General Theory of Relativity question...


New member
Aug 11, 2012
Visit site
Ok, I went through a video to understand it more simply. But there is one point I just don't understand.

Which is this:

How does Adam think others' clock is SLOW?

Shouldn't it be FAST?

Here's my breakdown of the video:

Adam is in- Space

Sarah is on - Ground

Beam of light shone from the spaceship

According to the person on EARTH - Takes a longer time for the light to reach the spaceship back due to a V shape

According to the person on the SPACESHIP - Takes a shorter while for the light to return back to the ship

The person on EARTH sees the time passed as MORE

The person on the SPACESHIP sees the time passed as LESS

The person on EARTH sees the time passed as MORE (1 hour),

The person on the SPACESHIP sees the time passed as LESS (1 minute)

FOR the person ON EARTH, the clock on the spaceship is SLOWER (BECAUSE for her, the time passed on HER clock is 1 HOUR, but the time passed on HIS clock is JUST 1 MINUTE, hence his is SLOWER compared to hers, hence time is moving SLOWER on HIS clock and FASTER on HER clock)

FOR the person ON THE SPACESHIP, the clock on the EARTH is FASTER - Right??

How does Adam view the clock on Earth as SLOW?

Can someone please explain this..



Active member
Jan 12, 2013
Visit site
Quite singular to find a topic such as this posted here!:wink:

I could not watch the video you're referring to, but here's an attempt to make things a bit clear in three points: the speed of light; the perceived change in the rate of time; and the reference body,

The speed of light in a vacuum is constant, unless affected by a gravitational field strong enough (such as in an event horizon). Therefore, regardless of your speed, light will travel in a fixed speed, i.e. consider yourself in a space craft and a second observer on earth: the time light will take to travel from a point x miles away from earth to reach earth would be the same whether you're speeding away from earth or not - the only difference would be in the perceived color* of the light. Similar to the Doppler Effect in sound, speeding away from earth would stretch the wavelength of light and vice versa - this method is now being used to detect stars that are oscillating in an attempt to identify other planetary systems.

Time itself does not change in "rate" with respect to the speed, rather it is the perceived rate of time that changes. Imagine looking at your clock, then suddenly you start travelling at the speed of light away from your clock. Since you're traveling at the speed of light, you would constantly be seeing your clock telling the time at which you left it (assuming you don't deplete the photons you're traveling with). Hence, time appears to be frozen, but that doesn't mean that time was frozen just because your traveled at the speed of light - otherwise, how could the length (distance) of a light-year be measured? This also means that when you travel at the speed of light, your perception of time would be delayed by as much time you had been traveling. Travel at the speed of light for five minutes and you'll see your watch be late by five minutes although you know that you have been traveling for five minutes. If time stopped, you wouldn't even know you have been traveling at all.

The perceived change in the rate of time is given by a surprisingly simple equation that requires two reference points. The first is the speed of light; the second is the speed of a reference body. The reference body must have a larger gravitational footprint than your object of concern. So, in the case of your example, the reference body is the earth. This prevents the counter argument: "If the space ship is travelling at 100 miles per second with respect to the earth, isn't true that the earth is also travelling at 100 miles per second with respect to the space ship?" This means that the perceived time of a body would be slower with respect to the perceived time on the reference body. If you're on earth, the time on the space ship would appear to be slower (this adjustment is being used in the internal clocks of GPS satellites since these objects are travelling at very large speeds - otherwise their clocks would be off).

*- If the resulting wavelength is still visible.


Retired Moderator
Jul 25, 2009
Visit site
We've had a little bit of a science thread going on here: that we add to from time to time.

X0LARIUM, I am not sure if any of these videos will be available to you, but the Science Channel in the states explores the issue you're talking about in quite a few episodes. Recently, they had an episode on gravity, which touched on the subject of how gravity effects time, It's worth a look:
Science Channel : Science Channel

Members online

No members online now.

Forum statistics

Latest member