# Is it possible to tell whether ship is moving or not?

1. Jun 1, 2014

### Meson080

Consider a hypothetical bus drifting along at a uniform speed in the vaccum and a hypothetical insect which crawls inside the bus without any physical contact with it.

I think in a moving bus the insect suffers lot of crashes, which can be used to know whether the bus is in motion or not. Instead of the insect, robotic model can be used which stands still at a particular point inside the bus, to have better results.

I hope the above experiment is neither optical nor electrical.

So, according to this thought experiment. I think we can tell, by making mechanical experiments (other than optical or electrical), whether the system is moving or not.

2. Jun 1, 2014

Why?

3. Jun 1, 2014

### Meson080

Consider a robotic fly standing still in the space inside the bus, flapping its wings. Here the fly has no physical contact with the bus. If the bus is stationary, I think the fly stays where it was. If the bus is drifting with uniform speed, even if the fly was staying where it was, I think the backside of the moving bus hits it.

Similarly, I feel that an insect crawling inside a moving bus suffers lot of crashes (vaccum has to be considered). In fact, I have seen a fly suffering from lot of crashes inside a moving bus (but there is no vaccum here).

4. Jun 1, 2014

### Nathanael

But that only reveals that the bus is moving RELATIVE to the fly. You still can't tell if the bus is moving forward, (and the fly is still,) or if the fly is moving backward, (and the bus is still).
(Sure you can say "well we said the fly was standing still..." but you can't actually tell which is "true")

5. Jun 1, 2014

### Meson080

Sorry I didn't understand your comment.

6. Jun 1, 2014

### Drakkith

Staff Emeritus
The key to relativity is that motion is relative to something else. In other words, a fly that is stationary inside a bus is stationary relative to the bus. It may or may not be stationary relative to another object. To another object both the bus and the fly may be moving, yet the fly is still stationary with respect to the bus.

Not true. If we take our stationary bus and fly, accelerate them both to 100 mph (relative to the ground), the fly is still stationary with respect to the bus and will not be hit by it.

7. Jun 1, 2014

### Meson080

No. I am not saying to accelerate both.

8. Jun 1, 2014

### A.T.

If you accelerate just one, then you have effects of differential acceleration, not of uniform movement.

9. Jun 1, 2014

### Meson080

What do you mean by differential acceleration? Google search of DIFFERENTIAL ACCELERATION: https://www.google.de/search?q=DIFFERENTIAL+ACCELERATION&rlz=1C1DFOC_enIN544IN544&oq=DIFFERENTIAL+ACCELERATION&aqs=chrome..69i57.7179j0j7&sourceid=chrome&es_sm=122&ie=UTF-8, didn't give me any clue.

I hope you meant accelerating both to different extent. Isn't it?

10. Jun 1, 2014

### A.T.

Yes. If on the other hand both have been accelerated to the same velocity, then there is no relative velocity between them. So they fly will not bounce into the back side.

11. Jun 1, 2014

### Meson080

The bus is drifting at uniform speed. I am performing the experiment in it by making the insect stay at a particular point inside the bus (but not physically in contact). What's the problem?

12. Jun 1, 2014

### Nick O

Suppose you are in this bus. It is moving at a uniform velocity relative to the earth, with a perfect suspension system such that you feel no movement. There are no windows, and you were asleep while the bus was accelerating and therefore do not know whether or not the bus is moving.

Then you observe a fly drifting toward the back of the bus. Who is to say that the bus is moving, and not the fly?

Now, my own query:

Is there a measurable "absolute velocity" that requires no reference velocity for complete definition? We know about relativistic effects that occur as an object approaches the speed of light, which have a zero point (I think?) and an asymptotic limit.

By observing the rate of change of relativistic effects on an object as it accelerates to some velocity, can we somehow produce a velocity measurement that is the same from all inertial frames of reference? Wouldn't it be noticeable when the object reaches the point at which relativistic effects reduce to an absolute minimum, and start to increase again?

Edit:
If this absolute velocity is measurable, how is it defined? With respect to the center of the universe, or perhaps space itself? Does the expansion of space have any effect on its value?

Last edited: Jun 1, 2014
13. Jun 1, 2014

### Meson080

I can say whether the bus is moving or not.

14. Jun 1, 2014

### Nick O

Using what measurements?

Note also that as soon as your fly makes contact with your bus, it will be accelerated to the velocity of the bus. Your experiment really depends on recent acceleration. An accelerating observer can already sense the acceleration. However, decreasing velocity is indistinguishable from increasing velocity.

15. Jun 1, 2014

### A.T.

If the insect stays at a particular point relative to the bus, then it's just as if the bus was at rest.

16. Jun 1, 2014

### Meson080

Suppose the bus is at rest in vaccum. Send a robotic fly using a remote and make it stand still at some point without any contact with the bus. As the bus is at rest, the fly stands still where I have made it to stand. If the bus was drifting along with uniform velocity. The fly hits the backside of the bus. Using these observations, I can say whether the bus is moving or not.

17. Jun 1, 2014

### Nick O

How do you know that the fly is still, and with respect to what? You have to measure that!

What you can determine here is the bus's velocity with respect to the fly. But you still need knowledge of the fly's velocity with respect to some other reference point. If you have communication with the outside world, then radar is more than sufficient.

18. Jun 1, 2014

### jbriggs444

You cannot make it stand still if you don't know what "standing still" is. Since the point of this exercise is to determine a reference for "standing still", your experiment cannot succeed.

19. Jun 1, 2014

### Staff: Mentor

And how do we know that the fly is standing still and not drifting slowly itself?

20. Jun 1, 2014

### Meson080

Yes. If the fly crashes, then the bus is moving with uniform speed.