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11 votes

Is a photon truly massless?

Although everyone has heard of the famous equation: $$ E = mc^2 \tag{1} $$ few realise that this is a special case and applies only in limited circumstances. Specifically it applies only to a massive ...
John Rennie's user avatar
9 votes

Is a photon truly massless?

The correct version of your syllogism is: $E=mc^2$ for a particle at rest. For a photon, $E>0$. For a photon, $m=0$. The correct conclusion is that a photon can never be at rest.
WillO's user avatar
  • 15.9k
6 votes

Is a photon truly massless?

Light has inertia: it takes a force to change the direction in which they travel, and if you have a box with light bouncing around inside of it, it takes more force to change the speed of the box than ...
g s's user avatar
  • 14k
5 votes

Is a photon truly massless?

I like pictures, and the relevant picture is: You can ignore the formulae, they are just high school trig, tho, so nothing prohibitive. (The do look bad, though, I think it's because they are crammed ...
JEB's user avatar
  • 35.4k
2 votes

Why would speed of light be directional if spacetime is discrete?

The book cited in the question (contrary to much of the discussion in the comments) is about simulating physics on computers. When you go to a discrete grid and simulate with finite differences ...
Brick's user avatar
  • 5,095
2 votes

How can space stretch a wave?

Space does not stretch waves. That is a common misconception of the cosmological redshift, which is based on erroneously treating a particular coordinate choice as reality. As Bunn & Hogg (2009) ...
Sten's user avatar
  • 6,395
2 votes

What is the problem with this method to measure one-way speed of light?

The first time that you do this experiment, you have six unknowns, the speed along each of the three legs of your triangle ($c_{SO}$, $c_{SM}$, and $c_{MO}$) plus the time take to transit each leg ($...
Brick's user avatar
  • 5,095
2 votes
Accepted

When you are in a gravitational field, do object far away get physically closer to you as you get closer to the mass?

This is to answer the latest version of the question, as phrased in the comments: I should have clarified in the question [...] The only thing that truly matters is clock ticks counted by each ...
safesphere's user avatar
  • 12.7k
2 votes

Velocity of light in different directions

The statement is really stronger. Not only have we not been able to measure the one-way speed of light. It is impossible to measure even in theory. To make this measurement you would need ...
Brick's user avatar
  • 5,095
1 vote

Is this a valid experiment to measure the one-way speed of light?

Your definition of index of refraction has built into it an assumption that the speed of light is the same on both paths, $\mu = c/v$, where $c$ as usual is the speed of light in the vacuum. That ...
Brick's user avatar
  • 5,095
1 vote

Physical meaning of $vx/c^2$ in Lorentz transformation

You can multiply through by $c$ and get the equivalent relation $$ ct' = \gamma \left( ct - \beta x \right) $$ where $\beta = v/x$ is the fraction of the speed of light between the frame. In that ...
Brick's user avatar
  • 5,095
1 vote

When 'speed of light' changes, is it because the pendulum moves slower or the transition of caesium-133 gets more sluggish or?

Your question is really about how to choose appropriate standards of measurement. You are essentially correct in your understanding that if you choose standards that have elements that change over ...
Brick's user avatar
  • 5,095
1 vote

Increasing the view of observable universe

This won't work. To increase the observable universe for someone earth-bound, the probe needs to observe events and relay information about that event to earth. The probe may see the event before ...
Brick's user avatar
  • 5,095

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