# What is Light clock: Definition and 64 Discussions

In physics and relativity, time dilation is the difference in the elapsed time as measured by two clocks. It is either due to a relative velocity between them (special relativistic "kinetic" time dilation) or to a difference in gravitational potential between their locations (general relativistic gravitational time dilation). When unspecified, "time dilation" usually refers to the effect due to velocity.
After compensating for varying signal delays due to the changing distance between an observer and a moving clock (i.e. Doppler effect), the observer will measure the moving clock as ticking slower than a clock that is at rest in the observer's own reference frame. In addition, a clock that is close to a massive body (and which therefore is at lower gravitational potential) will record less elapsed time than a clock situated further from the said massive body (and which is at a higher gravitational potential).
These predictions of the theory of relativity have been repeatedly confirmed by experiment, and they are of practical concern, for instance in the operation of satellite navigation systems such as GPS and Galileo. Time dilation has also been the subject of science fiction works.

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1. ### B SR equation seems to depend on orientation of the 'light clock'

One of my first exposures to special relativity was looking at a 'light clock' where a photon is emitted and captured by a detector whilst traveling on a train. The passenger sees the photon go straight up. The distance traveled by this photon in a given time, t, is c t An observer however...
2. ### B Why do you need a light clock?

what is wrong with the thought experiment below?! If I consider a light beam going straight towards an observer in motion it makes it look the moving clock ticks faster, but if you use a light clock it makes it look like the clock turns slower. What am I missing here? “Two observers convene in...
3. ### B Light Clock: Why Aren't Delta T and 2D/c Equal?

With this light clock delta t' in S' would be 2 Ls proper time. Since time measurements were done in same location delta x'=0 . Then from stationary S frame delta t = gamma delta t' . With gamma equal to 1.25 then that is 2.5 Ls. However the path the light takes as calculated from S frame in...
4. ### B Light Clock & Length Contraction in GR

Hi, can i use a light clock made out of mirrors a distance appart to measure whether there is length contraction in different regions of spacetime? If the clock speeds up then the distance between the mirrors decreased. If the clock slows down the distance between mirrors increased.
5. ### I Light Clock Animation App: Find, Start & Feedback

I have written a Javascript/HTML5 app that simulates a Light Clock as a way to understand the kinematics of relativity. It shows special and general relativistic effects, and how they are related. I would appreciate any feedback on the app, it's correctness, and it's usefulness as a learning...
6. ### Relativity light clock problem

1. I'm fine with this one, I simply calculated L = L0/γ = 5/1.2 5= 4m. 2. This is the one I'm having problems with. My approach was to say that both observers would measure the light beam to have velocity of ‘c’. Therefore, if 4m is the distance between the mirrors as observed in the frame of...
7. ### Change in time due to accelerated light-clock

Homework Statement A light-clock (a photon traveling between two mirrors) has proper length l and moves longitudinally through an inertial frame with proper acceleration ##\alpha## (ignore any variation of a along the rod). By looking at the time it takes the photon to make one to-and-fro...
8. ### B Light Clock Traveling in Z Direction

Hi, I'm studying GR these days. But then I got to thinking about something about SR and got kinda stuck. I know that if a light clock is moving in the x direction(from left to right), the light beam has to traverse more space and then you will see this person's clock running slower in the...
9. D

### I Einstein's light clock thought experiment

I'm an engineer who has an amateur interest in physics. I have been reading about Einsteins light clock experiment. I understand the principal that when a light clock on a train etc is moving relative to a standing still observer then the light must travel a longer distance per tick. given that...
10. ### Insights Relativity Variables: Velocity, Doppler-Bondi k, and Rapidity - Comments

robphy submitted a new PF Insights post Relativity Variables: Velocity, Doppler-Bondi k, and Rapidity Continue reading the Original PF Insights Post.
11. ### B Derivation of time dilation without light clocks

In the way I was taught about special relativity, time dilation is like the fundamental building block from which you derive things like relativistic mass and length contraction. So it has always struck me as quite odd, that the derivation of time dilation (in some sense the basis of special...
12. ### B How can a light clock tick once per meter even when in motion?

Hello P.F. I have previously raised the idea of a light clock that ticked each time the light traveled one metre. Dale has said if I could explain how it could be done, I could open a new thread. Here goes. A normal light clock ticks each time the light goes mirror to mirror. I propose a light...
13. ### I Light Clock Problem on the Train: Exploring Time Dilation in Special Relativity

Hello PF. Re. the light clock on the train thought experiment. If the mirrors are one metre apart in both frames. And the speed of light is the same in both frames. Why isn't the time it takes light to travel one metre, the same in both frames? Thanks.
14. ### Color of light in the Light Clock

In another thread, I came across the question of how the photon's energy and momentum appears in the moving frame. The question is best explained with the standard light clock. Lets have 2 horizontal mirrors, that is, one above the other. In the rest frame, the photon is set up to be linearly...
15. ### I Understanding Light Clock Confusion on Moving Trains - A Discussion

Hello PF. Sometime ago I posted a question about the light clock train thought experiment. I didn't understand the answer, so if I may I'll ask it again. In the light clock on the train thought experiment, the clock is calibrated to each time the light strikes the mirrors, which are 1 metre...
16. ### The Light Clock Problem: Resolving Observer Discrepancies

Lets suppose there's two obsrver O observer is in rest and A is moving with a constant velocity. A observer has a light clock which every click it makes sound.So A observer sees everything normal. Let's suppose A observer heard 10 tics but O sees him moving so O observer will heard 5 tics.But...
17. ### Light clock on Train experiment.

In the moving frame, relative to the stationary frame, if time is slowed and the speed of light isn't, why doesn't the light travel further in less elapsed time?
18. M

### Understanding Light's Motion in a Moving Clock

When light is moving up and down in a light clock, it has no horizontal speed to an observer at rest relative to it. If that light clock starts moving, the light must be moving with the clock because at rest, the light and clock both have no horizontal speed. How does the light convert its...
19. ### Time dilation -- light clock on a train thought experiment

Who first came up with the light clock on a train thought experiment.
20. ### Is the Light Clock Calibration the Key to Understanding Relativity?

Hello PF. Re. the light clock / train thought experiment. It seems to me that the clock calibration chosen, ( light's mirror to mirror path ), is determining the outcome of the experiment. And that a different calibration would give a different result. e.g. assuming the mirrors are one metre...
21. ### Solving the Horizontal Light Clock Puzzle: Length Contraction & Frame Shifting

Consider a Horizontal light clock of length ##L_0## lying at rest in a frame K. There are two important events: (A) a photon gets emitted from the left mirror and (B) it gets reflected at the right mirror. Another frame K' is moving by at velocity v and the frames are in standard configuration...
22. ### Light clock - Galilean transformations

We have two parallel mirrors, which are located at y=0 and y=l in the (x,y) plane. A photon is traveling between the mirrors, up and down along the y-axis. Consider an observer O at rest w.r.t. the mirrors. What's the time (Δt) measure by O for the photon to make a full period. Consider an...
23. ### Questions on length contraction in train light clock experiment?

Hi guys, I've been thinking about this experiment for the last few hours and I have a few questions which I am deeply confused about. 1) In the light clock experiment, where light clock is set up so that light travels vertically, and the train carrying the clock travels horizontally, wouldn't...
24. ### Need help understanding Einstein's Light Clock experiment

A general summary of the experiment is that when the light clock is stationary on the platform it runs as normal, up and down, but when someone is looking at a clock on a moving train, it appears to move slower because the light has further to travel, and thus, time runs slower. What confuses me...
25. ### Track of the light in a moving light clock.

There is a moving light clock at a certain velocity. For an outside observer, the light just seemed to be traveling in a slant path. So far, it is right for the outside observer, the light is going in a slant path and hitting the top bar and then again returning and hitting the bottom bar...
26. ### Light Clock Confusion in Special Relativity

When students are first introduced to special relativity, the first thing they are taught is the derivation of the gamma factor and time-dilation, and it goes something like this: Suppose there are 2 observers in space, let's call them Alice and Bob. Let's assume Alice is stationary from our...
27. ### Why does the frequency of a light clock change when it is accelerated?

This is an analysis of how a classic Einstein light clock behaves when accelerated orthogonal to its main axis. The results might be considered slightly controversial, so I am putting the details of the calculation here so that they can be checked.Consider a light clock that has its long axis...
28. ### Time-dialation and light clock

Can someone explain exactly(or to the best of your abilities) the changes that the light clock goes through from being in a stationary position, to reaching the speed of light? How does the light clock work, and how does the distance between "ticks" change to result in the slowing down of time...
29. ### Light clock with spaceship moving vertical at .99c

So what would happen if there was a spaceship moving at 99% the speed of light straight up. The spaceship has a light clock, so a photon of light moving bouncing between 2 mirrors in such a way that the light moves ALMOST in sync with the spaceship moving up. In theory wouldn't the light photon...
30. ### The light clock and the simple clock

This is probably a simple question but I can't seem find a convincing enough explanation. Many books on introductory Special relativity use what are called "light clocks" to explain Time dilation. I myself learned this from Arthur Beiser's Modern Physics. This is a simple clock that measures...
31. ### Exploring the Paradoxical Light Clock Problem in Special Relativity Analysis

In a recent thread, a discussion developed on the subject of how we observe light and how this affects our understanding of SR. I called attention to the famous light clock diagrams: http://home.comcast.net/~peter.m.brown/sr/image_gif/sr05-im-01.gif In my view, the problem here is not...
32. ### Who came up with the Light Clock Thought Experiment

Everyone is familiar with the Light Clock, a thought experiment which forms the entire basis of Special Relativity. Through the Light Clock, the Lorentz factor can be derived. So who was the genius who first came up with the Light Clock? Was it Lorentz himself? Or someone else? When did it...
33. ### Light Clock Query: Explaining why Light Does Not Escape

So my uncle recently asked me about a phenomenon that I have been unable to explain, but there that there must surely be a good answer for. A light clock is being observed on an airplane, with the light bouncing back and fourth between the plates of clock. The plates of the clock have the...
34. ### Time dilation - Lorentz transformation using light clock

Homework Statement I am trying to derive the time dilation formula using Pythagorean theorem: L = ct (in clocks rest frame) D = ct'(in observer's frame, the clock moving with velocity u) The Attempt at a Solution So D^2 = L^2 +v^2t^2 c^2{t'}^2=c^2t^2+u^2t^2 Sot'=t*\sqrt{1+v^2/c^2}...
35. ### Time dilation explaination with mechanical clock instead of light clock

This page http://en.wikipedia.org/wiki/Time_dilation clearly explains time dilation by exampling with the light clock. But how can it be explained if suppose there was a simple mechanical clock instead of light clock. So, now for moving observer there is no light (in light clock) to follow...
36. ### Why Does Stationary Observer See Light Clock Traverse Longer Distance?

Why does the stationary observer see the spaceship light clock traverse a longer distance? How can you prove this? I don't understand why he sees the light take a longer path. I understand it because you tell me this is so but I don't understand why it actually happens.
37. ### Wgat would a light clock measure in free fall?

Wgat would a "light clock" measure in free fall? I've read in a couple of different books that the similarities between acceleration from gravity and rockets or whatever is only local. Both books said one of the reasons is because with acceleration from gravity two objects in free fall starting...
38. ### Light Clock Example this is not HW

Ok I am only a physics undergrad so if this is obvious I am sorry. I understand the concepts of SR pretty well but one thing has bugged me. In the most basic light clock example used to describe the effects of time dilation, where 2 mirrors are placed 1 ls apart on a moving boxcar with an...
39. ### Why is the light clock accurate?

My friend and I have debated about this for some time and, because the vast majority of the scientific community backs him, I presume him to be correct but still do not understand why. I have far from an extensive knowledge of relativity, but I will try not to make a complete fool of myself...
40. ### Light Clock Experiment - questions from a newbie?

Hi all, firstly, I must apologise for my level of physics knowledge and how this might be reflected in my question and the terminology used. I am trying to understand particle physics at an advanced age my neurons are not as elastic as they once were! I have been listening to an excellent...
41. ### Question on the reasoning behind the light clock thought experiment.

I read that the speed of light is independent on the relative velocity of the source. So why is it that the beam zig-zags if there is relative motion between an observer and a light clock. This zig-zagging implies the vector addition of velocities. I know that the speed c is kept constant during...
42. ### Understanding Light Clock & Time Dilation

Hi guys, I am having trouble getting my head around the light clock scenario which demonstrates time dilation. I can't help but intuitively feel that it seems like a fault of the clock, rather than time dilation. I hope you can understand what I'm getting at. If someone could try to give an...
43. ### Why do clocks appear to run at different speeds when traveling at high speeds?

I don't understand this: http://casa.colorado.edu/~ajsh/sr/timevbig_gif.html if the mirror is moving fast enough, why won't the light just miss it? why is the light traveling towards at an angle instead of straight?
44. ### Solving Light Clock Question: Calculations for a Relativistic Light Clock

I am trying to understand the calculations for a relativisitic light clock and am doing something wrong. I am the stationary observer. A light clock goes by at c/2, or 93000 miles per second. The length of the light clock 'rod', which is perpendicular to the line of motion is 186,000 miles...
45. ### Can a light beam stay trapped in a moving cart with ideal mirrors indefinitely?

Consider a light clock, i.e. two ideal mirrors, mounted in parallel with a vacuum in between, such that, in principle, a beam of light or just a single photon will keep bouncing between the two forever. Now consider the light clock on a cart. If the cart, from the beginning, has constant...
46. ### Time Dilation Light Clock Example

I'm missing something here on the light clock example from http://en.wikipedia.org/wiki/Time_dilation. I understand the math on the picture, but I'm missing how that applies to time dilation. Why does the light pulse take on a diagonal path for the moving observer? Is the light pulse still...
47. ### How Does Light Behave in a Moving Light Clock?

The two parallel mirrors in the light clock are 1 light year apart, vertically. My reference frame which encompasses this clock, is traveling horizontally. The mirrors are 1cm wide. My reference frame is traveling much faster than 1cm/year. Does the light in the light clock reach the second...
48. ### Light clock with something else than light

I think I understand how the classical thought experiment of the light clock (light bouncing between two mirrors work). The typical explanation that is given is "the stationary observer sees the light travel a longer distance (the diagonal), so since light can't go faster than the speed of...
49. ### Revisiting the Flaws of the Light Clock in Special and General Relativity

Why "light clock" is flawed. I have been considering this thought experiment for some time, and the principle seems "broken". I'm wondering if by playing devil's advocate you can show me what is going wrong. -In SR, the light clock was used to predict time dilation in a "moving" object...
50. ### Light clock moving to demonstrate time dilation

Hello all, having decided that I wish to apply to Oxford to study Physics (as well as Imperial and, I am still considering Natural Sciences at Cambridge), I have been informed that extra reading and independent study would be advised, so I'm delving into the world of relativity (I'm a first year...