The speed of light is a fundamental universal constant that marks the maximum speed at which energy and information can propagate. Its value is $299792458\frac{\mathrm{m}}{\mathrm{s}}$.

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Does the speed of light change? [duplicate]

I know that there is a similar questions, but I think mine is a bit different. I wonder if with the expansion of the universe the speed of light changes. It seems that the speed of light is very ...
6
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1answer
366 views

How is it that distortions in space can be measured as distances? [duplicate]

Imagine yourself and anything you're able to perceive to be drawn onto a rubber band. Any means of determining distances would be affected by the stretching/contracting of the rubber band. The ...
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1answer
45 views

Special Relativity and the Light Clock and which direction was the laser pointing? [closed]

If I were to make a light clock, I would have some sort of laser pen attached to the top of one of two mirrors. That laser pen would be perpendicular to the surface of either/both mirrors. I would ...
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2answers
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Why is the speed of light exactly $299,792.458$ m/s and not faster or slower? [duplicate]

Why is the speed of light $299,792,458$ metres per second, and not faster or slower. Why not $500$ trillion kilometers per second or $120$ miles per hour? This has been 'bothering' me for a while. ...
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2answers
144 views

A classical explanation of why there is a speed limit in the universe [closed]

The question is - is there any flaw in this explanation, or it is the right explanation in general language? Let me break the explanation into two parts - Why there is a certain speed of ...
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1answer
55 views

When travelling in the direction of light, what is my relative velocity?

If suppose I am traveling in the same direction as that of light, the relative velocity I am traveling with is speed of light + my speed in the direction of light. Is this correct? Does this have ...
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0answers
45 views

Explanation about the speed of light according to Maxwell from the book : “ A brief History of Time”

I just started reading Hawking's book: " A brief history of time" and there is a section that I don't understand referring to the speed of light moving through the "ether". In the second chapter, ...
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1answer
59 views

Cold Solid Matter at the End of a Black Hole's Acceleration Disk?

Is it possible that black holes magnetic field are rapidly cooling matter using a type of magneto caloric effect and cold fusion creating new matter only possible with a black hole's gravity? A type ...
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1answer
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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0answers
31 views

How can you find the $u$ using time dilation if you only know $\Delta t_o$? [closed]

I've come across two problems dealing with time dilation now that ask how fast should an astronaut go in order to travel EG: 400 LYears in 10 Years. What I can not wrap my head around is how this is ...
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1answer
121 views

Why does time stop its flow? [duplicate]

According to the "list verse" time gets to a pause when someone or something travels at the speed of light. But a question still lies in my mind, if time always travels forward and does not stop at ...
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1answer
82 views

Interpretation of time dilation

Is there any interpretiation of time dilation for photons? I mean, how does time work for them? Do photons have any distinction between their own and observer's time? Or is it something like ultimate ...
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2answers
88 views

Help understanding why can't we measure absolute velocity?

(This is probably a clone of some other question, but I haven't been able to find a satisfactory answer, so I hope this is okay.) So, I know that in special relativity, there's no such thing as a ...
3
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2answers
306 views

Why isn't a Photon justified in concluding that it is at rest and everything else is moving past it at c? [duplicate]

Why isn't a photon moving at c not justified in concluding that it is at rest and everything else is moving past it at c since relativity postulates that the laws of physics are invariant (i.e. ...
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2answers
56 views

How fast can video/ transmission waves travel through space? [duplicate]

Something that has been on my mind for quite a while now and has sprouted lots of thought, is whether we can create a machine to put into space, far away from the earth, and use it to see back in ...
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1answer
44 views

light in black holes and space time

I watched videos, read articles, learned a lot about black holes and got confused: Lets say that a light is doing its way inside a black hole toward the singularity. If it's inside the event ...
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1answer
54 views

Implication of Relativity [duplicate]

According to relativity, no useful matter can travel faster than light. Suppose that your are a light beam travelling. Now you have a parallel light beam travelling beside you. Suppose you look at the ...
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2answers
89 views

Speed of light invariance (once again)

The answer to this very good question seems to be favored by a large amount of users. Yet it seems to imply that the constancy of the speed of light and its finiteness stems from the underlying ...
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0answers
40 views

Impossible? Or not? [closed]

If the universe is expanding at light speed, and you stand on the edge moving out, if you look across the universe, then you will see twice the light speed, why is that?
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3answers
117 views

If our eyes see at the speed of light, how do we know about the current states of stars and galaxies far away?

As you guys know that we see at the speed of light, it means that we see the past of stars and galaxies. So say a star went supernova right now, how are we able to know current state of that star that ...
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1answer
77 views

Field of view from travelling at (almost) the speed of light?

As we go faster, our field of view increases, but would happen if we travelled at (almost) the speed of light? My questions: Can we see a 360 degree view while travelling at (almost) the speed of ...
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3answers
2k views

Can we tell when an established theory is wrong?

I was reading the following answer from this question: In physics, you cannot ask / answer why without ambiguity. Now, we observe that the speed of light is finite and that it seems to be the ...
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0answers
67 views

Would a “better” speed of light be possible? [closed]

The vast distances in space make it almost impossible to reach anywhere in a reasonable timeframe, since we have the speed of light as an upper bound and even getting close to that speed takes a lot ...
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1answer
97 views

Thought experiments about $c$ [closed]

Poincare had a brilliant thought experiment about what if everything in the universe doubled in size, would anybody notice any difference or even be able to measure anything that can be compared with ...
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1answer
40 views

Can absolute speed of object be measured? [closed]

Let's say there are two objects in space - Station and huge non-transparent sphere. Sphere starts to move from station 5000 km/h. Inside the sphere, beam of light is produced in direction the sphere ...
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3answers
107 views

Why is the speed of light not 3 x $10^{8}$ m/s, or why not a round figure for the meter [duplicate]

This question is not about why the speed of light is not greater than 299,792,458 m/s. It's about why it's that exact value. This was a question from a book(I know homework questions are frowned upon ...
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26 views

How much is the maximum posible information travel speed in medium?

Light has slower speed in medium. Should we consider this when we use special relativity in atmosferic(or any other type of medium) motions? If we should use vacuum light speed, does it mean photons ...
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1answer
75 views

Gravity is Instant [duplicate]

Without being able to manipulate gravity. How do we know that gravity is restricted to the speed of light? or gravitational effects from an exploding star would be felt (if we were near) before we ...
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1answer
114 views

Speed of light or speed of time?

Proxima Centauri is about 4.22 Light Years, so if there is anyone that could see us, the image of us would be 4.22 years old, if we depart at 99.99% the speed of light we would arrive almost instantly ...
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1answer
73 views

Constancy of speed of light in vacuum? [closed]

Have scientists tried their best to calculate speed of light in vacuum? It seems very absurd to me that light of all frequencies have same velocity in space. Can someone find in future if this speed ...
182
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27answers
26k 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 ...
2
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1answer
114 views

Is near light speed travel possible? Likely?

I was contemplating the possibility of sending an interstellar probe that can produce results in a reasonable time frame. For such a mission to be realistic, the spacecraft would have to travel at ...
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6answers
162 views

Light in mass moving at high velocity

Imagine if you will, a strand of fiber optic cable 186,000 miles long. A pulse of light is sent through the stationary cable: it takes 1 second for light to travel the entire length of the cable. Now ...
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3answers
109 views

Is mass directly or inversely proportional to time? [closed]

From a Newtonian perspective: F = ma F dx = ma dx E = m (dv/dt) dx m = (E dt) / (dv dx) Mass is directly proportional to time, if time slows down then mass goes down or decreases. From a ...
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30 views

Variable speed of light impact on spectral absorption lines in distance luminous objects?

There are many other arguments as to the constancy of the speed of light (or more precisely, c). One thing I have been curious is what would the impact be on spectral measurements from distant ...
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1answer
47 views

Time and velocity

Suppose two cars are traveling at a velocity of $c/2$ ($c$=speed of light) in two straight lines parallel to each other but in opposite direction in vacuum. Then what will a person sitting in one of ...
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1answer
60 views

Variable speed of light in loop quantum gravity

Loop quantum gravity predicts a variable speed of light. What is the range of such a variation in its speed?
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1answer
117 views

Is it experimentally proven that photons travel at speed $c$ in vacuum?

Are there experiments which show that single photons (not classical em waves) travel exactly at $c$ in vacuum? What is the error bar in that case? The question is posed due to the fact that loop ...
2
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1answer
169 views

Slow light that does not go back to it's original speed in vacuum [closed]

I have watched today episode of scishow where Hank claims that photons can be permanently slowed down to speed that is less than speed of light. ...
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2answers
143 views

Is it possible to go faster than the speed of light in vacuum?

If it is possible for particles to go faster than the speed of light in certain events, would it be possible to create a situation in which the barrier of the speed of light can be crossed?
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0answers
43 views

Does time dilation affect the speed of a moving object?

Imagine you are traveling at 99% of the speed of light. Will the speed of the moving object slow down for both objects and for the resting observer or either one?
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1answer
66 views

Objective velocity (Relativity theory) [closed]

Suppose that I have a velocity of (approximately) $c$. Then I decelerate with $c$ per hour for exactly one hour. Then I would know that I would be standing still in the end, because the decelleration ...
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2answers
68 views

Does the speed of light differ for two frames moving at different speeds relative to an non moving observer?

Wikipedia says that: The speed of light in a vacuum is the same for all observers regardless of the motion of the light source. How can this be true and and returning to my question, if there is ...
2
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1answer
87 views

Is this argument valid as to why we can (not can't) reach the speed of light?Please do explain [closed]

I have already asked this question on quora, with difficulty but I have not yet obtained a "works for me" answer. Please discuss with me about this.:) Q:Why can we not touch the speed of light? ...
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3answers
122 views

Light cone argument for speed of light

Using light cones to state that nothing can travel faster than light to me seems like a flawed argument. (I understand other reasons why the speed of light cannot be broken.) However, it seems to me ...
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3answers
129 views

If photon can bend spacetime how does it exchange graviton?

Say two photons moving beside each other through space at speed of light, since it carries energy it can bend spacetime. So both photons will exert gravitational force on each other at speed of light ...
4
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4answers
240 views

Can there be acceleration without velocity?

If I was in a spaceship continually accelerating at $9.81m/s^2 =1g$ in a straight line, I would reach near light speed within a year. On Earth, we are in a gravitational field of $1g$, which ...
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23 views

Is photon massless? [duplicate]

if photon is massless then in E2=m2c4+p2c2 is m=0 ? and p=mv1-mv2 shouldnt be zero too. if both the statements are true then does photon has no energy?
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Highest temperature [duplicate]

The temperature of a body varies with the KE of its particles. The maximum KE can be obtained by making the atoms move about near the speed of light. Let us take the body as a gas so that the body can ...
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0answers
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Are gravitational waves constrained by special relativity (speed of light)? [duplicate]

In Lawrence Krauss's excellent book "Universe from Nothing" and his related lectures, he cites findings of the BOOMERANG-instrument observations in Anarctica as used to measure the diameter of ...