# Time as a series of interactions

1. Sep 8, 2009

### hwpage

John Wheeler made the statement - “Time is what prevents everything from happening at once”

However, at a quantum level (and other levels too, but let's focus on quantum and disregard gravity for now) particles can only change state if they interact with other particles. The particles that carry the interaction force cannot exceed the speed of light so "everything" (meaning interactions) cannot happen at the same time because of a bound on the speed of the particles that carry the force of the interaction. This seems to imply that everything doesn't happen at once because of a finite speed of light and we interpret the sequencing of interactions as time.

Since John Wheeler knows more about this than I do, I believe I have missed something here.

Could someone help me out?

2. Sep 12, 2009

### daisey

I would expect such an insightful quip from a fellow Longhorn. :tongue2:

It appears that no one has yet responded to your question. I suspect it is because your question is both very general, and because how Time works is not well understood. Surely not by me, a casual reader of texts on subjects such as this. I'm not sure this is pertinent, but here is my two-cents on the subject of Time.

I've heard Time best described as simply something that clocks measure. I now understand it to be just another dimension. I learned this clearly by reading something the other day that explained this feature of Time in a fascinating way.

The author of this article stated that "we all travel through space-time at the speed of light". We humans devote the large majority of our momentum traveling through the dimension of Time, and very little through the three spacial dimensions. On the other hand, photons (for example) spend all of their momentum traveling through the dimension of space, and none through the dimension of Time.

I think the speed of light is just a natural speed limit on how much three dimensional things can change in a given duration of Time, limiting how far, as you state, force carriers can travel in any given duration. It keeps our perception of three-dimensional change as a smooth transition from second to second. So I don't see Time as a series of interactions. It's more just a perception of change in spacial dimensions. (Boy, that sounds obvious. :uhh:)

Like I said, this is only my two-cents worth. I hope it was worth at least that.

3. Sep 12, 2009

### pep_i

Time is a thought of man and simple it does not exist. Each person exist within frames and each frames contains movements of each individaul movements or actions, such actions is described as elapses.
Noone or particle can travel at the speed of light because general realativity prohibits such behaviour. that is according to einstein theory however, but such notion is false because if time is simple a though, how can it exist? and if it does not exist how can something "gravity" acts on "i"t (time nothing).

4. Sep 13, 2009

### hwpage

I'd like to read the article on time as an extra dimension and how we allocate our energy moving through the different dimensions of space-time. It just seems like a kludge to add another dimension for time when it isn't necessary. At the sub-atomic level, there are numerous examples of interactions that are reversible, because the interactions and state variables of the particles are so limited. This indicates to me that time doesn't exist at all for sub-atomic particles, but is something that emerged as systems and interactions became more complex.

5. Sep 13, 2009

### muppet

First, relativity. Let's clear up what we mean when we describe time as a "fourth dimension". "Space-time" is perhaps best thought of as "the mathematical space of events". You need four co-ordinates (x,y,z,t) to describe both when and where something happens, so this space is four-dimensional. Picture every event that ever happens in a big four-dimensional box, and the theory of relativity is a set of rules about what co-ordinate systems you're allowed to use to describe the contents of the box, and the relations between different points in the the box. These relations are mathematically expressed most naturally as the geometry of this space.
In this geometry, time and space aren't treated in quite the same way. Spatial intervals are measured with rulers, and time intervals are measured with clocks (and multiplied by c, in part so that we're using the same units for everything). The reason for this seeming preoccupation with the process of measurement is that it turns our intuition about what time and space are into something we can analyse more carefully, and this is where interactions first creep in.
We could use the above formalism to describe classical physics if we wanted, but it'd be pretty pointless- everyone would use the same time-coordinate. What messes up this classical picture is that everyone, regardless of how fast they are moving, agrees on the speed of light in a vacuum, denoted c. This means that a man driving a car sees the light from his headlamps move away from him at c; it also means that the person on the pavement he's overtaking sees the light moving away from the point on the road at which it was emitted at c. I'm sure careful explanations of the consequences of this idea abound on the relativity forum, so I'll be brief here: this leads to the driver and pedestrian measuring different intervals, of both time and space, so that they agree on the speed of light. It's the propagation of light that leads to relativity, and the classical details of how this works are laid down by Maxwell's equations. So there's the first example of how physical interactions determine the relationship between different events. However, it's still taken as as a given that a time co-ordinate is necessary. So relativity doesn't tell us where time comes from; rather, it tells us about the relationship between time and space.

Now, let's talk about quantum mechanics. In QM we don't picture particles as physically travelling from A to B the way we do classically. Rather, in orthodox QM, you either have to picture the particle following every possible trajectory at the same time (-the "path integral" approach due to Feynman) or you have to abandon the very idea of trajectories altogether (the more common approach), and talk about a transition from some state A to some state B without asking too much about what goes on in the middle. So, the idea of the "speed" of the particles determining the ordering of events is a conceptual non-starter. (In any event, you can't talk about the speed of particles independently of time- the very idea of speed is dependent upon that of time.) The idea of a transition from A to B is built into the theory, so it's not yet really possible to describe time as emerging from the formalism rather than being used to construct it.

I think, however, that string theories have something to say about the interesting idea of time as an emergent phenomena- but precisely what, ask me again in a year!

6. Sep 13, 2009

### Hernik

If that was the case, how can we age differently depending on relative velocity. How can decay of radioactive material depend on relative velocity?

- Henrik

7. Sep 13, 2009

### daisey

I would agree with this part of your statement.

Good point. It is not true that we age slower at higher velocities because everything moving through space has farther to travel. And even though, as muppet stated, things don't literally travel through space from A to B, the distance from A to B changes at different speeds.

8. Sep 13, 2009

### muppet

... But the rest of it was rubbish? :tongue:

Just to be nitpicking, it wasn't me Hernik was quoting there. But with the twin paradox in a quantum mechanical context we're touching upon the elephant in the room: quantum gravity.

When I talked about relativity before, I was really talking about special relativity. In SR, when you make statements like "time slows down for someone moving quickly", what you mean is this: "a clock in an inertial frame that moves with respect to an observer's frame will be perceived to run slowly by that observer". The symmetry of the situation is such that two observers moving relative to each other both consider the other's clock to be running slowly! In the famous "twin paradox", however, this symmetry is broken. One twin gets in a static spaceship, flies off, then changes direction to fly back, and is younger than the other on her return. (Let's challenge the masculine bias in my hypothetical examples, why not? ) The reason for this is that one of the twins is accelerated. The point of departure of general relativity is that in accelerating frames light appears to travel in curved paths, and you end up equating gravity with curvature.

As you're probably aware, we don't have a quantum theory of gravity yet. Some people are trying to construct one in what to someone familiar with the maths behind all of this might consider the "obvious" way- which is physically tantamount to turning spacetime into a blurry thing that can exist in superpositions of states. There's some considerable technical difficulties to this however, which is why string theory takes a different route to solving the problem- gravity becomes a force like all the others, and spacetime becomes an emergent concept when you're talking about interactions of low energy.

9. Sep 14, 2009

### daisey

I am also talking about SR. My point about the reason for things running slower at higher speeds has to do (I think) with relative distances travelled. See picture (taken from this site: http://www.astronomy.ohio-state.edu/~pogge/Ast162/Unit5/sr.html" [Broken]).

The photon in Jane's clock is moving just as fast as any other photon, but because Jane is moving faster relative to Dick, Jane's photon is covering more ground than a photon is Dick's similar clock (not shown in picture). So Jane's clock will tick slower (relative to Dick's clock), and Jane's bodily functions will move slower (with respect to Dick's) for basically the same reason. Although to Jane, she feels no difference in her metabolism.

So how can one say (as pep_i did earlier) that time does not exist?

Daisey

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10. Sep 14, 2009

### hwpage

Thanks for the dialog. I certainly am not saying time does not exist but that it makes more sense to view time as a human construct. I think we'll see that our current notion of time is an emergent property that came about as the universe cooled enough for matter to come together into sizes (larger than particles) and speeds (much slower than the speed of light) that facilitated gravitational interactions. Gravitational interactions between singleton particles traveling at high speeds (which was the case when the universe's expansion began) are too small to make any difference. I believe the process that allowed these particles to "come together" will be much more complicated than expected and will follow a similar path to other emergent properties - like life, species and human intelligence. String theory may weigh in on this and complexity theory deals with these sorts of emergent properties. Neither theory has come up with a well defined set of rules for these sorts of interactions and certainly will not have the predicative powers we have come to expect from quantum physics and SR/GR. If gravitational interactions emerged, it would go a long way towards explaining why we've had such a difficult time coming up with a unified theory. It would be like defining a universal explanation of life and non-life - which seems like a nonsensical idea to me.

11. Sep 14, 2009

### Hyrox

Just out of curiosity, it was Einstein who originally said that, right? I understood that was what the Special Theory of Relativity was all about, how everything traveled through space/time at the speed of light.

12. Sep 15, 2009

### muppet

Yes, but you've got to be careful about what it means to travel through time at the speed of light

13. Sep 15, 2009

### daisey

Of course he did, but I never heard it explained that way before.

14. Sep 15, 2009

### Hyrox

The first time I heard it explained like that (I think) was when I read The Elegant Universe by Brian Greene.

15. Sep 15, 2009

### daisey

That is the one and ONLY place I've seen it explained that way. Good book, but I found the latter chapters on string theory too confusing. :-)

16. Sep 15, 2009

### Hyrox

I didn't think it was that bad. I loved the book. I'm currently reading The Fabric of the Cosmos. I can't say he's done a great job of convincing me to believe in string theory though...I'm rather skeptical about it.

17. Sep 23, 2009

### DougW

Wow, I'm more confused after reading this thread than I was before! Can I ask a few 'amateur' questions to help clear it up for me?

1) Saying time doesn't exist seems a bit of an overstatement. Clearly the universe right now is not in the same state that it was in one million years ago, and if we need to describe the variable that this change is a function of, wouldn't time be that variable? So do we really mean that "time as a special dimension with properties different from any other dimension is simply a construct of man..."?

2) If time is a dimension no different from other dimensions, it would seem that our 'perception' is somehow linked to the variable of time. We experience things, and store memories across a function of time. If that is true, can we model a universe where perception instead flows across one of the other dimensions? What would the existence of a being traveling and experiencing perception through one of the other dimensions be like? To this being, only matter moving at the same velocity as him would seem to exist. Matter moving at any different speed would exist in his frame of reference for an 'instant' too small to be measured (I use the word 'instant' not in the context of time, but to describe a 3-dimensional frame in the 4-dimensional existence described above...). Has there been any discussion/thought of looking at the universe this way?

3) If we consider that time is simply another of N dimensions that exist, one of 4 that we happen to be able to observe in some manner, then what of those we can't observe? Are there an infinite series of time/spaces existing around us, separated by a variable we can't understand? Could interaction with 1 or more of these extra dimensions be how quantum entanglement is 'communicated'?

4) Since our entire observation of the universe is constrained by our functional existence across time, how do we think that limitation affects our view about the fundamental 'information' that makes up the universe? Is 'matter' simply information that is also constrained by time? Is energy the same basic information constrained by a different function, and so exists only in 'instants' in our observable universe?

Anyway, this stuff is fascinating! I've recently started reading as much as I can about dimensions/time/etc. and will definitely pick up the books referenced above!

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