He said what?! Rebelscience.org

drachefly · Post by **drachefly** » 2006-02-13 01:32pm

Darth Wong wrote:It's kind of funny how he dismisses evidence for time dilation by arguing that all of the systems of particles are merely acting as if there is time dilation.

He doesn't say that time dilation is fake; merely that it is not the best way of thinking about what's going on. I mean, look at what I recently pointed out with proper time being what's left over of time after gross motions are done with.
If somethings internal motions are what allow it to perceive time (and I think we'll all agree that they are), then coherent motion to one side will slow down the internal motions, analogously to how the current will slow down one's rowing across a river. So without considering multiple reference frames, it is possible to re-derive time dilation! It works as a stretching of processes without needing to invoke relativity as a principle.

In other words, I think what he's getting at is that people are too enamoured with relativity as a principle rather than as an effect. When we look at it as an effect, time travel sounds dumb. When we look at it as a principle, it sounds like it's just more weirdness of the theory.

Now, given how the most successful theories we have are based on it as a principle, I can't say I agree completely; but it's not a crazy PoV, and to some extent I agree with it.

drachefly · Post by **drachefly** » 2006-02-13 01:33pm

(in the rowing analogy above, I meant that the rower is rowing against the river, trying to keep from being pulled downstream)

Kuroneko · Post by **Kuroneko** » 2006-02-13 02:29pm

Drachefly, it's one thing to say his philosophical concerns are compelling. I don't disagree that they are. However, the real issue is that a large fraction of his concerns are actually supported by relativity, not in contradiction with it. For example, he doesn't like simply describing motion and wants, like Zeno, to have a physical distinction between stationary and moving but otherwise identical objects. Spacetime does that directly. He rails against Newtonian absolute space and time and backs Leibniz's relational approach instead. That is very sensible--in Leibniz's words, the universe wouldn't be absolute space). Fine, relativity not only does that but in fact forces it--the metric does not uniquely determine a spacetime, so that there must be an equivalence class of metrics which describe the same physics from a different perspective.

drachefly · Post by **drachefly** » 2006-02-13 05:05pm

If you clarify what you mean by Leibnitz's approach, that would help; I know what Newton said, but there are all sorts of ways one could deviate.

He seems to be proposing that the universe has its current state, and the past and future are not a part of the universe. Thus time-travel and affiliated subjects are bogus, since the past doesn't exist anymore.
(note: field theories, which are normally defined over all time, still work if you allow the present state of the universe to include the first derivative of all the fields, and consider it to be a history of the field rather than the field itself in 1+3 dimensions)

Now, relativity has some nontrivial things to say about simultaneity, but all that means is we can't figure out which frame is 'real'.

This may sound like needless multiplication of entities, but he considers it not to be any more of one than gauge freedom is.

I consider all of this so far to be something which kind of needs to be said. Yes, relativity is great, I use it all the time; but that doesn't mean that it should be forgotten that there are multiple ways of getting there.

Then he gets to GR, and I'm left wondering what the heck he's trying to say.

Zero · Post by **Zero** » 2006-02-13 05:58pm

lsavain@rebelscience.org

Anyone want to e-mail this guy, and set him straight? I imagine several of you already have.

Kuroneko · Post by **Kuroneko** » 2006-02-13 07:30pm

drachefly wrote:If you clarify what you mean by Leibnitz's approach, that would help; I know what Newton said, but there are all sorts of ways one could deviate.

I intended to post Lebniz's critique of "how would the universe be different if East and West directions were switched?" For some reason, that part of my post disappeared mid-sentence. For Newton, there would indeed be a difference, since there is such a thing as absolute space and time [1]. Under Leibniz's relational view, space is an abstraction secondary to the relations between objects. In other words, there would be no difference whatever if the entire universe was mirrored exactly, since the relational properties of objects would be unchanged. My observation is simply that general relativity deals with such philosophical issues fairly directly. Two different solutions may in fact represent the same physics, as long as they are related by a diffeomorphism--a straightforward implication of general covariance. Not only that, but the spacetime metric is completely inseparable from either the manifold itself or its energy-momentum content.

[1] One might note that Newton might have actually changed his mind somewhere in the middle of writing the Principia. He started out much more sympathetic to Galileo's principle of relativity. However, he did have the water-buchet argument done precisely to argue the need for absolute space.

drachefly wrote:He seems to be proposing that the universe has its current state, and the past and future are not a part of the universe. Thus time-travel and affiliated subjects are bogus, since the past doesn't exist anymore.
(note: field theories, which are normally defined over all time, still work if you allow the present state of the universe to include the first derivative of all the fields, and consider it to be a history of the field rather than the field itself in 1+3 dimensions).

Can't be true in general. What happens if the spacetime admits no Cauchy surface at all?

drachefly · Post by **drachefly** » 2006-02-14 11:54am

It is not an unreasonable suppostion that space-time might admit a Cauchy surface (if that means what I think it does). I think that is the only physical prediction that what he says contains.

Kuroneko · Post by **Kuroneko** » 2006-02-14 09:22pm

Well, I agree that if he ever looked into gtr seriously, he may hold that the universe admits a Cauchy surface, but I don't see any reason for it. For example, the anti-de Sitter spacetime (negative-curvature vacuum) seems to me to be quite reasonable, in the sense that I don't see any other a priori reason to dismiss it, and yet it does not have such a surface. Among other examples, perhaps somewhat less compelling ones, are Reissner-Nordström black holes, gravitational plane waves, and Taub-NUT space.

drachefly · Post by **drachefly** » 2006-02-15 11:17am

I don't mean to give him the credit for carefully examining GR; but it is painfully obvious that at least one valid spacetime is the fairly straightforward one in which time keeps going forward and space keeps going off to the sides.

In any case, all I meant was, no one apparently valid point of view should hegemonically dominate over the other apparently valid points of view, as each may suggest distinct useful research directions that others do not. Presently, the word we're getting from the cosmologists is all about weird-ass shit the universe could be pulling; but it should be remembered that we haven't ruled out the universe doing the perfectly prosaic instead.