Is There a Connection Between Movement Through Space-time and Aging?

[ChrisXenon]

I'm getting older and dumber, it seems.

In his book "The Fabric of the Cosmos", from page 49, Green points out some basic vector thinking. Given a fixed speed over the ground, as you head more West of North, your speed North decreases, whilst your speed West increases. He suggests that this is analogous to our movement through space-time. As our speed through space increases, our speed through time slows down - i.e. time slows down.

From page 53, Greene develops the model of space-time being a block of slices of 2D space. In this model, the third dimension is time, the two dimensions of the page are space, and the entity as a whole is space-time.

Here's my problem, and it hits before we even get into the interesting stuff. He says that a photon, moving through space at light-speed, will experience no time. This would be analogous to moving across one of his bread slices without moving in the time dimension. However, in his end note 9, he says that something moving at light speed sweeps out a line at 45 degrees to the crust-to-crust axis of the loaf. So on the one hand, the angle is 90 degrees and on the other it's 45 degrees.

Where have I gone wrong?

Please be gentle )

 

[Dale]

You haven't gone wrong. The analogy is an analogy, it isn't perfect or it wouldn't be an analogy. You have uncovered one of the limitations.

The source of the limitation is geometry. In Euclidean space the geometry is defined by the Pythagorean theorem $ds^2=dx^2+dy^2+dz^2$, but in spacetime it is defined by the Minkowski metric $ds^2=-dt^2+dx^2+dy^2+dz^2$. In Euclidean space the only way to have 0 length is to have 0 on all three axes, but in Minkowski spacetime you can also have 0 length by having equal dx and dt, which corresponds to 45 deg.

 

[A.T.]

Here's my problem, and it hits before we even get into the interesting stuff. He says that a photon, moving through space at light-speed, will experience no time. This would be analogous to moving across one of his bread slices without moving in the time dimension.

Yes, if you put proper-time on the time axis. That is a so called Epstein-diagram.

However, in his end note 9, he says that something moving at light speed sweeps out a line at 45 degrees to the crust-to-crust axis of the loaf.

That refers to a Minkowski-diagram, where coordinate time is on the time axis.

In this applet you can compare the two diagrams:
http://www.adamtoons.de/physics/twins.swf

See this thread on the same topic:
https://www.physicsforums.com/showthread.php?t=614282

 

[Naty1]

Hey Chris...
that's a great book! with what i found to be some nice conceptual analogies to get one started with the ideas of relativity. Those confounded time slices were, for me, however, more trouble than they were worth. I must have read that part three or four times on different days and decided to give up... but the rest of his ideas, like figure 3.7 [paths through spacetime] were ones I still use in thinking about relativity.

Some of his ideas are disliked in these forums...If something bothers you with his explanations I found checking in Wikipedia sometimes provides a different perspective that can be helpful trying to figure out Greene's view...

He says that a photon, moving through space at light-speed, will experience no time.

"Eternity is no time at all for a photon" is a nice quote I found to think about that...

It's also helpful to remember that in many frames, YOU are right now moving at near light-speed...and 'SOMEBODY' thinks you are really,really 'time dilated' for example...But I don't think that, I'm just sitting here 'stationary' typing...

 

[Chestermiller]

Dale Spam's comment is right on target. Nowhere in Brian Greene's development does he mention that he is not looking at the actual universe, but at a 4D Euclidean analog of the real universe. His discussion would be right on target if the actual universe were Euclidean, but it is not. Still, it is sometimes helpful to study Greene's Euclidean analog universe because it exhibits many of the same phenomena as actual spacetime: relativity of simultaneity, length expansion (rather than contraction), time contraction (rather than dilation), a 4-velocity, and many other interesting features of behavior analogous to the real universe. I have found it very helpful in studying accelerated frames of reference, for example.

Chet

 

[bobc2]

I'm getting older and dumber, it seems.

In his book "The Fabric of the Cosmos", from page 49, Green points out some basic vector thinking. Given a fixed speed over the ground, as you head more West of North, your speed North decreases, whilst your speed West increases. He suggests that this is analogous to our movement through space-time. As our speed through space increases, our speed through time slows down - i.e. time slows down.

From page 53, Greene develops the model of space-time being a block of slices of 2D space. In this model, the third dimension is time, the two dimensions of the page are space, and the entity as a whole is space-time.

Here's my problem, and it hits before we even get into the interesting stuff. He says that a photon, moving through space at light-speed, will experience no time. This would be analogous to moving across one of his bread slices without moving in the time dimension. However, in his end note 9, he says that something moving at light speed sweeps out a line at 45 degrees to the crust-to-crust axis of the loaf. So on the one hand, the angle is 90 degrees and on the other it's 45 degrees.

Where have I gone wrong?

Please be gentle )

Hi ChrisXenon, Greene's illustration is actually quite good and very consistent with special relativity. And remember that the slices at the various angles represent the cross-section views of the universe for the observer in motion at various velocities. So, remember that this means that as the moving guy's time axis rotates clockwise with increasing speeds from the long axis of the loaf (rotation with respect to rest time axis), the spatial axis (X1-axis of the moving observer) is rotating counter clockwise, such that the world line of a photon would always bisect the angle between X4 and X1. Hopefully, the sketch below illustrates this.

Your problem with Greene's description is assuming he meant that the moving observer's time axis would rotate all of the way through 90-degrees to become colinear with the at-rest X-axis. You can see in the sketch below a sequence of diagrams where the moving observer moves faster and faster in the sequence. If the moving guy reaches the speed of light his time axis would have to become colinear with his X axis. I'm afraid I can't give you any ideas about what goes on at that point (others here will explain why that would not happen). So, his time axis needs to rotate just 45-degrees, not 90-degrees.

Notice that by the photon worldline always bisecting the angle between t and x, all observers measure the ratio, dX/d(ct), to equal 1.0, i.e., speed of light is the same for all observers, not matter their speed.

 

[ChrisXenon]

Thanks people, for all your helpful comments.

DaleSpam (err, why??), do analogies have to be imperfect? Surely, it's not an analogy's imperfections which label it an analogy - but it's similarities to that which is an analog of.

Nevertheless, knowing that the analogy is imperfect and that I can look there for my failure to understand Greene's analogy is something of a relief. It does seem astonishing that he would say two mutually contradictory things about an angle in the same section of the book and not mention the problem though.

I may have to lever myself up of my forelimbs and waddle into Mincowski's world. Or I may accept my own ignorance and return to the thorny topic of earning a living... A.T. Greene made no mention of this "proper time" malarkey. I want a refund... The applets are very ... colourful...

Naty1, well, on page 56 I find he did the dirty on me. He done lied! So I hate him, but I'll try to man up and press on.
I do find his talk of Itchy, Scratchy et al an annoying distraction. I ahve a 3K brain to work with here, and he's filled 1K of it with cartoon characters. Those other frames you mention...you may think you're sitting there stationary, typing, but - pah - ask your wrist, buddy. You was rattlin' round like some mad man with dyspensia.

Chestermiller, I've always understood that these analogies drop dimensions in order to illustarte points, but I've never heard before that they also use THE WRONG GEOMETRY. This is new to me. I may look into this further if I can find an appropriate vvector.

bobc2. You say Greene's analogy is quite good, but for me, if it contradicts itself on its own terms, without apologising, then it's always going to be, well, rude and annoying. As for your diagrams. No, sorry - too many lines... I always thought I was smart. Thanks you so much for ermoving that amusing little notion.

Hehe. But seriously, I appreciate your efforts folks. I'm thinking next stop - cross stitch...

 

[A.T.]

A.T. Greene made no mention of this "proper time" malarkey. I want a refund... .

If he talks about fixed speed in space time where only direction changes, and relates the movement along the time dimension to individual aging, then he must be thinking in terms of a space-propertime diagram. The rate of advance in space-propertime is the same for all objects and the Euclidean distance corresponds to coordinate time. Here is another applet using this diagram:
http://www.adamtoons.de/physics/relativity.swf

 

[Dale]

If he talks about fixed speed in space time where only direction changes, and relates the movement along the time dimension to individual aging, then he must be thinking in terms of a space-propertime diagram.

It is hard to tell with Greene, but I think he is just talking about a spacetime diagram, not a space proper time diagram. The "fixed speed" in spacetime is the unit norm of the tangent vector, the four-velocity.

 

[Dale]

DaleSpam (err, why??), do analogies have to be imperfect? Surely, it's not an analogy's imperfections which label it an analogy - but it's similarities to that which is an analog of.

When we make an analogy we are saying that this new unfamiliar thing X has some of the same features of this different familiar thing Y. If X has some features that Y does not have (or vice versa), then the analogy is imperfect. If it does not, then X is Y, and it isn't an analogy, they are the same thing. Therefore, all analogies are imperfect.

Nevertheless, knowing that the analogy is imperfect and that I can look there for my failure to understand Greene's analogy is something of a relief. It does seem astonishing that he would say two mutually contradictory things about an angle in the same section of the book and not mention the problem though.

Out of all of the pop-sci authors, Brian Greene causes more confusion and questions here than all of the others put together. This analogy, in particular, is a common problem, so you are not alone in this.

 

[Naty1]

Naty1, well, on page 56 I find he did the dirty on me. He done lied! So I hate him, but I'll try to man up and press on.

What did you find on page 56? can you quote a few lines of the offense??

My book is numbered differently than yours...

I do find his talk of Itchy, Scratchy et al an annoying distraction.

Even Leonard Susskind in his Youtube videos resorts to 'Alice and Bob' observers...
didn't help me grasp much of anything any better.

 

[bobc2]

bobc2. You say Greene's analogy is quite good, but for me, if it contradicts itself on its own terms, without apologising, then it's always going to be, well, rude and annoying. As for your diagrams. No, sorry - too many lines... I always thought I was smart. Thanks you so much for ermoving that amusing little notion.

You seem like a pretty smart guy. Greene is definitely using the 4-dimensional Minkowski space-time model. If you really have a burning desire to understand his loaf of bread, google "space-time diagram" and "block universe."

 

[PeterDonis]

Greene is definitely using the 4-dimensional Minkowski space-time model.

That model underlies what he is saying, yes, but he also mixes in concepts from his space-proper time model. Mathematically, the two models are equivalent, but conceptually Greene's way of putting things seems to lead to a lot of misinterpretation.

I don't know why Greene insists on putting things the way he does, of course, but I wonder if it's because he doesn't actually expect his audience to do any interpretation at all; he just expects the audience to nod their heads and say "wow, neat!" and look no further. So when people actually try to reason things out further from what he's saying, naturally they have problems; he wasn't intending for anyone to reason things out further from what he's saying.

Ok, I'll stop ranting now.

 

[nitsuj]

I don't know why Greene insists on putting things the way he does, of course, but I wonder if it's because he doesn't actually expect his audience to do any interpretation at all; he just expects the audience to nod their heads and say "wow, neat!" and look no further. So when people actually try to reason things out further from what he's saying, naturally they have problems; he wasn't intending for anyone to reason things out further from what he's saying.

Ok, I'll stop ranting now.

Ah so a physics bible of sorts, perhaps his book would be better placed in Religion or Art & Poetry; as opposed to the physics section.

 

[Naty1]

Complaining about a few of Greene's descriptions seems to me like ranting about the 'balloon analogy' in cosmology...even if some people draw incorrect conclusions, seems to me a much larger group is intrigued and undertakes additional study...

 

[PeterDonis]

Complaining about a few of Greene's descriptions seems to me like ranting about the 'balloon analogy' in cosmology...even if some people draw incorrect conclusions, seems to me a much larger group is intrigued and undertakes additional study...

I think the problem with Greene's descriptions is the relative size of the two groups--many more people seem to draw incorrect conclusions.

 

[Alain2.7183]

Out of all of the pop-sci authors, Brian Greene causes more confusion and questions here than all of the others put together.

The simultaneous time slices used by Brian Greene are the same as the lines of simultaneity that Taylor and Wheeler use in their Example 49 (pages 94 and 95) of their "Spacetime Physics" book.

 

[Dale]

The simultaneous time slices used by Brian Greene are the same as the lines of simultaneity that Taylor and Wheeler use in their Example 49 (pages 94 and 95) of their "Spacetime Physics" book.

Sure. But that doesn't change the fact that Greene's explanation is the one that causes problems. I don't know if it is primarily due to his wording or his popularity or what, but Greene's explanations bring decidedly more confused people here than anyone else.

 

[WannabeNewton]

You can only sacrifice so much mathematics for "intuition" in an exposition of a complicated physical theory before it starts confusing the hell out of people. It's like trying to use diagrams and "real - world" physical examples to explain the relationship between C* - algebras and physical systems in QM.

 

[bobc2]

The simultaneous time slices used by Brian Greene are the same as the lines of simultaneity that Taylor and Wheeler use in their Example 49 (pages 94 and 95) of their "Spacetime Physics" book.

Good point, Alain2.7183. Brian Greene tries to get across one of the most profound aspects of special relativity, cutting the loaf at different angles to illustrate the notion of different cross-section views of a 4-dimensional universe--a discussion very tough to take on with those non-math and non-science folks with curiosity about special relativity. I thought it was a jolly good effort.

 

[pervect]

I'm not terriby fond of some of other of Greene's efforts, but I applaud his "loaf" analogy.

I often talk about "cutting" space-time into space+time, and I believe I recall MTW using the word "cutting" to describe this process as well.

 

[robphy]

I remember in high-school when I was frustrated by reading pop books on relativity... to the point where I decided I wanted to do physics so I can understand better what is really going on.

 

[ChrisXenon]

It's getting worse. Greene's chapter on entropy is thoroughly confusing. Firstly, as before, he's cluttering the place up with spurious anaolgies. OK, illustrate entropy with the idea of a breaking egg or of CO2 escaping Coke bottle or with [whatever the third one was] - but not all three every single time it's analogy time - it just gives you more crud to load into your brain which serves no useful purpose. I think, as PeterDonis says, Greene is aiming at wowing the masses, rather than imparting real understanding to people. It may be that real understanding is simply not possible this side of a Maths doctorate, in which case - he should say so; better that than m banging my head on his text trying to make it understandable.

Aside from that - entropy is so depressing...

On a more serious note, I simply don't understand the part where he says entropy OUGHT to get higher back in time just as it does as you go forwards in time. Why? His explanation seems to be to be a non-sequiter - but of course, he's a world-famous physicist, and I'm an idiot, so the fault is likely to be be in my head. He goes on to disprove it by another route, so the whole discourse turns out to be a dead end, but it would still be nice if it made sense.

And actually, this takes me back to Maths at school. I did OK because I could follow the rules but I never really understood it at a deep level. Now I teach Maths privately, and I DO understand it at a deeper level and I realize that my teachers probably never did.

I'm seeing now how I will probably not finish this book now.

 

[DarkMatterHol]

On a more serious note, I simply don't understand the part where he says entropy OUGHT to get higher back in time just as it does as you go forwards in time. Why?

Hi ChrisXenon. Maybe what he is getting at is that 'time' is reversible in many scenarios, and entropy aught to get higher regardless of the 'so called' direction of time.

I'D still like to know, in Greene's 'loaf' analogy, what we see when we look at people on planets in distant galaxies with a telescope, when we switch directions on our unicycle, and why we would not see planets reversing their orbits in far away galaxies, when we switch from seeing them in the future to seeing them in the past. The last time i inquired about that i was informed that Greene's analogy was completely wrong. That was a year ago.

 

[Alain2.7183]

I'D still like to know, in Greene's 'loaf' analogy, what we see when we look at people on planets in distant galaxies with a telescope, when we switch directions on our unicycle, and why we would not see planets reversing their orbits in far away galaxies, when we switch from seeing them in the future to seeing them in the past.

In your telescope, you won't see time at the distant location running backwards. But when you properly allow for the transit time of the image, you will conclude that the current time at the distant location is rapidly moving backward and forward as you ride your bike around in a circle.

 

[Dale]

In your telescope, you won't see time at the distant location running backwards.

I agree.

But when you properly allow for the transit time of the image, you will conclude that the current time at the distant location is rapidly moving backward and forward as you ride your bike around in a circle.

That depends on what you mean by "properly allow". One method will produce the results you suggest, but there are other equally "proper" methods which do not have that feature. E.g. http://arxiv.org/abs/gr-qc/0104077

 

[Alain2.7183]

That depends on what you mean by "properly allow". One method will produce the results you suggest, but there are other equally "proper" methods which do not have that feature.

When Brian Greene gave that result in his NOVA TV series on "The Fabric of the Cosmos", and also in his book of the same title, he didn't present it as "one possible result among many" ... he didn't seem to imply any ambiguity or arbitrariness in the result at all.

 

[Dale]

When Brian Greene gave that result in his NOVA TV series on "The Fabric of the Cosmos", and also in his book of the same title, he didn't present it as "one possible result among many" ... he didn't seem to imply any ambiguity or arbitrariness in the result at all.

Which is one reason that Brian Greene's non-peer-reviewed pop-sci material is not considered authoritative here on PF. His material is, bar none, the largest source of confusion here compared to any other single author.

 

[UberGoober]

I'm glad someone brought this up

Greene's loaf analogy has stirred in the back of my mind ever since I read the book years ago.

If I recall correctly, there was a physical consequence of these skewed world lines - basically that the past & future physically exist simultaneously and for some reason we just experience time linearly.

Did the book say this, or did I fall asleep and dream that part? I've tried asking here before about the nature of the time dimension and got conflicting responses. Some will say its a physical dimension, others will say a mathematical one only.

 

[nitsuj]

What do you mean by "say this"? as straight forward as your wording; No. There wouldn't be confusion with wording that clear.

Did he "poetically" blur the distinction between your wording of a "block" concept and that of spacetime; yes.

 

[Naty1]

I simply don't understand the part where he says entropy OUGHT to get higher back in time just as it does as you go forwards in time. Why?

Maybe what he is getting at is that 'time' is reversible in many scenarios, and entropy aught to get higher regardless of the 'so called' direction of time.

that's just what Greene describes ...in my book, page 160:

Since Newtons laws ..have no built in temporal orientation...the reasoning we used to argue that systems will evolve from lower to higher entropy toward the future works equally well when applied toward the past...not only is there an overwhelming probability that the entropy of a physical system will be higher in what we call the future, but there is the same overwhelming probability that it was higher in what we call the past.

Page 161:

...entropic reasoning yields accurate and sensible conclusions when applied...to the future, but gives apparently inaccurate and seemingly ridiculous conclusions...when applied to the past...we will shortly find a way out of this strange place...entropic reasoning has taken us...

In my opinion, he spends far too many words on this incorrect view...I see I even have margin notes written in my book to remind myself of some of the incorrect conclusions based on the above reasoning...which he corrects later...


I wish he would have mentioned more modern ideas...

We discussed one set here:
emergent gravity
https://www.physicsforums.com/showthread.php?t=618397&page=2

involving:
Thermodynamics of Spacetime:
The Einstein Equation of State
Ted Jacobson, 1995

http://arxiv.org/abs/gr-qc/9504004


Ted Jacobsen, for example, has these insights:

The origin of large entropy is the vacuum fluctuations of quantum fields. ...and he points out that classical GR 'knows' that causal horizon area would turn out to be a form of entropy...and that entropy is proportional to causal horizon area...which increases over time...

 

[dubiousraves]

Greene on Spacetime

Greene explains time dilation using his analogy about a car having less eastward motion because it heads more northward. So, he says, we are all moving at the speed of light through spacetime. If we thus increase our speed through space, we have less speed for time, so time slows down. All good so far. But doesn't this assume we are naturally at rest moving 0 speed through space? I.e., could we move slower through space, and thus move faster through time? Why not? Also, why can we only control our motion through through the space half of spacetime, only affecting time corollarily?

 

[Naty1]

All good so far. But doesn't this assume we are naturally at rest moving 0 speed through space? I.e., could we move slower through space, and thus move faster through time? Why not? Also, why can we only control our motion through through the space half of spacetime, only affecting time corollarily?

It does not appear anyone or anything can 'move slower through space' than sitting still.
You are asking more than science can explain. We don't know a lot...like the origin of space and time, the mass of the electron, and why there is gravity.

All we can say is that so far in this universe, the speed of light seems to be the maximum velocity that information can be communicated...and no mass can travel quite that fast.

 

[Haelfix]

In my opinion, he spends far too many words on this incorrect view...I see I even have margin notes written in my book to remind myself of some of the incorrect conclusions based on the above reasoning...which he corrects later...

This is not an incorrect statement, just incomplete. In classical physics, and in quantum mechanics, one can show that entropy increases in either direction in time. The operation of sending t--> - t does not reverse the H theorem's arrow.

The reason there is a distinction between the past and the future is understood to be simply a statement about asymmetric boundary conditions between the past and the future. You simply have to postulate a low entropy initial condition in order to have any distinguishable directionality.

 

[Chestermiller]

For conceptualization purposes, some of us like to visualize space-time as a stationary absolute entity which replaces absolute space as it is described by the pre-relativistic Newtonian model. Within this framework, the 4 velocity of a particle represents its velocity relative to stationary space-time, and is equal to the speed of light when reckoned from the particle's rest frame of reference. But, as reckoned from a different inertial frame of reference, the particle has components both in the time direction, and in the spatial directions. The component in the time direction is γc. Unlike Euclidean 4D space where the Pythagorean theorem is satisfied, space-time is non-Euclidean and features a metric in which the square of the differential time component is opposite in sign to the squares of the differential spatial components. This is why, from the standpoint of an observer in another inertial frame of reference, the component of 4 velocity in the time direction (γc) is greater than the speed of light. Of course, this component can't be measured directly since, as mere 3D beings, we suffer from the inherent physical limitation of not being able to see into our own time dimension. I hope this visualization works for some of the PF readers that are struggling with these concepts.

Chet