# I Relativity's "time dilation" or clock accuracy alteration

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1. Jan 7, 2017

### stevendaryl

Staff Emeritus
If you're on a train moving at a constant speed, and you start bouncing a basketball up and down, then to you, it is traveling a path that is perpendicular to the floor. To someone outside the train, the basketball travels a diagonal path. Light is no different in this respect. So the conclusion is that the angle that the bouncing object follows is relative to the rest frame of the observer. Special Relativity is no different from pre-relativistic physics in this regard.

I understand the intuition: In the light clock, when the clock is moving, it appears that the light must be "aimed" ahead of the mirror. But the same thing is true of the person bouncing a basketball inside a moving train: From the perspective of someone at rest looking at the train move past, it seems that the person with the basketball must "aim" it slightly ahead of the point on the floor where he wants it to hit. But the person aboard the train isn't doing anything differently than if the train were at rest.

2. Jan 7, 2017

### Crowxe

thanks for the effort and explaining , it cleared a lot of the confusing things to me . if you still have energy left , i would like to tweak a bit the example that you put here.

lets say it's one observer with light beaming vertically to the plan he's standing on and then him along with the light source gained speed horizontally , he and the light source gained horizontal speed but did the light gain that horizontal speed?

3. Jan 7, 2017

### Crowxe

there's one significant difference comparing the bouncing ball and the bouncing beam regarding the direction (according to the person in the same reference frame) . the ball can gain the train's horizontal speed but light cant gain speed

4. Jan 7, 2017

### stevendaryl

Staff Emeritus
Well, there is the speed in the horizontal direction, and there is the speed in the vertical direction. The total speed (which is defined by: (total speed)2 = (horizontal speed)2 + (vertical speed)2) is constant, but light can change its horizontal speed.

5. Jan 7, 2017

### Crowxe

thats a simple language i can understand , so the horizontal speed is through aiming the beam forward or gained by the horizontal source movement ? (i believe in both cases time dilation will take place and relativity should still hold)

6. Jan 7, 2017

### PeroK

Yes, because it's all about reference frames. If the light (or anything) is moving vertically in his frame of reference, then it must be moving vertically and horizontally in another frame of reference in which he and the light source are moving horizontally. That's essentially a physical/geometric reality - and, in fact, has to do with all physics, not just relativity.

It's better not to think about the light source or light "gaining" horizontal speed, but to see that it has a horizontal component to its motion in one frame. If its horizontal speed in frame A is 0, then in a frame B moving horizontally at speed $v$ with respect to frame A, its horizontal speed must be $v$.

This was standard, classical physics from Netwon's time at least. In classical physics, you can then take the vertical speed $w$, say. which is the same in both frames and then:

The speed of the object is $w$ in frame A.

The speed of the object is $\sqrt{w^2 + v^2}$ in frame B.

However, there was experimental evidence that if the "object" was a beam of light, then this did not apply. In particular:

The speed of the light is $c$ in frame A.

The speed of the light is $c$ in frame B. It is not $\sqrt{c^2 + v^2}$.

This was a terrible puzzle for classical physics. By using this experimental fact and his light-clock thought experiment, Einstein showed that one explanation was that "time itself was suspect" and that with respect to one frame, time in the other frame is dilated by the factor of $\frac{c}{\sqrt{c^2 + v^2}}$.

This time dilation explains why, even thought the observer in frame A sees the light moving only vertically and B sees the light moving vertically and horizontally, they measure the same speed $c$.

There was never any thought that somehow the light was moving vertically in both frames.

7. Jan 7, 2017

### PeroK

It can't increase its speed, but its speed will and must have a horizontal component in some frames! You need to give up this idea that light can only travel vertically!

8. Jan 7, 2017

### Crowxe

thanks a lot , you've been great help , i think i'm confusion free for now

9. Jan 7, 2017

### stevendaryl

Staff Emeritus
A mirror that is oriented vertically cannot change the light's horizontal speed. So if the light is initially traveling vertically, it'll keep traveling vertically. If it is initially traveling at a diagonal, it will keep traveling diagonally. After bouncing off the mirror, the only change is to the vertical velocity: that switches direction.

So the mirror doesn't need to "aim" for where the light should go.

Now, you could ask the question of how the light gets started traveling diagonally, in the first place. Although it's maybe not intuitive, the fact is that if the person on the train takes an ordinary flashlight, and aims it straight up, the light coming out of the flashlight will travel "diagonally" as seen from someone outside the train.

10. Jan 7, 2017

### Crowxe

i guess the confusion is almost gone but i'll just state it again in case it wasnt clear. i wasnt talking about 2 farme references , just the one on the train and my prediction was the light beam would appear diagonally backward to him

11. Jan 7, 2017

### Staff: Mentor

I don't have a particular favorite light clock presentation. It is nothing more than a teaching aid, so if it doesn't help you, then I would move on to something else. Personally, learning about four-vectors was what made relativity "click" for me, not any of the usual thought experiments.

Did you ever read the poem about the descriptions of the elephant? All of the descriptions you have received are correct, they each correctly describe part of the relativity "elephant". It is not "chaos" it is a bunch of pieces of a big picture that you currently haven't seen. For a full and coherent description of relativity you will need a textbook.

The experiments are pretty conclusive that there is no such thing as an "actual speed", only "relative speed".

Last edited: Jan 7, 2017
12. Jan 7, 2017

### PAllen

And this would be a counterfactual prediction. Any simple optics experiment disproves this because the earth's motion in one season relative to its motion at a different season is substantial, thus one or the other should see your proposed effect. This is exactly what the many variations of the Michaelson-Morely experiment refute.

13. Jan 7, 2017

### stevendaryl

Staff Emeritus
Well, that's not the way that light works. If you shine a flashlight, the light comes out traveling straight away from the flashlight, in the frame in which the flashlight is at rest (the frame of the train). (Actually, the light coming from a flashlight is spread out in all directions, so maybe we should talk about a laser pointer, instead of a flashlight.)

14. Jan 7, 2017

### Vitro

@Crowxe, I think you are still holding on, whether consciously or subconsciously, to a notion of some absolute universal "rest" frame relative to which everything has an actual speed (in an absolute sense), and probably trying to figure out ways you can detect or measure this speed, like what's the actual speed of the Earth through space.

Many physicists over many centuries have wondered and tried the same and, at least for mechanics, they have realized since Galileo that there was no such thing, or at least mechanics didn't offer any means for detecting such absolute speed or rest. The laws of mechanics obeyed the Principle of Relativity, they were the same in every inertial reference frame.

But there was one thing that wasn't covered by Galilean relativity and which could potentialy be used to measure this absolute speed, light (electromagnetism), thought to be carried by a medium (luminiferous aether) which determined its speed. Many experiments were performed, first to mesure the speed of light with ever increasing accuracy and then to try to measure the speed of the Earth through this hypothetical aether by detecting variations in the speed of light measured at different times of the day or year and in different directions as the Earth rotated on its axis and orbited the Sun. But the speed of light always turned out to be the same no matter what, all attempts to detect or measure speed relative to the aether were unsuccessful.

Einstein realized that the Principle of Relativity must also apply to the laws of electromagnetism extending the PR to say that all laws of physics are the same in every inertial frame, which also implied that the speed of light must be the same in every inertial frame (the two postulates of Special Relativity). Basically what this means is that if you are in a closed box floating somewhere in empty space (and you cannot see outside to notice stars changing position) there is no experiment whatsoever that you could perform inside the box to determine its absolute speed, so the notion simply doesn't apply. And even if you do look outside and see the stars moving that doesn't mean that the box is moving, or that the stars are moving (in any absolute sense), it just means that they are moving relative to each other. It makes no sense to ask "but which one is really moving?". The answer would be "from who's perspective?". From a star's perspective the box is moving, from the box's perspective the star is moving, from some other perspective both the star and the box are moving, and all those perspectives are equally valid.

As a first step towards understanding relativity you must give up any notion of absolute rest or motion, speed is purely relative, anything can be considered moving or at rest simply by changing one's point of view or perspective.

Regarding the speed of light being constant, you must differentiate between two notions: speed and velocity. Velocity is a vector, it has a direction. Speed represents the magnitude of the velocity vector, and is just a value (has no direction). Speed is the one that's constant and cannot be changed, the velocity is observer dependent. The direction of the same pulse of light can be different for different observers, no physical aiming of the source in different directions is necessary for this.

15. Jan 7, 2017

### Crowxe

i've been talking 2 points lately :
1. the light direction / path if beamed perpendicularly to the platform line of motion and that is from the observer in same frame reference not another observer stationary nor any other reference frame.
2. we all presume things and examples to reach conclusions , so i think it's valid to assume we are stationary in an example or presumed test just to simplify it and then later apply it to our situation of not knowing what is our actual speed. i think that question is valid even if it cant be answered until now

16. Jan 7, 2017

### PAllen

This exact question has been answered numerous times in this thread. As I mentioned, you could test it yourself with any beam source you like. Just do the experiment in winter and also in summer. Further, the experiment has been done to great precision for many decades because many types of precision optical devices would need continual adjustment if your hypothesis were true. In fact your garage door opener safety beam would need regular adjustment due to earth's changing state of motion, if your hypothesis were true. This is, in fact, a moderately precise implementation of your question.

17. Jan 7, 2017

### Mister T

There is no experiment that has ever been done, or observation ever recorded, that distinguishes between a state of rest and a state of steady motion. Any distinction that you make is therefore unsupported.

The present state of physics includes the assumption that no such distinction can be made.

The very notion that the ball "gains" the train's horizontal speed implies, to my way of thinking, that the ball's inertia is somehow a property of the ball. It's not. It appears simply because of an observation made in a frame of reference that is in motion relative to the ball.

Likewise, the claim that the train is in motion is just a result of the train being observed from a frame of reference that is not at rest relative to the train.

Last edited: Jan 7, 2017
18. Jan 7, 2017

### Staff: Mentor

The reason that it is valid to make the assumption that we are stationary is precisely because there is no such thing as "our actual speed".

19. Jan 7, 2017

### Vitro

Except for light and other massless things which always move at $c$, so not really anything.

20. Jan 7, 2017