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Nov 13, 2020, 3:02:56 PM11/13/20

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The bridge and the train have the same length at rest.

The bridge collapses only if the entire weight of the train rests on

it.

In the reference of the train (traveling at relativistic speed) the

bridge (contract) is shorter and the weight of the train never rests

entirely on the bridge: the passengers are safe.

Instead, in the reference of the ground, the train is shorter and there

is a time interval in which the weight of the train rests entirely on

the bridge which, therefore, collapses: for the observer on the ground

the passengers of the train are doomed.

Who's right? Are train passengers saved or not?

[Moderator's note: This is essentially the same puzzle as the ladder

paradox, which even has its own Wikipedia entry. In fact, it is closer

to the "man falling into grate" version originally discussed by the

late, great Wolfgang Rindler. -P.H.]

The bridge collapses only if the entire weight of the train rests on

it.

In the reference of the train (traveling at relativistic speed) the

bridge (contract) is shorter and the weight of the train never rests

entirely on the bridge: the passengers are safe.

Instead, in the reference of the ground, the train is shorter and there

is a time interval in which the weight of the train rests entirely on

the bridge which, therefore, collapses: for the observer on the ground

the passengers of the train are doomed.

Who's right? Are train passengers saved or not?

[Moderator's note: This is essentially the same puzzle as the ladder

paradox, which even has its own Wikipedia entry. In fact, it is closer

to the "man falling into grate" version originally discussed by the

late, great Wolfgang Rindler. -P.H.]

Dec 12, 2020, 1:13:40 PM12/12/20

to

On 2020-11-13, Luigi Fortunati <fortuna...@gmail.com> wrote:

> The bridge and the train have the same length at rest.

>

> The bridge collapses only if the entire weight of the train rests on

> it.

[...]
> The bridge and the train have the same length at rest.

>

> The bridge collapses only if the entire weight of the train rests on

> it.

> [Moderator's note: This is essentially the same puzzle as the ladder

> paradox, which even has its own Wikipedia entry. In fact, it is closer

> to the "man falling into grate" version originally discussed by the

> late, great Wolfgang Rindler. -P.H.]

barn with the doors opening/closing just in time. The answer, of

course, is relativity of simultaneity.

--

ciao, Bruce

Dec 14, 2020, 4:08:16 PM12/14/20

to

Bruce Scott sabato 12/12/2020 alle ore 19:13:37 ha scritto:

>> The bridge and the train have the same length at rest.

>>

>> The bridge collapses only if the entire weight of the train rests on

>> it.

> [...]

>

>> [Moderator's note: This is essentially the same puzzle as the ladder

>> paradox, which even has its own Wikipedia entry. In fact, it is closer

>> to the "man falling into grate" version originally discussed by the

>> late, great Wolfgang Rindler. -P.H.]

>

> The version we got in class (way back when) was the train entering the

> barn with the doors opening/closing just in time. The answer, of

> course, is relativity of simultaneity.

But does the bridge collapse or does it not collapse?
>> The bridge and the train have the same length at rest.

>>

>> The bridge collapses only if the entire weight of the train rests on

>> it.

> [...]

>

>> [Moderator's note: This is essentially the same puzzle as the ladder

>> paradox, which even has its own Wikipedia entry. In fact, it is closer

>> to the "man falling into grate" version originally discussed by the

>> late, great Wolfgang Rindler. -P.H.]

>

> The version we got in class (way back when) was the train entering the

> barn with the doors opening/closing just in time. The answer, of

> course, is relativity of simultaneity.

[Moderator's note: Answer per moderator's note here, as this has been

solved long ago. The bridge collapses. Forget the complication of the

bridge and the weight of the train causing it to break; just have a gap

where the bridge should be. Does the train fall into the gap? Yes.

See the "paradox" due to Rindler above. Check this out:

https://www.youtube.com/watch?v=Xrqj88zQZJg I think that some of the

confusion comes from first assuming that when the train is on the bridge

or the gap then it will fall, but in practice if the train were moving

that fast then it would just sail over the gap. But if you assume that

it would fall when positioned over the gap, you also have to assume that

gravity is strong enough to pull it down. -P.H.]

Dec 16, 2020, 3:15:20 AM12/16/20

to

follow "straight lines" near worldlines of light on local light cones.

If light rays traverse the bridge without beeing bend down to touch the

opposite wall, any massive train will reach the other side, too, in the

limit v->c. Since bridges are constructed using light rays, no extra

engeneering art is necessary.

For slow trains, the engineer should form the bridge as a ballistic

parabola in order to save steel.

The question of the bridge collapse is a question of energy-momentum

transfer in its rest system.

The train at the speed of light is releivistically compressed to a point

and acts like a point mass on a ballistic hyperbole at its perigaeum. It

does not transfer energy-momentum to the bridge. It will leave the earth

surface tangentially and will disappear somwhere behind Uranus.

--

Roland Franzius

Dec 16, 2020, 5:53:40 PM12/16/20

to

On Monday, December 14, 2020 at 4:08:16 PM UTC-5, Luigi Fortunati wrote:

> Bruce Scott sabato 12/12/2020 alle ore 19:13:37 ha scritto:

> Bruce Scott sabato 12/12/2020 alle ore 19:13:37 ha scritto:

> >> The bridge and the train have the same length at rest.

> >>

> >> The bridge collapses only if the entire weight of the train rests on

> >> it.

> > [...]
> >>

> >> The bridge collapses only if the entire weight of the train rests on

> >> it.

> >

> >> [Moderator's note: This is essentially the same puzzle as the ladder

> >> paradox, which even has its own Wikipedia entry. In fact, it is closer

> >> to the "man falling into grate" version originally discussed by the

> >> late, great Wolfgang Rindler. -P.H.]

> >

> >> [Moderator's note: This is essentially the same puzzle as the ladder

> >> paradox, which even has its own Wikipedia entry. In fact, it is closer

> >> to the "man falling into grate" version originally discussed by the

> >> late, great Wolfgang Rindler. -P.H.]

> >

> > The version we got in class (way back when) was the train entering the

> > barn with the doors opening/closing just in time. The answer, of

> > course, is relativity of simultaneity.

> > barn with the doors opening/closing just in time. The answer, of

> > course, is relativity of simultaneity.

> But does the bridge collapse or does it not collapse?

>

> [Moderator's note: Answer per moderator's note here, as this has been

> solved long ago. The bridge collapses. Forget the complication of the

> bridge and the weight of the train causing it to break; just have a gap

> where the bridge should be. Does the train fall into the gap? Yes.

> See the "paradox" due to Rindler above. Check this out:

> https://www.youtube.com/watch?v=Xrqj88zQZJg I think that some of the

> confusion comes from first assuming that when the train is on the bridge

> or the gap then it will fall, but in practice if the train were moving

> that fast then it would just sail over the gap. But if you assume that

> it would fall when positioned over the gap, you also have to assume that

> gravity is strong enough to pull it down. -P.H.]

Simultaneity says that if the bridge collapses in one frame of relativity
>

> [Moderator's note: Answer per moderator's note here, as this has been

> solved long ago. The bridge collapses. Forget the complication of the

> bridge and the weight of the train causing it to break; just have a gap

> where the bridge should be. Does the train fall into the gap? Yes.

> See the "paradox" due to Rindler above. Check this out:

> https://www.youtube.com/watch?v=Xrqj88zQZJg I think that some of the

> confusion comes from first assuming that when the train is on the bridge

> or the gap then it will fall, but in practice if the train were moving

> that fast then it would just sail over the gap. But if you assume that

> it would fall when positioned over the gap, you also have to assume that

> gravity is strong enough to pull it down. -P.H.]

that it collapses in all frames. Sort of like that a life destruction

cannot alter to a many universes model.

Say a train is given a barn door pair to travel thru. And the train has

explosives in it, triggered by closed doors firing them if the train

trigger is centered in it. The train velocity to trigger is findable

by trial and error? But in fact the train is never at c. Meaning

that the trigger never fires. Indicating that the barn viewer

can never know when the barn doors are closed.

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