Do different frames of reference experience time dilation at the same rate?

kweagle
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Hi Everyone, first I want to say I have no formal education or background on these topics, but find them very interesting and research and learn as much as I can on my own. With that in mind, I am hoping some of you will have the patience to explain what I don't seem to understand.

What I am curious most about it 'what is time' from a scientific standpoint. I can't seem to find a definitive answer to this, which maybe there is not one. It would however help me in understanding the next part of my question.

I am thinking of this in term of GPS positioning satellites. I understand and accept that these satellites experience a time dilation relative to the Earth's surface due to the speed they travel and the gravitational difference. I understand that they use atomic clocks which are the most accurate way we have of measuring time. What I don't understand is how do we know that speed and gravity are actually affecting 'time itself', and not just affecting the atomic vibrations we are measuring instead?
 
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Time dilation was predicted by special relativity, and later proved to be real.
As for what time actually is, Einstien said that it's the thing that gets measured by clocks.
 
Hi, kweagle. Welcome to PF!

The usual definition of time used in physics is 'time is what clocks measure'. It is the definition Einstein adopted when developing his special and general theories of relativity.
So there's no difference between saying that 'time slows down' and 'all physical processes slow down', since a clock is just some physical process (usually a multitude of those).

We could posit a hypothesis that it's not time (all processes) slowing down, but just one specific type of a process (e.g., radioactive decay), but so far all observations agree with the 'all processes' assertion. The hypothesis would need to specify which kind of process is not affected, and then test it.

But that would be a bit backwards, since the predictions of time dilation/contraction of SR and GR follow from the definition that treats time as encompassing all processes.
 
So with the idea that clocks are just a physical process, is correct to say there a difference between 'time' and a 'physical process'?

I guess what I don't understand is if clocks are affected by speed and gravity, how can they be a reliable tool to measure time?
 
To define time as what a clock measures is an example of a philosophy called operationalism: http://plato.stanford.edu/entries/operationalism/

kweagle said:
I guess what I don't understand is if clocks are affected by speed and gravity, how can they be a reliable tool to measure time?

But we don't have a more reliable tool.
 
kweagle said:
I guess what I don't understand is if clocks are affected by speed and gravity, how can they be a reliable tool to measure time?
Because the physical processes change their (apparent) speed, in a way which is consistent with what relativity describes.
 
Hmmm... I accept GR and all that, I am not trying to dispute it, but can it not be said that time is a constant, and its the tools we use to measure time that change with speed and gravity, and therefore the time dilation we see with a clock is not necessarily a change in time itself, and only in the tools we are using to measure it?
 
kweagle said:
Hmmm... I accept GR and all that, I am not trying to dispute it, but can it not be said that time is a constant, and its the tools we use to measure time that change with speed and gravity, and therefore the time dilation we see with a clock is not necessarily a change in time itself, and only in the tools we are using to measure it?

If this approach were to be useful, you would need some way to define and measure this true or undistorted time. Nobody has any way to do that.
 
Is my way of looking at it somehow flawed though? Is it possible that the time dilation we see is a result of the effect on the instruments we use to measure time, and not time itself? This has always been my biggest problem with understanding time dilation.
 
  • #10
One way of putting it is that there is no universal absolute clock which can be referred to.
Relativity makes predictions (such as time dilation) which are useful in some cases (GPS), and more accurate than Newtons (equally amazing earlier) proposals, which make the assumption of 'time' as being a universal absolute constant.
 
  • #11
So can the following statements all be said to be true?

-Clocks measure time
-The definition of time is a measurement of a physical process
-All physical processes are affected by speed and gravity
 
  • #12
1, Yes, 2,Yes, and 3.I don't know( but it seems to be so).
 
  • #13
kweagle said:
So can the following statements all be said to be true?

-Clocks measure time
-The definition of time is a measurement of a physical process
-All physical processes are affected by speed and gravity

The first one is true but can be qualified to 'clocks measure their own time'. In SR the time elapsed on a clock has a clear mathematical definition and is an invariant.
The nearest thing to a deifinition of time is 1. So probably 2 is redundant.

Your third point is true as regards gravity. Relative velocities are important physically if two things collide or interact in some way. Otherwise relative velocity causes Doppler shifts and strange length measurements.
 
  • #14
kweagle said:
So can the following statements all be said to be true?

-Clocks measure time
-The definition of time is a measurement of a physical process
-All physical processes are affected by speed and gravity
The third one is difficult because it lacks a mechanism or even a theoretical basis. According to the principle of relativity that has been a part of physics since physics was first invented by Galileo, speed is only relevant as measured between two objects. So at any time, any object can have an infinite number of different speeds depending on what frame of reference you choose to measure it in. That makes the clock rate an essentially arbitrary choice and not a single value for a particular clock.

Even with gravitational time dilation, there is a difficulty in detecting an actual impact of gravity itself on the processes. That is wholly different from the way gravity affects a pendulum clock, for example.

So the idea that gravity/relative speed affects time and not just individual physical processes is theoretically simpler -- the opposite view is highly problematic and has no evidence for it whatsoever. It isn't necessarily wrong, but it is scientifically inferior due to its complexity and lack of support (Occam's razor).
Is it possible that the time dilation we see is a result of the effect on the instruments we use to measure time, and not time itself?
The way you say that implies that you think that some other instrument, not yet invented, might be able to measure how time "really" works and not be influenced by gravity/speed. While not completely impossible, that would be a pretty huge coincidence that all current clocks show exactly the same "error" (if accurate enough to measure it), even though we have methods of measuring time that use vastly different operating principles.
 
  • #15
kweagle said:
So can the following statements all be said to be true?

-Clocks measure time
-The definition of time is a measurement of a physical process
-All physical processes are affected by speed and gravity

All those statements can be said to be true - but the third one is not generally accepted even though it works as a model (the theoretical basis is nowadays often called "LET"). As no absolute speed can be determined (measurements of uniform speed are relative, that's relativity in a nutshell), many people hold that it doesn't exist, and of course physical processes can not be affected by the speed with respect to your freely chosen reference system! Debates on this forum on that rather philosophical topic have been ended with a formal interdiction, see the FAQ here, https://www.physicsforums.com/threads/what-is-the-pfs-policy-on-lorentz-ether-theory-and-block-universe.772224/

Concerning gravity, indeed according GR, a "clock goes therefore slower when it is placed in the neighbourhood of ponderable masses" - https://en.wikisource.org/wiki/The_...Perihelion-motion_of_the_paths_of_the_Planets. [edit: slight correction mine]
 
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  • #16
kweagle said:
Hmmm... I accept GR and all that, I am not trying to dispute it, but can it not be said that time is a constant, and its the tools we use to measure time that change with speed and gravity, and therefore the time dilation we see with a clock is not necessarily a change in time itself, and only in the tools we are using to measure it?

This is wrong because there are different tools for measuring time and they all yeild the same results for time dilation. Theres no way, I'm sorry, that its just our tools and not varying time. Your totaly disputing GR your saying Einstien was wrong and time is constant.

Why would gravity make a clock go slower, I could use a sand clock and gravity would make time go faster.

Think of time as changes, and we are in a high entropy universe so everything is changing. That the best way to think of time as rate of change .
 
  • #17
Hi Kweagle, welcome to PF forum.
I'm no physicist either, much less scientist. But let me try to help you with a layman point of view.
kweagle said:
-Clocks measure time?
Yes.
But,
Grandfather clocks/pendulums measure time? Yes, but it depends on gravitation
Digital clocks measure time? Yes, it doesn't depend on gravity but it depends on the power of its battery
Bacteria fermentation measures time? Yes, it does not depend on electric force but it depends if you put it in a refrigerator or not.
Atomic clocks measure time? Yes, but a moving atomic clocks run faster than stationary atomic clocks. SR theory.
So I think the standard clock is atomic clock without acceleration force applied to it. It can move, as long as it does not experience acceleration.
I'm at lost here also. What defines standard clock? The vibration of caesium atom or the time it takes for light to travel 1 metre? Or something else?

kweagle said:
-The definition of time is a measurement of a physical process
Yes.
1 second is the time it takes for the second hand in analog clock to rotate 60 clock wise (of course)
caveat: If somehow the clock axis has rust and dragging the second hand, the time is slow according to the clock.
1 second is the time it takes for the temperature of 1cc of water to be raised 10 Celcius if applied by 4.2 watt.
1 second is the time it takes for caesium to vibrate 9,192,631,770 times. https://en.wikipedia.org/wiki/Caesium_standard
The definition of time is a measurement of a physical process. But first, you have to define the physical process.
You just can't put 4.2 joules in 1 cc of any water to increase it 10 celcius. It has to be pure H2O in certain pressure and in liquid state.
But the definition of standard time is usually by atomic clock.

kweagle said:
-All physical processes are affected by speed and gravity
Gravity perhaps..., But speed? I don't think so. Speed is relative. Of course if you move one object speed is affective, or in the case of kinetic energy. Perhaps someone else can answer...?
 
  • #18
Stephanus said:
[..] [+rearrange]
What defines standard clock? The vibration of caesium atom or the time it takes for light to travel 1 metre? Or something else?
[..]
But the definition of standard time is usually by atomic clock.
Right. The original standard clock was the Earth's rotation in its orbit, giving the solar day as reference (divided in 24 hours->min->s). Sundials were replaced by pendulum clocks and nowadays as atomic clocks are more regular than the Earth's rotation, they have mostly replaced the solar clock for precise time keeping. [edit:] The basis for long periods remains the solar day, that's why we need to insert a leap second now and then - the next one will be in a few days from now!
-https://en.wikipedia.org/wiki/Leap_second
but a moving atomic clocks run faster than stationary atomic clocks. SR theory.
[..] speed? I don't think [that all physical processes are affected by speed]. Speed is relative. Of course if you move one object speed is affective, or in the case of kinetic energy. Perhaps someone else can answer...?
Those two statements sound contradictory to me (and "faster" should be "slower") :wink:. Several clarifications have been given already, mine is in #15
 
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  • #19
harrylin said:
Those two statements sound contradictory to me (and "faster" should be "slower") :wink:. Several clarifications have been given already, mine is in #15
The moving clock runs SLOWER wrt rest. Sorry, sorry, sorry :smile:
 
  • #20
kweagle said:
Is it possible that the time dilation we see is a result of the effect on the instruments we use to measure time, and not time itself?
What is "time itself"? That is something pretty hard to pin down, if you don't want to use the operational definition given above.

However, let's look at this alternative as a possibility. Suppose that there is a background "time itself" which is not subject to time dilation. If that were the case then we would need some mechanism to explain why clocks based on EM (e.g. atomic clocks) are coincidentally time dilated to exactly the same degree as we would expect from relativity, despite time itself continuing unaffected. OK, so we propose such a mechanism.

But some clocks use the weak force as their clock mechanism, and we find that such clocks also are coincidentally time dilated by exactly the same degree as expected from relativity. But since the mechanism is different then we need a separate mechanism for the time dilation. OK, so we propose another mechanism which accomplishes that.

But some clocks use the strong force as their clock mechanism, and we find that such clocks are also coincidentally time dilated by the same degree as expected from relativity. I'm sure you see the point.

The number of theories that you would have to derive and have to tune exactly correctly for it to all coincidentally turn out to be the same is not something which is taken seriously. It is asking for coincidence upon coincidence upon coincidence, all to arrive at the same place as you get from the postulates of relativity.
 
  • #21
kweagle said:
-All physical processes are affected by speed and gravity
Can I ask a question here?
Gravity affects physical process, that is intuitively correct I think. I read somewhere that giving birth in space is impossible if not difficult. The baby can't orient his/her head to birth canal. Is just one example.
But what about speed? We could have move 300km/s from Andromeda, but all the physical/chemical processes are not affected. Is this right?
Of course if you count the kinetic energy if Andromeda hits us is ##\frac{1}{2} * Andromeda_{mass}? or Milkyway_{mass} * 300000^2## (or I should calculate it with momentum equation?) and assuming Andromeda and Milkyway are just two supermassive black holes, not two galaxies with sparse stars.
Seriously, does speed affect physical process INSIDE a closed system that moves? Because motion is relative, right?
 
  • #22
Stephanus said:
Can I ask a question here?
Gravity affects physical process, that is intuitively correct I think. I read somewhere that giving birth in space is impossible if not difficult. The baby can't orient his/her head to birth canal. Is just one example.
That intuition is quite useless: clock frequency is a function of gravitational potential energy. It is not a function of gravitational strength.
But what about speed? We could have move 300km/s from Andromeda, but all the physical/chemical processes are not affected. Is this right?
Of course if you count the kinetic energy [..]
You cannot go as fast as light. According to a reference system in which you are moving almost as fast as light, your physical processes are very slow; and indeed, you also have a very high kinetic energy as measured in that system. This slowdown of your body clock to nearly zero at nearly the speed of light can in principle be used for intergalactic travel, so that it should be possible for an astronaut to arrive alive at a distant star that is hundreds of light years away according to us. Thus Einstein already mentioned in his 1905 paper that "we shall, however, find in what follows, that the velocity of light in our theory plays the part, physically, of an infinitely great velocity."
if Andromeda hits us is ##\frac{1}{2} * Andromeda_{mass}? or Milkyway_{mass} * 300000^2## (or I should calculate it with momentum equation?) and assuming Andromeda and Milkyway are just two supermassive black holes, not two galaxies with sparse stars.
Seriously, does speed affect physical process INSIDE a closed system that moves? Because motion is relative, right?
Because our measurements of uniform motion are relative, no effect of constant linear speed can be detected inside a system that moves; no observation from a different perspective can turn a star into a black hole. :oldwink: It doesn't matter that the mass density goes to infinite according to you; the equations relating to black holes are wrt the centre of mass reference system.
For calculating kinetic of very high speed objects (with always v<c), the classical equation is not accurate enough. For v=c, Ekin=infinite
 
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  • #23
kweagle said:
Hi Everyone, first I want to say I have no formal education or background on these topics, but find them very interesting and research and learn as much as I can on my own. With that in mind, I am hoping some of you will have the patience to explain what I don't seem to understand.

What I am curious most about it 'what is time' from a scientific standpoint. I can't seem to find a definitive answer to this, which maybe there is not one. It would however help me in understanding the next part of my question.

I am thinking of this in term of GPS positioning satellites. I understand and accept that these satellites experience a time dilation relative to the Earth's surface due to the speed they travel and the gravitational difference. I understand that they use atomic clocks which are the most accurate way we have of measuring time. What I don't understand is how do we know that speed and gravity are actually affecting 'time itself', and not just affecting the atomic vibrations we are measuring instead?

Hey , same with me and it is nice that even if we have no formal education we are still interested in science and particulary in physics. It is nice to try know and understand our universe. Well , time if I don't missremember , as Michio Kaku said : Time it is not the Tick Tick Tick of a clock , it is the Click Click Click of an electromagnetism itself and no wonder that it will always be affected by other forces like gravity for example. Time , spacetime is always there or here or just around and we just move in it or just live in it. Humans invented clocks or watches to measure it and remember , the very fist clock that was "invented" it is our Earth herself : day and night, day and night :) but again just to meausre it :)
 
  • #24
kweagle said:
So with the idea that clocks are just a physical process
All of physics is just about physical processes. If what you call "time itself" cannot by measured by physical processes, then it's not part of physics.
 
  • #25
Thank you everyone for the replies, I really appreciate it. I have been scouring the web trying to learn more about time and what it really is, and I think I am starting to get some of the theories a bit more. I do have some follow-up comments/questions if you don't mind.

First let me explain what my personal concept of time has always been. To me, 'time' is a constant. It is linear, and it always passes at the same rate, no matter where you are, what you are doing, or what is around you, time never stops or slows down. What does change is what you are able to observe happening in time as a result of the time it takes for light to travel. Time keeping, however, is something developed by humans as a way to measure the passage of time. It has always been my thought that it was the measuring devices (the physical/quantum world) that is affected by speed/acceleration and gravity. I accept that this view is not correct, which is why I am here to try and understand it.

Relativity does make perfect sense to me, but it is the time dilation that does not. It seems to me that time dilation is simply a result of the time it takes light to travel based on relative speeds of two objects, therefore, I think a more accurate name would be 'light dilation'.

With that being said, here are my questions...

If the speed of light is considered to be a constant speed, no matter who is observing it, and no matter what velocity they are traveling or what gravitational forces they may be experiencing, would it be accurate to say that time as we know it in physics is derived directly from the speed of light, lightspeed being the base line for how time is measured, and this is why time changes with the speed of light?

If the speed of light is constant, why do we observe a redshift in stars that are moving away from us? Shouldn't they appear to look the same no matter how they are moving if light is always traveling at a constant speed?

If two people left Earth traveling at near the speed of light, one going in a straight line and back, and the other simply orbiting the earth, both returning to the surface at the same moment, would time have passed the same amount for both of them when they got back? How much time would have passed for them and how much time would have passed on earth?
 
  • #26
I think I have a simplified analogy for time dilation. Due to motion is relative to the velocity of light, due to gravity is the amount a ray curves through warped space at the speed of light.
 
  • #27
kweagle said:
Thank you everyone for the replies, I really appreciate it. I have been scouring the web trying to learn more about time and what it really is, and I think I am starting to get some of the theories a bit more. I do have some follow-up comments/questions if you don't mind.
You bet, I don't understand it either. I'll answer what I know is POSITIVELY TRUE, okay...?
kweagle said:
To me, 'time' is a constant.
Yes, for me, too.:smile: And for my brain, and for my neuron system, how I perceive things around me, for my cloth and for my watch for that matter. No matter where I am. But, if I come close near the singularity, time at my feet could be different with the clock in my head. But of course as Albert said, sitting for 1 minute on a hot stove seems verrry long time. Talking to a beautiful girl for an hour seems very short.
kweagle said:
If the speed of light is considered to be a constant speed, no matter who is observing it, and no matter what velocity they are traveling or what gravitational forces they may be experiencing, would it be accurate to say that time as we know it in physics is derived directly from the speed of light, lightspeed being the base line for how time is measured, and this is why time changes with the speed of light?
That's what I think, too. But perhaps someone more reliable can answer that for both of us.
kweagle said:
If the speed of light is constant, why do we observe a redshift in stars that are moving away from us? Shouldn't they appear to look the same no matter how they are moving if light is always traveling at a constant speed?
Red shifted is the Doppler effect that you see, not the speed of light, it's the light FREQUENCY that moves to the red area according to Franhover spectrum lines. The speed of light is always the same, now and ever and ever in the same medium. For instance in a vaccuum
kweagle said:
If two people left Earth traveling at near the speed of light, one going in a straight line and back, and the other simply orbiting the earth, both returning to the surface at the same moment, would time have passed the same amount for both of them when they got back? How much time would have passed for them and how much time would have passed on earth?
I think, to know this answer, you have to study Lorentz Factor, Lorentz Transformation, Space Time diagram, Light Cone, World line, as I do. First, I want to know about Twins Paradox, but the explanation is not that simple, at least for me.
 
  • #28
kweagle said:
If the speed of light is considered to be a constant speed, no matter who is observing it, and no matter what velocity they are traveling or what gravitational forces they may be experiencing, would it be accurate to say that time as we know it in physics is derived directly from the speed of light, lightspeed being the base line for how time is measured, and this is why time changes with the speed of light?
What do you mean by 'time changes with the speed of light' ? Whose time ? It is important to realize that every clock measures its own passage through spacetime.

If the speed of light is constant, why do we observe a redshift in stars that are moving away from us? Shouldn't they appear to look the same no matter how they are moving if light is always traveling at a constant speed?
No. If the receiver is receding from the source there is a red shift. As predicted and observed.

If two people left Earth traveling at near the speed of light, one going in a straight line and back, and the other simply orbiting the earth, both returning to the surface at the same moment, would time have passed the same amount for both of them when they got back? How much time would have passed for them and how much time would have passed on earth?
This can be calculated from the proper intervals of the various clocks involved. There is no mystery here.
 
  • #29
Ok... Let me rephrase some of that.

Lets say someone is able to teleport instantaneously from Earth to 1 light year from earth. If they look at earth, they will only see the light from the Earth from one year ago, would you consider that person to have traveled a year into the past?

Lets say someone leaves Earth traveling at the speed of light for one year. While they are moving, it will appear time has stopped on earth. If they return to Earth at the speed of light, will time on Earth appear to move twice as fast? Two years will have passed on earth, but no time would have passed for the person who left? Even though they are able to see time passing on earth?
 
  • #30
kweagle said:
will time on Earth appear to move twice as fast?
It would be infinitely sped up at c which is why it can't be conceived logically.
 
  • #31
kweagle said:
Ok... Let me rephrase some of that.

Lets say someone is able to teleport instantaneously from Earth to 1 light year from earth. If they look at earth, they will only see the light from the Earth from one year ago, would you consider that person to have traveled a year into the past?
A year after arriving they could see themselves leaving. It is not possible so don't even think about it.

Lets say someone leaves Earth traveling at the speed of light for one year. While they are moving, it will appear time has stopped on earth. If they return to Earth at the speed of light, will time on Earth appear to move twice as fast? Two years will have passed on earth, but no time would have passed for the person who left? Even though they are able to see time passing on earth?
It is not possible to travel at the speed of light. Rephrase the question in terms of sub-light speed travel and the answer is 'the person whose worldline had the shorter proper time will age less.'.
 
  • #32
kweagle said:
Lets say someone is able to teleport instantaneously from Earth to 1 light year from earth.
What do you mean by "instantaneously"? Two observers in relative motion have different concepts of "simultaneous"...

kweagle said:
If they look at earth, they will only see the light from the Earth from one year ago, would you consider that person to have traveled a year into the past?
...which is why whatever definition of "instantaneously" you use there will be someone who sees the person having traveled backwards in time. It's got nothing to do with the light arriving.

kweagle said:
Lets say someone leaves Earth traveling at the speed of light for one year. While they are moving, it will appear time has stopped on earth. If they return to Earth at the speed of light, will time on Earth appear to move twice as fast? Two years will have passed on earth, but no time would have passed for the person who left? Even though they are able to see time passing on earth?
Travelling at the speed of light is impossible. One of the fundamental principles of relativity is that light always travels at the same speed viewed from any inertial frame. If you are traveling at the speed of light, you are at rest with respect to light. How can light travel at 0 and 3x108m/s at the same time? The whole concept of travel at the speed of light is self-contradictory in a universe described by relativity.
 
  • #33
kweagle said:
Ok... Let me rephrase some of that.

Lets say someone is able to teleport instantaneously from Earth to 1 light year from earth. If they look at earth, they will only see the light from the Earth from one year ago, would you consider that person to have traveled a year into the past?

Leaving aside the impossiblity of instanaenous teleportation, your question reveals a common misconception that relativity is related to or depends upon the finiteness of the speed of light delaying observations. Consider a problem from classical physics: a piledriver is working at some distance from you. You see the impact of the piledriver before you hear it. But, you do not conclude that the sound is produced later than the impact. Your observations are delayed by the finiteness of the speed of light (negligible in this case) and the speed of sound (not negligible). But, in your reference frame the impact and the sound generation are simultaneous. In other words, you must take the delay in the respective observations of light and sound into account when deciding when something happened.

To rephrase your question:

If two observers are at rest with respect to each other and a light year apart, and they exchange signals, then these signals take a year before they are observed, but neither observer is "in the past" with respect to the other.
 
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  • #34
kweagle said:
Ok... Let me rephrase some of that.

Lets say someone is able to teleport instantaneously from Earth to 1 light year from earth. If they look at earth, they will only see the light from the Earth from one year ago, would you consider that person to have traveled a year into the past?
Ignoring the fact that you cannot travel any distance instantaneously, No. No more than you would consider a person standing 300 meters away from you as being 1 microsecond "in the past because that is how long it took the image of him to reach you.
Lets say someone leaves Earth traveling at the speed of light for one year. While they are moving, it will appear time has stopped on earth. If they return to Earth at the speed of light, will time on Earth appear to move twice as fast? Two years will have passed on earth, but no time would have passed for the person who left? Even though they are able to see time passing on earth?

When you try and assume travel at the speed of light you bring in all kinds of problems. (for instance if you try and work out what he sees on the return leg, you end up with a division by zero, which it undefined.)

You can work it out fro any speed up to, but not including c, Say for instance 0.999999999999999 c.

In this case, if he travels to a point 1 light year from Earth ( as measured by the Earth), he will see the Earth age 3.156e-8 sec during the 1.411 sec he measures for the trip. On the return leg he will see the Earth age 63103756.7084 sec while he ages another 1.411 sec. Total amount he ages 2.822 sec, total time he sees the Earth age, just a tad over 2 years.
 
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  • #35
Ibix said:
What do you mean by "instantaneously"? Two observers in relative motion have different concepts of "simultaneous"...

With all due respect, how is "instantaneously" not clear? I know it is impossible, but we are talking hypothetically to make it easier to explain what I am asking. I could rephrase the same question to ask what does an observer 1 light year away from the Earth see? They see what Earth looked like 1 year ago, right?
Ibix said:
Travelling at the speed of light is impossible. One of the fundamental principles of relativity is that light always travels at the same speed viewed from any inertial frame. If you are traveling at the speed of light, you are at rest with respect to light. How can light travel at 0 and 3x108m/s at the same time? The whole concept of travel at the speed of light is self-contradictory in a universe described by relativity.

Again, I know traveling at the speed of light is impossible. If it makes the question any more acceptable, let's say you are a photon.
 
  • #36
PeroK said:
..in your reference frame the impact and the sound generation are simultaneous. In other words, you must take the delay in the respective observations of light and sound into account when deciding when something happened.

Exactly. I understand that perfectly, but it seems to contradict some of the notions about relativity and time dilation. Perhaps it is because I do not understand it fully.
PeroK said:
If two observers are at rest with respect to each other and a light year apart, and they exchange signals, then these signals take a year before they are observed, but neither observer is "in the past" with respect to the other.

So what you are saying is that a moment in time is the same for everyone no matter where you are, correct? Then how is it that time can pass slower for someone traveling compared to someone stationery? Would that not mean they would be in different moments in time? Again, I understand they will perceive things differently relative to each other due to the speed of light, but they must be in the same moment in the universe.

Janus said:
In this case, if he travels to a point 1 light year from Earth ( as measured by the Earth), he will see the Earth age 3.156e-8 sec during the 1.411 sec he measures for the trip. On the return leg he will see the Earth age 63103756.7084 sec while he ages another 1.411 sec. Total amount he ages 2.822 sec, total time he sees the Earth age, just a tad over 2 years.

So even though it takes ~1 light year to reach the destination, he is unable to observe anything during that time? Why is that?
 
  • #37
kweagle said:
Exactly. I understand that perfectly, but it seems to contradict some of the notions about relativity and time dilation. Perhaps it is because I do not understand it fully.

It contradicts nothing about relativity, as the finiteness of the speed of light delaying observations is not a factor in relativity. And, yes, that's because you don't yet understand it fully.

kweagle said:
So what you are saying is that a moment in time is the same for everyone no matter where you are, correct? Then how is it that time can pass slower for someone traveling compared to someone stationery? Would that not mean they would be in different moments in time? Again, I understand they will perceive things differently relative to each other due to the speed of light, but they must be in the same moment in the universe.

Not at all. The critical difference between relativity and classical physics is when you have two reference frames moving with respect to each other. If you have two observers at rest with respect to each other, then they will agree on measurements of length and elapsed time.

But, if the two observers are moving with respect to each other, then they will not agree on lengths and elapsed times.

Notice that I say "elapsed time", as the concept of an absolute "moment" in time is essentially meaningless. Elapsed time as measured between two events is the vital concept.
 
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  • #38
kweagle said:
With all due respect, how is "instantaneously" not clear?
It means "you depart and arrive simultaneously". But "simultaneously" is only an unambiguous concept if there is an absolute universal definition of time - which is not the case in relativity.
kweagle said:
I know it is impossible, but we are talking hypothetically to make it easier to explain what I am asking. I could rephrase the same question to ask what does an observer 1 light year away from the Earth see? They see what Earth looked like 1 year ago, right?
Typically, we would say yes, what you are seeing now is what happened one year ago. But in doing so we would have implicitly chosen to use Einstein's definition of simultaneity. It's the most natural one in this circumstance and perfectly reasonable, but it's just a choice. That's why I was criticising your teleportation - because "simultaneous" (even using Einstein's convention) is different for observers in relative motion. Some people would say your "instantaneous" transit took time to complete; some would say it took negative time to complete.

Relating that to you sitting a light year from Earth looking at it through a telescope, someone traveling at 0.866c would say that the Earth is only half a light year from you, so you are observing it six months ago. Such an observer would have a coherent explanation in terms of your clocks being set differently from the ones on Earth (because his definition of "simultaneous" is different from yours) for why you are seeing the calendars showing a year ago, even if it's only six light months to Earth in his frame (6 months' travel time plus six months offset).

kweagle said:
Again, I know traveling at the speed of light is impossible. If it makes the question any more acceptable, let's say you are a photon.
How is the photon going to measure time or distance? It can't build rulers or clocks.

I'm not just being glib here - the operational definition of time is "whatever it is clocks are measuring". Since you can't have a clock moving at light speed, you can't define time at the speed of light. That's a kind of mirror version of my earlier argument. I like the first one better because it's more or less a direct statement of the relevant truth: it makes no sense to talk about the perspective of anything traveling at the speed of light. You will only get nonsense if you try.
 
  • #39
Mentz114 said:
What do you mean by 'time changes with the speed of light' ? Whose time ? It is important to realize that every clock measures its own passage through spacetime.
I think what kweagle meant is this. If you don't have a measuring time device and you only have sand clock. How can we determine how much grain of sands we need to measure 1 second. So all you have to do is put a mirror at 150000 km away and send a light and also put some sands in the sand clock. And when the light comes back it is how 1 second is. Or you want to calibrate your other time measure device, is through light and distance.
 
  • #40
Mentz114 said:
It is not possible to travel at the speed of light. Rephrase the question in terms of sub-light speed travel and the answer is 'the person whose worldline had the shorter proper time will age less.'.
The longer worldline you mean? Come on, it's you who taught me space time diagram. :smile:
 
  • #41
Mentz114 said:
It is not possible to travel at the speed of light. Rephrase the question in terms of sub-light speed travel and the answer is 'the person whose worldline had the shorter proper time will age less.'.
Sorry, you're right. The shorter proper time = the longer worldline.
 
  • #42
kweagle said:
Ok... Let me rephrase some of that.

Lets say someone is able to teleport instantaneously from Earth to 1 light year from earth. If they look at earth, they will only see the light from the Earth from one year ago, would you consider that person to have traveled a year into the past?/QUOTE]
If I may rephrase your question.
Not teleported. But two person. A on earth, B 1 light year away from earth. At the same time B will see the clock on Earth is 1 year late. On the other hand, A will see B clock is 1 year late also.
Let's say if there's an astronout on the moon. The astronout would see the Earth as it was 1.3 seconds ago. But we see the moon as it was 1.3 seconds ago, too. It doesn't mean that from the moon we are 1.3 sec in the past and vice versa.
 
  • #43
kweagle said:
Lets say someone leaves Earth traveling at the speed of light for one year. While they are moving, it will appear time has stopped on earth. If they return to Earth at the speed of light, will time on Earth appear to move twice as fast? Two years will have passed on earth, but no time would have passed for the person who left? Even though they are able to see time passing on earth?
Janus said:
You can work it out fro any speed up to, but not including c, Say for instance 0.999999999999999 c.

In this case, if he travels to a point 1 light year from Earth ( as measured by the Earth), he will see the Earth age 3.156e-8 sec during the 1.411 sec he measures for the trip. On the return leg he will see the Earth age 63103756.7084 sec while he ages another 1.411 sec. Total amount he ages 2.822 sec, total time he sees the Earth age, just a tad over 2 years.
I think the answer of your question is this...
##63103756.7084 / 1.411 = 44,722,719## Not twice as fast, but 44 millions faster. It's just a simple math :smile:
 
  • #44
kweagle said:
So what you are saying is that a moment in time is the same for everyone no matter where you are, correct? Then how is it that time can pass slower for someone traveling compared to someone stationery? Would that not mean they would be in different moments in time? Again, I understand they will perceive things differently relative to each other due to the speed of light, but they must be in the same moment in the universe.
Perhaps this can answer your question...
Janus said:
Consider Einstein's Train example...
https://www.physicsforums.com/threads/length-contraction.817911/page-2#post-5135255
 
  • #45
kweagle said:
So even though it takes ~1 light year to reach the destination, he is unable to observe anything during that time? Why is that?
Oh, he is ABLE observers anything, all right. During his return trips, he is ABLE to see everything on Earth moves 44 millions times faster! But on away trip, he is ABLE to see everything on Earth much slower. How much slower? I think it is ##\frac{1}{1.411}## slower. And when he stops, he'll see everything on Earth moves normally. During the return trip -> 44 millions faster.
 
  • #46
Stephanus said:
The shorter proper time = the longer worldline.

No, the shorter proper time = the shorter worldline, hence less aging. The amount of proper time, the amount of aging, and the length of the (timelike) worldline are all just different ways of describing the same thing.
 
  • #47
Stephanus said:
Gravity affects physical process, that is intuitively correct I think. I read somewhere that giving birth in space is impossible if not difficult. The baby can't orient his/her head to birth canal. Is just one example.

This is an example of proper acceleration affecting physical processes, not gravity. Giving birth in a rocket accelerating at 1 g would work the same as giving birth in a room at rest on the surface of the Earth. But giving birth in a rocket in free fall in space would have the difficulty you describe. That difference has nothing to do with gravity; it has to do with the presence or absence of proper acceleration.
 
  • #48
Stephanus said:
...impossible if not difficult
I like that.
 
  • #49
kweagle said:
[..] First let me explain what my personal concept of time has always been. To me, 'time' is a constant. It is linear, and it always passes at the same rate, no matter where you are, what you are doing, or what is around you, time never stops or slows down. What does change is what you are able to observe happening in time as a result of the time it takes for light to travel. Time keeping, however, is something developed by humans as a way to measure the passage of time. It has always been my thought that it was the measuring devices (the physical/quantum world) that is affected by speed/acceleration and gravity. I accept that this view is not correct, which is why I am here to try and understand it.
Relativity does make perfect sense to me, but it is the time dilation that does not. It seems to me that time dilation is simply a result of the time it takes light to travel based on relative speeds of two objects, therefore, I think a more accurate name would be 'light dilation'.
[..]
If the speed of light is considered to be a constant speed, no matter who is observing it, and no matter what velocity they are traveling or what gravitational forces they may be experiencing, would it be accurate to say that time as we know it in physics is derived directly from the speed of light, lightspeed being the base line for how time is measured, and this is why time changes with the speed of light?
Instead, all physical processes are affected the same - the speed of light was merely a known fact of observation that was used as input to the derivation (and perhaps you overlook that the estimated travel time is accounted for in the calculations).

This is just as when you have an equation 2x + c = 5 and you know that the constant c equals 3. With that information you can solve for the correct value for x, but that doesn't mean that x is physically a result of c!
If the speed of light is constant, why do we observe a redshift in stars that are moving away from us? Shouldn't they appear to look the same no matter how they are moving if light is always traveling at a constant speed?
The "constant speed of light" means that the speed of light is a constant, independent of the speed of the source - just like the speed of sound in air at standard conditions. The Doppler effect is well known with sound.

This is often confounded with the invariance of the speed of light: standard inertial reference systems that are in relative motion will each measure the speed of light as the same constant (this follows from the relativity principle). Those two facts of observation seem in contradiction to each other.
This is well presented in the intro here: https://www.fourmilab.ch/etexts/einstein/specrel/www/
If two people left Earth traveling at near the speed of light, one going in a straight line and back, and the other simply orbiting the earth, both returning to the surface at the same moment, would time have passed the same amount for both of them when they got back? How much time would have passed for them and how much time would have passed on earth?
In his 1905 paper to which I gave you the link, Einstein gave a prediction that a clock that is moved in a circle will be slow compared with a clock that stays in place. The answer to your questions are very well explained there (in the second half of §4): it makes no difference at all.

Similarly, Langevin gave in 1911 an example of an astronaut who travels fast to a nearby star and back; upon arrival on Earth he may find that all his friends already died of old age.
 
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  • #50
kweagle said:
Thank you everyone for the replies, I really appreciate it. I have been scouring the web trying to learn more about time and what it really is, and I think I am starting to get some of the theories a bit more. I do have some follow-up comments/questions if you don't mind.

First let me explain what my personal concept of time has always been. To me, 'time' is a constant. It is linear, and it always passes at the same rate, no matter where you are, what you are doing, or what is around you, time never stops or slows down. What does change is what you are able to observe happening in time as a result of the time it takes for light to travel. Time keeping, however, is something developed by humans as a way to measure the passage of time. It has always been my thought that it was the measuring devices (the physical/quantum world) that is affected by speed/acceleration and gravity. I accept that this view is not correct, which is why I am here to try and understand it.

Relativity does make perfect sense to me, but it is the time dilation that does not. It seems to me that time dilation is simply a result of the time it takes light to travel based on relative speeds of two objects, therefore, I think a more accurate name would be 'light dilation'.

With that being said, here are my questions...

If the speed of light is considered to be a constant speed, no matter who is observing it, and no matter what velocity they are traveling or what gravitational forces they may be experiencing, would it be accurate to say that time as we know it in physics is derived directly from the speed of light, lightspeed being the base line for how time is measured, and this is why time changes with the speed of light?
I'm going to try and paint a picture of what is going on through an analogy.
Imagine two men( M1 and M2) walking side by side on a featureless plane. Without changing his pace, one of the men(M2) changes the direction in which he is walking. Each man judges forward progress as progress in the direction he is facing. So from each Man's perspective, the other man is now making less forward progress (even though by the other man's judgement nothing has changed and he is still walking at the same rate as before.), and falls further and further "behind".
Now M2 changes direction again, turning so that he is walking in the same direction as M1. What does he perceive? As he turns, the M1's apparent position with respect to the direction M2 is facing changes and goes from being "behind" M2 to Being "ahead" of M2, as judged by M2. ( as far as M1 is concerned, M2 remains behind him). Both men are now walking in the same direction, and their relative positions are constant and bot Men agree that M1 is ahead of M2.

If M2 continues his turn until his forward path intersects M1's path, As he turns, M1 will move a bit more ahead of him. After the turn, M1 will be ahead of him, but now not progressing "forward" as quickly. M2 will start to gain on him a bit. However, by the time he intersects M1's path, he will not have caught up to him. If he then turns to match M1's walking direction he will still find himself behind M1.

The above analogy is like the twin paradox, While moving at different speeds (walking in different directions), Time in the other frame runs slow ( the other man makes less forward progress). If a spaceship travels away from the Earth at some speed, turns around and comes back, (M2 changes direction so that he returns to the path of M1), It will find that it has aged less than the Earth has. (M2 finds himself behind M1)

In the analogy I gave, there is no universal fixed direction to which the concept of "forward progress" can be applied. each man has his own measurement of forward progress. In Relativity, there is no universal fixed "direction" to time measurement. Put another way, instead of a universe with 3 spatial dimensions and 1 time dimension which are fixed and separate from each other, we have a universe of 4 dimensional Space-time, where the "direction" for time depends on the relative motion's of the frame measuring it. Two observers with relative motion with respect to each other will judge the position of an event in time and space differently. Observer 1 could say that two events are separated by x meters and y sec, while Observer 2, with a relative motion with respect to Observer 1 would judge the same two events as being separated by w meters and z sec. Much as the way that two people facing in different directions would disagree as to how far to the right or left and how far to the front or back two objects are from each other.

The speed of light comes in in that it determines just how these differences perspective relate to each other. ( or conversely, the relationship between space and time determines what the speed of light is.)

If the speed of light is constant, why do we observe a redshift in stars that are moving away from us? Shouldn't they appear to look the same no matter how they are moving if light is always traveling at a constant speed?
I've heard this question a lot, and I've never quite understood the reasoning behind it. Doppler shift is not a result of how fast the light is traveling with respect to you, but how closely the Light waves are bunched together according to the observer.
Consider the following animation. In the first, the light source is stationary wit respect to the observers. The waves move out as circles, and hit both observers at the same frequency
doppler1.gif


In the second one, the source is moving towards the blue dot and away from the red.
doppler2.gif


The waves still move out as circles and at the same speed as they did before. ( the first wave even hits both observers at the same time, just as in the last animation.) However, as each successive wave is emitted, the source is closer to the blue dot than it is the red dot and the waves are more crowded together in this direction and more spread out in the other the other direction than they were in the first animation. The blue dot sees an Increase in frequency, and the red dot a decrease in frequency.
If two people left Earth traveling at near the speed of light, one going in a straight line and back, and the other simply orbiting the earth, both returning to the surface at the same moment, would time have passed the same amount for both of them when they got back? How much time would have passed for them and how much time would have passed on earth?

As long as we ignore gravitational effects and assume that the orbital speed and "straight line" speeds are the same, Then both people will have aged the same and less than someone on the Earth. The main difference is that the straight line traveler does all his acceleration turning the turn around while the "orbiting" traveler is under constant acceleration. (he wouldn't be in a true orbit as he would have to be constantly firing his engines inward towards the Earth in order to maintain a circular motion around it at such a high speed. )
 

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