Einstein's Train Example: Speed of Light and Simultaneity

[ahmeeeeeeeeee]

hello

About Einestien's train example




I can't get what it really does with the constancy of the speed of light , what would be the difference if , instead of two bolts , we have two balls thrown at the same angle ?!

the observer in the train would still see the front ball first

Or we are using light because it is what determines simultaneity in our eyes , which means when I see two balls passing by me at different times , I don't have to conclude they weren't thrown simultaneously but when I see Light doing that I will ?

I need basically to know what difference does the constancy of the speed of light make here

thanks in advance ;

 

[harrylin]

hello

About Einestien's train example



I can't get what it really does with the constancy of the speed of light , what would be the difference if , instead of two bolts , we have two balls thrown at the same angle ?!

the observer in the train would still see the front ball first
[..]
I need basically to know what difference does the constancy of the speed of light make here

thanks in advance ;

Nice animation!

You are right that the observer in the train will still see the front ball first. The difference here is that according to her, the front ball has a higher speed than the rear ball (that is, relatively to the train). But she will still think (if she measures it unbelievably precisely and uses the train as reference system) that the front ball left a tiny little bit earlier than the rear ball.

That is easy to understand if you think of two balls shot at the train in an identical way by the impacts of the two lightning bolts: everyone agrees that the front ball left immediately after the front lightning bolt, and in her reckoning that happened just before the rear one.
If the balls were launched at almost the speed of light, then she would think that the balls had almost the same speed relative to the train.

Those calculations are complex. For light rays and radio waves the calculation becomes simple!

 

[ahmeeeeeeeeee]

You mean that the observer will see the front ball first , but also she will see the speed differences so she will not say a thing about simultaneity

But in the light example , she will see the front first , and also she will notice the same speed of light so she will say the events aren't simultaneous

Am I right ?!

If so , can she notice that speed of light in order to determine they are equal ?! or Are we just supposing she has proper instruments to measure it on the train , and with these instrument she will arrive at the result of non-simultaneity ?!

 

[harrylin]

You mean that the observer will see the front ball first , but also she will see the speed differences so she will not say a thing about simultaneity

But in the light example , she will see the front first , and also she will notice the same speed of light so she will say the events aren't simultaneous

Am I right ?!

Almost - with the balls, the difference is mostly due to the speed difference.

If so , can she notice that speed of light in order to determine they are equal ?! or Are we just supposing she has proper instruments to measure it on the train , and with these instrument she will arrive at the result of non-simultaneity ?!

Almost so! In fact, she can not notice that the speed of light in both directions is equal - on that point the video is slightly misleading. In fact it is the opposite: she claims that the speed of light is the same in both directions relative to the train, and she adjusts any clocks so that the clocks agree with that assumption. And the guy on the platform sets his clocks in disagreement with her clocks. Of course, after she made her clocks say that the speed of light is the same in both directions, she will also "measure" that this is so.

You can read about it in Wikipedia:
http://en.wikipedia.org/wiki/One-way_speed_of_light

We also had long discussions about that topic, see for example (I think that there is a better link though):
- https://www.physicsforums.com/showthread.php?t=641102
- https://www.physicsforums.com/showthread.php?t=544366

 

[sardar]

The concept of the constancy of the speed of light is a fundamental principle in Einstein's theory of relativity. It states that the speed of light in a vacuum is the same for all observers, regardless of their relative motion. This means that the speed of light is a universal constant that cannot be exceeded by any object or particle.

In the train example, the observer on the train and the observer on the platform will both measure the same speed of light, even though they are moving at different speeds. This is because the speed of light is not affected by the motion of the observer. This is in contrast to the speed of the balls, which would be different for the observer on the train and the observer on the platform.

The significance of this is that it challenges our common sense notion of simultaneity. In classical mechanics, events that occur at the same time for one observer will also occur at the same time for another observer. However, in Einstein's theory of relativity, this is not always the case. The constancy of the speed of light means that the concept of simultaneity is relative and depends on the observer's frame of reference. This is why the observer on the train sees the front ball first, while the observer on the platform sees both balls at the same time.

In summary, the constancy of the speed of light is a fundamental principle that has a profound impact on our understanding of the universe and the concept of time. It highlights the relativity of simultaneity and challenges our previous understanding of how events occur simultaneously for different observers.