[ries van Twisk]

ries van Twisk  asked the Naked Scientists:
   
Chris,

I am a frequent listener to your programme and it's great!

i do have a question for you :

since everything seems to vibrate at some sort light light has waves, electrons move around atoms, and I believe that string theory is based on these vibrations as well.

I was wondering if there is evidence that Time Ticks, or is time continuously?

Thanks,
Ries van Twisk
Quito, Ecuador

What do you think?

[syhprum]

Quantum mechanics assumes that the smallest unit of time is that which light takes to travel the planck unit of distance.

Planck time 5.391 * 10^-44 seconds

Planck length 1.616 * 10^-35 meters

See http://en.wikipedia.org/wiki/Planck_units#Base_Planck_units

[DoctorBeaver]

If time is quantised then it wouild not flow smoothly but, rather, it would take discrete steps. These steps would obviously need to be incredibly tiny; maybe the Planck time - 10^-43 seconds.

I think syhprum meant to say "Quantum mechanics assumes that the smallest unit of time is that which light takes to cross the planck unit of distance". I believe that is how the Planck time is defined.

[swansont]

Planck scale units can be derived from solving for the point where a quantum theory of gravitation is needed (for some, but not all, formalisms).  What actually happens at the Planck scale is quite another story — we don't have the ability to do experiments at that level.  Planck's original determination happened before QM, and were just a convenient unit system from setting c, G and hbar to 1.  So take any statement that says the Planck time is the smallest unit of time with a quantum of salt.

[DoctorBeaver]

OK, I shall bear that in mind.

[Vern]

I think the second is defined in relation to the number of times the caesium atom changes states under a defined condition. I guess you could count those changes of states of the caesium atom to be ticks.

Those ticks would be pretty short though; I'm not sure how they would compare to a Planck length.

A "cesium(-beam) atomic clock" (or "cesium-beam frequency standard") is a device that uses as a reference the exact frequency of the microwave spectral line emitted by atoms of the metallic element cesium, in particular its isotope of atomic weight 133 ("Cs-133"). The integral of frequency is time, so this frequency, 9,192,631,770 hertz (Hz = cycles/second), provides the fundamental unit of time, which may thus be measured by cesium clocks.

[DoctorBeaver]

That isn't time ticking, though, is it. It's just atomic decay.

[Vern]

That isn't time ticking, though, is it. It's just atomic decay.

I think the second is defined in those terms. They synchronize the time piece with the vibrations of the atom; vibrations we used to think of as orbiting electrons.

[DoctorBeaver]

But a second is an artificial construct. It isn't "time" per se.

[yor_on]

There is no experimental evidence for time being events.

You could say that Feynman diagrams could be seen as some sort of proof for time being 'events' perhaps, as they can go 'both ways' in time?

But we have tested the idea down to 'attoseconds'
(One quintillionth (10^-18) of a second) without being able to see any proof for it being 'events'.
I think it's a 'flow' like we are 'fishes' inside 'times ocean'

[DoctorBeaver]

I've heard it said, although I'm not sure of the reasoning behind it, that the Planck length is the smallest unit of length that has any meaning. If time is just another dimension, as is thought by some, why should that also not have a smallest measurement that is meaningful? If that is indeed the case, then it would not flow linearly but would have to pass in discrete steps.

[yor_on]

Yes DB, you are quite correct, I read the same.
Even though it gives me problem to see spacetime as having a 'smallest length'.
But then the definition seems to be more like 'having a meaning for us'.
http://www.scribd.com/doc/11157955/Planck-Length-and-Time

[DoctorBeaver]

Yes, that is certainly what that article seems to imply (paragraph 3).



(I had to take that from a screenshot as it doesn't allow copy & paste.)

I must have misunderstood what I read about the Planck length.

[syhprum]

Dear Doc

If you sign up (free) you can download as text instead of PDF and then you can cut and paste from the text document

[DoctorBeaver]

A free sign? I wasn't trying to download anything, just copy & paste from the site.

[LeeE]

This question falls within an area of theoretical physics that is little discussed but which is of special interest to me.  I think it is little discussed because it concerns an aspect of reality that we don't, in general, even think to question.

Cutting to the chase, if you consider an object that has zero length in the direction that it is moving, it is apparent that it can only move in discrete steps in that direction; any change of position of a zero-length object must result in a displacement greater than the length of the object (which is zero) i.e. there must be a discrete (non-zero sized) gap between it's former and current positions.

In fact, having zero length just high-lights the broader issue of the change of precisely known values; any precisely known value can only change by values greater than zero, so once again, there must be a discrete and non-zero sized gap between any two precisely known values.

The significance of the Planck Length and Time units, is I believe, probably one of the single most counter-intuitively understood aspects of science and physics.  While the Planck Length and Time units are part of a scheme that allows the normalisation of many of the constants in physics to the range of values between 0 and 1, it is the interpretation of the Planck Length and Time units as being the smallest divisions of those dimensions that has any meaning that is most interesting here; if the Planck Length is the smallest measure of distance that has any meaning, and the Planck Time is the smallest duration of time that has any meaning, then the slowest possible constant speed would appear to be 'c'.  It would, of course, be possible to achieve lower speeds, but only by moving less than the Planck Length in the Planck Time duration, or by only moving the Planck Length in multiples of the Planck Time, but this then begs the question; 'if it only moves one Planck Length in two Planck time units, where was it after only one Planck Time unit?

In any case, the idea of something moving just one Planck Length distance in multiples of the Planck Time unit soon runs in to other problems; to achieve a perfectly smooth and continuous acceleration from zero would require an infinite number of Planck Time units for the initial movement of one Planck Length away from it's original position; any other solution still results is a series of discrete jumps.

The logical solution then, to describe constant movement at lower speeds than 'c', is to require the locations and velocities of the moving objects to be imprecise, rather along the lines of the Uncertainty Principle.

[DoctorBeaver]

LeeE has managed to baffle me with science yet again.

[justaskin]

Me to DB.I am trying to get my head around how something with zero length can exist.For that matter zero height or zero width?.

Cheers
justaskin

[Vern]

But a second is an artificial construct. It isn't "time" per se.

Yes; you're right; maybe the ticking of time is simply a conceptual construct to allow us to think about it. Time itself may flow smoothly along, impervious to our attempts to explain it.

[yor_on]

Perhaps one could see time as 'whole', but in it we have phenomena relating to either QM or our macroscopic spacetime? What we see as distance seems to be connected to our 'relative' motion and acceleration macroscopically. It seems to me that we can't say that distance is anything else than a 'property' between two frames of reference. And as we can't say that anything is 'standing still' in spacetime either, as our 'frames of reference', as far as I understand, always will be defined as being relative someone else's frame.

--

I'm not sure I'm expressing myself 'explicitly enough' here (no surprise huh:)
What I try to say is that to me there is no correct 'definition' of 'distace'.

It's just a concept describing a relation between two frames.
You and whatever you are measuring against. That as you will be at some frame, not able to define its possible motion without referring to another 'frame of reference'.
That we have an 'Earth standard' doesn't make it a 'preconception universale'.

---
Well, thinking of it I do have one for the 'shortest path' though
The one costing you the least energy
Like light through spacetime's geodesics.

---------

It's like the whole question of plank length. there is nothing hindering us from saying that a plank length is double the distance we have measured, relative those other definitions Plank used for defining.
That is, as long as we change those too to keep the relative 'balance' in check between them, sort of:)
Or is there anything that we can say won't be possible to change numerically as long as we keep their 'intergroup' relations correct?

So if we agree on that the frames of reference we use are 'arbitrary' in that meaning then there is no 'gold standard' for 'distance'. That we think so comes from what we are used to historically it seems to me.

Now one could say that the 'intergroup' relations will be there anyway, and I would agree on that. But what I want to put forward is that the concept of 'distance' doesn't have any meaning in spacetime other than defining a relation between two frames. So ' meaningful for us' seems a very appropriate concept here, like describing a 'tool' we use for giving us a 'meaningful' comparison.