Time dilation for a clock thrown vertically

gnieddu
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Homework Statement
Two clocks are placed on the earth's surface. One is kept still while the other is thrown upwards with an initial small velocity ##v_0##. How will the time measurements differ between the two clocks according to SR only and to GR only?
Relevant Equations
Equation describing the motion of a body in earth's gravitational field following Newton's laws: ##z=v_0t-\frac{1}{2}gt^2##

Proper time under Minkowski metric (with (+, -, -, -) signature): ##c^2d{\tau}^2=-ds^2=c^2dt^2-dz^2##
The non-moving clock will see the other one move upwards and land as predicted by Newton's laws, so using the equation ##z=v_0t-\frac{1}{2}gt^2##, and assuming the moving clock starts at ##t=0##, it will land at ##t=\frac{2v_0}{g}##.

Now, using SR only, and the Minkowski metric (with signature (-,+,+,+)) the time ##\tau## measured by the moving clock is obtained by using the equation:

$$c^2dτ^2=−ds^2=c^2dt^2−dz^2$$

where dz can be obtained from the equation of motion by differentiating:

$$dz=d[v_0t−\frac {1} {2} gt^2]=(v_0−gt)dt$$

In the end, we need to evaluate:

$$τ=\frac {1} {c} \int_0^{\frac {2v_0}{g}}[c^2−(v_0−gt)^2]dt $$

If we consider GR only, the solution follows the same line, but the metric would be the one for the weak field approximation:

$$ds^2=−(1+\frac{2gh}{c^2})c^2dt^2+dz^2$$

with the added complexity of expressing h as ##v_0t-\frac{1}{2}gt^2##.

In both cases, the integrals are not nice, but can possibly be simplified by using the approximation ##\sqrt{1+x}=1+\frac{x}{2}##. All of this, of course, if my plan of action makes sense and I'm not missing anything.

Thanks in advance

Gianni
 
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gnieddu said:
Now, using SR only, and the Minkowski metric (with signature (-,+,+,+)) the time ##\tau## measured by the moving clock is obtained by using the equation:

$$c^2dτ^2=−ds^2=c^2dt^2−dz^2$$

where dz can be obtained from the equation of motion by differentiating:

$$dz=d[v_0t−\frac {1} {2} gt^2]=(v_0−gt)dt$$

In the end, we need to evaluate:

$$τ=\frac {1} {c} \int_0^{\frac {2v_0}{g}}[c^2−(v_0−gt)^2]dt $$
There is an important typographical error in the last equation. Note that the right-hand side as written does not have the dimensions of time. Otherwise, I think this is correct for the "SR only" calculation.

gnieddu said:
If we consider GR only, the solution follows the same line, but the metric would be the one for the weak field approximation:

$$ds^2=−(1+\frac{2gh}{c^2})c^2dt^2+dz^2$$

with the added complexity of expressing h as ##v_0t-\frac{1}{2}gt^2##.
OK. It's not clear to me what they mean by "GR only". In the above expression, the ##dz^2## term will reproduce the "SR only" contribution. By "GR only" they might mean the contribution due to only the ##(1+\frac{2gh}{c^2})c^2dt^2## part of the expression above. I don't know.

gnieddu said:
In both cases, the integrals are not nice, but can possibly be simplified by using the approximation ##\sqrt{1+x}=1+\frac{x}{2}##.
Yes.
gnieddu said:
All of this, of course, if my plan of action makes sense and I'm not missing anything.
I think you are on the right track.
 
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TSny said:
There is an important typographical error in the last equation. Note that the right-hand side as written does not have the dimensions of time. Otherwise, I think this is correct for the "SR only" calculation.OK. It's not clear to me what they mean by "GR only". In the above expression, the ##dz^2## term will reproduce the "SR only" contribution. By "GR only" they might mean the contribution due to only the ##(1+\frac{2gh}{c^2})c^2dt^2## part of the expression above. I don't know.Yes.

I think you are on the right track.
Hi TSny, thanks for your feedback (and apologies for my late reply). You're right about the typo in my formula: the expression under the integral should be square-root-ed. Regarding the meaning of "GR only", my assumption is that I should compute the time dilation using only the GR metric vs. also considering special relativistic effects connected with the clock's speed (i.e. the ##\sqrt{1-\frac{v^2}{c^2}}## factor normally found in SR). But perhaps I'm only unnecessarily messing up things here...
 
gnieddu said:
Regarding the meaning of "GR only", my assumption is that I should compute the time dilation using only the GR metric vs. also considering special relativistic effects connected with the clock's speed (i.e. the ##\sqrt{1-\frac{v^2}{c^2}}## factor normally found in SR). But perhaps I'm only unnecessarily messing up things here...
The GR metric ##d\tau^2=(1+\frac{2gh}{c^2})dt^2-dz^2/c^2## includes the SR effect of "moving clocks run slow". For example, imagine letting the gravitational acceleration ##g## go to zero in the metric. Then the metric reduces to the SR Minkowski metric ##d\tau^2=dt^2-dz^2/c^2 ##. Thus ##d\tau= dt\sqrt{1 - (v_z/c)^2}## which is the SR time dilation.

So, in the GR metric, the ##dz^2/c^2## accounts for "moving clocks run slow". The ##\frac{2gh}{c^2}## part takes care of "clocks at higher gravitational potential run faster". GR handles everything. We wouldn't add the SR effect to the result of the GR metric.
 
TSny said:
The GR metric ##d\tau^2=(1+\frac{2gh}{c^2})dt^2-dz^2/c^2## includes the SR effect of "moving clocks run slow". For example, imagine letting the gravitational acceleration ##g## go to zero in the metric. Then the metric reduces to the SR Minkowski metric ##d\tau^2=dt^2-dz^2/c^2 ##. Thus ##d\tau= dt\sqrt{1 - (v_z/c)^2}## which is the SR time dilation.

So, in the GR metric, the ##dz^2/c^2## accounts for "moving clocks run slow". The ##\frac{2gh}{c^2}## part takes care of "clocks at higher gravitational potential run faster". GR handles everything. We wouldn't add the SR effect to the result of the GR metric.
Thanks. I thought I'd have to have some expression for ##dz^2## in order to account for the fact that the clock's z coordinate is changing in time (and not uniformly due to the accelerated motion), but I not I get yor point.
 
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