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Unified space-time starts to make sense if we think that the speed of light is a relationship between the distance traveled over time.
Because the speed of light can't change, your laser beam won't go any faster. The measurement of distance and time must be changed instead, depending on the state of motion. This leads to effects known as "space contraction" and "time dilation."
As you work at your desk, you move through time, but not through space. A cosmic ray moves over vast distances at nearly the speed of light but takes little time.
Many insights of Albert Einstein are now part of popular imagination: black holes, time warps, and wormholes show up in movies and books.
Less famous, but probably the most revolutionary part of Einstein's phenomena, is a simple idea that shows how pieces fit together and illuminate the road ahead.
Einstein's special theory of relativity applies only to steady, unchanging motion through space-time, not accelerating motion like an object falling toward Earth.
After Einstein, the pull of symmetry became more powerful.
We often think of things as the heart of reality. But most often the relationship is more important, not the stuff.
We may think "stuff" like space and time are unchangeable aspects of nature. In reality, the relationship between space and time stays the same.
Symmetry, as it is understood, seems not to answer the biggest questions in physics. In some cases, symmetries show the underlying laws of nature that do not show up in reality.
For example, when energy congeals into matter (E = mc2), the result is an equal amount of matter and antimatter - a symmetry. Yet if the energy of the Big Bang created both matter and antimatter equally, they should have destroyed each other, leaving nothing behind.
The most fundamental aspects of nature stay the same.
For example, Einstein's papers on relativity show that the relationship between energy and mass is invariant, even though energy and mass can take on many different forms.
Even though matter produces energy, the energy-matter content of the universe never changes. Matter and energy are less fundamental than the underlying relationship between them.
The idea of symmetry proved very powerful. Giving it up would mean giving up on naturalness - the idea that the universe has to be exactly the way it is for a reason.
But inside black holes, the speed of light (which grounded Einstein's work) will not play a vital role in the future. "The speed of light can't remain constant if space-time is crumbling," says physicist Stephon Alexander.
Albert Einstein did not think about symmetry when he wrote his first relativity papers in 1905. He was considering several seemingly unrelated puzzles and connecting the dots.
The relationship that mattered most to Einstein's ideas was symmetry. Scientists describe symmetry as changes that don't really change anything. More complicated symmetries have led to the discoveries from neutrinos to quarks.
Symmetry is at the root of our description of nature. But symmetry has not been able to explain why gravity is so weak or vacuum energy is so small. The idea of symmetry may be very powerful, but we may have to give up on these principles that have worked so well.
From the 1950s, invariances took on a life of their own. New symmetries, known as "gauge" invariances, became productive by requiring the existence of everything from W and Z bosons to gluons.
Gauge symmetry dictates what other ingredients you have to introduce. Gauge symmetries describe the internal structure of the system of particles in our world. Physicists can move, rotate and distort their equations without changing anything important. The result is a look at the hidden structures that supports the basic ingredients of nature.
Duality is a closely related idea to symmetry. Wave-particle duality has been around since the beginning of quantum mechanics. But newfound dualities have shown interesting relationships. For example, a three-dimensional world without gravity can be mathematically equivalent to a four-dimensional world with gravity.
Certain dualities suggest that space-time emerges from something more basic, what Einstein called the "spooky" connection between entangled quantum particles.
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