# Tag Info

27

The light that we see from the Andromeda galaxy was emitted 2.5 million years ago. During those 2.5 million years the Andromeda galaxy may have moved one or two thousand light years closer to us. However, the uncertainty in the 2.5 million light year distance estimate is of the order of 100 thousand light years (just because measuring the distance to other ...

13

This is a more complicated question than you might think because in relativity there is no unique definition of time and therefore no unique definition of now. You have probably heard of time dilation, and this arises because different observers will in general define time in different ways. However there is a natural choice for the time coordinate known as ...

7

for energy to be transferred from destructive region to the constructive region, the energy must go faster than its initial traveling speed (the speed of light) so as to cover up the extra distance This is incorrect. The key problem is the idea of “extra distance”. There is no extra distance. A region of interference only exists where EM waves from both ...

6

When we look at the Andromeda galaxy, we see it as it was $2.5 \times 10^6$ years in the past, because the light from there was emitted $2.5 \times 10^6$ years ago. In that time, the galaxy may have changed its distance, but probably not by a significant amount. Distances to cosmic objects are obtained by astronomers using the cosmic distance ladder methods. ...

6

You are making a common mistake. The speed of light is constant, not its direction. The direction of any motion can be frame dependent. If I walk at right angles across the carriage of a moving train, in the frame of a person standing on the platform as the train passes, the path I take is at an acute angle to the platform, even though it is at a right angle ...

6

My initial intuition after watching this video was that Derek's claim was technically true but only for an uninteresting reason. Namely, he stated that the lamp will light up once any current at all flows through it. Well, of course an infinitely sensitive amperemeter will react to the flip of the switch with the speed of light! (It will probably react to ...

5

during this time space-time expanded with a speed faster than the speed of light This statement actually is the problem here. The expansion of the universe is not measured in units of speed, so it cannot really be compared to c in the first place. Saying that it is faster than the speed of light is “comparing apples and oranges”. The expansion of the ...

4

The Einstein synchronization convention produces a one-way speed of light that is c. So the second postulate is based on the one way speed. This is justified by the isotropy of the two way speed of light and the isotropy of all known laws of physics. In Einstein’s seminal paper he says “we establish by definition that the “time” required by light to travel ...

4

The null result of the Michelson Morley experiment rules out a rigid aether. The Michelson Gale experiment rules out a dragged aether. Currently only the Lorentz aether, which is an aether that is designed to be experimentally indistinguishable from no aether, is compatible with experiment.

4

All differences are going to be counterbalanced, levelled in the journey back. I think you have misunderstood the Michelson-Morley experiment. It does not attempt to measure the time that light takes to travel along one arm of the interferometer and back. Instead it shows that the difference in travel time along the two perpendicular arms of the ...

3

You have this the wrong way around. Time changes because the speed of light is constant. Consider this simple thought experiment, bearing in mind that the speed of light is always 299 792 458 metres per second... You and I stand together and flash a light to the east and west at t=0. At t=1, the light is exactly 299 792 458 metres away from us in each ...

3

To your first question, you are right : the answer is just that the universe is really really empty. To your second one : Did the expansion of the universe somehow 'outrun' the photon? the answer is more complicated. In some sense, the photons are catching up. Consider a very small volume, 13 billions years ago, and all the photons is that volume. When time ...

3

Your understanding is wrong, and a common misconception. The speed of light is constant, but its direction, and therefore its velocity, is frame dependent. Suppose the two objects moving in parallel directions are the two sides of a railway carriage. Suppose I stand against one wall and aim a laser at a spot marked on the other wall directly across from me ...

3

A non-Lorentz coordinate transformation on Minkowski space can always change one-way speeds in various directions (exercise for OP: write down a transformation law that changes the speed of light in positive $x^1$ and negative $x^1$ directions). That of course has absolutely no physical significance because all coordinate systems are equally good for ...

3

There are a number of alternative theories to SR that posit absolute simultaneity (and hence reject SR's relative simultaneity). There are often referred to as Lorentzian relativity or neo-lorentzian relativity, and were originally developed by Lorentz and others (such as Poincare). They preceded SR by approximately 10 years. These theories presuppose that ...

2

Hypothesis 1 is not actually a hypothesis but a principle that helps select inertial frames. Without it, every frame is possible because a law of physics in one frame is also a law of physics in another frame - but they will look different. Further, it's only the second sentence in Hypothesis $\alpha$ that is roughly equivalent to Hypothesis 1. And this is ...

2

In your spacetime diagram, you can't have a light ray go at an arbitrary angle. Before you can make use of your diagram, you'd need to go set up an experiment, and measure the speed of light in sticks per hour. Once you obtain the value, you'd go that number of sticks to the right, and one hour up, and mark that point. Then you'd connect the origin with the ...

2

Look at relativistic aberration of light and relativistic photon momentum. This means, in special relativity, the light beam's velocity components $c_x,c_y,c_z$ as well as momentum components $p_x, p_y, p_z$ ("inertia") depend on the chosen inertial frame by the Lorentz transformation. Only the magnitude of light speed remains the same in all ...

2

The Michelson-Morley experiment wasn't designed to rule out aether. It was designed to establish it, according to the terms of how the aether was then conceived. What it showed instead was that the aether could not have the properties ascribed to it at the time. Einstein later showed that for the purpose of his theory of special relativity, it was not ...

2

There is no experiment which can measure the one-way speeds of light. Standard SR adopts the Einstein clock synchronisation procedure, which tacitly assumes that the one-way speeds of light are equal to the two-way speed (i.e., the round-trip speed). Professor John D. Norton has an excellent website that discusses this topic. Norton is considered an ...

2

I am evading your question here and instead answer by saying: You only need to consider this if over the course of the propagation of gravity your positions change notably. Typically in galaxies and stars that is not the case, since speeds are very non-relativistic, and the distances are large, i.e. the change in the force vector is tiny and the Newtonian ...

2

In classical electromagnetic waves that are described by solutions of the Maxwell equations, the energy carried by the wave is given by the Poynting vector, as a rate of energy transport and is in term of averages per unit area, and cannot be used for determining any direction in energy,afaik. To answer your question, one has to go to the quantization frame ...

1

The speed of light will always appear to be c to you, regardless of how quickly you move relative to other things or whether you are moving towards or away from the direction of motion of the light. Time dilation doesn't explain it, as you have seen for yourself. The point you need to take into account is the relativity of simultaneity. I strongly recommend ...

1

We generally mean "is now", because the word "is" means present. The problem is that where we see it now, is where it really was 2.5 million years ago, because the light from Andromeda which we are seeing now, has taken 2.5 million years to reach us. So Andromeda has of course moved since then. But the galaxy moves much slower than the ...

1

Since the relative speed between the Miliky Way and the Andromeda system is much smaller than the speed of light the difference is in this relatively very small.

1

Your question is very vague. There are four inertial reference frames involved, so the answer depends upon which of the frames the specified times and distances are related to- you really should spell that out to avoid confusion. It is also not clear what you mean when you say that ship A travels at 0.5c for 1 second- what happens at the end of the 1 second? ...

1

I will write in general on how to approach the issue. First: The context is spacetime as described in terms of special relativity. In terms of special relativity it is assumed/granted that the Minkowski metric is the appropriate metric. In order to meaningfully approach the issue of speed of light it is necessary to absorb the concept of Minkowski metric. ...

1

The starting point is different, the ether exists and its effect would be to affect the velocity due to a counteracting force (what they actually wanted to prove). Changing the direction of the arms implies a different effect on the light path and so on the shift of the fringes. But the shift has to be of a given quantity, they instead obtained a very small ...

1

Yes, the length does change as well. This is called "length contraction". Space and time are linked, so any changes to how we measure one will also affect the other. The exact formula for how they are linked is called the Lorentz transformation and actually predates Einstein's special relativity.

1

In the Newtonian framework, the force acting on Earth now reflects the position of the Sun $8$ minutes ago. When the Earth takes a little time to move a little bit, the new force acting on it reflects the position of the Sun a little bit less than $8$ minutes ago. Edit - After David Hammen's comments, I read Speed of gravity. I need to expand on the answer ...

Only top voted, non community-wiki answers of a minimum length are eligible