89

I am not extremely knowledgeable about both subjects so I'm going to leave that question for someone more experienced ;)

8. Dec 2, 2008

### pallidin

With my limited understanding as well, one of the theories I heard is that travelling back in time does not permit interaction. A secret, passive observer if you will.
But who knows? I sure don't.

9. Dec 2, 2008

### Math Jeans

Well, if you are just a secret, passive observer, that would make it even easier to measure the particle's position without altering the particle from where it was in the other timeline.

10. Dec 2, 2008

### ThomasT

The uncertainty relations don't refer to single measurements. The HUP regarding measurements of position (x) and momentum (p) is: (delta x)(delta p) >= h. This says that the statistical spread around an average value for a large number of position measurements multiplied by the statistical spread around an average value for a large number of momentum measurements is greater than or equal to Planck's constant.

General relativity is a source of speculations about time travel. But those speculations are based on math and geometry that doesn't actually describe, in a qualitative sense, the real world.

Talking about physically travelling backward in time requires a general concept of what the word, time, means physically.

If we define time as motion, the changing arrangements or configurations, etc. of physical objects, and if we live in an evolving universe, then travelling backward in time is a physical impossibility, because it would require rewinding the evolution of the universe. Put another way, if there is an arrow of time, then there can be no backward time travel. Available evidence strongly suggests that there is an arrow of time.

11. Dec 3, 2008

### pallidin

Something I find interesting is that it is proven beyond any doubt that we can view astronomical events which have occurred before our current time.
I do fully understand the process(and it's current limitations), and I'm not implying that it means anything more, but it is interesting.

12. Dec 3, 2008

### Vanadium 50

Staff Emeritus
Why is this surprising? If you hear thunder you are also observing a terrestrial event that happened before your current time. Same idea, different scales.

13. Dec 3, 2008

### pallidin

It's not surprising to me at all; I understand the physics of each phenomenon.
I just find it interesting.

14. Dec 3, 2008

### Gnosis

Time travel into the past is utterly impossible and deserves no serious waste of thought and energy. It is best left to science fiction books and movies where virtually anything is possible, as well as entertaining.

The passing of time is marked by the orientation of all heavenly objects relative to one another. Their orientation is merely a singular and unique position on our infinite clock. Their relative orientations are never identical from one passing moment to the next nor will they ever be repeated in any future time frame due to expansion of the universe and the complex relative motion of all objects. Each moment of orientation is unique and the recorder of an instant in time.

Examine the Earth's motion through space. A year from now the Earth will be in almost the same orbital position around our Sun however, the Earth (and Sun) will NOT be in the same orientation in space nor will it be precisely in the same orientation relative to all other heavenly objects. This is because the Sun is streaking through the great expanse of space at roughly 12 to 12.5 miles per second (43,200 MPH to 45,000 MPH) and the Earth has no choice but to follow, as it has for eons now. A year from now, the Earth and Sun will have traveled roughly 378,691,200 miles deeper into space. The motion of all heavenly objects continually marks unique orientations in time that will NEVER be repeated.

So, anyone desiring to travel 100 years back in time to Earth as it were 100 years previously would have to travel 37.8 billion miles back through space to where the Earth was actually located 100 years previously (in which case, the Earth won't be there). In order for the Earth’s night skies to appear just as they did 100 years previously, EVERY viewable heavenly object would ALSO have to be returned to its 100 year previous orientation. Needless to say, this isn’t going to happen. A feat of this nature would require a magnitude of control over the entire universe that could only be comparable to that which is thought to be God-like and limitless in ability. We humans aren’t going to be pulling off such a feat, you can be certain of that. We can’t even live peaceably amongst ourselves on our own planet. Profit margins dictate which scientific products are feasible and which ones are deemed too costly to pursue.

The continually forward progression of all heavenly objects over billions of years has demonstrated that time advances in one direction only, and that direction takes us ever deeper into the universe. The past is gone forever, so make the most of the present to make it a memorable past…

15. Dec 3, 2008

### Nick89

I can't see how the motion of heavenly objects has anything to do with time travel. Sure, time travel is probably impossible, but as far as I know it has never been proven impossible.

Suppose for a minute that time travel was possible. If you could somehow manipulate time so it runs backwards and you end up a few years before, than that action would automatically move everything back in time, even the heavenly objects you are talking about, and they would be in exactly the same position.

Of course this is all fiction, I have no idea how we could control time to run backwards. I am merely pointing out that IF you can travel back in time, than everything would have to travel with you (otherwise, you haven't traveled back in time, you have gone to another reality, dimension, whatever...)

16. Dec 3, 2008

### scupydog

Travel into the past is permitted by the laws of physics and science fiction has a knack of becoming science fact all to often nowadays which i agree is very entertaining.

17. Dec 3, 2008

### scupydog

now that's food for thought, I love this forum

18. Dec 3, 2008

### Gnosis

If you were able to travel 100 years back into the Earth's past, its night sky WOULD have to appear just as it did 100 years ago or you wouldn't otherwise actually be experiencing the Earth of 100 years ago! The heavens in any other orientation indicates a progression in time has occurred and the time frame has changed.

Tonight's heavens will only appear precisely as they do on this night ONLY. Any other night in the past or the future will NOT provide the exact same orientation and naturally, the further into the past (or the future), the greater are the visible differences in their orientation. The past is only the past if ALL is actually as it once was and this includes the orientation of all heavenly objects or it's simply NOT the actual past.

19. Dec 3, 2008

### Nick89

Of course. But that doesn't mean that it is impossible for everything (including the heavenly objects) to go back to the exact same 'state' as they were before. Note how I use the words "go back to" instead of "go to". Time traveling to the past indicates that you are changing TIME, not the 'states' of every single object in the universe.

You seem to think that, if it was possible that I could hit a button here which would make everything (including heavenly objects) become the same state as it was 5 seconds ago, that I traveled 5 seconds back into time? Of course not! I have merely changed every single object to reflect how they were 5 seconds ago, that does not mean I have changed time...

If you are actually changing TIME, than everything would AUTOMATICALLY follow and go back to the same state. If that is not true, than you are not changing time.

20. Dec 3, 2008

### Gnosis

You're clearly not paying attention and you need to actually read what I've stated from the beginning. I stated that time travel into the past IS IMPOSSIBLE! You cannot travel into the past. You can travel whatever distance, but that isn't traveling into the past. Time travel into the past is impossible.

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