4 ms·

> So if a photon is emitted from the sun, it's passing earth immediately? Then why does sunlight need 7 or 8 minutes to get to earth? From the photon's perspec

> So if a photon is emitted from the sun, it's passing earth immediately? Then why does sunlight need 7 or 8 minutes to get to earth? From the photon's perspective, it passes earth immediately. From our perspective, it takes 7 or 8 minutes. > And let's say, i travel one light-year at the speed of light, that should be instantaneous, right? The odometer would show 1 light-year, my watch would should 0 seconds and some decimals. How much would I have aged by the end of the journey? You would have aged as much as your watch says you would have aged. Zero seconds. >If I travelled at one percent of speed of light, same distance, i suppose 100 years would elapse for me, how much would elapse on earth? I don't know how to do the math, but a very, very long time would have passed on earth. > And what if I left earth at 50percent of light speed , traveled 1 light-year away and did a turn and came back to earth at same speed. For me, it would be 1 year, but if I had a twin brother who was waiting on earth, would i now be a year younger than him? And how is this possible? Yes, you would be younger than him, and it's possible because that's just how relativity and time dilation work. It's even practically measurable in "real" life: http://www.leapsecond.com/great2005/tour/ http://www.leapsecond.com/great2005/tour/

throw_away 5y ago
> > If I travelled at one percent of speed of light, same distance, i suppose 100 years would elapse for me, how much would elapse on earth? > I don't know how to do the math, but a very, very long time would have passed on earth. Eh, at .01c, not so much: https://www.omnicalculator.com/physics/time-dilation?c=USD&v=equation:0,t:100!yrs,v:3000!kmps https://www.omnicalculator.com/physics/time-dilation?c=USD&v... (=100.005y, not even two days extra)
- wruza 5y ago
  Yeah, all interesting things only begin at roughly 0.5c and go crazy at 0.9c. Google images on “lorentz factor” to get the quick idea. https://www.google.ru/search?q=lorentz+factor&tbm=isch https://www.google.ru/search?q=lorentz+factor&tbm=isch
  - deleted 5y ago
    [deleted]
senectus1 5y ago
just curious... does "time" have a "speed" ? Is travelling at the speed of light, actually travelling a a fraction of the speed of time?
- aaron-santos 5y ago
  Yes, one second per second.
  - ithinkso 5y ago
    I know this might seem like a joke answer but I love it because it's the truest answer. The 'speed of light' is nothing but a coefficient between seconds and meters (or in general, between units of time and units of distance and is equal to 1 in any sensible measurement system) since the spacetime in GR is unified
  - TedDoesntTalk 5y ago
    We are all time travelers traveling at one second forwards in time.
- Rayhem 5y ago
  It can be said that everything is traveling through spacetime "at c (the speed of light)". The math works out such that the faster you move through space, the slower you move through time and vice versa.
  - yccs27 5y ago
    The faster you move through space, the faster you move through time as well, actually! That's because distance in spacetime is defined with a negative sign for time periods. The profound statement is now that the subjective time (which can be measured by a clock moving along with you) matches the theoretically-defined spacetime distance (which is constructed to be invariant under Lorentz transformations).
- gizmo686 5y ago
  > Is travelling at the speed of light, actually travelling a a fraction of the speed of time? This is probably one of the more counter-intuitive simple calculations you can do in physics. In special relativity, distance is given by: ds^2 = dx^2 + dy^2 + dz^2 - (c^2)dt^2 if X is your total distance in space, you have: dX^2 = dx^2 + dy^2 + dz^2 Which is just the standard Pythagorean theorem of Euclidean geometry. Further, your velocity is given by dX/dt. If you are traveling at the speed of light, you have: dX/dt=c From which you can derive dX^2 = (c^2) dt^2 dX^2 - (c^2) dt^2 = 0 ds^2 = 0 In other words, the "distance" light travels in space time is 0. > just curious... does "time" have a "speed" ? It is not clear how to parse this question. Traditional "speed" is defined as distance over time. We can give this meaning for time itself by realizing that there is no single notion of time in relativity. As such, you could consider the line parallel to the time axis in the coordinate system of observer A. Since dt=0 in the coordinates of observer A, the speed of this line is not well defined. However, we could consider the coordinates of observer B. Assuming B is moving relative to A, he would see this line as being slanted, with both a time component, and a space component. As such, B could compute the speed of this line as dX'/dt', where X' is the total displacement along B's 3 spatial dimensions, and dt' is the displacement in B's time dimension. As such, B could meaningfully answer "what is the speed of A's time". Assuming I didn't mess up on the math, dX'/dt' turns out to be the velocity of A relative to B. This is a curious result that I have never seen before, but I can't really see any physical significance to it. B could also compute dt/dt', where t' is the time axis in B's coordinate system. This computation seems more useful as it gives a direct measure of time dilation. Unsurprisingly, it also works out to be the Lorentz factor.
archeantus 5y ago
So it sounds like they did a decent job sticking to the science on the Interstellar movie?
- qazpot 5y ago
  Interstellar correctly demonstrates how general relativity will work. Even the 5-dimension tesseract was a decent representation as well. Matthew Mcconaughey using love to navigate through the 5-dimension is classic hollywood.
  - adriancr 5y ago
    'love' might have been their interpretation. Another one would be cause and effect, if that had already happened it means it must happen again... and so whatever he does there it will succeed in setting events in motion again... Interesting bit would have been how it all started.
hota_mazi 5y ago
> From the photon's perspective, it passes earth immediately. To go even further: to a photon's perspective, it is able to teleport instantly anywhere in the universe, because time doesn't elapse for it.
paraschopra 5y ago
Actually there's no perspective or reference frame in which light is standstill (that's one of the postulates of special relativity). So, saying time stands still for the light is a misnomer.

throw_away 5y ago

> > If I travelled at one percent of speed of light, same distance, i suppose 100 years would elapse for me, how much would elapse on earth? > I don't know how to do the math, but a very, very long time would have passed on earth. Eh, at .01c, not so much: https://www.omnicalculator.com/physics/time-dilation?c=USD&v=equation:0,t:100!yrs,v:3000!kmps https://www.omnicalculator.com/physics/time-dilation?c=USD&v... (=100.005y, not even two days extra)

wruza 5y ago
Yeah, all interesting things only begin at roughly 0.5c and go crazy at 0.9c. Google images on “lorentz factor” to get the quick idea. https://www.google.ru/search?q=lorentz+factor&tbm=isch https://www.google.ru/search?q=lorentz+factor&tbm=isch
- deleted 5y ago
  [deleted]

senectus1 5y ago

just curious... does "time" have a "speed" ? Is travelling at the speed of light, actually travelling a a fraction of the speed of time?

aaron-santos 5y ago
Yes, one second per second.
- ithinkso 5y ago
  I know this might seem like a joke answer but I love it because it's the truest answer. The 'speed of light' is nothing but a coefficient between seconds and meters (or in general, between units of time and units of distance and is equal to 1 in any sensible measurement system) since the spacetime in GR is unified
- TedDoesntTalk 5y ago
  We are all time travelers traveling at one second forwards in time.
Rayhem 5y ago
It can be said that everything is traveling through spacetime "at c (the speed of light)". The math works out such that the faster you move through space, the slower you move through time and vice versa.
- yccs27 5y ago
  The faster you move through space, the faster you move through time as well, actually! That's because distance in spacetime is defined with a negative sign for time periods. The profound statement is now that the subjective time (which can be measured by a clock moving along with you) matches the theoretically-defined spacetime distance (which is constructed to be invariant under Lorentz transformations).
gizmo686 5y ago
> Is travelling at the speed of light, actually travelling a a fraction of the speed of time? This is probably one of the more counter-intuitive simple calculations you can do in physics. In special relativity, distance is given by: ds^2 = dx^2 + dy^2 + dz^2 - (c^2)dt^2 if X is your total distance in space, you have: dX^2 = dx^2 + dy^2 + dz^2 Which is just the standard Pythagorean theorem of Euclidean geometry. Further, your velocity is given by dX/dt. If you are traveling at the speed of light, you have: dX/dt=c From which you can derive dX^2 = (c^2) dt^2 dX^2 - (c^2) dt^2 = 0 ds^2 = 0 In other words, the "distance" light travels in space time is 0. > just curious... does "time" have a "speed" ? It is not clear how to parse this question. Traditional "speed" is defined as distance over time. We can give this meaning for time itself by realizing that there is no single notion of time in relativity. As such, you could consider the line parallel to the time axis in the coordinate system of observer A. Since dt=0 in the coordinates of observer A, the speed of this line is not well defined. However, we could consider the coordinates of observer B. Assuming B is moving relative to A, he would see this line as being slanted, with both a time component, and a space component. As such, B could compute the speed of this line as dX'/dt', where X' is the total displacement along B's 3 spatial dimensions, and dt' is the displacement in B's time dimension. As such, B could meaningfully answer "what is the speed of A's time". Assuming I didn't mess up on the math, dX'/dt' turns out to be the velocity of A relative to B. This is a curious result that I have never seen before, but I can't really see any physical significance to it. B could also compute dt/dt', where t' is the time axis in B's coordinate system. This computation seems more useful as it gives a direct measure of time dilation. Unsurprisingly, it also works out to be the Lorentz factor.

archeantus 5y ago

So it sounds like they did a decent job sticking to the science on the Interstellar movie?

qazpot 5y ago
Interstellar correctly demonstrates how general relativity will work. Even the 5-dimension tesseract was a decent representation as well. Matthew Mcconaughey using love to navigate through the 5-dimension is classic hollywood.
- adriancr 5y ago
  'love' might have been their interpretation. Another one would be cause and effect, if that had already happened it means it must happen again... and so whatever he does there it will succeed in setting events in motion again... Interesting bit would have been how it all started.

hota_mazi 5y ago

> From the photon's perspective, it passes earth immediately. To go even further: to a photon's perspective, it is able to teleport instantly anywhere in the universe, because time doesn't elapse for it.

paraschopra 5y ago

Actually there's no perspective or reference frame in which light is standstill (that's one of the postulates of special relativity). So, saying time stands still for the light is a misnomer.