# Tag Info

Accepted

### Is it possible for a Ricci-flat manifold to have a negative energy measured by distant observers?

The positive energy theorem tells us that the ADM mass of an asymptotically flat spacetime is non-negative as long as the energy momentum tensor satisfies the dominant energy condition. In particular ...
• 9,357

### Is there anything truly "stationary" in the universe?

Your question is like asking whether there is a particular point that is the centre of the surface of the Earth. The answer is that the surface of a sphere has no unique centre, so it is meaningless ...
• 22.3k
Accepted

### Is a blinking pole moving through a barn at relativistic speeds striped?

The forward direction of the barn / pole thought experiment is that the pole is length contracted, as you state. Resolution of the reverse direction (the frame of reference of the pole) is a matter of ...
• 1,506

### Special Relativity and Hypersurface of Simultaneity?

Nothing happens. The hyperplane perpendicular to your worldline does not figure into the laws of nature in any way. It's an invention of human beings who have trouble thinking fourth-dimensionally. It'...
• 22k

### Special Relativity and Hypersurface of Simultaneity?

No combination of Lorentz boosts exists which reverses any part of the order of any process. Only unrelated events (for which distance $s \gt c\Delta t$ as measured in any frame) may have their ...
• 11.3k

### In this time dilation explanation, shouldn't gravity and mass affect the example?

Or does it somehow assume that gravity is non-existent? The derivation explicitly assumes that the reference frame is inertial. This, in turn, assumes that: the clock is in free fall tidal effects ...
• 81.8k

### Does proper time have two different values or meanings?

A better definition of proper time is the time measured by a clock. Then the clock hypothesis is an actual testable hypothesis. The formula you used has some limitations. It can be used for an ...
• 81.8k

### Is there anything truly "stationary" in the universe?

The laws of physics do not allow us to distinguish who in a pair is moving and who is stationary. As such, the question of whether or not there is anything in our world that is truly stationary is ...
• 82.6k
1 vote

If both twins were born at the big bang then the twin that is at rest in the CMB rest frame will be the oldest when they meet, while the twin which is moving relative to local comoving observers and ...
• 9,572
1 vote

### Does proper time have two different values or meanings?

I'm trying to intuitively understand proper time which is defined as the time that is always measured in a moving observer’s or particle's rest frame... It is right. It is the time measured by the ...
• 13.7k
1 vote

### Does proper time have two different values or meanings?

Both $\Delta t$ and $\Delta \tau$ are measurements of time interval during an event, or perhaps the time between two events. Proper time is a measure of an event that starts and stop at the same ...
1 vote

### In this time dilation explanation, shouldn't gravity and mass affect the example?

Gravitational time dilation is incredibly small. Even on Earth it's only fractions of a nanosecond per year. It was shown that the time dilation on earth compared to 10,000 km altitude is only one ...
• 4,013
1 vote

### Special Relativity and Hypersurface of Simultaneity?

Benrg's 'Nothing happens' is exactly right. One bit of advice I would like to share is that whenever you are contemplating the meaning of SR, bear in mind that effects such as time dilation, length ...
• 22.3k
1 vote

### Special Relativity and Hypersurface of Simultaneity?

If you consider the Universe to be everywhere, now, then yes: your definition of now changes. Details are known as the Rietdijk-Putnam Argument (aka: The Andromeda Paradox), of which David Mermin ...
• 28.2k
1 vote

### Special Relativity and Hypersurface of Simultaneity?

It depends on your stance on the philosophical question of Presentism versus Eternalism. 'Presentism' is the philosophical position that only the present exists, and the past and future don't. The ...
1 vote

### Principle of relativity and the train and ball example

According to the principle of (classical) relativity, the laws of mechanics are valid for all inertial frames of reference. The train is moving with constant velocity, so it is an inertial frame. From ...
• 13.7k
1 vote

### Principle of relativity and the train and ball example

As others have noted, this could be called "classical" or "Galilean" relativity, which is not what the Einstein theory is that most people associate with the word. Assuming this is ...
• 1,506
1 vote

### Is there anything truly "stationary" in the universe?

The word "stationary" in the modern (post-Newton) physics requires a reference frame: Object A is stationary relative to e.g. a non-rotating frame attached to the center of mass of the ...
• 6,528
1 vote

### Is there anything truly "stationary" in the universe?

As far as is understood, there is not absolute sense of "stationary" so the question you link is not even really well defined. All that could be asked is "do the galaxies move relative ...
• 1,506
1 vote

### Confusing definition of proper time – which is correct?

A 'propre' French word for own, propre time later polluted as proper time of an object is the time that is measured by a clock which is at rest w.r.t. the object, not only at rest but is must be ...

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