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

[Meson080]

The principle of relativity was first stated by Newton, in one of his corollaries to the laws of motion: "the motions of bodies included in a given space are the same among themselves, whether that space is at rest or moves uniformly forward in a straight line." This means, for example, that if a spaceship is drifting along at a uniform speed, all experiments performed in the spaceship and all the phenomena in the spaceship will appear the same as if the ship were not moving, provided, of course, that one does not look outside...in a moving spaceship the electrical and optical phenomena should be different from those in a stationary ship.

Consider a hypothetical bus drifting along at a uniform speed in the vacuum 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.

 

[A.T.]

I think in a moving bus the insect suffers lot of crashes,

Why?

 

[Meson080]

I think in a moving bus the insect suffers lot of crashes,

Why?

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 vacuum here).

 

[Nathanael]

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 vacuum here).

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")

 

[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 vacuum here).

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")

Sorry I didn't understand your comment.

 

[Drakkith]

Sorry I didn't understand your comment.

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.

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.

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.

 

[Meson080]

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.

No. I am not saying to accelerate both.

 

[A.T.]

No. I am not saying to accelerate both.

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

 

[Meson080]

No. I am not saying to accelerate both.

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

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?

 

[A.T.]

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

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.

 

[Meson080]

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

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.

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?

 

[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?

 

[Meson080]

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?

I can say whether the bus is moving or not.

 

[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.

 

[A.T.]

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?

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

 

[Meson080]

I can say whether the bus is moving or not.

Using what measurements?

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.

 

[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.

 

[jbriggs444]

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.

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.

 

[Nugatory]

Send a robotic fly using a remote and make it stand still at some point without any contact with the bus.

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

 

[Meson080]

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

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

 

[Nick O]

How do you know that the bus is moving, and not the point with respect to which the fly is motionless? The bus might be perfectly still with the fly's reference point moving at a constant velocity.

 

[Nick O]

What we are trying to say is that you can't know that the fly actually comes to rest. And if you could know, then it would be exactly as if you had a window to see whether external reference points were moving, because the fly would be synchronized with a known external reference.

 

[Meson080]

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

My eyes can say whether the fly is still or not. The fly is still with respect to me or the bus.

 

[Nick O]

Are you moving at the same velocity as the bus? If so, this tells you nothing. If not, then why do you need the fly?

 

[Nugatory]

My eyes can say whether the fly is still or not. The fly is still with respect to me or the bus.

Right, your eyes will tell you whether it is the bus or the fly that is moving relative to you. But how do you know whether you are still or moving? How would you convince someone else who happens to be nearby, also watching the bus and the fly, but moving at the same speed relative to you as the bus, that it is him and the bus that is moving and not you and the fly?

 

[Nick O]

Okay, what we need are scenarios. Tell me where these are wrong, and why.

Scenario 1: Bus moving at 100mph north.

The fly enters bus and comes to rest w.r.t. the bus. Therefore, the bus and fly are now moving 100mph north. The fly appears motionless, so you conclude that the bus is not moving.

Scenario 1: Bus motionless.

The fly enters bus and comes to rest w.r.t. the bus. Therefore, the bus and fly are now motionless. The fly appears motionless, so you conclude that the bus is not moving.

 

[SteamKing]

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?

How do you make the insect stay at a particular point inside the bus? It's not as easy as it seems.

 

[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.

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.

Sorry, I didn't understand your comment.

 

[Nick O]

He means this: define "standing still".

 

[Meson080]

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

If the bus is still, I can make the fly stand at point. I can know that it is still, because I have the remote.

 

[A.T.]

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

Or the fly was moving, and the bus was still.

 

[Nick O]

So you and the remote are inside the bus, and you are flying a reference point from outside into the bus. That will indeed tell you whether or not the bus is moving with respect to the reference point, but it won't tell you which of the two is moving with respect to the earth.

 

[A.T.]

I can know that it is still, because I have the remote.

How does your remote determine if the fly is "still"?

 

[Meson080]

How do you know that the bus is moving, and not the point with respect to which the fly is motionless? The bus might be perfectly still with the fly's reference point moving at a constant velocity.

I am the one who has made the fly stand still using my remote. So, I know that bus is moving and not the fly.

 

[Meson080]

What we are trying to say is that you can't know that the fly actually comes to rest. And if you could know, then it would be exactly as if you had a window to see whether external reference points were moving, because the fly would be synchronized with a known external reference.

What happens if it is synchronized?