Is this a science?
- Thread starter seivtcho
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Darby
Epochal Historian
Yes, it's based on real physics - particularly Special Relativity. And Dr. Greene correctly states that the physics can be run in reverse just as "lawfully" as in forward. That's T-Symmetry or time reversal symmetry. The time signature and velocities, for instance, in the equations of physics can be reversed and you will get a valid solution. You've run the problem in reverse.
But he tends, in this popular science treatment intended to make the producers of the TV show money, to quickly gloss over thermodynamics and entropy in order to "stir the imagination" of...and entertain...the audience. It's only during the last few minutes that he touches on thermodynamics and entropy. The real world isn't a math equation. Our broken egg might fly up off the floor and reassemble itself because the laws of physics don't preclude the possibility. Wait around several trillion years watching broken eggs and maybe one fill fly back together. Thermodynamics and entropy aren't easily reversed.
So take it with a grain of salt. It's a show about the arrow of time intended for people who have little or no background in physics.
Will please someone explain me the part of the video with the alien? I watched it over and over and I can't understand it. So.. the time-space? is a loaf of bread and one man is in on the earth and an alien is far away. Then the alien starts to ride a bycicle and ... happens what?
Darby
Epochal Historian
You can't even know what the previous state of the universe was. The speed of light precludes you from receiving any information from a distant event such that you are seeing it "now" as defined by a clock at it's origin and your clock, i.e. this is the fall of the notion of absolute simultaneity of two or more events. There's no way for you to know the exact state of the Moon "now". The information that you receive from the Moon is at least 1.2 seconds old when it arrives on Earth.
As I said before, asking if something is impossible is the wrong question. You have to ask, through a theory, if it is possible and then develop an experiment to prove or disprove the theory.
There are many theories that say that under very specific circumstances time travel to the past is possible. But each of those theories are set to specific circumstances that are not known to exist in our universe. This doesn't make them bad theories. It just means that the theorist was eliminating everything from the universe that would prevent the theory from being valid. Thereafter they add back into the mix the real world, one item (parameter) at a time, until the theory fails or is shown to be a valid description of the world (which allows for time travel).
Darby
Epochal Historian
Unfortunately its a great bit of motion picture magic with some eye candy graphics, this part of the show goes from Special Relativity straight into General Relativity. The show takes a hugely complex idea and presents it as pop-sci. It doesn't imply time travel. The alien is still 10 billion LY away and the signals that he is receiving will be 10 billion years old. The idea that his "now" is the same "now" as ours is mathematically correct but it has nothing to so with his reality. But there's no way, not even in theory, for you or he to know his state of motion "now" - he's 10 billion LY away. It would have been boring for him to present the basic math with the trig functions that shows the space-time metric for moving frames of reference. But t he trig functions define the angles and that's the angle at which he slices his loaf of bread. All he was trying to say was that if two events are billions of light years apart then a tiny difference of relative velocity between them (thus a tiny angle in the bread slicing) become a huge space-time separation when you extend the hypotenuse and base across 10 billion LY. (If you think that I'm implying that the Pythagorean Theorem plays a part in the space-time metric you are correct.
)ds = sqrt(-ct^2 + x^2 + y^2 + z^2) where ds is the space-time interval between the events. This is the Pythagorean Theorem in 3 spatial dimensions (x,y,z axes) plus a time dimension (t axis)...3D+1 dimensions.
The bottom line is that to really understand this episode in detail you'd have to have a PhD and have studied at length Special Relativity, General Relativity and Quantum Mechanics. The above equation is "baby math" for lower division first year physics. It doesn't even have the trig functions in it.
Just take the video for what it is intended to be - entertaining and a little bit educating.