Moving Clocks - Questions

Moving Clocks































For an object moving near the speed of light, dimensions perpendicular to its direction of motion

(A) stay the same. (B) shrink. (C) expand.

(D) sometimes shrink and sometimes expand.


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For an object moving near the speed of light, dimensions perpendicular to its direction of motion

(A) stay the same. --- Yes.

Any other assumption leads to contradictions.

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For an object moving near the speed of light, dimensions perpendicular to its direction of motion

(B) shrink. --- No.

That leads to contradictory predictions by observers in different inertial frames.

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For an object moving near the speed of light, dimensions perpendicular to its direction of motion

(C) expand. --- No.

That leads to contradictory predictions by observers in different inertial frames.

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For an object moving near the speed of light, dimensions perpendicular to its direction of motion

(D) sometimes shrink and sometimes expand.

No. There is no consistent way to do that.

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The photon clock which is used to derive the time dilation formula works by

(A) counting photons emitted by Cesium atoms.
(B) using light to count the swings of a pendulum.
(C) bouncing a light pulse between two mirrors.
(D) trapping light in a stable circular orbit.


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The photon clock which is used to derive the time dilation formula works by

(A) counting photons emitted by Cesium atoms.

No. It is much simpler than that.


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The photon clock which is used to derive the time dilation formula works by

(B) using light to count the swings of a pendulum.

No. Pendulums are too complicated!


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The photon clock which is used to derive the time dilation formula works by

(C) bouncing a light pulse between two mirrors.

Yes. Because all observers
agree on how a light pulse moves.

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The photon clock which is used to derive the time dilation formula works by

(D) trapping light in a stable circular orbit.

No. You need a black hole for that
and they are hard to find.

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The starship Enterprise is travelling at 5/13 light speed. For every two minutes which pass on board the ship, a clock back on earth will read an interval close to

(A) two minutes.
(B) one minute and fifty seconds.
(C) two minutes and ten seconds.
(D) two minutes and thirty seconds.


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The starship Enterprise is travelling at 5/13 light speed. For every two minutes which pass on board the ship, a clock in the reference frame of the earth will read an interval close to

(A) two minutes. --- No.

There will be time dilation.

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The starship Enterprise is travelling at 5/13 light speed. For every two minutes which pass on board the ship, a clock in the reference frame of the earth will read an interval close to

(B) one minute and fifty seconds. --- No.
Earth sees moving clocks run slow
and ship-board intervals as longer.

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The starship Enterprise is travelling at 5/13 light speed. For every two minutes which pass on board the ship, a clock in the reference frame of the earth will read an interval close to
(C) two minutes and ten seconds. --- Yes.
Use v = 5/13, T = 120s and calculate:

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The starship Enterprise is travelling at 5/13 light speed. For every two minutes which pass on board the ship, a clock in the reference frame of the earth will read an interval close to

(D) two minutes and thirty seconds.
No. Use the time dilation formula
to calculate the answer.

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In earth-year 2010, a star-wisp is accelerated by a short burst of microwave radiation and travels to a star 5 light years away. In earth-year 2028, radio signals containing pictures from the star-wisp's close encounter with the star are received on earth. The pictures have labels supplied by the wisp's on-board clock which indicate that they were taken in wisp-year

(A) 2015. (B) 2020. (C) 2022. (D) 2023.

Hint:The signal took 5 years to reach earth, so the probe must have taken 13 years to reach the star. How much time elapsed on the wisp's own clock during the journey?

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In earth-year 2010, a star-wisp is accelerated by a short burst of microwave radiation and travels to a star 5 light years away. In earth-year 2028, radio signals containing pictures from the star-wisp's close encounter with the star are received on earth. The pictures have labels supplied by the wisp's on-board clock which indicate that they were taken in wisp-year

(A) 2015. --- No.

Going 5 light years in 13 years, you do not expect
that much time dilation.

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In earth-year 2010, a star-wisp is accelerated by a short burst of microwave radiation and travels to a star 5 light years away. In earth-year 2028, radio signals containing pictures from the star-wisp's close encounter with the star are received on earth. The pictures have labels supplied by the wisp's on-board clock which indicate that they were taken in wisp-year

(B) 2020. --- No.

Calculate the proper-time interval for x=4, t=13,
and add it to 2010.

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In earth-year 2010, a star-wisp is accelerated by a short burst of microwave radiation and travels to a star 5 light years away. In earth-year 2028, radio signals containing pictures from the star-wisp's close encounter with the star are received on earth. The pictures have labels supplied by the wisp's on-board clock which indicate that they were taken in wisp-year

(C) 2022. --- Yes.
Use x= 5, t=18-5 = 13, to get the proper time T:
T = t - x = 13 - 5 = 169-25=144.
T = 12 years. Wisp-date = 2010+12 = 2022.

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In earth-year 2010, a star-wisp is accelerated by a short burst of microwave radiation and travels to a star 5 light years away. In earth-year 2028, radio signals containing pictures from the star-wisp's close encounter with the star are received on earth. The pictures have labels supplied by the wisp's on-board clock which indicate that they were taken in wisp-year

(D) 2023. --- No.

That would only be right if there were
no time dilation at all.

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