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5

When you are given acceleration, density, area and time, you can indeed find an expression for mass in terms of these. Here is how you go about it: Make a table of the units that occur in each, and their exponents: L M T a 1 -2 D -3 1 A 2 t 1 As you can see, you need to use D (density) as the only one that contains mass. But ...


4

The geometry of the expanding universe is (approximately) described by the FLRW metric, and to express this there is a natural space/time split called comoving coordinates. The comoving time coordinate is roughly speaking the time measured by an observer who is at rest with respect to the cosmic microwave background. Since most galaxies have peculiar ...


4

A photon traveling at speed of light has a lightlike worldline. It has one place of emission and one place of absorption. The spacetime interval between both points is empty (=0), that means that no spacetime is between them. That means, if a photon would experience something, it would experience both points as simultaneous. But there is no reference frame ...


4

It should be clarified that the Higgs boson does not carry mass. The correct statement is that the Higgs field (not boson) is giving mass to some (not all) particles. In fact most of your mass is not given by the Higgs field. Most of the mass of atomic nucleus (protons and neutrons) is due to the binding energy of strong interaction. The Higgs field is ...


3

Let $\mathcal{H}$ be the space of states of our theory. Then, time evolution is given by a unitary operator $U(t_2,t_1) : \mathcal{H}\to\mathcal{H}$ that evolves "stuff" from time $t_1$ to time $t_2$. For time-independent Hamiltonians it is just $\mathrm{e}^{\mathrm{i}H(t_2 - t_1)}$. If we are in the Schrödinger picture, we say states "carry the time ...


3

Due to the quantized energy levels of allowed electron orbitals, single atoms can easily absorb energy around certain narrow wavelengths. A cesium atom has one of these narrow absorption bands at a frequency of 9,192,631,770 Hz. A cesium clock can produce EM radiation in this region (microwaves) and detect how well the cesium atoms are absorbing it. So ...


2

It's quite common to use time to parameterise an equation. For example suppose you have a particle moving in a circle (or radius $r$). One way of describing its motion would be to say the trajectory describing its motion is: $$ x^2 + y^2 = r^2 \tag{1} $$ but an alternative description would be to use the pair of equations: $$\begin{align} x &= ...


2

If you are abit more careful about making the statements, then "both perspectives" are actually correct. Let me be more concrete and explain. Let $E$ be the one who stays on Earth and $S$ be the one who is on the spaceship. The first issue you have is you said "let's say seven years passes on Earth" - this is an ambiguous statement: from whose perspective? ...


2

To address the question of whether the age estimates are in years on Earth or years for a comoving observer in deep space, I could tell you the same thing John Rennie said. But that would make this redundant, unproductive, and redundant. Instead, let me show you that it doesn't really matter. The equation for time dilation due to gravity is as follows: ...


1

You may be speaking of the subjective experience of time, which is not a physics question, but rather a psychological one. Otherwise, the last paragraph of your question doesn't seem to make any physical sense. Time is an arbitrary standard by which a motion or a process is measured relative to other motions and processes. For example, you can synchronize ...


1

Quantum mechanics has an absolute time concept, and general relativity has the dynamic time concept of spacetime. The incompatibility of both concepts is called the problem of time in quantum gravity. Yes, but that Wikipedia article needs attention from an expert. It says "therefore, we arrive at the conclusion that 'nothing moves' ('there is no time') ...


1

If you don't want to violate causality then you can only have a few types of faster than light travel. One is to not allow it to slow down. Another is to only allow it to go in one direction and hope the universe is infinite. Otherwise it is rather trivial to use regular slower than light motion between some FTL trips to generate a time machine. As for ...


1

regarding the first half: Density is Mass/Volume (not the other way around), so you need a Volume.. You could achieve that by a sufficient power of the area or find yourself a length-scale from acceleration and time regarding the second half: that's the whole point of dimensional analysis, a mass-like quantity is a mass-like quantity and must come from ...


1

Entanglement experiment performed in one frame of reference guarantees that the two measured results are synchronized in this frame of reference. If we try to perform the same entangled experiment while the two ends of the same length fiber optic lines are attached to two frames of references moving with a constant relative velocity, the two measured results ...



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