the effect of gravity

[maxlongo]

Hi, I have actually just signed because it is a while I have a question in my mind I can’t find an answer on the net and I hope that some of you could help me out.

 

I start from this quote off another topic by a member of the forum, quote that I find quite interesting for the question I have in mind…

 

…

I have found that a useful analogy to help grasp the concept is the familiar one of visual perspective. Anna walks into the distance and Betty sees her as smaller. Anna sees Betty as smaller. But they stay the same size. How is this possible???

…

 

In this case it is clear that distance does not affect the size of the two girls but only their perception of each other.

 

And if the analogy is correct as I think it is that it comes the question.

 

It is accepted that gravity affect time. But has it been demonstrate without any doubts that what it is affected is not actually our perception of it?

 

A while ago in a program about the research into the graviton and quantistic gravity.

 

They shown, as proof of the effect of gravity on the time, the painstaking job of some military men to keep the clocks on the GPS satellites in line with those on Earth.

 

But how can we esclude that gravity does not affect time but rather the mechanisms we used to measure its flowing?

 

 

 

 

PS: i apology if i posted in the wrong place, or in the wrong way or other similia, and if any of you have problem with my english (not my mother tongue, nor i am really into physics. just curious.)

[Janus]

 

 

But how can we esclude that gravity does not affect time but rather the mechanisms we used to measure its flowing?

 

If it were gravity affecting the mechanism then the time difference would vary according to the local strength of the gravitational field. Instead, Relativity predicts, and we find, that the time difference varies according to the difference in gravitational potential.

IOW, it is quite possible to have two clocks experiencing exactly the same gravitational force, yet be at different gravitational potentials and to run at different rates.

[maxlongo]

thanks for the answer Janus, very helpfull although it open the door to another tricky question.

 

a question that probably is more philosophical and speculative that scientific.

 

i getting a bit deeper into the matter and i noticed that many of the experiments, if not almost all of them, to confirm the time dilation predicted by the theory of relativity use the Doppler effect.

 

An effect which we could "feel" with our senses, hearing and sight, before it was scientifically "discovered" and explained.

 

so now i am wondering.

 

What if time rather than being the 4th dimension as we consider it now, it is actually a 6th sense. (not in the "classical" o Shamalayan way). which actually would cut the "possibility" of travelling in time, since it is not a dimension.

 

i mean with our five senses we take notice of changes. in radiation with the eyes, in air pressure with the hears, in surface texture with the skin, ecc.ecc.

 

i just throw it as an idea if anyone is interested in developing it, but as i said i know is more philosophical than scientific.

 

i should first demontstrate that we have an organ that register the change in "state" of the universe around us. a change that we call time.

 

but i have no intention in studying biology.

 

thanks again

[Iggy]

…

I have found that a useful analogy to help grasp the concept is the familiar one of visual perspective. Anna walks into the distance and Betty sees her as smaller. Anna sees Betty as smaller. But they stay the same size. How is this possible???

…

 

In this case it is clear that distance does not affect the size of the two girls but only their perception of each other.

 

Relative size difference is an effect of perspective or perception only. Relative time dilation is not. We know this because when Anna and Betty approach each other Betty no longer seems smaller--they both appear the same size again. When time dilated clocks are brought back together they are no longer synchronized. It's not just perception.

 

On the issue of time being a sixth sense... it seems clear that things without senses are affected by time.

[swansont]

They shown, as proof of the effect of gravity on the time, the painstaking job of some military men to keep the clocks on the GPS satellites in line with those on Earth.

 

Civilians, too. And women.

 

If it were gravity affecting the mechanism then the time difference would vary according to the local strength of the gravitational field. Instead, Relativity predicts, and we find, that the time difference varies according to the difference in gravitational potential.

IOW, it is quite possible to have two clocks experiencing exactly the same gravitational force, yet be at different gravitational potentials and to run at different rates.

 

And, of course, we see the effect of this different gravitational potential without needing to go into space. Relativistic corrections need to be made for clocks at different altitudes.

 

(and not just at government timing labs. http://www.wired.com/science/discoveries/news/2007/12/time_hackers )

[Pete]

But how can we esclude that gravity does not affect time but rather the mechanisms we used to measure its flowing?

Actually gravity is affecting the mechanisms that we are using to measure the "flow" of time. The reason that we say that gravity affects time itself is because gravity affects all mechanisms in the exact same way. Since an operational definition of time is what we measure with a clock and we know that gravity affects clocks then we say that time itself is affected by gravity.

 

Pete

[swansont]

Actually gravity is affecting the mechanisms that we are using to measure the "flow" of time. The reason that we say that gravity affects time itself is because gravity affects all mechanisms in the exact same way. Since an operational definition of time is what we measure with a clock and we know that gravity affects clocks then we say that time itself is affected by gravity.

 

Pete

 

There are those who tend to misconstrue this, however, so to clarify: on some clocks there are some mechanical effects due to gravity that do not affect time. The prime example of this is a pendulum clock, whose period explicitly depends [math]\sqrt{g}[/math]

 

Nobody is claiming that moving a pendulum clock to a different elevation changes the flow of time in that fashion. One would compensate for the different value of g for a pendulum clock, just as one would compensate for a change in length of the pendulum. As Janus has pointed out, you can have the same value of g but be at a different gravitational potential, and time will flow at a different rate under that circumstance.

 

External physical effects (mechanical, electrical, temperature, humidity etc.) are taken into account. Relativistic effects are what is left over; they are a result of what frame of reference you are in, and independent of the clock you use. In fact, this is tested, because different types of clocks respond to physical perturbations in different ways, and yet the relativistic effects are the same, just as predicted.

[maxlongo]

I realised that i should have probably specified that i was using the word "mechanism" both in the classical and the quantistic way.

 

Nowadays, we use atomic clock, but do we know enough about gravity to be 100% sure that it doesn’t have any effect at atomic and/or subatomic level?

 

Especially, if we consider the acceptance by many scientist of the existence of the graviton.

 

Civilians, too. And women.

...

 

Most definitively.

 

...

On the issue of time being a sixth sense... it seems clear that things without senses are affected by time.

 

Depends if it is time to be cause, affecting things without sense, or simply a “perception” of the event happening.

 

If a car crashes into a wall would you say that time cause the damage?

 

If nothing is created, nor destroyed but only transformed every change can be considered as part of a flow with no end nor beginning. Nor birth or death.

 

similarly nor light or sound exist on their own. they are vibrations that we "record" with our senses and that makes them real for us.

 

but we can't put light or sound in a box.

Edited September 1, 2008 by maxlongo

[Kyrisch]

but we can't put light or sound in a box.

 

You can, too. Both of these phenomena are waves and as such can be contained by using reflective surfaces.

[maxlongo]

You can, too. Both of these phenomena are waves and as such can be contained by using reflective surfaces.

 

i have my most profound doubts

[Kyrisch]

i have my most profound doubts

 

Well, you can with light. It'll eventually all be absorbed into the walls of the box but it'll bounce around for a bit. I'm not sure if anyone's tried with sound...

[swansont]

Nowadays, we use atomic clock, but do we know enough about gravity to be 100% sure that it doesn’t have any effect at atomic and/or subatomic level?

 

Especially, if we consider the acceptance by many scientist of the existence of the graviton.

 

Cesium clocks, Rubidium clocks and Hydrogen masers (and others, but these have flown in space) are all affected the same way as predicted by relativity. It does not scale with the atomic mass, nor is the effect constant for an electron (as one might think it would be, were it a direct effect of gravity) undergoing different transitions.

[Pete]

i have my most profound doubts

It is not uncommon for physicists to speak of a box of photons. There is no law of physics which prevents it. There may be practical problems though. But it is possible to have multiple reflections. A laser is a perfect example of this. Photons bounce back and forth between two mirrors inside a laser tube but will eventually escape.

 

There is an article in the American Journal of Physics which discusses a box containing a gas of photons. The author explains the mechanism responsible for the photons contribute to the mass of the gas/box system.

 

The mass of a gas of massless photons, H. Kolbenstvedt, Am. J. Phys. 63, 44 (1995). The abstract reads

Using minimal formalism, we demonstrate that the massless photons, constituting the radiation in a cavity, contribute to the mass of the cavity in agreement with Einstein's mass–energy formula. We assume that a photon has energy and momentum related to frequency by E=h[math]\nu[/math], p=h[math]\nu[/math]/c. Restricting velocities to nonrelativistic values, we impart a uniform acceleration to the cavity. Reflection from a moving mirror produces a Doppler shift, and thus the momentum delivered to the front and rear walls can easily be calculated in the laboratory frame. A simple application of F=ma leads to the usual conclusion that the mass of a gas of photons of total energy U is U/c². An alternative argument based on calculations in the co-accelerating frame is also given.

 

Pete

[lancelot]

Relative size difference is an effect of perspective or perception only. Relative time dilation is not. We know this because when Anna and Betty approach each other Betty no longer seems smaller--they both appear the same size again. When time dilated clocks are brought back together they are no longer synchronized. It's not just perception.

.

 

Very good point Iggy. The analogy is only strictly 'correct' if both A and B move away from each other at identical acceleration.

 

Still it's useful to get people over the intial disbelief of time dilation by comparison with an everyday experience. Having got that sorted, I suppose the analogy could be extended to "now imagine, if perspective worked like relativity, if B runs into the distance and back again, she will actually come back smaller!"