Can time run differently for the same observer at the speed of light?

[Rishavutkarsh]

99% The Speed of Light??

1. if we move at this speed time is slow for us so what about the outer world does it seems faster to us?
2. people say that space also contracts in the direction so if we move at this speed for long time then will it contract constantly or will it just stop this process at one point?
3. and the last thing is if we light a torch from our space craft(we are moving at 99%C) then we will see it fasten and at the speed of C(times must be slower for us to see light faster than it really is) and at the same time any other spaceship moves to our opposite direction and light's a torch then how will we see that light ? our time must fasten for us to see light at that speed of C but at the same time our time must be slow to see our torch light at the speed of C! so how can these both processes take place at the same time? remember time can run differently 4 two observers but it can't run differently for 1 observers simultaneously

in third question i know that time can run differently 4 two objects but how can it travel at different speeds for the same object simultaneously
u know what i am asking this question third time because i don't get good answers anytime!

 

[Dale]

Hi Rishavutkarsh, welcome to PF!

SMS messaging shorthand is not acceptable, please use the best English you can.

1) We always see our own time as normal and (assuming that we are inertial) we always see other people's time as going slowly.

2) Again, assuming you are inertial, the amount of length contraction is constant.

3) We always see all light at c regardless of the direction. What changes based on direction is the frequency of the light (i.e. Doppler shift).

 

[nitsuj]

in third question i know that time can run differently 4 two objects but how can it travel at different speeds for the same object simultaneously
u know what i am asking this question third time because i don't get good answers anytime!

Consider the different perspectives. Observer #1, Observer #2 and the object that is traveling at C.

Perspective of observer one is there perspective only, not that of the other two. Spacetime is the variable that leads to different measurements that all calculate the object traveling to be moving at C.

From the perspective of the object traveling at C, it's speed never changed. What's cool, is the two observers would agree. But the observers would have different measurements of time/distance (ie spacetime) assuming one observer is moving and the other relatively isn't.

 

[Rishavutkarsh]

Consider the different perspectives. Observer #1, Observer #2 and the object that is traveling at C.

Perspective of observer one is there perspective only, not that of the other two. Spacetime is the variable that leads to different measurements that all calculate the object traveling to be moving at C.

From the perspective of the object traveling at C, it's speed never changed. What's cool, is the two observers would agree. But the observers would have different measurements of time/distance (ie spacetime) assuming one observer is moving and the other relatively isn't.

i understand that ! the problem arises when an observer observes 2 rays of light at the same time from opposite direction consider that we are moving at 99%C in a direction and 2 rays of light are emitted from our from and back simultaneously , time can run different for 2 observers but can it be different for a single observer simultaneously huh!?

 

[Rishavutkarsh]

1) We always see our own time as normal and (assuming that we are inertial) we always see other people's time as going slowly.

really!? but it's highly confusing i mean think about twin paradox if we would see everything outside slow then everything happens there slow so we should be more aged than them!

 

[George Jones]

really!? but it's highly confusing i mean think about twin paradox if we would see everything outside slow then everything happens there slow so we should be more aged than them!

Moving clocks are not always seen visually to run slow, sometime they are seen visually to run fast.

In reality, the phrase "a moving clock runs slow" does not necessarily mean "a moving clock is seen visually to run slow." A clock moving directly away from an observer appears visually to run slow, but a clock moving directly towards an observer appears visually to run fast. In both cases, what is seen visually is given by the Doppler expression, which is always different than the time dilation expression. In both cases, the time dilation expression, used appropriately, does apply.

Consider the following example.

Assume that Alice is moving with constant speed directly towards Ted. When Ted uses his telescope to watch Alice's wristwatch, he sees her watch running at a faster rate than his watch. Ted sees Alice's moving watch running fast, not slow! Ted sees this because of the Doppler shift. Because Alice moves towards Ted, the light that Ted sees from her watch is Doppler-shifted to a higher frequency. But the rate at which a clock or watch runs is like frequency, i.e., a second-hand revolves at a certain frequency, and all frequencies are Doppler-Shifted., so ted see Alice's wristwatch running fast.

To explain what "A moving clock runs slow." means, I first have to explain how Ted (with help from Bob) establishes his frame of reference.

Starting from Ted, a series of metre sticks, all at rest with respect to Ted, are laid end-to-end by Bob along the straight line joining Alice and Ted. At each joint between two consecutive metre sticks, Bob places a small clock. The metre sticks and clocks all are at rest with respect to Ted. Initially, none of the clocks are running; before turning them on, the clocks have to be synchronized. To do this, Ted directs a laser pointer along the line joining Ted and Alice, and then sends a flash of light. Each clock is turned on when the flash of light reaches it. The speed of light is not infinite, so the time taken for the light to travel from Ted to each clock has to be taken into account. To do this, the clocks' hands are set initially as follows. The clock one metre away from Ted is set to the time taken for light to travel one metre; the clock two metres away from the tower is set to the time taken for light to travel two metres; ... .

This whole setup of metre sticks and clocks establishes Ted's reference frame.

Now, As Alice moves toward Ted, Ted uses his telescope to watch Alice's wristwatch, and to watch his clocks. First, he watches one of the distant clocks in his reference frame. The time he sees on the clock is the time at which the light he sees set out from the clock, so Ted sees an earlier time on the distant clock than he sees on his wristwatch. Because the clock is stationary in his frame, Ted does, however, see the distant clock running at the same rate as his watch. Similarly, Ted's sees all the clocks in his frame running at the same rate as his watch.

As Alice approaches Ted, she whizzes by clock after clock of Ted's reference frame. Using his telescope, Ted sees that Alice is beside a particular clock, and he notes the time on her watch and the time on the clock adjacent to her. Some time later, Ted sees Alice beside a different clock, and he again notes the time on her watch and the time on the clock adjacent to her.

Ted checks his notes, and he finds that the time that elapsed on Alice's watch as she moved between these two clocks of his frame is less than the difference of the readings of the two clocks. This what is meant by "A moving clock runs slow."

 

[Rishavutkarsh]

Moving clocks are not always seen visually to run slow, sometime they are seen visually to run fast.

i get it thank you but clock can move slow or fast in respect to speed of light *time dilation* like if we are 90% speed of light in a spaceship and light a torch then our time moves 10 times slower than a object at 0%C (rest) but if at 99% C in a ship then it moves 100 times as slow as 0%C as these conditions let us see light at C the outer world is faster so it should appear fast rather than slower and also if we are slow in time our 1 year=10 years (of any object at 0%C )
twin paradox the outer world must be faster

 

[George Jones]

i get it thank you but clock can move slow or fast in respect to speed of light *time dilation* like if we are 90% speed of light in a spaceship and light a torch then our time moves 10 times slower than a object at 0%C (rest) but if at 99% C in a ship then it moves 100 times as slow as 0%C as these conditions let us see light at C the outer world is faster so it should appear fast rather than slower and also if we are slow in time our 1 year=10 years (of any object at 0%C )
twin paradox the outer world must be faster

Here is what happens.

I wrote a(n unreadably) terse quantitative analysis of what atty posted,

https://www.physicsforums.com/showthread.php?p=1384776#post1384776.

A simpler version:

Two twins, Alfred and Betty, are together on the planet Omicron 7. After synchronizing their watches, Betty sets off on a return trip from Omicron 7 to Earth and back to Omicron 7, and Alfred remains the whole time on Omicron 7. The distance between Omicron 7 and Earth is 3.75 lightyears in the (approximately) inertial reference frame of Omicron 7 and the Earth. Betty takes the most direct route and moves at a constant speed of 3/5 lightspeed during both the outgoing and incoming segments of the trip. Both the outgoing an incoming legs of Betty's trip take 5 years according to her watch.

As Betty travels, Betty uses a telescope to watch Alfred's wristwatch. During Betty's outgoing leg, she sees Alfred's second hand spin slower than hers by a factor of 2. Thus, Betty sees Alfred's watch advance by 5/2 = 2.5 years during the outgoing leg. During Betty's incoming leg, she sees Alfred's second hand spin faster than hers by a factor of 2. Thus, Betty sees Alfred's watch advance by 5*2 = 10 years during the incoming leg.

During the whole trip:

Betty's watch advances by 5 + 5 = 10 years;

Alfred's watch advances by 2.5 + 10 = 12.5 years.