# time dilation

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#### mickeyl

##### Guest
If a light photon begins traveling from a star located 2-light-years from

earth....it will reach earth in 2-years....a person with a telescope on earth

would have aged 2-years during the length of it's journey from the star to

the reflector on his telescope....and (if it was possible) a spaceship with

an astronaut onboard is traveling at the speed of light, and following the

photon to earth....he would travel (11,749,178,300,000 miles), or 1.1749 x

10^13 miles and age 2-years....there is no time-dilation when traveling

through "space". The photon aged 2-years, the astronomer aged 2-years, and

the astronaut aged 2-years....light-speed (C) is a constant, and it's effect

in "space" is constant.

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#### ramparts

##### Guest
If a person were on a spaceship traveling at the speed of light (there are very good reasons that this is impossible, but I'll humor you) then he wouldn't age at all. Photons similarly do not "age" - traveling at c through space precludes traveling at any rate through time, funny as that is to think about.

O

#### origin

##### Guest
If you were on a transport that could go, lets say 99.99999999%, the speed of light, and it could instantly accelerate and decelerate here is the announcement you would here from the flight attendent, " Welcome on board flight 9946, our flight time to earth will be, oh, nevermind prepare for landing, make sure your tray tables....."

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#### weeman

##### Guest
origin":19esdvsr said:
If you were on a transport that could go, lets say 99.99999999%, the speed of light, and it could instantly accelerate and decelerate here is the announcement you would here from the flight attendent, " Welcome on board flight 9946, our flight time to earth will be, oh, nevermind prepare for landing, make sure your tray tables....."

:lol: :lol:

No jet lag after flying two lightyears would be nice!

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#### Shpaget

##### Guest
Well, jet lag doesn't come from the travel itself. It comes from the change in duration of day.
So, instantaneous travel (teleportation) around the Earth would also cause jet lag.

What I would like to know is why mickey thinks that "there is no time-dilation when traveling through "space"."
There are some pretty convincing experiments and measurements that say otherwise.

Edit:
weeman, happy 1k[super]th[/super] post
this is my no. 666 :twisted:

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#### csmyth3025

##### Guest
origin":1tt7xkjb said:
If you were on a transport that could go, lets say 99.99999999%, the speed of light, and it could instantly accelerate and decelerate here is the announcement you would here from the flight attendent, " Welcome on board flight 9946, our flight time to earth will be, oh, nevermind prepare for landing, make sure your tray tables....."

This concept is very hard for me to envision. Let's suppose that our hypothetical transport started their trip today from a planet 4.5 billion light years from Earth and had on board a very powerful telescope trained on our solar system. It seems to me that the observers on our transport would see a very blue-shifted version of the evolution of our sun and its planets. The travelers would see the clock on their ship ticking off the seconds, days, and years as the ship travels to Earth.

A complication in this scenario is the effect of cosmological expansion on the trip of our observers. A planet 4.5 Bly from Earth would be, I believe, 1,379.5 Mpc from Earth. Assuming a Hubble Constant of ~72.5 km/sec per Mpc, the recessional velocity of the starting point would be ~100,014 km/sec (about 1/3 c). This "stretching" of space between the starting point and earth will, I think, lengthen the travel time as seen by the travelers on the ship. How much time will the travelers see on their clock by the time they reach Earth after traveling for billions of years through a constantly expanding section of space?

Chris

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#### SpeedFreek

##### Guest
csmyth3025":vb6wx0h6 said:
origin":vb6wx0h6 said:
If you were on a transport that could go, lets say 99.99999999%, the speed of light, and it could instantly accelerate and decelerate here is the announcement you would here from the flight attendent, " Welcome on board flight 9946, our flight time to earth will be, oh, nevermind prepare for landing, make sure your tray tables....."

This concept is very hard for me to envision. Let's suppose that our hypothetical transport started their trip today from a planet 4.5 billion light years from Earth and had on board a very powerful telescope trained on our solar system. It seems to me that the observers on our transport would see a very blue-shifted version of the evolution of our sun and its planets. The travelers would see the clock on their ship ticking off the seconds, days, and years as the ship travels to Earth.

Well, with a velocity of 99.99999999% of c, time on that spaceship would be passing 70710.6 times slower when compared to a clock on Earth. Obviously, for a long journey of 4.5 billion light-years, the journey time in the reference frame of the spaceship would still be quite long.... it would take something around 64,000 years!

But I think origin was referring to a slightly shorter journey. A trip to Alpha Centauri at 99.99999999% of c would take just over half an hour for the occupants of the spaceship!

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#### csmyth3025

##### Guest
SpeedFreek":2qg33ts6 said:
Well, with a velocity of 99.99999999% of c, time on that spaceship would be passing 70710.6 times slower when compared to a clock on Earth. Obviously, for a long journey of 4.5 billion light-years, the journey time in the reference frame of the spaceship would still be quite long.... it would take something around 64,000 years!

But I think origin was referring to a slightly shorter journey. A trip to Alpha Centauri at 99.99999999% of c would take just over half an hour for the occupants of the spaceship!
Well, here's the rub. The occupants of the transport would see their clock ticking normally and they would see the clocks on Earth ticking very slowly. Conversely, the folks on Earth would see their clocks ticking normally and the clocks on the transport ticking very slowly. I believe the accepted solution to this "twin paradox" kind of situation has to do with the acceleration and subsequent deceleration of the transport.

I suppose the central question is, if Alpha Centauri is 4.3 LY from Earth, would the travelers see their clock tick off slighty more than 4.3 years over the course of the journey?

Chris

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#### Shpaget

##### Guest
It is already known that acceleration (no matter if it comes from movement or gravity) affects time.

csmyth3025":omogtorw said:
I suppose the central question is, if Alpa Centauri is 4.3 LY from Earth, would the travelers see their clock tick off slighty more than 4.3 years over the course of the journey?

No, the travelers' clock would show that only half an hour had passed.

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#### csmyth3025

##### Guest
Shpaget":3obfi4zq said:
It is already known that acceleration (no matter if it comes from movement or gravity) affects time.

csmyth3025":3obfi4zq said:
I suppose the central question is, if Alpha Centauri is 4.3 LY from Earth, would the travelers see their clock tick off slighty more than 4.3 years over the course of the journey?

No, the travelers' clock would show that only half an hour had passed.

It occurs to me that, from the perspective of the travelers, the distance along their path of travel would appear to be shorter than 4.3 LY due to the foreshortening relativistic effect. Would they measure the distance from Alpha Centauri to Earth as being ~1/2 Lhr? I'm not sure how they could measure this distance from inside their vessel. Any ideas?

I suppose they could "ping" a reflector on Earth with a laser beam as they pass Alpha Centauri. The trouble with this method is that they would receive the return ping almost at the same time as they arrive at Earth (I think).

Chris

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#### Shpaget

##### Guest
No one is talking measuring distance. We talk about apparent time passage.

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#### darkmatter4brains

##### Guest
csmyth3025":348fk5wn said:
It occurs to me that, from the perspective of the travelers, the distance along their path of travel would appear to be shorter than 4.3 LY due to the foreshortening relativistic effect.
Chris

That is essentially correct. If you examine the Lorentz formulas in the limit that v approaches c, it basically hints at the fact that something traveling at c, would see zero distance and zero time between any two events. It's everywhere before it ever left. Crazy, huh.

O

#### origin

##### Guest
csmyth3025":3su3y72g said:
I suppose they could "ping" a reflector on Earth with a laser beam as they pass Alpha Centauri. The trouble with this method is that they would receive the return ping almost at the same time as they arrive at Earth (I think).

Chris

Correct - even though they would measure the speed of the light leaving the ship at ~186,000 mph.

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#### csmyth3025

##### Guest
origin":14obmloc said:
csmyth3025":14obmloc said:
I suppose they could "ping" a reflector on Earth with a laser beam as they pass Alpha Centauri. The trouble with this method is that they would receive the return ping almost at the same time as they arrive at Earth (I think).

Chris

Correct - even though they would measure the speed of the light leaving the ship at ~186,000 mph.

As I understand it, this is essentially the mechanism by which the detection at the Earth's surface of short-lived particles created by cosmic "ray" collisions with atoms high in the atmosphere is explained. The characteristic half-life of certain such secondary particles is observed in the laboratory to be too short for them to last long enough to reach the ground. The particle itself decays normally according to its own internal "clock", which observers on the ground perceive to be running slow. Additionally, to the particle, the ground is a lot closer than we see it to be due to its relativistic speed. It therefore has enough time (as measured by its own clock) to reach the ground before it decays.

Is this description accurate, or am I off-base?

Chris

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#### Mee_n_Mac

##### Guest
csmyth3025":ed69uw9e said:
origin":ed69uw9e said:
csmyth3025":ed69uw9e said:
I suppose they could "ping" a reflector on Earth with a laser beam as they pass Alpha Centauri. The trouble with this method is that they would receive the return ping almost at the same time as they arrive at Earth (I think).

Chris

Correct - even though they would measure the speed of the light leaving the ship at ~186,000 mph.

As I understand it, this is essentially the mechanism by which the detection at the Earth's surface of short-lived particles created by cosmic "ray" collisions with atoms high in the atmosphere is explained. The characteristic half-life of certain such secondary particles is observed in the laboratory to be too short for them to last long enough to reach the ground. The particle itself decays normally according to its own internal "clock", which observers on the ground perceive to be running slow. Additionally, to the particle, the ground is a lot closer than we see it to be due to its relativistic speed. It therefore has enough time (as measured by its own clock) to reach the ground before it decays.

Is this description accurate, or am I off-base?

Chris

You're no "muon", you're right on !

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#### csmyth3025

##### Guest
Thanks Mee_n_Mac.

So traveling at very close to the speed of light would result in observers on our transport seeing the Earth ~1/2 light hour away looking out their front window as they zoom past Alpha Centauri. If they look out their back window as they approach Earth, would they also see Alpha Centauri as being ~1/2 light hour away?

Chris

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#### SpeedFreek

##### Guest
Let's just back up a little...

We are not talking about what they would see out of the window, we are talking about what they would measure using scientific instruments. Due to an apparent effect called aberration, as you approach the speed of light your view of the universe seems to curve around from behind you, and stretch away in front of you.

http://www.fourmilab.ch/cship/aberration.html

But using pulses of light reflected back at themselves, they would measure (i.e. calculate) the universe to be contracted in their direction of motion, both behind and in front.

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#### csmyth3025

##### Guest
SpeedFreek":smg9zy51 said:
Let's just back up a little...

We are not talking about what they would see out of the window, we are talking about what they would measure using scientific instruments. Due to an apparent effect called aberration, as you approach the speed of light your view of the universe seems to curve around from behind you, and stretch away in front of you.

http://www.fourmilab.ch/cship/aberration.html

But using pulses of light reflected back at themselves, they would measure (i.e. calculate) the universe to be contracted in their direction of motion, both behind and in front.

Thanks SpeedFreek for the link - it's very instructive. More impotantly (for me), I can follow the logic of the presentations. The related article on the "Oh-My-God" particle is also very helpful in explaining these concepts.

It seems that intergalactic travel in reasonable transit times is theoretically possible (for the spacefarer, anyway) provided you have an almost inexhaustable source of power on your ship and you don't mind the highly probable risk of running into a mote of dust and being blown to smithereens.

Chris

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#### darkmatter4brains

##### Guest
Something that's important to be aware of and Speedfreek sorta mentioned is that the "observer" mentioned in Special Relativity is not a person - it's an infinite lattice work of rods and clocks. It's unfortunate that all SR problems and examples in texts seem to have the observers depicted as humans as it just spreads the misconception that the observer is a person. But, in reality they're not.

The infiite latticework of rods and clocks are synchronized in such a way that takes out any effects that are due to the finite speed of light. So all the effects that are encountered are not due to when a photon hits your eyes and based on what you then see. It's how reality really works. All optical illusions are taken out of the picture.

So as far as what you would see outside the window on your spaceship, unless your an infiite lattice work of rods and clocks, you might not see what SR predicts ;-) As speedfreek mentiioned they're are many optical effects that can happen, but SR is not concerned with these.

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#### csmyth3025

##### Guest
darkmatter4brains":2nkgf0hw said:
Something that's important to be aware of and Speedfreek sorta mentioned is that the "observer" mentioned in Special Relativity is not a person - it's an infinite lattice work of rods and clocks. It's unfortunate that all SR problems and examples in texts seem to have the observers depicted as humans as it just spreads the misconception that the observer is a person. But, in reality they're not.

The infiite latticework of rods and clocks are synchronized in such a way that takes out any effects that are due to the finite speed of light. So all the effects that are encountered are not due to when a photon hits your eyes and based on what you then see. It's how reality really works. All optical illusions are taken out of the picture.

So as far as what you would see outside the window on your spaceship, unless your an infite lattice work of rods and clocks, you might not see what SR predicts ;-) As speedfreek mentioned they're are many optical effects that can happen, but SR is not concerned with these.

John Walker's C-ship site specifies:
Let's board the C-ship Bradley to explore relativistic aberration. To avoid being confused by other relativistic effects, we've set the main viewer to compensate for Lorentz contraction and Doppler shift, leaving only aberration to affect our view.
The "main viewer" is described as a computer screen - thus eliminating the complication you mention. I'm guessing that Walker's simulations are accurate renderings of the effects he describes. For me, seeing a picture is much more helpful than reading a description.

Chris

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#### darkmatter4brains

##### Guest
csmyth3025":1eicm84x said:
John Walker's C-ship site specifies:
Let's board the C-ship Bradley to explore relativistic aberration. To avoid being confused by other relativistic effects, we've set the main viewer to compensate for Lorentz contraction and Doppler shift, leaving only aberration to affect our view.
The "main viewer" is described as a computer screen - thus eliminating the complication you mention. I'm guessing that Walker's simulations are accurate renderings of the effects he describes. For me, seeing a picture is much more helpful than reading a description.

Chris

Well, I didnt actually read that link, but that sounds like the opposite of what I mentioned. Aberration is not a "true" relativistic effect in terms of special relativity (SR). (Yeah but it does have a formula with a relativistic correction discovered by Einstein, I know) It sounds like his screen is compensated to show only aberration and eliminate any of the relativistic effects that one normally encounters in SR.

The infinite lattice work of rods and clocks - the "official" observer in SR - are designed to eliminate effects like aberration, that are dependent on the finite speed of light - and only allow through the relativistic ones like length contraction, time dilation, etc. Aberration is an optical effect based on how light hits your eyes (and it does hit your eyes differently at non-relativistic speeds as compared to relativisitc). Time dilation, etc is how reality actually works independent of that.

So, again Special Relativity is NOT concerned about what you as a human literally sees with his eyes. That depends on the finite speed of light, propagation delays, etc. The observer in SR is designed to not have these limitations so that it can see truly what reality is like. That's the reason the true observer in SR is the rods and clocks and not a human.

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#### csmyth3025

##### Guest
It never ceases to amaze me that, by taking the rather well accepted concept of equivalence in inertial frames of reference and the added postulate that the speed of light is observed to be the same in all inertial frames of reference, that Einstein was able to craft such an elegant and far-reaching theory. From my reading, I understand that some of his contemporaries were thinking along the same lines, but in my eyes that doesn't diminish his stunning achievement.

To me, it's like an unknown post-doctoral graduate making a simple (in hindsight) assumption and explaining the existence and nature of dark matter and dark energy.

Chris

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#### mickeyl

##### Guest
Yes, we all know that no massive object can travel at the speed of light. So, as an approximation then, close to the speed of light, time-dilation does not occur in open-space. Gravity is dark-energy-radiation, that surrounds any massive object in the universe as the radiation is slightly slowed while passing through it. The disturbance/slowing of radiation flow creates a massless-radiation-force surrounding the object. The slightly slowed radiation-flow, creates a minuscule slowing of time (atom-vibration) on the object, as compared to atom-vibrations in empty-space away from any massive object.
In empty-space, time-flow is uniform. A non-accelerating spaceship, traveling in uniform-motion in empty-space experiences no time compression (as expressed by Einstein's elevator thought experiment, or the gravitational effect of an accelerating spaceship). The light-photon would age 2-years, the astronaut would age 2-years, and the astronomer with the telescope would also age approximately 2-years. Because of the slowing of dark-energy-flow through the earth, the astronomer here would age 2-years minus 7.3 x 10^-7 second (730 days x 1^-9 seconds) = 7.3 x 10^-7 sec.). Or, approximately one-billionth second per day as determined by the global-positioning-satellite system.
You can check out my discussion of dark-energy-radiation at: http://www.dark-energyuniverse.com

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#### SpeedFreek

##### Guest
mickeyl":2qfcurqr said:
Yes, we all know that no massive object can travel at the speed of light. So, as an approximation then, close to the speed of light, time-dilation does not occur in open-space.
Sorry, but time-dilation occurs between objects in relative motion, whether they be travelling close to the speed of light or relatively slowly (there is time-dilation between clocks on the Earth and clocks on GPS satellites in orbit around the Earth, this is a confirmed effect of both relative motion and the difference in gravitational potential).

mickeyl":2qfcurqr said:
Gravity is dark-energy-radiation, that surrounds any massive object in the universe as the radiation is slightly slowed while passing through it. The disturbance/slowing of radiation flow creates a massless-radiation-force surrounding the object. The slightly slowed radiation-flow, creates a minuscule slowing of time (atom-vibration) on the object, as compared to atom-vibrations in empty-space away from any massive object.
Why does time "run slowest" at the centre of any massive object? Why does your dark-energy-radiation converge at the centre of any massive object, if it is the cause of gravity itself? If it were just passing through, from every direction, we would not see a concentration of this energy at the centre, would we?

mickeyl":2qfcurqr said:
In empty-space, time-flow is uniform. A non-accelerating spaceship, traveling in uniform-motion in empty-space experiences no time compression (as expressed by Einstein's elevator thought experiment, or the gravitational effect of an accelerating spaceship).
Time-dilation is relative. A spaceship travelling in uniform motion will be time dilated relative to anything else that is not "at rest" in relation to that spaceship.

mickeyl":2qfcurqr said:
The light-photon would age 2-years, the astronaut would age 2-years, and the astronomer with the telescope would also age approximately 2-years.
Why does a clock at the top of a mountain run faster than a clock at sea level? Why does distance from the centre of the massive object affect time, if dark-energy-radiation is passing through it from all directions? Why a centre of gravitational potential and not the same gravitational potential throughout the object?

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#### csmyth3025

##### Guest
I'm certainly no expert on relativity. My understanding, though, is that the gravitational force of a large body - at the surface of the body (assuming the body has a solid surface) - is measured by the acceleration of a relatively small mass (a kg, for instance) if it's allowed to free fall and the effects of atmospheric drag are disregarded. For Earth this acceleration amounts to ~9.8 m/sec^2.

For most normal large bodies (a nearly spherical body like the Earth, for instance) this force acts as if it is emanating from the center of the body (for an observer standing on the surface). I'm afraid I don't follow the logic proposed for dark energy in this regard. As far as I know, only dark matter has a gravitational effect (although dark energy can be thought of as having a negative gravitational effect, I suppose).

As I understand it, dark matter is invoked to explain the otherwise unexplainable excessive rotational speed of stars in the outlying regions of galaxies and the equally unexplainable observation that there are groups of galaxies which seem to be gravitationally bound even though there doesn't seem to be enough visible mass to account for this effect. Is there any observational evidence that the gravitational effects of dark matter can be seen on scales smaller than that of galaxies?

I know that the orbits of the major planets of our sun, for instance, have been calculated with great precision. Is there any indication that they're going faster than their orbits would theoretically require them to go according to our currently accepted methods of calculating such things?

Chris

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