Modified Car and ditch paradox

  • Context: Graduate 
  • Thread starter Thread starter I_am_learning
  • Start date Start date
  • Tags Tags
    Car Paradox
Click For Summary

Discussion Overview

The discussion revolves around a modified version of the car and ditch paradox in the context of special relativity. Participants explore the implications of simultaneity and frame-dependent observations regarding a car moving at relativistic speeds and its interaction with a ditch that is larger than the car when at rest.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Exploratory

Main Points Raised

  • One participant proposes that the car will fall into the ditch if all its tires are in the air at the same time, while others question the physicality of this assumption due to the frame-dependent nature of simultaneity.
  • Another participant argues that the modification contradicts reality, suggesting that the car will always crash into the opposite edge of the ditch regardless of the observer's frame of reference.
  • A later reply introduces a scenario with pressure sensors on either side of the road, proposing that the car would be snatched by a robotic arm if all sensors are not pressed simultaneously from the road's perspective, indicating a potential solution to the paradox.
  • Participants discuss the implications of relativistic speeds on the car's downward velocity when hovering above the ditch, noting that the car's behavior appears different depending on the observer's frame of reference.

Areas of Agreement / Disagreement

Participants express disagreement regarding the validity of the proposed modifications to the paradox. There is no consensus on whether the car will fall into the ditch or how the scenario should be interpreted under special relativity.

Contextual Notes

The discussion highlights limitations related to the assumptions made about simultaneity and the physical implications of the proposed modifications. The complexity of the mechanical engineering aspects of the problem is also noted.

I_am_learning
Messages
681
Reaction score
16
Modified Car and ditch paradox!

Suppose there is a car moving on a road in relativistic speed. The road has a ditch (hole) which is litter bigger than the car when both are at rest.

Modification: Assume that the car will fall on the ditch if all of its tyres are in the air at the same time. If two of the rear tyres rest on the road and two of the front tyre hang on the air, the car won't fall in the ditch; it can move as if all of its tyres are on road.

Now the problem is, car seem to fall on the ditch on the
roads perspective whereas in the cars' perspective it can pass the ditch without trouble.
The question is Will the car fall in the ditch?

(Though I have much understood much of S.R. I got stuck on this. I know that: though in the road's frame of reference the two rear and the two front wheels seem to be in air at the same instant its not the case in the car's frame of reference. In the car's perspective they occuur one after the other.
But anyway, it can't be that --> for the car it can escape the ditch AND for the road it can trap the car.? )
Have attached picture for visualization.
 

Attachments

  • carandditch.jpg
    carandditch.jpg
    10.9 KB · Views: 459
Physics news on Phys.org


thecritic said:
Modification: Assume that the car will fall on the ditch if all of its tyres are in the air at the same time. If two of the rear tyres rest on the road and two of the front tyre hang on the air, the car won't fall in the ditch; it can move as if all of its tyres are on road.
"at the same time" according to whom? Since simultaneity is frame-dependent, your modification is unphysical.
 


thecritic said:
Suppose there is a car moving on a road in relativistic speed. The road has a ditch (hole) which is litter bigger than the car when both are at rest.

Modification: Assume that the car will fall on the ditch if all of its tyres are in the air at the same time. If two of the rear tyres rest on the road and two of the front tyre hang on the air, the car won't fall in the ditch; it can move as if all of its tyres are on road.

Now the problem is, car seem to fall on the ditch on the
roads perspective whereas in the cars' perspective it can pass the ditch without trouble.
The question is Will the car fall in the ditch?

This is a variation of a familiar problem. And it suffers from the same weakness. Your modification to the problem contradicts reality. That makes any solution meaningles. The truth is, this is a very difficult Mechanical Engineering problem. The other side of the ditch must be lower by a certain amount for the car to make it across no matter whose Physics you use (Newton's or Einstein's). That means it will always crash into the other edge. So why not just mark a spot on the opposite edge where it hits. Then the problem is to verify that it will hit the same spot wheather you observe the action from the ditch or as a passenger in the car.

Hint: Remember that velocities don't add the same intuitive way using Special Relativity. The vector sum can never reach "c". So, the faster the car goes along the road, the less downward velocity is available for it to acquire when it's hovering above the ditch. At .9c it will drop much less than expected before it hits the opposite edge. Now switch and be the observer in the car. The car is at rest and can acquire all the downward velocity it wants once its above the ditch. But the ditch is moving at .9c and is much narrower. So the car hits the opposite edge in the same spot. Make sense?
 


Doc Al said:
"at the same time" according to whom? Since simultaneity is frame-dependent, your modification is unphysical.
To tell According to roads view.
To make the modification sensible assume that on the both side of the roads are pressure sensors. As long as at least one sensor is kept pressed all is fine. But whenever there is no pressure in all of them at the same instant (from the road's frame) a robotic arm that emerge from the ditch will snatch the car.

I think this is the answer now ? --> The car's driver, Having known the arrangement of the road, but now happy about the contracted ditch's length, would try to cross the ditch casually. But alas, the contracted length is only a mirage, he is snatched away!
 

Similar threads

  • · Replies 6 ·
Replies
6
Views
2K
  • · Replies 23 ·
Replies
23
Views
2K
  • · Replies 24 ·
Replies
24
Views
6K
  • · Replies 47 ·
2
Replies
47
Views
5K
  • · Replies 98 ·
4
Replies
98
Views
9K
  • · Replies 29 ·
Replies
29
Views
6K
  • · Replies 26 ·
Replies
26
Views
20K
  • · Replies 25 ·
Replies
25
Views
2K
  • · Replies 20 ·
Replies
20
Views
4K
Replies
8
Views
3K