# Time Dilation in the ISS: 2 Seconds in a Year

• Aristarchus_
The problem is that you are incorrectly using the gravitational constant, gamma. The gravitational constant is not a velocity, it is a strength of the gravitational force. To find the velocity of the ISS, you would need to use the gravitational force at the two points and divide the sum of the two forces by the distance between them.f

#### Aristarchus_

Homework Statement
One twin is on the Earth's surface, at water level. The other is in the ISS 408km above Earth's surface. If they are separated for one year, how much longer does the twin on Earth age?
Relevant Equations
##t= \frac {t_0}{\sqrt{1- \frac{2 \gamma M}{rc^{2}}}}## (Edited)
Where r is the distance from the centrum of Earth. The answer is supposed to be 0.007 sec
Using the above formula I get that the time goes 6.5∗10−86.5∗10−8 percent faster in ISS. Thus, this is approximately 2 seconds in a year. But the answer is much lower. Where am I making a mistake?

One year according to which reference system?

Can you show how you came up with the speed of the ISS station in your formula?

Is gamma Newtons gravitational constant?

It would help if you showed your working, specifically the values you input, and stated the answer you were expecting. With a quick back of the envelope calculation I get around ##4.4×10^{-9}\%## difference.

Also, ##6.5×10^{-8}## of a year is about 2s, but ##6.5×10^{-8}\%## of a year is about 0.02s. Which did you mean?

Thoughts:

Did you take the radius of the Earth into account?

Did you take the rotation of the Earth into account? If so, it matters in which direction the ISS orbits.

Did you take gravitational time dilation into account?

• Ibix
Is the answer supposed to be ##0.01s##?

Homework Statement:: One twin is on the Earth's surface, at water level. The other is in the ISS 408km above Earth's surface. If they are separated for one year, how much longer does the twin on Earth age?
Relevant Equations:: ##t= \frac {t_0} {\sqrt{1- \frac{2 \gamma M}{rc^{2}}}##
Try adding one more curly brace after that ##\LaTeX##:

##t= \frac {t_0} {\sqrt{1- \frac{2 \gamma M}{rc^{2}}}}## renders as:$$t= \frac {t_0} {\sqrt{1- \frac{2 \gamma M}{rc^{2}}}}$$
However, I could not immediately follow this formula. You've left out a fair number of steps. It seems that you are working to calculate relativistic Gamma and needed a ##v^2## in the formula. You have calculated that from the mass of the Earth ##M## and the radius of orbit ##r##. Probably, as @malawi_glenn suspects, your use of ##\gamma## is intended to represent Newton's gravitational constant ##G##.

So you have ##\frac{GM}{r}## which represents the potential energy deficit of a satellite with unit mass. You may know that the orbital kinetic energy (##\frac{1}{2}mv^2##) will be half of that. So ##\frac{GMm}{r} = mv^2## and ##v^2 = \frac{GM}{r}##

Nicely done.

But I think that you've bungled and left a factor of 2 in there. Orbital kinetic energy is only 1/2 of the potential energy deficit. Not the whole thing.

• PeroK
One can also use centripetal acc

• jbriggs444
One can also use centripetal acc
Yes, nicer. I just grabbed the first tool in my bag. [Second, actually. If I'd been working the problem, a Google search for "velocity of iss in meters per second" would have been first -- 7778 m/s, about right for low Earth orbit]

Try adding one more curly brace after that ##\LaTeX##:

##t= \frac {t_0} {\sqrt{1- \frac{2 \gamma M}{rc^{2}}}}## renders as:$$t= \frac {t_0} {\sqrt{1- \frac{2 \gamma M}{rc^{2}}}}$$
I would tend to use ##\Delta t_s, \Delta t_o## for the proper times on the surface and in the ISS orbit. A useful approach is to rewrite this as:
$$\Delta t_o = \Delta t_s \sqrt{1- \frac{v^2}{c^2}} = \Delta t_s \sqrt{1- \frac{GM}{rc^2}}$$Then expand that term to first order using a binomial expansion.

That said, I suggest the stress of space travel would age someone more than a tenth of second!

That said, I suggest the stress of space travel would age someone more than a tenth of second!
Surely the stress tensor for a body in free fall is more nearly zero than that for a [live] body at rest... in the Ukraine. :-)

It would help if you showed your working, specifically the values you input, and stated the answer you were expecting. With a quick back of the envelope calculation I get around ##4.4×10^{-9}\%## difference.

Also, ##6.5×10^{-8}## of a year is about 2s, but ##6.5×10^{-8}\%## of a year is about 0.02s. Which did you mean?
4.2*10^{-9} is the correct answer for the percentage of time dilation in relation to Earth, how did you come up with it?

Try adding one more curly brace after that ##\LaTeX##:

##t= \frac {t_0} {\sqrt{1- \frac{2 \gamma M}{rc^{2}}}}## renders as:
t=t01−2γMrc2−−−−−−−√t=t01−2γMrc2​
However, I could not immediately follow this formula. You've left out a fair number of steps. It seems that you are working to calculate relativistic Gamma and needed a ##v^2## in the formula. You have calculated that from the mass of the Earth ##M## and the radius of orbit ##r##. Probably, as @malawi_glenn suspects, your use of ##\gamma## is intended to represent Newton's gravitational constant ##G##.

So you have ##\frac{GM}{r}## which represents the potential energy deficit of a satellite with unit mass. You may know that the orbital kinetic energy (##\frac{1}{2}mv^2##) will be half of that. So ##\frac{GMm}{r} = mv^2## and ##v^2 = \frac{GM}{r}##

Nicely done.

But I think that you've bungled and left a factor of 2 in there. Orbital kinetic energy is only 1/2 of the potential energy deficit. Not the whole thing.
Yes, I used that formula but I am getting the wrong answer by plugging in the values of r = 408km + 6371km. What is it that I am missing here?

Thoughts:

Did you take the radius of the Earth into account?

Did you take the rotation of the Earth into account? If so, it matters in which direction the ISS orbits.

Did you take gravitational time dilation into account?
I did not take gravitational time dilation into account. How would one do so? Is it a separate formula from the one provided above?

One year according to which reference system?

Can you show how you came up with the speed of the ISS station in your formula?

Is gamma Newtons gravitational constant?
How would I take the speed into account? Would I need the orbital velocity or the escape velocity in the Lorentz formula?

I did not take gravitational time dilation into account. How would one do so? Is it a separate formula from the one provided above?
I wasn't sure whether you were supposed to or not. It turns out that for the ISS the velocity-based effects are significantly greater than the gravitational effects. So, you can neglect both gravitation and the Earth's rotation. It wasn't immediately obvious to me that you could do so.

• malawi_glenn and Aristarchus_
Did you take gravitational time dilation into account?
One can not do that at this level, I give exactly this problem (well I do say in what frame "1 year" is measured in). I do mention that one can take into account also GR effects and what the actual timedilation is for ISS. I also mention the Hafele & Keating experiment.

Would I need the orbital velocity or the escape velocity in the Lorentz formula?
You need the relative velocity between the two frames, and assuming that Earth frame is an inertial frame.
What is it that I am missing here?
First tell us what value you get for ##v##

First tell us what value you get for ##v##
There's no reason I can see to calculate ##v##.

• Aristarchus_
Try adding one more curly brace after that ##\LaTeX##:

##t= \frac {t_0} {\sqrt{1- \frac{2 \gamma M}{rc^{2}}}}## renders as:

t=t01−2γMrc2−−−−−−−√t=t01−2γMrc2​

However, I could not immediately follow this formula. You've left out a fair number of steps. It seems that you are working to calculate relativistic Gamma and needed a ##v^2## in the formula. You have calculated that from the mass of the Earth ##M## and the radius of orbit ##r##. Probably, as @malawi_glenn suspects, your use of ##\gamma## is intended to represent Newton's gravitational constant ##G##.

So you have ##\frac{GM}{r}## which represents the potential energy deficit of a satellite with unit mass. You may know that the orbital kinetic energy (##\frac{1}{2}mv^2##) will be half of that. So ##\frac{GMm}{r} = mv^2## and ##v^2 = \frac{GM}{r}##

Nicely done.

But I think that you've bungled and left a factor of 2 in there. Orbital kinetic energy is only 1/2 of the potential energy deficit. Not the whole thing.
But I don't know why I'm getting the wrong answer. The value of radius should be 408km +6371km right?

But I don't know why I'm getting the wrong answer. The value of radius should be 408km +6371km right?

There's no reason I can see to calculate ##v##.
If he/she can not get ##v## correct, then its hard to get everything else correct.
I am getting the wrong answer by plugging in the values of r = 408km + 6371km.
Would be intersted to see what value of ##v## he/she gets.

One can not do that at this level, I give exactly this problem (well I do say in what frame "1 year" is measured in). I do mention that one can take into account also GR effects and what the actual timedilation is for ISS. I also mention the Hafele & Keating experiment.

You need the relative velocity between the two frames, and assuming that Earth frame is an inertial frame.

First tell us what value you get for ##v##
If we have the radius of the orbit, we shouldn't need V

If we have the radius of orbit, we shouldn't need V
No but you can still calculate it and compare with the number that was given in #8
Sometimes making small steps are good to "debug" caclulations. Once you understand and make sure every ingredient is properly calculated, one can do the entire thing.

Also, what calculator are you using?

• • jbriggs444 and Aristarchus_
6.53*10^{-8} in percent, which makes roughly 2 seconds of a year

6.53*10^{-8} in percent, which makes roughly 2 seconds of a year
It was already pointed out that "percent" is wrong here. Did't you read post #3?
Don't use percentages unless you have to. 0.0005 is 0.05% why is "%" more helpful here? It's just another layer of confusion.

Yes, I used that formula but I am getting the wrong answer by plugging in the values of r = 408km + 6371km. What is it that I am missing here?
No the forumula you wrote had a factor of 2 in it.

It was already pointed out that "percent" is wrong here. Did't you read post #3?
When values are plugged into the formula, I get 1.000000000653, which is t = t_0 * 1.000000000653. Then I converted this into percentage and estimated the time of the year by multiplying by 3600*24*365. But it was pointed out that 6.5*10^{-8} is not the correct percentage...How so?

When values are plugged into the formula, I get 1.000000000653, which is t = t_0 * 1.000000000653. Then I converted this into percentage and estimated the time of the year by multiplying by 3600*24*365. But it was pointed out that 6.5*10^{-8} is not the correct percentage...How so?
Why do you need to convert into %? It's pretty pointless.

You have still not answered my question in what frame 1 year is measured in. Is it 1 year on Earth or 1 year on ISS?

6.53*10^{-8} in percent, which makes roughly 2 seconds of a year
That's definitely wrong. I get about ##0.01 s## taking into account only the speed of the ISS.

• malawi_glenn
Why do you need to convert into %? It's pretty pointless.

You have still not answered my question in what frame 1 year is measured in. Is it 1 year on Earth or 1 year on ISS?
I am not sure, I am supposed to compare the two. For example how much younger or older is the twin in ISS after a year in comparison to the one on Earth

You have still not answered my question in what frame 1 year is measured in. Is it 1 year on Earth or 1 year on ISS?
That can't be important, given we are dealing with tiny difference in proper times.

That's definitely wrong. I get about ##0.01 s## taking into account only the speed of the ISS.
I miscalculated than, can you provide your calculation?

That can't be important, given we are dealing with tiny difference in proper times.
It's not important here, but its not all about getting the correct answer.

I miscalculated than, can you provide your calculation?
It's in a spreadsheet. That said I don't like your method. Do you know how to use the binomial theorem for the square root?