# A problem with time dilation help?

• Rishavutkarsh
In summary: The presence or absence of light beams has nothing to do with time dilation, which says (for instance) that Earth observers will measure our clocks to be running slow and that we will measure the Earth clocks to be running... normal.
Rishavutkarsh
A problem with time dilation help!?

when we move with light our time is slower and when we move against it then it's slower but what if we move at 99%C in a direction and a light ray is move with us and another light ray moves opposite from our direction simultaneously?

Rishavutkarsh said:
when we move with light our time is slower and when we move against it then it's slower but what if we move at 99%C in a direction and a light ray is move with us and another light ray moves opposite from our direction simultaneously?
When you move with respect to some frame, then that frame will observe your clocks to run slow compared to their own. (You, of course, see your own clocks as running normally.) And the speed of light is the same with respect to anyone, regardless of their speed with respect to some frame.

Doc Al said:
When you move with respect to some frame, then that frame will observe your clocks to run slow compared to their own. (You, of course, see your own clocks as running normally.) And the speed of light is the same with respect to anyone, regardless of their speed with respect to some frame.

i know but if light comes from two opposite directions at the same time and we are moving at 99%c in one direction then what will happen huh?

Rishavutkarsh said:
i know but if light comes from two opposite directions at the same time and we are moving at 99%c in one direction then what will happen huh?
What do you mean 'what will happen'?

Say you are moving to the right at 0.99c with respect to me. And there are two beams of light heading towards you.

I see: You moving to the right at 0.99c and the light beams moving left and right at speed c.

You see: The light moving towards you at speed c.

Doc Al said:
What do you mean 'what will happen'?

Say you are moving to the right at 0.99c with respect to me. And there are two beams of light heading towards you.

I see: You moving to the right at 0.99c and the light beams moving left and right at speed c.

You see: The light moving towards you at speed c.

well i mean that two beams of light not coming 2wards me but one going with me (from my side) and other from opposite side simultaneously

Rishavutkarsh said:
well i mean that two beams of light not coming 2wards me but one going with me (from my side) and other from opposite side simultaneously
You tell me. Using language similar to what I used in my last post, how would you describe it?

Doc Al said:
You tell me. Using language similar to what I used in my last post, how would you describe it?

ok now listen we are moving with 99% C -
case 1- a light beam comes from opposite direction towards us
case 2- a light beam goes with us
i know u can answer these but what if these both things happen simultaneously huh?

Rishavutkarsh said:
when we move with light our time is slower and when we move against it then it's slower

Hi Rishavutkarsh,

Correct me if I am wrong, but it sounds like you are thinking that time runs slower when we move relative to light. This is not really correct.

For you, your time always passes at the same rate, despite how fast you are moving around the universe.

What you will notice if you compare your passage of time to someone else’s passage of time that is moving relative to you, is that their time will be running slower than yours.

Rishavutkarsh said:
but what if we move at 99%C in a direction and a light ray is move with us and another light ray moves opposite from our direction simultaneously?

So if you accept what I've written above, you can see that this question doesn't really make any sense, as you would only notice a difference in the rate of time if you compare your time to someone moving relative to you.

Does that make sense?

Rishavutkarsh said:
ok now listen we are moving with 99% C -
case 1- a light beam comes from opposite direction towards us
case 2- a light beam goes with us
Is this what you mean:
We are moving to the right at .99c with respect to Earth (say)
case 1 = there's a light beam coming towards us (moving to the left)
case 2 = we shine a light in the same direction we are traveling (to the right)
i know u can answer these but what if these both things happen simultaneously huh?
The presence or absence of light beams has nothing to do with time dilation, which says (for instance) that Earth observers will measure our clocks to be running slow and that we will measure the Earth clocks to be running slow.

Rishavutkarsh said:
when we move with light our time is slower and when we move against it then it's slower but what if we move at 99%C in a direction and a light ray is move with us and another light ray moves opposite from our direction simultaneously?

This same question you asked before...

Perhaps you don't understand (or even know!) the definition of the one way speed of light, related to the definition of simultaneity? No doubt that that's where your problem is.

- From our perspective, you are moving along with a light ray, only a little slower. And you are heading towards another light ray with a speed difference of almost 2c.

- From your perspective, if you set up your own reference system, you will use (for example) light rays to set your clocks. And you will assume by definition that the light rays move at the same speed in both directions wrt you.

See: section 1, "Definition of simultaneity",
http://www.fourmilab.ch/etexts/einstein/specrel/www/

As you set your clocks using that assumption, you will next also measure with your clocks that the speed of light is the same wrt you, both ways - you simply measure your own synchronisation!

And from our perspective (not yours), thanks to the contraction of your rulers and the slowdown of your clocks, you will find that speed to be equal to c.

Is there still a problem left?

Harald

seems like u don't get my question and i need to be clearer
let a observer at Earth be stationary ,let's consider time passing here is proper (tp) now that we are moving with 99% c and a light beam is going with us (note 10 secs tp= 1 sec our time) just an assumption
so we see the light beam at c because of clock slowing and length contraction
now in second case we move in other way of light shouldn't the length expand and time fasten to let us see the light at C (10secs tp = 1sec of our time)
it means in one case length is expanding and other contacting and in one case time is faster than TP and in other slower can these two cases be applicable simultaneously huh?
i wish you get my question and please correct if i am assuming some wrong

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Rishavutkarsh said:
seems like u don't get my question and i need to be clearer
let a observer at Earth be stationary ,let's consider time passing here is proper (tp) now that we are moving with 99% c and a light beam is going with us (note 10 secs tp= 1 sec our time) just an assumption
so we see the light beam at c but clock slowing and length contraction

To you the clock would run at normal speed and there would be no contraction. The observer on Earth would see your clock running slow and contraction.

now in second case we move in other way of light shouldn't the length expand and time fasten to let us see the light at C (10secs tp = 1sec of our time)
it means in one case length is expanding and other contacting and in one case time is faster than TP and in other slower can these two cases be applicable simultaneously huh?
i wish you get my question and please correct if i am assuming some wrong

I think your confusion here is that whether you are moving the same or opposite directions to light is irrelevant to time dilation. Time dilation occurs because of your speed relative to an observer at rest.

ryan_m_b said:
To you the clock would run at normal speed and there would be no contraction. The observer on Earth would see your clock running slow and contraction.

I think your confusion here is that whether you are moving the same or opposite directions to light is irrelevant to time dilation. Time dilation occurs because of your speed relative to an observer at rest.

well if light comes from another direction then we should see it at 199%C right? but we don't because length expands and time fastens relative to a observer on earth

the reverse happens when light comes from our side we should see it at 1%c but we don't due to length contraction and time slowing relative to the same observer on Earth

these both can't take place simultaneously do u get my point now
please tell if am assuming something wrong

Rishavutkarsh said:
well if light comes from another direction then we should see it at 199%C right? but we don't because length expands and time fastens relative to a observer on earth

the reverse happens when light comes from our side we should see it at 1%c but we don't due to length contraction and time slowing relative to the same observer on Earth

these both can't take place simultaneously do u get my point now
please tell if am assuming something wrong

Classically this would be true. For example if we are both passengers on a train and I walk past you I am moving 3mph relative to you. For an observer standing by the tracks the train is moving at 70mph and I am moving at 73mph. That's very simple to work out, we just add the two velocities together.

However adding velocities in special relativity uses a different formula. http://math.ucr.edu/home/baez/physics/Relativity/SR/velocity.html" might help you as it explains the formula and the reasoning behind it, essentially when adding together speeds it is never possible for something to exceed the speed of light.

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ryan_m_b said:
Classically this would be true. For example if we are both passengers on a train and I walk past you I am moving 3mph relative to you. For an observer standing by the tracks the train is moving at 70mph and I am moving at 73mph. That's very simple to work out, we just add the two velocities together.

However adding velocities in special relativity uses a different formula. http://math.ucr.edu/home/baez/physics/Relativity/SR/velocity.html" might help you as it explains the formula and the reasoning behind it, essentially when adding together speeds it is never possible for something to exceed the speed of light.

thanks for the link but i think there is a reason why it happens it happens to conserve the speed but man time can't run slow and fast simultaneously right?

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Rishavutkarsh said:
thanks for the link but i think there is a reason why it happens it happens to conserve the speed but man time can't run slow and fast simultaneously right?

I'm not sure what you mean. Can you give a clear example to what you are referring to? i.e Alice is moving at 99% C relative to Bob towards the right...

Rishavutkarsh said:
thanks for the link but i think there is a reason why it happens it happens to conserve the speed but man time can't run slow and fast simultaneously right?
Why do you think time must 'run slow and fast simultaneously'?

Your time runs normally. When you observe something moving, you will measure their clocks to run slow. (They, of course, think their clocks are running normally and your clocks are slow.)

ryan_m_b said:
I'm not sure what you mean. Can you give a clear example to what you are referring to? i.e Alice is moving at 99% C relative to Bob towards the right...

sure, like candice is at 99% relative to alice on Earth . if a light ray moves from the side of candice then both of them see it at C right? so that means that alice's clock is moving faster than candice . note - candice should see the light at 1%C but she isn't because of length contraction and clock slowing.

now if a light ray comes from opposite direction of candice both of them see it at C but candice
should see it at 199%c that means that in this way the clock of alice is slower than candice
and length of candice expands rather than contraction to see the light at C .

i hope i get these both points right feel free to correct them

now my question is can these both cases be applicable simultaneously ? as time can run slow and fast for a single observer simultaneously

i have a request that can we use the chat to clear my doubts?

Doc Al said:
Why do you think time must 'run slow and fast simultaneously'?

Your time runs normally. When you observe something moving, you will measure their clocks to run slow. (They, of course, think their clocks are running normally and your clocks are slow.)

thanks but i already know this concept please see my next post and tell me that am wrong as i want to be proved wrong but i want to proved

Rishavutkarsh said:
thanks for the link but i think there is a reason why it happens it happens to conserve the speed but man time can't run slow and fast simultaneously right?
Yes it can! :) This is one of the central points of relativity: simultaneity. At relativistic speeds, time moves differently. Time is not constant across the universe. Suppose you had two identical clocks. You place one on a spaceship and the other at rest next to you on earth. If the spaceship is moving at 0.5c, you would observe its clock to run at 86.6% the speed of your clock.

However, the astronauts would think that your clock is running slow. They would see that your clock was also running at 86.6% of its normal speed.

Therefore, between two different frames of reference, you cannot agree on velocities, lengths, times, simultaneity, or momenta. The only thing you can agree on (and this is pivotal) is the speed of light. Both you (being on Earth) and the astronauts (traveling at 0.5c) will agree on the speed of light.

This explains simultaneity very elegantly:

If any of this confuses you, pinpoint the statement that didn't make sense, and we'll go from there. :)

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Rishavutkarsh said:
well if light comes from another direction then we should see it at 199%C right? but we don't because length expands and time fastens relative to a observer on earth

the reverse happens when light comes from our side we should see it at 1%c but we don't due to length contraction and time slowing relative to the same observer on Earth

these both can't take place simultaneously do u get my point now
please tell if am assuming something wrong
Have you ever considered how it is possible to see the speed of light? You cannot see light in transit, can you? You can only see it when it gets to you, correct?

Let's forget about relativity for the moment and see what happens if there were no time dilation and no length contraction.

Now, if a flash of light were approaching you from the front, it would be correct that it would be traveling toward you at 199%C, just like you said and if another flash of light were approaching you from behind, it would be correct that it would be traveling at 1%C, again, just like you said. But how would you measure these two speeds?

In order to measure the speed of light, you need to have a timing device and a mirror placed some fixed, measured distance away from you and traveling with you. You need one mirror behind you for light approaching you from the front and another mirror in front of you for light approaching you from behind.

So for the light coming from in front of you, you start your timer when the light first hits you, then you wait until the light hits the mirror that is a fixed distance behind you and reflects off it and starts coming back toward you from behind. When it gets to you, you stop the timer. Then to calculate the average speed of light, you take double the fixed, measured distance to the mirror and divide by the time on your timer. Does this make sense to you?

And in a similar way, for light coming from behind you, you start the timer when the light first hits you, then you wait until the light hits the mirror that is a fixed distance in front of you and reflects off it and starts coming back toward you. When it gets to you, you stop the timer. Then to calculate the average speed of light, you take double the fixed, measured distance to the mirror and divide by the time on your timer.

Now if you think about what will happen when you make these two measurements, you will see that you get the same answer for the light coming toward you at 199%C and the light catching up to you from behind at 1%C. Do you see that?

If not, consider this: Let's take the first case where the light is coming from in front of you. It will be traveling at 199%C when it goes from you until it hits the mirror behind you. It won't take very long for this to happen but whatever time it is, let's call it t1. Keep in mind that you don't know what this time is because you cannot yet see that the light has hit the mirror. Now when the light comes back to you, it will be traveling at 1%C and will take a very long time to reach you. Let's call this time t2. Again, you don't know what this value is but you do know the sum, t1+t2, is the time on your timer.

Now if we take the second case for light coming from behind you, it will be traveling at 1%C from the time it hits you until it hits the mirror in front of you but you cannot yet see that happening so you don't know when it happens. Assuming the two mirrors are the same fixed, measured distance away from you, one in front, one behind, this time will be the same as t2, a really long time, correct? And for the trip back at 199%C, the time will be t1, a really short time, correct? So the total time is t2+t1 which matches the time on your timer for the first case.

So now when you calculate the speed of light for the first case and for the second case, you get the same result for both because they both have to take the same time to make the round trip, it's just that the order is different for the two cases.

Remember, we are assuming that time dilation and length contraction don't happen but what if they did? All that would be different is that you would get a different time on your timer and your measured distance would be different but you would still calculate the speed of a flash of light coming toward you exactly the same as the speed of a flash of light catching up to you.

Do you understand all of this?

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Rishavutkarsh said:
well if light comes from another direction then we should see it at 199%C right? but we don't because length expands and time fastens relative to a observer on Earth [...]

No that is not the main reason, as I already explained in my post; the main reason is the clock synchronization that you (should have) performed. The problem is therefore not a lack of explanation of the problem that you have. Please read my answer again together with the elaborations by yoshtov and ghwellsjr and ask questions about what you don't understand in the answers.

Harald

I think what he wants to know is that if you shoot a light in front of yourself and behind yourself simultaneously while moving at 99% c relative to earth, how does time dilation and length contraction work to make both of the lights move at c in both cases. Eg time would have to move slower when you shoot a light in front of yourself, to give the photon more time so it can move at c, but when you shoot it out the back time for you would have to be faster to make the photon move at c.

chingel said:
I think what he wants to know is that if you shoot a light in front of yourself and behind yourself simultaneously while moving at 99% c relative to earth, how does time dilation and length contraction work to make both of the lights move at c in both cases. Eg time would have to move slower when you shoot a light in front of yourself, to give the photon more time so it can move at c, but when you shoot it out the back time for you would have to be faster to make the photon move at c.
Time dilation and length contraction aren't what make both lights move at the same speed. Didn't I make that clear?

ghwellsjr said:
Time dilation and length contraction aren't what make both lights move at the same speed. Didn't I make that clear?

Several of us already stressed that, explained that and made it clear - but of course, only if what we wrote was:
and
2. understood!

yoshtov said:
Yes it can! :) This is one of the central points of relativity: simultaneity. At relativistic speeds, time moves differently. Time is not constant across the universe. Suppose you had two identical clocks. You place one on a spaceship and the other at rest next to you on earth. If the spaceship is moving at 0.5c, you would observe its clock to run at 86.6% the speed of your clock.

However, the astronauts would think that your clock is running slow. They would see that your clock was also running at 86.6% of its normal speed.

Therefore, between two different frames of reference, you cannot agree on velocities, lengths, times, simultaneity, or momenta. The only thing you can agree on (and this is pivotal) is the speed of light. Both you (being on Earth) and the astronauts (traveling at 0.5c) will agree on the speed of light.

This explains simultaneity very elegantly:

If any of this confuses you, pinpoint the statement that didn't make sense, and we'll go from there. :)

oh come on i know time can move faster and slower for two observers simultaneously but it can't run both fast and slow for the same observer simultaneously

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Rishavutkarsh said:
oh come on i know time can move faster and slower for two observers simultaneously but it can't run both fast and slow for the same observer simultaneously

Perhaps you should clearly and concisely state what you are confused about. Under what situation do you think time would run both slow and fast? You gave a scenario earlier but I cannot make sense of it.

ghwellsjr said:
Have you ever considered how it is possible to see the speed of light? You cannot see light in transit, can you? You can only see it when it gets to you, correct?

Let's forget about relativity for the moment and see what happens if there were no time dilation and no length contraction.

Now, if a flash of light were approaching you from the front, it would be correct that it would be traveling toward you at 199%C, just like you said and if another flash of light were approaching you from behind, it would be correct that it would be traveling at 1%C, again, just like you said. But how would you measure these two speeds?

In order to measure the speed of light, you need to have a timing device and a mirror placed some fixed, measured distance away from you and traveling with you. You need one mirror behind you for light approaching you from the front and another mirror in front of you for light approaching you from behind.

So for the light coming from in front of you, you start your timer when the light first hits you, then you wait until the light hits the mirror that is a fixed distance behind you and reflects off it and starts coming back toward you from behind. When it gets to you, you stop the timer. Then to calculate the average speed of light, you take double the fixed, measured distance to the mirror and divide by the time on your timer. Does this make sense to you?

And in a similar way, for light coming from behind you, you start the timer when the light first hits you, then you wait until the light hits the mirror that is a fixed distance in front of you and reflects off it and starts coming back toward you. When it gets to you, you stop the timer. Then to calculate the average speed of light, you take double the fixed, measured distance to the mirror and divide by the time on your timer.

Now if you think about what will happen when you make these two measurements, you will see that you get the same answer for the light coming toward you at 199%C and the light catching up to you from behind at 1%C. Do you see that?

If not, consider this: Let's take the first case where the light is coming from in front of you. It will be traveling at 199%C when it goes from you until it hits the mirror behind you. It won't take very long for this to happen but whatever time it is, let's call it t1. Keep in mind that you don't know what this time is because you cannot yet see that the light has hit the mirror. Now when the light comes back to you, it will be traveling at 1%C and will take a very long time to reach you. Let's call this time t2. Again, you don't know what this value is but you do know the sum, t1+t2, is the time on your timer.

Now if we take the second case for light coming from behind you, it will be traveling at 1%C from the time it hits you until it hits the mirror in front of you but you cannot yet see that happening so you don't know when it happens. Assuming the two mirrors are the same fixed, measured distance away from you, one in front, one behind, this time will be the same as t2, a really long time, correct? And for the trip back at 199%C, the time will be t1, a really short time, correct? So the total time is t2+t1 which matches the time on your timer for the first case.

So now when you calculate the speed of light for the first case and for the second case, you get the same result for both because they both have to take the same time to make the round trip, it's just that the order is different for the two cases.

Remember, we are assuming that time dilation and length contraction don't happen but what if they did? All that would be different is that you would get a different time on your timer and your measured distance would be different but you would still calculate the speed of a flash of light coming toward you exactly the same as the speed of a flash of light catching up to you.

Do you understand all of this?
oh thank you now i get it all ( i think so eh!) listen what i get-

light would first take t1 time and next t2 time with both being 1% and 199% respectively and the reverse in both cases (forget the serial) so we see both at C . does this mean that time dilation and length contraction has no influence by speed of light and i had an misconception right?

Rishavutkarsh said:
does this mean that time dilation and length contraction has no influence by speed of light and i had an misconception right?

Correct

Rishavutkarsh said:
oh thank you now i get it all ( i think so eh!) listen what i get-

light would first take t1 time and next t2 time with both being 1% and 199% respectively and the reverse in both cases (forget the serial) so we see both at C . does this mean that time dilation and length contraction has no influence by speed of light and i had an misconception right?
No, I didn't say that. I was talking about comparing the measured round trip speed of light for two flashes approaching you from opposite directions. I said that even without time dilation and length contraction, you will measure the same value for the two flashes. I did not say that you would get a value of c, which you will not, if there weren't any time dilation and length contraction. But with time dilation and length contraction, which is what happens in our real world if you actually made the measurement, you would get the value of c for both measured round trips.

ghwellsjr said:
No, I didn't say that. I was talking about comparing the measured round trip speed of light for two flashes approaching you from opposite directions. I said that even without time dilation and length contraction, you will measure the same value for the two flashes. I did not say that you would get a value of c, which you will not, if there weren't any time dilation and length contraction. But with time dilation and length contraction, which is what happens in our real world if you actually made the measurement, you would get the value of c for both measured round trips.

oh.. can you please elaborate so a 13 year old can understand it?
if time dilation and length contraction is still what you insist on things get cocky. i

OK, you were claiming that when you are traveling at a high speed (99%C) and two light rays approach you from opposite directions, one from in front of you and one from behind you, that you will be able to detect something different about the speed of these two light rays because time dilation couldn't cause one to slow down and the other one to speed up at the same time, correct?

So I showed you in post #21 that even without time dilation (or length contraction) you wouldn't be able to detect any difference in the speed of those two light rays because you can only measure the round trip speed, not the one-way speed, and you understood that, correct?

And then I pointed out that the speed that you would measure if there were no time dilation (or length contraction) would not be equal to C but since we don't live in a world like that, you would, in fact, measure the round trip speed of both rays as traveling at C because we do live in a world that has time dilation and length contraction. I'm not trying to explain why you measure C in our real world, only that you cannot detect any difference in the round trip speed of light approaching you from opposite directions, no matter how fast you are traveling.

This, by the way, has nothing to do with the Theory of Special Relativity. This concept was known by all scientists prior to Einstein and understood by them, it's just logical reasoning: you cannot measure the one-way speed of light, you can only measure the round-trip speed of light and you will always get the same answer for light approaching you from opposite directions, no matter what your own speed is.

Do you understand all of this so far?

ghwellsjr said:
OK, you were claiming that when you are traveling at a high speed (99%C) and two light rays approach you from opposite directions, one from in front of you and one from behind you, that you will be able to detect something different about the speed of these two light rays because time dilation couldn't cause one to slow down and the other one to speed up at the same time, correct?

So I showed you in post #21 that even without time dilation (or length contraction) you wouldn't be able to detect any difference in the speed of those two light rays because you can only measure the round trip speed, not the one-way speed, and you understood that, correct?

And then I pointed out that the speed that you would measure if there were no time dilation (or length contraction) would not be equal to C but since we don't live in a world like that, you would, in fact, measure the round trip speed of both rays as traveling at C because we do live in a world that has time dilation and length contraction. I'm not trying to explain why you measure C in our real world, only that you cannot detect any difference in the round trip speed of light approaching you from opposite directions, no matter how fast you are traveling.

This, by the way, has nothing to do with the Theory of Special Relativity. This concept was known by all scientists prior to Einstein and understood by them, it's just logical reasoning: you cannot measure the one-way speed of light, you can only measure the round-trip speed of light and you will always get the same answer for light approaching you from opposite directions, no matter what your own speed is.

Do you understand all of this so far?
can't say i don't! but what if a high tech device is made which can measure speed of light when it falls on it ?

Rishavutkarsh said:
can't say i don't! but what if a high tech device is made which can measure speed of light when it falls on it ?
In order to measure the speed of anything, you have to know how long it took for that something to get from its starting point to its ending point and how far away those two points are, correct?

ghwellsjr said:
In order to measure the speed of anything, you have to know how long it took for that something to get from its starting point to its ending point and how far away those two points are, correct?

yes you correct. but technology can evolve anything correct?

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