Re: Time Machines

hansen (
Sun, 22 Aug 1999 22:08:45 -0700

On Sun, Aug 22, 1999 at 11:04:31AM -0400, John Clark wrote:
> In looking over the Extropian archives I noticed that the subject of time machines
> and faster than light travel came up on the list about a year and a half ago, I didn't
> have time
[.... snip]

> A less spooky example could be found in the idea of Phase Speed. I'm standing
> in the center of a huge hollow sphere 2 light years in diameter, I've been
> there for a long time and I'm holding a powerful LASER that makes a spot of
> light on the distant wall of the sphere one light year away. Suddenly, still
> holding the LASER and in the space of one second I make a complete 360
> degree turn. Exactly 2 years later an observer standing at the same place
> would see the spot move much faster than light, it would travel the entire
> circumference of the sphere, 2PI or 6.28 light years in only one second.
> No photon moved faster than light however, and no energy or information
> between any two points traveled faster than light. A photon of light moves
> at light speed and carries energy and information, a spot of light can move
> at any speed but carries neither energy nor information.

Nice example. To those who are confused and are inevitably going to ask for an explanation (I peeked into the future), try thinking about the path in time and space of the photons being spewed out from the laser. Think of them as rubber bullets that bounce off the inner surface of the sphere and into your eye.

I agree that phase speed (which is unobservable directly) can "travel" at >c, but I don't see what that has to do with the example. But it doesn't matter.

[... snip]

> nature is totalitarian, if it's not forbidden then it's mandatory.
> What about the logical paradoxes that would result from communicating with
> the past, wouldn't that be enough to rule out Tachyons? It would if anybody
> saw them, but suppose nature rubbed out any witnesses to her crime and
> brought a universe to an end that was about to see a paradox.
> Damn, I just knocked my coffee cup off the table, what a mess! I'm really not
> in the mood to clean it up, instead I'll use my Gateway 14,400 Tachyon modem
> and send myself some E mail 2 minutes ago. I'll just hit the send key and
> .....brought a universe to an end that was about to see a paradox. Pardon me,
> I just got some E mail from John, let's see what it says " Dear John: Be
> careful with that coffee cup near your elbow, you're about to knock it over."
> Wow, John is right, that cup is dangerously near the edge, I'll put it in a
> safe place. It was nice of John to warn me about it, it's too bad that means
> oblivion for him and his entire universe but that's life, nature just will
> not allow anybody to observe a paradox.
> I know what you're thinking, how could John be so stupid, he must be
> completely out of his mind, why else would he deliberately buy an obsolete
> 14,400 Tachyon modem? Well, call me cheap if you want but I still think the
> 28,800 model is too expensive, besides I have it on very good authority that
> Gateway will drop the price next year.
> John K Clark
[... snip]

Don't worry, it's worse; there are universes in which John is using an Amiga.

That is to say, in the many-worlds interpretation, the idea of time travel is non-paradoxical, since you really travel to an alternate universe what is similiar in most, but not all, aspects from the one you came from, and don't cause any paradoxes.

See _The Fabric of Reality_. David Deustch. the chapter "Time, the first quantum concept". He explains it better than I can. Or his Scientific American article. (I don't remember "when".)

[fyi, Gateway bought the rights to whatever is left of the Amiga technology...]

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