# Are present and future perspectives different?

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1. Mar 9, 2015

### HZY

It is said that when an observer look up the sky and observe the cosmos, the further out he looks, the further back in time he sees, allowing him to peer into the past. However, there is a catch, namely, the observation must take place at present time. In other words, a distant object 100 light years away that he is just now observing is probably going be very different than what it actually is at present time. By that scenario, one can say that the entire universe at it exists at present time will have to be different than the "observable" universe as it is being observed now at present.

Now, let's say observer is somehow able to time travel ahead by 1-day into the future, will his perspective be different? Will he know he is in the future and not the present by looking up the sky and observe the cosmos? Is the a case of multi-version universe as supposed to multi-verse universe?

2. Mar 9, 2015

### DaveC426913

Yes. I can do so easily, by going to sleep for a day*. I will wake up tomorrow and I will observe the universe to be slightly different. For one, the sun will appear in a slightly different part of the sky because the Earth will have moved.

*you can't really distinguish this from time travel. I tell you over the phone that I am stepping into my time machine and will set it for 24 hours from now. In fact, I simply climb into bed. How would you know the difference?

This question is flirting dangerously with philosophy - an inappropriate topic for PF. It may stay open; it may get locked.

3. Mar 9, 2015

### Staff: Mentor

Yes, of course; he is seeing it now as it was 100 years ago, whereas "at present time" it is 100 years older and has had 100 years to change. (I put "at present time" in quotes because that actually doesn't have an absolute meaning; see below.)

Yes, of course; the entire universe "at present time" is a spacelike hypersurface, while what we call the "observable" universe--what we actually see at a given instant--is our past light cone, which is a null hypersurface. They're two different things.

Also, what "at present time" means depends on which coordinates we choose; there is no absolute meaning to "the entire universe as it exists at present time", because different observers will slice spacetime up into space and time differently. So you have to resist the temptation to think that there is a unique answer to the question of what is happening "now" to a distant object 100 light years away (100 light years in coordinates in which we are at rest--how far away the object is also depends on which coordinates we choose). The only thing we can say for sure is that nobody's "now" is the same as our past light cone.

4. Mar 9, 2015

### HZY

Is it possible to travel through space between two adjacent past light cones? And it is also possible to travel through time between your past light cones at different time?

5. Mar 9, 2015

### Staff: Mentor

These questions don't make sense. By definition, your past light cone at a given instant is the set of events that emitted light rays that you are receiving at that instant. It's just a way of referring to that particular set of events. It's not something you can change or "travel between".

6. Mar 10, 2015

### HZY

The difference is that time travel allows the 24-hour future to concurrently exist at present.

So based on the logic of past-light-cone, one could say that the reason we are living in the present and not the future is because the future light rays haven't gotten to us yet, today. But what if it had gotten to us today, could you then say we are now living in the future, today? Could it be possible that we are actually living in the future and not knowing it? And if we did know, could we then figure out a way to connect the two states, namely, "living in the future, today" and "living in the today. today", and engineer a pathway (spatial or temporal) between the two, thereby making time travel (or spatial jump travel) possible?

7. Mar 10, 2015

### Staff: Mentor

No, it doesn't. You need to think more carefully about what "time travel" actually means.

You have this backwards. The future light rays haven't gotten to us yet, today, because today it's today and not yet the future. "Today" is just a label we give to a particular point in spacetime, one in which the light rays we are calling "future light rays" today have not yet reached us. The paths of the light rays don't cause spacetime; spacetime is the underlying geometry that determines the paths of the light rays, and which ones reach us at which points on our worldline.

This question makes no sense. It's like asking "if New York were Los Angeles, could you then say that New Yorkers are living in Los Angeles?" It's asking about how things are labeled instead of asking about physics, about what actually happens. Again, "today" is just a label we give to a particular point in spacetime. Changing the label does not change the physics of what happens.

What does this even mean? I think you need to take a step back and think very carefully about what you are asking. You seem to be confused about what terms like "today" and "future" mean.

8. Mar 10, 2015

### HZY

Basically, what I am trying to find out is that is it possible for all times, be it past, present, or future, to co-exist concurrently in the same time frame relative to different arrival points of a particular light ray from a distant object, measured against the present time frame, within the same past light cone coverage? If such concurrent co-existence is possible, then wouldn't you say that past, present, and future could be merged into a single time frame (the Present, let's say), thus making time travel possible?

9. Mar 10, 2015

### Staff: Mentor

I still don't understand what this means; it doesn't make sense to me. I have a suggestion: try to draw a spacetime diagram illustrating what you are asking about. What would such a diagram look like if what you are asking about were possible?

10. Mar 11, 2015

### DaveC426913

How so? If I time travel 24 hours into your future, I am no longer in your present.

11. Mar 11, 2015

### HZY

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12. Mar 11, 2015

### HZY

Isn't bringing the future time frame into the present time frame so you can just step into the future the idea of time travel? If two past light cones representing two versions of the observable universe could be brought toward each other, then their individual now's be made to overlap, creating a potential time travel point, as indicated in the diagram below:

13. Mar 11, 2015

### Staff: Mentor

No. Your diagram makes it clear what you are describing, and what you are describing is not how spacetime works. The point in spacetime where observer 1 is when his clock reads $t_p$, and the point in spacetime where observer 2 is when his clock reads $t_f$, are fixed; they don't change. So you can't "move" the light cones to overlap the way you are drawing them; since the points in spacetime that mark the apexes of the light cones are fixed, the cones themselves are fixed as well.

What "time travel" means can be described in terms of your diagram as follows. Suppose that the points "observer 1 at $t_p$" (point 1p) and "observer 2 at $t_f$" (point 2f) are placed in spacetime such that point 2f lies within the future light cone of point 1p (and therefore point 1p lies within the past light cone of point 2f). Then "time travel" would involve observer 1 jumping from point 1p to point 2f (for example) without passing through any intervening points. In other words, what "moves" in time travel is not the points in spacetime or their light cones; it is only the "now" of the traveler that moves, i.e., which point the traveler calls "now" is what moves.

And, as DaveC426913 pointed out, even if observer 1 moves from point 1p to point 2f and does pass through the intervening points, he is still "time traveling" in a sense. In this sense, we all are constantly "time traveling" along our worldlines; which point in spacetime we call "now" is constantly changing. But the points themselves and their light cones don't change; all that changes is which point each observer calls "now".

14. Mar 11, 2015

### HZY

You mean like this?

15. Mar 11, 2015

### Staff: Mentor

You have drawn the light cones the way I was describing them, yes. However, your interpretation of what this diagram is telling you is not correct.

The key thing you are leaving out is that this is a diagram of spacetime, not space. The points at the apex of each set of light cones are the points in spacetime at which observer 1 and observer 2 are located at one single instant of their own experienced time. Observer 1 does not stay at the point at the apex of the red light cones, and observer 2 does not stay at the point at the apex of the yellow light cones. Those are each just one point in the entire history of observers 1 and 2.

To draw the entire history of observers 1 and 2, you would draw two curves in this diagram, one for observer 1 and one for observer 2. Each curve would pass through the points at the apex of the appropriate cones (red for observer 1, yellow for observer 2). And you could draw similar cones at any point on each curve, describing the light cones of observer 1 or observer 2 at that point.

So the correct way to describe the spacetime relationships in the diagram is as follows:

* Call the point at the apex of the red cones point 1p, and the point at the apex of the yellow cones 2f.

* Point 1p is in the past light cone of point 2f, and conversely, point 2f is in the future light cone of point 1p. Therefore, observer 1, at the instant of his experienced time at which he is at point 1p, is in the causal past of the point of observer 2's experienced time at which he is at point 2f. That means observer 1, at the instant of his experienced time at which he is at point 1p, could do something to causally affect what happens to observer 2 at the instant of his experienced time at which he is at point 2f: he could send a signal, dispatch a package, fire a gun, even adjust his own motion to ensure that he is co-located with observer 2 at the instant of observer 2's experienced time at which he is at point 2f.

* But note that in all the statements of the above, I carefully specified the specific instant of experienced time involved for each observer. None of what I said applies to different instants of experienced time for either observer. To say things about those other instants, you would need to draw the entire worldlines of observers 1 and 2 on the spacetime diagram, and look at how different points on them are related.

16. Mar 12, 2015

### HZY

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17. Mar 12, 2015

### Staff: Mentor

Once again, you've drawn possible "historical paths" ("worldlines" is the usual term) for observers 1 and 2 (at least for part of the diagram--note that, since observers can't move at or faster than the speed of light, the slope of worldlines can't be different from vertical by more than 45 degrees, assuming we're using units in which the speed of light is 1), but your interpretation of what they're telling you is wrong. The "historical path" of observer 1p does not "represent one single instant of observer 1's own experienced time". It represents all of the instants of observer 1's experienced time, in their appropriate order as he experiences them. Similarly for observer 2. The different cones at each point on observer 1's worldline represent the past and future light cones for him at that particular instant of his experienced time--which is placed on the worldline in its appropriate order relative to all the other instants of his experienced time. Similarly for observer 2.