It was not necessary to understand the Theory of Relativity to complete this puzzle but Hedgesparrow meant us to grasp at least one of its elements. The grid represented a region containing a number of interconnected entities, which formed the theme of the puzzle. Extra letters in the wordplay of down clues gave us SHORTCUTS FOR SPACE TIME TRAVEL. The answers to across clues started at their number but finished at another space, always creating new words. Finally, the word WORMHOLES could be highlighted diagonally in the grid.

Wikipedia explains thus - In physics, a wormhole is a hypothetical topological feature of spacetime that is basically a 'shortcut' through space and time. A wormhole has at least two mouths which are connected to a single throat or tube. If the wormhole is traversable, matter can 'travel' from one mouth to the other by passing through the throat.

Extra words in across clues are underlined. Extra letters in wordplay are shown in square brackets.

ACROSS
1 Back Rector to spread Biblical extract around Spain (7)
REVERSE R + VERSE containing E (Spain) 40 DISPERSE
8 Dad rows south to catch river fish (4)
PARS R (river) in PA + S 35 OARS
11 Poem fellow discovered, in essence (4)
COVE Hidden 34 ODE
12 Rescinding last bit of article disclosing changes in syndicate (9)
REPEALING E (last letter in ARTICLE) + PEAL in RING 1A REVEALING
13 Lead window-frame has wood on the end (4)
SASH S (has) + ASH 38 LEASH
14 Choose the wrong moment to stop drinking -- Vermouth's come back into play! (7)
MISTIME IT’S (reversed) in MIME 21 CALL TIME
17 Students' clubs being supported by head of school following university cuts (6)
UNIONS ON + S after UNI 30 DOCKS
18 Eagle, perhaps, in story's first to reach rocky outcrop (6)
INLIER IN + LIE + R(each) 47 FLIER
19 Isolated, left alone, accepts rest cure finally (6)
LONELY L + ONLY containing E 48 RELY
21 Conspiracy to demand repayment of cash up front -- everything included (6, two words)
CALL IN C(ash) + ALL-IN 21 COVIN
25 In Germany, scold orchardman -- one scattering seed (6)
GROWER G + ROWER 13 SOWER
27 One answering awful lies before king is more artful (5)
SLIER Anagram LIES + R (king) 12 REPLIER
28 Plant here, one in total (4)
SIUM I in SUM 44 ADSUM
30 One loading ships (one without friends) cut short ends of rope ladder (6)
DOCKER DOCK + (rop)E (ladde)R 19 LONER
34 Conservative poet is on Tritium, following overdose (5)
ODIST OD (overdose) + IS + T 17 UNIONIST
35 Exposed to view, became more inclined to write in dictionary (6)
OPENED PEN in OED 49 STEEPENED
37 To make front of house sound, mistakenly value compound of Arsenic and Silver, perhaps? (8)
ASPIRATE AS (arsenic) + PIRATE (ref. Long John Silver!) 14 MISRATE
38 Don't retire -- relax, and improvise lute pieces on piano (5, two words)
LET UP Anagram LUTE + P 28 SIT UP
39 York perhaps to seal maiden over -- look, duck (4)
SMEE M in SEE 27 SEE
40 To amuse girl, play bagpipe composition (7)
DISPORT DI + SPORT 8 PORT
43 Intelligent person hit by end of pike -- Jock's to blame! (4)
WITE WIT + (pik)E 39 SMITE
44 Notices blank panel once put in sphere of (eg) solid material, to accumulate thin surface layer (6)
ADSORB ADS + ORB² 18 INORB
46 Making a mistake eagerly desiring to possess a piece of jewellery (7)
EARRING A in ERRING 37 ASPIRING
47 Removing unwanted growth (Dijonaise fairy ring) in pasture (7)
FEEDING FEE (fée = fairy in French) + DING 43 WEEDING
48 Removing tare, neaten unappealing border (3)
RIM TRIM less T 25 GRIM
49 Harder to control older vehicle around railway siding (8)
STEELIER lie in STEER 46 EARLIER

DOWN
1 After mixing, less rust follows as a consequence (7)
RESULTS Anagram LE[S]S RUST
2 Repeat start of series about customary financial management (9)
ECONOMICS EC[H]O + S containing NOMIC
3 Making a promise: "After five, there's nothing to be paid" (6)
VOWING V + O + [O]WING
4 First Lady, at all times right at the top (3)
EVE EVE[R]
5 Train moving iron around Inverness (4)
AIRN Anagram [T]RAIN
6 Girl is consuming a lot of cereal (4)
IRIS RI[C](e) in IS
7 A tangled mass putting us in difficulty (4)
NEST [U]S in NET
8 Locate the once lost traditional learning about arrowheads and suchlike (9)
PLANT-LORE PLAN[T] (locate) + T (the) + LORE (lost)
9 Taking top off releases bird (3)
REE (F)REE[S]
10 Makes an effort to involve British following in races (6)
TRIBES B + [F] in TRIES
15 Irritated Romeo's made sly move! (5)
RILED R (Romeo) + [O]ILED
16 Angus's turn to get nothing in net (4)
TOIL² O in TI[R]L
20 Learnedly manipulated slide-rule (substituting "time" for "length") to get unknown variable (9)
ERUDITELY Anagram [S]LIDERULE changing L to T + Y (unknown)
22 Pale-faced female enters saltern (5)
ASHEN SHE in [P]AN
23 Thin bit of oriental fabric (4)
LENO LE[A]N + O (of)
24 Secures mask with secure twist (9, two words)
MAKES SURE Anagram MASK + SE[C]URES
26 One in support of modern European manners (4)
MODI MOD + I + [E]
29 Pondered and turned left (5)
PORED POR[T]ED
31 Planned school day to include English (current and Middle English) (7)
SCHEMED SCH (school) + D (day) containing E + [I] (current) + ME
32 Fellow teacher's more rash (6)
FASTER F + [M]ASTER
33 Old watch has almost multi-coloured base (4)
SPIE S + PIE(d) + [E]
36 Rising artists forwarded sandstone block (6)
SARSEN RAS (reversed) + SEN[T]
40 Tried reorganising national assembly (4)
DIET Anagram T[R]IED
41 Concerning central section of main arena (4, two words)
IN RE Hidden maIN [a]REna
42 Feel very soft cuddling Virginia Lambert (4)
PALP [V]A + L in PP
43 Dejected Highland sheep eating bit of apple (3)
WAE A(pple) in [E]WE
45 Golf ball's position following initially difficult hazard (3)
DIE D(ifficult) + [L]IE