@inkican Going back in time would require more energy than is available in the universe, for going faster than the speed of light, which is impossible. That's the basis in the theory (should be called 'law' by now) of relativity.
Backwards time travel would obviously interfere with causality in creating an alternative past, which would lead to a different present. So it's a good thing that it's physically impossible, despite what a desperate astrophysicist believes.
The idea is that you can go backwards in time by going faster than light. The speed of light is actually the speed of information itself - so if you could go faster than that, you’d be going backwards in time.
However, Einstein showed that nothing can accelerate to the speed of light. You’d need infinite energy to actually reach the speed of light, and infinite energy is assumed to not be possible.
I wonder if there is a meaningful difference between your example, and the technology with which the JWST uses to view light in the past. Rather, if the later is something we can use for time travel ;)
Good for you commenting on the title alone! If you looked at the actual article you would know that one of the limitations is, that the furthest point you can reach going back in time is when the “time machine” was first activated.
Can only go back to when you started operating the device. So, basically the Primer time machine, except the math says it has to be done at galactic black hole amounts of energy sort of scale.
Mallett’s vision for a time machine centers on what he calls “an intense and continuous rotating beam of light” to manipulate gravity. His device would use a ring of lasers to mimic the spacetime-distorting effects of a black hole.
Wildass hypothesis I just pulled out of my ass with an undergraduate degree in applied physics: maybe interaction with particles emerging from quantum vacuum?
Okay, that sounds like great technobabble. I’m going to watch star trek now ;)
It’s probably not that the light is losing energy it’s just that the distance it travels over time (the time we “know” is supposed to take for a given distance) appears compressed because of unknown/unseen gravitational forces.
Think of it like this: If there were only one star in the universe and it emits a particle of light we could calculate the distance it would travel over time. Yet we know that star will still have a gravitational effect on that light… No matter how far away it gets.
That’s what they mean by light “losing energy”. Is the energy actually “lost”? Not really. Is this slowing (aka appearance of lost energy) caused by dark energy/dark matter or something more fundamental like spacetime itself being stretched or compressed due to the gravity of astronomical objects we can see or “dark matter”/“dark energy” or… ? We don’t really know for certain yet!
It’s probably not that the light is losing energy it’s just that the distance it travels over time (the time we “know” is supposed to take for a given distance) appears compressed because of unknown/unseen gravitational forces.
This doesn’t seem to be at all what tired light proposes though. What you’re explaining sounds like red-shift due to an expanding universe. From what I can tell they claim it actually loses energy through interaction with “other things” in the universe.
This doesn’t answer the question in the context of this theory, but the current understanding is that light does lose energy as it travels through expanding space. As the space it’s in expands, the wavelength gets longer, and the energy goes down. It doesn’t go anywhere; energy just isn’t conserved in an expanding space-time.
If the light loses energy, then it must surely lose it to something? And if your last point that energy isn’t being conserved in our universe, in which case we are either in some deep shit with the first law of thermodynamics, or our universe isn’t an isolated system.
The thing about photons is that they redshift, losing energy as space expands. If we keep track of a certain fixed number of photons, the number stays constant while the energy per photon decreases, so the total energy decreases.
Ok. Smarter people probably thought of this, and probably found my hypothesis to be impossible. But what if… It is the the other way around. What if photons are losing energy because they are expanding spacetime. Like tiny little springs expanding out.
Further into the article he says that, "It would be irresponsible of me not to mention that plenty of experts in cosmology or GR would not put it in these terms. We all agree on the science; there are just divergent views on what words to attach to the science. In particular, a lot of folks would want to say “energy is conserved in general relativity, it’s just that you have to include the energy of the gravitational field along with the energy of matter and radiation and so on.” "
So energy is conserved on the whole, it’s just not conserved if you consider photons apart from their greater context.
The energy is actually not conserved across the universe in general relativity, as it is currently understood. Conversation of energy is due to the time symmetry, which the expansion of space breaks.
BTW, thanks! This comment sent me down a fascinating rabbit hole. It had simply never occurred to me that energy conversation didn’t apply in an expanding universe!
“Energy is conserved in general relativity, it’s just that you have to include the energy of the gravitational field along with the energy of matter and radiation and so on.”
Man, lots of people in this thread seem happy to accept any wild, physics-breaking idea rather than accept that there’s just a bunch of matter we can’t see.
I think it goes beyond not being able to “see” it and goes to we can’t detect it at all. Doesn’t dark matter just fill in the mathemagical holes with some numbers to make it all work?
We can detect its gravitational influence, as it interacts via gravity. The issue being that gravity is a weak force, and so there’s a lot of room for speculation.
But there is a lot of evidence backing up dark matter existing. But it’s not definitive yet.
I get that but it still sounds woo-woo since we can’t directly detect it. I’m not naysaying since I realize it’s the best we have and I’m not smart enough to come up with anything better.
I mean, I guess it depends on what you mean by “directly detect”. We measure neutrinos by having photoreceptors in huge tanks of very pure water deep under old salt mines… which hardly seems more direct than looking at where galaxies and stars are moving and calculating the gravitational pull and noticing that something is missing…
Dark matter is matter that we infir to exist only on its gravitational effects. We’ve observed its existence by the fact that it seems to clump up in the middle of two massive super-solar structures following a collision.
We can indirectly detect dark matter thru gravitational lensing. That is how NASA created this map showing the actual locations of dark matter in tinted blue.
earth.com
Aktywne