All Questions
2,360 questions
107
votes
10
answers
19k
views
What is time, does it flow, and if so what defines its direction?
This is an attempt to gather together the various questions about time that have been asked on this site and provide a single set of hopefully authoritative answers. Specifically we attempt to address ...
95
votes
9
answers
19k
views
Why is there a controversy on whether mass increases with speed?
Some people say that mass increases with speed while others say that the mass of an object is independent of its speed.
I understand how some (though not many) things in physics are a matter of ...
- 1,296
121
votes
16
answers
19k
views
Is it possible for information to be transmitted faster than light by using a rigid pole?
Is it possible for information (like 1 and 0s) to be transmitted faster than light?
For instance, take a rigid pole of several AU in length. Now say you have a person on each end, and one of them ...
- 6,987
85
votes
6
answers
17k
views
If a mass moves close to the speed of light, does it turn into a black hole?
I'm a big fan of the podcast Astronomy Cast and a while back I was listening to a Q&A episode they did. A listener sent in a question that I found fascinating and have been wondering about ever ...
- 977
113
votes
12
answers
16k
views
How can time dilation be symmetric?
Suppose we have two twins travelling away from each other, each twin moving at some speed $v$:
Twin $A$ observes twin $B$’s time to be dilated so his clock runs faster than twin $B$’s clock. But twin ...
212
votes
11
answers
273k
views
If photons have no mass, how can they have momentum?
As an explanation of why a large gravitational field (such as a black hole) can bend light, I have heard that light has momentum. This is given as a solution to the problem of only massive objects ...
- 2,814
164
votes
7
answers
33k
views
A list of inconveniences between quantum mechanics and (general) relativity?
It is well known that quantum mechanics and (general) relativity do not fit well. I am wondering whether it is possible to make a list of contradictions or problems between them?
E.g. relativity ...
- 7,071
82
votes
12
answers
15k
views
What is time dilation really?
Please will someone explain what time dilation really is and how it occurs? There are lots of questions and answers going into how to calculate time dilation, but none that give an intuitive feel for ...
64
votes
7
answers
13k
views
If I run along the aisle of a bus traveling at (almost) the speed of light, can I travel faster than the speed of light?
Let's say I fire a bus through space at (almost) the speed of light in vacuum. If I'm inside the bus (sitting on the back seat) and I run up the aisle of the bus toward the front, does that mean I'm ...
- 775
112
votes
17
answers
20k
views
What is the proper way to explain the twin paradox?
The paradox in the twin paradox is that the situation appears symmetrical so each twin should think the other has aged less, which is of course impossible.
There are a thousand explanations out there ...
68
votes
10
answers
173k
views
Would time freeze if you could travel at the speed of light?
I read with interest about Einstein's Theory of Relativity and his proposition about the speed of light being the universal speed limit.
So, if I were to travel in a spacecraft at (practically) the ...
46
votes
7
answers
10k
views
Does a photon in vacuum have a rest frame?
Quite a few of the questions given on this site mention a photon in vacuum having a rest frame such as it having a zero mass in its rest frame. I find this contradictory since photons must travel at ...
- 3,148
60
votes
13
answers
42k
views
Why and how is the speed of light in vacuum constant, i.e., independent of reference frame?
I was told that the Galilean relative velocity rule does not apply to the speed of light. No matter how fast two objects are moving, the speed of light will remain same for both of them.
How and why ...
- 5,427
31
votes
6
answers
8k
views
(Almost) double light speed
Let's say we have $2$ particles facing each other and each traveling (almost) at speed of light.
Let's say I'm sitting on #$1$ particle so in my point of view #$2$ particle's speed is (almost) $c+c=...
- 537
35
votes
4
answers
9k
views
How does a photon experience space and time?
To an an external observer it appears that time has stopped for photon. But this relation is reflexive, so for an observer travelling with the photon it appears the universe has stopped everywhere.
...
- 13.4k
33
votes
3
answers
108k
views
How is light affected by gravity?
Light is clearly affected by gravity, just think about a black hole, but light supposedly has no mass and gravity only affects objects with mass.
On the other hand, if light does have mass then doesn'...
- 441
205
votes
7
answers
247k
views
How do moving charges produce magnetic fields?
I'm tutoring high school students. I've always taught them that:
A charged particle moving without acceleration produces an electric as well as a magnetic field.
It produces an electric field ...
- 7,463
55
votes
2
answers
25k
views
How long would it take me to travel to a distant star?
Suppose I wanted to travel to one of the recently discovered potentially Earth-like planets such as Kepler 186f that is 490 light years away. Assuming I had a powerful rocket and enough fuel, how long ...
- 363k
52
votes
8
answers
15k
views
How is the classical twin paradox resolved?
I read a lot about the classical twin paradox recently. What confuses me is that some authors claim that it can be resolved within SRT, others say that you need GRT. Now, what is true (and why)?
- 3,771
83
votes
8
answers
19k
views
Why is there no absolute maximum temperature?
If temperature makes particles vibrate faster, and movement is limited by the speed of light, then I would assume that temperature must be limited as well.
Why is there no limit?
- 1,465
37
votes
5
answers
9k
views
What is so special about speed of light in vacuum?
I will try to be as explanatory as possible with my question. Please also note that I have done my share of googling and I am looking for simple language preferable with some example so that I can get ...
- 673
38
votes
8
answers
5k
views
Interval preserving transformations are linear in special relativity
In almost all proofs I've seen of the Lorentz transformations one starts on the assumption that the required transformations are linear. I'm wondering if there is a way to prove the linearity:
Prove ...
- 3,802
37
votes
16
answers
10k
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 "...
- 3,453
33
votes
6
answers
9k
views
Is time travel possible? Is it possible to go back in time?
I read somewhere that according to relativity, it is possible - involving black holes and other stuff - to jump into the past. Is it possible for anything to go back in time either continuously or by ...
- 5,427
28
votes
1
answer
3k
views
The choice of measurement basis on one half of an entangled state affects the other half. Can this be used to communicate faster than light?
It is often stated, particularly in popular physics articles and videos about quantum entanglement, that if one measures a particle A that is entangled with some other particle B, then this ...
- 135k
47
votes
8
answers
12k
views
If I am travelling on a car at around 60 km/h, and I shine a light, does that mean that the light is travelling faster than the speed of light?
The title says it all.
If I was on a bus at 60 km/h, and I started walking on the bus at a steady pace of 5 km/h, then I'd technically be moving at 65 km/h, right?
So my son posed me an interesting ...
- 1,435
37
votes
8
answers
11k
views
What determines which frames are inertial frames?
I understand that you can (in principle) measure whether "free particles" (no forces) experience accelerations in order to tell whether a frame is inertial. But fundamentally, what determines which ...
Joss L
54
votes
9
answers
48k
views
How can a photon have no mass and still travel at the speed of light?
I've read a number of the helpful Q&As on photons that mention the mass/mass-less issue. Do I understand correctly that the idea of mass-less (a rest mass of 0) may be just a convention to make ...
- 565
36
votes
14
answers
135k
views
Why does the (relativistic) mass of an object increase when its speed approaches that of light?
I'm reading Nano: The Essentials by T. Pradeep and I came upon this statement in the section explaining the basics of scanning electron microscopy.
However, the equation breaks down when the ...
- 1,483
22
votes
2
answers
8k
views
How do I derive the Lorentz contraction from the invariant interval?
While reviewing some basic special relativity, I stumbled upon this problem:
From the definition of the proper time:
$$c^2d\tau^2=c^2dt^2-dx^2$$
I was able to derive the time dilation formula by using ...
- 16.5k
21
votes
2
answers
3k
views
Vector spaces for the irreducible representations of the Lorentz Group
EDIT: The vector space for the $(\frac{1}{2},0)$ Representation is $\mathbb{C}^2$ as mentioned by Qmechanic in the comments to his answer below! The vector spaces for the other representations remain ...
- 1,892
16
votes
9
answers
12k
views
Can One-Way Speed of Light be Instantaneous?
I recently watched this video by Veritasium where he talks about the One Way Speed of Light and talks about the limiting case where in one direction the speed of light is $c/2$ while it's ...
- 807
27
votes
7
answers
22k
views
Why are objects at rest in motion through spacetime at the speed of light? [closed]
I read that an object at rest has such a stupendous amount of energy, $E=mc^2$ because it's effectively in motion through space-time at the speed of light and it's traveling through the time dimension ...
- 4,637
26
votes
9
answers
32k
views
Accelerating particles to speeds infinitesimally close to the speed of light?
I'm in a freshmen level physics class now, so I don't know much, but something I heard today intrigued me. My TA was talking about how at the research facility he worked at, they were able to ...
- 1,158
73
votes
3
answers
10k
views
Idea of Covering Group
$SU(2)$ is the covering group of $SO(3)$. What does it mean and does it have a physical consequence?
I heard that this fact is related to the description of bosons and fermions. But how does it ...
- 27.2k
127
votes
13
answers
24k
views
Can Maxwell's equations be derived from Coulomb's Law and Special Relativity?
As an exercise I sat down and derived the magnetic field produced by moving charges for a few contrived situations. I started out with Coulomb's Law and Special Relativity. For example, I derived the ...
- 7,488
159
votes
7
answers
15k
views
Is temperature a Lorentz invariant in relativity?
If an observer starts moving at relativistic speeds will he observe the temperature of objects to change as compared to their rest temperatures?
Suppose the rest temperature measured is $T$ and the ...
- 2,803
66
votes
6
answers
9k
views
What keeps mass from turning into energy?
I understand the energy and mass can change back and forth according to Einstein. It is fluid; it can go from one to the other. So, what keeps mass from just turning into energy? Is there some force ...
- 945
15
votes
6
answers
5k
views
Rotate a long bar in space and get close to (or even beyond) the speed of light $c$
Imagine a bar
spinning like a helicopter propeller,
At $\omega$ rad/s because the extremes of the bar goes at speed
$$V = \omega * r$$
then we can reach near $c$ (speed of light)
applying some ...
Hernan Eche
344
votes
34
answers
58k
views
Do we know why there is a speed limit in our universe?
This question is about why we have a universal speed limit (the speed of light in vacuum). Is there a more fundamental law that tells us why this is?
I'm not asking why the speed limit is equal to $c$ ...
- 7,860
42
votes
3
answers
16k
views
Is it really possible to break the speed of light by flicking your wrist with a laser pointer?
Minutephysics has a popular YouTube video called "How to break the speed of light". In the video it states that if you flick your wrist while pointing a laser that reaches the moon, that the spot of ...
- 551
5
votes
2
answers
3k
views
What is the relativistic action of a massive particle?
all Lorentz observers watching a particle move will compute the same value for the quantity
$$ds^2 = -(c \, dt)^2 + dx^2 + dy^2 + dz^2,$$
$$ds^2 = g_{\mu\nu}dx^{\mu}dx^{\nu},$$
and ''ds/c'' is then ...
- 1,025
12
votes
20
answers
5k
views
The origin of the value of speed of light in vacuum
Meaning, why is it the exact number that it is? Why not $2\times10^8$ m/s instead of $3$? Does it have something to do with the mass, size or behavior of a photon?
To be clear, I'm not asking how we ...
Tom Info
12
votes
13
answers
9k
views
understanding time: Is time simply the rate change?
Is time simply the rate of change?
If this is the case and time was created during the big bang would it be the case that the closer you get to the start of the big bang the "slower" things change ...
- 334
0
votes
2
answers
442
views
Decayed object energy into kinetic energy
An object of mass M has half life of T, if the energy released by the decayed half(mc^2) converts into kinetic energy of the rest of object what would the speed be over time? I tried and found ...
- 199
38
votes
3
answers
5k
views
Symmetrical twin paradox in a closed universe
Take the following gedankenexperiment in which two astronauts meet each other again and again in a perfectly symmetrical setting - a hyperspherical (3-manifold) universe in which the 3 dimensions are ...
- 3,771
27
votes
1
answer
17k
views
Lorentz Invariant Integration Measure [closed]
When we canonically quantize the scalar field in QFT, we use a Lorentz invariant integration measure given by
$$\widetilde{dk} \equiv \frac{d^3k}{(2\pi)^3 2\omega(\textbf{k})}.$$
How can I show that ...
- 3,043
24
votes
5
answers
65k
views
Why don't electromagnetic waves require a medium?
As I understand it, electromagnetic waves have two components which are the result of each other, i.e., when a moving electric charge creates a changing magnetic field at point X then a changing ...
- 493
10
votes
3
answers
4k
views
Extended Rigid Bodies in Special Relativity
I was reading Landau & Lifshitz's Classical Theory of Fields and I noticed that they mention that an extended rigid body isn't "relativistically correct".
For example, if you consider a rigid ...
- 3,747
22
votes
9
answers
11k
views
Minkowski Metric Signature
When I learned about the Minkowski Space and it's coordinates, it was explained such that the metric turns out to be $$ ds^{2} = -(c^{2}dx^{0})^{2} +(dx^{1})^{2} + (dx^{2})^{2} + (dx^{3})^{2} $$ ...
- 1,049