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2 votes
Accepted

How to show only direction is changed in collision between unequal atoms?

It comes from including conservation of energy. Let's work in the centre of momentum frame where we have: $$ m_1 v_1 + m_2 v_2 = 0 $$ so: $$ v_2 = -\frac{m_1}{m_2} v_1 $$ Then the initial total ...
John Rennie's user avatar
0 votes

What is the meaning of universal speed limit?

Consider two objects with mass: A baseball and a train. A train has more mass than the baseball. So it is harder to move the train than the baseball. It requires more power to move the train. When an ...
qa test's user avatar
  • 29
1 vote

What is the meaning of universal speed limit?

Causality ensures that cause precedes effect, so any event can't influence others outside its future light cone. The speed of light is the universal speed limit, as dictated by special relativity, ...
Zuir Hakiz's user avatar
1 vote

What is the meaning of universal speed limit?

According to the postulate of special relativity (and to the many confirming experiments) light speed $c$ does not depend on the relative speed of a moving source or the detector. Hence, no matter how ...
Agnius Vasiliauskas's user avatar
6 votes

What is the meaning of universal speed limit?

Does exceeding the speed of light mean exceeding the speed of light relative to all other objects in the Universe? I think a better statement is exceeding the speed of light means exceeding the speed ...
KDP's user avatar
  • 10.1k
11 votes

What is the meaning of universal speed limit?

Fun fact: Suppose that astronaut Allen has a laser mounted to his spacecraft that sends pulses of light off toward infinity. Allen performs an experiment proving that the pulses travel at speed, C. ...
Solomon Slow's user avatar
  • 16.2k
15 votes

What is the meaning of universal speed limit?

It means the following: suppose the object has a good clock and a light that can flash in all directions (and for simplicity assume everything is transparent to this light). At $t=t_0$ on the object’s ...
Dale's user avatar
  • 109k
0 votes

Is conservation of momentum and energy valid for non-inertial frames?

Balance of momentum and energy (and also angular momentum) is valid in any frame and any coordinate system. It's important here to make a distinction between a balance law and a conservation law. For ...
pglpm's user avatar
  • 3,833
1 vote

Is Schwarzschild spacetime in Eddington-Finklestein coordinates flat at Schwarzschild radius?

weeab00 wrote: "Is the spacetime really flat to an observer hovering around Schwarzschild radius?" Locally the spacetime is flat at every r except r=0, but the local patch becomes smaller ...
Yukterez's user avatar
  • 13.1k
3 votes

Is Schwarzschild spacetime in Eddington-Finklestein coordinates flat at Schwarzschild radius?

Choice of coordinates does not change the nature of the spacetime around a black hole. Spacetime always looks flat locally. How locally such an approximation will work depends on the second ...
ProfRob's user avatar
  • 137k
2 votes

How do different observers decide if they are looking at the same thing?

I think a way to visualize this is if you imagine that the ball suddenly popped into existence at time $t$ and position $x$ for observer $A$. Then observer $A'$ would see the ball pop into existence ...
Tob Ernack's user avatar
6 votes

How do different observers decide if they are looking at the same thing?

I see a leaf fall from a tree. You see a leaf fall from a tree. We want to know whether we both saw the same leaf fall from the same tree. Here's how we can tell: I use my meter sticks and clocks ...
WillO's user avatar
  • 17.1k
10 votes
Accepted

How do different observers decide if they are looking at the same thing?

Many presentations of relativity start with this business of reference frames and observers. Those concepts are useful, but before invoking them, first get thoroughly fixed in your mind that there is ...
Andrew Steane's user avatar
3 votes

How do different observers decide if they are looking at the same thing?

Coordinates, like $x$ or $x’$ are not physical quantities. They are convenient mathematical labels that we define. Specifically they are labels that label real physical events with quadruples of ...
Dale's user avatar
  • 109k
6 votes

How do different observers decide if they are looking at the same thing?

What you ask is not so much a special relativity thing as it is a metrology thing. How do you set up an experiment such that you can measure what you seek to measure. If you are trying to confirm ...
Cort Ammon's user avatar
  • 51.7k
9 votes

How do different observers decide if they are looking at the same thing?

You are over-thinking it. An event is a position in spacetime, so in any given frame it has three spatial coordinates and a time coordinate. Consider the event of me typing the letter Q in this ...
Professor Sushing's user avatar
0 votes

Order of events is invariant in timelike intervals

You have set this problem up in the most confusing way possible. It's best to address these things systematically, with actual events using coordinates, not just ambiguous sentences. Now we have 3 ...
JEB's user avatar
  • 39.6k
1 vote
Accepted

Order of events is invariant in timelike intervals

Eric's comment is correct. If two spatially separated events are simultaneous in some frame, then they must be space like separated, so their order can be reversed in other frames. If they were time ...
Professor Sushing's user avatar

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