Relativity discussion from “What is Time?” thread

[CraigD]

Moderation note: the first 14 posts of this thread were moved from the philosophy of science thread “What is Time?”, because they’re a discussion of the theory of relativity, which, while pertinent to the ontological question “what is time?”, is a distinct enough subject to warrant its own thread.

 

A traveller taking off with a clock and a measuring tape takes off after having synchronized his timepiece with his friend. He goes flying at relativistic speeds around the sun a few times, and does very careful measurements of the inside of his spaceship, and the ticking of his clock. His buddy on Earth does exactly the same in his absence. In all the time he's away, the inside of his spaceship measures exactly the same according to his measuring tape. It changes not with an inch. His clock ticks away, at a perfect one second per second. He does not measure any change in the flow of time nor the length of a centimeter. Same with his buddy on Earth. Now, once he gets back on Earth, there's a problem. His clock is out of whack with his buddy's clock. How did this happen?

I *think* it happened because the different *forces of acceleration* on the traveler's clock slowed it down, while his buddy's clock did not have those forces acting on it. (Just in case you missed my countless repetitions of the above.)

I understand your thinking, Michael. It’s one that every physics student should have, and consider deeply. As a conclusion about physical reality, however, it’s one that every physics student but the most neophyte should know is wrong. It’s very important, I think, for all thoughtful people to understand this, as the ability to realistically imagine things on astronomical scales requires it.

 

A good way, IMHO, is to consider a specific example – that is, have a though experiment - about the twins paradox in which both observers experience nearly exactly the same acceleration at all times.

 

Let’s say the earthbound one – and all his clocks - calmly experiences Earth’s surface acceleration of gravity of 9.8 m/s/s, while the astronaut experiences the same acceleration because his spaceship accelerates at a constant 9.8 m/s/s. Nonetheless, assuming only the few equivalency principles of the theory of relativity, an out-and-back round trip of 2 years duration by the astronaut’s clock takes about 2.37 years by the Earthbound clock (the “time dilation at constant acceleration” wikipedia section presents some derived formulae useful in calculating this).

 

I velocity time dilation was the result of the acceleration experienced by the astronaut’s clock, it would have to be detectably different than that experienced by the earthbound clock, but the example has been chosen so that almost no difference in acceleration can be detected by the two observers.

 

I strongly encourage anyone who’s not though their way past the common, but incorrect, resolution of the twins paradox Michael presents, to think deeply of though experiments such as the above, posting questions and comments as needed.

[watcher]

I strongly encourage anyone who’s not though their way past the common, but incorrect, resolution of the twins paradox Michael presents, to think deeply of though experiments such as the above, posting questions and comments as needed.

 

so what's with this differing "frame of references" that give us these time paradoxes?

[HydrogenBond]

Acceleration is an important consideration for the twin paradox, because it brings something to the table that is usually ignored in this thought experiment. One can not go from the starting conditions of the earth reference to relativistic speed without adding energy. The normal thought experiment never does an energy balance. It starts with final states and ignores the energy and the acceleration to get there.

 

What is different between the two references is the energy added to the reference that will show time dilation. The more energy we add, to generate more kinetic energy (V) the more time dilation we will get.

 

Time dilation is not relative, or both clocks would always be the same. If it was relative each twin would see the other twin appearing to time dilate, so when they met, both clocks would differ the same way and remain synchronized. In reality, only the one with energy added will show any real effect when they compare clocks.

 

 

Energy is needed to create the velocity state causing the time dilation via SR. This shows time dilation is energy dependent. The constraint is the energy needs to use an acceleration as a mediator for velocity. Energy without acceleration will not work quite the same way. Acceleration and velocity differ dimensionally by the extra t in acceleration; d/t vs d/t/t. This is the time potential added at terminal velocity d/t.

[CraigD]

so what's with this differing "frame of references" that give us these time paradoxes?

A frame of reference, as the term’s used in both modern relativity and it’s 17th century root principle Galilean (AKA classical or Newtonian) relativity, also known as an inertial frame, or just frame, simply means a system for representing points in space that doesn’t need a time term to represent a point in its frame. If a time term is required, the point described isn’t in that inertial frame.

 

For example, using the axes of the floor and walls of a house, one can describe the location of an objects in it with 3 numbers, eg: (x, y, z). Describing the position of this same object using the axes of the floor and walls of a compartment of a moving train requires at least one additional numeric time term, eg: (x+v_xt, y, z). We say, then, that the house and the moving train are different frames.

[Little Bang]

First, the earth bound clock's frame of reference is not changing so there is no time dilation as stated by Hydro. The only way to synchronize the two clocks is if they are in the same frame of reference. Hydro is correct, the twin in space is experiencing a changing frame of reference and that explains the difference in the two clocks.

[watcher]

synchronicity of clock time there fore is a function of the internal oscillator of a particle.

the internal oscillator of the particle is the same for all particles at say rest, with same acceleration or same velocity. iow, all particles that share a resonant frequency will always have/keep the same time.

 

dissynchronity of time happens because of doppler's effect on the moving particle. the particle's overall structure such as wavelengths and frequency are distorted. a constantly accelerating particle will therefore naturally have a different frame of reference compare to the one at rest. imo

[Little Bang]

We tend to think that velocity is the reason for relativity but that is not true. Acceleration is the reason for time dilation and length contraction.

[PerfectLiquid]

If you give a little more thought to this issue, you may find that reliance on either velocity or acceleration in trying to understand time dilation has a built-in problem. They are both measurements that are indexed by time...

You may want to take a step back (or is it down?) to displacement. Both gravity and motion share the phenomenon of time dilation as well as this fundamental quality of displacement. You may argue, What are they displacing? I would answer that they both represent matter displacing space-time. Gravity is the result of a static displacement, while motion (assuming little gravitational effect) represents dynamic displacement. This, in my mind at least, converges Clerk Maxwell's approach with Einstein's understanding.

Alternative views?

 

From the aether,

Al

[Little Bang]

I disagree. Motion has nothing to do with time dilation. Acceleration is the only thing that does cause dilation. A clock accelerated with respect to me slows. A clock dropped into a gravity well slows with respect to the dropee. Static displacement?

[PerfectLiquid]

Hello Little Bang,

 

It seems that you disagree with the good Dr. Einstein as well:

 

"From this there ensues the following peculiar consequence. If at the points A and B of K there are stationary clocks which, viewed in the stationary system, are synchronous; and if the clock at A is moved with the velocity v along the line AB to B, then on its arrival at B the two clocks no longer synchronize, but the clock moved from A to B lags behind the other which has remained at B by 1/2 tv^2/c^2 (up to magnitudes of fourth and higher order), t being the time occupied in the journey from A to B." [On The Electrodynamics Of Moving Bodies, June 30, 1905]

 

Note that only constant velocity is invoked in this example, and this characteristic of Special Relativity has been tested and confirmed. Naturally, for an object to exhibit constant velocity, it has to exhibit positional displacement (i.e., motion) as well.

 

All the best,

Al

[watcher]

I disagree. Motion has nothing to do with time dilation. Acceleration is the only thing that does cause dilation. A clock accelerated with respect to me slows. A clock dropped into a gravity well slows with respect to the dropee. Static displacement?

 

time dilation is relative. iow, you can't say it occurred without a comparison of two clocks. so two particles accelerating exactly at the same rate, will keep the same time. ergo no time dilation.

[watcher]

We tend to think that velocity is the reason for relativity but that is not true. Acceleration is the reason for time dilation and length contraction.

a particle accelerates when force or energy is introduced to the particle.

all measurements of time are basically measurements of frequency.

energy is direct proportional to frequency and inverse proportion to time ( clock slows down in high frequency)

 

any time keeping device made of electrons will slow down every time energy is introduced to its surroundings whether in the form of acceleration or gravitational force for the simple reason that frequency and wavelength change with varying energy states. .

[CraigD]

We tend to think that velocity is the reason for relativity but that is not true. Acceleration is the reason for time dilation and length contraction.

Motion has nothing to do with time dilation. Acceleration is the only thing that does cause dilation. A clock accelerated with respect to me slows.

Little Bang, speaking as a former intro to Physics teacher, I’ll just say your claims are wrong. Time dilation occurs whenever two observers have non-zero velocity relative to one another, even in the absence of any force or change in velocity (acceleration). It even occurs, theoretically, in a universe in which force and acceleration is impossible.

 

Rather than my or others initially explaining this, I believe you’d do better to try supporting your claims with references to actual physics. I’m guessing you’re not ready to rigorously mathematically derive a theory of relativity from a collection of fundamental postulates (eg: classical mechanics and the invariance of the speed of light), so would do better reading some introductory modern physics textbooks’ or website’s treatment of special relativity (It’s best, in my experience, to confine yourself to the special theory, which doesn’t account for forces or accelerations, only positions and velocities, before learning the general theory, which accounts for gravitational and other forces and accelerations) for anything that supports your claims.

[PerfectLiquid]

...to confine yourself to the special theory, which doesn’t account for forces or accelerations, only positions and velocities, before learning the general theory, which accounts for gravitational and other forces and accelerations) for anything that supports your claims.

 

I believe this is sound (or is that light) advice :). One thing to watch out for are treatments that suggest that time dilation is completely relative (i.e., symmetrical). The twin moving at the higher rate of speed (relative to space-time?) should exhibit the slower rate of time as measured by his/her clock. Note that in Einstein's original example, he avoids this issue by making one object stationary by definition. He was not the first to consider relativistic time dilation (e.g., Lorentz and Poincare), but he nicely brought together this with other relativistic aspects into his famous special relativity package. Note that he also tossed out the need for an aether in this special package, but later realized that an aether (or physical structure associated with space-time) was required by general relativity. Funny that this latter idea was never brought forward through time... ;)

 

Regards,

Al

[Michael Mooney]

CraigD:

I understand your thinking, Michael. It’s one that every physics student should have, and consider deeply. As a conclusion about physical reality, however, it’s one that every physics student but the most neophyte should know is wrong. It’s very important, I think, for all thoughtful people to understand this, as the ability to realistically imagine things on astronomical scales requires it.

 

A good way, IMHO, is to consider a specific example – that is, have a though experiment - about the twins paradox in which both observers experience nearly exactly the same acceleration at all times.

 

Let’s say the earthbound one – and all his clocks - calmly experiences Earth’s surface acceleration of gravity of 9.8 m/s/s, while the astronaut experiences the same acceleration because his spaceship accelerates at a constant 9.8 m/s/s. Nonetheless, assuming only the few equivalency principles of the theory of relativity, an out-and-back round trip of 2 years duration by the astronaut’s clock takes about 2.37 years by the Earthbound clock (the “time dilation at constant acceleration” wikipedia section presents some derived formulae useful in calculating this).

 

I velocity time dilation was the result of the acceleration experienced by the astronaut’s clock, it would have to be detectably different than that experienced by the earthbound clock, but the example has been chosen so that almost no difference in acceleration can be detected by the two observers.

 

I strongly encourage anyone who’s not though their way past the common, but incorrect, resolution of the twins paradox Michael presents, to think deeply of though experiments such as the above, posting questions and comments as needed.

 

Excuse me but how does your reply address the ontological question, "What is Time?"

We all know that clocks keep time differently under different conditions, such as you described above. But what is being posited as an entity here which exists in and of itself. What is it that "dilates?"

Would you please read or re-read my post 584 on page 59 of your "What is Time" thread and tell me how I am wrong in my ontology of time?

http://hypography.com/forums/philosophy-of-science/3650-what-is-time-59.html

Thank you.

BTW, Modest And I went around a few times about whether acceleration is a factor in clocks slowing down , and I do understand the concept of momentary co-moving inertial frames (as to how a lot of quick "snapshots" factor out acceleration artificially.)

 

Also, didn't your astronaut above and his clock experience acceleration different than the earhbound clocks just to escape earth's gravity well in the first place? It is well and good that the spaceship maintains an earth-equivalent acceleration of 9.8 m/s/s, during his journey, but in the real world, were the clocks not subjected to different forces?

Thanks for your "time.";)

Michael

[PerfectLiquid]

CraigD:

 

 

Excuse me but how does your reply address the ontological question, "What is Time?"

We all know that clocks keep time differently under different conditions, such as you described above. But what is being posited as an entity here which exists in and of itself. What is it that "dilates?"

...

Thanks for your "time.";)

Michael

 

Hi Michael,

 

This is a good question that may not be so easy to answer, or that will likely produce many answers. IMO - time is the rate at which entropy increases, that is, it is a parameter that reflects how fast order moves towards disorder. This would represent time from a thermodynamic perspective. I expect that others may offer reasonable alternatives, but this works for me.

A partial quote from Einstein: "People like us, who believe in physics, know that the distinction between past, present, and future is only a stubbornly persistent illusion.":)

 

Regards,

Al

[CraigD]

Excuse me but how does your reply address the ontological question, "What is Time?"

It doesn’t. For that reason, I’ve moved it and its thread of replies from the “What is Time?” thread to this thread, which is meant for a discussion of the formalism of relativity. IMHO, at least a rudimentary grasp of mathematical physics is an essential prerequisite for ontology and epistemology. Although formalism is itself a philosophical position, the use of a particular formalism is not, but rather may informs many different philosophies. Regrettably, as Snow noted half a century ago in his lecture The Two Cultures, philosophical literacy and practical mathematical skill appears be found in an increasingly rare intersection of practitioners of the two disciplines.

Also, didn't your astronaut above and his clock experience acceleration different than the earhbound clocks just to escape earth's gravity well in the first place? It is well and good that the spaceship maintains an earth-equivalent acceleration of 9.8 m/s/s, during his journey, but in the real world, were the clocks not subjected to different forces?

For the sake of having the force experienced by the two observers be identical at all times in the example in post #1, assume that the astronaut begins and ends his trip in space, never landing on Earth.

 

Note that what astronauts and clocks can experience is technically force, not acceleration. The Earthbound observer experiences force due to gravitational attraction to the Earth that is nearly identical to the force experienced by the astronaut due to the acceleration of his spaceship. If both were locked in “windowless rooms” devoid of any means of measuring the universe outside of them (other than the required comparison of their clocks), they only way they would know who was the astronaut would be upon comparing clocks. Thus, their clocks couldn’t have been affected by any difference in experienced forces, because there was almost no difference in the forces experienced by them.

 

Alternately, the thought experiment could involve the “home” observer (it’s no longer appropriate in this version to call him “Earthbound”, so let’s just call him “Homer”) or the “traveler” (let’s call him “Rover”) experiencing no outside forces. Imaging Homer is in a motorless ship in deep space, experiencing practically no gravitational force. Rover is in an identical ship positioned 200 km behind an Earth-radius and mass spheroid being towed by a ship accelerating at about 9.2 m/s², an arrangement that results in Rovers ship freefalling toward the sphere with at precisely the same acceleration as it and the ship.

 

Both Homer and Rover experience a period of weightlessness. After a 2 year out-and-back trip, Homer and Rover compare clocks, and discover that Homer’s clock reads about 65.62 days later than Rover’s.

 

Notice I’ve used the phrase “nearly identical” a lot in these examples. Though not important to the outcome of the experiments or the theory of relativity, bonus points to whoever explains what’s not quite identical about the forces experienced by Homer and Rover.