# Illusionary time

1. Apr 15, 2006

### Ratzinger

Why is according to relativity the flowing of time an illusion (as Einstein put it)?

2. Apr 15, 2006

### pervect

Staff Emeritus
Sounds like philosophy to me...

Out of curiosity, what was the exact quote of Einstein you had in mind, anyway?

3. Apr 15, 2006

### Staff: Mentor

Perhaps the OP is referring to the so-called "block universe" idea.

4. Apr 15, 2006

### Andrew Mason

I believe the quote the OP is referring to is the remark Einstein made in a letter to the family of his long-time friend, Michele Besso on Besso's death. He said that although Besso had died before him, it was of no consequence because: "...for us physicists who believe, the separation between past, present, and future is only an illusion, although a convincing one."

This hardly qualifies as a statement that as a matter of physics, Einstein believed that past, present and future was an illusion. Certainly, in a single inertial frame of reference it is not true and Einstein certainly did not believe that.

In different reference frames moving at high relative speeds, there is some limited truth to the suggestion that past, present and future is an illusion.

So I think Einstein the poet rather than Einstein he physicist penned those remarks.

AM

5. Apr 15, 2006

### JesseM

Yes, but the standard idea of the "flowing time" people is that there is a single objective present which is the same for everyone. Once you accept that one's definition of simultaneity depends on an arbitrary choice between equally valid coordinate systems, you've basically accepted the "block time" view.

Of course, in theory it would be possible to believe that although no coordinate system is physically preferred, maybe one could be "metaphysically preferred" in a way that would have no experimental consequences but which would single out the single metaphysically true definition of simultaneity, so we could hold on to the view that future events somehow "don't exist yet" as in the flowing time view. But this does seem pretty inelegant, so I think it's fair to say that relativity makes the "block time" view a lot more appealing even though it can't settle such a philosophical question.

Here's a pretty good article by physicist Paul Davies on the subject of relativity and flowing time vs. block time:

http://urgrue.org/lib/mysterious-flow.html [Broken]

Last edited by a moderator: May 2, 2017
6. Apr 16, 2006

### -Job-

I just use the z-axis as the time dimension...

7. Apr 16, 2006

### Ratzinger

Yes, that’s the quote I was referring to.

And what JesseM and Paul Davies say is what I intended to ask about.

Accepting that there are no preferred nows, i.e. accepting relativity leads automatically to block time, so it’s not a view and no philosophy but inevitable consequence of Einstein’s theory. Or not?

But that is a disturbing consequence, to me far more disturbing than twin paradoxes and shrinking meter sticks.

8. Apr 16, 2006

### pervect

Staff Emeritus
I'm not convinced that one actually has to philosophically asssume "block time" given that one believes in relativity.

My personal philsophical views go something like this. Because I think of myself as a unit (and not an assembly of parts), my perception of the world is based on events, which happen in a definite order. But I recognize that this is an oversimplfiication.

A closer analysis will indicate that my consciousness is synthesizing a notion of "when" events happen based on signals from my nervous system, which all have different propagation delay times. This is part of the process by which I construct a model of reality based on various signals from my senses.

My perceptions aren't accurate on the nanosecond scale, but for everyday life they don't have to be.

Physically, my eyes, my feet, my fingers, all have somewhat different notions of time (especially if they are moving with respect to one another), but part of the function of my brain is to reconcile all these different inputs into one sensory experience that's easy to deal with.

On a related note, I often work physics problems based on the same idea that we specify the state of some system at a particular instant in "time", and study its evolution as "time" progresses. However, while I do do this, this process is actually somewhat suspect, as it requires me to adopt some specific notion of "time". Because it is very useful, though, I do it anyway

In my opinion, my mental models when I perform this sort of analysis aren't actually reflecting the true causal structure (at least, what I consider to be the true causal structure) of the universe, which is a series of events related via lightcones. But, as I said, I do it anyway.

This is a bit rambling, and perhaps a bit on the philosophical side, but I hope it helps.

9. Apr 17, 2006

### Garth

IMHO the question of whether the 'block time universe' exists or not is a matter of perspective.

SR has taught us to look at the world as a 4D continuum, with proper time $\tau$ replacing t, and 4D geometric objects, such as 4-momentum, replacing their classical equivalents.

However we are creatures of a conscious (3+1)D world, and as observers on a particular world-line through that 4D continuum, we define our own foliation of space-time into a particular 3D space + time. The only world that is 'real' to our consciousness is that particular foliation with its sensation of the passing of time.

Which perspective do you therefore use to describe the universe? 4D block space-time, or (3+1)D space and time? It depends on what is appropiate to the task in hand. Both are equally valid descriptions, however each is not necessarily the most appropiate description for a particular use.

To do physics from a geometric perspective choose 4D, to describe a particular observer's experience of the universe (e.g. our own perspective) choose (3+1)D.

Garth

Last edited: Apr 17, 2006
10. Apr 17, 2006

### Ratzinger

So to have flowing time consiousness or some cognitive system is needed.
If you look at the physics alone block time is inescapable. But isn't the perspective that does not assume conscious observer more fundamental, more valid?

Is not the conscious view, the flowing time view as Einstein said it only a convincing illusion?

11. Apr 17, 2006

### Garth

The word 'illusion' is a loaded term, it suggests that perspective is less real than the 4D block time one, yet we can only observe and test the nature of time as conscious observers - hence it might well be said that it is the (3+1)D perspective that is 'real' and the 4D block time that is the illusion.

I say they are both equally valid in their respective perspectives.

The 'block' nature of space-time is simply an automatic consequence of regarding time 'from outside' so to speak, that is, including it in a space-time diagram observed objectively 'from outside'.

Garth

Last edited: Apr 17, 2006
12. Apr 17, 2006

### pervect

Staff Emeritus
What is "real" and what is "illusion" is a matter of philosophy, not physics. One is generally free to regard different aspects of physics as "real" or "illusion", as long as one gets the right answers in the end.

As far as block time goes, my position is that there is (for instance) nothing about relativity that suggests that events outside one's past light cone are necessarily "predestined".

There is IMO nothing that suggests that they are not predestined, either, the choice of predestination vs free-will is IMO a philosophical choice that is not testable by experiment.

13. Apr 17, 2006

### JesseM

If you want to describe an observer's experience of the universe, you should only talk about how the observer's past light cone changes as their proper time increases along their worldline--after all, the the idea of distant events with spacelike separation being "simultaneous" is just an abstraction based on Einstein's clock synchronization convention, you don't actually experience these "simultaneous" events until later when light from them has had time to reach you. I think it is fair to say that relativity makes the notion of any sort of "universal now" problematic, although you are free to imagine that each observer has their own separate "now" which is moving forward along their worldline, or even take the solipsistic position that it is only your now that is real and everything outside of your past light cone is undetermined. But a question like "what is my friend on Mars doing right now" cannot have any sort of single objective answer unless you introduce something like the "metaphysically preferred reference frame" idea I talked about in my last post.

14. Apr 17, 2006

### Garth

Garth

15. Apr 17, 2006

### Ratzinger

That was the next thing I wanted to ask about. Is that safe to say that relativity does not imply predestination?

But why is that? Is not everything on my now list, no matter how far away it takes place equally real? If a glass drops in my room now or if that happens on Andromena now, both are unchangeable facts and I can say a second later they happened.

16. Apr 17, 2006

### JesseM

Only if you accept that there can be no universal truth about which events have already happened and which haven't (assuming you don't use the 'metaphysical preferred reference frame' idea)--either each observer would have his own separate truth, or you could adopt that solipsistic view that only your point of view is the truth.
Because there's no physical reason why you must to associate each observer with a reference frame constructed in the way Einstein does it, that's just a human convention. A given observer could use a different clock synchronization convention than Einstein's for his rest frame (although this would be less elegant because it would mean the equations to represent the laws of physics in different inertial frames would no longer be the same), or he could even define "his" frame as a frame where he was in motion. Also, what about an accelerating observer? If you say that his "now" at each moment is defined by the inertial frame in which he is instantaneously at rest, then it will be possible for events that were in his past at an earlier time according to his clock to be in his future at a later time according to his clock.

17. Apr 17, 2006

### Andrew Mason

I think that, by definition, time can only flow in one direction, as I shall explain:

Suppose we take the space time co-ordinates (in our frame of reference) of every quanta of energy and matter in the universe. That defines the present universe (for us) which we will call Present(1). We then take the space-time coordinates of all the quanta of energy and matter in the universe exactly one day later, which we call Present(2).

If at some time in the future all of those quanta of matter and energy should have exactly the same space time co-ordinates as Present(1) (highly improbable, but let's suppose it happened), would we conclude that the flow of time had reversed? No. Because our brains and all of the information devices needed to record the fact that Present(2) had already occurred would be lost and we would be completely unaware that time had flowed backward. So even if time flows backward, it would appear to flow forward!

The reason we can remember a past is because as some things change, other things are preserved. It is the preservation of a record that creates a past. If time actually reversed its flow, the record of the past is lost there is no past to return to. Therefore, there is only one direction that time can flow.

AM

18. Apr 17, 2006

### pervect

Staff Emeritus
My philosophical position is that when the light reaches you from the dropping glass, only then is it an unchangable fact.

Before then, there is room for doubt.

When you drop a glass in your room, the difference doesn't make much practical difference. The same is not the case when the glass is in Andromeda.

Note that as many others have said, the notion of simultaneity is observer dependent. This is one of the reasons why I do not choose to include "now" for distant events as part of "reality", because I define reality, as much as I can, to be obsever-independent. If we consider SR, the notion of "now" for distant events depends on the state of motion of an observer. Thus the SR notion of "now" is observer dependent. This observer dependence is reason enough for me to exclude it from "reality", which I define as things which are not dependent on the obsever. (Note that this approach has some problems with regards to quantum mechanics, which is much more challenging philosophically :-().

In GR, the situation with regards to simultaneity is probably even worse. By addopting different coordinate systems, it is possible for the same observer to assign a different set of events as happening "now".