Special Relativity Question ;)

September 21, 2009 10:43 PM

This one's for us physics nerds. - Is the speed of light our universe's only true constant? And does this explain why time dilation occurs most for extremely massive objects? - Does time dilation only occur when an observer is present?
posted by jfid1 to science & nature (14 comments total)

The chandrasekhar limit could be considered a constant, no?

As for observers- does anything occurr when nobody is there to observe it? Does the concept of "Occurring" even make sense when thinking of spacetime?
posted by TravellingDen at 10:50 PM on September 21

Can you clarify what you mean by a "true constant" as opposed to regular old constants?

What is the relation between this and time dilation?

Time dilation does not require an observer.
posted by ssg at 11:00 PM on September 21

No, there are more indipendent constants, see John Baez' essay about this. Consequently the answer for your second question is no. The third one cannot be answered by science, at least not as I understand science.
posted by dhoe at 11:06 PM on September 21 [2 favorites]

"observer" effects are a quantum mechanics thing, and typically refer to the presence of other particles to interact with, not to an 'observer' in the person sense. If you hit it with a beam of light, it interacts with those photons, etc. With relativity problems we talk about "observers" but that's just shorthand for "reference frames". "Traveling at .8c" doesn't mean anything - speed is relative. "Traveling at .8c relative to some other point" can be said as "Traveling at .8c relative to an observer", etc. The 'little person' icon is useful in diagrams for "this is the reference frame we're operating from".

Time dilation mostly can be derived from "the speed of light is constant in all reference frames" - it doesn't matter how fast I'm going, how fast you're going, you'll see the light traveling at c and I'll see it traveling at c, even though that isn't true if I throw a ball or anything other than light. From that you can do 'light clock' thought experiments to get time dilation, which IS subjective - I'll think your clock is slower and you'll think mine is slower. Conservation of momentum in both reference frames gives you relativistic mass, which I think is what you're referencing with the mass comment; things moving close to the speed of light relative to an observer will seem to be more massive to that observer. So the "effective mass" increases. But mass does not have any effect on time dilation.
posted by Lady Li at 11:20 PM on September 21

"Is the speed of light our universe's only true constant?"

No. There's also Planck's constant, the universal electrical constant, the universal gravitational constant, equivalent constants relating to the weak and strong forces, and I'm pretty sure that's not all.

"And does this explain why time dilation occurs most for extremely massive objects?"

Your question assumes a fact not in evidence. Time dilation takes place equally for all objects solely as a function of relative velocity, irrespective of their mass. "Extremely massive objects" are not special in any regard.

"Does time dilation only occur when an observer is present?"

I rue the day that some physicist started using the word "observer", because it has mislead millions of laymen into thinking that an "observer" is an living intelligence. In reality it has nothing whatever to do with Bishop Berkeley.

In quantum theory, an "observer" is anything that can cause a wave function to collapse. That's all it means. It has nothing whatever to do with intelligence.

In relativity, "observers" have no effect at all. The predicted relativistic behaviors take place whether they're observed or not.
posted by Chocolate Pickle at 11:37 PM on September 21 [3 favorites]

The Chandrasekhar limit is emergent, not fundamental.
posted by Chocolate Pickle at 11:39 PM on September 21 [1 favorite]

If you have two electrons sitting still, they repel one another - no field.

If you have two electrons moving in parallel they product a field.

If the two electrons and I are all at rest relative to one another, I see a repulsion, no field. If you go zipping past, you measure a field, but see no repulsion. That's Einstein's WTF moment in a nut shell.



If I'm in a lab doing the Schroedinger's cat thing, and you're waiting in the hall, what's in the lab once I open the box? Half of me finding a dead cat and half of me finding a live cat. I'lll collapse into one or the other state when you stick your head in to see how it went.

So much for being a special snowflake.
posted by Kid Charlemagne at 12:25 AM on September 22 [2 favorites]

Are pi and e not constants too?
posted by nonspecialist at 4:31 AM on September 22

Are pi and e not constants too?

Well if you include mathematical (dimensionless) constants, then yes, but you don't need to get that fancy. 0, 1, and 9.43 are all constants as well.
posted by DevilsAdvocate at 4:57 AM on September 22 [2 favorites]

Surely the gravitational constant, G, is a "true" constant too?

DevilsAdvocate: "Are pi and e not constants too?

Well if you include mathematical (dimensionless) constants, then yes, but you don't need to get that fancy. 0, 1, and 9.43 are all constants as well."

There are physical constants that are dimensionless too - e.g. α, the fine-structure constant and various ratios.
posted by turkeyphant at 5:11 AM on September 22

The speed of light isn't a constant, only the speed of light in a vacuum is. You can make light go real slow with the right gear.
posted by jenkinsEar at 6:53 AM on September 22

jenkinsEar, thanks for pointing out what is painfully obvious to the physics-educated here, but not necessarily the rest of us. Important distinction.

I would argue that all of these physical constants are invariant insofar as we currently understand the universe, but any of them are subject to possible variation as our knowledge increases.

Theories are constantly being proposed that subject one or more of these constants to some level of variation (moments after the Big Bang, for instance, if not since...), but so far none of these theories have gained widespread acceptance. Certainly none of them have been tangibly demonstrated - which of course is pretty difficult for some situations (like the Big Bang-vicinity phenomena).
posted by IAmBroom at 7:59 AM on September 22

Yeah there's a whole bunch as has already been said. However, it turns out to be a bit of a pain to figure out if the speed of light really is a constant - when you try to make measurements of it you end up having to make measurements that are a bit less direct and are mixed up with other things. For example, you might not be measuring c at some other time, but hc, and then you're not sure if it's the h that changed or the c, or some combination of the two.
It turns out to be much easier to measure dimensionless physical constants though, which is why much of the research effort in this sort of direction looks at the variation of the fine structure constant (new paper I saw this morning on that actually...).
Incidentally, constraints on the variation of that constant are extremely tight, and have been for quite a while. If it changes over the lifetime of the universe it doesn't change much at all.
posted by edd at 8:31 AM on September 22

If the two electrons and I are all at rest relative to one another, I see a repulsion, no field. If you go zipping past, you measure a field, but see no repulsion. That's Einstein's WTF moment in a nut shell.

This is a famous problem that is interesting enough to state correctly.

Electrons at rest tend to move apart due to electric repulsion. But parallel currents produce a magnetic field which causes them to move closer together. Suppose I drive my car past two electrons at rest. I see them as currents approaching me; do they come closer together or move apart?

The right way to ask the question is: at what speed does the magnetic attraction exactly balance the electrical repulsion? The threshold speed turns out to be -- oh, don't peek.

Deciding what might be a fundamental constant and whether you can actually measure it is an interesting exercise. Baez's essay linked above is a good place to start
posted by fantabulous timewaster at 10:12 AM on September 22