Why time dilation?

[Qfwfq]

I just want to understand why time dilation occurs - this is not a bad thing. Have any physicists attempted to answer this question?

Physics has no answer, at least not currently. Let a physicist tell you to consider it a metaphysical question, for the "time being".

:shrug:

[DrProctopus]

Well, ok...

 

But, it just seems like a big glaring hole.

 

Is there any reason why the idea that time is a component of spacetime, divisible into smallest possible units, would not work as an explanation? I discussed this with a guy who called himself an "information physicist" once, and he explained it to me in some detail - but I confess to not understanding much of what he said.

 

I mean - Einstein considered spacetime to be a real thing, not just an abstract concept, didn't he? If it is a real thing, and it is divisible...

 

And Stephen Hawking refers to matter and energy as information - if that is so, the information must be encoded into some sort of frame work...

 

So, why can't time be a component of that framework?

[Qfwfq]

:shrug:

 

But spacetime is a real thing, it isn't just an abstract concept...

[DrProctopus]

Well, that is what I always believed...

 

So, if spacetime is a real thing, is time a component of spacetime?

[Qfwfq]

is time a component of spacetime?

It is one of the four dimensions. There are timelike and spacelike directions. The directions we perceive as spatial directions are spacelike ones. A particle is described by a timelike curve through spacetime.

[DrProctopus]

So then...

 

given that there is such a thing as a smallest meaningful unit of space and a smallest meaningful unit of time (planck lenghth and time) - does this mean that spacetime is composed of discreet quanta?

[Perspicacious]

given that there is such a thing as a smallest meaningful unit of space and a smallest meaningful unit of time (planck lenghth and time)

The proposition is widely held but is meaningless. One has to be able to define what is meant by a spacetime geometry with those properties before you ask about implications.

[DrProctopus]

What I mean is this:

 

Spacetime is a framework within which matter and energy is defined.

 

Is this correct?

[Perspicacious]

What I mean is this: Spacetime is a framework within which matter and energy is defined. Is this correct?

Defining matter and energy is not in the purview of known physics. You need to ask easier questions.

[Bo]

as for the initial question: what is the physical mechanism that causes time dilation?

i think that the problem might be that this question as such is meaningless. The reason i say this is that the question sugegsts that time is something fixed for anyone, and then when there is a relative speed between two observers, there is something magical that causes time dilation. The problem with this view is that time is not something fixed and the should be seen as a dynamical concept. furthermore it should be seen not as something on it's own, but part of the larger (dynamical) structure of space-time. Now as for the question: what causes time dilation? the answer lies in the way this spacetime is structured. It has certain symmetries (so called isometries, spanned by the lorentz or poincare group) that determine in what way the perception of an observer of spacetime changes. So there is no physical mechanism; it is a property of spacetime itself. (for a deeper understanding it is probably necessary to truly understand what spacetime is, and as far as i know nobody has given a sensible answer to that one...)

 

Bo

[erKa]

The flat Minkowski space, of course, because the group preserves the Minkowski metric.

 

The geometry is defined by choosing a metric, afaik, and once a metric G is chosen we can write

 

T^ G T = G

 

and find the group of Ts that are isometries. Mathematicians have a habit of glossing over things, a posteriori, and saying things a bit in the reverse, but they understand what they mean because the are accustomed to decrypting it.

 

To specify the metric by specifying the group of isometries is somewhat contorted, but mathematicians are a rather odd species.

You are right both: Sir Russell in Principiae Mathematicae (written 1903; nobel prized 1950) stated that such operation (Space-1 to Space-2)is perfectly allowable and the rendering from a Geometry-1 to a Geometry-2 must be done by substituting any term one by one without lacking any property. Exactly the operation you did:

T^ G T = G

the problem is in stating the real TOPOLOGY of Space, not the Geometry.

[DrProctopus]

If time is divided up into quanta, then this leads to very intuitive mechanism for time dilation.

 

Are there any theories that treat time and space as quanta, and attempt to figure out the properties of those quanta that define matter and energy? I had thought that string theory and its descendants did that?

[Bo]

in a sense any quantum theory does this trick.

 

Bo

[Qfwfq]

In which sense? ;)

 

Neither basic QM nor field theory treat time and space as quanta.

[Bo]

hmm I misread the question ;)

I was referring to the fact that all quantum theories have a smallest time and length scale (and so in a sense 'quantized' space time), but that doesn't mean that they treat spacetime as a particle (in the not-realy-a-particle sense of QM).

 

theories that do this are qantum theories of gravity; the carier of the gravitational force, the graviton, is basicly a disturbence in space-time (just as the photon is a disturbance in the electromagnetic field). A quantum theory of gravity sees the graviton as a quantum.

 

Bo

[Qfwfq]

I was referring to the fact that all quantum theories have a smallest time and length scale (and so in a sense 'quantized' space time)

The Planck length and time depend on G and are only relevant to attempts at quantizing gravity. :doh: They are really just the scale at which we should expect phenomenology to distinguish things. Not much to do with being "quanta" of space and time. ;)

[DrProctopus]

If there is such a thing as a spacetime moment - a unit of spacetime - then it provides a pefectly intuitive explanation for time dilation.

 

To me, it seems like all things must have a smallest unit. Otherwise they would be infinitely divisible, which leads to all sorts of philosophical difficulties (but which I cannot prove is impossible)