Talk:Eternalism (philosophy of time)

Regarding this statement:

For example, relativity has shown that the concept of simultaneity is not universal, with different frames of reference having different perceptions of which events are in the future and which are in the past; there is no way to definitively identify a particlar point in univeral time as "the present".

As a layman, I'm having trouble getting this, and the entry on relativity isn't helping any. I can see how frame of reference can influence how I perceive the passage of time and how far in the past something happened, but I don't see how it affects my perception of which events are in the past. The article goes on to mention that people aren't able to observe future events, so there seems to be a contradiction here.

I've read nothing on the topic besides this article, and some introductions to relativity many years ago, and watching Cosmos.. so.. grain of salt :) --mjb

Simultenaity makes my head hurt too, when I pay attention to the details. :) Basically, it is my understanding that if there are two events "A" and "B" that occur some distance apart, there are some frames of reference in which an observer will see that event A happened before event B and some frames of reference in which an observer will see that event B happened before event A. The observer isn't observing anything in his own future, just observing two distant events that happened in his past and determining which one happened first. Since no frame of reference is more important than any other frame of reference, this isn't just an optical illusion.

I can't say why this works the way it does, offhand, I'd have to read up on it some more before I'd feel confident about writing anything over at special relativity. It does need to be explained more clearly, though, so if nobody with more physics knowlege than I gets to it before me I'll see if I can write something that I can understand. :) Bryan

Your confusion is understandable as it doesn't confirm to our common sense understanding of the world around us. The light clock thought experiment is an excellent way to refine your understanding of it. So here goes:

Empirical evidence supports the proposition that a given lightwave / photon particle will travel at a constant velocity irrespective of your interial reference frame; that means if you're going at c/2 (c equals the speed of light in a vacuum) relative to another frame of reference in the same direction as the lightwave, the lightwave will still be measured as going at c by parties in both reference frames.

Let's put a light clock in the frame moving at c/2. What happens when someone in the other reference frame looks at the light clock? He sees a ball of light moving along a diagonal path bouncing up and down between the two mirrors of the light clock. What does the person in the clock's reference frame see? He sees a ball of light moving straight up and down between the two mirrors. The ball of light that moved along the diagonal path had to travel further, and given that additive velocities only works in Newtonian mechanics, i.e. the ball of light is moving at the same speed relative to parties in both reference frames, that means the ball of light will take longer to bounce between the two mirrors for observers in the outside reference frame of the clock than observers inside the reference frame.

So time as measured by this light clock is moving slower in that reference frame relative to the other reference frame. (If parties in both reference frames had light clocks, both parties would see the other as moving slower. But this all gets reconciled, although it's a little tricky and beyond the scope of this reply.)

For a more tangible example of different parties seeing a different order of events happen, let me go over a train example:

A train is moving along in direction x. There are two laser sensors situated at opposite ends in one of its compartments. At the center of the compartment there is a laser that will fire beams at both sensors when turned on. When the sensors detect a laser beam, they light up. What happens when a person on the train in the compartment watches this process unfold? Both sensors light up at the same time. But what happens when someone on the ground, looking at the train go by, watches this process unfold? Since the train is moving in direction x, the laser beam moving in direction x towards the sensor nearer the front of the train must traverse a longer distance to reach the sensor than the laser beam that is moving towards the sensor at the back of the train. In Newtoniam mechanics, both observers would see the same thing as additive/substractive velocities would add to the velocity of the laser moving in the same direction as the train is and substract from the other laser, but we don't apparently live in a Newtonian world.

All sorts of interesting thought experiments can be done with this. What happens if the spaceship with the light clock moved at c? The ball of light in the light clock would use up all its velocity just to keep up and time would be completely still. And since we could use the light clock as a way to measure distance, space / distance itself is just as relativistic as time. If you were in a ship traveling at c, the whole Universe would be the size of a singularity.

Not only light travels at the speed of light; *everything* does. That is, everything moves through space-time at the speed of light. That means, the faster you move through space the slower you must move through time.

Can someone please provide links to validate this. I've never heard this before, and I find it interesting. -- Olathe November 17, 2003

That is the implication of special relativity, which follows from the finite speed of light. However, that is all relative to a detector, and assumes points in spacetime do not have a timestamp. The temperature of the microwave background seems to provide such an absolute timestamp, but I have seen no discussion of that. I guess the accelerating detector sees the universe expanding faster. Does that have any implications for our observations of the Hubble "constant"? ----

Anyway, hope that clarified some things for you. Relativity is fairly logical once you accept the premise of the constancy of the speed of light. It's light reading compared to quantum mechanics! No one understands it! :) --Spinoza