# Time dilation and relative speedJuly 13, 2004 4:53 AM   Subscribe

Time dilation and relative speed. If the speed we travel is relative to the thing we're travelling away from, then how does the universe know which of the two things is actually moving? (so time dilation can occur) Is there a known universal "zero-speed"? If so, at what speed is the earth moving, and would it make a difference to the time dilation depending on the direction a spaceship moved away from the earth?
posted by seanyboy to Science & Nature (17 answers total) 1 user marked this as a favorite

It doesn't, there isn't, it's not, and it wouldn't. ;)
posted by LairBob at 5:44 AM on July 13, 2004

how does the universe know which of the two things is actually moving?

It doesn't. Each of the two observers knows that the other is moving, and a third observer might know that both of the first two are moving, but you can't "know" anything without observing it. And to observe something, you have to stand somewhere.
posted by gleuschk at 5:50 AM on July 13, 2004

Seriously, though, you're making a (very common and understandable) mistake in the way you've framed the question. The universe doesn't "know" anything--time dilation is an effect that you can only see when something is measured, and it just refers to the fact that, as the measurer, you'd get different measurements in different circumstances.

You're moving in a frame of reference on the Earth, so your time measurements are going to remain consistent with everything that's moving with you. If a spaceship were to travel near the speed of light, then any clocks or stopwatches travelling with it are going to remain consistent. There's no "master" time that the spaceship is judged by.
posted by LairBob at 5:50 AM on July 13, 2004

True, the key here is that the speed of light is constant and everything else is relative to it. If you were to measure the difference in speed between two laser beams travelling in opposite directions then the difference in speed is not twice the speed of light - it's the speed of light. In practice, when you approach the speed of light in a spaceship, time essentially slows down (when compared to something moving at a slower pace). Is there a known universal "zero-speed"? Not really, everything is in orbit around something, except for the direct centre of the universe, which is difficult to pinpoint. Confused yet? You will be.
posted by BigCalm at 5:57 AM on July 13, 2004

what everyone else has said, but the cosmic microwave background does define a "zero-speed" frame. however, this is more to do with cosmology/general relativity than special relativity and, as far as i can see, is only local (distant points each at rest wrt the local microwave background will be moving relative to each other unless the universe is stationary).

in other words, it's not connected to what you asked, but you might find it interesting, and i might be wrong.
posted by andrew cooke at 6:12 AM on July 13, 2004

I took a shower between writing this and posting, so everyone beat me to it, but I've got some links:

Back up a second. Relativity is *relative*, there is no universal objective viewpoint. That's the point of it. The universe doesn't need to know.
Experiments.
More.
posted by planetkyoto at 6:14 AM on July 13, 2004

posted by andrew cooke at 6:14 AM on July 13, 2004 [1 favorite]

Also, most problems in special relativity can be answered by remembering that *all* observers measure the speed of light to be the same. It doesn't matter how fast they're moving, or more precisely, which inertial reference frame they happen to currently reside in.
posted by bshort at 6:58 AM on July 13, 2004

IAMNAPh, but as far as I can tell, Time Dilation can only be measured against some other inertial frame of reference, i.e.: you can say "my clock is running X times faster/slower than his (some other frame of reference) one", but there is no absolute time dilation factor, no absolute time, either.
posted by signal at 7:57 AM on July 13, 2004

BigCalm, that doesn't sound right. If I were standing still and measured the speed of two light beams travelling past me in opposite directions, I would indeed see them moving apart at twice the speed of light.

If I were travelling along with one of those beams of light, though, then from that reference frame the other beam of light would be moving away from me at the speed of light, not twice the speed of light.

As for there being a zero speed, it's not because we can't pinpoint the center of our galaxy, or the center of the universe, or whatever. It's because speed, mass, etc. can only be measured with respect to a particular inertial frame of reference. Even if we knew the exact center of the universe, that would still not be a "better" reference frame than any other. As relativity goes, there are no absolute frames of reference.
posted by Khalad at 8:03 AM on July 13, 2004

Thanks all. The twins paradox was exactly what I was looking for. Now if I can only get my head around this idea of "Frames", then I'll have it.

posted by seanyboy at 8:10 AM on July 13, 2004

Actually, I think the twin paradox is perhaps what's causing some of seanyboy's confusion.

Time dilation is fairly nice and neat as long as both observers are in inertial reference frames.

Let's say A and B are moving relative to each other at near the speed of light--and both of them are in inertial reference frames (i.e., no acceleration). Both of them perceive that time dilation is occuring to the other person. A sees A's clock as running normally, and A sees B's clock as running slower than it should. Meanwhile, to B, B's clock runs normally, and A's clock appears to run slow.

The twin paradox only arises when one of the ship turns around and later comes back to the other--but for that to happen acceleration must be involved, so the ship which turns around is no longer in an inertial frame of reference, and the (relatively) simple rules of special relativity no longer apply. Without going into a lot of detail, although the universe doesn't "know" which of two things in different inertial reference frames is moving, it does "know" when acceleration is applied to something.
posted by DevilsAdvocate at 8:14 AM on July 13, 2004

The first point to understand is that acceleration in SR is a little tricky (it's actually handled better in Einstein's Theory of General Relativity - GR). I don't mean to say that SR can't handle acceleration, because it can.
So it's not that SR doesn't apply when acceleration occurs, but my point stands that things get more complicated when acceleration is involved.
posted by DevilsAdvocate at 8:26 AM on July 13, 2004

Here's another page containing more than you ever wanted to know about the Twin Paradox.
posted by Johnny Assay at 8:43 AM on July 13, 2004

i suggested the twins paradox because it's sufficiently meaty to make you think clearly about what's happening. for me, at least, drawing the world lines involved was what sparked my "aha" moment for sr.
posted by andrew cooke at 9:04 AM on July 13, 2004

Sorry; I didn't mean to suggest that trying to understand the twin paradox isn't worthwhile in this context; rather, that hearing about the twin paradox without fully understanding it might lead to exactly the sort of confusion seanyboy expressed in his original question. (i.e., how does the universe "know" which twin is supposed to be older, and which younger?)
posted by DevilsAdvocate at 10:25 AM on July 13, 2004

For a good overview of the history of theories of motion, I highly recommend Julian Barbour's The Discovery of Dynamics. It isn't light, but it's thorough, and its central topic is the absolute/relative debate that's really behind your question. Though it stops with the post-Newtonians and never gets to an exhaustive account of relativity, I found it extremely helpful as background information. Perhaps it's most useful as a model of how to think about these things.
posted by amery at 4:10 PM on July 13, 2004

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