The union of special (SR) and general (GR) relativity. Use this tag if both SR and GR apply.

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3answers
8k views

How can time be relative?

I don't understand how time can be relative to different observers, and I think my confusion is around how I understand what time is. I have always been told (and thought) that time is basically a ...
-5
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1answer
41 views

How does relativity work? [on hold]

How does Einstein's theory of relativity lead to objects at high speed or low mass moving faster through time relative to objects at high mass or slow speed?
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0answers
31 views

Point View and the relativity of everything, also regarding seemingly neutral pen and paper formulas - also, the nonexistence of black holes [on hold]

I am just a programmer with a love for math/physics and ... science fiction, haha! Anyway, to state it right in the beginning: This question is related to the following postulation (of mine, but ...
5
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1answer
557 views

Doesn't dating the universe violate the concept of spacetime's inseparability?

It would seem that measuring an age of the universe from the big bang requires separating spacetime into a 3D coordinate system and a time track. I fail to understand why it is appropriate to take ...
0
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1answer
91 views

Is the intuition of “absolute simultaneity” necessarily wrong?

I read about Einstein’s theories of relativity with all the interesting stuff like time dilation, mass increase and whatnot. And also about the relativity or conventionality of simultaneity. The ...
0
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3answers
96 views

What is the pace of absolute time

How fast or slow compared to my alarm clock time does time passes in a place where there is no gravitational pull and where you are not moving. I don't know if my question is clear, but is there some ...
2
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1answer
118 views

Perfect fluid and Cauchy momentum equation

The stress-energy tensor of a perfect fluid is given by $$T^{\mu\nu}=\left(\rho+pc^{-2}\right)u^\mu u^\nu+pg^{\mu\nu}$$ The divergence of the stress-energy tensor is zero: $\nabla_\mu T^{\mu\nu}=0$. ...
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0answers
31 views

Is the Universe a 4D elastic shell that is contracting with time, where particles are traveling waves or standing waves of energy density? [closed]

In gravitational red shift, “the phenomenon known as the red shift of a photon is really the blue shift of an atom.” This conclusion also applies to Galactic redshift. According to Hubble’s law, when ...
1
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2answers
578 views

If time stops for an object, does that object feel gravity?

As far as I understand The GTR, it is said that Mass bends space-time which causes gravity. So every Mass in this universe is flowing through space-time example earth is moving along with space-time ...
14
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5answers
1k views

Can we find actual rest mass of things on Earth

Earth moves around the Sun and the Sun moves around the galaxy and the galaxy moves with unknown speed and direction. We have speed so the mass of us all altered. Can we know the real rest mass? If ...
2
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3answers
113 views

Is it possible to express various nonlinear motions as straight lines in transformed spacetime?

I am trying to understand simple examples of space-time curvature. Assume for the moment that $c$ is infinite (classical curvature due to Newton's laws). Also, I will only consider 1+1-dimensional ...
3
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1answer
91 views

Perpetual Mobile and Gravitation

I have fundamental question about what is called the “law of conservation of energy”. We all hear about the tidal power stations which using the tidal power. The source of the tidal power came from ...
1
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1answer
86 views

Since the Earth is moving through space, do we experience time dilation?

What I mean is, the Earth orbits around the sun at around 30km/s, while I understand that this isn't a very high velocity does time dilation still occur to say a person who is completely stationary in ...
0
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0answers
44 views

Which is the corrispondent of the Lorentz's transformation in general relativity?

The Lorentz's transformations tell us how space and time change in a flat case? There are a more general and powerfull transformation for general relativity?
8
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3answers
736 views

Special Relativistic approximation to GR

Some time ago I was talking to a professor in college about some of the fundamental aspects and origin of General Relativity. I was surprised to learn, in fact, that a pretty good approximation to GR ...
0
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3answers
85 views

How can you explain objects of unequal masses falling at the same rate using GR?

Isn't gravity caused by the curvature of space-time, and the stronger it's curved the stronger the attraction? it makes more sense to me that if a heavier object is falling on earth it would fall ...
0
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2answers
45 views

Did at least one photon go straight and never interact, and is that the one true clock?

This is a late night thought, and I apologize if it's something many have already thought of. Did at least one photon go straight out from the origin of the big bang and continue at C until now? If ...
1
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2answers
95 views

Question about basic formalism of GR and the metric tensor

I really don't know much about GR, but I've come across a few rough sketches of its formalism in my DG books. I'm trying to piece it together to get a very basic intuition of what spacetime is in GR. ...
1
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1answer
46 views

Proper acceleration of a stationary object

What is the proper acceleration measured for a stationary object? is it 9.8 or 0? Thank you.
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0answers
26 views

what are the relativistic/cosmological implications on a long rigid body? [duplicate]

This is my first question. Hope it's in respect of quality standards. Let's suppose to have a "long" rigid almost unbreakable bar of fixed negligible diameter . (we can imagine it as a rigid wire) ...
0
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2answers
63 views

Meaning of Proper time

Sorry for a bit of a basic question, but want to clarify things in my head. Is proper time quantified by the amount of physical process that an object, or physical system undergoes, for example the ...
1
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0answers
33 views

Does quantum mechanics have any insight into why and how photons get adsorbed into atoms? [duplicate]

Relativity holds that energy and mass are equivalent via the equation $E=mc^2$ also I know in quantum mechanics we talk about electrons jumping into bigger orbitals if they gain just the right amount ...
0
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0answers
28 views

Does a fixed amount of gas have slighty more mass when at a higher temperature?

According to relativity, energy and mass are equivalent. Does this mean that the energy added to a fixed amount of gas when it is heated adds slightly to the mass of the gas? The difference would be ...
3
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2answers
138 views

Relativity conclusions from Michelson-Morley Experiment

This is what my textbook says about the Michelson-Morley experiment: "This invariance of the speed of light between inertial reference frames means that there must be some relativity principle that ...
1
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1answer
49 views

Is there a name for the linear quantity corresponding to the (quadratic) “interval $\Delta s^2$”?

Recently it has been affirmed here (again) that the quantity called "interval (also 'spacetime interval' or 'invariant interval')" is referring to two (in general distinct) events as arguments, such ...
1
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1answer
30 views

Relative Movements Agreement on Object speed

I am confused about Relative Motion.Lets suppose we have one train and two observer.One of them inside the train and the other one is outside.Lets think the outside observer see the train moves a ...
3
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1answer
367 views

Is time the property of an object?

I don't know if the title makes much sense, but hopefully it will become clear with the text. Temperature is not a property of a point in the three dimensions, but actually of the object occupying ...
0
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1answer
43 views

Influence of spacetime curvature on electromagnetic wave propagation [duplicate]

Classical physics assumes that spacetime is evenly distributed in the sense that Coulomb's Law predicts that a charged particle will create a spherically symmetric electric field around its location. ...
7
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9answers
1k views

Did relativity make Newtonian mechanics obsolete?

Did Einstein completely prove Newton wrong? If so, why we apply Newtonian mechanics even today? Because Newton said that time is absolute and Einstein suggested it relative? So, if fundamentals are ...
1
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1answer
145 views

Is there any relationship between the $E=mc^2$ equation and the $a_n=\kappa v^2$ formula for the normal component to acceleration?

To clarify, I know very little about physics and don't pretend to have any insight whatsoever into relativity beyond what has entered the popular imagination; my knowledge is more or less at the level ...
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3answers
100 views

Negative energy of free particle: classical and quantum picture

Classically, the energy of a free particle consists of only the kinetic energy given by $E=\frac{|\textbf{p}|^2}{2m}$ Since $|\textbf{p}| $is real and $m>0$, $E\geq 0$. However, since ...
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2answers
73 views

Lorentz Transformations and time of event

Consider two inertial frames, $F$ and $F'$, such that $F'$ moves at $\mathbf{v} = (v,0,0)$ with respect to $F$ (assume $v > 0$). Suppose tat $x = x\prime = 0$, when $t = t' = 0$, where $x,t$ refer ...
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0answers
58 views

Is it possible for someone traveling at the speed of light to see at all? [closed]

I know that the speed of light is the same for all observers regardless of your speed, But this question is really worth thinking about. If someone let's say managed to reach the speed of light, ...
8
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2answers
563 views

Momentum and spacetime

My apologies up-front for the naive question and my rudimentary understanding, but I should be delighted if someone would enlighten me :) The question has to do with the application of the ...
3
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0answers
20 views

Is gravitational attraction increased by relativistic mass as well as rest mass? [duplicate]

Is gravitational attraction increased by relativistic mass as well as rest mass? If so what would happen if an observer travelled at relativistic speeds? Would it appear to him that the whole ...
3
votes
2answers
68 views

Is spacetime absolute?

As I understand it Newton's Laws imply that space is relative, as the laws of physics are the same in all inertial frames and as such there is no way, even in principle, to distinguish a frame that is ...
2
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2answers
235 views

Finding wave-fuctions of a Dirac particle for given 4-momentum and spin 4-vector

I've been reading through various materials on relativistic quantum mechanics, but I find the lack of simple examples disturbing. I'm acquainted with the general form the solutions to the Dirac ...
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0answers
43 views

Relativity - Diffraction Of Light

Following thought experiment. Inside a rocket, right in the middle, we place two lasers pointing in opposite directions. Both emitting the same wavelength. The Left wall of the rocket is L1 while ...
1
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3answers
92 views

Time dilation, reference frame

If we have two objects (clocks) in (empty) space and they separate (the distance between them increases) at relativistic speeds then get back together (just for easier comparison of their times), what ...
0
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3answers
76 views

Simultaneity and quantum indeterminism

How can Relativity talk about simultaneity planes when the events are actually quantum in nature so they are indetermined ? How can you say that if you walk on the street with 1m/s you are ...
24
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5answers
3k views

Will 1 gram of matter moving at relativistic speeds completely annihilate a larger quantity of stationary antimatter?

This is a question about the relativistic mass concept which I am having trouble understanding, mainly because of the scenario below. Simple scenario: Suppose 1 gram of matter is accelerated to 99% ...
1
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1answer
46 views

Relative motion at almost the speed of light

My title might not be precise but I didn't know how to decribe it better. I've been reading about the special and general theories of relativity for a short while and I started wondering. First, ...
1
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1answer
54 views

Derivation of the energy-momentum tensor for an imperfect fluid

In chapter 7 of the "Physical Foundations of Cosmology" Mukhanov uses this energy-momentum tensor for an imperfect fluid: $T^\mu_\nu = (\rho + p)u^\mu u_\nu - p\delta^\mu_\nu - \eta(P^\mu_\gamma ...
2
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3answers
88 views

Is simultaneity testable?

I was reading about Andromeda paradox, and I started wondering. How can we know that the situation in the Andromeda paradox is real ? How can you know that simultaneity is real ? How can you say that ...
2
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2answers
125 views

What would a nuclear explosion look like at relativistic speeds where time dilation and length contraction become significant?

Suppose a rocket carrying a nuclear weapon is moving past our planet at very near the speed of light while a stationary observer watches from here on Earth. As the rocket passes directly overhead, an ...
3
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3answers
80 views

space contraction near lightspeed

I read a lot of the Time-Life Science books when I was a kid (well, mostly the pictures and captions). One of the things that always stuck in my mind was an explanation of the twin paradox, using ...
2
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3answers
94 views

Which is more fundamental,''constancy of speed of light'' or ''space-time''? [closed]

My friend asked me,''which is more basic or fundamental? constancy of speed of light which makes space-time behave dynamically OR dynamical behavior of space-time that makes light to travel with ...
0
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1answer
63 views

General Relativity - Four Velocity Derivative Question

I am trying to get my head around a small point used in a book I am reading about General Relativity. The book states that because $u_au^a = c^2$ it follows that $u_a \nabla_b u^a = 0 $ The first ...
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0answers
23 views

Does relativity in any way “disprove” Newton? [duplicate]

I was reading a science magazine and it claimed that Einstein's theory of relativity disproves all of Newtonian physics. I was highly skeptical because all we have been learning at school is Newtonian ...
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7answers
500 views

“Reality” of length contraction in SR

I was in argument with someone who claims that length contraction is not "real" but only "apparent", that the measurement of a solid rod in its rest reference frame is the "real length" of the rod and ...