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

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Light's oscillation in time

Electromagnetic waves have electric (and magnetic) fields that oscillate spatially and with time. But light, moving at the universal speed limit, is a "space-like" object according to relativity since ...
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What are the equations of motion that model near light speed orbits of a massive body about incredibly massive bodies?

In Kip Thorne's recently published book, The Science of Interstellar, he describes, by means of an illustration, the complex nature of a spacecraft orbiting a massive black hole with velocities ~ ...
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Is there a theory that physically explains the origin equivalence principle / explains gravity via acceleration?

I've been thinking about how gravity could arise from a 4th dimensional spinning cylinder with space-time that has pliability like rubber (which I think is a generally accepted analogy). The ...
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65 views

Measuring time dilation

After watching a certain recent film, I started wondering whether it would be possible to create a watch (or, more likely due to the sizes of the components, a clock) capable of measuring one's time ...
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1answer
73 views

Relativistic Induced Magnetic Field in particle's frame

Suppose there is a constant magnetic field: $\vec{\mathbf{B}} = B \hat{z}$. A charged particle orbits that magnetic field perpendicular to the magnetic field, and induces a magnetic field in the ...
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69 views

How is force felt at relativistic speeds?

Hypothetically, should a force feel the same as its measured force at relativistic speeds? Obviously a human body would not survive the force of a rotation at say, 0.6 the speed of light, but it’s ...
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313 views

Relativity of simultaneity and the second law of thermodynamics? [closed]

Has the connection been noted between these two? I have been studying special relativity and in learning Minkowski diagrams I have noticed that as v approaches c, the relativity of simultaneity is so ...
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83 views

Is the cross section of a relativistic water hose or string always a perfect circle?

Given is a very long tube, such as a water hose or a tubular string with finite thickness, that has a constant circular cross section of radius $r$ along the length and that is at rest in an inertial ...
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56 views

Accelerated charge in relativistic frame

An accelerated charge radiates energy as electromagnetic waves. Imagine a point charge at rest in an inertial frame We are standing on a frame accelerating with respect to the charge. Now with respect ...
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42 views

How do we generalize limitations on the propagation speed of sinusoidal waves to limitations for transmitting any kind of information?

Our calculations on EM waves basically deal with harmonic waves and through such calculations and relations, we derive some relations and limitations for wave like phenomena. Of these relations are ...
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129 views

How to name different approaches to relativistic quantum theory

In the introductory chapter of the QFT book by Mark Srednicki the author notes that [p. 26] So now we have two different approaches to relativistic quantum theory [...] Which [one of those two] we ...
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194 views

How do I extend the Lorentz transformation metric to dimensions>4?

How do I extend the general Lorentz transformation matrix (not just a boost along an axis, but in directions where the dx1/dt, dx2/dt, dx3/dt, components are all not zero. For eg. as on the Wikipedia ...
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127 views

Does Gravity Warp Coulomb's force?

Does gravity effect the "sphericalness" of the Coulomb force? For example, is the field 1 meter above a charge weaker than the field 1 meter below the charge? Could a 0.014% difference in the field ...
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Wouldn't backwards time travel break the law of conservation of mass? [closed]

We know from the law of conservation of mass that the amount of mass in the universe is constant. Suppose there were a way for a person to travel backwards in time. Let's call this mass $m_t$ for some ...
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4answers
124 views

How should observers determine whether they can be described as being “defined on a Lorentzian manifold”?

Consider infinitely many distinguishable observers, no two of whom ever meet; and who generally "keep sight of each other", but not necessarily "each keeping sight of all others". How should they ...
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1answer
169 views

Are we all moving at $c$? [duplicate]

Is it true to say that all matter in the universe is travelling with velocity c through spacetime, but that for baryonic matter most of that velocity is through the time dimensions rather than the XYZ ...
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2answers
111 views

Particle accelerators reaching light speed [duplicate]

I just have a question in mind that's stuck in my head, and it's why particle accelerators cannot accelerate any particle to the speed of light. I'm assuming it involves Einstien's theory of ...
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2answers
553 views

Relativistic Energy

The question stated: By what percentage does your rest mass increase when you climb 30m to the top of a ten-story building? New to the concept of relativistic energy, I was a bit confused with ...
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527 views

Can a scalar field model gravity? How accurate would be the results? Are there any difficulties with such a model?

Newtonian gravity can be described by the equation: $$ \nabla^2 \phi = 4 \pi \rho G $$ where $\rho$ is the mass density, $\phi$ is the gravitational potential, and G is the universal gravitational ...
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67 views

What is the $ds^2$ notation in relativistic physics?

Could someone please explain me intuitively how $ds^2$ represents distance in relativistic physics?
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4answers
123 views

Is there anything we are aware of that can't be slowed by time dilation?

While thinking about the ambitions of this post: Can radioactivity be slowed through time dilation? I was asking myself, is there even anything that is dependent on time and can't be slowed through ...
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107 views

Precedence and quantum entanglement: The Alain Aspect experiment in spacetime

Recall that the spin components of a spin-entangled pair do not exist until one of the pair undergoes quantum observation, at which time both of the pair immediately obtain quantum random opposing ...
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1answer
168 views

Does spin-0 or spin-2 describe massive or massless particles?

spin-0 is massive or massless? How does we separate the massive and massless degrees of freedom for spin-2? What is the partially massive?
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2answers
152 views

How can a photon have energy when its mass is zero? [duplicate]

How can a photon have energy when its mass is zero? According to Einstein's equation $E = mc^2$ energy depends on $mass*c^2$ Light has zero mass so the energy would be zero too but solar cells use ...
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1answer
77 views

How much real and how much relative is our universe?

Apologies in advance if my question appears more philosophical than practical. It is indeed philosophy, but a philosophy relating to our scientific understanding of the universe as a whole. Special ...
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1answer
59 views

Derivation of superluminality in astronomy

Can somebody provide derivation of the relativistic explanation of (apparent) superluminality in astronomy? The derivation on wikipedia seems to be a bit confusing.
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3answers
290 views

Are some events simultaneous in all reference frames? (Einstein goes drinkin')

If simultaneity is not a universal characteristic (eg. events are not simultaneous in all reference frames), then why do some events seem to be simultaneous in all reference frames as in the following ...
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1answer
203 views

Time Dilation Question

I know physicists don't often like to hear about "photon's frame of reference", so I apologize in advance :) If the time dilation at the speed on light is infinite, it should mean that if Bob is ...
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52 views

Is light relative? [duplicate]

I was wondering, if two cars (A and B) are going at different velocities let's say A=90km/h, B=80km/h then if we add the two speeds we get 170km/h meaning they are relative to each other. But if one ...
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1answer
124 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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2answers
87 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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2answers
160 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. ...
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4answers
283 views

Which one is inertial frame?

I'd like to ask a question which made me confused. I think that, e.g., a man falling to earth could think himself as stationary and the Earth as accelerating towards him. But I, as an observer, I ...
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4answers
611 views

Why are we still teaching Newtonian physics if it's “false”? [duplicate]

Why are we teaching an outdated theory when the math in general relativity isn't that hard? I really don't see a reason why we should teach a highly oversimplified theory in our schools. For me, it's ...
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1answer
128 views

What does $\nu$ mean in relativity?

I decided to teach myself relativity over the Christmas holiday, and I've gotten a bit stuck. Coordinates in space time can be defined by a collection of coordinates, $$ x^0 = ct \\ x^1 = x \\ x^2 = ...
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677 views

How to prove the raising/lowering indices operation?

I've read this related question, though it didn't satisfy me; I hope this complements it. I know that if I contract a covariant tensor ${A_{\alpha\beta}}$ with a vector ${B^\beta}$, I get some other ...
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1answer
145 views

Does anything apart from gravity and relative velocity affect time?

I have been learning about time and the different dilation effects of gravity and relative velocity. I was wondering, are there any other factors that can affect time?
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134 views

Points in Spacetime

Assume there are two points in spacetime $a=(t,x,y,z)$ and $a'=(t',x',y',z')$. Let's say that the first one is in the origin of spacetime i.e. $a=(0,0,0,0)$. The point $a'$ has two possibilities ...
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672 views

What is the Lorentz tensor with a superscript and subscript index?

I have been reading about symmetries of systems' actions, e.g. the Polyakov action, and I have encountered Lorentz transformations of the form: $\Lambda^{\mu}_{\nu} X^{\nu}$. I am moderately familiar ...
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1answer
372 views

Time travel to future [duplicate]

Possible Duplicate: Is time travel possible? Is time travel possible? According to my friend, it is possible to go to the future but not the past. In Physics, particles move faster than ...
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1answer
68 views

Why can an object not reach the speed of light by falling in a gravitational field with constant acceleration?

I recently learned in school that the gravitational potential energy of an object is equal to $mgh$. I also learned that the kinetic energy of an object is $\frac12 mv^2$. If an object falls in a ...
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1answer
174 views

Why invariance is important?

The concept of invariance seems to have a great importance. Indeed, the fact that the laws of Electrodynamics are not invariant in every inertial reference frame led to the theory of Special ...
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4answers
78 views

Relativity: stationary vs moving

In the book The Elegant Universe, Greene describes a situation in which there are two space travelers, George and Gracie, moving in relationship towards each other at a constant velocity with no other ...
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195 views

Spacelike to timelike four vectors

First at all, let me just say that I'm not a Physicist, I study mathematics. So, I have this question. If you have a spacelike four vector, is there any transformation that could change it to be a ...
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1answer
81 views

What are the two dimensions of relativity that are added to string theory?

Based on the Ramanujam's modular functions, somehow these magic numbers 10 and 26 spacetime dimensions appear in string theory. The dimensions can be viewed as 8 + 2 and 24 + 2. The number 2 is added ...
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130 views

Time dilation if earth was stationary

Being on earth, we are constantly flying through space at incredible speeds. We revolve around a sun in a system that revolves around the center of the galaxy that is hurling itself through space. ...
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1answer
161 views

What happens if a body free-falls at a certain speed?

It is known that a body falling to the ground is affected by gravity, and its velocity increases by 9.8 m/s per second. But when this body is falling, and it reaches the speed of 340 m/s (the speed of ...
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1answer
297 views

Why do charge on an object remains unaffected by the motion of the object?

Unlike mass, the charge on an object is said to be unaffected by the motion of the object. This statement in my text book, is not yet understood by me. I don't know theory of relativity. On ...
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2answers
117 views

What is a geometrical object?

From the Wikipedia link for Geometry: Geometry (Ancient Greek: γεωμετρία; geo- "earth", -metron "measurement") is a branch of mathematics concerned with questions of shape, size, relative position ...
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884 views

Does an accelerating proton also lose mass?

A proton accelerated with electric field gives off E.M. radiation and therefore should lose mass. Larmor's formula gives us a value for the power emitted (varies as acceleration squared). However, as ...