6 ms·

Which thought experiment?

by ralfd 7y ago

WhompingWindows 7y ago
What would it be like to ride on a beam of light? What would happen if you're on a motorcycle that's going the speed of light, that then has a headlight that's throwing light forward at the speed of light?
- cperciva 7y ago
  There is no ride. From the photon's perspective, it is emitted and instantaneous absorbed at its destination.
- PeterisP 7y ago
  It's not possible even in a though experiment, as nothing that has mass (i.e. a motorcycle) can be going at the speed of light. It can be going (theoretically) at 99.999999% of lightspeed, but that's conceptually different.
brilee 7y ago
Special relativity is the set of equations that arises when you take a hypothesis: "you always perceive light as traveling at a speed `c`, no matter how fast you're going" and work out what would have to change in order to allow this statement to hold true.
- carapace 7y ago
  Sorry to be pedantic, it was: 1.) Maxwell's equations permit no other speed for light than c. 2.) All physical laws (such as Maxwell's) are the same in all inertial reference frames. From those two the rest flows.
  - ralfd 7y ago
    I found that useful. But why is 2 true? Was that proven by experiment or by math equations? It doesn’t seem intuitively true to me.
    - carapace 7y ago
      I think both, I'm not a physicist, so I may get this wrong. If it wasn't true, if there was a difference in some physical law due to your motion, you would be able to detect the One True Frame of the Universe. Now here's where I get a little fuzzy, I think it was up for grabs, and then they tried (Michelson–Morley experiment https://en.wikipedia.org/wiki/Michelson%E2%80%93Morley_experiment https://en.wikipedia.org/wiki/Michelson%E2%80%93Morley_exper... ) and couldn't do it. (See also Mach https://en.wikipedia.org/wiki/Ernst_Mach#Philosophy_of_science https://en.wikipedia.org/wiki/Ernst_Mach#Philosophy_of_scien... ) If you're in a spaceship with no windows, out in space, and you are in free-fall (not accelerating), there's no experiment you can do that will tell you whether you are moving or not. If your spaceship has a window, and you look out and see another spaceship passing by (but nothing else) you can't tell whether you're still and it's moving, or you're moving and it's still, or you're both moving. You can detect the relative velocity between you and the other ship, but not the absolute velocity (because there isn't one.) Intuitively we are used to a "absolute" reference frame provided by the bulk of the Earth but that's just a (highly) local context. In space (which is to say, in our Universe) there's no absolute reference frame, so you literally cannot have an absolute velocity, and obviously you can't have a physical law described by an equation that includes a term that doesn't exist, n'est-ce pas? Cheers!
0x445442 7y ago
The one I learned was this: Person A is on a moving train and tosses a ball into the air of height H. To this person the ball travels a distance of 2H to in a time T to return to his hand. Person B is beside the tracks and sees the same event. To this person the ball travels 2 * (x^2 + H^2)^1/2, which we'll later call D, in the same time T where x is the distance the train traveled during this time. Since velocity is a measure distance divided by time the velocity of the ball in Person A's frame of reference is 2H/T (V1). The velocity of the ball in Person B's frame of reference is 2D/T (V2). Obviously V2 is greater because the distance traveled is greater for the same amount of time. Now suppose instead of tossing a ball into the air of height H, Person A shines a flashlight to a mirror at height H. The new constraint is this; the velocity of light is a constant and cannot be faster for Person B. Now what? Length contraction / time dilation is what.

Which thought experiment?

by ralfd 7y ago

Which thought experiment?

What would it be like to ride on a beam of light? What would happen if you're on a motorcycle that's going the speed of light, that then has a headlight that's throwing light forward at the speed of light?

cperciva 7y ago
There is no ride. From the photon's perspective, it is emitted and instantaneous absorbed at its destination.
PeterisP 7y ago
It's not possible even in a though experiment, as nothing that has mass (i.e. a motorcycle) can be going at the speed of light. It can be going (theoretically) at 99.999999% of lightspeed, but that's conceptually different.

brilee 7y ago

Special relativity is the set of equations that arises when you take a hypothesis: "you always perceive light as traveling at a speed `c`, no matter how fast you're going" and work out what would have to change in order to allow this statement to hold true.

carapace 7y ago
Sorry to be pedantic, it was: 1.) Maxwell's equations permit no other speed for light than c. 2.) All physical laws (such as Maxwell's) are the same in all inertial reference frames. From those two the rest flows.
- ralfd 7y ago
  I found that useful. But why is 2 true? Was that proven by experiment or by math equations? It doesn’t seem intuitively true to me.
  - carapace 7y ago
    I think both, I'm not a physicist, so I may get this wrong. If it wasn't true, if there was a difference in some physical law due to your motion, you would be able to detect the One True Frame of the Universe. Now here's where I get a little fuzzy, I think it was up for grabs, and then they tried (Michelson–Morley experiment https://en.wikipedia.org/wiki/Michelson%E2%80%93Morley_experiment https://en.wikipedia.org/wiki/Michelson%E2%80%93Morley_exper... ) and couldn't do it. (See also Mach https://en.wikipedia.org/wiki/Ernst_Mach#Philosophy_of_science https://en.wikipedia.org/wiki/Ernst_Mach#Philosophy_of_scien... ) If you're in a spaceship with no windows, out in space, and you are in free-fall (not accelerating), there's no experiment you can do that will tell you whether you are moving or not. If your spaceship has a window, and you look out and see another spaceship passing by (but nothing else) you can't tell whether you're still and it's moving, or you're moving and it's still, or you're both moving. You can detect the relative velocity between you and the other ship, but not the absolute velocity (because there isn't one.) Intuitively we are used to a "absolute" reference frame provided by the bulk of the Earth but that's just a (highly) local context. In space (which is to say, in our Universe) there's no absolute reference frame, so you literally cannot have an absolute velocity, and obviously you can't have a physical law described by an equation that includes a term that doesn't exist, n'est-ce pas? Cheers!

0x445442 7y ago

The one I learned was this: Person A is on a moving train and tosses a ball into the air of height H. To this person the ball travels a distance of 2H to in a time T to return to his hand. Person B is beside the tracks and sees the same event. To this person the ball travels 2 * (x^2 + H^2)^1/2, which we'll later call D, in the same time T where x is the distance the train traveled during this time. Since velocity is a measure distance divided by time the velocity of the ball in Person A's frame of reference is 2H/T (V1). The velocity of the ball in Person B's frame of reference is 2D/T (V2). Obviously V2 is greater because the distance traveled is greater for the same amount of time. Now suppose instead of tossing a ball into the air of height H, Person A shines a flashlight to a mirror at height H. The new constraint is this; the velocity of light is a constant and cannot be faster for Person B. Now what? Length contraction / time dilation is what.